- Email: [email protected]
Closed photo-bioreactors as tools for biofuel production
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Closed photo-bioreactors as tools for biofuel production Florian Lehr and Clemens Posten Production of biofuels from microalgae is a promising sustainable option for the future. Unfortunately, until now production of algae biomass is too expensive owing to costly plant designs or high demand of auxiliary energy. These problems are addressed in recent developments. Basic ideas that are followed in different novel pilot plants are efficient mixing, high light dilution via large external surfaces or internal light conducting structures and gas transport via membranes. Other attempts are directed towards cheaper constructions. These endeavours have brought microalgal biofuel production closer to economic viability as has been shown in some pilot plants. But until now, these plants operate only on a small area and a limited time frame, making economic assessment difficult. The next years will show, whether these promises can be kept on a pure commercial basis for a whole process chain from algae cultivation to oil extraction during a whole year and on a real hectare. Address University of Karlsruhe, Institute of Engineering in Life Sciences, Department Bioprocess Engineering, D-76131 Karlsruhe, Germany Corresponding author: Posten, Clemens ([email protected])
Current Opinion in Biotechnology 2009, 20:280–285 This review comes from a themed issue on Energy biotechnology Edited by Peter Lindblad and Thomas Jeffries
probably only be possible with high efficient closed reactor designs, for example, water and nutrient recycling. But until today only about 100 tons (t) per year, which is 10% of microalgal biomass, is derived from closed photobioreactors, while the biggest part of a few thousand tons p.a. is cultivated in open ponds [7,8]. Closed reactors could be tubes, plates or bags made of plastics, glass or other transparent materials, in which the algae are supplied with light, nutrients and carbon dioxide [9,10]. However, only a few of these designs can be practically used for mass production of algae [11,12]. For energy production, algal biomass is too much expensive up to now. Algal biomass is traded for more than 5.000 s/t. Reasons therefore are, on the one hand, that this price is governed by the perceived nutritional value of algal biomass that is mostly produced for animal feed and not for energetic usage. On the other hand, it is caused by the low productivities of open ponds, the high demands of auxiliary energy and high costs of classical photo-bioreactor designs. But the problems are being addressed by engineering and science. Encouraging results have been obtained using new reactor geometries, optimized aeration and mixing strategies. Several pilot plants have been set up in the past two years that show high yields related to the available sunlight. This review will give ideas about the bottlenecks to achieve economical feasibility and will present some of the reactor concepts that are recently under discussion.
Available online 6th June 2009
Principal considerations
0958-1669/$ – see front matter # 2009 Elsevier Ltd. All rights reserved.
Introduction
Cultivation of microalgae for the production of energy implies harvesting of solar energy over large areas, bringing light and CO2 to the algae cells and care for moderate conditions inside the medium. The efficiency of this process – the so-called photoconversion efficiency (PCE) – is limited by the efficiency of photosynthesis and cell anabolism.
For decades, microalgae have been cultivated on a commercial basis for the production of high value compounds for food, feed, cosmetics and pharmaceutical products. The need for alternative renewable energy feedstocks that are not in competition to food production has drawn the attention to microalgae. The advantages of microalgae as an alternative biomass feedstock have been discussed extensively [1–6] in terms of higher areal yields compared with terrestrial crops, less or no use of fresh water making cultivation in arid regions possible, less dependency on seasonal variations or the cell composition with high oil yields or lack of non-fermentable compounds for biogas production. The high yield cultivation on tremendously large areas sufficient for production of a reasonable part of the world’s energy demand is a new challenge. This will
Because of the fact that only a small band of the whole solar radiation is photosynthetically active (PAR, 400–740 nm) and through energy losses caused by the reflectance of active light, the relaxation of higher excitation states and the energy demand for carbohydrate synthesis, the PCE is reduced to 12.6% [13]. The portion of respiration within the whole algal metabolism differs from species to species, depends on culturing conditions and can therefore only hardly be estimated. If we assume a minimum average respiration portion of 20% (Tredici, International Algae Congress 2008, Amsterdam, December 2008), the PCE would further decrease to 8.8%. Furthermore, the key enzyme of photosynthesis (Ribulose-1,5-bisphosphate carboxylase) shows an oxygenation side-activity at low
DOI 10.1016/j.copbio.2009.04.004
Current Opinion in Biotechnology 2009, 20:280–285
www.sciencedirect.com
Closed photo-bioreactors as tools for biofuel production Lehr and Posten 281
