PROMOTION OF PHOTOBIOREACTORS VIA THIRD AND FOURTH
GENERATION ENGINEERED ALGAE AS A BIOFUEL SOURCE
William Smith (WDS10@pitt.edu), Nasir Sharaf (NAS155@pitt.edu)
Abstract— At the dawn of the new millennium it became
quickly apparent that we globally could not meet the energy
demands of the ever increasing population at the current
rate of consumption. Clearly alternative fuel sources must be
examined in order to provide sustainable energy for future
generations.
One possible source of fuel rests in oil-producing algae.
Certain algae produce oil naturally and efficiently and thus
are an obvious potential source for meeting humanity’s fuel
needs. One issue is providing an environment for the growth
and cultivation of biofuel producing algae. A solution is
found through bioreactors.
This paper will describe and evaluate appropriate
bioreactor
technologies,
specifically
those
in
photobioreactors (PBRs). The engineering and technologies
of systems especially appropriate for biofuel-producing
algae will be described and explained. The value of
technologies and systems in use will be assessed, and the
direction and value of current research and development of
PBRs with particular applications to biofuel-producing
Algae will be discussed.
Key Words- algae, biofuel, bioreactors, fourth-generation,
oilgae, Photobioreactors
WHAT ARE PHOTOBIOREACTORS AND WHY ARE
THEY IMPORTANT?
As the year 2000 ushered in, the world and the U.S. faced a
new range of unique, energy-related problems that they are
still dealing with. These include the rising price of oil, the
energy crisis, and the possibility of global warming. As a
result of the environmental and economic implications of
these growing issues, alternative fuel sources have been
suggested as an attempt to mitigate the detrimental effects of
the U.S. dependence on petroleum based fuel. Algae for oil
production have been suggested [1] to best handle these ever
present and rising problems and bioreactors have been
suggested to best grow algae. But what is a bioreactor?
A bioreactor is defined as a system that supports and
maximizes biological processes. By extension, a
photobioreactor also supports and maximizes biological
processes by incorporating light. Admittedly, this is a rather
broad definition, but still stands as legitimate because such a
definition accounts for varied natural phenomenon such as
calf stomachs, ponds, airlift reactors, membrane reactors,
and termite guts [2] to be rightly included as bioreactors. In
fact, as this paper will detail further, a pond (known as an
open system) is one type of bioreactor that could potentially
mass-produce algae [3]. The first half of this paper will
continue to clarify and define what photobioreactors are, and
the second part will aim to answer the question “why must
photobioreactors be accessed to grow algae?”
BIOREACTOR DESIGN
In order to fully develop a definition for bioreactors and
understand its intricacies, the hardware of a photobioreactor
must inevitably be discussed. However, by how we
previously defined bioreactors (and by extension
photobioreactors) we cannot make any conclusive, definitive
statements about bioreactors. We must limit our scope then
to discussing bioreactors only used by industries. This will
mean open systems, such as ponds, will still be included for
discussion but natural phenomenon such as calf stomachs
can be disregarded.
This still leaves discontinuities as to what physically
constitutes as a PBR since each reactor is different. For
example, a vertical tubular biocoil design has to be fitted
multiple external components including a heat exchanger,
airlift, and back flow system while concentric loop airlift
PBRs and Carboy PBRs had to be fitted with only a
magnetic stirrer [4]. Although the physical structures vary
with each type of PBR, all PBRs must accommodate for
certain factors which in turn influences the design of the
reactor. Instead of addressing the technical differences
between every type of design, it would be far wiser to
address the design theory that goes behind every PBR and
how this theory is applied to various designs. After
discussing the intimacies of design theory, this paper will
examine “case studies” of flat panel, annual, and tubular
designs [5].
General Design and Overview
Photobioreactors are designed to maximize biological
processes which in this paper are stringent on the proper
growth and maturation of certain algal species. This means
factors such as light, nutrient supplies, CO2, O2, pH, and
temperature, among many other concerns, must be balanced
perfectly to maximize algal growth and development [1].
