EnErgy and ChEmistry – an allianCE for thE futurE
From Fuel Cells to
Light-Emitting Diodes
Energy and Chemistry –
an Alliance for the Future
prEFaCE
E
nergy is one of the most fundamental resources of all: given sufficient
energy, we can solve almost all the other problems facing mankind,
for instance desalinate sea water in order to avoid water scarcity, develop
fertilizers to enhance soil productivity in order to produce sufficient food, or
concentrate very dilute resources in order to render them utilisable. at present
our civilisation depends to a considerable extent on fossil energy sources
which, unfortunately, are finite. We will, therefore, be forced to revise our energy management strategies. On the one hand we need to make better use of
known energy sources and thus conserve energy, on the other hand we have
to open up new energy sources. Without research and innovation in chemistry
it will not be possible to master this great challenge to society. For example,
chemistry provides the basis for modern fuels: they can only be produced by
complex chemical processes in refineries. Biofuels, too, depend on chemical conversion processes: thus, biodiesel is produced from rapeseed oil by
a chemical reaction; and new generations of biofuels are even more heavily
dependent on chemical processes. Fuel cells, which are now being tested in
cars as an alternative to conventional combustion engines and are already
partly in use, are inconceivable without chemistry.
This publication presents application fields in which chemistry contributes
to an efficient energy industry. Experts introduce leading-edge developments
and vividly demonstrate, for example, how a tremendous amount of energy
can be saved in road traffic by using novel light-weight materials or how organic solar cells make highly efficient use of sunlight. Similarly they discuss
how hydrogen technology and fuel cells can be applied in future to make
heating or driving a car more environmentally friendly.
Besides these already feasible improvements, the booklet also pinpoints
potential options for highly efficient energy use which will be indispensable if
we are to maintain our standard of living in the decades to come.
The rationale for this publication is to provide an interesting, varied overview of the versatile contributions of chemistry to the energy industry. We
particularly hope that the topics presented will inspire some readers to carry
on the work of developing new solutions so that our children and children’s
children will have an adequate energy supply.
You are sure to derive new insights and impulses from reading this booklet
and I hope that you will find it both pleasurable and informative!
prof. Dr. Ferdi Schüth
Chairman
Coordination Group
Chemical Energy research
3
FOrEWOrD
The human population is growing and resources are becoming scarce.
The resilience of the ecosphere turns out to be limited. This is not a
scenario, but a factual stock-taking, especially as today’s technologies
cannot adequately solve tomorrow’s problems. Never before have the
challenges to science and technology been so great and so pressing –
at least if our standard of living and the foundation of our existence are
to be secured. Here energy plays a pivotal role.
C
hemistry already makes vital contributions to the basic needs of mankind. Food, health, clothing, housing, mobility and communication
are already dependent on chemical products today. This will be even more
the case in the future. But how can chemistry contribute towards solving the
energy problem? Only insiders are aware of the crucial role that chemistry and
chemical engineering play in energy conversion and storage and particularly
in the efficient use and conservation of energy. However, considerable additional efforts are required to further develop and refine our skills to unlock the
full potential that chemistry holds as an even more significant factor in solving
the imminent energy problems.
Energy is a physical quantity. That explains why the intensive public debate
lacks awareness of the importance of chemical conversion processes and of
materials. For instance, a battery is a source of energy. Who would conceive
that it involves a complex chemical system at a very advanced level? Solar
energy, wind-power stations and lighter cars: all rely on chemical products.
In 2008, the German national societies in the area of chemistry and the
chemical industry association published a compilation of essays, addressing
the contribution of chemistry towards solutions for today’s and future energy
problems. These essays demonstrated how research and development in the
area of chemistry and adjacent fields provide smart solutions to the pressing
societal needs in the area of energy. This booklet is meant to serve a general public audience as an interesting compilation, hoping to stimulate further
public engagement in the debate about energy problems and their solutions.
Since the publication of the booklet two years ago, several new initiatives
have been launched to unravel the complex systemic structures of the topic.
Most notably, on a national level, the German national academies of sciences
published a blueprint for a national energy research concept. also, the German societies in the area of chemistry compiled, for the first time, a detailed
quantitative analysis on the impact of improvements within relevant technology areas in the energy sector. Both publications are currently only available
in German.
On an European level, two major research conferences will address the
topic of materials and energy in 2010 while proposed solutions enter the international political debate, as the discussion about biomass conversion to
fuel or the installation of (multinational) smart grids clearly demonstrate.
Even though these new developments have provided significant new insights into the area during the last two years, it remains the conviction of the
publishers that this booklet provides a useful resource to familiarise oneself
with the multiple facets of the topic. It is especially meant to serve as auxiliary
material for educational purposes and hopes to provide through its various
examples an engaging approach for everyone interested in the topic.
Each one of us can help to minimise energy consumption in our everyday
life. One of the best-known examples is the low-energy light bulb. There are,
however, many other means of conserving energy by using innovative materi4
als and technologies at home, at work and at leisure. The first chapter “Energy Consumption in Everyday Life” describes some of the options and shows
that, although energy efficiency requires experts, it concerns everyone.
The subsequent chapters provide an overview of various means of energy
production and storage today and in the future. Will we produce our energy
by means of solar cells or from biomass? are batteries an appropriate storage device or should we use hydrogen as a source of energy? admittedly, it is
not possible to give definitive answers to these questions, but this publication
presents the various options clearly and vividly and provides food for thought.
Energy efficiency and resource protection play an important role not only
in our everyday lives, but above all in industry. New technologies to optimise
products and processes are essential for the future and for the competitiveness of the chemical industry. The last chapter, “New Technologies for Greater Energy Efficiency”, introduces some pertinent examples.
Since the German version of the booklet was very successful and received
very positive responses from various areas of society, the stakeholders decided to prepare an English version to lend support to the international debate
with engaging material and fascinating examples.
We would like to express our gratitude to our authors for all the time
and work they have invested. We are also indebted to professor QuadbeckSeeger for supporting the compilation of this brochure with his help and advice. Finally we would like to thank all those who participated in the conception and realisation of this publication.
prof. Dr. Wolfgang von rybinski
First Chairperson, Deutsche Bunsen-Gesellschaft für physikalische Chemie
(German Bunsen Society for physical Chemistry)
prof. Dr. Michael Dröscher
präsident, Gesellschaft Deutscher Chemiker (German Chemical Society)
Dr. Hans Jürgen Wernicke
Chairman of DECHEMa Gesellschaft für Chemische Technik und Biotechnologie
(DECHEMa Society for Chemical Engineering and Biotechnology)
Dr. andreas Kreimeyer
Chairman, research, Science and Education Committee of
Verband der Chemischen Industrie (German Chemical Industry association)
5
CONTENTS
ENERGY CONSUMPTION IN EVERYDAY LIFE
SMART ENERGY HOME
Energy-efficiency and healthy living
Elmar Keßenich
8
ORGANIC LIGHT-EMITTING DIODES
Colourful pocket-size displays
Frank Voges
11
ORGANIC LIGHT-EMITTING DIODES
Saving energy watching TV
Frank Voges
14
ENERGY EFFICIENCY IN GERMANY
We can all make a difference
Hermann pütter
16
ENERGY FOR A MOBILE SOCIETY
LIGHTWEIGHT MATERIALS
Textiles in vehicle construction
Werner Hufenbach, Martin Lepper and Heiko richter
20
LIGHTWEIGHT MATERIALS
A paper and plastic sandwich
Karsten Müller, Detlef Mies and Stefanie Eiden
22
ENERGY-STORING DEVICES
From a frog’s leg to the lithium-ion battery
Jürgen Janek
24
ENERGY-STORING DEVICES
Lithium-ion batteries for electric cars
Ernst-robert Barenschee
27
HYDROGEN TECHNOLOGY
Tailored storage materials
Michael Hirscher
30
METHODS OF ENERGY SUPPLY
PHOTOVOLTAICS WITH SILICON
From sand to solar cell
Christina Modes
33
ORGANIC PHOTOVOLTAICS
Electricity from plastic
Klaus Griesar
36
SOLAR CELLS
It doesn’t necessarily have to be silicon
Derck Schlettwein
39
THERMOELECTRICS
Wonderful thermal converters
Harald Böttner
6
42
METHODS OF ENERGY SUPPLY
THERMAL WASTE TREATMENT
44
Waste to energy
Marcell peuckert
FUEL CELLS
47
Emission-free energy production
Klaus Funke
COMBUSTION RESEARCH
52
Soon on the back burner?
Katharina Kohse-Höinghaus
RENEWABLE RESOURCES
RENEWABLE FUELS
56
Biofuel from the fields and the ocean
G. Herbert Vogel
BIOFUELS
59
How “bio” is biofuel?
Hermann pütter
BIOGAS
63
Fuel, heat and electricity from the bioreactor
peter Weiland
WHITE BIOTECHNOLOGY
67
Microorganisms in the chemical industry
roland Ulber
BIOREFINERIES
70
Chemical plants and power stations modelled on nature
Thomas Hirth, Walter Trösch and Steffen rupp
NEW TEC HNOLOGIES FOR IMPROVED ENERGY EFFICIENCY
HYDROGEN ECONOMY
72
Many questions still remain
Christian Sattler and Hermann pütter
CATALYSIS
76
Thrifty reaction catalysts
Hans-Joachim Freund
IONIC LIQUIDS
Saline, safe and incredibly versatile
Michael Schmidt
79
REFERENCES AND FURTHER READING
82
LEGAL NOTICE
86
7
SMarT ENErGY HOME
Improved thermal insulation, energy-saving domestic appliances,
light-emitting diodes instead of light bulbs – a great deal of energy
can be saved in house construction and in the household. The Smart
Energy Home project aims to launch innovative technologies on the
mass market.
This low-energy house in Nottingham, England,
has no need of air-conditioning on a hot day,
since the roof has a coating that reflects heat,
thus significantly reducing the temperature
inside.
(Image: BaSF)
Energy Efficiency and
Healthy Living
T
he scarcity of fossil resources,
climate protection, import dependence in the energy sector and the
desire for constant economic growth
in order not to be left behind in global
competition – these are the challenges
which will confront Europe in the next
few years. according to richard E.
Smalley, the 1996 Nobel prize Laureate
in Chemistry, the availability of sufficient
energy is the number one problem facing humanity in the next 50 years.
Initiated by SusChem, the European Technology platform for Sustainable Chemistry (www.suschem.org),
the “Smart Energy Home” (SEH) is a
visionary project committed to sustainable chemistry. SusChem aims to influence European society with respect to
8
energy consumption and thus make a
substantial contribution to climate protection. Some of the technologies necessary for implementing the vision of
energy-efficient, healthy and comfortable living are already in place, others
still remain to be developed.
Despite all these success stories, the
following still holds: as far as climate
protection and energy efficiency are
concerned, products and processes
- both in our everyday lives and in industry - will have to satisfy even higher
standards in the future.
Compared with houses in other regions of the world, European buildings
are among the most energy-efficient.
No wonder, since Europe is the global
leader for new, energy-efficient building
materials and alternative energy systems, thermal insulation and intelligent
control technology for the housing sector. European companies have started
to integrate new technologies into prototype homes and to develop them
into commercially available products.
Energy-efficient buildings, such as
the ‘passivhaus’ (passive house) and
buildings with solar energy plants,
which have a neutral or even a positive energy balance, make an important contribution to climate protection
and protect resources. The savings
potential in commercial and residential
properties is huge. It is imperative that
these advantages and the available
knowledge be promoted. Only if the
market acceptance of new, energy-
E N E R G Y
E F F I C I E N C Y
A N D
efficient technologies is accelerated
will the demand increase and mass
production pay off. and only then will
innovative products be cost-effective
and competitive.
Passivhaus to be the norm
Energy savings on the one hand
and comfortable, healthy living on the
other are integral parts of the SEH concept which controls resources intelligently, thus making sustainable working and living possible.
The most powerful economic lever to encourage energy savings and
mitigate greenhouse gases cost-effectively is to equip buildings with a sound
system comprising highly efficient insulation, heating and ventilation components. according to a recommendation in a recent study by McKinsey,
the management consultancy, in new
housing construction the passivhaus
standard should become the norm.
In houses built prior to 1979 average
energy consumption could be reduced
by approximately one third if three per
cent of the existing building was to be
modernized accordingly each year –
about the rate of older houses due for
modernization each year, i.e. the mod-
H E A L T H Y
L I V I N G
ernization should be linked to maintenance for its financial implications.
Over the past 20 years the lowenergy house and the passivhaus have
reached market maturity. Implemented
in new houses, this is hardly more expensive than in corresponding buildings using conventional construction
methods – yet so far they have failed
to achieve market acceptance in Germany. In the case of existing buildings,
energy-efficient systems, products and
processes are used even more rarely.
although energy savings in buildings
meanwhile rank high on the priority list
of the public and politicians, the average consumer still has reservations in
terms both of cost-effectiveness and
of the comfort and health benefits of
such buildings. The SEH partners aim
to change all that – their holistic approach gives equal weight to optimising comfort, health and energy conservation. The overriding strategic goals
are: enhanced quality of life, lower CO2
emissions and increased economic efficiency extending over the whole lifecycle of a building.
Zero energy consumption
To achieve these targets and come
up with a holistic solution, large industrial companies have joined forces to
form an SEH consortium. architecture
adapted to local needs, conditions and
traditions plays an important role in
the project. To be precise: instead of
standardised, prefabricated houses, a
package is offered, comprising materials, systems, integrative concepts and
integral building management methods, enabling any architect to design
a house conforming to SEH standards.
The buzzword of SEH is “living innovation”, meaning that the project
partners are developing a wide range
of materials – including nanomaterials –
appliances, equipment and integrated
technologies that are more efficient,
for example use less energy and are
more user-friendly than conventional
products. Future products aim to use
all resources efficiently throughout
their life cycle besides taking into account the needs of the end-user, thus
enabling them to have a chance on the
mass market and not end up as niche
products.
Before and after refurbishment: this apartment house in Hopferau, Bavaria, is now a passive house, requiring only one-seventeenth of its former
energy consumption. (Image: röthele)
9
E N E R G Y
E F F I C I E N C Y
A N D
These developments will create
a portfolio of industrial-scale product
innovations, such as next-generation
domestic appliances, for the mass
market. To cite one example: the development of low-temperature detergents together with the appropriate
washing machine. The ambition of
SEH is no less than to pool an array
of technologies in order to reduce net
domestic energy consumption to zero.
Light-emitting diodes
The energy efficiency of light bulbs
leaves much to be desired: in 2005 the
global figure for lighting amounted to
around 2650 terawatt hours – high-efficiency lighting systems could save one
third to one half of this energy, thereby
reducing carbon dioxide emissions by
more than 450 million tons worldwide.
The light sources with the greatest potential are light-emitting diodes (LEDs).
a US Department of Energy report of
December 2006 predicts that, compared with conventional fluorescent
10
H E A L T H Y
L I V I N G
materials, organic light-emitting diodes
(OLEDs) will be twice as efficient, while
having the same lifespan. OLEDs are
set to revolutionise the lighting market,
rendering conventional incandescent
bulbs partly obsolescent in the next
few years and opening up completely
new applications (see the contributions
on pages 11 and 14). OLEDs produce
two-dimensional light that is easier on
the eyes and, since they do not contain
any toxic mercury, they are simpler to
recycle than fluorescent lighting.
a key research area in Germany,
supported by public funds, targets the
development of stable, efficient semiconductors for the ultra-thin LEDs. In
order to exploit the innovation potential
of OLEDs and thus effect their breakthrough to the mass market, universities and large industrial companies are
working together in close collaboration.
Energy consumption in Europe
could be slashed and be decoupled
from economic growth if products like
insulation foams, OLEDs, highly efficient, low-cost photovoltaic systems
and other promising technologies were
to be used in new and existing buildings, in schools, hospitals, administrative buildings and other public buildings. Since such new developments
enhance comfort and are beneficial
to the health of the consumer, there
should be no problem about public acceptance – given a modicum of public
awareness.
Elmar Keßenich
The author is Senior Manager OLED,
BASF Future Business GmbH in Ludwigshafen am Rhein. The author acknowledges the support of his colleagues Uwe Wullkopf and Timothy
Francis in compiling this article.
OrGaNIC LIGHT-EMITTING DIODES
Energy-efficient, flat and video-enabled – organic light-emitting diodes
are ideal as displays in mobile telephones and other small devices.
Surf and phone longer: screens made of organic
light-emitting diodes save the batteries of mobile
appliances.
(Image: Doreen Salcher, Fotolia.com)
Colourful,
Pocket-Size Displays
I
n the past, mobile phones were
just used for making phone calls,
today this fantastic little box of tricks is
a miniature communications centre for
sending short messages and e-mails, it
houses sophisticated photo and video
cameras, plays music and films. Even
television is possible on the little multimedia screens. It is now hard to find
a mobile that does not come with a
full-colour, high-resolution display with
more than 200 pixels per inch (2.54
cm). The pixel size has shrunk to less
than 100 micrometres, the sub-pixel
size for every colour to below 50 micrometres.
Organic
light-emitting
diodes
(OLEDs) are equipped for even higher
resolutions with greater colour depth.
These self-illuminating displays consist
of organic semiconductor layers less
than 100 nanometres thick, positioned
between two electrodes. When a voltage is applied at the electrodes, the
charge carriers are injected into the organic semiconductor which begins to
light up. One of the electrodes is transparent so that the light is visible.
The extremely energy-efficient
OLEDs may one day prolong the running time of mobile appliances since
they consume less electricity, thereby
saving the battery. They are also practical, being ultra-light and flat. If suitable supporting substrates are used,
they are only a few hundred microme-
tres thick. a further advantage is that
such ultra-thin components are relatively flexible and thus less sensitive to
shock.
Customised organic layers
The small displays are produced
by thermal evaporation: organic molecules are deposited on a cold substrate
where they form a thin amorphous film.
an OLED for small displays consists of
several organic layers, each layer having a special function for which organic
molecules are customised.
The layers bordering directly on the
electrodes are responsible for injecting
the carriers – the negatively charged
11
OrGaNIC LIGHT-EMITTING DIODES
xxx
P O C K E T- S I Z E
electrons from the cathode, the positively charged holes from the anode
– into the organic layers. For this reason the molecules of these layers are
chemically constructed so that they
can easily absorb charge carriers from
the electrodes. The next layers serve
to transport charge carriers. They have
to ensure that the positively-charged
holes and the negatively-charged electrons converge as perfectly as possible.
Finally, fluorescent or phosphorescent
emitter molecules, i.e. substances that
can emit light, are located in the emitter
layer. This is the layer where the electrons and holes converge and recombine to form an excited state. When the
excited state returns to the lower-energy ground state, the excess energy is
emitted as light – the display lights up.
The colour of the light depends on the
energy difference between the excited
and the ground state and can be varied
by modifying the molecules.
D I S P L A Y S
0,9
5 20
530
0,8
540
0,7
5 50
5 60
0,6
5 70
0,5
Y
C O L O U R F U L ,
5 80
5 90
0,4
0,3
600
610
6 20
63 0
680
49 0
0,2
480
0,1
470
0
4 60
0
0,5
X
All colours on the display
Colour triangle, in which the colour points for the display emitters are charted: in the colour triangle every colour occurring in nature is determined by two coordinates. For an OLED display the
three primary colours, red, green and blue, should preferably be situated as close as possible to
the edge so that a broad range of colours can be achieved on the display.
For as full a range of colours as
possible to be achievable on the display, the three primary colours, red,
green and blue, are necessary. They
should be located as close as possible
to the edge of the standardised colour
triangle (see image) since their second-
ary colours will then cover a broad colour palette. For red and blue emitters, a
compromise has to be found between
efficiency and location on the colour triangle, since the human eye cannot well
perceive extremely deep blue and red
hues. OLED displays cover the colour
Cathode
space excellently and render nearly all
the colours recognised as standard in
colour television.
The colours on the screen, however, depend not only on the emitter
molecules used but also on the device
Electron transport
Emitter
Hole transport
U–
Hole injection
U+
Substrate
Transparent anode
Light
A Merck employee investigates the lifetime
of materials for organic light-emitting diodes.
(Image: Merck)
12
Schematic layout of an organic light-emitting diode (OLED): different materials are deposited on
a substrate. The active layers of the OLED are sandwiched between two electrodes, the cathode
and the anode. When a current flows, the diode lights up.
C O L O U R F U L ,
P O C K E T- S I Z E
set-up, since the wavelength of light
emitted corresponds more or less to
the layer thickness of the component.
This can cause interference which can
strongly impact the emission spectrum
of the emitter molecules.
In mobile phones, OLED displays
have hitherto mainly been used as
sub-displays, i.e. as small information
displays in addition to the high-resolution, full-colour liquid crystal main
displays (LCD). Since the end of 2007,
more and more appliances have been
launched with top-quality OLED displays which replace the liquid crystal
main displays. If OLED technology lives
up to its promise to combine better
D I S P L A Y S
performance with lower material and
production costs, it will prove fierce
competition to LCDs.
Frank Voges
The author is Head of Laboratory at
Merck, the pharmaceutical and chemical company in Darmstadt. He also
wrote the contribution on the following
pages.
How fluorescence and phosphorescence work
The matter is complex because electrons have a spin which can assume
two possible values. according to a
law of quantum chemistry, the total
spin of a system must not change.
Fluorescence and phosphorescence,
the two forms of luminescence, are
distinguished by their adherence to
this law of spin retention: in fluorescent materials, in fact, electrons do
not need to change their spin. Since
three-quarters of all electrically generated excited states in fluorescent materials have the “wrong” spin direction,
they cannot fall back into the ground
state, and so they do not emit light
– as far as the display is concerned,
therefore, they are lost. In phosphorescent materials, on the other hand,F
charge carriers can change their spin
due to what is referred to as spin-orbit
coupling. In this case, all electrons
can contribute to light productionF
and the efficiency is about four times
greater. For this reason, phosphorescent dyes are preferentially used in
xxx
the emitter layer of an OLED. admittedly, the lifetime of these intricately
structured compounds still has room
for improvement. Only red and green
phosphorescent emitters achieve operational lifetimes in the order of several tens of thousands of hours, even
with greater luminous intensities of
1000 candela per square metre. For
the higher-energy blue, on the other
hand, there are as yet no stable phosphorescent dyes. To date, therefore,
long lifetimes are only possible for
blue OLEDs with less efficient fluorescent emitters.
F
F
F
N
N
Ir
NF
N
O
N
N
Ir
O
N
F
F
Ir
O
O
N
N
Ir
N
N
N
Chemical structure of two phosphorescent emitters known as Flrpic (left) and Irppy (right).
13
OrGaNIC LIGHT-EMITTING DIODES
Large-screen televisions consume a tremendous amount of energy.
What a good thing that organic light-emitting diodes are catching on in
the world of television and are making screens more energy-efficient!
Well, what can we expect next? Nextgeneration television screens are based
on organic light-emitting diodes and thus
consume less energy.
(Image: soupstock, Fotolia.com)
Saving energy watching TV
H
omes designed for today’s lifestyles are inconceivable without televisions and displays. although
the present generation of popular flat
screens are becoming increasingly
energy-efficient, at the same time they
are becoming increasingly large. When
all’s said and done, modern displays
consume more energy than their predecessors. Large plasma screens consume up to 500 watts of electricity, and
in doing so negatively impact both the
environment and one’s wallet.