CO2 partial pressure, leading to metabolic pathways known as photorespiration [14,15], which lowers photosynthetic efficiency of terrestrial C3 crops significantly [13,16]. But as this side-activity is competitively inhibited by CO2, these losses can be avoided for microalgal cultures by appropriate gassing. Another reason, why the theoretical upper PCE-value of 9% is not reached in praxis, is that the photosynthesis rate increases linearly with light intensity only at low photon fluxes and diminishes at higher levels. Again, this is a starting-point for elaborate reactor and process design and underlines the necessity of appropriate closed reactor systems to achieve high conversion efficiencies. This maximum PCE of approximately 9% clearly shows the potential of microalgae compared to terrestrial crops (maximum PCE 4.6% (C3), 6.0% (C4) [13], real average values <1% [13,17]). But on the one hand, for reaching the ceiling of 9% PCE, photo-bioreactors have to be almost ideal concerning mass and light transfer. On the other hand, this has to be fulfilled with minimal auxiliary energy demand and minimal areal-based reactor costs to meet economic efficiency constraints. These conflicting priorities raise different requirements for closed photobioreactors as tools for biofuel production. Auxiliary energy
The demand for auxiliary energy is one major issue. Growth of microalgae in closed photo-bioreactors depends on appropriate mass transfer of CO2 and O2. In addition, good mixing substantially helps to prevent the cells from staying too long in dark or bright zones of the reactor. Slow light/ dark cycles (e.g. >1 Hz) will lead to reduced productivity as has been observed already for some time. For a recent model based investigation see [18]. Thus, a thorough understanding of fluid dynamics is needed for reactor optimization and assessment. A rigorous mathematical analysis of the relation between productivity and hydrodynamics is given by Pruvost et al. [19]. Sufficient mass transfer can be assured by bubbling, and in the case of plate reactors or bubble columns the ascending bubbles assure sufficient mixing at the same time. To achieve an appropriate degree of aeration and mixing, energy inputs of about 50–70 W/m3 are reported [20,21] for the case of carefully bubbled plate and airlift reactors. This means that the auxiliary energy demand could eventually count up to one third of the possible chemical energy finally harvested (see Box 1). For tubular reactors, values of more than 2000 W/m3 are usually employed to achieve sufficient fluid velocities inside the tube [22–24]. Therefore, tubular reactors are excluded from further consideration for biofuel production. Cost considerations
One cornerstone to define an upper limit for the cost of closed reactors per footprint area is the amount of energy www.sciencedirect.com
Box 1 Realistic case scenario based on current applications and designs Assumptions: Total incident solar energy (Central Europe) Photoconversion efficiency Auxiliary energy demand (flat plate reactor) [21] Areal water coverage Energy content of algal biomass (30% oil content) Results: Areal energy harvest Biomass calculated from energy balance Areal auxiliary energy demand
I = 150 W/m2 PCE = 5% 50 W/m3 50 L/m2 20 MJ/kg
7.5 W/m2 = 237 MJ/(m2a) 118 t/(haa) 2.5 W/m2
that could be collected from a microalgae plant. A revenue of approx. 120 t dry biomass per hectare and year seems to be achievable for Central Europe (see Box 1). Assuming a biomass price of 1000 s/t for energy use, the annual revenue is in the range of 12 s/m2. Roughly speaking, the investment for the photo-bioreactor should not lead to annual costs higher than that amount. This includes beside the reactor itself ground, stillage, pumps, piping, among others. Thus, a greenhouse covering the reactor is already a knockout criterion. It has to be mentioned, that these cost constraints vary with the biomass yield and are therefore dependent on the location (incident solar radiation, climate) and photoconversion efficiency of the plant. With improved photo-bioreactors, even nowadays, biomass yields up to 200 t per hectare and year seem to be achievable [25]. But the net energy ratio (NER), which is the ratio between the energy output (biomass) of a photo-bioreactor and the energy content of all its construction materials plus the energy needed for operation during its lifespan [26], has to be substantially greater than one to be economically coevally. One major advantage of closed reactors is the higher yield compared with open systems. But this can only be achieved, if the incident light is somehow diluted over the surface of the reactor to allow for cultivation at the lower linear range of the growth kinetics. The transparent reactor surface area is therefore chosen to be up to ten times larger than the footprint area. While in this way the yield will be higher, the amount of material to be spent is higher as well. Integrated processes
The reactor itself is only part of the challenge. Only the integration into a system of closed material flows can bring an economically feasible solution. Three major points are given here: Carbon dioxide is not for free as sometimes assumed. It has to be purified and transported from the place of origin over large distances to the algae plants, while the saving of carbon credits (e.g. 20 s/t) is nearly insignificant. Harvesting and extraction includes Current Opinion in Biotechnology 2009, 20:280–285
282 Energy biotechnology
Figure 1
Green Wall Panel photo-bioreactors (Picture by courtesy of E. Molina Grima, University of Almeria, Spain).