With regards to light, an optimal reactor enhances light
intensity, light penetration, the wavelength of light, and
frequency of cell exposure to light. Mixing helps distribute
radiation evenly to all the cells, and also allows the cells to
evenly get the various nutrients they need. The algae also
needs CO2 at faster rate than the simple diffusion from the
Nasir Sharaf
William Smith
air to the medium and must similarly dispose of the excess
O2 which is a byproduct of photosynthesis [6]. This is also
accounted for through mixing. There are many more factors
that can effect algae growth, but addressing everyone merits
a paper unto itself. For simplicities sake, we will keep these
discussed factors in the back of our minds and continue on to
more the more pressing concerns of bioreactor design
As alluded to previously, there are two species of
systems, open and closed. Open systems include ponds,
ponds fitted with rotating mechanisms, and raceway ponds
(ponds designed to continue the flow of the algae) [1]. Open
systems, like closed systems, must be attached to a
harvesting system. Open systems present several technical
issues such as preventing contamination and maintaining a
mono-cultured environment which is just another added
challenge on top of maximizing production, sometimes in
the face of predation. Evaporation also has been known to
offset production estimates [1]. In essence, the primary
fallacy with open systems can be summed up as lack of
control over the process.
Closed systems are generally limited to (at least in the
scope of this paper) devices containing the algae and the
materials necessary to produce biofuels. Closed systems
offer the advantages of control over the entire process, but
are often stated to be more financially costly. Nevertheless,
the potential for lowered costs in the future have generated
interest in closed systems as the major choice for biofuel
production [1]. Closed systems can be further divided into
closed outdoor systems and closed indoor systems. Closed
outdoor systems offer the advantages of maintaining a
monoculture, increased biomass concentration for easier
harvesting, and control over certain features like pH and
CO2 levels [6]. Closed indoor systems offer greater control
over environmental stress that can negatively influence even
closed outdoor systems. For example, closed outdoor
systems are still reliant on “seasonal, latitudinal, and diurnal
variations in light conditions” which closed outdoor systems
accommodate with “florescent lights, optical fibers or light
emitting diodes or plates” [6]. Clearly, closed systems afford
far more options with regard to design and must be studied
further. The three most common types of PBRs must be
addressed in order to address the variation on closed system
designs (all of which are closed systems).
reactor types. They hold the disadvantages of low
photosynthetic efficiency and are more easily damaged from
aeration [1], which actually costs more to do [7].
Tubular (horizontal) types are long pipes that are
generally mounted in parallel loops on scaffolds with pumps
to provide circulating flow throughout the system (Figure
1C). Tubular designed systems are noted to have the highest
biomass concentrations (up to 6 g/L) [7] and the highest
volumetric biomass density [1]. This design can arguably
represent the central problem with all PBR designs – though
so effective that the tubular type is the choice for the world’s
largest PBR, it is extremely expensive and requires a lot of
auxiliary energy [7]. Horizontal types are also noted to have
“very limited possibilities for commercial scale
applications” [1].
Annual designs, or column or vertical designs, are the
final designs to be discussed. Generally, they consist of one
large hollow cylinder with an additional internal surface to
discourage dark zones (Figure 1B) [7]. This structure gives
them the best light/ dark cycles, and thus a high photoefficiency, while taking up the least space. While, on a small
scale, or in a laboratory, these PBR designs may be best
suited for algal growth, industrially they face the most
challenges for scalability and have high support costs [7].
Figure 1 A shows a flat panel PBR, B shows an annual PBR,
and C shows a tubular [7]
Conclusively, we can say little to nothing about which
PBR is best to use. The reality is that it depends on the
situation and what purpose PBRs are to be implemented for.
Annual designs are indicated to be best for laboratory
research and small scale production [7], but industrial, largescale endeavors may benefit more from flat plane [8]. In the
end, however, regardless of design each PBR does the same
exact thing. This paper has covered the hardware and theory
into the construction of PBRs, but little of how PBRs
function. Indeed, if innovation is to be expected, a thorough
understanding of the operation of a PBR is necessary.
