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Green Energy -An Introduction
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12
Green Energy - An Introduction
SAMEER SAADOON AL-JUBOORI*
ABSTRACT
Green energy is at the heart of all ecological strategies because it affects
companies in three vital areas: environmental, economic, and social.
Conventional energy sources based on oil, coal, and natural gas have proven
to be highly effective drivers of economic progress, but at the same time
damaging to the environment and to human health. The potential of
renewable energy sources is enormous as they can in principle meet many
times the world’s energy demand. Renewable energy sources such as biomass,
wind, solar, hydropower, and geothermal can provide sustainable energy
services, based on the use of routinely available, indigenous resources.
Renewable energy sources currently supply somewhere between 15 percent
and 20 percent of world’s total energy demand. The supply is dominated by
traditional biomass, mostly fuel wood used for cooking and heating,
especially in developing countries in Africa, Asia and Latin America. A
major contribution is also obtained from the use of large hydropower; with
nearly 20 percent of the global electricity supply being provided by this
source. New renewable energy sources (solar energy, wind energy, modern
bio-energy, geothermal energy, and small hydropower) are currently
contributing about two percent. A number of scenario studies have
investigated the potential contribution of renewables to global energy
supplies, indicating that in the second half of the 21st century their
contribution might range from the present figure of nearly 20 percent to
more than 50 percent with the right policies in place.
Key words: Green energy, Renewable energy, Sustainable, Conventional
energy, Energy scenarios
Head of Electronic and Contol Engineering Dept. and Head of Climate change group at
Kirkur Technical college, Kirkuk, Iraq.
*Corresponding author: E-mail: Samir19592003@ieee.org.
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317
1. INTRODUCTION
Renewable Energy RE is any form of energy from solar, geophysical or
biological sources that is replenished by natural processes at a rate that
equals or exceeds its rate of use. RE is obtained from the continuing or
repetitive flows of energy occurring in the natural environment and includes
resources such as biomass, solar energy, geothermal heat, hydropower,
tide and waves, ocean thermal energy and wind energy. However, it is
possible to utilize biomass at a greater rate than it can grow or to draw
heat from a geothermal field at a faster rate than heat flows can replenish
it. On the other hand, the rate of utilization of direct solar energy has no
bearing on the rate at which it reaches the Earth. Fossil fuels (coal, oil,
natural gas) do not fall under this definition, as they are not replenished
within a time frame that is short relative to their rate of utilization.
Renewable energy sources are often considered alternative sources
because, in general, most industrialized countries do not rely on them as
their main energy source. Instead, they tend to rely on non-renewable
sources such as fossil fuels or nuclear power. Because the energy crisis in
the United States during the 1970s, dwindling supplies of fossil fuels and
hazards associated with nuclear power, usage of renewable energy sources
such as solar energy, hydroelectric, wind, biomass, and geothermal has
grown.
Renewable energy comes from the sun (considered an “unlimited” supply)
or other sources that can theoretically be renewed at least as quickly as
they are consumed. If used at a sustainable rate, these sources will be
available for consumption for thousands of years or longer. Unfortunately,
some potentially renewable energy sources, such as biomass and
geothermal, are actually being depleted in some areas because the usage
rate exceeds the renewal rate. Fig. 1, shows paths of energy from source to
service[1-4].
2. WHY RENEWABLE ENERGY?
Today we primarily use fossil fuels to heat and power our homes and fuel
our cars. It’s convenient to use coal, oil, and natural gas for meeting our
energy needs, but we have a limited supply of these fuels on the Earth.
We’re using them much more rapidly than they are being created.
Eventually, they will run out. And because of safety concerns and waste
disposal problems, the United States will retire much of its nuclear capacity
by 2020. In the meantime, the nation’s energy needs are expected to grow
by 33 percent during the next 20 years. Renewable energy can help fill the
gap. Even if we had an unlimited supply of fossil fuels, using renewable
energy is better for the environment. We often call renewable energy
technologies clean or green because they produce few if any pollutants.
Energy Sci. & Tech. Vol. 1: Opportunities and Challenges
Fig. 1: Paths of energy from source to service[4]
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319
Burning fossil fuels, however, sends greenhouse gases into the atmosphere,
trapping the sun’s heat and contributing to global warming. Climate
scientists generally agree that the Earth’s average temperature has risen
in the past century. If this trend continues, sea levels will rise, and scientists
predict that floods, heat waves, droughts, and other extreme weather
conditions could occur more often. Other pollutants are released into the
air, soil, and water when fossil fuels are burned. These pollutants take a
dramatic toll on the environment and on humans. Air pollution contributes
to diseases like asthma. Acid rain from sulfur dioxide and nitrogen oxides
harms plants and fish. Nitrogen oxides also contribute to smog.
Renewable energy will also help us develop energy independence and
security. Replacing some of our petroleum with fuels made from plant
matter, for example, could save money and strengthen our energy
security[5,6].
3. SOLAR ENERGY
Solar energy is the ultimate energy source driving the earth. Though only
one billionth of the energy that leaves the sun actually reaches the earth’s
surface, this is more than enough to meet the world’s energy requirements.
In fact, all other sources of energy, renewable and non-renewable, are
actually stored forms of solar energy. The process of directly converting
solar energy to heat or electricity is considered a renewable energy source.
Solar energy represents an essentially unlimited supply of energy as the
sun will long outlast human civilization on earth. The difficulties lie in
harnessing the energy. Solar energy has been used for centuries to heat
homes and water, and modern technology (photovoltaic cells) has provided
a way to produce electricity from sunlight. There are two basic forms of
radiant solar energy use: passive and active.
Passive solar energy systems are static, and do not require the input of
energy in the form of moving parts or pumping fluids to utilize the sun’s
energy. Buildings can be designed to capture and collect the sun’s energy
directly. Materials are selected for their special characteristics: glass allows
the sun to enter the building to provide light and heat; water and stone
materials have high heat capacities. They can absorb large amounts of
solar energy during the day, which can then be used during the night. A
southern exposure greenhouse with glass windows and a concrete floor is
an example of a passive solar heating system.
