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Photobioreactor Systems for Concentrating Solar Energy in the Lipids of Photosynthetic Algae: A Renewable Source of Microbial Biodiesel
by Joline El Chakhtoura
American University of Beirut Department of Civil & Environmental Engineering
ENSC 660: Environmental Technology
January 2009
[1]
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Abstract
With energy consumption and combustion pollutants drastically augmenting, we need to
develop clean and renewable fuel sources with inconsequential effects to both human and
environmental health. One of the main biofuels currently being produced is biodiesel synthesized
by transesterification of the oils contained in algae. This happens by collecting solar energy and
allowing for high photosynthetic efficiencies. Large-scale algal cultivation takes place in
outdoor, relatively inexpensive, open systems whereas closed photobioreactors are more
productive and highly controlled, but costly. The process takes place in a solar collector
connected to an airlift pump and usually follows Monod kinetics. Third generation algal fuels
have proven to be beneficial in numerous areas but come with many drawbacks too. The
feasibility of this process will be discussed along with major technical and economic challenges.
Research and development is being conducted by major industrial firms and governmental
establishments before biodiesel will be fully commercialized. Genetic and bioreactor engineering
may be the solutions to perfect biodiesel production and consequently may have a massive
impact on the future welfare of our planet.
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Introduction
“No longer the pond scum, the environmental menace”, algae are now “the darling of the
biofuels industry [2].” With overall populations and energy consumption drastically augmenting,
we have already devoured more than half the existing fossil fuels on earth. Combustion
pollutants have engendered global climatic changes and this calls for the development of clean
and renewable fuel sources with inconsequential effects to both human and environmental
health. Can algal fuel or oilgae become the sustainable alternative to petroleum, and will its
commercialization boost global prosperity and energy security?
Bioenergy
Fuels today constitute approximately 67% of the global energy demand while nearly all
the renewable sources of energy (solar, wind, hydroelectric…) account for only 33% of the
demand, targeting mainly the electricity market [3, 4]. “According to Oil and Gas Journal
estimates, at today’s consumption level of about 85 million barrels per day of oil and 260 billion
cubic feet per day of natural gas, the reserves represent 40 years of oil and 64 years of natural gas
[5].” (See figure 2) Biofuels are thus being inspected and developed rapidly, representing
Figure 2 [6] Figure 3 [3]
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renewable energy derived from biological materials through photosynthesis. The main biofuels
currently being produced include biohydrogen, bioethanol, biomethane and biodiesel- the subject
of this paper. Photosynthesis is responsible for converting sunlight into chemical energy and
hence generates the feedstock needed for bioenergy synthesis: protons and electrons for
biohydrogen, starch and sugar for bioethanol, biomass for biomethane, and oil for biodiesel [4].
(See figure 3)
Biodiesel and Algae
The most common biofuel in Europe, biodiesel is a non-petroleum-based diesel fuel
synthesized from animal fats and edible/nonedible/waste oils. These are extracted from
soybeans, rapeseed, corn, sunflower, canola, Jatropha plants… etc. by transesterification. In the
presence of an alcohol and an alkali or acid catalyst the reaction transforms triglycerides into
fatty acid alkyl esters (biodiesel) with glycerol as a byproduct [5]. It is possible however to use
lipid-accumulating microorganisms such as cyanobacteria, yeast and microalgae to produce oils
for biodiesel manufacture. In all cases any biofuel represents “a means of collecting solar energy
and storing it in an energy dense chemical” [5].
Algae are a diverse group of eukaryotic, aquatic and typically photoautotrophic
organisms, ranging from unicellular to multicellular forms (See figure 4), and they have copious
applications. “The worldwide annual production of algal biomass is estimated to be 5 million
kilograms per year with a market value of about 330 USD per kilogram” [7]. High-value
microalgal products include nutritional supplements, aquaculture feeds, biofertilizers,
Figure 4 [1]
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pharmaceuticals, β-carotene and cosmetics, and they also have the potential to be used as edible
vaccines through genetic recombination [8, 9]. Microalgae play an imperative role in
bioremediation and wastewater treatment. They can eliminate heavy metals, uranium, nitrogen,
phosphorous and other pollutants from wastewater and they can degrade carcinogenic
polyaromatic hydrocarbons and other organics. Furthermore, algae are accountable for at least
50% of the photosynthetic biomass production on our planet and they are great sources of
biofuels because they can accrue 70% or more of their dry biomass as hydrocarbons [9].