Three Specific Types of Bioreactor Design
The most commonly seen PBRs are the Flat Plate, Annual
(also referred to as column or vertical) , and the tubular (or
horizontal) models [1] [7]. A flat plane is the most common
setup for its simplicity. It is described as “a plastic bag
shaped by a wire netting” into a green wall panel which are
often scaled up in the horizontal direction or setup in parallel
with other fences (see Figure 1) [7]. Flat plane reactors hold
the advantages of a shorter oxygen path, which means less
dissolved oxygen buildup. They are also known for their low
power consumption especially when compared to tubular
BIOREACTOR PROCESSING AND OPERATION
In addition to understanding the components and so-called
“nuts and bolts” of our biofuel producing equipment we
must also understand how the equipment works. This is
important because knowing the reasoning behind whatever is
being produced will allow for innovation to streamline that
process to work towards optimal levels. As we will see,
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William Smith
proper process management can “further improve the
performance of the system and meet energy demands” [7].
Recall how certain principles must be accommodated for
in order to construct a properly functioning PBR. We
discussed how design accounts for light, particularly with
the annual design. But what of the other factors, factors such
as temperature and pH? How does one receive
measurements concerning biomass and how does one feed
and provide the nutrients that the algae require for optimized
growth? Though we lightly touched on how mixing provides
some of the nutrients need by the algae and how mixing
provides CO2, we would do well to further explain the
process.
Understanding the processes behind PBRs serves a
purpose beyond academic understanding. As also noted
processes can be streamlined to decrease energy and fiscal
costs [7] and thus merits scrutiny. By doing so one will
inadvertently have to analyze and assess the processes in
order to fit the whole process into an ecological, economical
and social scenario [7]. Thus, the following sections will
showcase the internal processes and provide a hint towards
the economic feasibility of PBRs.
along the strongest mass transfer gradient are recommended
[7].
Process Integration
Recall from the General Design and Overview section how
photobioreactors must account for variables such as
temperature, oxygen removal, etc. Several of these factors
are useful in certain situations but a hindrance in others. One
can streamline the process by carefully organizing the
process to become an integrated process that could
potentially have cost saving benefits [7]. This adds another
feasible, pragmatic financial gain to bioreactors.
As alluded to previously, atmospheric CO 2 is not enough
for proper algal cultivation. One suggested source of CO2 is
combustion plant emissions and their steady supply of CO 2
waste [7]. Though certain restriction apply (natural gas
byproducts are perfectly acceptable, but coal-powered
emissions have detrimental amounts of sulfur and nitrogen
oxide), the synthesis of combustion power waste streams and
PBRs have the potential to decrease costs and promote so
called “green technologies”.
Another possible energy optimizing process is membrane
gassing. In this method, a “gas-permeable membrane creates
the surface for CO2 dissolving and O2 removal directly at the
gas intake” to allow for more degrees of freedom in process
operation [7]. This lowers the loss of CO2 via leakages and
results in a homogenous dispersion of CO2, which minimizes
mixing. This process thus saves energy.
Process Management
As there are several factors that influence PBR performance
and algal growth, it is better to focus on a few factors whose
presence is strongly felt. Temperature is one such factor.
Though temperature can be affected by the PBR itself and
certain algae strains can compensate for temperature changes
better than others, temperature and its effects are largely
reliant on the global position of the PBR. For example, in
Southern regions of the U.S. and Australia excess heat is a
large issue, whereas in northern climates like Central Europe
and the northern regions of the U.S. one must find
innovative ways to warm up the PBRs [7]. One possible
solution is phase changing materials, which are
commercially available in “wallpapers for flats” that control
the room temperature at the value of phase transition. This
means PBRs could be controlled to an adjustable level and
not in a more haphazard night/day average [7].
Managing what and when the PBR is fed is also another
way to control outputs. The medium that is fed to the PBR is
composed of the minimum nutrients (e.g. phosphate) the
algae needs, with the exception of nitrogen. Nitrogen is
intentionally lowered to prevent the production of nucleic
acids and proteins and instead forcing the microalgae to
store CO2 and light energy in lipids, which are central to oil
formation [7]. However, this has only been done in
laboratory settings thus far [7]. Other nutrients like CO2 can
be gassed in. It is recommended that gassing occurs the most
during the day, when there is the most photosynthetic
activity, and reduced to bare minimum at night, when there
is the least. To monitor these and other nutrient intakes and
subsequent productivity, two pO2, pH and pCO2 sensors
ENGINEERED ALGAE PRODUCTION
Algae is easily grown, developed, and sustained in many
diverse habitats all by itself. The problem is that algae that
excrete oil is not only difficult to grow in large amounts, but
it is not found everywhere, especially in areas that don’t
receive sunlight for at least half a day’s time [9]. Once the
algae are successfully cultivated though, the metabolism
process begins.