energy usage as well. Finally the nutrients like nitrogen or phosphorous have to be recycled from the place of biomass processing back to the medium. Direct usage of microalgae biomass in anaerobic digesters for production of biogas requires the separation of methane and carbon dioxide as well as the extraction of minerals from the effluent. The co-production of high value compounds has been proposed to support the production. This would indeed be a good start, but even if it was possible to produce a fine chemical for usage in chemical synthesis the residual algal biomass would make up only a minor part of the feedstock required to cover the total energy demand. The final challenge is to produce algal biomass cost effective enough to be used only for energy purposes as is nowadays tried with sugar cane, rape seed or corn.
Options for increased performance Low cost designs
Several attempts have been undertaken to produce cheap reactors. Simple designs like hanging bags or plastic tubes have been used for a long time. Novel attempts aim towards simplified installation, automation and maintenance. One example is the V-shaped bag reactor from Novagreen working as bubble column (pictures at URL: http://www.novagreen-microalgae.com). A vertical plate reactor with plastic bags shaped by a wire mesh has been presented by Tredici (Green Wall Panel, patent WO 2004/074423 A3) [26] and is now widely used (Figure 1). It shows a net energy ratio around one. So called annual reactors [27,28] are another option already available showing promising efficiencies of 9.3% overall areal productivity referred to the visible fraction of global solar Current Opinion in Biotechnology 2009, 20:280–285
radiation, which corresponds to a PCE value of approximately 4.5% (referred to the whole solar spectrum). Efficient mass and light transfer
Other designs aim to higher efficiency in mixing. A specific photo-bioreactor design based on the airlift principle is described by Merchuk [18]. While the reactor is an inclined tube, riser and downcomer are separated by a flat baffle inserted in its bore. One air inlet produces the typical airlift flow while the second one evokes turbulences in the riser by a countercurrent of medium and air. In experimental set-ups and with the employment of CFD and tracer trajectory measurements the authors showed the good mass transfer qualities of this design and that the turbulences induced in the riser are good enough to support sufficient fast light/dark cycles. Mixing energy (superficial gas velocity close to 0.1 m/s) is not calculated here. The countercurrent principle was in detail described by Vunjak-Novakovic et al. [29]. Here the downcomer is performed as separate vertical tube behind the inclined riser leading to a ‘‘triangular’’ air-lift reactor (ALR) configuration (US Patent Application 20050239182). Tests have been carried out with a set of 30 ALRs each having a volume of 30 L at MIT. An assessment of Pulz [25] of the so called 3D matrix system (3DMS) with Greenfuels at the Redhawk Power Station, Arizona, revealed very high productivities of nearly 100 g/(m2d) on average of 19 days. As this design exhibits other advantages like effective suppression of fouling, reservations are discussed with respect to harvesting problems, energy demand for bubbling and negative effects of slow secondary light/dark cycles induced by circulation through the shaded downcomer (values not clear or not available). www.sciencedirect.com
Closed photo-bioreactors as tools for biofuel production Lehr and Posten 283
Another reactor type used for the installation of bigger plants during the past years is the Flat Panel Airliftreactor (FPA, patent WO 02/31102) from Subitec GmbH, Germany (URL: http://www.subitec.com/technik.html). The reactor consists of two deep-drawing film half-shells, which are welded together to form internal static mixers. Thus, the riser is subdivided into interconnected chambers by means of horizontal baffles. Therefore, the ascending bubbles induce vortices that lead to short light/dark cycles of the algae cells [30,31]. The downcomer is arranged at the same level in between to raiser units. From the points outlined above there are several starting points for reduction of auxiliary energy demand. To reduce pressure and volumetric energy demand the reactor design should change from quite elevated plates mounted in series on a given area to humble units located close to each other. That minimizes the hydrodynamic pressure and reduces the space between the units, which is usually necessary to avoid too much mutual shading between the single units. This idea has been followed by the G3 design of Solix (Figure 2, Bryan Willson, paper submitted for publication). Flat bags are floating in a water basin that acts as cheap mechanical support for the bags, temperature control and as a means for light distribution over the bags surfaces. Carbon dioxide enriched air is pumped through tubes along the reactor axis and is sparged only against a few centimetres of medium. The next version (G4) will avoid bubbles at all and provide gas exchange by membrane aeration. A second basic point to consider is light dilution. State of the art is distribution of light over large outer reactor surfaces. This principle has already been followed for
years. However, the combination with the idea of vertical thin films has the potential to lead to high biomass concentration [8]. This is besides high productivities another very important issue as the increase from 1 g/L to 10 g/L could cut the cost for solid/liquid separation in downstream processing to 10%.