One remedy for this is organic lightemitting diodes (OLEDs) which convert
electricity very efficiently into light, only
consuming energy when a pixel is lit.
Moreover an OLED is an ideal flat light
source which can be less than one
14
millimetre thick. Sony was the first flatscreen manufacturer to produce an
OLED television, which has been on
the market since 2007.
OLEDs are not only light, flat, energy-efficient and mass-producible, they
also offer a picture that is almost independent of the viewing angle and exciting features, such as a high contrast
ratio and brilliant colours.
Ultra-thin organic
semiconductors
What is organic about these lightemitting diodes (developed by Kodak
scientists Ching Tang and Steve van
Slyke) are the materials at their heart.
These materials consist of carbon-
based, i.e. organic, molecules. Contrary to most organic polymers, which
do not conduct electricity, OLED materials are electric semiconductors. Semiconducting polymers such as these are
also used in the construction of transistors and solar cells. In an OLED, the
organic semiconductor is sandwiched
between a reflective and a transparent
electrode. The semiconducting layer is
one hundred times thinner than a human hair.
Based on the method of preparation, organic semiconductors can be
divided into two classes: there are firstly
small organic molecules, layers of which
are prepared by vacuum thermal evaporation (VTE) and subsequent deposition on a carrier material, and there are
S A V I N G
E N E R G Y
W A T C H I N G
secondly polymers deposited from solution. polymers, in everyday language
referred to as plastics, are macromolecules composed of many individual
building blocks, called monomers,
which are linked together. an OLED
polymer consists of many different
monomers which fulfil various functions.
Some of the monomers, for example,
facilitate the passage of the charge carriers from the electrodes to the organic
layer. Others are designed to transport
the charge carriers. Yet others light up
when a current is applied.
Deposition from solution is particularly suitable for inexpensive production
of large-sized displays. However, the
requisite polymers are still in development and the lifetime of semiconductor devices produced by this method
leaves much to be desired. On the other
hand, for certain colours, such as green
or red, OLEDs produced from small
molecules using VTE already have a lifetime of several hundred thousand hours
even with high luminous intensities. The
lifetime of a small full-colour display produced by VTE, however, depends on
T V
the most unstable colour – usually blue
– and is about ten times shorter.
Optimising molecules
Organic chemistry provides the
means of tailoring molecules, and thus
of varying, for example, the colour they
emit. By tailoring the molecular structure it is possible to control the colour
emitted by the molecule in wide limits.
However, this also has an impact on
the current flow in the light-emitting
diode.
Those who are currently planning to
launch OLEDs on the television market
will still find themselves in a dilemma:
the production of large-area organic
semiconductors by VTE is still too expensive, and the quality of semiconductors made for the less expensive
liquid deposition is not satisfactory. Not
until OLEDs have cleared one of these
technical hurdles will they be in a position to hold their own on the market.
Yet other properties of the molecules, for instance stability towards
the electrochemical and electrooptical
processes in an OLED, can be adjusted by modifying the molecule. By
these means the lifetime of light-emitting
diodes can be increased.
Tailoring the side chains also facilitates the production of OLEDs. For
example, the solubility of molecules
can be adapted to the solvents used in
individual production processes.
Frank Voges
The author is head of laboratory at
Merck, the pharmaceutical and chemical company in Darmstadt. He also
wrote the contribution on the preceding pages.
Not only flat, but also energy-efficient: the first OLED television, manufactured by Sony.
(Image: Sony)
15
ENErGY EFFICIENCY IN GErMaNY
Cars are becoming more and more fuel-efficient, German industry is
stepping up its energy efficiency – the Federal Republic seems to be on
the right track to solving the energy problem. However, when we scrutinise the data, it becomes evident that the efficiency jumps belie reality.
(Image: bilderbox, Fotolia.com)
We can all make a difference
I
we private citizens indirectly use even
more energy.
n 2007 the United Nations Environment programme (UNEp)
published the report Global Environment Outlook GEO-4. This almost
600-page report summarizes the impact of economic, political, technical
and ecological interdependence on
sustainable development. The introductory overview cites two threats to
the energy supply: the insecurity of
supply and the environmental impacts
of excessive exploitation.
of other nations. It is simply a matter
of fairness: if there is to be sufficient
energy for the whole human race, we
should confront wasteful inefficiency
with energy efficiency. This is not just
the view of the German chancellor, it
is to be found in numerous handbooks
on climate change and energy saving.
Well, where do we stand?
20 tonnes of waste per capita
per year
Hence, 82.5 million individuals consume the lion’s share of Germany’s energy volume. If it is taken into account
that we pass on a share of our energy
with our export goods, our energy balance slightly improves. Nevertheless,
the bottom line is that the average
inhabitant accounts for 5.5 tonnes of
coal units per year.
Both prompt the question: do German households use energy wisely?
This is not a question of whether we
Germans should play a pioneering role
as model pupils or whether all our efforts are meaningless, given the dramatic rise in the energy consumption
private households are Germany’s
largest energy consumers. Households
rank just above transport and industry
and far ahead of trade, commerce and
services. In addition, a large share of fuel
is used by private vehicles. Moreover,
by consuming goods and services,
The consumption of energy is
closely linked to the consumption of
material resources. The average German consumes 20 tonnes of material annually, including goods whose
production is highly energy-intensive,
such as steel, cement, aluminium,
16
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M A K E
A
D I F F E R E N C E
copper and glass. In fact, these materials generally go into the manufacture
of durable goods, cars, houses and
household appliances. particularly in
the case of glass and certain metals,
however, the market has developed all
the symptoms of a throw-away culture.
Consumers simply view packaging as
rubbish, even though they separate it
for recycling. The collection of glass in
waste glass containers shows how our
consumer culture has eroded: gone
are the days when empty wine bottles
were regarded as something of value
or as returnable empties.
more water for cooling than the drinking water supplied to our households
by the waterworks.
Our consumption of energy closely
interacts with our resource requirements and our lifestyle. There are good
ecological, economic and political reasons for increasing energy efficiency in
our country. after all, we are not alone
on our planet and the number of people who are closing the gap to our
standard of living is rising rapidly.
If all the material that Germans
consume daily was delivered to them
at the breakfast table, it would correspond to the average German’s body
weight. By adding the material that is
required to extract and process the
necessary resources, part of which is
left over as residues and waste, the
amount is far higher.
There are two ways by which each
individual can reduce his or her energy
needs: by economising on energy and
by dematerialising consumption. What
first comes to mind is to do both with
moderation, since action for action’s
sake is not the right strategy. Our technology and our awareness represent a
foothold, but not a launching pad.
Water consumption, too, is connected with our energy supply. German power stations require five times
at first glance Germany seems to
be on the right track. after all, the specific fuel consumption of a car dropped
Energy efficiency and lifestyle
from 8.9 litres per 100 kilometres in
1995 to 7.7 litres in 2005. Furthermore, the national energy intensity,
which is recognised as a measure of
energy efficiency, has decreased by
11 per cent since 1995. although the
gross domestic product has risen by
14 per cent, energy consumption has
remained almost stable, increasing by
only 1.5 per cent. Industry is further
ahead: between 1995 and 2004 the
energy intensity of German exports fell
by almost 24 per cent. resource intensity has also decreased. Even the federal government’s target of halving this
figure between 1994 and 2020 seems
to be within reach. By 2005 resource
intensity had already declined by 21
per cent.
Such figures give the impression
of efficiency jumps, but unfortunately
they belie reality. Technology and the
changeover from petrol to diesel have
certainly reduced the specific fuel consumption of cars. Overall fuel consumption on the roads, however, has
not declined, but remained constant
since more and more goods are trans-
A symptom of throw-away culture: empty
wine bottles used to be returnable empties
for cleaning and refilling; now, at best, they
land in the waste glass container. Such
changed behaviour patterns also cause
energy consumption to increase.
Since more and more goods are transported by road, overall fuel consumption by road traffic is
not decreasing – even if individual cars use less fuel.
(Image: bilderbox, Fotolia.com)
(Image: Manfred Steinbach, Fotolia.com)
17
ENErGY EFFICIENCY IN GErMaNY
Energy consumption in Germany
ported by road and there have been no
significant reductions in private vehicle
use. There can be no question of an
efficiency strategy.
Overview of energy consumption in Germany in 2006
(HCU = hard coal units)
Primary energy consumption 2006: 498 million tonnes HCU 1)
Final energy consumption 2006: 319 million tonnes HCU
Share of final energy consumption
Private energy consumption
Direct
28,6 %
Households
Energy for
and/or
in goods
and
services
Traffic
28,2 %
27,9 %
Indirect
Industry
Energy in
export goods
(252 m t HCU)
Energy in
imported
goods
(211 m t HCU)
15,3 %
Trade, commerce, services
Values for 2004 2)
1) arbeitsgemeinschaft Energiebilanzen (energy balance working group) 09/2007
2) Statistisches Bundesamt (Federal Statistical Office), press conference 13.11.2007
Efficiency achievements in Germany
Change since 1995
-20%
-10%
Fuel consumption (2005) 1)
private vehicles
passenger traffic
freight traffic
Resource use (2005) 1)
resource intensity
resource consumption
gross domestic product
National energy efficiency
energy intensity
energy consumption
gross domestic product
Energy intensity in export trade (2004) 2)
total exports
paper % pulp
chemistry
engineering
+10% +20%
1) Stat. Bundesamt (Federal Statistical Office): Environmental Data Germany 2007
2) Stat. Bundesamt (Federal Statistical Office): Environmental-economic aspects of Globalisation, press conference 13.11.2007
18
The data on energy intensity are
also misleading since the increase in
national energy efficiency has stagnated at one per cent per annum –
increasing consumption counteracts
any technological advances. Moreover
exports owe the main share of their
efficiency gains on the one hand to a
different basket of commodities and
on the other to drivers from the paper
and pulp industry, the chemical industry and engineering. Nor is the picture
so bright in terms of resource intensity,
since the statistics only account for part
of material consumption – they do not
include agricultural products or metal
imports, the latter amounting to 50 million tonnes per year. The extraction of
metals itself is highly energy-intensive,
it generates huge amounts of residues,
and often leaves critical waste behind.
Through our consumerism we export
environmental problems.
On balance, these examples reveal
a strange phenomenon: technology
and consumerism go their separate
ways. Despite many positive exceptions, our society as a whole is stuck in
a business-as-usual rut. In contrast to
the world of employment, our private
lives are scarcely affected by energy
balances and notions of sustainability.
Efficiency evolves from technological
change, not from more rational behaviour on the part of consumers.
This is also borne out by our greenhouse gas balance: it is anticipated
that Germany will be one of the few
countries to achieve the Kyoto targets. Three factors have contributed
towards the reduction in greenhouse
gases: the optimisation of electricity
generation – this includes the changeover from coal to natural gas, the ex-
W E
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M A K E
A
D I F F E R E N C E
pansion of renewable energy sources,
and efficiency improvements in industry, particularly in the chemical industry. Nevertheless, the upward trend of
carbon dioxide emissions and domestic energy consumption in Germany is
slow but steady.
Convincing concept
It is evident that technological innovations alone are not sufficient to
curb individuals’ demands on nature.
These demands go far beyond the
production of goods and energy, since
increasing prosperity goes hand-inhand with increasing expectations of
an intact, aesthetically attractive environment with tourism potential. Many
regard technology as a nuisance: wind
generators, it seems, impair the landscape, power stations of any kind are
viewed with suspicion, action groups
militate against the expansion of the
electricity grid.
The chemical industry is also in
a dilemma. Its services are traditionally only indirectly geared towards the
consumer. In everyday life chemistry is
invisible although it is ubiquitous. More
factual information cannot resolve this
paradox. Would consumers change
their habits with regard to energy efficiency if they had an in-depth knowledge of chemistry? probably not –
even chemists tend to behave like the
average citizen within their own four
walls. What is true of chemistry applies
equally to other areas of technology:
knowledge alone has little power of
persuasion.
Those who step forward with one
foot, but drag the other behind, do
not get very far. Different attempts are
being made to resolve this challenging predicament, some using appeals,
others graphic descriptions. The intention of both is to develop a convincing
concept.
Two current experiments in the
German-language region should serve
to illustrate this point. The technical, scientific, medical and social science academies of Switzerland have
compiled an “energy memorandum”,
“Denk-Schrift Energie”, which tackles
the question of how Switzerland can
bring about an “efficiency revolution
in terms of resource and energy use”.
The text is an “appeal to those responsible in Switzerland” for sustainable development. Not only are technical and
scientific aspects put forward, but also
ethical and ecological viewpoints.
Last year a project on a much
larger scale was initiated in Germany.
The foundation “Forum für Verantwortung” (forum for responsibility) pursues
the same aim with a series of twelve
paperbacks focusing on the topics
energy, resources and the consumer
society. In this case, too, the authors
represent a variety of academic disciplines. They reveal how complex our
attitude towards our planet is. When
pieced together, their arguments make
up a picture that is as impressive as it
is alarming. The two publications are
written in a clear, comprehensible style.
The question remains whether the
lightweight or rather the heavyweight
will provide our society with the necessary impulse. We can only wish both
success.
Hermann Pütter
The author worked for many years until
his retirement for BASF, his last position being that of Scientific Director
and Head of the Electrochemical Processes Research Group. Since 2007
he has been the Energy Coordinator
of Gesellschaft Deutscher Chemiker
(German Chemical Society). Hermann
Pütter also wrote the contribution on
page 59 and is co-author of the article
on page 72.
19
LIGHTWEIGHT MaTErIaLS
The weight of a car determines its fuel consumption. For this reason lightweight materials are increasingly catching on in vehicle construction. The
novel materials contain, for example, tube-shaped fabric – and are no less
robust than conventional sheet metal.
Image: Fantasista, Fotolia.com
Textiles in vehicle construction
M
obility is a prerequisite for the
economic development of
a country as well as for the personal
development of every human being. It
permits division of labour by location,
satisfies individual and social needs
and, against the background of globalisation, is more than ever a basic
function of our existence and economic activity. Individual mobility, however,
costs energy – road traffic accounts for
28 per cent of total energy consumption in Germany – and causes carbon
dioxide and other air pollutants.
In these times of climate change
and dwindling oil reserves, lightweight
materials and structures are becoming increasingly important in vehicle
construction, since lighter cars use
20
less fuel and consequently emit less
exhaust fumes. Lightweight materials
have long since been standard in aircraft construction, however the mass
production of cars calls for different
materials and processing techniques.
Flexible composites
aluminium instead of steel, polymers instead of metal, thin instead of
thick sheet metal - lightweight engineering knows no end of variations.
One special example of lightweight
construction materials is fibre composites. Fibre composite materials
consist of a basic material, the matrix,
reinforced by a fibre skeleton. In the
main, polymers, but also ceramics or
metals, serve as the matrix. The rein-
forcements generally consist of carbon
or glass fibres. These composites are
extremely flexible: both their form and
their properties, such as strength or
stiffness, can be well adapted to the
design of components and the stress
they will be exposed to. In fact, they
seem to be predestined for vehicle
construction, the more so as new fabrication techniques facilitate mass production of such components at competitive prices.
Composites made of technical
textiles, a two-dimensional structure
woven from hybrid yarn, require more
sophisticated production. Hybrid yarn
consists of two or more different fibres.
The reinforcing carbon fibres are, for instance, swirled together with a polymer
T E X T I L E S
I N
V E H I C L E
C O N S T R U C T I O N
inforced polymers are ideal for lightweight engineering of machinery and
automobile parts subject to particular
stress since they clearly reduce their
weight without impairing their function
and performance.
cars could weigh about one third less.
It is not possible to determine exactly
how much fuel they save. One thing is
certain, however: lighter materials are
one of the most important levers in vehicle construction to cut fuel consumption and carbon dioxide emissions.
Integrated sensors
Carbon-fibre-reinforced composite: roof and
front end for a car.
(Image: ILK, Dieffenbacher)
fibre that later forms the matrix material. Subsequently technical textiles
with specific structures are woven from
such yarns to form, for instance, a tubular or honeycomb skeleton. Further
on in the process the textile structure
is heated in a die so that the polymer
fibres melt and the polymer flows between the carbon fibres. This is how
the matrix of the composite is formed.
The reinforcing carbon fibres, on the
other hand, remain in the ordered skeletal structure, making the light material
particularly stable.
Composites made of textile-re-
In vehicle construction not only
lighter weight, but also other aspects
of textile-reinforced composites are of
interest: the new materials have better
damping and crash-protection properties than conventional ones since they
are better able to absorb energy, such
as impact or vibrational energy. another practical feature is that sensors can
be embedded directly in the composite
materials, for example sensors that activate airbags can be integrated directly
into bumpers. Moreover safety components made of composites can be permanently controlled by integrated sensors. This renders periodic inspections
superfluous, thus cutting maintenance
costs. The sensors can be either applied to the composite or integrated
directly into it during fabrication.
This combination of enhanced
consumer convenience and fuel savings due to reduced weight makes
lightweight engineering such a promising trend in the automotive industry.
Thanks to novel materials, tomorrow’s
Werner Hufenbach, Martin Lepper
und Heiko Richter
Werner Hufenbach is Director of the Institute of Lightweight Engineering and
Polymer Technology at Technische Universität Dresden, where Martin Lepper
and Heiko Richter are co-workers.
From hybrid yarn to technical textile: lightweight vehicle components are manufactured from such fabrics.
(Image: Sonderforschungsbereich 639)
21
LIGHTWEIGHT MaTErIaLS
Polyurethane systems, for example from BaySystems, make it possible
to manufacture lightweight, stable car components at a competitive
price. They reduce fuel consumption and carbon dioxide emissions as
well.
Opel Zafira roof module
A paper and plastic sandwich
T
he dwindling supply of fossil resources combined with
the increase in global carbon dioxide
emissions are the key drivers of advances in car production. Their aim is
to reduce fuel consumption and thus
emissions. The automobile industry is
optimising established drive concepts
and exploring novel ones, increasing
the efficiency of energy users inside
vehicles, reducing their air and rolling
drag, and cutting the weight of parts
and components. Only the application
of lightweight materials can achieve the
last-mentioned target.
Up to 50 per cent lighter
One example of lightweight structural elements that automotive suppli-
22
ers are already manufacturing in large
quantities is polyurethane sandwich
components. To this end, lightweight
paper honeycomb cores are placed
between two glass fibre mats, spraycoated on each side with a polyurethane reaction mixture and subsequently compressed in hot moulds.
The slightly foaming reaction mixture
bonds the individual sandwich constituents, after curing, securely and
permanently. By this means structural
components weighing significantly less
than three kilograms per square metre
can be manufactured and used in cars,
for example as floor panels for the passenger room, the boot and the sparewheel recess, and as sliding roof modules. With these materials components
can be 25 to 50 per cent lighter than
those made from conventional materials. The combination of light, pressureresistant paper honeycombs and fibreglass-reinforced outer layers a few
tenths of a millimetre thick enables the
sandwich components to bear heavy
loads with only moderate deformation.
Even components with large wall
thickness variations, thick edges, force
transmission points and inserts can be
fabricated without difficulty. at sharp
angles and edges larger amounts of
polyurethane and a simultaneous addition of glass fibres ensure good surface
qualities and the necessary reinforcement in the load transmission and insert areas.
A
P A P E R
A N D
P L A S T I C
S A N D W I C H
Novel car roofs – lighter and
more impact-resistant
an additional development is that
of polyurethane systems for roof modules reinforced with long glass fibres.
In this case a thermoplastic polymer
film forms the outer skin. Fasteners
and inserts can be integrated and also
textile designs on the inside of the vehicle interior. In the production all elements are placed in a foaming mould.
On heating, the polyurethane reaction
mixture, introduced into the lower shell
together with the glass fibres, bonds all
the components.
Since these roofs are extremely
robust and impact-resistant and do
not splinter when overloaded, they enhance passive safety. The advantage
for the design engineer is that, similar to aluminium or steel, heat causes
polyurethane panels to expand. This
promotes adhesion, for example of the
metal body to the roof module. Bayer
MaterialScience and partners from the
automotive sector are currently developing a roof module that is at least 25
per cent lighter than a conventional
metal roof.
Light thanks to the voids: sandwich material with paper honeycomb core.
Karsten Müller, Detlef Mies
und Stefanie Eiden
Karsten Müller and Detlef Mies are
experts for the development of lightweight car parts at BaySystems, and
Stefanie Eiden works in Product Design and Nanotechnology at Bayer
Technology Services.
Sandwich load floor of the VW Tiguan, fabricated by a polyurethane spray system.
23
ENErGY-STOrING DEVICES
Primary and secondary batteries have been part of our everyday lives
for decades. They supply the digital society with takeaway energy. The
development of efficient electric cars may well add a thrilling chapter
to the success story of the battery.
From a frog’s leg
to the lithium-ion battery
O
ver 200 years ago in the anatomy laboratory of Bologna
University, Luigi Galvani discovered
that if two different metals, attached
to a dissected frog’s leg, were brought
into contact, the leg twitched. Galvani
called this phenomenon “animal electricity”. Meanwhile the galvanic cell,
the voltaic pile and the first simple
batteries of individual cells connected
in series have given way to efficient,
rechargeable electrochemical energystoring devices in all different shapes
and sizes. Tiny batteries are used in
chip cards, button cells supply power
to clocks and watches, and lithium-ion
batteries power portable devices and
cordless power tools. annual sales of
primary and secondary batteries probably amount to several billion world-
24
wide. Over 90 per cent of them come
from Japan, Korea and China. In the
race to produce efficient hybrid drives
and battery-powered cars there is also
a renewed surge of interest in the development of new batteries in Europe
and the USa. In this context electrochemical research assumes considerable importance.
More power and safety
The principle of a galvanic element
is surprisingly simple, yet the development of batteries that store more energy, supply more power and, in the case
of secondary batteries, are rechargeable represents a challenge to research.
When all’s said and done, modern batteries still consist of two electrodes –
the cathode and the anode – and the
electrolyte, but stability, performance
and safety standards necessitate additional components. The modern
lithium-ion battery is a good example:
it requires inert separators to separate
the anode and cathode and to accommodate the electrolyte, electronically
conductive aggregates to ensure contact between the electrode materials
that store lithium and the current collectors, and yet other components to
improve the configuration of stabilising
insulating layers. Like the fuel cell, the
battery is an electrochemical system
whose parts have to be perfectly coordinated if it is to function at an optimal
level.
F R O M
A
F R O G ’ S
L E G
T O
Current research on electrochemical energy-storing devices focuses
primarily on the synthesis and function of new battery components. On
the other hand, the development of
the maintenance-free lead acid battery
shows that even well-established battery types still have tremendous potential for improvement. Besides standard
batteries for the mass market, many
special systems have been developed
for the most diverse applications – from
the zinc-air battery as a power source
for electric fences to the rechargeable
nickel metal hydride battery for hybrid
vehicles.
Beltways for ions
The quest for novel ion conductors
and for an understanding of the mobility of ions in liquids is a particularly
challenging task for battery researchers. The ion-conducting electrolyte
ensures the internal electrical contact
between the anode and the cathode.
With current flow, i.e. when charging
and discharging, it makes a vital contribution to the internal resistance of the
battery and to power loss through heat
emission. For this reason electrolytes
must necessarily be good ion conductors. They are generally liquid or
at least so viscous that ions can travel
in them without generating too much
heat. There are, however, a number of
solids, glass or crystals which, thanks
to their atomic structure, provide beltways for ions.