Splicing genes together and producing environments for
the algae to adapt in order to create a strand of algae that
produces fuel on a much grander scale that can survive harsh
climate changes and shifts in diet easily. Now that the algae
are adapted to the environment and a larger population of
algae exists, the catalyst of carbon dioxide is pumped
through tubes that mix with the algae. These translucent
tubes allow light to pass through and provide the algae the
light needed for photosynthesis. The sugar created from
photosynthesis added with the carbon dioxide in accordance
to the various ways the algae produces these materials
results in either ethanol, butanol, isoprene, or biodiesel. The
processes that provide each of these outputs are specific to
the algae strand that is being utilized; therefore the time and
energy spent developing each fuel is completely unique from
the rest, making the engineering and creation process
strenuous and critical to success.
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demonstrates the growth efficiency affected by the switch
from one PBR to the next.
Cultivation & Growth
Algae grow in many places in the world, but only certain
algae can produce oil. Algae that possess lipids can excrete
oil and are now being genetically metabolically engineered
to be less susceptible to climate change, to withstand other
symptoms, such as infection or disease, causing loss of
algae, and to enable a production of greater amounts of oil
for the same amount of nutrients. This effectively creates
“super algae” that can reach peak efficiency in the
production of oil. Currently, 30 specific brands of algae are
being researched and tested to find the algae strand that is
most compatible with the ever-growing human need for fuel
[10].
The engineered algae’s growth starts out in a lag and
works into an exponential, linear, and finally a peak growth
life cycle. It is believed that the initial lag is used as time for
the algae to adjust to the surroundings and to develop a large
population to use the resources that are given. Theoretically
once the initial lag has ended, the amount of sunlight should
proportionally yield the growth rate of the algae and the
constitution of the algae. There is, however, a slight dip in
the rate of growth which is labeled as “maximum
occupancy” [9]. This term refers to the point where the
general mass of the algae divided by the density of the algae
approaches the maximum volume of the PBR. Like most
organisms, algae needs sufficient room to grow, live, and
function. Without the necessary space, the algae growth rate
declines. As expected if more space were provided, which is
entirely dependent on necessary funding, the algae growth
rate will continue to rise.
Figure 2 is based on lipid production from light intensity
illustrates the level of importance of light intensity in the
algae growth process. The cultivation potential map almost
produces the same range of information but yields to specific
points on the map.
Figure 3 This graph is a general representation of the initial
growth vs. time for the fourth generation of microalgae. The
growth units are in duplication speed and the time units are in
seconds [9].
Sustainability & Treatment
Since the algae uses sun irradiation to grow and produce
fuel, it is essential for the algae to be exposed to intense rays
usually from three hundred to three hundred and sixty-five
days year [12]. The microalgae also depend on the structure
of the PBR and its composition to enable the flow and even
distribution of light, heat, nutrients, and catalysts to each
algal structure. For a better observation of these systems, see
the general design and overview section of this paper. The
loss of microalgae is only attributed to the extreme cases of
light, pH levels, nutrient and temperature depravation and
supplementation which in a controlled environment happen
rarely [12].
Because energy is such a readily used accommodation by
most all citizens, inaccessibility of the suns light for the
microalgae could provide fatal blows to the daily life of the
individual and, subsequently, the economic market. To
ensure that never happens, many research facilities have
implemented the use of lighting system for cloudy and foggy
days. The use of carbon dioxide, nitrogen, and phosphorous
the main ingredients for predictable growth rates of algae
and for a metabolite process of obtaining oilgae [10]. The
amount of supplementation for the algae is subject to the
overall health of the algal community. The issue with going
into any detailed specifics on the general spectrum of algae
is that every stand of algae is dependent on different
catalysts, amounts of nutrients, and environmental structure.
Sadly, the specific information needed for each brand of
algae is beyond the scope of this paper.