Future perspectives Another promising idea is to supply light via conducting structures inside the reactor volume. This idea as such is not new [32–38] but has never been brought even to pilot scale. A recent example is given by Zijffers [39,40]. Besides optimum light distribution, this reactor type offers a remarkable saving of transparent material. This makes employment of glass reasonable with advantages in shelf life cycle and gas impermeability. The consequent end of this development trend could be a photobioreactor where light is collected over a large area and conducted to a central closed reactor. Inside the algal suspension it is distributed again to an appropriate intensity. Sterile and controlled operation could be achieved quite easily. Also for hydrogen production or operation under overpressure, this is a promising approach. However, this seems to be up to now too costly and too ineffective [11,37]. As only approximately 50% of the total solar irradiation can be used for photosynthesis, maybe the residual fractions of the sunlight can be used with other techniques. If for example the energy of the IR-fraction of the sunlight could be employed for mixing purposes, this could shift the energy balance significantly to the safe side. Not only mechanical engineering but also genetic engineering approaches can solve parts of the problems. New
Figure 2
G3 photo-bioreactor design of Solix Biofuels. Left: schematic drawing; right: picture of outdoor facility (Picture by courtesy of Bryan Willson, Solix Biofuels). www.sciencedirect.com
Current Opinion in Biotechnology 2009, 20:280–285
284 Energy biotechnology
microalgae strains with reduced antenna size could show high efficiency even at high irradiance, making light dilution less important [41,43].
Conclusions A photo-bioreactor that is cheap enough, energy efficient enough and stable enough to be used for commercial biofuel production over large areas does not presently exist. Nevertheless, the specific problems are addressed and partially solved. In some pilot plants productivities have been reached which were thought some months ago to be possible only in lab scale. A perfect adjustment between algal physiology and sophisticated engineering has the potential to solve the problems in cost and performance. According to the current opinion, it is a question of a few years to show economical feasibility of biofuel production by algae on a whole hectare and during a whole year [42].
References and recommended reading Papers of particular interest, published within the period of review, have been highlighted as: of special interest of outstanding interest 1.
Chisti Y: Biodiesel from microalgae beats bioethanol. Trends Biotechnol 2008, 26:126-131.
2.
Behzadi S, Farid MM: Review: examining the use of different feedstock for the production of biodiesel. Asia-Pac J Chem Eng 2007, 2:480-486.
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Chisti Y: Biodiesel from microalgae. Biotechnol Adv 2007, 25:294-306.
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Gouveia L, Oliveira A: Microalgae as a raw material for biofuels production. J Industrial Microbiol Biotechnol 2009, 36:269-274.
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Hu Q, Sommerfeld M, Jarvis E, Ghirardi M, Posewitz M, Seibert M, Darzins A: Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances. Plant J 2008, 54:621-639.
6.
Li Y, Horsman M, Wu N, Lan CQ, Dubois-Calero N: Biofuels from Microalgae. Biotechnol Prog 2008, 24:815-820.
7.
Becker EW: Microalgae: Biotechnology and MicrobiologyCambridge University Press; 1994.
8.
Pulz O, Gross W: Valuable products from biotechnology of microalgae. Appl Microbiol Biotechnol 2004, 65:635-648.
9.
Carvalho AP, Meireles LA, Malcata FX: Microalgal reactors: a review of enclosed system designs and performances. Biotechnol Prog 2006, 22:1490-1506.