Some of the most intensive research is concentrating on discovering
solids with good lithium-ion conductivity at room temperature. In the case of
sodium, chemists made a discovery as
long as fifty years ago: using a special
sodium-aluminium oxide, which has
a characteristic layered structure, a
rechargeable battery was built based
on the reaction of sodium with sulphur
to produce sodium sulphide. Since
T H E
L I T H I U M - I O N
B A T T E R Y
the battery works with liquid sodium
at temperatures in excess of 300 degrees Celsius and the ceramic sodiumaluminium oxide membrane is relatively
sensitive, safety reasons preclude its
suitability for cars. In Japan, however,
research is now concentrating on its
potential as an alternative energy storage medium for stationary and largescale applications.
It is quite possible that, in the future, hybrid materials made of highly
viscous polymers and inorganic substances, and therefore with a high internal surface area, will facilitate the
development of new batteries. Such
hybrid materials could even act as both
electrolyte and separator, thus simplifying the cell structure. Chemists are
currently working at full steam on such
materials.
Nanomaterials for maximum
performance
Crucial processes take place at the
electrodes during battery discharge: at
the anode one substance releases electrons, accordingly another substance
at the cathode takes up electrons. at
the cathode, therefore, the supply of
electrons from the metallic input line is
necessary. at the same time, to maintain electroneutrality the electrolyte has
to provide or take up ions. Here a fundamental kinetic problem is exposed:
charging and discharging of batteries
at a high rate requires correspondingly
rapid mobility of ions, coupled with
satisfactory electronic conductivity not
only in the (generally) liquid electrolytes,
but also in the solid electrodes. The
fact that ions mostly move very slowly
in solids at room temperature is a basic
problem for the rate of charge and discharge cycles. To increase the rates
necessitates shortening the paths for
the ions in the solid. Consequently, in
batteries the materials of choice consist of very small particles, known as
crystallites. The logical strategy of materials research at present is to focus
on nanostructured electrode materials,
particularly in the case of electrodes
for lithium-ion batteries. Nanostructur-
The dawn of battery research: Luigi Galvani’s experiments with frogs’ legs.
25
F R O M
A
F R O G ’ S
L E G
T O
T H E
L I T H I U M - I O N
B A T T E R Y
Scarcity of resources
for batteries?
Over two centuries after Luigi Galvani’s frog experiment the battery has
developed from a disposable article to
an indispensable item of the technical
stock-in-trade of our mobile society.
assuming that this equipment will rapidly expand to encompass electronic
books, electromechanical aids for the
elderly, robots and other items, the last
thing we can expect is that this development will grind to a halt.
as a result of mass production the
availability of important chemical substances could pose a problem for the
development of novel batteries. The
supply of many elements, in particular
transition metals like cobalt and nickel,
is not infinite and prices have risen significantly over the past few years. For
this reason chemists and other scientists are working not only on improving
known batteries, but also increasingly
on recycling them and on discovering
completely new electrochemical solutions.
Jürgen Janek
The author is Professor for Physical Chemistry at the Justus-LiebigUniversität in Gießen and Head of the
TransMit-Zentrum für Festkörperionik
und Elektrochemie.
ing entails ball-milling materials so that
individual crystallites measure only a
few nanometres (10-9 metres) or preparing them by customised synthesis
as nanoparticles or as materials with
nanopores. This electrode architecture
with nanoscale structures considerably
enhances the power of batteries.
a do-it-yourself battery: two small metal plates, one of copper, the other of zinc, are placed in half an orange and half a
lemon respectively. as soon as the electric circuit is closed –
here through the wires and the electrolyte bridge at the front of
the picture – a chemical reaction takes place. Since zinc atoms
more readily give up their electrons than copper atoms, zinc releases electrons to copper – an electric current flows and the
bulb lights up. The lemon juice conducts the electricity and
serves as the electrolyte
(Image: Eva Mutoro/Gießen)
26
ENErGY-STOrING DEVICES
Lithium-ion batteries have long since been the power source of choice
for mobile telephones and other portable devices. In the future they
could even serve as an energy storage system in electric cars. To do
so they have to be become safer.
This test hybrid car has run on a lithium-ion
battery for five years.
(Image: Evonik)
Lithium-ion batteries
for electric cars
O
f all electric energy storage
systems, the lithium-ion battery is accepted as the most promising one. On account of their high cell
voltage of 3.6 volts, rechargeable
lithium-ion batteries store more energy
per volume and supply more power
than conventional batteries. These two
parameters are an important factor,
including with respect to their application in vehicles: the amount of energy
stored determines the driving range
of an electric vehicle. power density,
in turn, must be high enough to accelerate it. Thanks to the high energy
density, high-performance lithium-ion
batteries can be composed of relatively
few cells. Moreover, in contrast to nickel-metal-hydride batteries, they exhibit
neither a memory nor a lazy battery effect. With conventional batteries these
two effects reduce the overall capacity
if the batteries are not fully discharged
before recharging. Lithium-ion batteries, on the other hand, can be fully or
partly discharged without compromising the lifetime. a further benefit of the
lithium system is its low rate of selfdischarge. This enhances the battery’s
efficiency and prevents damage during
storage.
ers and other portable devices are
powered by rechargeable lithium-ion
batteries. Modern large-format energy-storing devices, however, are also
suitable for many other applications,
primarily for vehicles with hybrid or fully
electric drive and also for electric boats,
bicycles and scooters. In view of the
steady rise in fuel prices and targeted
reduction of carbon dioxide emissions,
these drive systems are predicted to
have good market potential.
In mobile telephones, cars
and solar systems
another promising application of
large-format lithium-ion batteries is in
mobile and stationary storage systems
in industry – for instance in the electric
drive trains of machinery and transport equipment – and also in industrial
Meanwhile over 99 per cent of
small-format batteries in mobile telephones, laptops, cameras, Mp3 play-
27
ENErGY-STOrING DEVICES
and domestic energy-storage systems
powered by renewable energy from
wind and solar plants. The advantage
is that large-format, lithium-ion-based
stationary batteries can act as power
buffers: during times of energy production surplus they charge, while they are
discharged when the energy demand
is higher than production. Thus they
are capable of reducing peak loads
and of supplying power to domestic
and administration buildings without
interruptions – this would significantly
boost efficiency on the electricity market.
according to prognoses, the market volume for large-format lithium-ion
batteries is set to exceed the 10 billioneuro mark in 2015. Strong market
penetration depends on a continuous
increase in the power output, lifespan
and safety of lithium-ion batteries coupled with a decrease in costs.
Specially structured electrodes
Electrodes for lithium-ion batteries
generally consist of tiny particles, a few
Ceramics off the roll: the ceramic separator
increases the safety of rechargeable lithiumion batteries.
(Image: Evonik)
28
microns large, of active material bonded on a metal foil, the current collector, in a layer about 20 to 200 microns
thick. On the negative electrode there
are, for example, graphite particles, on
the positive electrode tiny lithium-metal
oxide particles. The particles can store
and release lithium ions, thus facilitating the charge and discharge of the
battery.
For the coating, the particles are
finely distributed in a solvent together
with a binder. The exact composition
of this dispersion is adapted to the individual coating technology and the desired electrochemical parameters of the
electrodes. The weight per unit area of
the layer, its height, width, porosity and
contour determine the properties of the
rechargeable lithium-ion battery. If the
influence of the structure of the electrodes on the properties of the cell is
known, it is possible to tailor the electrodes to the individual application. For
instance, cells destined to store large
amounts of energy per volume require
thick electrodes with a large amount of
active material. The thick layer, how-
ever, exhibits high electrical resistance,
hence the power density is lower. Thin
layers, on the other hand, produce
more powerful batteries, but can store
comparatively less energy.
Ceramic membrane instead
of polymer film
In the lithium-ion battery a membrane separates the negative from the
positive electrode, thus ensuring that
the cell functions. at present porous
films made from polymers like polyethylene or polypropylene are used as
separator membranes which, however,
melt at temperatures above 140 and
170 degrees Celsius respectively. The
films cannot withstand overheating,
which can arise in large-format batteries due to heavy loads or abuse. Such
high temperatures can then cause a
short circuit, which leads to battery
failure.
Scientists at Evonik have developed a flexible ceramic membrane for
high-performance batteries. It consists
of a polymer support with a ceramic
A roll of cathode material for rechargeable lithium-ion batteries.
(Image: Evonik)
L I T H I U M - I O N
B A T T E R I E S
coating. On account of its ceramic
properties this separator membrane is
far superior to the polyethylene or polypropylene films applied hitherto. The
ceramic membrane exhibits greater
resistance to heat and chemicals, can
be more rapidly wetted by electrolytes
and displays an excellent charging
performance. Safety tests have shown
that the ceramic membrane makes
large lithium-ion batteries so safe that
they can be used in modern hybrid and
fully electric drives.
In the past, all attempts to bring
electric vehicles to market have failed,
the crux being the battery technology.
The batteries tested included lead,
nickel-cadmium and the less known
“zebra”, a sodium-nickel chloride battery. Today’s hybrid vehicles use nickelmetal hydride batteries, but in terms
F O R
E L E C T R I C
C A R S
of energy and power density they are
significantly inferior to lithium-ion batteries. a hybrid car equipped with a
lithium-ion battery has meanwhile travelled more than 90 000 kilometres in a
test run over five years, proving that the
new battery technology is suitable for
everyday use.
Ernst Robert Barenschee
The author is Managing Director of
Evonik Litarion, a producer of electrodes
and separators for lithium-ion batteries, located in Kamenz, Saxony.
How the lithium-ion battery works
+
a lithium-ion battery comprises an anode, a cathode, a separator and an
electrolyte. The separator insulates
the anode and the cathode electrically,
while the electrolyte, which infuses the
electrodes and the separator, guarantees ionic conductivity.
-
Separator
Cu
Cathode
+
Li
Anode
Al
+
Li
+
Li
Legend
Carbon (graphite)
Non-aqueous
electrolyte
Lithium-metal oxide (cobalt)
Lithium
Charge method
Oxygen
Discharge method
The anode and the cathode consist of
thin metal foils as current collectors.
They are coated with active material
capable of inserting and releasing lithium ions. During discharge the lithium
ions move to the cathode. as soon as
a current is applied, the ions migrate
through the electrolyte to the anode
where they are stored. During charge
this process is reversed. By using specific active materials, this process is
highly reversible, permitting a lithiumion battery to handle a high number of
charge/discharge cycles.
29
HYDrOGEN TECHNOLOGY
Hydrogen cannot be used as a fuel source for cars until the appropriate tanks have been developed. Novel metal-organic storage materials
could solve the problem.
Viewed under the electron microscope:
nanocubes made of metal-organic framework materials could solve the problem of
hydrogen storage.
(Image: BaSF)
Tailored storage materials
F
ossil resources are limited and
their combustion has a drastic
impact on climate change. an alternative energy economy based on regenerative sources requires an efficient
energy carrier, especially for mobile
applications. Its high weight-specific
energy content makes hydrogen an
attractive candidate. Its competitive
edge: when used in a combustion engine or, with even greater conversion
efficiency, in a fuel cell, only water is
produced as exhaust.
One of the crucial challenges still
facing the use of hydrogen technology for mobile applications is hydrogen
storage: the volumetric and gravimetric
storage density of hydrogen tanks cannot yet compete with today’s petrol and
diesel tanks. Furthermore, refuelling
30
should take no longer than five minutes
and the storage system should cost no
more than 100 euros per kilogram of
stored hydrogen.
Currently available tanks store hydrogen either as a high-pressure gas
or as a liquid at extremely low temperatures. However, they have fundamental disadvantages: liquid tanks are too
bulky and most consumers feel uneasy
about gas tanks on account of the high
pressures. One solution to these problems would be to store hydrogen by
chemically or physically bonding it to
lightweight solids.
Chemical storage as a hydride
In the case of chemical storage,
known as chemisorption, research
mainly concentrates on hydride storage systems that can be reversibly
loaded in the vehicle. Hydrides are
chemical compounds formed by the
reaction of hydrogen and metals. With
this type of storage the hydrogen molecules break up on the surface of the
metal and dissolve into the metal. By
this means more hydrogen per volume
can be stored in a metal than in the
liquid state.
The storage of hydrogen in a metal
was observed for palladium as far
back as 1866. admittedly, relative to its
weight, palladium stores comparatively
little hydrogen. Other elements, for instance titanium and magnesium, certainly absorb more hydrogen, but they
only release it at temperatures above
250 degrees Celsius. These metals
T A I L O R E D
S T O R A G E
M A T E R I A L S
cannot, therefore, be used for hydrogen storage in cars.
Complex hydrides based on alkali
metals and aluminium are proving to
be more suitable candidates. In the
mid-nineties the researchers Borislav
Bogdanovic and Manfred Schwickardi
of the Max-planck-Institut für Kohlenforschung (coal research) in Mülheim
an der ruhr discovered that, when
doped with small amounts of titanium, a sodium-aluminium hydride can
readily absorb and release hydrogen
at around 100 degrees Celsius. Since
this discovery, this substance – sodium
alanate or NaaIH4 – has been the subject of intensive investigations and is
today the most highly developed complex hydride storage system.
peratures, round about the temperature of liquid nitrogen (-196 degrees
Celsius), and, in fact, best by nanoporous materials. On account of their
innumerable tiny pores, nanoporous
materials have a large internal surface
area providing many adsorption sites
for hydrogen molecules. The internal
surface area is many times larger than
the external surface of the material.
activated carbon, for instance,
has an internal surface area of over
2000 square metres per gram, in other
words: a sugar-cube sized amount of
activated carbon has an internal surface area the size of half a football field.
For crystalline materials, so far zeolites
– framework structures consisting of
aluminium, silicon and oxygen – have
exhibited the highest values with some
900 square metres per gram. a few
years ago, however, completely new
materials were produced providing
hydrogen molecules with an accessible surface area of, in some cases,
over 4000 square metres per gram.
These novel storage materials consist of clusters of metal oxides linked
by organic molecules, thus creating a
three-dimensional porous network. By
reason of their chemical structure the
new compounds are referred to as
metal-organic frameworks, or MOFs
for short. MOFs have the lowest density of all crystalline materials known
at present. First measurements have
revealed that their extremely porous
structure enables them to absorb much
more hydrogen at low temperatures
than any other crystalline microporous
material. Hydrogen storage in MOFs is
Theoretically this titanium-doped
sodium alanate can reversibly store 5.6
per cent by weight of hydrogen, or 56
grams of hydrogen per kilogram storage material. Many other lightweight
metal hydrides that are also currently
being intensively researched store
more hydrogen, but only release it at
temperatures above 200 degrees Celsius.
Hydride storage systems have one
fundamental problem in common: the
latent heat that is generated when
quickly filling up with fuel; this requires
a complex tank construction with an
additional external cooling system to
remove the heat during refuelling.
Physical bonding at
low temperatures
With the second type of hydrogen
storage in solids, physisorption, the hydrogen does not bond chemically, but
by means of attractive force, the van
der Waals force, on the surface of the
solid. Since this van der Waals force is
relatively weak, large amounts of hydrogen can only be stored at low tem-
Structure of MOF-5: at each of the corners there are four zinc oxide (Zn4O) tetrahedra (yellow)
bonded by benzene derivatives. (Image: MPI für Metallforschung)
31
T A I L O R E D
S T O R A G E
M A T E R I A L S
based on physisorption, which means
that hydrogen molecules adsorb on
the large internal surface area.
Industrial-scale production
of MOFs
Over 2000 different MOFs have
been synthesised to date. However,
only those with a large internal surface
area and, moreover, whose production is economically viable qualify for
technical application. One example is
a framework known as MOF-5 which
stores a good five per cent by weight of
hydrogen at a pressure of about 50 bar
and liquid nitrogen temperature (-196
degrees Celsius). This storage capacity
is more than twice that of the best zeolites and comparable to that of the best
carbon materials. recently MOFs with
a surface area of up to 4700 square
metres per gram, capable of storing up
to seven per cent by weight of hydrogen, have been produced.
Even cost-efficient industrial-scale
production of MOFs has been suc-
32
cessful. BaSF, the chemical company,
uses an electrochemical process for
the synthesis of the framework materials at room temperature and standard
pressure. This makes hydrogen storage by physisorption on MOFs a viable
alternative to chemical storage in hydrides.
Chemists can tailor the chemical
composition, pore size and pore size
distribution of MOFs. By this means
the interaction between hydrogen and
the storage material can be examined
in detail. The great challenge is to optimise MOF pore sizes without reducing
the surface area. The storage material
can be tailored, as it were, on the energy carrier hydrogen itself.
Michael Hirscher
The author is Head of the Hydrogen
Research Group at Max-Planck-Institut
für Metallforschung in Stuttgart.
pHOTOVOLTaICS WITH SILICON
In times of rising energy prices, solar systems are experiencing a boom,
since the sun is a virtually inexhaustible source of energy. Moreover
sand, the raw material for solar cells made of silicon, is abundantly
available. It only remains for the conversion of sunlight into electricity
to become more efficient.
From sand to solar cell
T
he sun’s radiation directly or
indirectly affects all the essential processes of the whole living world.
This energy source will outlast the human species – and, apart from the cost
of the plant, it is free, clean and independent of political influence.
Various concepts for using solar
energy exist. One of them is photovoltaics, by which solar cells convert
the sun’s radiation into electricity. photovoltaics exploits the photoelectric effect, that is, the fact that sunlight can
release positive and negative charge
carriers in a solid.
Cheaper solar cells
are on the way
Today’s solar cells consist of a
semiconductor material. Semiconductors are substances that conduct electricity when supplied with light or heat,
whilst at low temperatures they act as
electrical insulators. The semiconductor absorbs sunlight irradiation and
builds up an electric potential, since
the incidence of light releases negatively charged electrons that move
about. as a result, at the same time
positively charged holes develop that
are also mobile. These positive and
negative charges are collected, as it
were, by means of contact areas located on the front and back surfaces
of the solar cell. The contact area on
the back, the positive pole, is a thick
holohedral aluminium layer. On the
front, contact fingers consisting of thin
silver conductor lines form the negative
pole. The fingers conduct the collected
charge carriers to bonding sites, also
known as busbars. The electric energy
accrues as low-voltage direct current
and, on being converted into alternating current, is used directly or fed into
the grid.
Over 90 per cent of solar cells produced worldwide consist of the semiconductor material silicon, the second
most abundant element of the earth’s
crust, sufficient quantities of which are
found in quartz sand – the chemical
term is silicon dioxide – and in other
minerals. However, the production
33
pHOTOVOLTaICS WITH SILICON
should be minimised. For this reason
the front surface of the silicon wafer is
chemically treated, normally by etching, so that it reflects less light. Furthermore it is coated with an antireflective
layer comparable to the coating of
spectacle lenses. This layer consists
of silicon nitride, which gives solar cells
their deep blue colour.
of high-purity silicon for solar cells is
expensive. at the moment this factor
still limits the competitiveness of solar electricity from silicon-based solar
cells. The tremendous rise in demand
for solar cells has triggered a tenfold
expansion of solar silicon production
capacities. Consequently prices have
now fallen by one fifth, bringing solar
power production closer to price parity
with fossil fuels.
Limited efficiency
The proportion of incident light that
a solar cell converts into electric energy
indicates its efficiency. The theoretical
efficiency of silicon-based solar cells is
about 28 per cent, although for industrially manufactured solar cells it has so
far only been between 13 and 18 per
cent. There are several reasons for this.
Wafers cut from purified silicon
have a silvery surface which reflects
about 40 per cent of incident sunlight.
In order to convert as much sunlight as
possible into electricity, the reflection
Solar cells on an apartment building in Munich.
(Image: Gehrlicher Umweltschonende Energiesysteme GmbH)
Solar systems are not only installed on roofs.
Photovoltaic power stations, seen here in
Miegersbach, Bavaria, are up-and-coming.
(Image: Oxaion)
A co-worker at TÜV Rheinland tests the stability of photovoltaic modules with the hailstone impact test: standard-size hailstones are shot
at the module at a precisely determined speed.
(Image: TÜV rheinland)
34
For optimum light trapping, the
front surface of the silicon wafer should
be completely exposed to incident
light. For this reason, solar cells are
installed where solar radiation is greatest, for example on roofs facing south,
south-east or south-west. However,
the contacts deposited on the front reduce the surface area that can be irradiated, since sunlight cannot penetrate
the contact structures. Efforts are being made, therefore, to reduce shading losses to a minimum by making the
lines of the contact fingers as thin as
possible. Too thin contact fingers, on
the other hand, increase the electrical
resistance, while reducing the solar
cell’s efficiency. Today screen printing
F R O M
S A N D
T O
S O L A R
C E L L
is used to deposit silver contact fingers
0.02 to 0.035 millimetres thick and
0.08 to 0.11 millimetres wide.
The efficiency of the silicon solar
cell is also limited by the fact that it
only uses a certain fraction of the full
sunlight spectrum, since silicon does
not absorb light over the sun’s whole
sphere of radiation.
Still much development
potential
a single silicon solar cell measuring
156 by 156 millimetres has a potential of about 3.8 to 4 watts. as a rule,
eight to over 72 such individual cells
are connected in series to form solar
modules. To protect them from precipitation, dust and other impacts the photovoltaic arrays are hermetically encapsulated, that is, they are mounted in a
frame behind tempered glass at the
front, the back being sealed with synthetic resin. Established manufacturers
of solar cell modules give a guarantee
term of up to 20 years.
Solar technology works, but it has
not yet reached its physical limits.
Comprehensive research and development coupled with optimised production could consolidate the market position of photovoltaic energy production.
How solar silicon is produced
To produce a solar cell, dopants
such as phosphorus or boron, which
provide more or less electrons than
silicon, are diffused into the semiconductor material silicon. an excess of
negatively charged atoms produced
in this way is referred to as n-doping;
with a deficiency, in other words, with
an excess of positively charged holes,
p-doping.
The p-doped layer contains a small
amount of boron which is added to
the silicon melt during the production process. Subsequently the melt
crystallises to form monocrystalline
silicon ingots consisting of a single
silicon crystal, or polycrystalline silicon blocks. The ingots or blocks are
sawn into 0.15 to 0.3 millimetre-thick
discs, called silicon wafers. n-doping
the wafers takes place at over 800
degrees Celsius in a gas atmosphere
with phosphoric substances. In the
process a phosphorus layer about
0.001 millimetres deep diffuses into
the silicon wafer. Thereafter this phosphorus doping only remains on the
front side, that is, the side of the silicon wafer that will later face the sunlight.
Doping is crucial to the conversion
of solar energy into electricity: with
photon radiation, electric charge carriers – negatively charged electrons
and positively charged holes – are
released in the silicon; they must
not recombine otherwise they would
neutralise each other and no macroscopic electric current would flow.
at the interface between the p- and
n-doped layers the two charge carriers separate: the holes migrate to the
p-doped rear surface of the silicon
wafer, the electrons to the n-doped
front. This process ceases if there is
no incidence of sunlight, but restarts
immediately with sunlight.
Negative electrode
n-Doped silicon
n-Doped silicon
positive electrode
Christina Modes
The author is Head of the Thick Film
Business Unit of the Thick Film Materials Division of Heraeus, the precious
metals and technology group in Hanau.
National grid
Diagram of a wafer: sunlight releases negatively charged electrons and positively charged
holes which migrate to the electrodes.
(Image: guukaa, fotolia.com)
35
OrGaNIC pHOTOVOLTaICS
The market for solar energy is booming. Most photovoltaic systems are
still based on the inorganic semiconductor silicon, yet organic solar
cells are gaining ground.