Figure 2 Displays potential lipid production across the U.S.
color specific light intensity (Red representing most intense and
blue representing least intense) [11]
GENERATION-BASED EXTRACTION METHODS
Many factors, such as climate change, pressure shifts,
contamination, and the structure of the PBR affect the
growth efficiency of the algae though not as drastically as
the un-engineered counterparts. The table below
This section will discuss the particular types of engineered
algae along with the benefits and disadvantages of each. Past
attempts have been made to use algae as a biofuel and with
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William Smith
recent technology advancements new breeds of algae are
available [10]. The discussion and implementation of such
new age algae is important if the project should succeed.
As expressed in table the variance of steps between
growing and gathering oil in the third and fourth generation
biofuel are definitively significant. In the third generation
the need to separate the algae from the fuel is required by
either a sedimentation type process or by various draining
methods. Also with the addition of separating the oil from
the algae, the process of converting the oil into a biodiesel
incorporates more steps that initially made the use of this
technology unattractive. In the latest generation however, the
oil naturally, through genome influence, divides from the
algae. Harvesting the algae and preparing for commercial
use is the only step after oil is being produced in this new
biofuel production method. Within the processes of the third
generation algae, the last step of trans-esterification, which
converts fat into biodiesel oil, requires the use of strong
bases, acids and a treatment center for this method to occur.
Just by eliminating one step many appliances and wasted
space is saved, making the entire scope of the project
incredibly more efficient in converting to fourth generation
algae.
Generation Variance
The fourth generation of engineered algae exists as follows:
Chlorophyta, rhodophyta, phaeophyta, Bacillariophyceae,
Euglenids, Dinoflagellates. These are the specific phylum
that incorporates the strands of algae that are complete,
verifiably operational, and are currently being tested. An
average of about 7 different strands of algal genome
sequences per phylum, of which there are over a hundred,
exist that are being industrially utilized and considered. The
third generation of engineered algae did not progress far, the
entirety of research spent in that series of time was on the
basic characteristics of the algae and it’s general
behaviors.[13] Of the complete eukaryotic microalgae that
have been investigated with gene transformation, which
share the same phylum as the complete list of sequences,
only thirty are actually stable enough to be tested and
considered viable for commercial use.
There are two different generation of algae. These two
are the third and fourth generation of production possible
biofuels, the first two being sugar cane and cellulosic
bioethanol. “The main difference between fourth generation
biofuel production and the previous generations is that only
the former applies the “cell factory” concept for the
production of gaseous and liquid biofuels. The
photosynthetic organisms are thus driven by solar energy
from the sun for continuous production of biofuels using
CO2 as the raw material. The common feature of 4th
generation biofuel production methods is that they will
secrete the end products out of the cells, which will avoid
the costly fermentation and/or processing of biomass
feedstock applied in current biofuel production” [14]. The
third generation was discontinued because of the lack of
algae that could sustain intense environments, making
research difficult and area specific, and the prospective
integration of lipid advancements.
Figure 4 displays the four generations of biofuel and their step
by step process lifecycle. [10]
Fuel Extraction
ETHICAL AND ECONOMIC CONCERNS IN LIGHT
OF COMMERCIALIZATION
Over the course of a generation the technological
advancements of microalgae has eliminated most all steps
from cultivation to harvesting fuel. If such a short time had
such a monumental effect that energy corporations, such as
algae tec., are being utilized and governmentally funded then
it’s only a matter of time before algae energy become stable,
renown method of providing clean efficient energy. Not only
that, but going from genetically engineered algae that
produces fuel to genetically engineered algae that
instantaneously produces fuel is progress that cannot, and
has not, been ignored by any and all interested parties that
can benefit from this technology.
This part of our conference paper will discuss the positive
and negative aspects to implementation, establish a
theoretical basis to predict risks and chance of successful
production, and how this new technology will affect the
economic status of United States. Incorporating this into our
paper is essential; in any large project every risk and
possible result must be accounted for to insure the greatest
yield of success.
Introducing a new energy technology into the world of
lobbyists, media, private and public interest groups and with
the competition of other technologies that may or may not
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William Smith
already control the market is full of risks and uncertainty.
Some corporations could take measures to stay on top of the
energy market by stunting the growth of many alternative
sources. A prime example of the failure of alternative green
technology by big industries is the electric car.
ultimately decide the accessibility of PBR technology [15].