Active solar energy systems require the input of some energy to drive
mechanical devices (e.g., solar panels), which collect the energy and pump
fluids used to store and distribute the energy. Solar panels are generally
mounted on a south or west-facing roof. A solar panel usually consists of a
glass-faced, sealed, insulated box with a black matte interior finish. Inside
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are coils full of a heat collecting liquid medium (usually water, sometimes
augmented by antifreeze). The sun heats the water in the coils, which is
pumped to coils in a heat transfer tank containing water. The water in the
tank is heated and then either stored or pumped through the building to
heat rooms or supply hot water to taps in the building.
Photovoltaic cells generate electricity from sunlight. Hundreds of cells
are linked together to provide the required flow of current. The electricity
can be used directly or stored in storage batteries. Because photovoltaic
cells have no moving parts, they are clean, quiet, and durable. Early
photovoltaic cells were extremely expensive, making the cost of solar electric
panels prohibitive. The recent development of inexpensive semiconductor
materials has helped greatly lower the cost to the point where solar electric
panels can compete much better cost wise with traditionally-produced
electricity.
Though solar energy itself is free, large costs can be associated with the
equipment. The building costs for a house heated by passive solar energy
may initially be more expensive. The glass, stone materials, and excellent
insulation necessary for the system to work properly tend to be more costly
than conventional building materials. A long-term comparison of utility
bills, though, generally reveals noticeable savings. The solar panels used
in active solar energy can be expensive to purchase, install and maintain.
Leaks can occur in the extensive network of pipes required, thereby causing
additional expense. The biggest drawback of any solar energy system is
that it requires a consistent supply of sunlight to work. Most parts of the
world have less than ideal conditions for a solar-only home because of their
latitude or climate. Therefore, it is usually necessary for solar houses to
have conventional backup systems (e.g. a gas furnace or hot-water heater).
This double-system requirement further adds to its cost[2],[4],[5]&[6].
3.1. Photovoltaic Systems
A photovoltaic system is composed of the PV module, as well as the balance
of system (BOS) components, which include an inverter, storage devices,
charge controller, system structure, and the energy network. The system
must be reliable, cost effective, attractive and match with the electric grid
in the future.
At the component level, BOS components for grid-connected applications
are not yet sufficiently developed to match the lifetime of PV modules.
Additionally, BOS component and installation costs need to be reduced.
Moreover, devices for storing large amounts of electricity (over 1 MWh
or 3,600 MJ) will be adapted to large PV systems in the new energy network.
As new module technologies emerge in the future, some of the ideas relating
to BOS may need to be revised. Furthermore, the quality of the system
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needs to be assured and adequately maintained according to defined
standards, guidelines and procedures. To ensure system quality, assessing
performance is important, including on-line analysis (e.g., early fault
detection) and off-line analysis of PV systems. The knowledge gathered
can help to validate software for predicting the energy yield of future module
and system technology designs.
To increasingly penetrate the energy network, PV systems must use
technology that is compatible with the electric grid and energy supply and
demand. System designs and operation technologies must also be developed
in response to demand patterns by developing technology to forecast the
power generation volume and to optimize the storage function.
Moreover, inverters must improve the quality of grid electricity by
controlling reactive power or filtering harmonics with communication in a
new energy network that uses a mixture of inexpensive and effective
communications systems and technologies, as well as smart meters.
Photovoltaic applications include PV power systems classified into two
major types: those not connected to the traditional power grid (i.e., off-grid
applications) and those that are connected (i.e., grid-connected applications).
In addition, there is a much smaller, but stable, market segment for
consumer applications.
Off-grid PV systems have a significant opportunity for economic
application in the un-electrified areas of developing countries. Of the total
capacity installed in these countries during 2009, only about 1.2% was
installed in off-grid systems that now make up 4.2% of the cumulative
installed PV capacity of the IEA PVPS countries.
Off-grid centralized PV mini-grid systems have become a reliable
alternative for village electrification over the last few years. In a PV minigrid system, energy allocation is possible. For a village located in an isolated
area and with houses not separated by too great a distance, the power may
flow in the mini-grid without considerable losses[2],[4],[5]&[6].
4. HYDROELECTRIC ENERGY
Hydroelectric power is generated by using the energy of flowing water to
power generating turbines for producing electricity. Most hydroelectric
power is generated by dams across large-flow rivers. A dam built across
river creates a reservoir behind it. The height of the water behind the dam
is greater than that below the dam, representing stored potential energy.
When water flows down through the penstock of the dam, driving the
turbines, some of this potential energy is converted into electricity.
Hydroelectric power, like other alternative sources, is clean and relatively
cheap over the long term even with initial construction costs and upkeep.
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But because the river’s normal flow rate is reduced by the dam, sediments
normally carried downstream by the water are instead deposited in the
reservoir. Eventually, the sediment can clog the penstocks and render the
dam useless for power generation.
Large-scale dams can have a significant impact on the regional
environment. When the river is initially dammed, farmlands are sometimes
flooded and entire populations of people and wildlife are displaced by the
rising waters behind the dam. In some cases, the reservoir can flood
hundreds or thousands of square kilometres. The decreased flow
downstream from the dam can also negatively impact human and wildlife
populations living downstream. In addition, the dam can act as a barrier
to fish that must travel upstream to spawn. Aquatic organisms are
frequently caught and killed in the penstock and the out-take pipes. Because
of the large surface area of the reservoir, the local climate can change due
to the large amount of evaporation occurring.
