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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***

Figure 10 [21] Figure 11 [22]

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