While these estimates provide a possible trajectory towards
the future of PBR technology, further advancements in this
technology will force the future to unfold either in
accordance with our estimates or against.
Overview of Production Costs
Effect on Socioeconomic Market
This part will relate the cost of materials, labor, and time
spent to the amount of fuel produced. This range of
information is necessary for a scaled numerical comparison
between risks versus benefits. Note, this will solely be
statistical data to provide objective comparison insofar as
that is possible.
Recall the factors that decided the technological design of
PBRs. These factors also will decide the economic
feasibility of PBRs. Initial investment costs are estimated at
$74.03 per m2 or a fixed cost of $169,543.81 [15]. The fixed
cost of $169,543.81 can be considered an initial
approximation as costs are likely to rise as external
infrastructure such as pipelines are added and “wear and
tear” damages such as leakages in the nitrogen and CO 2
sources will add to such costs. Profitability is depended on a
wide variety of the aforementioned factors and each has a
specific impact on profitability with minimum impact from
$0.09 per megawatt hour (the variable being H 2O) to a
maximum impact of $71.01 megawatt hour from predictably
post-biodiesel [15]. It should be noted that these profitability
estimates have been made without other potentially
profitable byproducts estimates included.
Integrating other fuel sources into the fuel market will
drastically change the possibilities for this project and affect
the daily lives of many making it a key topic for
consideration. The effect on the economic market will be
characterized by hypothetical scenarios that will represent
predictions on stock variance, shifts in the energy industry,
and swings in international affairs [15].
In the past there have been a number of green
technologies that have failed due to lack of social
acceptance, economic uncertainty, and from corporate
influence. The need to consider the electric car becomes
apparent when we observe the magnitude of money and time
spent on it and the private support it received. The electric
car, in just three short years, went from potential success to
an abrupt failure [16]. Some attribute the fault completely on
private companies while others say it was simply before its
time [16].
Photobioreactors, like many technologies today, would be
more efficient if design specific advancements were made
but based on recent research results it is not before its time.
PBR’s are receiving support and funding from many
industries and countries such as the United States, the
Australian company Algae Tec., and various Middle Eastern
countries are now implementing PBR’s into green specific
research facilities [17]. There has been no evidence that
companies have been making similar moves to buy out,
eliminate, or otherwise marginalize PBR technology. The
aim of our statement is not to create conspiracy, only to
recognize the possibility of another collapse of a clean,
efficient technology.
Feasibility of Large Scale Commercialization
Production on a large scale is necessary to cause even a
noticeable effect on the fuel market therefore making it
imperative to discuss the likeliness of this project gaining
momentum. Here, the aforementioned statistical data will be
analyzed and applied to the possibility of large scale
commercialization.
Numbers are meaningless with analysis. Though PBRs
have a high yield per unit area, land is possible the single
factor deciding whether PBRs produce more than they
consume. Even if PBRs “break even” compared with
technologies such as solar PV and solar heat PBRs occupy a
small portion of the economic market [15]. This isn’t to say
PBRs don’t have their place as PBRs have certain
advantages give them a competitive edge compared to other
green technologies. One mentioned previously was the O2
produced as a byproduct that may be marketable. Other
advantages include the efficient use of land (even
inhospitable lands traditional agriculture could not thrive in)
and the directly produced hydrogen, which could potentially
fuel future devices (think hydrogen fuel cells) [15].
Succinctly, one could say that the PBR market does not
look bleak, but equally so, not promising. Of course, the
sensitivity to energy prices with regard to investment will
FUELING THE FUTURE: PROSPECTIVE
APPLICATIONS
Through our careful examinations of the technologies of
photobioreactor design and through its interactions with the
algae it cultivates we have established it as a strong potential
source of alternative energy. We have discussed its designs
to great length and the process underwent in the PBR. We
have seen the cultivation, harvesting, and genomic
engineering of various algal cultures. Through all this we
can contend that PBR’s may potential be the source for a
cleaner, more environmentally stable future.
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Nasir Sharaf
William Smith
ACKNOWLEDGMENTS
We would like to thank Roger Wyss, Mike Anzaldo,
Mike Galla, Blair Suter, and Diane Kerr for their insightful
aid and consistent assistance.
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