The total worldwide technical potential for hydropower generation is
14,576 TWh/yr (52.47 EJ/yr) with a corresponding installed capacity of
3,721 GW, roughly four times the current installed capacity. Worldwide
total installed hydropower capacity in 2009 was 926 GW, producing annual
generation of 3,551 TWh/y (12.8 EJ/y), and representing a global average
capacity factor of 44%. Of the total technical potential for hydropower,
undeveloped capacity ranges from about 47% in Europe and North America
to 92% in Africa, which indicates large opportunities for continued
hydropower development worldwide, with the largest growth potential in
Africa, Asia and Latin America. Additionally, possible renovation,
modernization and upgrading of old power stations are often less costly
than developing a new power plant, have relatively smaller environment
and social impacts, and require less time for implementation. Significant
potential also exists to rework existing infrastructure that currently lacks
generating units (e.g., existing barrages, weirs, dams, canal fall structures,
water supply schemes) by adding new hydropower facilities.
Only 25% of the existing 45,000 large dams are used for hydropower,
while the other 75% are used exclusively for other purposes (e.g., irrigation,
flood control, navigation and urban water supply schemes). Climate change
is expected to increase overall average precipitation and runoff, but regional
patterns will vary: the impacts on hydropower generation are likely to be
small on a global basis, but significant regional changes in river flow
volumes and timing may pose challenges for planning.
Hydropower can provide important services to electric power systems.
Storage hydropower plants can often be operated flexibly, and therefore
are valuable to electric power systems. Specifically, with its rapid response
load-following and balancing capabilities, peaking capacity and power
quality attributes, hydropower can play an important role in ensuring
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323
reliable electricity service. In an integrated system, reservoir and pumped
storage hydropower can be used to reduce the frequency of start-ups and
shutdowns of thermal plants; to maintain a balance between supply and
demand under changing demand or supply patterns and thereby reduce
the load following burden of thermal plants; and to increase the amount of
time that thermal units are operated at their maximum thermal efficiency,
thereby reducing carbon emissions. In addition, storage and pumped storage
hydropower can help reduce the challenges of integrating variable
renewable resources such as wind, solar photovoltaics, and wave power[2],[4].
4.1. Environmental and Social Impacts
Although hydroelectricity is generally considered a clean energy source, it
is not totally devoid of greenhouse gas emissions (GHG) and it can often
have significant adverse socio-economic impacts. There are arguments now
that large-scale dams actually do not reduce overall GHG emissions when
compared to fossil fuel power plant. To build a dam significant amounts of
land need to be flooded often in densely inhabited rural area, involving
large displacements of usually poor, indigenous peoples. Mitigating such
social impacts represents a significant cost to the project, which if it is
even taken into consideration, often not done in the past, can make the
project economically and socially unviable. Environmental concerns are
also quite significant, as past experience has shown. This includes reduction
in biodiversity and fish populations, sedimentation that can greatly reduce
dam efficiency and destroy the river habitat, poor water quality, and the
spread of water-related diseases. In fact, in the U.S. several large power
production dams are being decommissioned due to their negative
environmental impacts. Properly addressing these issues would result in
an enormous escalation of the overall costs for producing hydropower
making it far less competitive than is usually stated. As many countries
move toward an open electricity market this fact will come into play when
decisions regarding investments in new energy sources are being made. If
the large hydro industry is to survive it needs to come to grips with its poor
record of both cost estimation and project implementation[1].
5. WIND POWER
Wind is the result of the sun’s uneven heating of the atmosphere. Warm
air expands and rises, and cool air contracts and sinks. This movement of
the air is called wind. Wind has been used as an energy source for millennia.
It has been used to pump water, to power ships, and to mill grains.
Areas with constant and strong winds can be used by wind turbines to
generate electricity. Wind energy does not produce air pollution, can be
virtually limitless, and is relatively inexpensive to produce. There is an
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initial cost of manufacturing the wind turbine and the costs associated
with upkeep and repairs, but the wind itself is free.
The major drawbacks of wind-powered generators are they require lots
of open land and a fairly constant wind supply. Windmills are also noisy,
and some people consider them aesthetically unappealing and label them
as visual pollution. Migrating birds and insects can become entangled and
killed by the turning blades. However, the land used for windmill farms
can be simultaneously used for other purposes such as ranching, farming
and recreation[1],[2]&[4].
5.1. Technology and Applications
Modern, commercial grid-connected wind turbines have evolved from small,
simple machines to large, highly sophisticated devices. Scientific and
engineering expertise and advances, as well as improved computational
tools, design standards, manufacturing methods, and O&M procedures,
have all supported these technology developments. As a result, typical wind
turbine nameplate capacity ratings have increased dramatically since the
1980s (from roughly 75 kW to 1.5 MW and larger), while the cost of wind
energy has substantially declined. Onshore wind energy technology is
already being manufactured and deployed on a commercial basis.
Nonetheless, additional R&D advances are anticipated, and are expected
to further reduce the cost of wind energy while enhancing system and
component performance and reliability. Offshore wind energy technology
is still developing, with greater opportunities for additional advancement.
Specifically, modern large wind turbines typically employ rotors that
start extracting energy from the wind at speeds of roughly 3 to 4 m/s. A
wind turbine increases power production with wind speed until it reaches
its rated power level, often corresponding to a wind speed of 11 to 15 m/s.
At still-higher wind speeds, control systems limit power output to prevent
overloading the wind turbine, either through stall control, pitching the
blades, or a combination of both. Most turbines then stop producing energy
at wind speeds of approximately 20 to 25 m/s to limit loads on the rotor and
prevent damage to the turbine’s structural components.
Wind turbine design has centered on maximizing energy capture over
the range of wind speeds experienced by wind turbines, while seeking to
minimize the cost of wind energy. Increased generator capacity leads to
greater energy capture when the turbine is operating at rated power. Larger
rotor diameters for a given generator capacity, meanwhile, as well as
aerodynamic design improvements, yield greater energy capture at lower
wind speeds, reducing the wind speed at which rated power is achieved.
Variable speed operation allows energy extraction at peak efficiency over
a wider range of wind speeds. Finally, because the average wind speed at a
Green Energy - An Introduction
325
given location varies with the height above ground level, taller towers
typically lead to increased energy capture. To minimize cost, wind turbine
design is also motivated by a desire to reduce materials usage while
continuing to increase turbine size, increase component and system
reliability, and improve wind power plant operations. A system-level design
and analysis approach is necessary to optimize wind turbine technology,
power plant installation and O&M procedures for individual turbines and
entire wind power plants.