Common genera utilized in oilgae or ‘third generation biofuel’ production include Botryococcus,
Chlamydomonas, Chlorella, Dunaliella and Neochloris [8].
Cultivation Scheme
1) Open Systems
To be able to generate biological petroleum or algaeoleum, these “sunlight-driven cell
factories” [9] must be cultured in suitably designed systems that allow for high photosynthetic
efficiencies. Culturing conditions that should be controlled include temperature, irradiance level,
turbulence, fluid dynamics, gas exchange, pH, salinity, cell density, growth inhibition,
hydrodynamic stress and carbon/mineral availability [3, 9]. According to Chisti (2006)
conventional large-scale cultivation takes place in outdoor open systems such as ponds and
lagoons and these may be circular or raceway-designed (See figures 5 and 6).
Figure 5 Southern Japan farm growing Chlorella in circular ponds [10]
Figure 6 green and red algae growing in raceways in Hawaii- 75 ha [11]
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Open culture systems comprise a relatively cost-effective method for growing
microalgae. Paddle wheels are usually employed to drive water flow and various construction
materials can be used for building the ponds. They are relatively inexpensive and easy to operate
and maintain. Major drawbacks however include loss of water by evaporation, difficulties in
controlling cultivar parameters, and susceptibility to competition and contamination by bacteria,
viruses and invasive algae. Consequently, only specific algal strains can be cultured in open
systems, especially extremophiles such as Spirulina and Dunaliella that favor highly alkaline and
saline environments and therefore outcompete other species. [3, 9]
2) Photobioreactors
Closed systems are highly controlled cultivars for algae that incorporate sunlight or
artificial illumination, thus the term photobioreactor (PBR). They are contaminant-free but have
a much higher capital cost compared to open cultures (gross annual revenue of $23,200 -
$49,600/acre vs. $10,500 - $22,500/acre). This is compensated for by higher productivity as
recent studies reveal: lipid annual production= 9,300 gal/acre vs. 4,200 gal/acre for open ponds
[12]. Several PBRs have been designed; tubular reactors, vertical alveolar panels, flat panels,
bubble column reactors… etc. (See figures 7 a,b,c).
Figure 7a vertical tubular [13]
Figure 7b flat panels [13]
Figure 7c horizontal tubular [13]
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Tubular photobioreactors have proven most successful for large-scale production. They
are made of one or more small-diameter transparent tubes that can be designed in several ways
and represent the reactor’s solar collectors [9]. To increase efficiency light should be distributed
over a large surface area to prevent photoinhibition and light/dark cycles “should be in
frequencies of 10 Hz or faster with the dark period being up to ten times longer than the light
period” [14]. In addition, constant mixing is required to prevent cell sedimentation and to
distribute photosynthetic gases [3].
Chisti (2006) delineates an airlift-driven tubular photobioreactor shown in figures 8 a and
b. This PBR is made of a transparent continuous-run solar collector attached to an airlift pump
which provides the energy necessary to circulate the culture fluid or broth. The degasser ensures
that bubble-free broth returns to the solar tubes. Carbon dioxide is pumped into the solar
collector in response to the pH. The necessary nitrogen, phosphorus and carbon concentrations
can be estimated from the molecular formula of the algal biomass. Temperature must be between
20 and 30 ˚C for most strains, regulated by passing the broth through a heat exchanger. The tubes
Figure 8a [13] Figure 8b pilot plant at BioKing, Finland [15]
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are constructed with stable and sturdy materials that “transmit light in the photosynthetically
active wavelength range”, such as glass, polyvinyl chloride and Teflon, with plastics generally
preferred. Algal wall growth diminishes bioreactor efficiency and thus is controlled by
increasing turbulence, regularly scouring the tubes with air, or by suspending abrasive grit
particles. [9]
In a well-designed photobioreactor culture, algal growth rate depends on the average
illumination (Iav) and may follow Monod kinetics:
where the light saturation constant depends on the algal strain and culturing conditions. PBR
tubes are usually 0.01 to 0.1 meters in diameter and biomass productivity drops as the diameter
increases unless the tubes are internally irradiated. This is because algae need light to grow and
deep regions are dark. Initially, PBRs operate under batch mode but the cultures are then
maintained under “pseudo steady state” conditions. The dilution rate (D) should not surpass the
maximum specific growth rate or else the culture will wash out. Xb being the steady state
biomass concentration, biomass productivity P can be measured by P = DXb and it isn’t usually
greater than 2 kg.m-3d-1. Irradiation slightly higher than the saturation light intensity will damage
the apparatus and inhibit growth resulting in photoinhibition, which is usually the case in hot
summer days. In addition, dissolved oxygen concentrations higher than the air saturation level
inhibit photosynthesis and can cause photooxidative damage to the algal cells in the presence of
strong sunlight. This is controlled by keeping the tube length below 80 meters to remove oxygen