15. Spalding MH: Microalgal carbon-dioxide-concentrating mechanisms: Chlamydomonas inorganic carbon transporters. J Exp Botany 2008, 59:1463-1473. 16. Bolton JR, Hall DO: The maximum efficiency of photosynthesis. Photochem Photobiol 1991, 53:545-548. 17. Boyer JS: Plant productivity and environment. Science 1982, 218:443-448. 18. Merchuk JC, Rosenblat Y, Berzin I: Fluid flow and mass transfer in a counter-current gas-liquid inclined tubes photobioreactor. Chem Eng Sci 2007, 62:7414-7425. In experimental set-ups and with the employment of CFD and tracer trajectory measurements, the authors show the good mass transfer qualities of this photo-bioreactor design. 19. Pruvost J, Cornet JF, Legrand J: Hydrodynamics influence on light conversion in photobioreactors: an energetically consistent analysis. Chem Eng Sci 2008, 63:3679-3694. 20. Alias CB, Lopez MCGM, Fernandez FGA, Sevilla JMG, Sanchez JLG, Grima EM: Influence of power supply in the feasibility of Phaeodactylum tricornutum cultures. Biotechnol Bioeng 2004, 87:723-733. 21. Sierra E, Acien FG, Fernandez JM, Garcia JL, Gonzalez C, Molina E: Characterization of a flat plate photobioreactor for the production of microalgae. Chem Eng J 2008, 138:136-147. In this paper the performance of a flat panel photo-bioreactor is characterized with respect to optimum orientation of the panel and the influence of the aeration rate on mixing, mass and heat transfer that finally determines the power supply of the reactor. 22. Perner-Nochta I, Lucumi A, Posten C: Photoautotrophic cell and tissue culture in a tubular photobioreactor. Eng Life Sci 2007, 7:127-135. 23. Perner-Nochta I, Posten C: Simulations of light intensity variation in photobioreactors. J Biotechnol 2007, 131:276-285. 24. Sierra E, Acie´n FG, Ferna´ndez JM, Garcı´a JL, Gonza´lez C, Molina E: Characterization of a flat plate photobioreactor for the production of microalgae. Chem Eng J 2008, 138:136-147. 25. Pulz O. Performance Summary Report: Evaluation of GreenFuel’s 3D Matrix Algae Growth Engineering Scale Unit 2007. 26. Rodolfi L, Zittelli GC, Bassi N, Padovani G, Biondi N, Bonini G, Tredici MR: Microalgae for oil: Strain selection, induction of lipid synthesis and outdoor mass cultivation in a low-cost photobioreactor. Biotechnol Bioeng 2009, 102:100-112. 27. Zittelli GC, Rodolfi L, Biondi N, Tredici MR: Productivity and photosynthetic efficiency of outdoor cultures of Tetraselmis suecica in annular columns. Aquaculture 2006, 261:932-943. This paper reports the determination of the overall areal productivity of 120 L annular columns in a simulated full-scale arrangement to investigate mutual effects compared to single reactors. 28. Zittelli GC, Rodolfi L, Tredici MR: Mass cultivation of Nannochloropsis sp in annular reactors. J Appl Phycol 2003, 15:107-114. 29. Vunjak-Novakovic G, Kim Y, Wu X, Berzin I, Merchuk JC: Air-lift bioreactors for algal growth on flue gas: mathematical modeling and pilot-plant studies. Ind Eng Chem Res 2005, 44:6154-6163.
10. Pulz, Pulz O: Photobioreactors: production systems for phototrophic microorganisms. Appl Microbiol Biotechnol 2001, 57:287-293.
30. Degen J, Uebele A, Retze A, Schmid-Staiger U, Trosch W: A novel airlift photobioreactor with baffles for improved light utilization through the flashing light effect. J Biotechnol 2001, 92:89-94.
11. Janssen M, Tramper J, Mur LR, Wijffels RH: Enclosed outdoor photobioreactors: light regime, photosynthetic efficiency, scale-up, and future prospects. Biotechnol Bioeng 2003, 81:193-210.
31. Meiser A, Schmid-Staiger U, Troesch W: Optimization of eicosapentaenoic acid production by Phaeodactylum tricornutum in the flat panel airlift (FPA) reactor. J Appl Phycol 2004, 16:215-225.
12. Ugwu CU, Aoyagi H, Uchiyama H: Photobioreactors for mass cultivation of algae. Bioresource Technol 2008, 99:4021-4028.
32. An J-Y, Kim B-W: Biological desulfurization in an optical-fiber photobioreactor using an automatic sunlight collection system. J Biotechnol 2000, 80:35-44.
13. Zhu XG, Long SP, Ort DR: What is the maximum efficiency with which photosynthesis can convert solar energy into biomass? Curr Opin Biotechnol 2008, 19:153-159. 14. Spalding MH: Photosynthesis and photorespiration in freshwater green algae. Aquatic Botany 1989, 34:181-209. Current Opinion in Biotechnology 2009, 20:280–285
33. Chen C-Y, Lee C-M, Chang J-S: Feasibility study on bioreactor strategies for enhanced photohydrogen production from Rhodopseudomonas palustris WP3-5 using optical-fiberassisted illumination systems. Int J Hydrogen Energy 2006, 31:2345-2355. www.sciencedirect.com
Closed photo-bioreactors as tools for biofuel production Lehr and Posten 285
34. Chen C-Y, Saratale GD, Lee C-M, Chen P-C, Chang J-S: Phototrophic hydrogen production in photobioreactors coupled with solar-energy-excited optical fibers. Int J Hydrogen Energy 2008, 33:6886-6895. 35. Feuermann D, Gordon JM: Solar fiber-optic mini-dishes: a new approach to the efficient collection of sunlight. Solar Energy 1999, 65:159-170. 36. Gordon JM: Tailoring optical systems to optimized photobioreactors. Int J Hydrogen Energy 2002, 27:1175-1184. 37. Ono E, Cuello JL: Design parameters of solar concentrating systems for CO2-mitigating algal photobioreactors. Energy 2004, 29:1651-1657.