Moreover, optimizing turbine and power plant design for specific site
conditions has become common as wind turbines, wind power plants and
the wind energy market have all increased in size; site-specific conditions
that can impact turbine and plant design include geographic and temporal
variations in wind speed, site topography and access, interactions among
individual wind turbines due to wake effects, and integration into the larger
electricity system. Wind turbine and power plant design also impacts and
is impacted by noise, visual, environmental and public acceptance issues[1].
5.2. Regional and National Status and Trends
The countries with the highest total installed wind power capacity by the
end of 2009 were the USA (35 GW), China (26 GW), Germany (26 GW),
Spain (19 GW) and India (11 GW). After its initial start in the USA in the
1980s, wind energy growth centered on countries in the EU and India during
the 1990s and the early 2000s. In the late 2000s, however, the USA and
then China became the locations for the greatest annual capacity additions
as shown in Fig. 2[1][4].
Fig. 2: Top-10 countries in cumulative wind power capacity
6. BIOMASS ENERGY
Biomass is the term used for all organic material originating from plants
including algae, trees and crops and is essentially the collection and storage
of the sun’s energy through photosynthesis. Biomass energy, or bioenergy,
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is the conversion of biomass into useful forms of energy such as heat,
electricity and liquid fuels.
Biomass for bioenergy comes either directly from the land, as dedicated
energy crops, or from residues generated in the processing of crops for food
or other products such as pulp and paper from the wood industry. Another
important contribution is from post consumer residue streams such as
construction and demolition wood, pellets used in transportation, and the
clean fraction of municipal solid waste (MSW). The biomass to bioenergy
system can be considered as the management of flow of solar generated
materials, food, and fiber in our society. These interrelationships are shown
in Fig. 3, which presents the various resource types and applications,
showing the flow of their harvest and residues to bioenergy applications.
Not all biomass is directly used to produce energy but rather it can be
converted into intermediate energy carriers called biofuels. This includes
charcoal (higher energy density solid fuel), ethanol (liquid fuel), or producergas (from gasification of biomass)[1][6].
Fig. 3: Biomass and bioenergy flow chart[1]
The use of biomass as a fuel source has serious environmental effects.
When harvested trees are not replanted, soil erosion can occur. The loss of
photosynthetic activity results in increased amounts of carbon dioxide in
the atmosphere and can contribute to global warming. The burning of
biomass also produces carbon dioxide and deprives the soil of nutrients it
normally would have received from the decomposition of the organic matter.
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Burning releases particulate matter (such as ash) into the air which can
cause respiratory health problems.
In 2008, biomass provided about 10% (50.3 EJ/yr) of the global primary
energy supply. Major biomass uses fall into two broad categories:
• Low-efficiency traditional biomass such as wood, straws, dung and
other manures are used for cooking, lighting and space heating,
generally by the poorer populations in developing countries. This
biomass is mostly combusted, creating serious negative impacts on
health and living conditions. Increasingly, charcoal is becoming
secondary energy carrier in rural areas with opportunities to create
productive chains.
• High-efficiency modern bioenergy uses more convenient solids, liquids
and gases as secondary energy carriers to generate heat, electricity,
combined heat and power (CHP), and transport fuels for various
sectors. Liquid biofuels include ethanol and biodiesel for global road
transport and some industrial uses. Biomass derived gases, primarily
methane, from anaerobic digestion of agricultural residues and
municipal solid waste (MSW) treatment are used to generate electricity,
heat or both. The most important contribution to these energy services
is based on solids, such as chips, pellets, recovered wood previously
used and others. Heating includes space and hot water heating such
as in district heating systems. The estimated total primary biomass
supply for modern bioenergy is 11.3 EJ/yr and the secondary energy
delivered to end use consumers is roughly 6.6 EJ/yr.
Additionally, the industry sector, such as the pulp and paper, forestry,
and food industries, consumes approximately 7.7 EJ of biomass annually,
primarily as a source for industrial process steam.
Global bioenergy use has steadily grown worldwide in absolute terms
in the last 40 years, with large differences among countries. In 2006, China
led all countries and used 9 EJ of biomass for energy, followed by India (6
EJ), the USA (2.3 EJ) and Brazil (2 EJ). Bioenergy provides a relatively
small but growing share of Total Primary Energy Supply: TPES (1 to 4% in
2006) in the largest industrialized countries (grouped as the G8 countries:
the USA, Canada, Germany, France, Japan, Italy, the UK and Russia).
The use of solid biomass for electricity production is particularly important
in pulp and paper plants and in sugar mills. Bioenergy’s share in total
energy consumption is generally increasing in the G8 countries through
the use of modern biomass forms (e.g., co-combustion or co-firing for
electricity generation, space heating with pellets) especially in Germany,
Italy and the UK.
By contrast, in 2006, bioenergy provided 5 to 27% of TPES in the largest
developing countries (China, India, Mexico, Brazil and South Africa), mainly
through the use of traditional forms, and more than 80% of TPES in the
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poorest countries. The bioenergy share in India, China and Mexico is
decreasing, mostly as traditional biomass is substituted by kerosene and
liquefied petroleum gas within large cities. However, consumption in
absolute terms continues to grow. This trend is also true for most African
countries, where demand has been driven by a steady increase in wood
fuels, particularly in the use of charcoal in booming urban areas.
Three principal categories are more or less comprehensively considered
in assessments of biomass resource potentials:
• Primary residues from conventional food and fiber production in
agriculture and forestry, such as cereal straw and logging residues;
• Secondary and tertiary residues in the form of organic food/forest
industry by products and retail/post consumer waste; and
• Plants produced for energy supply, including conventional food/fodder/
industrial crops, surplus round wood forestry products, and new
agricultural, forestry or aquatic plants. Given that resource potential
assessments quantify the availability of residue flows in the food and
forest sectors, the definition of how these sectors develop is central
for the outcome[1].