efficiently before re-entering the solar collector. [9]
3) Hybrid Systems
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To achieve both cost-effective and high-yield cultures, a combination of open ponds and
photobioreactors may be applied. This has been tested by Huntley and Redalje (Aquasearch) on
a privately funded commercial scale (2 ha) in Hawaii. The first stage requires sterility and
continuous cell division, thus a PBR, and the second stage requires high oil production under
environmental stress, thus an open pond. [16] Results show that “the average biomass energy
production … works out to a net photosynthetic efficiency of just over 1% … However, the
average oil yield reported was over 1,200 gal of biodiesel per acre-year, far better than
conventional oil bearing crops. While their trials can be counted a success by many measures, it
is worth pointing out how low the yield is in terms of comparison to the potential yield based on
the quantum limits of photosynthetic efficiency, as well as compared to other means for
harnessing solar energy. [5]”
Lipids to Biodiesel
1) Processing Challenges
Algae normally have a lipid content of approximately 10 to 30% dry weight. However
this amount can double or triple during nitrogen depletion since cell division ceases and storage
products keep accumulating. Therefore higher oil strains are those with lower reproduction rates,
and hence these species take over photobioreactors. It is cost-effective to use high oil strains
during processing, both at the economic and energetic levels. So trying to maximize oil
production is only achieved when the algae are stressed, due to nutrient limitations, and at the
same time this restricts growth and thus the net photosynthetic efficiency. [3, 5]
2) Harvesting and Transesterification
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The high water content of algae must be removed to enable harvesting. “In existing algal
aquaculture the most common harvesting processes are flocculation, microscreening and
centrifugation.” These must be energy-efficient and relatively inexpensive so selecting easy-to-
harvest strains is important. Pure sedimentation is time- and space-consuming and although it’s
used in some algal farms it shouldn’t be considered for biodiesel synthesis. “Organic cationic
polyelectrolyte flocculants” are less expensive than aluminum or ferric chloride or lime.
Furthermore, adding flocculants isn’t considered sustainable and thus cell self-flocculation has
been recently studied by regulating carbon and the pH. Lipid extraction is facilitated by
combining methyl esterification with the “use of immobilized lipases … and mechanical
crushing followed by squeezing” can be carried out. A modern technique used to disrupt the cells
is electroporation where a strong electric field is applied to the biomass in order to perforate the
cell wall and better extract the lipids. “Chemical solvents can be chosen in one- or two-step
extraction approaches … and methanol and a catalyst such as sodium methoxide” can then
produce glycerol and biodiesel. [3] The required steps are shown in figure 9.
Figure 9 [1]
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Pros and Cons
Third generation algal fuels have proven to be beneficial in numerous areas. They have a
high photon conversion efficiency implying high biomass yields. They can be harvested in all
seasons providing us with a continuous supply of biofuel. Algae can be utilized in saline water
and wastewater thus helping to conserve fresh water resources and to reduce pollution. They can
be cultivated on non-arable land and in arid regions such as deserts and they don’t require much
land to be cultured, as opposed to other crops (See table 1). The use of oilgae rather than first
generation biofuels will
help alleviate the pressure of utilizing agricultural crops meant to feed the developing world in
particular. This in turn helps prevent food dearth. [3]
Algae can “couple CO2-neutral fuel production with CO2 sequestration” by utilizing
carbon dioxide from power plants as the input and remediating combustion exhausts [3]. The
yield is generally a non-toxic biodegradable fuel lacking sulfur, heavy metals and polycyclic
aromatic hydrocarbons thus making it “a safe alternative for storage and transportation”. It also
has a higher flash point than diesel making it safer to handle and store. [5] Microalgae have the
ability to select their substrate from a mixture of chemicals thus there is no need to refine the raw
material beforehand. Furthermore, these ‘microbial energy technologies’ can produce energy
[3]
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locally thus reducing the cost of energy transfer [6]. Similar to petrodiesel, biodiesel is
compatible with regular engines either in its pure form or blended with petroleum. Since it is
oxygenated, it constitutes a better lubricant than diesel, amplifying the life of engines and
burning more completely. [5]
Biodiesel comes with many drawbacks too. Some argue that the yield is in fact
insufficient to cover global fuel requirements [3]. Contrary to other references, the American
Academy of Microbiology states that “emissions from biodiesel-powered vehicles are worse, in
terms of their impacts on human and environmental health, than emissions from petrodiesel-
powered vehicles” [6]. Moreover, biofuels are utilized in the transportation sector only because
using them for electricity generation is “an inefficient means of harnessing solar energy” [5].