This paper describes a promising approach to realize an area-efficient photobioreactor by supplying light via conducting structures inside the reactor volume. 40. Zijffers J-WF, Salim S, Janssen M, Tramper J, Wijffels RH: Capturing sunlight into a photobioreactor: ray tracing simulations of the propagation of light from capture to distribution into the reactor. Chem Eng J 2008, 145:316-327. 41. Mussgnug JH, Thomas-Hall S, Rupprecht J, Foo A, Klassen V, McDowall A, Schenk PM, Kruse O, Hankamer B: Engineering photosynthetic light capture: impacts on improved solar energy to biomass conversion. Plant Biotechnol J 2007, 5:802-814.
38. Suh IS, Lee SB: A light distribution model for an internally radiating photobioreactor. Biotechnol Bioeng 2003, 82:180-189.
42. Hankamer B, Lehr F, Rupprecht J, Mussgnug JH, Posten C, Kruse O: Photosynthetic biomass and H2 production by green algae: from bioengineering to bioreactor scale-up. Physiologia Plantarum 2007, 131:10-21.
39. Zijffers J-WF, Janssen M, Tramper J, Wijffels RH: Design process of an area-efficient photobioreactor. Mar Biotechnol 2008, 10:404-415.
43. Beckmann J, Lehr F, Finazzi G, Hankamer B, Posten C, Wobbe L, Kruse O: J Biotechnol 2009, doi:10.1016/j.jbiotec.2009.02.015, in press.
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Current Opinion in Biotechnology 2009, 20:280–285
Closed photo-bioreactors as tools for biofuel production Florian Lehr and Clemens Posten Production of biofuels from microalgae is a promising sustainable option for the future. Unfortunately, until now production of algae biomass is too expensive owing to costly plant designs or high demand of auxiliary energy. These problems are addressed in recent developments. Basic ideas that are followed in different novel pilot plants are efficient mixing, high light dilution via large external surfaces or internal light conducting structures and gas transport via membranes. Other attempts are directed towards cheaper constructions. These endeavours have brought microalgal biofuel production closer to economic viability as has been shown in some pilot plants. But until now, these plants operate only on a small area and a limited time frame, making economic assessment difficult. The next years will show, whether these promises can be kept on a pure commercial basis for a whole process chain from algae cultivation to oil extraction during a whole year and on a real hectare. Address University of Karlsruhe, Institute of Engineering in Life Sciences, Department Bioprocess Engineering, D-76131 Karlsruhe, Germany Corresponding author: Posten, Clemens ([email protected])
Current Opinion in Biotechnology 2009, 20:280–285 This review comes from a themed issue on Energy biotechnology Edited by Peter Lindblad and Thomas Jeffries
probably only be possible with high efficient closed reactor designs, for example, water and nutrient recycling. But until today only about 100 tons (t) per year, which is 10% of microalgal biomass, is derived from closed photobioreactors, while the biggest part of a few thousand tons p.a. is cultivated in open ponds [7,8]. Closed reactors could be tubes, plates or bags made of plastics, glass or other transparent materials, in which the algae are supplied with light, nutrients and carbon dioxide [9,10]. However, only a few of these designs can be practically used for mass production of algae [11,12]. For energy production, algal biomass is too much expensive up to now. Algal biomass is traded for more than 5.000 s/t. Reasons therefore are, on the one hand, that this price is governed by the perceived nutritional value of algal biomass that is mostly produced for animal feed and not for energetic usage. On the other hand, it is caused by the low productivities of open ponds, the high demands of auxiliary energy and high costs of classical photo-bioreactor designs. But the problems are being addressed by engineering and science. Encouraging results have been obtained using new reactor geometries, optimized aeration and mixing strategies. Several pilot plants have been set up in the past two years that show high yields related to the available sunlight. This review will give ideas about the bottlenecks to achieve economical feasibility and will present some of the reactor concepts that are recently under discussion.
Available online 6th June 2009
Principal considerations
0958-1669/$ – see front matter # 2009 Elsevier Ltd. All rights reserved.