(Image: Tobias Marx, fotolia.com)
Electricity from plastic
r
egenerative
energies
are
steadily becoming more important. The German renewable Energies act and associated promotion
of solar power have triggered annual
increases of up to 40 per cent on the
solar energy market in Germany alone.
Currently some 90 per cent of solar
cells sold are silicon-based. Yet other
processes are becoming increasingly
relevant, since the production of silicon
solar cells is energy-intensive and the
existing production plants are scarcely
able to meet demand.
the thinner the photoactive layer can
be designed, and thus also the solar
cell. Organic cells are, therefore, more
flexible, more adaptable with regard
to form, and lighter than those made
of silicon. a further advantage is that
they are more cost-efficient and less
energy-intensive to produce since no
high-temperature and vacuum processes are required. In addition, by
tailoring the dye molecules, the solar
cell’s properties, in particular its absorption and thus its efficiency, can be
modified.
This is where organic solar cells
become interesting. In their photoactive layer they use organic dyes which
absorb an extreme amount of light.
Basically, the higher the absorption,
In an organic solar cell the layer
that absorbs sunlight is sandwiched
between two electrodes. The negative electrode consists of aluminium,
calcium or magnesium, the positive
36
electrode of a transparent, conductive
oxide. Expensive indium-tin oxide is
mainly used. Glass carriers or polymer
films are already commercially available; they are coated with indium-tin
oxide, which gives the whole stability.
Sophisticated charge separation
In the photoactive layer of an organic solar cell three fundamental
steps take place: first, the absorption
of sunlight excites the electrons of the
organic molecules so that they are
promoted from the highest occupied
molecular orbital (HOMO) to one step
higher, namely to the lowest unoccupied molecular orbital (LUMO). Thus
free spaces arise in the HOMO. The
electrons are bound to these holes by
E L E C T R I C I T Y
F R O M
P L A S T I C
electrostatic interactions and are thus
captured in the molecule – however,
no current will flow until the electrons
are mobile. The holes and the electrons
must, therefore, be separated.
This charge separation takes place
in the next step, in which the excited
electrons fall back again into lower
energy levels. The trick with the solar
cell is not to let the electrons in “their”
molecules jump back onto a lower energy level, but into molecular orbitals of
other molecules. The solar cell, therefore, is constructed from two kinds of
molecules, one type readily donating
electrons, the other readily accepting
them. In order for electron transfer to
function – namely without the hole migrating with the electron – the energies
of the orbitals of the various molecules
must be absolutely compatible.
In the third and last step the holes
and electrons separated by this method migrate to the respective electrodes
– a current flows. Since charge carriers in organic semiconductors move
significantly slower than in inorganic
layers, the organic layers may not be
Apparatus for characterising organic solar cells
(Image: © Universität Freiburg)
more than a few hundred nanometres
thick. This does not present a problem,
however, since organic dyes absorb
extremely high amounts of light, which
compensates for the deficiency in conductivity.
The different types of organic solar
cells can be distinguished by the lightabsorbing substances. plastics can be
used, that is, large organic polymers,
or small molecules, or combinations
of the two. The decisive factor is that
the various types of molecules are so
compatible that they readily exchange
electrons.
In production the simplest method
is first to deposit one type of molecule
as a layer on the carrier and then the
other. However, this two-layer concept
is not very efficient since the two molecules only interact at the interface. It
is better, therefore, to mix them in one
layer.
polymers, that are highly light-absorbent and cover different areas of the
sunlight spectrum. However, the difficulty lies in depositing the two polymers on the carrier since they do not
readily mix and thus form separate areas, measuring a few microns, on the
carrier. This may not seem dramatic,
yet even this phase separation on the
microscale results in a loss of sunlight
absorption.
an alternative is to use block copolymers. They consist of two polymers linked by chemical bonds. admittedly, not many block copolymers are
suitable for application in solar cells.
another option is the deposition of
polymers as nanoparticles measuring
between 40 and 100 nanometres. This
method requires either nanoparticles
that already contain two polymers, or
a mixture of two types of nanoparticles
that are each composed of only one
polymer.
Plastic solar cells
With plastic solar cells, the photoactive layer contains two different
The efficiency of plastic solar cells
is still low, being so far under two per
cent. By way of comparison: industri-
Organic solar cells are more flexible than
those made of silicon.
(Image: © Siemens pressebild)
37
E L E C T R I C I T Y
F R O M
P L A S T I C
How organic
p-i-n solar
cells work
p-i-n solar cells owe their name
“p-i-n” to their structure: the photoactive layer is sandwiched inside
(i) two organic semiconductor layers,
which are also called transport layers. These semiconductor layers are
specifically doped with molecules to
create an excess of positive (p) and
negative (n) charge carriers. This
doping facilitates the charge carrier transport to the electrode, thus
increasing the cell’s efficiency. The
semiconductor layer facing the light
is transparent and its thickness selected so that the interference maximum of incident and reflected light
lies in the centre of the photoactive
layer. In addition, this layer protects
the photoactive layer from damage
during vapour deposition of the top
contacts. The photoactive layer of a
p-i-n cell consists of a deep blue dye
and a C60 fullerene as the electron
collector.
On account of the low mobility of
the positive charge carriers, p-i-n
cell layers should not be thicker than
60 nanometres. However, higher absorption can be obtained by stacking several thin cells, which are then
referred to as tandem cells.
Organic solar cells are flexible.
(Image: Merck KGaa)
38
ally produced silicon solar cells achieve
efficiencies of between 13 and 18 per
cent.
Smaller molecules,
higher efficiencies
Higher efficiencies of almost six per
cent have been attained by solar cells
with small molecules as light absorbers. They are produced by high-vacuum thermal evaporation, a process that
was developed for serial production of
displays made of organic light-emitting
diodes (see contributions on pages
11 and 14). In older solar cells of this
type the electron-donor and electronacceptor molecules are in separate
layers. a more recent concept is ‘p-i-n’
solar cells (see box), in which doped
semiconductor layers improve charge
transfer performance.
energy yield. To date five per cent efficiency has been achieved.
Comparable to amorphous
silicon
Many ideas have been put forward to boost photovoltaics. The key
is the organic semiconductor, the heart
of any plastic solar cell. They have to
be better understood and optimised
so that maximum electricity can be
produced from solar energy. Organic
semiconductors were long considered
to be unattractive because their physical properties were not reproducible.
They may not yet have achieved the
conductivity of crystalline silicon, but a
polymer that conducts electricity similarly well to amorphous silicon in thinfilm solar cells is already available.
Organic solar cells combining polymers and small molecules promise the
highest efficiencies. For example, by
mixing suitable polymers and fullerenes, efficiencies of up to ten per cent
should even be feasible. Fullerenes are
spherical compounds composed of
carbon. The most well-known of them
is C60, also known as the football molecule on account of its shape. To counteract the energy loss during electron
transfer of the polymers to the fullerenes, the compatibility of the materials
needs further improvement. Moreover,
this type of cell is not yet capable of
fully exploiting the red and infrared areas of sunlight.
progress is anticipated from new
production variables, such as different process temperatures or a different
ratio of polymers to fullerenes. Even
simple chemical or thermal annealing of the solar cell can, for instance,
significantly improve the assembly of
the molecules, thus enhancing parameters like the absorption or mobility of
the charge carriers and increasing the
Klaus Griesar
The author is Senior Manager of Business Development at Merck, the pharmaceutical and chemical company in
Darmstadt.
SOLar CELLS
The sun supplies its energy free of charge, but its conversion into
electricity is expensive. This is mainly due to silicon which is costly to
process for application in solar cells. Novel fabrication processes and
materials are set to reduce costs.
Too expensive? Novel electrochemical
processes could make solar cell production
more cost-efficient.
(Image: Thaut Images, Fotolia.com)
It doesn’t necessarily
have to be silicon
T
he sun has always played a
central role in man’s energy
supply. plants store solar energy, for
example in wood which is then used
for heating. Modern approaches go
far beyond that, using, for instance,
rapeseed oil as fuel. Solar collectors
for heating water in domestic heating
circuits, for use as heat for industrial
processes and for steam generation
in solar-thermal power stations are becoming increasingly popular.
In the past few years the direct
conversion of sunlight into electricity
without temporary storage of the heat
generated has met with growing interest. More and more solar systems are
being installed on houses, agricultural
buildings and industrial facilities. admittedly, in a land that is not exactly
spoiled by the sun this boom would
hardly have been conceivable without
the funding policy of the federal German government.
photovoltaics is an elegant technology that performs efficiently even in
small units and with lower solar radiation. However, the electricity produced
is comparatively expensive since the
cost of producing crystalline silicon
modules, which currently dominate
the market, is high. This holds particularly for monocrystalline silicon wafers,
namely those consisting of one single
crystal, that are otherwise only used by
the computer industry for microprocessors. Yet even the less elaborate process for polycrystalline silicon, which is
similarly ultra-pure, is costly.
Technologies beyond crystalline
silicon
photovoltaics will only play an important role in the future energy supply
if there is a substantial drop in the cost
of manufacturing solar cells. This has
already partly been achieved by the
development of thin-film cells, in which
the active materials are not produced
separately and subsequently interconnected to form cells and modules, but
39
SOLar CELLS
are directly deposited in the modular
structure. This type includes modules
made of amorphous and microcrystalline silicon, which can be produced,
for example, by vapour deposition of
silicon films from the gas phase. Other
materials besides silicon already figure
in the production of thin-film cells, primarily cadmium telluride – a semiconductor composed of cadmium and
tellurium – and also certain coppersemiconductor compounds, such as
copper indium selenide.
and, as a bonus, use cost-efficient
polymer films or technical textiles as
the substrate promise a further major
leap towards reasonably priced solar
systems. Solar cells made of organic
semiconductors (see contribution on
page 36), on the one hand, come under this category. On the other hand,
many inorganic semiconductors can
be deposited on such substrates. To
this end, electrodeposition, in which
the deposition is electrochemical, is an
interesting option.
These substances are deposited
from the gas phase or from a solution
on a relatively costly substrate material
and have to be stabilised at temperatures above 300 degrees Celsius. Even
so, such cells are cheaper to produce
than crystalline silicon cells. The time in
which the modules recoup the energy
required for their production is thus reduced. For thin-film solar cells this energy payback period is less than a year,
for crystalline silicon cells several years.
The advantage of electrodeposition: the growth of the films is easy to
regulate, since the concentration of the
precursors, the temperature of the solution and other parameters can be set
as desired. Moreover, the film formation
can be influenced by adding particular
chemical auxiliaries, or additives.
Controlled nanostructures
production processes that take
place at a constant, low temperature
Zinc oxide has proved to be particularly suitable for electrodeposition
since it is deposited from an aqueous solution as a crystalline film at
relatively low temperatures around 70
degrees Celsius. Furthermore it grows
on many different electrically conductive substrates. The morphology of
Cells made of monocrystalline silicon
zinc oxide can be controlled by additives that bond to its surface. The exact
adjustment of the film’s morphology in
the nanometre range is a decisive criterion in the optimisation of solar cells,
since the active interfaces form on the
nano-scale. They determine how well
absorbed light can be converted into
serviceable charge carriers – into negatively charged electrons and positively
charged holes – and how well these
charge carriers can be separated. If
electrons and holes collide, they recombine and no current flows.
The possibility of controlling the
nanostructure of the zinc-oxide film
is what makes zinc oxide an interesting electrode material. Coupled with
its low toxicity and good availability in
the earth’s crust, zinc oxide is regarded
as one of the potential substances for
large-scale photovoltaic power supply.
The combination of zinc oxide
with organic dyes
However, zinc oxide has a serious
downside: it does not absorb visible
light. For this reason its surface has to
be modified with intensive absorbers, a
Thin-film solar cells
Substrate
Deposition
Structuring
wafer
Single crystal
interconnected to form
a module
Complete module
Different types of solar cells: whereas conventional silicon cells consist of individually produced and then interconnected modules (left), thin-film
cells are produced as a module from the start (right), which makes them more cost-efficient.
40
N E C E S S A R I LY
process well-known from colour photography as sensitisation of silver bromide. Organic dyes are well suited as
sensitisers for zinc oxide: they absorb
light and release high-energy electrons
in zinc oxide and, in turn, are reduced
by lower-energy electrons from a second contact phase to close the electric
circuit.
Sensitisers work best when bound
as a monolayer film on the surface. In
order for them to absorb plenty of light,
however, the carrier electrode must exhibit a large surface area and is, therefore, deposited as a porous film. The
many pore walls cause a tremendous
increase in the surface area. Electrodeposition is also excellently suited
for the production of such porous films.
Hitherto the contact phase most
successfully applied for dye-sensitised
electrodes also consists of an electrochemical system, namely an iodinecontaining organic solution. These
cells would be easier to handle if they
contained a gel or an organic or inorganic semiconductor instead of the
liquid electrolyte. This is currently being
intensively researched. Solar energy
researchers, and even more so the
electrochemists among them, will not
run out of work too quickly.
H A V E
T O
B E
S I L I C O N
Air-purged solution
with zinc salt:
Substrate
D O E S N ’ T
½ O2 + Zn2+ + 2 e➞ ZnO
Structure-directing
agent
Zinc oxide (ZnO) is an interesting electrode material. The diagram shows the
principle of an electrochemical process by which zinc oxide is deposited from an
aqueous solution on a substrate. The additive (red circles) ensures pore formation in the zinc oxide film.
ZnO
sensitiser
Counter-electrode
Conductive redox
electrolyte
Substrate
I T
Load
Derck Schlettwein
The author is Professor for Applied
Physics at Justus-Liebig-Universität in
Gießen
Zinc oxide does not absorb visible light, thus it has to be sensitised with absorber
molecules (sensitisers). The diagram shows the functioning of a dye-sensitised
solar cell with light absorption (1), release of a high-energy electron to the semiconductor electrode (2), electron collection (3), transfer in the external circuit
(4) and completion of the electric circuit by the transmission of a lower-energy
electron to the contact phase (5), which transfers the electron to the sensitiser
molecule for its regeneration (6).
41
THErMOELECTrICS
Thermoelectric generators utilise temperature gradients to generate electricity. They could, for example, recover waste heat from the
exhaust to power a vehicle’s electronics. Researchers worldwide are
searching for thermoelectric materials that will make this thermal conversion economically viable.
Along with the waste gases from cars and
power stations, thermal energy is lost.
Thermoelectric materials can convert this
heat into electricity
(Image: DX, Fotolia.com)
Wonderful thermal converters
C
ars, machines and power stations are not fuel-efficient: on
average they utilise about one third
of the energy supplied to them. The
rest, in the form of heat, is dissipated.
If it were possible to recover the large
amounts of waste heat energy, the energy balance of combustion processes
would be significantly more favourable.
The quest for methods of, at least partly, recycling lost thermal energy leads
to a technology that has been known
for well over a hundred years: thermoelectrics.
Thermoelectric energy harvesting, the collection of waste heat from
combustion processes, is currently
enjoying high popularity and has trig-
42
gered a worldwide research race for
higher efficiencies, inspired by visions
such as the car as a mobile mini-power station. If it were possible at least
partly to power the vehicle electronics
with the exhaust heat, fuel savings of
around five to seven percent, claimed
by the automotive industry, could be
achieved. This would make a significant contribution to climate protection.
Even low temperature gradients
are effective
Thermoelectric converters work extremely reliably, without noise or vibration, and they produce no emissions.
Their heart is the semiconductor material. If two different, thermoelectrically
active semiconductors are connected
and they produce a temperature difference at the junction, an electric field
is created. Conversely: if a voltage is
applied at the junction, a thermal gradient is produced. How much electricity
is generated or how great the cooling
effect is, depends mainly on the materials used in each case.
a prerequisite for the conversion
of heat into electric energy is, therefore, an existing temperature gradient,
which does not need to be high. Thermoelectric watches and clocks, for
example, use just the low temperature
difference between body and room
temperature.
W O N D E R F U L
T H E R M A L
C O N V E R T E R S
Thermoelectric systems have been
deployed as energy sources in space
probes and satellites for decades.
However, for everyday applications
their production is still too expensive,
moreover they are not yet efficient
enough. One measure of its efficiency
is ZT, the figure of merit, which has
stagnated at 1 for decades – that is
not sufficient to make use of waste
heat. Values round about 1.5 to 2 are
regarded as the break-even point for
cost-effective use in a thermoelectric
generator. Novel materials, on the
other hand, have achieved values of up
to 3.5 in laboratory tests.
Nanotechnology outwits
natural laws
In their search for more efficient
thermoelectric materials and optimised
production processes, scientists are
focusing on modifying known thermoelectric materials or developing new
ones. They have an abundant choice
at their disposal, since the spectrum
of such materials ranges from alloys
to semiconductors and semi-metals to
ceramics.
The question whether a thermal
converter is economically viable depends, on the one hand, on its thermoelectric power, meaning, what the
voltage is at a certain temperature difference. On the other hand, efficient
thermoelectric converters should conduct electricity well, while being poor
heat conductors. The temperature gradient is as large as possible when the
material is heated on one side. Here,
the natural laws of physics put a spoke
in the material scientist’s wheel: according to basic physics, it is impossible to obtain high electrical conductivity simultaneously with low thermal
conductivity.
Currently the object of intensive
research, modern “high ZT” materials
Vehicle waste heat from the exhaust can power a vehicle’s electronics.
(Image: amridesign, Fotolia.com)
outwit nature, as it were. Their atomic
structure is so sophisticated that the
internal structure of the material limits
heat conduction, but without disturbing electron mobility, and perhaps even
promoting it. In the race for as high ZT
values as possible, nano-engineered
materials are regarded as particularly
promising. They consist either of pure
nanoparticles or of a matrix in which
nanoparticles are embedded.
Since the 1990s, materials scientists have been experimenting with
nano-dimensioned wires made of semimetals, such as bismuth, in which current only flows in one direction along
the wire axis. Such wires have a diameter of less than 15 nanometres
and attain ZT values of up to 3. The
drawback: the use of nanowires for the
construction of thermoelectric generators is limited.
Materials composed of many nanolayers, called superlattices, can be
processed better. In superlattices, heat
and electric current flow within the layers and also perpendicular to them.
What’s so special about them, however, is that thermal conductivity perpendicular to the layers is significantly
reduced, whereas electricity flows
largely unhindered in this direction.
This improves the ZT value.
For such thermoelectric nanomaterials scientists have to tailor the assembly of nanolayers or nanoparticles.
Other materials, however, form suitable
nanostructures by self-assembly when
various phases of the material separate.
Improved efficiencies can also be
achieved entirely without nanotechnology. One example of such non-nanostructured thermoelectric materials is
certain high-temperature materials,
called skutterudites after the Norwegian mineral deposit in Skutterud. In
skutterudites for thermoelectric materials, the elements cobalt and antimony
form a crystal lattice that can accommodate heavy atoms in its voids. Such
“filled” skutterudites have comparatively low thermal conductivity, but high
electrical conductivity.
The examples are conclusive: there
is no lack of thermoelectric materials
and energy harvesting is within tangible
reach. The new thermoelectric materials will result in improved fuel utilisation
in cars and power plants.
Harald Böttner
The author is Head of Department
Thermoelectrics and Integrated Sensor Systems at Fraunhofer-Institut für
Physikalische Messtechnik in Freiburg.
43
THErMaL WaSTE TrEaTMENT
Dirt, stench and dioxins – waste incineration has a negative image.
Quite wrongly, since modern incinerators emit hardly any pollutants
and, what’s more, they are capable of producing environmentally
friendly energy.
For some people it is refuse, for others
valuable fuel.
(Image: © Erwin Wodicka – Fotolia.com)
Waste to energy
N
owadays waste incineration
plants no longer disgorge
huge quantities of pollutants. In the
past 20 years incineration and flue gas
purification have undergone technological improvement to such an extent
that incinerators today only emit a fraction of the emissions they produced
well into the 1980s. In 2005 the German Federal Minister for the Environment at the time, Jürgen Trittin, declared that waste incineration plants
were no longer a significant factor in
terms of emissions of dioxins, dust and
heavy metals. and an article published
in april 2007 in the magazine of Greenpeace, the environmental organisation, entitled “Der Müll und die Mythen”
(waste and myths) stated that, due to
Germany’s strict Emission Control Or-
44
dinance, dioxin pollution had dropped
to one thousandth of the value for
1990 – although the number of waste
incineration plants in the preceding 20
years had almost doubled. In 2007
there were 72 waste incineration plants
in Germany.
Nevertheless, it goes without saying that, from an ecological point of
view, waste must be avoided as far
as possible, provided the avoidance
itself produces no negative effects.
The refuse that does accrue, however,
should be utilised with utmost efficiency since it contains many components
that can be used for energy and materials.
Waste rather than fossil fuels
Today waste is separated in an automated process and only minimally
by hand. Modern processes exploit
not only the magnetic and electrostatic
properties of materials, but other physical characteristics, such as their density or their capacity to absorb infrared
radiation. This enables ferrous metal,
aluminium, and non-ferrous metals and
also plastics, such as polyethylene,
polypropylene and polystyrene, to be
thoroughly segregated by type. Industrial, commercial and domestic waste
producers have already presorted the
bulk of paper, wood, glass, rubble,
and also green waste for composting.
Nevertheless around 26 million tons
of waste, whose separation requires
W A S T E
T O
E N E R G Y
more energy than that originally used
for its production, are produced annually in Germany alone. In the past this
waste ended up on landfills or in waste
incineration plants.
in production, refuse-derived fuels remain refuse and – in contrast to coal,
fuel oil, natural gas and wood – their
chemical composition is heterogeneous.
Up to 85 per cent efficiency
Since June 2005 landfilling of untreated waste has been prohibited
by law in Germany. The main reason:
in landfills, the landfill gas methane
develops in the absence of air and is
discharged into the atmosphere; the
greenhouse effect of methane is 23 times
stronger than that of carbon dioxide.
The main objective, therefore, is
to recover energy from non-recyclable
waste, thus deriving a substitute for
fossil primary energy carriers like coal,
oil or natural gas. The calorific value of
the waste determines how it can be recycled. Sorting processes separate residual waste into fractions with different
calorific values. The share with a value
of 10 megajoules per kilogram or less
does not burn independently, but requires supplementary firing with natural
gas or fuel oil. This is the principle of
conventional waste incineration plants.
The fraction with a calorific value
of at least eleven megajoules per kilogram, however, can be used as fuel,
also known as refuse-derived fuel. The
calorific value of refuse-derived fuels is
mostly between eleven and 25 megajoules per kilogram, which makes them
comparable to firewood and lignite.
refuse-derived fuels mixed with coal
can be burnt in coal-fired power stations or in cement kilns. Thanks to the
paper, wood and textiles they contain,
these fuels rank, at least to about 50
per cent, among the regenerative, climate-neutral fuels.
The recovery of refuse-derived fuel
requires close monitoring - from the
control of fuel deliveries through to
emission measurements. When all’s
said and done, despite the utmost care
How efficient waste-to-energy recycling is, depends not just on the calorific value of the waste, but crucially
on the efficiency of the power plant. In
general, the efficiency of energy conversion processes leaves much to be
desired. Tremendous optimisation potential of energy use, therefore, could
be tapped by increasing the efficiency
of these processes.