Feasibility and Barriers
“Of the renewable resources, incident solar energy is by far the largest (178,000
TW/year) and capable of supplying 13,500 times the total global energy demand (13 TW/year in
2000 predicted to rise to 46 TW in 2100)”. These values encourage us to attempt to make use of
this renewable energy source, especially since solar energy systems are advantageous even in
countries with poor irradiation. [17] According to the National Renewable Energy Laboratory,
economic modeling shows that the production price of oilgae ranges between 6.5 and 8 USD per
gallon [8], a cost competitive with corn- and sugar cane-based bioethanol prices. According to
Schenk et al. (2008), algal fuel prices range between 39 and 69 USD per barrel while another
study estimated a cost of 84 USD per barrel. Figures thus vary. Algal fuels produce a yield 15-
fold higher than food crops and are recognized as among the most efficient biomass producers.
Other studies reveal that “the theoretical maximum possible yield for algal productions is 365
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tons dry biomass per hectare per year” or “approximately 28,000 gallons per acre per year in the
US assuming 100% conversion of biomass to biodiesel which is infeasible”. [3, 5]
There are numerous technical challenges too that must be examined to make oilgae
profitable. Finding an algal strain that grows fast, contains high oil content, and is easy to
harvest, as well as constructing a cost-effective photobioreactor, are some of the difficulties.
Sustaining a high photosynthetic efficiency and decreasing the cost of installation, processing,
operation and maintenance are other barriers.
The answer to some of these challenges lies in bioengineering. Transgenic algal strains
have been recently created with their metabolism engineered to attain their maximum potential
capabilities. For example, scientists have succeeded in overexpressing acetyl-CoA carboxylase
which catalyzes lipid synthesis in algae. They have also used RNA interference technology to
produce mutants that can effectively capture sunlight and resist photodamage. [8]
Market, Commercialization, R&D
Biodiesel is growing into one of the most essential ‘near-market’ biofuels since all
industrial vehicles are diesel-based. “In the past decade, the biodiesel industry has seen massive
growth globally, more than doubling in production every 2 years” [3, 18]. Industrial and energy
giants such as Shell, Boeing and Airbus have immensely invested in research and development,
attempting to commercialize oilgae in next to no time [2]. “Markets for low-carbon energy
products are likely to be worth at least $500 billion per year by 2050, and perhaps much more”
[4]. Open algae cultures are used commercially in the US, Japan, Australia, China, India, Israel
and elsewhere. Moreover, Aquaflow Bionomic in New Zealand recently announced the first ever
commercial production of biodiesel from sewage pond microalgae. [9, 19]
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From 1978 till 1996, the US Department of Energy invested more than $25 million in its
Aquatic Species Program (ASP) to evaluate more than 3000 algal strains as a source of biodiesel.
The main conclusion was that oilgae is not limited by engineering designs but by culture
setbacks. It also pinpointed the significance of genetic manipulation. In the ‘90s Japan invested
around $117 million to conduct research on CO2 utilization by algae in photobioreactors. The
program was entitled Research Institute of Innovative Technology for the Earth (RITE). [16]
Other programs include MIT and GreenFuel Technologies Corp.’s triangular airlift reactor which
enables photomodulation and reduces the required physical space [20]. Rigorous research is
being conducted by private and governmental firms worldwide and conferences concerning
biodiesel are constantly being held. Algae World 2008 took place in Singapore a few months ago
to gain in-depth understanding of the potential of algae as a biofuel and to explore relevant
business opportunities. Algae World 2009 will take place in Rotterdam on the 27th and 28th of
April.
Conclusion
“The world faces a potentially crippling energy crisis in the next 30 to 50 years” [6].
Intensive funding and research in bioreactor engineering, biotechnology and perhaps
nanotechnology is needed urgently to find a clean and sustainable energy alternative of global
proportions. Solar energy is “evenly distributed and easily accessible to small, large, high-tech
and low-tech systems” [17] including Lebanon. Making use of this resource and microalgae may
have a profound impact on food and energy security, global warming and human health. As
depicted in the futuristic figures 10 and 11, algal bioreactors may become an integral part of
landscape and architecture, simultaneously soaking up pollution and generating energy.
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