Introduction
Cultivation of microalgae for the production of energy implies harvesting of solar energy over large areas, bringing light and CO2 to the algae cells and care for moderate conditions inside the medium. The efficiency of this process – the so-called photoconversion efficiency (PCE) – is limited by the efficiency of photosynthesis and cell anabolism.
For decades, microalgae have been cultivated on a commercial basis for the production of high value compounds for food, feed, cosmetics and pharmaceutical products. The need for alternative renewable energy feedstocks that are not in competition to food production has drawn the attention to microalgae. The advantages of microalgae as an alternative biomass feedstock have been discussed extensively [1–6] in terms of higher areal yields compared with terrestrial crops, less or no use of fresh water making cultivation in arid regions possible, less dependency on seasonal variations or the cell composition with high oil yields or lack of non-fermentable compounds for biogas production. The high yield cultivation on tremendously large areas sufficient for production of a reasonable part of the world’s energy demand is a new challenge. This will
Because of the fact that only a small band of the whole solar radiation is photosynthetically active (PAR, 400–740 nm) and through energy losses caused by the reflectance of active light, the relaxation of higher excitation states and the energy demand for carbohydrate synthesis, the PCE is reduced to 12.6% [13]. The portion of respiration within the whole algal metabolism differs from species to species, depends on culturing conditions and can therefore only hardly be estimated. If we assume a minimum average respiration portion of 20% (Tredici, International Algae Congress 2008, Amsterdam, December 2008), the PCE would further decrease to 8.8%. Furthermore, the key enzyme of photosynthesis (Ribulose-1,5-bisphosphate carboxylase) shows an oxygenation side-activity at low
DOI 10.1016/j.copbio.2009.04.004
Current Opinion in Biotechnology 2009, 20:280–285
www.sciencedirect.com
Closed photo-bioreactors as tools for biofuel production Lehr and Posten 281
CO2 partial pressure, leading to metabolic pathways known as photorespiration [14,15], which lowers photosynthetic efficiency of terrestrial C3 crops significantly [13,16]. But as this side-activity is competitively inhibited by CO2, these losses can be avoided for microalgal cultures by appropriate gassing. Another reason, why the theoretical upper PCE-value of 9% is not reached in praxis, is that the photosynthesis rate increases linearly with light intensity only at low photon fluxes and diminishes at higher levels. Again, this is a starting-point for elaborate reactor and process design and underlines the necessity of appropriate closed reactor systems to achieve high conversion efficiencies. This maximum PCE of approximately 9% clearly shows the potential of microalgae compared to terrestrial crops (maximum PCE 4.6% (C3), 6.0% (C4) [13], real average values <1% [13,17]). But on the one hand, for reaching the ceiling of 9% PCE, photo-bioreactors have to be almost ideal concerning mass and light transfer. On the other hand, this has to be fulfilled with minimal auxiliary energy demand and minimal areal-based reactor costs to meet economic efficiency constraints. These conflicting priorities raise different requirements for closed photobioreactors as tools for biofuel production. Auxiliary energy
The demand for auxiliary energy is one major issue. Growth of microalgae in closed photo-bioreactors depends on appropriate mass transfer of CO2 and O2. In addition, good mixing substantially helps to prevent the cells from staying too long in dark or bright zones of the reactor. Slow light/ dark cycles (e.g. >1 Hz) will lead to reduced productivity as has been observed already for some time. For a recent model based investigation see [18]. Thus, a thorough understanding of fluid dynamics is needed for reactor optimization and assessment. A rigorous mathematical analysis of the relation between productivity and hydrodynamics is given by Pruvost et al. [19]. Sufficient mass transfer can be assured by bubbling, and in the case of plate reactors or bubble columns the ascending bubbles assure sufficient mixing at the same time. To achieve an appropriate degree of aeration and mixing, energy inputs of about 50–70 W/m3 are reported [20,21] for the case of carefully bubbled plate and airlift reactors. This means that the auxiliary energy demand could eventually count up to one third of the possible chemical energy finally harvested (see Box 1). For tubular reactors, values of more than 2000 W/m3 are usually employed to achieve sufficient fluid velocities inside the tube [22–24]. Therefore, tubular reactors are excluded from further consideration for biofuel production. Cost considerations
One cornerstone to define an upper limit for the cost of closed reactors per footprint area is the amount of energy www.sciencedirect.com
Box 1 Realistic case scenario based on current applications and designs Assumptions: Total incident solar energy (Central Europe) Photoconversion efficiency Auxiliary energy demand (flat plate reactor) [21] Areal water coverage Energy content of algal biomass (30% oil content) Results: Areal energy harvest Biomass calculated from energy balance Areal auxiliary energy demand