6.1. Biomass Energy Conversion Technologies and Applications
There are a variety of technologies for generating modern energy carriers
electricity, gas, and liquid fuels from biomass, which can be used at the
household (~10 kW), community (~100 kW), or industrial (~MW) scale.
The different technologies tend to be classed in terms of either the
conversion process they use or the end product produced[1].
6.1.1. Combustion
Direct combustion remains the most common technique for deriving energy
from biomass for both heat and electricity production. In colder climates
domestic biomass fired heating systems are widespread and recent
developments have led to the application of improved heating systems which
are automated, have catalytic gas cleaning and make use of standardized
fuel (such as pellets). The efficiency benefit compared to open fireplaces is
considerable with advanced domestic heaters obtaining efficiencies of over
70 percent with greatly reduced atmospheric emissions. The application of
biomass fired district heating is common in the Scandinavian countries,
Austria, Germany and various Eastern European countries[1].
6.1.2. Gasification
Combustible gas can be produced from biomass through a high temperature
thermochemical process. The term gasification commonly refers to this high
temperature thermochemical conversion with the product gas called
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producer-gas, and involves burning biomass without sufficient air for full
combustion, but with enough air to convert the solid biomass into a gaseous
fuel. Producer-gas consists primarily of carbon monoxide, hydrogen, carbon
dioxide and nitrogen, and has a heating value of 4 to 6 MJ/Nm3, or 10–15
percent of the heating value of natural gas. The intended use of the gas
and the characteristics of the particular biomass (size, texture, moisture
content, etc.) determine the design and operating characteristics of the
gasifier and associated equipment. After appropriate treatment, the
resulting gases can be burned directly for cooking or heat supply, or can be
used in secondary conversion devices such as internal combustion engines
or gas turbines for producing electricity or shaft work. The systems used
can scale from small to medium (5–100 kW), suitable for the cooking or
lighting needs of a single family or community, up to large grid connected
power generation facilities consuming several hundred of kilograms of
woody biomass per hour and producing 10-100 MW of electricity[1].
6.1.3. Anaerobic digestion
Combustible gas can also be produced from biomass through the low
temperature biological processes called anaerobic (without air) digestion.
Biogas is the common name for the gas produced either in specifically
designed anaerobic digesters or in landfills by capturing the naturally
produced methane. Biogas is typically about 60 percent methane and 40
percent carbon dioxide with a heating value of about 55 percent that of
natural gas. Almost any biomass except lignin (a major component of wood)
can be converted to biogas animal and human wastes, sewage sludge, crop
residues, carbon laden industrial processing byproducts, and landfill
material have all been widely used.
Anaerobic digesters generally consist of an inlet, where the organic
residues and other wastes are fed into the digester tank; a tank, in which
the biomass is typically heated to increase its decomposition rate and
partially convert by bacteria into biogas; and an outlet where the biomass
of the bacteria that carried out the process and non-digested material
remains as sludge and can be removed. The biogas produced can be burned
to provide energy for cooking and space heating or to generate electricity.
Digestion has a low overall electrical efficiency (roughly 10–15 percent,
strongly dependent on the feedstock) and is particularly suited for wet
biomass materials. Direct non-energy benefits are especially significant in
this process. The effluent sludge from the digester is a concentrated nitrogen
fertilizer and the pathogens in the waste are reduced or eliminated by the
warm temperatures in the digester tank[1].
6.1.4. Liquid biofuels
Biofuels are produced in processes that convert biomass into more useful
intermediate forms of energy. There is particular interest in converting
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solid biomass into liquids, which have the potential to replace petroleumbased fuels used in the transportation sector. However, adapting liquid
biofuels to our present day fuel infrastructure and engine technology has
proven to be nontrivial.
Only oil producing plants, such as soybeans, palm oil trees and oilseeds
like rapeseed can produce compounds similar to hydrocarbon petroleum
products, and have been used to replace small amounts of diesel. This
“biodiesel” has been marketed in Europe and to a lesser extent in the U.S.,
but it requires substantial subsidies to compete with diesel.
Other alternative biofuels to petroleum-based fuels are alcohols produced
from biomass, which can replace gasoline or kerosene. The most widely
produced today is ethanol from the fermentation of biomass. In
industrialized countries ethanol is most commonly produced from food crops
like corn, while in the developing world it is produced from sugarcane. Its
most prevalent use is as a gasoline fuel additive to boost octane levels or to
reduce dependence on imported fossil fuels. In the U.S. and Europe the
ethanol production is still far from competitive when compared to gasoline
and diesel prices, and the overall energy balance of such systems has not
been very favorable. The Brazilian Proalcool ethanol program, initiated in
1975, has been successful due to the high productivity of sugarcane,
although subsidies are still required. Two other potential transportation
biofuels are methanol and hydrogen. They are both produced via biomass
gasification and may be used in future fuel cells.
While ethanol production from maize and sugarcane, both agricultural
crops, has become widespread and occasionally successful it can suffer from
commodity price fluctuation relative to the fuels market.
Consequently, the production of ethanol from lignocellulosic biomass
(such as wood, straw and grasses) is being given serious attention. In
particular, it is thought that enzymatic hydrolysis of lignocellulosic biomass
will open the way to low cost and efficient production of ethanol. While the
development of various hydrolysis techniques has gained attention in recent
years, particularly in Sweden and the United States, cheap and efficient
hydrolysis processes are still under development and some fundamental
issues need to be resolved. Once such technical barriers are surmounted
and ethanol production can be combined with efficient electricity production
from unconverted wood fractions (like the lignin), ethanol costs could come
close to current gasoline prices and overall system efficiencies could go up
to about 70 percent (low heating value). Though the technology to make
this an economically viable option still does not exist, promising technologies
are in the works and there are currently a number of pilot and
demonstration projects starting up[1].