In an incineration plant the firing
heat produces superheated steam
which drives a turbine and is converted
into electricity in a generator. By this
means only about one third of the primary energy used can be converted
into utilisable energy. By additionally
feeding heat into a steam or district
heating network, however, the efficiency can be increased to 80 to 85 per
cent. In chemical plants, paper mills
and other industries that require a great
deal of steam, waste-to-energy power
plants are an ideal supplement to existing supply systems.
One example of an integrated supply and waste management network is
Industriepark Höchst in Frankfurt am
Main which, with 90-plus companies
and 22 000 employees based there,
has an annual energy demand of almost two terawatt hours of electricity – the equivalent of the annual consumption of 500 000 households – in
addition to four million tons of process
steam. Besides classical energy and
steam generation in a coal-fired power
station and a gas turbine, the facility at
Industriepark Höchst utilises even the
waste heat from chemical production
processes and from the incineration of
sewage sludge and hazardous waste.
a waste-to-energy power plant is
under construction that will process
675 000 metric tons of refuse-derived
fuel annually. From mid-2010 it will
supply the companies based there
with energy and process steam. The
new plant will feed 70 megawatts of
electric power – corresponding to the
demand of about 150 000 households –
or 250 metric tons per hour of steam
into the park’s own grid.
additionally, in summer 2007 a biogas plant, the first of its kind in Europe,
which produces methane from the organic constituents of industrial waste-
Industriepark Höchst in Frankfurt am Main: here a plant that converts refuse-derived fuel into
electricity and steam will go on stream in 2010.
(Image: Infraserv GmbH & Co. Höchst KG)
45
W A S T E
T O
E N E R G Y
water, went on stream in Höchst. Three
combined heat and power plants convert the biogas produced into electricity. By this means it is even possible to
produce energy from wastewater.
Marcell Peuckert
The author is a Managing Director of
T2C, the operating company of the
waste-to-energy power plant at Industriepark Höchst in Frankfurt am Main.
How the waste-to-energy power plant works
Modern waste-to-energy power
plants achieve a high efficiency. They
are based on the process of a fluidised-bed furnace. The lower zone of
this vertically constructed furnace is
filled with sand which is whirled up by a
flow of hot air. The fluidised bed of hot
sand, which mixes well with the incinerator charge, has liquid-like properties
and provides good heat conduction.
In conventional incineration plants, by
contrast, the combustible material is
moved over a grate.
Compared with grate firing, fluidised-bed combustion permits better
temperature control. This is a distinct
advantage in the case of fuels with high
calorific values which are augmented
by high combustion temperatures.
Moreover an exact temperature range
of 850 to 950 degrees Celsius can be
set, which guarantees complete combustion, thus minimising the formation
of toxic nitrogen oxides. In order to further reduce nitrogen oxide emissions in
the flue gas, ammonia water is injected
Model of the waste-to-energy plant in Industriepark Höchst.
(Image: Infraserv GmbH & Co. Höchst KG)
46
into the boiler, causing the nitrogen
oxides to react to form nitrogen. alternatively the nitrogen oxides can
be catalytically converted in the
cooled flue gases shortly before
they reach the stack. The resulting
pressure drop, however, has a negative impact on total efficiency. For
this reason, in power plants whose
primary purpose is energy production rather than waste disposal,
injection of ammonia water is the
method of choice.
FUEL CELLS
Hydrogen and oxygen combine in a violent reaction to form water. Fuel
cells control this reaction, releasing the tremendous amount of energy
it produces in the form of electricity. The principle has been known for
over a century, the only drawback is that it is too expensive for application in everyday life.
NASA’s unmanned research aircraft Helios: during the day it flew on solar energy, at night it was
powered by fuel cells. Helios flew up to a height
of 30 kilometres – until in 2003 it sustained structural damage over the Pacific and crashed.
(Image: Nasa)
Emission-free energy production
I
n august 2003 one of those
spectacular electricity failures
happened in New York City, plunging
the whole of Manhattan into darkness.
In the midst of the dark skyscrapers,
however, one single building was lit up
in its habitual splendour: the Condé
Nast Building, 4 Times Square. How
was that possible? The building’s energy supply is based on the use of fuel
cells – and it was worth waiting for a
power failure to provide an effective
demonstration of its efficiency.
The principle of the fuel cell has
been known for a long time. In the year
1800 William Nicholson and anthony
Carlisle had discovered that water in
contact with electricity decomposes
into its constituents, namely into the
gases hydrogen and oxygen. In 1838
William robert Grove performed the
reverse step: he constructed an apparatus in which the gases hydrogen
and oxygen combined to form water,
thereby producing electricity – the first
fuel cell.
Controlled oxyhydrogen
explosion
The amount of energy that is released when hydrogen burns with
oxygen to form water is manifest in the
pictures of the catastrophe of 6 May
1937 when the airship the “Hindenburg”, that was filled with hydrogen,
burst into flames on landing in Lakehurst, New York. Within a matter of
seconds it was all over.
This combustion of hydrogen to
form water, also known as an oxyhydrogen explosion, is controlled in the
fuel cell and the energy released is
electrical energy. From Grove’s discovery of the fuel cell in 1838 several
decades passed until in 1893 Wilhelm
Ostwald, one of the founding fathers of
physical chemistry and a Nobel prize
winner, was able to explain the phenomenon. Ostwald realised that the
reacting gases in the fuel cell must not
come into direct contact with each
other. They have to be separated by a
substance – either liquid or solid. However, this substance must have the
unusual property of making the reaction possible yet in a controlled fashion.
How can this take place?
47
FUEL CELLS
Towards the end of the nineteenth
century the idea had taken root that
ions, namely electrically charged atomic units, transport electric charge, for
example in aqueous solutions. In this
field Ostwald was one of the pioneers
of his time. In fact he explained the
functioning of Grove’s fuel cell simply
by moving ions. Suitable candidates
are either the positively charged hydrogen ions, also known as protons, or the
negatively charged hydroxyl ions. The
driving force of the chemical reaction,
by which water is produced from the
elements hydrogen and oxygen, enables the moving ions to form on one
side of the fuel cell and be consumed
on the other side.
The positively charged protons are
formed from hydrogen molecules on
the hydrogen side and they combine
with oxygen molecules to form water
on the oxygen side. With the formation
of protons a negative charge remains in
the form of electrons, whereas with the
reaction to form water, electrons are
consumed. By this means separated
charges, and thus an electric potential,
form between the poles of the cell, between the anode and the cathode.
Nobel Prize winner Wilhelm Ostwald (1853
to 1932) explained the principle of the fuel
cell. He realised that the gases hydrogen and
oxygen must not come into direct contact.
48
If an external load is connected between the anode and the cathode and
current flows through it, the resulting
electrical power of the cell is the product of current and voltage. If several
of these elementary fuel cells are connected in series, the voltage increases
correspondingly. If they are connected
in parallel or if the surface areas of
the electrodes are enlarged, which
amounts to the same thing, the current
increases. With these measures the
power can be significantly increased.
With a fuel cell the available voltage
decreases with increasing current and
the power goes through a maximum.
This maximum power of a fuel cell
stack can amount, for instance, to five
kilowatts (about seven hp). However,
fuel cell aggregates with a voltage over
one thousand times higher already exist, which makes them small power
plants.
In buses, power plants and
apartment buildings
Let us take another look at the fuel
cells used in the Condé Nast Building:
these cells use phosphoric acid as the
This bus runs on fuel cell technology.
(Image: www.blueclick.com)
proton conductor and the operating
temperature is 200 degrees Celsius.
These phosphoric acid fuel cells have
achieved marketability, their purchase
price being around 3000 euros per kilowatt. 100-kilowatt modules are used,
for example, in buses. Units with significantly higher efficiency are used for
stationary applications, often as pointof-sale power stations supplying electricity and also heat to households in
the vicinity; this is known as combined
heat and power. On account of the
long heating-up time required, phosphoric acid fuel cells are not suitable
for domestic use or in cars.
It is generally acknowledged that
the ‘membrane fuel cell’ has far better
prospects. Here, a polymer interlayer
is the proton conductor; the operating temperature is between 70 and 80
degrees Celsius. at present the membrane fuel cell is still under development. However, prototypes of various
sizes and output power already exist.
The smaller units are suitable for portable electronic devices, the larger ones
with an output power of about five
kilowatts can supply private households with electricity. In albany, New
E M I S S I O N - F R E E
E N E R G Y
P R O D U C T I O N
York State, an apartment building that
meets its total energy demand in this
way has been built as a demonstration project. The automobile industry
is experimenting with membrane fuel
cells which have an output of about 20
kilowatts, and at a US navy airport in
Indiana a unit with output power of 250
kilowatts is in operation.
relation between the concentration of
carbon dioxide in the atmosphere and
the mean global temperature. There is
still, therefore, a tremendous discrepancy between the vision of emissionfree energy production and reality. Can
the processes which kept the research
aircraft Helios in the air be applied to
everyday life?
Combining solar and fuel cells
This question leads directly to the
concept of the hydrogen energy economy (see contribution on page 72). But
let’s not lose sight of the fuel cell. First
of all, it is generally accepted that a little
hydrogen goes a long way. recently a
test vehicle powered by a membrane
fuel cell travelled roughly 3000 kilometres from Valencia to Berlin on two
kilograms of hydrogen. There is just
one snag: a car with a 100-kilowatt engine powered by membrane fuel cells
needs about 50 grams of platinum
for the catalyst material. That is not
merely expensive – the current cost of
50 grams of platinum is about 2000
euros –it is simply not possible on an
industrial scale. The world’s production
of platinum would not suffice to equip
One promising idea is to combine
membrane fuel cells with solar cells.
With this combination, sunlight as the
sole source of energy is sufficient to
provide electrical energy round the
clock. This enables engines, for instance, to run continuously over a long
period of time.
at the beginning of this century the
unmanned research aircraft Helios attracted public attention by flying with
such a combination for weeks. By day
the electric energy from the solar cells
not only powered the vehicle’s electric
engines, but also served to produce
hydrogen from water. By night this
hydrogen was used to power the fuel
cells, which then supplied the engines
with energy besides recovering the water for use the next day.
Helios quite clearly demonstrates
an ideal: the ideal of emission-free energy conversion – without fossil fuels,
without loud combustion engines and
above all without polluting the environment with carbon dioxide. The reality
is still quite different. The arguments
against the combustion of coal, oil and
natural gas are overwhelming – and
the finite nature of fossil fuel supplies
is by no means the only one. The current global warming goes hand in hand
with the increased concentration of
greenhouse gases in the atmosphere,
to which the carbon dioxide resulting
from the combustion of fossil fuels is a
major contributor. There is a clear cor-
Germany’s new cars with fuel cells of
this type.
The alkaline fuel cell:
cobalt instead of platinum
The case of the alkaline fuel cell indicates a way out of the problem of the
costly catalyst. No positively charged
protons migrate in it from the anode
to the cathode, but negatively charged
hydroxide ions move from the cathode
to the anode. Here, too, at 80 degrees
Celsius water is formed from the elements and here, too, first of all platinum was used as the catalyst.
NaSa, the american air and space
agency, was not put off by the cost of
platinum and used alkaline fuel cells
not only in the apollo program, but
also in space shuttles. In the meantime it has become evident that cobalt,
which is much less expensive, can be
used rather than platinum. Meanwhile
prototype taxis in London run on alkali
fuel cells with cobalt catalysts. Nevertheless, compared with membrane fuel
cells, alkaline fuel cells have a funda-
Element composed of twenty stacked elementary solid-oxide fuel cells, permitting higher voltages.
(Image: Fraunhofer IKTS)
49
FUEL CELLS
mental disadvantage: they contain an
aggressive alkaline liquid.
according to today’s estimates, in
future alkaline fuel cells will not have
the largest market share anyway, but
those based on solid oxide, which are
predicted to have the largest share of
40 per cent by 2025, with 25 per cent
for the molten-carbonate fuel cell. Nevertheless these types are operated at
very high temperatures, which makes
them unsuitable for application in small,
portable devices. For local power stations, however, which supply energy to
urban districts and industrial facilities
by combined heat and power, they are
highly attractive. The high temperature
can be used to advantage: instead of
pure hydrogen, hydrocarbons can be
used which are gaseous at high temperatures and produce water and carbon dioxide as combustion products.
An engineer monitors the winding of a membrane for fuel cells. Once wound, it is brought to the
production plant and processed into membrane electrode units, the heart of a membrane fuel
cell.
(Image: BaSF)
e-
H2
½ O2
+
2H+
+
2e-
2H+ + 2e-
Graphite particles,
e--conducting
H+-Ion
H2O
H2
platinum
particles
O2
H2
Molecules
H2O
polymer,
H+-conducting
Electron
H atom
porous anode
polymer membrane
porous cathode
Diagram of a membrane fuel cell into which hydrogen (H2) (left) and oxygen (O2) (right) are fed.
Between them a membrane separates the gases, while allowing protons (H+) to pass through.
Hydrogen molecules cannot pass through the membrane. On the platinum catalysts on the
left hydrogen molecules are split into protons and electrons (e-). On the other side of the membrane the protons react with oxygen to form water (H2O). This reaction produces a tremendous
amount of energy.
50
In the molten-carbonate fuel cell
which operates at 650 degrees Celsius, carbonate ions take over the
charge transport. Here, too, water is
produced from the elements. On the
cathode side oxygen and carbon dioxide react with the electrons to form
carbonate ions. at the anode they
release their electrons and react with
hydrogen to form water and carbon
dioxide, which is then recycled to the
cathode. Inexpensive nickel is used as
the catalyst.
The molten-carbonate fuel cell is
still in the test phase and is intended
solely for use in stationary applications. Facilities with efficiencies from
below one megawatt up to about 100
megawatts are projected to supply
electricity and heat to smaller towns or
urban districts. The hot steam formed
is destined not only to provide district
heating for domestic users, but also for
application in steam turbines for cogeneration purposes.
E M I S S I O N - F R E E
E N E R G Y
The solid-oxide fuel cell has one
crucial advantage over the moltencarbonate fuel cell: its ion conductor is
a ceramic solid which is considerably
easier to handle at high temperatures
than a liquid. The operating temperature is 800 to 1000 degrees Celsius.
Only at this high temperature can sufficiently high oxygen-ion conductivity
be achieved. Materials that conduct
oxygen ions at 750 degrees Celsius or
less are being intensively sought worldwide.
The efficiency range of solid-oxide
fuel cells is greater than that of moltencarbonate fuel cells. Solid-oxide fuel
cells can be used both in local power
stations and in smaller stationary units
supplying electricity to individual houses or building complexes. The compact modules with a capacity of about
25 kilowatts are particularly suitable for
cities. In Tokyo they already feed electricity into the local grids.
P R O D U C T I O N
What remains is the question
of primary energy
Fuel cells are an environmentally
friendly means of converting chemical
energy into electrical energy. admittedly, this also holds for batteries, yet
the battery merely stores electrical
energy from the grid in chemical form
and releases it as needed as electrical
energy. It has to be charged as soon
as the chemically stored energy is
consumed. The fuel cell, on the other
hand, operates continuously. However,
the application of a hydrogen-powered
fuel cell also depends on electrical
energy, namely to produce hydrogen
from water. Ultimately, therefore, the
fuel cell also involves a system that
stores electrical energy chemically and
later releases it. What remains is the
question of the primary source of electrical energy. To protect fossil fuels and
avoid polluting the environment with
greenhouse gases, the candidates are:
nuclear energy, solar energy, hydro and
wind energy. The great challenge for
the future is to find the optimum mix.
Klaus Funke
The author is Professor for Physical
Chemistry at Westfälische WilhelmsUniversität Münster.
1.0
380
Atmospheric concentration of CO2
in ppm (vol.)
Deviation of the mean global
temperature (in °C) compared
with the mean value for 1960-1990
330
0.0
280
1600
1800
Year
2000
-1.0
Correlation between the concentration of
carbon dioxide in the atmosphere and global
warming. The red line depicts the concentration of carbon dioxide (CO2) in ppm. The unit
ppm stands for parts per million, meaning the
millionth part (as per cent stands for the hundredth part). The finely-spaced blue lines signify global warming. The reaction of hydrogen
and oxygen in a fuel cell produces emissionfree energy and is, therefore, climate-neutral.
51
COMBUSTION rESEarCH
The combustion of energy sources such as coal, oil and, more recently,
biofuels is one of the cornerstones of our prosperity. Although this form
of energy conversion is neither particularly effective nor clean, it has by
no means had its day. Combustion researchers are optimising current
processes.
Soon on the back burner?
F
ire is one of the four elements
of antiquity. Its mastery signified
an immense achievement for civilisation. In a world without fire there would
be no hot food, no heating and no other light but daylight. There would be a
general lack of visible flares for warning
and communication, and even tools
and instruments would be few and far
between – without calcining, forging,
annealing and smelting there would be
no needles, knives, rakes and blades
made of metal. Nor would coins and
other basic metal articles be in circulation. One could not afford to be ill – it
would neither be possible to sterilise
instruments in hot water nor to produce drugs or active substances for
ointments by distillation.
52
The mastery and controlled use of
fire plays a fundamental role in almost
all areas of life and it has gained even
more importance during the course of
industrialisation. Combustion produces hot steam for conversion into mechanical and electrical energy, without
which many operations and production
processes would not have been possible. Factories are no longer bound to a
specific location, for example at a river.
power stations for electricity generation and nationwide electrification, the
introduction of combustion engines for
vehicles, trains, ships and aircraft, the
establishment of international transport
facilities have all been associated with
tremendous changes – and they still
create problems for us today.
Far too valuable to burn
Our infrastructure depends predominantly on the combustion of coal,
oil and natural gas, that is to say: of fossil fuels. according to the Umweltbundesamt (German Federal Environment
agency), less than ten percent of the
energy produced in Germany is from
non-fossil primary energy sources.
However, it is not as though the problems connected with combustion were
unknown prior to the current climate
debate. The formation of environmentally harmful emissions, including the
greenhouse gas carbon dioxide, and
of dust and soot particles, but also the
finite nature and geographic location of
fossil reserves give reason to press for
alternatives.
S O O N
O N
T H E
B A C K
B U R N E R ?
Criticism of combustion of fossil
energy sources is levelled at various
aspects. The total efficiency of this
type of energy conversion, for example, is alarmingly low: the chemical energy in the fuel first has to produce hot
steam which expands in a turbine and
is then converted into electrical energy
by a generator. The electricity has to
be transformed and transported to the
user – who maybe uses it to boil water.
In conventional car or railroad engines, a large part of the chemical energy contained in fossil fuels is lost as
heat. also, it is not a particularly intelligent or efficient use of energy to move
a car weighing about one metric ton
only for the purpose to move a person
weighing about 75 kilograms. In fact,
oil and coal are really far too valuable
to burn – they would be put to much
better use as building blocks for the
synthesis of other products.
Has combustion still not had its
day?
There are clearly enough reasons
why a shift from combustion to other
processes for energy production is desirable. Yet no matter how we tackle
the energy problem in the next ten to
20 years: even if we manage to cover a
quarter of our (rising!) energy demand
from alternative – ideally: regenerative sources on a global scale, the remaining 75 percent will still originate from
the combustion of fossil resources. For
this reason combustion researchers
are working intensively on techniques
to render combustion as efficient and
low in emissions as possible. also, they
are developing combustor and engine
concepts to cope with different types
of fuel and to react flexibly according to
whatever is on offer.
The type of pollutants resulting
from combustion depends not only on
the fuels, but also on the combustion
conditions and the ensuing chemical reactions that take place. When
burned, the hydrocarbons contained
in coal, natural gas, aviation fuel, petrol and diesel produce water and the
greenhouse gas carbon dioxide. With
insufficient oxygen, toxic carbon monoxide and soot also develop. Moreover,
at very high combustion temperatures,
nitrogen oxides can develop from
atmospheric nitrogen, which is also to
be avoided. The reaction steps along
this chain of classical hydrocarbon
combustion are largely known and can
be computer-simulated. The respective
computer models can also be instrumental in optimising combustion chambers and catalytic converters.
Only some progress has been
made on the problem of soot formation. New particulate matter directives
for road traffic in congested areas
acknowledge health hazards represented not only by the clouds of dense
black smoke visible from afar, but even
more so by the tiny soot particles that
are invisible to the human eye. The formation of these minute soot particles
is extremely complex: aromatic compounds are produced as a first step
from fuel molecules with few carbon
atoms. From those, soot precursor
particles arise in the form of carbon
nanostructures, whose varied shapes
are reminiscent of wire netting. Details
of these processes remain the subject of intensive research. Interestingly,
similar reactions play a role in inter-
Soot precursors: small combustion molecules give rise to varied clusters reminiscent of wire netting.
(Image: from S.H. Chung, a. Violi, Carbon 45 (2007), 2400-2410, by courtesy of Elsevier)
53
COMBUSTION rESEarCH
stellar space, and an understanding of
these phenomena should also benefit
the development of carbon nanomaterials for molecular electronics.
Feasible, but expensive:
retrofitting power stations
In the context of pollution prevention, it is helpful to distinguish between
stationary and mobile combustion systems. In this connection “stationary”
stands for a power plant for electricity
production, for example, or a diesel
engine as an emergency generator for
which an appropriate control system
can monitor the combustion conditions. Once warmed up, a boiler can
be run long term under virtually ideal
conditions and power stations can be
retrofitted with large-volume, expensive, state-of-the-art filter systems.
retrofits in these dimensions are inconceivable for smaller mobile combustion systems.
detect important reaction partners at
the point of origin. Even the control of
back-up gas power stations with quick
start-up time for peak load operation
is possible using real-time monitoring
with modern laser technology.
Chemistry and process engineering
can help to reduce the carbon dioxide
emissions of stationary industrial facilities. Intensive research is focusing on
carbon dioxide capture, storage and
utilisation. The solution to the problem of carbon dioxide is the key to the
rehabilitation of coal power stations,
at least for a transitional period. It still
remains for alternative power stations
– regardless of whether they are based
on hydropower, sunlight, tidal currents
or geothermal heat – to demonstrate
their economic viability and environmental compatibility, whereas coal
energy production is an established,
technically well understood process.
Computerised engines
The efficiencies of stationary plants
can also be increased by coupling different processes. a stationary system
can be controlled by sensors that
Mobile drives can be clearly distinguished from stationary combustion plants, even in terms of pollutants:
Two co-workers at Daimler obtain detailed insight into combustion in
a vehicle’s engine by visualising the injection and combustion processes.
(Image: Daimler)
54
since an engine should be neither
particularly heavy nor expensive, the
weight and cost of additional filters and
retrofits must be kept within reasonable limits. Furthermore the similarity
of combustion in a mobile system to
that in a stationary system is limited:
a car engine runs in different operating cycles when started or during a
long-distance trip, and each individual
ignition results in a somewhat different
evolution of the combustion process.
New combustion concepts are
being developed with the aim of substantially suppressing the formation of
soot and nitrogen oxides in addition to
increasing efficiency – and thereby reducing carbon dioxide emissions. For
the engines and gas turbines of the
future combustion scientists propose,
among other things, lower temperatures and optimum mixing of fuel and
air. That may well sound simpler than
it is, since the flame will extinguish if
the combustion temperature is too
low; besides, a well-mixed combustible mixture can accidentally explode.
For such concepts to work reliably,
computer control of the combustion
Engineers fine tuning the DiesOtto engine, a petrol model with diesel
genes.
(Image: Daimler)
S O O N
O N
T H E
B A C K
B U R N E R ?
process is needed based on real-time
information on important operating parameters. In fact, such novel combustion engines are also being developed
in Germany: one example is Daimler’s
DiesOtto engine presented in July
2007, which combines petrol and diesel technology.
There are not so many alternatives
to the combustion engine as there are
for power generation systems. The
main types available are fuel cells,
electric motors and hybrid drive options, the crucial factor being how the
operating supplies – hydrogen, methane or electricity - are produced. None
of these concepts are in a position to
replace conventional combustion engines overnight. Instead, combustion
engines and also gas turbines that
use regenerative fuels will figure more
prominently. apart from problems
in their production, biofuels impose
special demands on the machinery in
which they are burned, independent of
whether they are used as an additive or
as neat fuel.
Since these molecules already contain oxygen in their skeletons, they are
said to have less of a tendency to form
soot than the related hydrocarbons – a
property which, however, is only now being investigated in depth. Chemists have
not yet even got to the bottom of the
combustion of ethanol although it has
long been used in Brazil as fuel. and butanol, which is similarly advocated as a
biofuel, has not just one but four isomeric
structures with different positions of the
alcohol group in the molecule. as a new
mass spectrometry process recently
demonstrated, these four structures produce different intermediates upon burning. and not only that: when butanol and
other alcohols are burned, air pollutants
such as formaldehyde and acetaldehyde
are released.
mechanical engineers face a great many
challenges – in fact, now more than ever
- since combustion will continue to play
an important role in mobile applications
in the future. We thus believe: If it has to
be combustion, make it as clean and efficient as possible!
With fuel mixtures, for instance petrol
with a small quantity of bioethanol and
even more so fuels composed of diverse biomass sources, it is still difficult
to predict the complete range of gases
emitted. Combustion scientists, chemists, physicists, process engineers and
Katharina Kohse-Höinghaus
The author is Professor for Physical Chemistry at Bielefeld University
and Second Chairperson, Deutsche
Bunsen-Gesellschaft für Physikalische
Chemie (German Bunsen Society for
Physical Chemistry).
Environmentally friendly
biofuels?
Just imagine undertaking a roundthe-world trip by car with biofuel being
available all the way: in some countries it
would be produced from corn, in others
from palm or olive oil, from wheat, turnips
or sugar beet, in countries with more advanced technology maybe even from
wood and compost. This hypothetical
case shows that there is a tremendously
broad range of substances that are suitable for biofuels with an equally broad
spectrum of energy content, ignition
limits, combustion temperatures, flow
characteristics and, of course, variability
in the formation of intermediates and end
products. Chemically, biofuels can be alcohols like ethanol and butanol, or they
can be ethers or esters such as biodiesel
produced from rapeseed oil.
Combustion research: two developers at Bosch test new engine control systems. With
controlled auto-ignition fuel and air are evenly mixed, distributed and completely combusted
throughout the combustion chamber.
(Image: Bosch)
55
rENEWaBLE FUELS
The use of oil revolutionised the world. Now there is a shortage of this
fossil resource and mankind is seeking alternatives. In future, fuels are
to be produced from wood, straw and other inedible plant residues,
and later on maybe even from algae.
Rapeseed: a feedstock for biodiesel.
Biofuel from the fields
and the ocean
T
he raw materials available in
the biosphere consist of 92
naturally occurring chemical elements.
If man wishes to utilise these raw materials, he will have to collect them
and modify them by physico-chemical techniques. This requires energy,
hence raw materials and energy are
inextricably interconnected.
Man has three sustainable primary
sources of energy at his disposition:
besides light from the fusion reactor,
the sun – in physical terms, a 5500
degrees Celsius hot, black body -,
and the heat of the earth (geothermal
energy), the tidal forces produced by
the planetary motion of the earth and
the moon can also be exploited. Solar
energy, which has been stored for mil-
56
lions of years in fossil resources, has
the lion’s share. We are about to deplete this reservoir in a period of just a
few generations.
gines was coke from coal. The coal
tar that accrued as waste formed the
material basis for the forerunners of today’s chemical industry.
Oats – the first superfuel
The Second World War, in turn, was
powered by fuel from coal liquefaction.
at the beginning of the 1950s German
industry still depended completely on
coal, since oil and natural gas were
not an economic option. Only after the
Suez Crisis of 1956 did oil pose a serious threat to coal. The development of
processes to convert oil into fuel coupled with increasing automobile traffic
boosted oil consumption in Germany,
until finally in the mid-1960s the demise of the era of coal-based fuel production became a fact.
In evolution, mobility was always
considered to provide a competitive
edge and in the age of globalisation it is
becoming increasingly important. The
basis of mobility is energy. Mankind’s
first superfuel was oats, a fuel from a
renewable energy source, and up to
the beginning of the last century a wellfed horse was a quick means of transportation for goods and information.
at the beginning of the industrial
era the fuel that powered steam en-
B I O F U E L
F R O M
T H E
F I E L D S
Since then, catalysed by oil, the
world has undergone an unprecedented transformation. The disadvantages
are obvious: environmental pollution,
climate change as well as the loss of
habitats and biodiversity.
Coal and natural gas are not
alternatives
Today we are being forced to realise that future planning on the basis
of oil is increasingly fraught with uncertainty. It is, therefore, a major challenge
to provide alternative resources to the
fuels that were hitherto based on oil. If
the distribution conflicts over oil lead to
ever increasing economic bottlenecks,
coal and natural gas can theoretically
serve as alternatives. Yet, since they
are also fossil-derived, they cannot really solve the problem. One sustainable
option is to exploit incident sunlight
and pave the way to a solar materials
and energy economy. From today’s
perspective, backing thermonuclear
fusion, which means bringing the sun’s
energy source to earth, is not economically viable: what the sun resolves
solely by means of its gravitational field,
namely by containing a hot plasma, is
by no means a simple problem to resolve here on earth.
Since man has not yet succeeded
in storing electricity or hydrogen costeffectively in cars, aircraft and ships,
the fuels of the near future will continue
to be organic liquids as only they can
readily store a high energy density.
Moreover this permits the use of the
existing infrastructure.
Initially from vegetable oils or
starch, later from wood and
straw
First-generation biofuels, whose
production uses only part of the plant,
have been on the market for some
years. The best-known representative
A N D
T H E
O C E A N
How biomass-to-liquid fuel production
works
Scientists at Karlsruher Institut für
Technologie are currently developing
a three-step process, consisting of
flash pyrolysis, gasification and fuel
synthesis, for the conversion of dry
plant waste into fuel. This concept involves a first step in which biomass is
Part of a pilot plant at Karlsruher Institut für
Technologie: here flash pyrolysis converts
biomass into an oil, from which subsequently
fuels are produced.
pyrolysed in the absence of oxygen.
The product is bio-crude oil, a suspension of pyrolysis oil and pyrolysis
char whose energy density is 15 to 20
times higher than that of the biomass
feedstock. pyrolysis should be carried out in small plants locally, in fact
as close as possible to the origin of
the biomass. The bio-crude oil is then
transported to a larger, central gasification plant for conversion into carbon monoxide and hydrogen. Chemists refer to the mixture of carbon
monoxide and hydrogen as synthesis
gas. Gasification uses a controlled
amount of oxygen, pressure of 30 to
80 bar and temperatures between
1200 and 1500 degrees Celsius. The
ratio of carbon monoxide to hydrogen in the synthesis gas depends on
the raw materials and the processes
used; a further step, in which carbon
monoxide reacts with water, enables
it to be adjusted to subsequent reactions. The synthesis gas produced by
this means is upgraded to diesel or
petrol by established chemical processes such as methanol or FischerTropsch synthesis.
(Image: Karlsruher Institut für Technologie)
is biodiesel, a rapeseed oil methyl ester
produced in Germany predominantly
from rapeseed oil and fossil methanol.
Bioethanol, on the other hand, which
is still mainly produced with starch derived from corn, lends itself to use in
Otto-cycle engines. Ethanol production from wood, more precisely: from
lignocellulose, or from municipal waste
is in development and is seminal. To
forestall competition between the food
and the fuel sector for raw materials,
future fuels should be produced from
crop residues, such as straw or wood
waste.
Despite intensive research efforts,
there are still no industrial-scale production plants for these second-generation fuels, which use any type of plant
material. One reason is that biomass
is a complex mixture of substances
whose composition varies, depending
on the plant species, condition of the
soil and climate of the growing area.
57
B I O F U E L
F R O M
T H E
F I E L D S
A N D
T H E
O C E A N
Biofuel from the ocean
Fossil energy sources
Oil
Coal
Renewable resources
Natural gas
Synthesis gas
FT
Co2-neutral sources
Reg. hydrogen
Methanol
Synfuels
Petrol
Bioethanol
Diesel
Biodiesel
Biobutanol
Designer fuel
The diagram shows the various routes for producing fuel from fossil and renewable energy sources.
FT stands for Fischer-Tropsch synthesis, by which synthesis gas is converted into liquid hydrocarbons. An alternative to this process is the indirect conversion of synthesis gas into synthetic fuels,
or synfuels, by way of the intermediate, methanol.
Various strategies are deployed for
second-generation biofuels, which use
every part of the plant. Using suitable
enzymes, wood waste, straw and other cellulosic materials can, for instance,
be converted into bioethanol, whereas
wet biomass, such as crop residues,
sewage sludge or grape pulp, should
be fermented to produce biogas, a
mixture of methane and carbon dioxide. The reaction of wet biomass in supercritical water to form hydrogen and
carbon dioxide, which yields the fuel
methanol, is a further option. Water is
designated as supercritical when it is
exposed to a pressure of at least 221
bar and a temperature of at least 374
degrees Celsius. Under these conditions innocuous water turns into an
aggressive substance. For dry plant
waste, a three-step process, consisting of flash pyrolysis, gasification and
fuel synthesis, is currently being developed (see box).
Fields for the production of thirdgeneration fuels will be the oceans.
This is the habitat of microalgae, which
consist mainly of cellulose. Light marine algae lack reinforcing lignin, which
is contained in the cell walls of woody
plants and interferes with conversion
into fuels. Furthermore, its cultivation
does not require any freshwater – a
crucial criterion in the future.
To reprogramme the world economy on the basis of sustainable development instead of growth, cooperation between science, engineering,
the humanities and social sciences, in
fact all the disciplines, is indispensable.
One of their key tasks for the future is
to organise a new, sustainable, postfossil age of materials and energy provision. To this end, the development
of processes for fuel generation from
renewable resources will play a central
role. Supporting measures – such as
improving the efficiency of all production processes and slowing down the
growth rate of the world’s population
– are essential to secure a sustainable
supply of materials and energy.
G. Herbert Vogel
The author is Professor for Technical
Chemistry at the Technische Universität Darmstadt.
Microalgae under the microscope: the raw material for third-generation fuels.
(Image: Fraunhofer IGB)
58
BIOFUELS
Is biofuel production forcing up food prices? Is this why the rain forests
are being deforested? There is no ‘ultimate’ answer since no two biofuels are alike. A differentiated view of the fuels of tomorrow.
Wheat: on the plate or in the tank? The
development of second-generation biofuels
will help to answer this question.
(Image: © H.-J. Sydow)
How “bio” is biofuel?
T
hose who have once experienced food shortages or even
famine react spontaneously with dismay on hearing that cereals are not
landing in bread, but in car tanks. Is
that an outmoded sentiment? Given
the huge amount of biomass that nature produces every year by photosynthesis, man’s fuel consumption looks
trivial in comparison. The global energy consumption of road traffic is only
three per cent of the energy bound in
biomass annually. We should also recall that with modern biofuels an ancient process is experiencing a comeback. Before the fossil age, biomass
was quite simply the only fuel, and it
was used as forage.
Nevertheless the question whether
our thirst for biofuels will aggravate
hunger in the world is justified. after
all, five times more people have to be
fed today than before the invention of
the car. Furthermore, man is already
stressing the biosphere to the limit.
terials, originating from atmospheric
carbon dioxide by photosynthesis, only
release as much carbon dioxide during
combustion in a vehicle as nature had
fixed in the plants beforehand.
The case for biofuels
Today the major biofuels are ethanol, biodiesel and vegetable oils. In response to the oil crises in the 1970s,
Brazil developed an efficient technology to extract ethanol from sugarcane
for transportation purposes. In Germany, by contrast, a different process
ranks foremost: here, biodiesel has
been produced for a decade by conversion of rapeseed oil into rapeseed
oil methyl ester.
What triggered the development of
biofuels in the first place was the endeavour to be independent of political
pressure by the Organisation of petroleum Exporting Countries (OpEC).
Fuels from the most varied sources of
biomass should replace or supplement
petrol and diesel.
Meanwhile the utilisation of biofuels pursues two different aims: climate
change and conservation of finite fossil
resources. Fuels from renewable ma-
In the meantime other countries
have followed suit. In the USa similarly large quantities of ethanol are
59
BIOFUELS
produced from maize and wheat as in
Brazil from sugarcane. In addition, the
USa derives large amounts of biodiesel
from rapeseed and soya. In countries
like Malaysia and Indonesia huge plantations of oil palms are grown to supply
palm oil for biodiesel.
Since the processes have proved
unsatisfactory in many respects, however, scientists worldwide are honing
the still emerging techniques to increase their efficiency. Enzymatic processes, for instance, aim to extract ethanol not only from sugar, but from all
plant carbohydrates, including starch,
cellulose and hemicellulose. The ultimate aim is to utilise the carbohydrates
of the whole plant mass.
German scientists, in particular, are
taking a completely different course:
they are attempting to utilise every
form of biomass – not only the carbohydrates – by biomass gasification at
high temperatures and catalytic conversion of the accumulating gas into
fuels in a subsequent reaction (see
contribution on page 56). Since these
fuels are derived from liquefaction of
biomass, they are called biomass-to-
liquid, or BTL, fuels. The conversion of
synthesis gas into liquid hydrocarbons
was developed by two German chemists, Franz Fischer and Hans Tropsch,
in the first part of the last century. This
process, which is still referred to as
Fischer-Tropsch synthesis today, was
used to convert coal-derived synthesis
gas into fuel. Biofuels, whose production utilises the whole plant and not just
its grain or oil, are known as secondgeneration fuels.
The catch: biomass is not a
primary energy source
at first sight the concept of biofuels
is convincing, but a differentiated view
reveals evident weaknesses, since biomass is not primary energy in the sense
of crude oil, coal or natural gas. It cannot simply be extracted, it first has to
be produced. Cultivation, harvesting,
transportation, storage, drying and
other processing steps as well as the
fuel synthesis itself consume a great
deal of energy. The actual demand for
biomass for fuels is, therefore, considerably greater than a comparison of the
energy contents implies.
This gives rise to two serious questions: are sufficient areas available to
grow crops for fuel production? and is
there sufficient water? The demand for
land makes it imperative to redesignate
forest areas or arable land; the water
demand increases the already exorbitant use of freshwater by agriculture.
Compared with the global volume
of biomass, its use for fuels may seem
utterly insignificant; nevertheless some
difficulties have already emerged. For
instance, tropical and sub-tropical forests are having to give way to sugarcane and oil palm plantations. Even
carbon neutrality turns out to be a mirage: the greenhouse gases emitted
during the production of biofuel are too
important to be disregarded. additionally, biofuels are putting pressure on
food production.
Exploiting the synergies between
biofuel and food production
The transformation of farms into
energy farms has resulted in increased
prices for wheat, maize and soya on
the world markets. Independent of
whether crop failures and speculation
Bioethanol plant in Zeitz, Saxony-Anhalt:
CropEnergies, a company of the Südzucker
Group, produces bioethanol from cereals and
sugar syrup at this site.
(Image: Martin Jehnichen)
60
H O W
“ B I O ”
I S
B I O F U E L ?
aggravate this development, they do
indicate a trend. Europe and the USa
will change their agricultural export
policy. This will give rise to painful price
increases in the short term, it may,
however, lead to farmers in developing
and emerging countries regaining their
local markets which they had lost to
the highly subsidised agricultural sector of the USa and the EU.
The production of biofuels and food
can even be mutually beneficial. For
instance, waste from biofuel production can be used as animal feed. On
the other hand, slurry, straw and other
agricultural waste as well as waste
fat and miscellaneous waste from the
food industry can be used as precursors for fuels.
a further synergy is achieved by
using undemanding crops, which can
stabilise or even rehabilitate the water balance and the quality of badly
degraded soils, for crop rotation or to
bolster the cultivation of food crops.
In India and Madagascar, for example,
experiments are being made with jatropha, a shrub-like wild plant, whose
fruits contain up to 40 per cent oil. It
even flourishes in arid soil, requires little
water and can favourably influence the
local microclimate, thus improving the
fertility of the land on which it is grown.
The future of biofuels
in Germany
In the year 2006 two-and-a-half
million tonnes of biodiesel, half a million
tonnes of bioethanol and one million
tonnes of vegetable oils were tanked
in Germany. accordingly, the share of
biofuel amounted to a good six per
cent of total German fuel consumption.
In 2007 over five billion litres of biofuel
were put into circulation.
Whereas in the past a large proportion of biofuel came from domestic production, now more and more is
having to be imported. True, in spring
2008 the biofuel boom experienced
a number of setbacks, prompting the
German federal government to curb
the pace of expansion. The economic
and ecological objections that had
been raised for years were supplemented by a technical problem: our
vehicles are not designed for high admixtures of biofuels.
according to statistics of Fachagentur Nachwachsende rohstoffe (FNr,
agency for renewable resources),
in 2007 an area of almost two million
hectares was used for growing energy
crops in Germany. This corresponds to
ten per cent of Germany’s agricultural
land. Fuel crops were cultivated on 1.4
million hectares, equivalent to seven
per cent of total arable land. Taking the
yields per hectare quoted by FNr for
biodiesel, vegetable oil and bioethanol,
this area could only partly satisfy Germany’s demand for biofuels.
Optimistic scenarios assume that
the area for energy crops in Germany
could be doubled. This would represent one fifth of German arable land for
energy crops, the greater part going
to biofuels. Whether these scenarios
will materialise or not depends on the
development of grain prices, grain
production and also on government
measures.
The demand for energy, water, fertiliser and pesticides of the new crops
underscores not only the opportunities,
but also the risks that they involve for
agriculture and forestry. In addition,
61
H O W
“ B I O ”
I S
B I O F U E L ?
technical details concerning transportation and storage of biomass have to
be solved. The changeover to secondgeneration biofuels will be favourable,
since utilising the whole plant substantially increases the fuel yield, and what
is more, all kinds of biowaste can serve
as feedstock.
The agenda for the years
to come
Competition between the basic
needs of the world’s growing population and the even more dramatically
rising demands for mobility is already
well underway. The market for biofuel
is growing and thanks to government
support programmes the new fuels
are playing an increasingly important
role worldwide. Forest and agricultural areas, once used to grow food,
are steadily being transformed into fuel
plantations.
Biofuel production is not an end
in itself, its underlying purpose is to
contribute to the development of a
transport strategy based on renewable fuels. In order to achieve this goal
by environmentally friendly and eco-
Loading sugarcane: in Brazil the production of
bioethanol from sugarcane is well established.
(Image: Shell)
62
nomically viable means, undemanding
crops with high yields have to be developed. protecting the soil from erosion
and salinity through intensive irrigation
should form part of this process.
What, therefore, needs to be done
in the years to come? ambitious efficiency strategies could strike a balance
between food and mobility. In this context, the reduction of fuel consumption should take priority on the agenda
since it sidesteps the feverish expansion of biofuels and defuses competition with the food supply. The levers
are more efficient drives on the one
hand, and the intensified use of lighter
materials in vehicles on the other. The
refinement of batteries for hybrid cars,
which take electricity from the national
grid, thus partly substituting fuels, is a
crucial step on the way to a more sustainable transport strategy.
Hermann Pütter
The author worked for many years until
his retirement for BASF, his last position being that of a Research Fellow
and Head of the Electrochemical Processes Research Group. Since 2007
he has been the Energy Coordinator
of Gesellschaft Deutscher Chemiker
(German Chemical Society). Hermann
Pütter is the author of the contribution
on page 16 and co-author of the article
on page 72.
BIOGaS
In Germany there are currently 4500 biogas plants. And the number is
growing. No wonder, since biogas is an all-rounder like natural gas –
except that it is renewable. Biogas from crops on one hectare of arable
land can cover the annual fuel consumption of five compact cars (e.g.
VW Golf).
Biogasification is an environmentally friendly
form of energy production from biomass.
(Image: EnviTec Biogas)
Fuel, heat and electricity
from the bioreactor
B
iogas is an energy-rich gas
consisting of 50 to 70 per cent
methane, 30 to 45 per cent carbon dioxide and small quantities of hydrogen
sulphide, hydrogen and nitrogen. It is
formed when organic matter is degraded by methane-forming bacteria,
which are among the oldest life-forms
on earth. This degradation process
takes place solely in an anaerobic environment, thus only in the absence
of oxygen. Besides methane-forming
bacteria, many other microorganisms
are involved in breaking down biomass. They first break down the various organic substances, which consist
mainly of carbohydrates, fats and protein, into their individual constituents.
Methane bacteria then convert them
into biogas. In the microbial conversion of biomass, methane is formed
either by decomposition of acetic acid,
which not only gives rise to methane
but also to carbon dioxide, or by oxidation of hydrogen with carbon dioxide,
depending on the raw materials.
Methane fermentation is an important link in the nature cycle of matter.
as far back as 1776 the Italian physicist alessandro Volta described biogas
as “combustible air over marshes”.
Only 110 years later, however, was the
development of biogas attributed to
the activity of microorganisms.
Used as fuel gas before the
Second World War
Methane fermentation was first
used in technical applications over a
century ago, however at first only in order to decompose organic matter into
sewage sludge. The resulting gas was
not yet used for energy.
It was only just before the outbreak
of the Second World War that sewage
sludge and agricultural residues were
systematically fermented into biogas
for use as fuel, preferentially for heavy
vehicles such as trucks. The crude
gas was dried, cleaned of carbon dioxide and hydrogen sulphide and then
compressed for storage in steel tanks.
63
BIOGaS
That worked; however, if the gas had
not been sufficiently dried and desulphurised, occasionally the pressure
tanks burst.
after the Second World War the
first larger biogas plants were developed in order to utilise agricultural organic residues for heat and electricity
generation. part of the gas was also
used as tractor fuel.
Up to the year 2000 the growth of
biogas plants in Germany was sluggish. Only since the introduction of the
renewable Energy Sources act ten
years ago, which regulates the supply
of electricity to the grid and guarantees
a fixed compensation, has there been
a sharp rise in the number of plants
and in the installed power. In the past
ten years the number of biogas plants
has increased by a factor of four. The
existing 4500 biogas plants with an
installed electrical power of over 1650
megawatts cover around 1.6 per cent
of German electricity production.
Energy crops for higher gas yield
What mainly distinguishes biogas
plants from other plants that convert
biomass is the broad spectrum of biomass that they convert into energy:
slurry, liquid manure, solid manure,
agricultural crop residues, domestic
waste, residues and by-products from
agriculture and the food industry, besides industrial wastewater and sewage sludge from wastewater purification.
In the course of the current expansion of renewable energy sources,
crops are being specifically cultivated
for biogas production. These energy
crops are characterised by a high energy density, they can be stored all year
round and can be readily fermented.
For biogas production all crops are
suitable that achieve high yields of biomass per unit area and are only slightly
lignified at harvest time. at present,
priority is given to silage maize and silage from whole cereals, forage rye or
grass. In terms of fresh mass, energy
crops attain significantly higher biogas
yields than slurry. Selected waste from
The biogas sector is booming: biogas park for electricity production in Penkun, MecklenburgWest Pomerania.