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7. GEOTHERMAL ENERGY
Geothermal energy uses heat from the earth’s internal geologic processes
in order to produce electricity or provide heating. One source of geothermal
energy is steam. Groundwater percolates down though cracks in the
subsurface rocks until it reaches rocks heated by underlying magma, and
the heat converts the water to steam. Sometimes this steam makes its way
back to the surface in the form of a geyser or hot spring. Wells can be dug
to tap the steam reservoir and bring it to the surface, to drive generating
turbines and produce electricity. Hot water can be circulated to heat
buildings. Regions near tectonic plate boundaries have the best potential
for geothermal activity.
The western portion of the United States is most conducive for
geothermal energy sources, and over half of the electricity used by the city
of San Francisco comes from the Geysers, a natural geothermal field in
Northern California. California produces about 50 percent of the world’s
electricity that comes from geothermal sources.
Entire cities in Iceland, which is located in a volcanically active region
near a mid ocean ridge, are heated by geothermal energy. The Rift Valley
region of East Africa also has geothermal power plants. Geothermal energy
may not always be renewable in a particular region if the steam is
withdrawn at a rate faster than it can be replenished, or if the heating
source cools off. The energy produced by the Geysers region of California is
already in decline because the heavy use is causing the underground heat
source to cool.
Geothermal energy recovery can be less environmentally invasive than
engaging in recovery methods for non-renewable energy sources. Although
it is relatively environmentally friendly, it is not practical for all situations.
Only limited geographic regions are capable of producing geothermal energy
that is economically viable. Therefore, it will probably never become a major
source of energy.
Global geothermal technical potential is comparable to global primary
energy supply in 2008. For electricity generation, the technical potential
of geothermal energy is estimated to be between 118 EJ/yr (to 3 km depth)
and 1,109 EJ/yr (to 10 km depth). For direct thermal uses, the technical
potential is estimated to range from 10 to 312 EJ/yr.
The heat extracted to achieve these technical potentials can be fully or
partially replenished over the long term by the continental terrestrial heat
flow of 315 EJ/yr at an average flux of 65 mW/m2. Thus, technical potential
is not likely to be a barrier to geothermal deployment (electricity and direct
uses) on a global basis. Whether or not the geothermal technical potential
will be a limiting factor on a regional basis depends on the availability of
EGS technology.
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There are different geothermal technologies with distinct levels of
maturity. Geothermal energy is currently extracted using wells or other
means that produce hot fluids from:
(a) Hydrothermal reservoirs with naturally high permeability; and
(b) EGS-type reservoirs with artificial fluid pathways. The technology
for electricity generation from hydrothermal reservoirs is mature and
reliable, and has been operating for more than 100 years. Technologies
for direct heating using geothermal heat pumps (GHP) for district
heating and for other applications are also mature.
Technologies for EGS are in the demonstration stage. Direct use provides
heating and cooling for buildings including district heating, fish ponds,
greenhouses, bathing, wellness and swimming pools, water purification/
desalination and industrial, and process heat for agricultural products and
mineral drying.
Geothermal resources have been commercially used for more than a
century. Geothermal energy is currently used for base load electric
generation in 24 countries, with an estimated 67.2 TWh/yr (0.24 EJ/yr) of
supply provided in 2008 at a global average capacity factor of 74.5%; newer
geothermal installations often achieve capacity factors above 90%.
Geothermal energy serves more than 10% of the electricity demand in 6
countries and is used directly for heating and cooling in 78 countries,
generating 121.7 TWh/yr (0.44 EJ/yr) of thermal energy in 2008, with GHP
applications having the widest market penetration. Another source
estimates global geothermal energy supply at 0.41 EJ/yr in 2008.
Environmental and social impacts from geothermal use are site and
technology specific and largely manageable.
Overall, geothermal technologies are environmentally advantageous
because there is no combustion process emitting carbon dioxide (CO2), with
the only direct emissions coming from the underground fluids in the
reservoir.
Historically, direct CO2 emissions have been high in some instances
with the full range spanning from close to 0 to 740 g CO2/kWhe depending
on technology design and composition of the geothermal fluid in the
underground reservoir.
Direct CO2 emissions for direct use applications are negligible and EGS
power plants are likely to be designed with zero direct emissions. Life cycle
assessment (LCA) studies estimate that full lifecycle CO 2 equivalent
emissions for geothermal energy technologies are less than 50 g CO2eq/
kWhe for flash steam geothermal power plants, less than 80 g CO2eq/kWhe
for projected EGS power plants, and between 14 and 202 g CO2eq/kWhth
for district heating systems and GHP. Local hazards arising from natural
Green Energy - An Introduction
333
phenomena, such as micro-earthquakes, may be influenced by the operation
of geothermal fields. Induced seismic events have not been large enough to
lead to human injury or relevant property damage, but proper management
of this issue will be an important step to facilitating significant expansion
of future EGS projects.
Several prospects exist for technology improvement and innovation in
geothermal systems. Technical advancements can reduce the cost of
producing geothermal energy and lead to higher energy recovery, longer
field and plant lifetimes, and better reliability. In exploration, research
and development (R&D) is required for hidden geothermal systems (i.e.,
with no surface manifestations such as hot springs and fumaroles) and for
EGS prospects. Special research in drilling and well construction technology
is needed to reduce the cost and increase the useful life of geothermal
production facilities. EGS require innovative methods to attain sustained,
commercial production rates while reducing the risk of seismic hazard.
Integration of new power plants into existing power systems does not
present a major challenge, but in some cases can require extending the
transmission network.
Geothermal-electric projects have relatively high upfront investment
costs but often have relatively low Levelized costs of electricity (LCOE).
Investment costs typically vary between USD2005 1,800 and 5,200 per
kW, but geothermal plants have low recurring ‘fuel costs’. The LCOE of
power plants using hydrothermal resources are often competitive in today’s
electricity markets, with a typical range from US cents2005 4.9 to 9.2 per
kWh considering only the range in investment costs provided above and
medium values for other input parameters; the range in LCOE across a
broader array of input parameters is US cents2005 3.1 to 17 per kWh.