(Image: NaWarO BioEnergie aG)
64
the food industry and agriculture –
such as stale bread and waste fats –
are even better suited, although their
availability is limited.
Biogas is a versatile energy carrier.
Depending on the degree of processing, it can produce heat in a boiler, or
electricity and heat in a combined heat
and power unit or a fuel cell. additionally, biogas can serve as fuel or can be
fed into the natural gas network.
Currently biogas is mainly applied in
cogeneration units for combined heat
and power production with the electricity being fed into the grid. a small
amount of waste heat from the power
generation unit maintains the process
temperature of the biogas plant, the
bulk is available for other purposes,
for instance for the provision of district
heating, drying processes or refrigeration. One cubic metre of biogas generates an average of 2.5 kilowatt-hours
of electricity and useful heat.
Biogas is converted into electricity
in special spark-ignition gas engines
Still a rare sight: biogas fuelling station.
(Image: Österreichischer Biomasse-Verband)
F U E L ,
H E A T
A N D
E L E C T R I C I T Y
and in series diesel engines to which
a small amount of ignition oil is added
for auto-ignition. Since biogas plants
are often located in remote areas, the
waste heat from a combined heat and
power plant can generally only be partly used or not the whole year round,
therefore other deployment uses are
becoming increasingly important.
One hectare for 68 000 kilometres
Biogas is an attractive fuel for powering vehicles and, besides the fact
that none of the energy is wasted, it
has many other environmental advantages. The gas has to be almost completely dried and desulphurised for use
as fuel. Furthermore, carbon dioxide is
separated, thus raising the methane
content to at least 97 per cent. In order
for conventional natural gas vehicles to
be able to tank biomethane and travel
as far as possible on one full tank, it
has to be compressed to a pressure of
about 250 bar.
The exhaust gases from the combustion of biogas in the engine contain
no particulate matter and significantly
less nitrogen oxides and sulphur dioxides than the exhaust gases from
conventional Otto-cycle and diesel
engines. Compared with other biofuels,
biogas energy is distinguished by its
higher area efficiency. When silage maize
is used, in which the whole plant is fermented, the amount of methane obtained per hectare is the equivalent of
some 4000 litres of diesel. This means
that the amount of methane from one
hectare is sufficient for a 68 000-kilometre trip or, using a different calculation, the average annual fuel consumption of five compact cars (e.g. VW Golf).
The previous first-generation biofuels, biodiesel and bioethanol, display a
lower acreage efficiency, however, their
advantage is that they can be readily
integrated into the existing fuel chain
F R O M
T H E
B I O R E A C T O R
as liquid fuel. While in Scandinavia and
Switzerland the use of biomethane for
fuel prevails, particularly in fleet operations, in Germany its use is still in its
infancy.
Biogas instead of natural gas
If biogas is upgraded to natural gas
quality, it can be fed into the existing
natural gas network. 35 upgrading
plants already existed in Germany at
the end of 2009 and a further 35 plants
are under development and construction. The plants are located over the
entire area of Germany. according to
the integrated energy and climate programme of the Government, the target
is to exploit the potential of 6 per cent
of today’s natural gas consumption by
2020. This requires the construction
of roughly 1500 biomethane plants
with an investment volume of at least
10 billion euros. By accelerating the
expansion of feeding biomethane into
the natural gas grid, gas providers are
aiming to reduce dependency on gas
imports and to cut back greenhouse
gas emissions.
The use of biogas in fuel cells is also
becoming increasingly attractive since,
compared with conventional utilisation
routes, considerably higher electrical
efficiency (of about 50 per cent) can
be achieved coupled with less maintenance and none of the disagreeable
noise emissions that are typical of engine combustion. Experiments with
different fuel cells have already been
successfully completed, however before they are implemented industrially
the techniques need to be optimised.
Moreover the production of fuel cells is
still too expensive.
The examples show that in future,
due to its similarity to natural gas, biogas will be capable of taking over a
wide range of tasks in a sustainable
energy economy. Since the application of biogas is spatially and temporally flexible, it can improve energy
efficiency, particularly in combination
with other renewable energy carriers.
Despite the wide range of potential applications, it should be borne in mind
that in all probability biogas will not be
able to cover more than ten per cent of
Biomethane
BTL (biomass-to-liquid)
Rapeseed oil
Biodiesel
Bioethanol
* Biomethane from by-products
(rapeseed cake, stillage, straw)
Fuel consumption of passenger cars: Otto-cycle engine 7.4 l/100 km, diesel 6.1 l/100 km
Comparison of biofuels: distance travelled by a car with biofuel from one hectare under cultivation. The production of rapeseed oil, biodiesel and bioethanol incurs waste that can still be
fermented into biogas. The additional distance that could be travelled with each type of biogas
is also given. (Image: Fachagentur Nachwachsende rohstoffe e. V.)
65
F U E L ,
H E A T
A N D
E L E C T R I C I T Y
primary energy consumption. In future,
therefore, it is essential to ensure that
the various renewable energy carriers
are applied where they will achieve the
highest efficiency. applied to biogas,
that is the cogeneration of heat and
power and its use as fuel in road traffic.
F R O M
T H E
B I O R E A C T O R
Peter Weiland
The author is Director and Professor at
the Institute of Agricultural Technology
and Biosystems Engineering, Johann
Heinrich von Thünen-Institut in Braunschweig.
How a biogas plant works
There are many different biomass fermentation processes. The heart of
every biogas plant is the fermenter, a
bioreactor in which bacteria convert
biomass into biogas in the absence
of oxygen. Fermenters are gas-tight,
thermally insulated tanks that are operated at a temperature between 35 and
55 degrees Celsius. The feedstock for
fermentation is conveyed into the fer-
Stable
the composition of the raw material,
contains traces of sulphur. Since sulphur can cause corrosion problems
in downstream equipment and the
biogas plant itself, the biogas has
to be thoroughly dried and desulphurised and, if its ultimate use so
requires, it has to be treated in further steps.
menter either by a pump or by a special feeding system. The fermentation
gas that forms is captured in the top
of the fermenter or in an external gas
tank before being put to use as energy.
Fermentation residues provide valuable
fertiliser.
The gas from the fermenter is saturated with water and, depending on
Residential building
Greenhouse
District heat
Heat
Manure
Heat storage
Interim storage
Co-substrates,
renewable resources
Diagram of the production and use of biogas.
66
Mixing tank
(Image: EnviTec Biogas)
Electricity
Cogeneration unit
Fermenter
Upgrading of
digestate
Grid
Digestate
Agricultural use
WHITE BIOTECHNOLOGY
The industrial use of microorganisms is by no means new. For the production of beer, bread and cheese, microorganisms have rendered their
services for centuries. Now they should show the chemical industry the
way out of the raw materials crisis.
The mould fungus Aspergillus niger produces
enzymes which serve as biocatalysts in
industry. Genetic engineering methods have
modified the fungus so that – rather like a
living factory – it produces large quantities of
enzymes. The picture shows the mycelium of
Aspergillus niger. The hyphae have a diameter
of about two to five microns.
(Image: BaSF)
Microorganisms in the
chemical industry
T
here is a general consensus
that our whole present economic system is dependent on the raw
material oil. It is widely held that this
resource will run out in the foreseeable
future. In fact, it is immaterial whether
this point in time will be reached in 40
or 100 years. The fact is, that it will
come one day – and the sooner alternatives are available for the production
of chemicals, the more serenely we
can await this day.
according to the International Energy agency, an increase in the price of
crude oil by just ten dollars per barrel
instantly slows down economic growth
in the euro zone by 0.5 per cent.
The global annual consumption of
crude oil is 3700 million tonnes, about
ten per cent of which is used by industry for chemical syntheses. The rest is
required for energy generation and for
transportation. It stands to reason that
priority should be given to alternatives
for the energy supply. However, renewable resources are also gaining ground
in the chemical industry. Besides the
chemical and physical conversion of
biomass into biofuel or into feedstock
for the chemical industry, the biotechnological conversion of starch, sugar
and cellulose by enzymes and microorganisms plays an increasingly significant role. In Europe, this area of research and development is referred to
as “white biotechnology”, in the rest of
the world as “industrial biotechnology”.
Industrial biotechnology spans the production of biogas, biodiesel and other
alternative energy carriers in addition
to the manufacture of bulk chemicals
and fine chemicals from renewable resources.
Oil refinery versus metabolism
processes in a classical oil refinery
are comparable to the metabolism of
a microbial cell. Both systems produce many substances that are used
as feedstock for further syntheses. In
a refinery diverse carbon compounds,
which are channelled into other proc-
67
WHITE BIOTECHNOLOGY
esses of the chemical industry or into
the energy sector, are produced from
the raw material oil. The cell acts in a
similar way: it transforms a raw material, sugar as a rule, into the most
varied carbon compounds which are
metabolically converted into further
products or energy.
The use of microbial processes
on an industrial scale calls for an adequate supply of raw materials, for
instance fermentable carbohydrates
from sugarcane, sugar beet, cereals,
wood and other renewable resources.
Bacteria, yeasts and fungi, which are
usually genetically modified for their industrial application, convert the carbohydrates and other feedstock into the
desired compounds. Some products,
for example antibiotics, amino acids and bioethanol, are already being
produced on a large scale by means
of such biotechnological processes.
These processes are also acquiring increasing importance in the production
of building blocks for pharmaceuticals
and polymers.
Enzymes conserve energy
The chemical and pharmaceutical
industries do not always, in fact, use
whole cells, but often only isolated enzymes. Enzymes are natural catalysts,
and they are numerous, since almost
every metabolic reaction is controlled
by an appropriate, specific enzyme.
The advantage of enzymes over classical chemical catalysts is that enzymes
unfold their catalytic effect under mild
conditions, in an aqueous environment and at moderate temperatures.
This saves organic solvents and energy. These properties make enzymes
highly interesting for industrial application – and in fact already many technical applications of biocatalysts are
deployed in the food and fodder industries, paper and textile industries, in
diagnostics and the chemical industry.
Thus, aided by various enzymes, about
Bioengineering in the chemical industry: a biotechnician monitors a routine fermentation in a
5000-litre bioreactor.
(Image: BaSF)
68
20 million tonnes of starch are converted into liquid sugar and several
million tonnes of paper are bleached
annually.
Sugar from wood
The market volume for enzymes is
estimated to be worth over 1.5 billion
US dollars and has almost doubled in
the past ten years. Currently one of the
research priorities worldwide is focusing on the quest for enzymes to convert wood – or rather: its main constituents, cellulose and hemicellulose – into
utilisable sugars by a microbial process
for use as feedstock by industrial biotechnology. Such enzymes that break
down wood have been known for a
long time. In nature they decompose
dead plant matter, enabling it to return
to the natural materials cycle. Bacteria
and fungi containing these enzymes
are already used in composting plants.
Nevertheless it has not yet been possible to isolate their enzymes efficiently
M I C R O O R G A N I S M S
I N
T H E
and cost-effectively for use in biotechnological processes.
Besides the quest for enzymes, the
chemical industry is also increasingly
concentrating on plant breeding. The
rationale is that plants should provide
raw materials recovered by low-energy
processes and which can readily be
converted by microorganisms and enzymes into the desired end products.
It would be even more attractive if the
plants themselves were to produce
a specific, industrially-relevant substance, for instance a pharmaceutical
active ingredient. This substance could
then be isolated directly from the plant
prior to further low-energy processing
of the rest of the plant. parts of plants
that are not chemically usable could
subsequently be utilised for energy
production in a biogas plant.
It is scarcely, or not at all, possible
to breed such plants using conventional methods, therefore genetic en-
C H E M I C A L
I N D U S T R Y
gineering processes would have to be
used. The public debate surrounding
green gene technology could thus make
a decisive contribution to a – publicly
funded – more widespread use of industrial biotechnology in the chemical
industry: conventionally bred plants will
very soon no longer be able to satisfy
the steadily rising demand for raw materials and ever higher expectations of
products. Efficient raw materials supply and utilisation will only succeed if
further advances are made in green
and industrial biotechnology in close
cooperation.
Roland Ulber
The author is Professor for Bioprocess
Engineering at Technische Universität
Kaiserslautern.
Microbes on a culture medium: a new gene was inserted into these bacteria. Only the cells
where gene transfer was successful proliferated on the special culture medium. Individual
bacteria colonies are now being isolated for further experiments.
(Image: BaSF)
69
BIOrEFINErIES
Biorefineries combine the conversion of materials and energy from renewable resources. In the future they are to produce chemicals and
biofuels. And also to extract biogas from the organic waste residues.
Feedstock grass
(Image: petra Blauensteiner)
Chemical plants and power stations
modelled on nature
N
ature has set the highest
standard for handling energy
and materials. In photosynthesis, water is split and the hydrogen produced
is bound to carbon derived from the
carbon dioxide in the air. The energyrich carbon compounds formed in this
way are an energy store which powers bioreactions. Once the energy has
been used, carbon dioxide and water
reform, by which, with respect to carbon, oxygen and water, the photosynthesis cycle is closed.
By means of photosynthesis, that
is, power generation from sunlight, nature produces a variety of renewable
raw materials that man can use to produce both materials and energy. Over
70
90 per cent of renewable raw materials
are carbohydrates or lignin. particularly
lignocellulose, which forms the cell wall
of woody plants, is available in abundance.
Already ten per cent of chemical
feedstock comes from fields
The use of sugar, starch, cellulose,
oils and other renewable resources for
materials and energy has a long tradition; in the industrial age they were
replaced by coal, oil and natural gas.
Until coal was used in industry, wood
was the chief source of materials and
energy. In Germany over twelve million tonnes of renewable resources,
including wood, are now used annually
for conversion into energy and more
than 2 million tonnes for the production of biofuels. The German chemical
industry processes a further two million
tonnes of biogenous material into polymers, surface-active agents and other
bio-based products. This corresponds
to ten per cent of its total raw materials input.
In the context of renewable resources, the starting point is highly
positive in Germany and in Europe. By
2020 Europe aims to derive a fifth of
all its fuels and over one quarter of its
energy carriers for electricity, heating
and cooling from renewable sources.
No figures are available in Europe for
the production of bio-based prod-
C H E M I C A L
P L A N T S
A N D
ucts, such as polymers, surface-active
agents and adhesives.
The situation is different in the
USa: there the stated national target
is to produce one quarter of all chemical products, based on base values of
1994, from renewable resources, besides ten per cent of fuels as well as
five per cent of energy carriers required
for heating, cooling and electricity production by 2030.
First biorefineries in ten years
at the earliest
The transformation into a bio-based
economy should be achieved by the
P O W E R
S T A T I O N S
M O D E L L E D
development of biorefineries which use
biomass both for valuable products
and for energy. after, or alongside, the
production of fuels and chemicals, the
biorefinery is intended for carbon-dioxide-neutral energy production by burning biowaste or by converting it into
biogas using biotechnological routes. In
the case of very wet residues, biogasification in a fermenter is the process of
choice from the point of view of energy
and with respect to climate protection,
since this option offers the most promising potential for reducing carbon dioxide. Furthermore, biogas production
is ideal in combination with new chemical processes that, just like biogasification, take place in aqueous phase and
GRASSLAND BIOMASS
e.g. grass, clover, alfalfa
Silaging
O N
N A T U R E
at low temperatures. Biogas can be
used for heating or cooling and it can
also be converted into methanol as fuel
or as building block chemicals.
Biorefinery concepts are debated
worldwide, nevertheless hitherto they
have only spawned isolated solutions,
such as cellulose, starch and oil facilities or bioethanol plants. It will in all
probability take ten to twenty years before the first larger biorefineries come
into existence.
Thomas Hirth, Walter Trösch und
Steffen Rupp
Thomas Hirth is Director of the Fraunhofer
Institute for Interfacial Engineering and
Biotechnology IGB in Stuttgart, where
Walter Trösch and Steffen Rupp are research scientists. Walter Trösch is Head
of Department Environmental Biotechnology and Bioprocess Engineering. Steffen
Rupp is Head of the Molecular Biotechnology Department.
(Solids
fermentation)
Mechanical fractionation
(pressing)
Press cake
Press juice
Fibre pulping
Amino acid
separation
Amino acids
Protein products
Lactic acid
separation
Lactic acid
Ethyl lactate, …
Fibre boards, biocomposites,
insulating materials, fodder
Electricity/power
heat
Biogas plant
Fine chemical
separation
Aromas, chlorophyll,
pigments
The principle of a biorefinery: the raw material biomass is converted into electricity and heat
and also into chemicals, fuels and other valuable products in biotechnological and chemical
processes.
Vision of the future: a biorefinery that converts biomass into energy and chemicals
could well look like this.
(Image: projektfabrik Waldhör)
(Image: projektfabrik Waldhör)
71
HYDrOGEN ECONOMY
Hydrogen is regarded as an alternative to fossil energy carriers. However, hydrogen is an energy storage medium, not an energy source –
thus it does not guarantee environmental compatibility.
In future, fill up with hydrogen and drive
clean: what part hydrogen will play in the
future energy economy depends on how
it is generated.
(Image: Fotomontage, Markus Fischer/pSI)
Many questions still remain
H
ydrogen has been used for a
long time, primarily as an important raw material for refineries and
in the chemical industry. Of the over 50
million tonnes of hydrogen produced
annually, fertiliser production and crude
oil refinement each absorb almost 50
per cent. Four per cent is placed on
the market and one per cent serves as
fuel for drives, for instance for the European launch rocket ariane V. In future,
hydrogen could also assume an important role in the general supply of fuels
and energy. In fuel cells, for example,
electricity could be produced from hydrogen. The great advantage is that the
only waste gas is steam. How hydrogen
should be transported (see contribution
on page 30) is still an open question:
under pressure, as a deep-frozen liquid
72
or bound to chemicals, for instance as
methanol? There is no lack of ideas, but
their suitability still remains to be determined.
Furthermore, hydrogen is not an
energy source, but an energy storage
medium. at most, therefore, it supplies
as much energy as was previously
used to produce it – and in reality only
a part of that. If hydrogen is to become
an attractive energy carrier, these losses and the costs, too, must be kept to
a minimum; this would provide an economic incentive to switch from carbonbased energy-carriers to hydrogen.
There are a number of options for
producing hydrogen. The energy required can originate either from nuclear
energy, from direct solar radiation – by
means of solar thermal, photovoltaic
or wind energy – or from stored solar
energy in the form of hydropower, biomass or fossil energy carriers.
How the production of hydrogen
should be assessed, is briefly demonstrated here taking the example of
biomass: to produce hydrogen from
biomass, two steps must be taken into
account, which from an energy standpoint are not advantageous. On the
one hand, with the energy from sunlight plants convert carbon dioxide into
organic matter with very poor efficiencies, on the other hand, to extract hydrogen from this plant matter requires
additional energy. This would necessitate huge areas for the cultivation of
M A N Y
Q U E S T I O N S
S T I L L
energy plants and that – as the current
debate surrounding biofuel shows –
leads to competition between food and
energy production.
additionally, hydrogen production
from biomass causes carbon dioxide,
which is detrimental to the climate
and is not directly converted back into
biomass. This criticism may sound
strange, since biomass is regarded as
a climate-neutral energy carrier – yet
it makes no difference to the climate
whether the carbon dioxide emitted
was bound a year ago or in earlier geological periods. It is crucial to compare
the alternatives: which one causes the
lowest total emissions? Not forgetting
that biomass, too, is an increasingly
scarce resource.
Hydrogen from natural gas
Hydrogen produced today is extracted almost exclusively from fossil
energy carriers like natural gas and
coal. Only five per cent are generated
by electrochemical splitting of water
into hydrogen and oxygen, that is, by
electrolysis.
R E M A I N
The present most important process for hydrogen generation is steam
methane reforming. In this process
natural gas reacts with water vapour
at temperatures of 750 to 850 degrees Celsius under pressure to form
hydrogen and carbon monoxide. The
necessary energy is supplied by the
combustion of natural gas, by which,
however, carbon dioxide arises whose
levels must be reduced. Steam reforming functions excellently and its carbon
dioxide emissions could be cut, enabling the process to be used as a first
step into the hydrogen economy. This
would be feasible if the heat were supplied not from the combustion of fossil resources, but from highly focused
sunlight – however, this is only economically viable in parts of the earth
with high levels of sunshine. Hightemperature thermal storage systems
should make the process independent
of solar radiation since even in deserts
the sun does not shine continually,
but the chemical process requires
constant conditions. The technology
has been under development since
the 1980s and, due to the favourable
business environment, is about to be
implemented industrially. The cost of
conventional and solar steam reforming are today already on a par.
One alternative is to use nuclear
energy. Hence, France, Japan and
other countries committed to nuclear
power are developing reformers to be
powered by process heat from hightemperature nuclear reactors. This
type of nuclear reactor alone generates
temperatures exceeding 850 degrees
Celsius necessary for hydrogen production. On account of their design,
high-temperature nuclear reactors that
use helium as the coolant and graphite
as the moderator are considered to be
safer and more efficient than conventional reactors. The idea is not new,
it was demonstrated in the 1980s on
the pebble-bed reactor at Forschungszentrum Jülich. To date, however, no
commercial high-temperature nuclear
reactors exist, the first are about to be
built in South africa and China.
In Almeria, Spain, scientists from Deutsches Zentrum für Luft- und Raumfahrt (German Aerospace Center) and the Spanish Energy Research
Centre Ciemat test solar thermal processes for hydrogen production. The photo on the left shows an array of mirrors designed to focus sunlight
constantly on one point, independent of the position of the sun in the sky. The concentrated solar energy powers a reactor in which water is decomposed into oxygen and hydrogen. The photo on the right shows parabolic mirrors to focus sunlight.
(Image: DLr)
73
HYDrOGEN ECONOMY
How thermal water splitting works
Direct thermal water splitting only
works at temperatures above 2500
degrees Celsius. Therefore it cannot
be said to have technical potential.
Efficiencies are low, there is a lack of
suitable materials for components and
plant construction and the oxygen
and hydrogen gases form an explosive mixture, known as oxyhydrogen.
These problems can be avoided by
spatially or temporally separating the
formation of hydrogen and oxygen,
thus circumventing the build-up of
oxyhydrogen. Since the 1970s investigations have focused on cycles in
which water splitting is divided into
several reaction steps. On the one
hand, this eliminates the formation of
oxyhygrogen, and on the other hand
the temperature can be reduced to
such an extend that it is technically
feasible. The two most significant cycles
are named after the companies that
developed them: the General atomics
process and the Westinghouse process. In terms of energy, the crucial
step in both processes is the decomposition of sulphuric acid (H2SO4) over
catalysts at temperatures above 800
degrees Celsius. Both cycles can
achieve thermal efficiencies of over
40 per cent. However, before larger
plants can be built a solution has to
be found to the corrosion problem.
Heat
2
O
800°C – 1200°C
H2SO4
H2SO4
H2O + SO3
SO3
SO2 + ½O2
Electrolysis (90°C)
H2SO4 + H2
SO2 + H2O
SO2 + 2H2O
2
H
O
H2
The Westinghouse process: sulphuric acid (H2SO4) is catalytically split into
water (H2O), oxygen (O2) and sulphur dioxide (SO2). The oxygen is removed from
the system. Water and sulphur dioxide are then converted into sulphuric acid
and hydrogen by electrolysis. Hydrogen is removed, sulphuric acid remains in
the closed loop.