These costs are expected to decrease by about 7% by 2020. There are no
actual LCOE data for EGS power plants, as EGS plants remain in the
demonstration phase, but estimates of EGS costs are higher than those for
hydrothermal reservoirs. The cost of geothermal energy from EGS plants
is also expected to decrease by 2020 and beyond, assuming improvements
in drilling technologies and success in developing well-stimulation
technology. Fig. 4 shows schematic diagram of a geothermal binary-cycle
power plant.
Current levelized costs of heat (LCOH) from direct uses of geothermal
heat are generally competitive with market energy prices. Investment costs
range from USD2005 50 per kWth (for uncovered pond heating) to USD2005
3,940 per kWth (for building heating). Low LCOHs for these technologies
are possible because the inherent losses in heat-to electricity conversion
are avoided when geothermal energy is used for thermal applications.
Future geothermal deployment could meet more than 3% of global
electricity demand and about 5% of the global demand for heat by 2050.
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Energy Sci. & Tech. Vol. 1: Opportunities and Challenges
Fig. 4: Schematic diagram of a geothermal binary-cycle power plant[1]
Evidence suggests that geothermal supply could meet the upper range of
projections derived from a review of about 120 energy and GHG reduction
scenarios. With its natural thermal storage capacity, geothermal energy is
especially suitable for supplying base-load power. By 2015, geothermal
deployment is roughly estimated to generate 122 TWhe/yr (0.44 EJ/yr) for
electricity and 224 TWhth/yr (0.8 EJ/yr) for heat applications. In the long
term (by 2050), deployment projections based on extrapolations of longterm historical growth trends suggest that geothermal could produce 1,180
TWhe/yr (~4.3 EJ/yr) for electricity and 2,100 TWhth/yr (7.6 EJ/yr) for
heat, with a few countries obtaining most of their primary energy needs
(heating, cooling and electricity) from geothermal energy. Scenario analysis
suggests that carbon policy is likely to be one of the main driving factors
for future geothermal development, and under the most favorable climate
policy scenario (<440 ppm atmospheric CO2 concentration level in 2100),
geothermal deployment could be even higher in the near and long term.
In 2009, the world’s top geothermal producer was the USA with almost
29% of the global installed capacity (3,094 MWe) as shown in Fig. 5. The
US geothermal industry is currently expanding due to state Renewable
Portfolio Standards (RPS) and various federal subsidies and tax incentives.
US geothermal activity is concentrated in a few western states, and only a
fraction of the geothermal technical potential has been developed so far[1][7]
and[8].
8. WAVE POWER
Very large energy fluxes can occur in deep water sea waves. The power in
the wave is proportional to the square of the amplitude and to the period of
Fig. 5: The world’s top geothermal producers
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the motion. Therefore the long period ~10 s, large amplitude ~2 m waves
have considerable interest for power generation, with energy fluxes
commonly averaging between 50 and 70 kWm–1 width of oncoming wave.
The possibility of generating electrical power from these deep water
waves has been recognized for many years, and there are countless ideas
for machines to extract the power. For example, a wave power system was
used in California in 1909 for harbour lighting. Modern interest has revived,
particularly in Japan, the UK, Scandinavia and India, so research and
development has progressed to commercial construction for meaningful
power extraction. Very small scale autonomous systems are used for marine
warning lights on buoys and much larger devices for grid power generation.
The provision of power for marine desalination is an obvious attraction. As
with all renewable energy supplies, the scale of operation has to be
determined, and present trends support moderate power generation at
about 100 kW–1MW from modular devices each capturing energy from
about 5 to 25 m of wave front. Initial designs are for operation at shoreline or near to shore to give access and to lessen, hopefully, storm damage.
It is important to appreciate the many difficulties facing wave power
developments.
1. Wave patterns are irregular in amplitude, phase and direction. It is
difficult to design devices to extract power efficiently over the wide
range of variables.
2. There is always some probability of extreme gales or hurricanes
producing waves of freak intensity. The structure of the power devices
must be able to withstand this. Commonly the 50 year peak wave is
10 times the height of the average wave. Thus the structures have to
withstand ~100 times the power intensity to which they are normally
matched. Allowing for this is expensive and will probably reduce
normal efficiency of power extraction.
3. Peak power is generally available in deep water waves from opensea swells produced from long fetches of prevailing wind, e.g., beyond
the Western Islands of Scotland (in one of the most tempestuous areas
of the North Atlantic) and in regions of the Pacific Ocean. The
difficulties of constructing power devices for these types of wave
regimes, of maintaining and fixing or mooring them in position, and
of transmitting power to land, are fearsome. Therefore more protected
and accessible areas near to shore are most commonly used.
4. Wave periods are commonly ~5–10 s (frequency ~0.1 Hz). It is
extremely difficult to couple this irregular slow motion to electrical
generators requiring ~500 times greater frequency.
5. So many types of device may be suggested for wave power extraction
that the task of selecting a particular method is made complicated
and somewhat arbitrary.
Green Energy - An Introduction
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6. The large power requirement of industrial areas makes it tempting
to seek for equivalent wave energy supplies. Consequently plans may
be scaled up so only large schemes are contemplated in the most
demanding wave regimes. Smaller sites of far less power potential,
but more reasonable economics and security, may be ignored.
7. The development and application of wave power has occurred with
spasmodic and changing government interest, largely without the
benefit of market incentives. Wave power needs the same learning
curve of steadily enlarging application from small beginnings that
has occurred with wind power[6].
9. Recent progress in Renewable Energy System Cost and Performance
as previously described there has been significant progress in cost
reduction made by wind and PV systems, while biomass, geothermal,
and solar thermal technologies are also experiencing cost reductions,
and these are forecast to continue. Fig. 6 presents forecasts made by
the U.S. DOE for the capital costs of these technologies, from 1997 to
2030.
Fig. 6: Capital cost forecasts for renewable energy technologies (Source: U.S. DOE,
1997)
Of course, capital costs are only one component of the total cost of
generating electricity, which also includes fuel costs, and operation and
maintenance costs. In general, renewable energy systems are characterized
by low or no fuel costs, although operation and maintenance (O&M) costs
can be considerable. It is important to note, however, that O&M costs for
all new technologies are generally high, and can fall rapidly with increasing
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Energy Sci. & Tech. Vol. 1: Opportunities and Challenges
familiarity and operational experience. Renewable energy systems such
as photovoltaics contain far fewer mechanically active parts than
comparable fossil fuel combustion systems, and therefore are likely in the
long term to be less costly to maintain. Fig. 7 presents U.S. DOE projections
for the levelized costs of electricity production from these same renewable
energy technologies, from 1997 to 2030[1].
Fig. 7: Levelized cost of electricity forecast for renewable energy technologies
(Source: U.S. DOE, 1997)
10. SOME EXAMPLES OF THE SUCCESS OF RENEWABLES
• Spain generated more than half its electricity demand on 9 November
2009 with wind energy.
• Spain’s wind energy overtook coal as its third largest producer of
power in 2009.
• During 2010, China built roughly one windmill every hour.
• The wind industry installed just over 41,000 MW of new clean, reliable
wind power in 2011, bringing the total installed capacity globally to
more than 238,000 MW at the end of last year. This represents an
increase of 21%, with an increase in the size of the annual global
market of just over 6%.
• Today, about 75 countries worldwide have commercial wind power
installations, with 22 of them already passing the 1 gigawatt (GW)
level.
Green Energy - An Introduction
339
• More than half of all new wind power was added outside the
traditional markets of Europe and North American in 2010, for the
first time.
• New Zealand generates 10% of its electricity needs from geothermal
power.
• Portugal’s renewables went from 15% to 45% in its electricity grid in
just five years.
Nuclear energy and the conflict with renewables
The nuclear industry often claims that nuclear energy is needed to combat
climate change. This is wrong. Research by Greenpeace and others shows
that continuing to operate nuclear plants prevents the large-scale
integration of renewable energy into the electricity grid. Nuclear also
channels investment away from renewables where investment can make a
difference in fighting climate change.
The argument that nuclear power could help fight climate change is
seriously flawed. If the entire global fleet of reactors was quadrupled, a
completely far-fetched scenario, this would lead to, at most, a 6% reduction
in global CO2 emissions, and only after 2020, well beyond the deadline that
climate scientists have set for avoiding catastrophic climate change.
A key problem with nuclear power is that it must run around the clock
with a constant output capacity, which is called ‘baseload’. The nuclear
industry presents this as an advantage, which it is not. First, a permanent
power generation mode–independent from the actual need in the power
grid is needed to generate as much electricity as possible to make generation
costs low. If the operational hours were reduced to half, the cost would
double. So the ‘base load’ strategy is more an economic than a technical
concept.
Second, unlike modern gas turbines, which can react within seconds to
fluctuating demand in the electricity grid, nuclear power stations are unable
to react to the demand curve, and demand must follow the operation mode
of nuclear power plants. This leads to the inefficient use of electricity. In
almost all countries with a winter heating demand, a large share of nuclear
in their power mix goes hand in hand with the expansion of highly inefficient
electrical heating systems. For example, France, with about 80% nuclear
in its power mix, had an overall power demand of 101 GW on a cold day in
February 2012, while Germany, which has 15 million more people than
France, with 20% nuclear in its power mix had a demand of just over 50
GW on the same cold day.
Germany has far better insulated houses and a significantly lower share
of electrical heating systems. The inflexibility of nuclear reactors has a
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Energy Sci. & Tech. Vol. 1: Opportunities and Challenges
negative effect on renewables. For technical and safety reasons, nuclear
plants cannot easily be turned down so wind operators are often told to
shut off their generators to give priority to electricity from nuclear plants,
an economic and ecological mistake. As a result, nuclear energy blocks the
development of renewable energy technologies by commandeering space
on the electricity grid and reducing income for wind operators.
Renewable power plants can be built much more quickly than nuclear
and are safe. In addition, renewables can replace several times more of the
carbon that is leading to climate change for the same cost as nuclear and
at a far faster pace.
At present, over 90% of the Japan’s reactors are offline. The rest may
be offline by May 2012. Given that only three of 54 reactors are operating
and there have been no significant problems with the electricity supply,
Japan has shown that it can survive without nuclear power[5],[9]&[10].
REFERENCES
[1] Antonia V. Herzog, Timothy E. Lipman and Daniel M. Kammen, Renewable Energy
Sources: http://www.eolss.com.
[2] Energy Sources, (2010). Paper published at: PROCESOS DE MERCADO. Volumen
VII, Número 1, Primavera ISSN: 1697-6797-13.
[3] Gabriel Calzada Álvarez, Study of the effects on employment of public aid to
renewable energy sources.
[4] Renewable Energy Sources and Climate Change Mitigation, Cambidge University
Press, 1st published, 2012.
[5] Farhad Islam, Institute for Advanced Analytics, NCSU, Raleigh, NC, SAS Global
Forum 2010, available at: http://support.sas.com/resources/papers/proceedings10/
208-2010.pdf.
[6] John twidell and Tony Weir, Renewable Energy Resources, available at: www.e
Bookstore.tandf.co.uk.
[7] IRENA, International renewable Energy Agency, IRENA Handbook on Renewable
Energy Nationally Appropriate Mitigation Actions (NAMAs) for Policy Makers and
Project Developers, 2012.
[8] William Moomaw,. Renewable Energy and Climate Change, Cambridge University
Press, 2011.
[9] REN21 Committee, Renewable Energy Policy Network for the 21st Century,
Renewables 2012 GLOBAL STATUS REPORT, available at www.ren21.net.
[10] US Department of Energy, Guide to Purchasing Green Power, Renewable
Electricity, Renewable Energy Certificates, and On-Site Renewable Generation,
available at: www1.eere.energy.gov/femp/technologies/renewable purchasing power
.html.
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