74
From water and the sun
in the long-term
The real goal of a future hydrogen economy is to replace carbon,
thus mitigating greenhouse gas emissions. In doing so, hydrogen generation must not pose a potential threat
to the climate or to human health. For
this reason the processes targeted for
hydrogen production in the long-term
are based on water as the raw material
and the sun as the energy source. The
snag: in regions with high incident solar radiation, water is generally scarce.
To produce the 50 million tonnes of
hydrogen needed today it would take
450 million tonnes of water, in reality the
demand would probably be far higher.
In moderate zones with ample freshwater, renewable energy can be used
to generate hydrogen by electrolysis; in
areas with high levels of sunshine solar
thermal processes combined with sea
water desalination are more promising.
The development of photochemical
processes for water splitting involves
both biochemical and catalytic pathways. research focuses primarily on
microorganisms and algae which produce hydrogen, and also on photocatalysts, such as titanium dioxide. Furthermore, both photoelectrochemical
processes, although they require electricity in addition to catalysts, and also
biomimetic processes have potential.
The latter require artificial biocatalysts
familiar, for instance, from photosynthesis. However, all these processes
are still in their infancy.
The only water-splitting technology
that is already available and mature is
electrolysis. Yet so far it has not been
able to establish itself either, since
steam reforming of natural gas is less
expensive and is superior to electrolysis from an energy standpoint. In times
of climate change and rising fossil fuel
prices, however, water splitting by
M A N Y
Q U E S T I O N S
S T I L L
R E M A I N
electricity from renewable sources is a
contender to be reckoned with.
will be no hydrogen economy, despite
the political targets.
Mature hydrogen economy
in 2050?
The critics face clear, political objectives from Brussels and the tightlyknit network of the hydrogen lobby.
The Deutscher Wasserstoff- und
Brennstoffzellen-Verband (the German
Hydrogen and Fuel Cell association),
which is linked to all European and
non-European hydrogen initiatives,
invokes the historical development
of energy systems: the energy carriers of the industrial age changed their
phase state from solid coal via liquid oil
to gaseous natural gas. Concurrently
the atomic ratio of hydrogen to carbon
rose from around 0.5 in coal to four in
natural gas. With hydrogen as energy
carrier this trend would continue, since
in a pure hydrogen economy the energy carrier would contain no carbon at
all. The consequence would be a completely carbon dioxide-free energy system. In addition, there would again be
a sharp increase in the calorific value of
the future energy carrier: for hydrogen
it is 120 megajoules per kilogram – that
is almost three times as much as for
petrol.
The question whether hydrogen
will ever prevail as an energy carrier,
depends on the political, ecological
and economic framework conditions.
admittedly, oil is becoming increasingly expensive and scarce, the climate
problems are urgent and the energy
demand is spiralling. The EU has set
the target of a total switch to a hydrogen economy by the year 2050 – in
view of the many open questions and
critical issues this is an ambitious, if not
an extremely ambitious, goal.
Electrolytic production of hydrogen
will probably be adopted in regions
with usable geothermal energy or hydropower, for instance in Norway or Iceland, for a regional hydrogen economy
from which other regions can learn a
lesson. Norway is already the world
leader in electrolytic water splitting
with hydropower as the energy source,
and Iceland is a model for a hydrogen
economy based on geothermy.
Natural gas could well function as
a stepping-stone until processes for
regenerative hydrogen production are
market-ready. Distinguished critics
doubt that regenerative hydrogen will
ever become a pillar of our energy system. They point out that the recovery,
storage and distribution of hydrogen
consume too much energy and incur
too high costs. Their hypothesis: an
energy system that is not only inefficient, but also expensive will not find
acceptance. These arguments cannot
simply be dismissed. If we do not succeed in demonstrating the suitability of
production processes, developing efficient hydrogen storage collectors and
upgrading fuel cells technically and
economically for use in vehicles, there
Christian Sattler
Hermann Pütter
Christian Sattler is Research Area Manager for Solar Materials Conversion
at Deutsches Zentrum für Luft- und
Raumfahrt (German Aerospace Center)
and Vice-President of N.ERGHY, the
research association in the European
Joint Technology Initiative for Hydrogen and Fuel Cells.
Hermann Pütter worked for many years
until his retirement for BASF, his last
position being that of Scientific Director
and Head of the Electrochemical Processes Research Group. Since 2007
he has been the Energy Coordinator
of Gesellschaft Deutscher Chemiker
(German Chemical Society). Hermann
Pütter is also the author of the contributions on pages 16 and 59.
and which scenario is the most
probable? There is no question: due to
the framework conditions, the energy
economy will radically change. Nonetheless, hydrogen will not achieve the
status that oil has today. There will be
certain areas where the use of hydrogen will be meaningful, provided the
technical prerequisites have been created. The inefficiency of production
and logistics could be partly offset by
the application advantages. This, however, necessitates intensive research
efforts
75
CaTaLYSIS
Catalysts allow chemical reactions to proceed under mild conditions,
causing fewer undesirable by-products. That saves energy, solvents
and costs. They also play a key role in fuel cells.
Catalysts in the chemical industry:
a laboratory technician fills a test reactor
with a catalyst in granular form. Over 80
per cent of all chemical products are
produced using catalysts.
(Image: BaSF)
Thrifty reaction catalysts
O
n hearing the buzz word ‘catalyst’, most people spontaneously think of the catalytic converter in
a car. However, the primary function of
catalysts is not to destroy emissions,
but to help conserve energy. Even in
the household: they are contained in
detergents, for example, and can eliminate stains even at 30 degrees Celsius
instead of in a boil wash. Moreover it is
not an exaggeration to claim that without catalysis most items in everyday
use would not exist: over 80 per cent
of all chemical products – from plastics
to medicines to body lotion – undergo
a catalytic process in the course of
their production. Even life itself is inconceivable without catalysis. In every
living cell, enzymes as natural catalysts
ensure that metabolic reactions can
76
take place at mild temperatures and
normal pressure.
Catalysts are like matchmakers
Catalysts are not a uniform class
of compounds. Their spectrum ranges
from precious metals to simple inorganic molecules through to proteins.
What they all have in common is that
they accelerate chemical reactions,
steer them in the right direction or even
make the conversion of Substance
a into product B possible in the first
place. With most chemical conversions
a mountain of activation energy has
to be surmounted on the way from
Feedstock a to product B. The higher
the mountain, the more energy has to
be fed into the system to trigger the
reaction. That is the reason why chemists generally heat their reaction flasks
vigorously when carrying out a synthesis.
Catalysts can serve to lower this
activation energy. Like a hiker in the
alps trying to avoid a high summit,
they show Feedstock a an easier, less
steep route to product B. Moreover,
they ensure that fewer undesirable byproducts arise.
Catalysts are chemically almost unchanged at the end of a reaction and,
like matchmakers at the end of a day’s
work, are again ready for the next reaction. Thus, a catalyst molecule can
aid the synthesis of millions of product
molecules. That also explains why an
T H R I F T Y
R E A C T I O N
C A T A LY S T S
automotive catalyst only has to be replaced after 100 000 kilometres driven.
The process in an automotive catalyst is an example of heterogeneous
catalysis, by which the catalyst exists
in a different physical state from the
substances to be converted. In the
automotive catalyst, gaseous nitrogen
oxides and carbon monoxide as well
as other incompletely burnt hydrocarbons react with platinum, deposited on
a ceramic base, to form carbon dioxide, nitrogen and water. Heterogeneous catalysis is also widespread in the
chemical industry, primarily in the combination “solid catalyst in liquid phase”.
In our body cells, on the other hand,
homogeneous catalysis takes place,
since the enzymes as well as the molecules to be metabolised are present in
dissolved form in the cytosol.
Key role in energy supply
Catalysts also play a key role in
the supply of energy carriers. petrol,
for example, is produced by a catalytic process. In catalytic cracking of
crude oil, the catalysts are aluminium
and silicon oxide powders which permit the process to run at temperatures
from 350 to 400 degrees Celsius and
normal pressure – instead of at 500 to
700 degrees Celsius and a pressure of
20 to 100 bar.
In addition, the successful production of alternative energy carriers
will depend on the use of catalysis.
Hydrogen, for example, could be an
important factor in our future energy
supply (see contributions on pages 30
and 72); it can be produced energyefficiently by catalytic splitting of water
into hydrogen and oxygen – for example using energy from sunlight and titanium dioxide catalysts. another option
for water splitting is based on photosynthesis, by which hydrogen is produced from water and then, together
with atmospheric carbon dioxide, is
further converted into sugar. The plant
cell accomplishes this biochemical wa-
ter splitting with the aid of elaborate
biocatalysts which are the model for
new catalysts in industrial hydrogen
production.
Hydrogen is set to be used as a fuel
to power fuel-cell vehicles in the future.
That is not really a problem, since a tremendous amount of energy is released
in the process by which hydrogen
combines with oxygen to form water
in the fuel cell. However, the problem
of hydrogen storage has not yet been
solved. Not only special tanks for
gaseous or liquid hydrogen are under
discussion, but also chemical storage
media, that is, compounds that contain
a large amount of hydrogen which can
be released, so to speak, by stepping
on the accelerator. The alcohol methanol is such a hydrogen carrier and it is
already in use in a Daimler fuel-cell vehicle. Hydrogen is separated on board
the vehicle by a hydrogen-permeable
membrane with a palladium-silver alloy
as the catalyst.
Catalysis research at Technische Universität Berlin (left) and in the chemical industry (right): new catalysts increase the energy efficiency of
chemical reactions.
(Image: TU Berlin, pressestelle; BaSF)
77
T H R I F T Y
R E A C T I O N
C A T A LY S T S
Renewable catalysts in the
chemical industry
In the chemical industry, too, catalysts play a central role en route to
energy-conserving processes. Energy
costs comprise a substantial share
of the chemical industry’s operating
costs, amounting to as much as 40 per
cent in the production of basic chemicals. However, this is not the only reason why chemists are interested in catalysts. It is rather the fascinating ability
of many catalysts to steer a reaction in
a specific direction, selectively guide it
to the end product, while avoiding byproducts.
Many chemical compounds are
present in two mirror-image structures
which behave like a right hand to a left
hand, while possessing distinctly different properties. One example is the
substance limonene which releases a
lemon-like aroma in one form, its mirror
image an orange-like aroma. However,
this difference is slight compared with
the properties of some pharmaceutical
active ingredients in which one form
is highly effective, the other harmful.
Catalysts which ensure that only a specific molecule forms, save solvents and
energy since they render the complex
separation of the mirror-image forms
superfluous. The best of these ‘stereoselective’ catalysts include natural enzymes which nature optimised during
millions of years of evolution.
additionally, thanks to sophisticated enzymes, nature often has a better
synthesis pathway in readiness than
classical chemistry, even for complex
biomolecules like vitamins, pharmaceutical agents and flavours. a prime
example is vitamin B which was once
produced on a large scale by a multistage synthesis, while nowadays industry uses microorganisms to manufacture the vitamin in a single stage.
The chemical company BaSF applies
78
the fungus Ashbya gossypii, for example. Compared with the classical
chemical process, the biotechnological
alternative produces about 30 per cent
less carbon dioxide with cost savings
of 40 per cent.
Magic no longer
The examples show that catalysts have become indispensable to
industrial processes, both for the production of energy carriers and for the
development of more energy-efficient
industrial processes. The outstanding
importance of catalysis for society is
documented by the award of the Nobel
prize for Chemistry in 2007 to the Berlin catalyst researcher Gerhard Ertl who
has made a significant contribution to
our understanding of catalytic processes on solid surfaces. His achievement
consists in liberating catalysis from its
reputation as magic and transforming
it into an exact science.
at which the particles move relatively
slowly, he was able to gain insight into
the surface chemistry and to interpret
fundamental catalysis phenomena.
The processes in real catalysts, however, are much more complex and, in
many cases, they have not yet even
been decoded. Moreover, not only the
processes at the surface are decisive,
but also the transport of the feedstock
to, and the products from, the catalyst.
The award of the Nobel prize marks a
milestone, but the final chapter on catalysis research is by no means done
and dusted.
In order to develop new catalysts
and optimise existing ones, chemists
need a detailed understanding of catalytic processes. Therefore they study
catalysis on the molecular level, using
spectroscopic, electron-microscopic
and other modern research methods.
They observe how base molecules accumulate on the catalyst, how chemical bonds split, new ones form and the
products leave the catalyst again. That
is no mean undertaking since many
catalysts are porous materials and the
centres of the catalyst are concealed
within the pores inside the solid. The
chemical reaction proceeds, as it were,
in a hiding place, where not even highdefinition analysis methods can just
take a look.
For his investigations, Gerhard Ertl
selected single crystals, that is, extremely regular, well characterised systems as simple models for catalysts.
In vacuum and at low temperatures,
Hans-Joachim Freund
The author is a Director of Fritz-HaberInstitut der Max-Planck-Gesellschaft in
Berlin, where he heads the Department
of Chemical Physics.
IONIC LIQUIDS
Salts which melt at ambient temperature are chemical all-rounders.
They are suitable as solvents, lubricants or heat storage media.
In modern energy technology they are in demand as electrolytes in
batteries, capacitors and fuel cells.
Liquid salts consist of large ions: top left, the
model of a typical cation for such salts, below,
an anion and beside it, for a comparison of the
size, a water molecule.
(Image: NIST)
Saline, safe and
incredibly versatile
I
onic liquids – or, put differently,
salts that are liquid at ambient
temperature – make up a new class of
substances with fascinating properties
that connect two worlds, that of salts
with that of solvents. Yet why are these
salts actually liquid?
Salts consist of positively and
negatively charged ions. For example,
common salt, chemically known as
sodium chloride, contains positively
charged sodium ions and negatively
charged chloride ions. These oppositely charged ions attract each other
and form a crystalline lattice structure
at ambient temperature. Only at high
temperatures – in the case of common
salt above 800 degrees Celsius – does
the lattice break up, the crystal disin-
tegrate and the substance melt. This
melt is comprised solely of ions, it is,
therefore, an ionic liquid.
Strictly speaking, today only salts
which melt at very much lower temperatures than common salt, often
far below ambient temperature, are
designated ionic liquids. In this case
the attractive forces between the positively and negatively charged ions are
so weak that the crystal lattice already
breaks up at low temperatures.
There are incredibly many variants
of ionic liquids – some experts speak of
1018 possibilities – with extremely versatile properties. Liquid salts are heatresistant, non-flammable and do not
evaporate. Thus they lend themselves
to the most diverse applications. They
are already deployed in process engineering, organic synthesis and analytical chemistry. In addition they serve as
lubricants or hydraulic fluids, as coolants or heat storage media. Because
they conduct electricity, they are in demand as electrolytes in modern energy
technology, such as in the construction
of sensors, batteries, capacitors, fuel
cells and organic solar cells.
New energy storage system
for hybrid vehicles
In future liquid salts could also play
a role in hybrid vehicles. Hybrid drives,
which combine a combustion engine
and an electric motor, already exist.
Market research institutes anticipate
79
IONIC LIQUIDS
Why certain salts even melt in the cold
Ionic liquids are salts whose crystalline lattices disintegrate even at low
temperatures. What makes these
salts that melt in the cold so special is
that the charge of the individual ions
is distributed over a relatively large ion
volume. Instead of the point charge of
a classical ion, in this case there is a
charge cloud. This weakens the attractive forces between the ions and
the crystal lattices are not as stable.
This charge smearing can be achieved
chemically by introducing nuclei that
attract electrons into the structure of
negatively charged ions (anions) or repel electrons into the structure of positively charged ions (cations). In ionic
liquids no small inorganic ions (like the
sodium ion in common salt) are used
as cations, but distinctly larger organic
cations such as tetra-alkylammonium
N
+
R1
N
or pyridinium. additionally these large
cations often absorb long alkyl chains
that strongly repel electrons, further
weakening the positive charge locally.
The anions of ionic liquids are also
far more complex than the simple
chloride ion in common salt: besides
the well-established tetrafluoroborat
(BF4-) and hexafluorophosphate (pF6-),
more complex structures are being increasingly applied, particularly anions
of superacids, such as trifluoro acetic
acid or trifluoro sulphonic acid. Only
recently Fap anions (tris(perfluoroalkyl)
trifluorophosphates) were developed.
Thanks to the fluoro and fluoralkyl
groups that strongly attract electrons,
these anions are ideal building blocks
for new ionic liquids.
N
+
R2
Bu
R
Pyridinium
N
+
Me
Tetra-alkylammonium
Imidazolium
Typical cations of ionic liquids.
C2F5
F
B
F
-
F
F
F F
P F
F
F F
F
P
F
-
C2F5
F
C2F5
FAP
Tetrafluoroborat Hexafluorophosphate Tris(perfluoralkyl)trifluorophosphate
Typical anions of ionic liquids.
80
that the market for hybrid vehicles will
boom in the next few years. The prerequisite is modern energy storage in
the form of batteries, possibly combined with double-layer capacitors.
Double-layer capacitors store more
energy than conventional capacitors,
which is why they are called supercapacitors. Whereas a battery absorbs
a great deal of energy over a long period of time, a supercapacitor stores
less energy in a very much shorter time.
This, for example, can benefit energy
production in electric vehicles when,
for instance, braking energy is converted into electricity. That takes place
for only a few seconds, but mainly with
high efficiency. In the case of batteries,
storage losses have to be reckoned
with. Double-layer capacitors and batteries in electric vehicles could, therefore, complement each other well.
all commercial hybrid vehicles
are still equipped solely with nickelmetal hydride batteries, but the next
generation of hybrid vehicles will see
the introduction of the lighter, more
efficient lithium-ion batteries which
already serve as energy storage devices in mobile phones. admittedly,
considerable obstacles still have to
be overcome, not only in terms of the
cost of the energy storage device, but
particularly with respect to lifetime and
safety. at least the new lithium-ion batteries should achieve output power of
up to 1500 watts per kilogram. By way
of comparison: a conventional nickelmetal hydride battery supplies about
200 watts per kilogram.
In the lithium-ion batteries currently
available on the market, organic electrolytes are applied as ion-conducting
fluids; they are relatively highly flammable and thus potentially hazardous.
By using them as an electrolyte or as
an additive to established electrolytes,
ionic liquids could provide more safety,
since they are not flammable.
S A L I N E ,
S A F E
A N D
I N C R E D I B LY
As viscous as olive oil
Liquid salts are already deployed in
double-layer capacitors as an electrolyte
or as a component of the electrolyte, especially in asia. Their use in lithium-ion
batteries, by contrast, is still in its infancy.
The reason is that today’s ionic liquids are
relatively syrupy. Some are as viscous as
coffee cream, most as viscous as olive oil,
a few even as syrupy as honey. In highly
viscous electrolytes, however, the lithium
ion is less mobile. That significantly prolongs the charging time of the battery.
V E R S A T I L E
Moreover, many ionic liquids are not compatible with the electrode materials in use
today. However, with newly developed
ionic liquids, containing, for instance, the
‘Fap’ ion (see box) and with special mixtures of liquid salts and organic solvents,
these obstacles can be overcome.
Michael Schmidt
The author is Senior Manager and
Project Manager for Mobile Energy
Storage at Merck, the pharmaceutical
and chemical company in Darmstadt.
81
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Handreichung Biogasgewinnung und –nutzung, FNr (ed.), Gülzow, 2005, ISBN 978-3-00-014333-5
Biogasanlagen, U. Görisch, M. Helm (eds.), Eugen Ulmer Verlag, Stuttgart, 2007, 3-8001-5573-7
Biogas from Waste and renewable resources, D. Deublein, a. Steinhauser (eds.), Wiley-VCH, Weinheim, 2008,
ISBN-10: 3-527-31841-0
Nutzung nachwachsender rohstoffe in der industriellen Stoffproduktion; r. Busch, T. Hirth, a. Liese, S. Nordhoff, J. puls,
O. pulz, D. Sell, C. Syldatk, r. Ulber; Chem.-Ing.-Tech. 3 (2006) 219-228
The Utilization of renewable resources in Industrial production; r. Busch, T. Hirth, a. Liese, S. Nordhoff, J. puls, O. pulz,
D. Sell, C. Syldatk, r. Ulber; Biotechnology Journal 1 (2006) 770–776
Weiße Biotechnologie – Energielösungen für die Zukunft?; D. Sell, J. puls, r. Ulber; Chem. Unserer Zeit 41 (2007) 108–116
Use of renewable raw materials for the chemical industry – beyond sugar and starch; K. Muffler, r. Ulber, Chem. Eng. Tech
31 (638-646) 2008
Wie aus Bio Chemie wird, r. Busch, B. Kamm, M. Kamm, Th. Hirth, J. Thoen; Nachrichten aus der Chemie 53, 130 (2005)
a number of scientific series and also information can be ordered from Fachagentur Nachwachsende rohstoffe (FNr,
agency for renewable resources) (www.fnr.de)
HYDROGEN – AN ENERGY CARRIER OF THE FUTURE?
Der Wasserstoffboom, Wunsch und Wirklichkeit beim Wettlauf um den Klimaschutz, J. J. romm (ed.), Wiley-VCH,
Weinheim, 2006, ISBN 978-3-527-31570-3
Wasserstoff – Der neue Energieträger; Deutscher Wasserstoff- und Brennstoffzellen-Verband e.V. (ed.), Hydrogeit Verlag,
Kremmen, 2006, ISBN 978-3-937863-08-5 (for download of 3rd revised edition 2009: www.dwv-info.de)
Hydrogen as a future energy carrier, a. Züttel, a. Borgschulte, L. Schlapbach (eds.), Wiley-VCH, Weinheim, 2008,
ISBN-13 978-3527308170
Hydrogen Technology: Mobile and portable applications, aline Léon (ed.), Springer-Verlag GmbH, 18 July 2008,
ISBN-13: 978-3540790273
Gasspeicherung in nanoporösen Materialien, r. E. Morris and p. S. Wheatley, angewandte Chemie, 2008, 120, 5044-5059
Untersuchungen der Desorption von Wasserstoff in metall-organischen Gerüsten, B. panella et al, angewandte Chemie,
2008, 120, 2169-2173
83
rEFErENCES aND FUrTHEr rEaDING
NEW TECHNOLOGIES FOR IMPROVED ENERGY EFFICIENCY
application of Ionic Liquids in the Chemical Industry, N. V. plechkova, r. Seddon, Chem. Soc. rev., 2008, 37, 123-150
What are Batteries, Supercapacitors and Fuel Cells?, M. Winter, r. Brodd, Chem. rev. 2004, 104, 4245-4269
Nonaqueous Liquid Electrolytes for Lithium-Based rechargeable Batteries, Kang Xu, Chem. rev. 2004, 104, 4303-4417
rahmenkonzept “Forschung für die produktion von Morgen”; Bundesministerium für Bildung (ed.); die kommende Zeit.
www.bmbf.de/pub/ffdpvm.pdf
Katalyse eine Schlüsseltechnologie für nachhaltiges Wirtschaftswachstum – roadmap der deutschen Katalyseforschung,
DECHEMa (ed.), February 2006, www.connecat.de
84
85
LEGaL NOTICE
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F R O M F U E L C E L L S T O L I G H T- E M I T T I N G D I O D E S
Publisher:
Deutsche Bunsen-Gesellschaft
für Physikalische Chemie
(German Bunsen Society for Physical Chemistry)
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Germany
with the participation of Gesellschaft Deutscher Chemiker (German Chemical Society),
DECHEMA Gesellschaft für Chemische Technik und Biotechnologie
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