I = 150 W/m2 PCE = 5% 50 W/m3 50 L/m2 20 MJ/kg
7.5 W/m2 = 237 MJ/(m2a) 118 t/(haa) 2.5 W/m2
that could be collected from a microalgae plant. A revenue of approx. 120 t dry biomass per hectare and year seems to be achievable for Central Europe (see Box 1). Assuming a biomass price of 1000 s/t for energy use, the annual revenue is in the range of 12 s/m2. Roughly speaking, the investment for the photo-bioreactor should not lead to annual costs higher than that amount. This includes beside the reactor itself ground, stillage, pumps, piping, among others. Thus, a greenhouse covering the reactor is already a knockout criterion. It has to be mentioned, that these cost constraints vary with the biomass yield and are therefore dependent on the location (incident solar radiation, climate) and photoconversion efficiency of the plant. With improved photo-bioreactors, even nowadays, biomass yields up to 200 t per hectare and year seem to be achievable [25]. But the net energy ratio (NER), which is the ratio between the energy output (biomass) of a photo-bioreactor and the energy content of all its construction materials plus the energy needed for operation during its lifespan [26], has to be substantially greater than one to be economically coevally. One major advantage of closed reactors is the higher yield compared with open systems. But this can only be achieved, if the incident light is somehow diluted over the surface of the reactor to allow for cultivation at the lower linear range of the growth kinetics. The transparent reactor surface area is therefore chosen to be up to ten times larger than the footprint area. While in this way the yield will be higher, the amount of material to be spent is higher as well. Integrated processes
The reactor itself is only part of the challenge. Only the integration into a system of closed material flows can bring an economically feasible solution. Three major points are given here: Carbon dioxide is not for free as sometimes assumed. It has to be purified and transported from the place of origin over large distances to the algae plants, while the saving of carbon credits (e.g. 20 s/t) is nearly insignificant. Harvesting and extraction includes Current Opinion in Biotechnology 2009, 20:280–285
282 Energy biotechnology
Figure 1
Green Wall Panel photo-bioreactors (Picture by courtesy of E. Molina Grima, University of Almeria, Spain).
energy usage as well. Finally the nutrients like nitrogen or phosphorous have to be recycled from the place of biomass processing back to the medium. Direct usage of microalgae biomass in anaerobic digesters for production of biogas requires the separation of methane and carbon dioxide as well as the extraction of minerals from the effluent. The co-production of high value compounds has been proposed to support the production. This would indeed be a good start, but even if it was possible to produce a fine chemical for usage in chemical synthesis the residual algal biomass would make up only a minor part of the feedstock required to cover the total energy demand. The final challenge is to produce algal biomass cost effective enough to be used only for energy purposes as is nowadays tried with sugar cane, rape seed or corn.
Options for increased performance Low cost designs
Several attempts have been undertaken to produce cheap reactors. Simple designs like hanging bags or plastic tubes have been used for a long time. Novel attempts aim towards simplified installation, automation and maintenance. One example is the V-shaped bag reactor from Novagreen working as bubble column (pictures at URL: http://www.novagreen-microalgae.com). A vertical plate reactor with plastic bags shaped by a wire mesh has been presented by Tredici (Green Wall Panel, patent WO 2004/074423 A3) [26] and is now widely used (Figure 1). It shows a net energy ratio around one. So called annual reactors [27,28] are another option already available showing promising efficiencies of 9.3% overall areal productivity referred to the visible fraction of global solar Current Opinion in Biotechnology 2009, 20:280–285
radiation, which corresponds to a PCE value of approximately 4.5% (referred to the whole solar spectrum). Efficient mass and light transfer
Other designs aim to higher efficiency in mixing. A specific photo-bioreactor design based on the airlift principle is described by Merchuk [18]. While the reactor is an inclined tube, riser and downcomer are separated by a flat baffle inserted in its bore. One air inlet produces the typical airlift flow while the second one evokes turbulences in the riser by a countercurrent of medium and air. In experimental set-ups and with the employment of CFD and tracer trajectory measurements the authors showed the good mass transfer qualities of this design and that the turbulences induced in the riser are good enough to support sufficient fast light/dark cycles. Mixing energy (superficial gas velocity close to 0.1 m/s) is not calculated here. The countercurrent principle was in detail described by Vunjak-Novakovic et al. [29]. Here the downcomer is performed as separate vertical tube behind the inclined riser leading to a ‘‘triangular’’ air-lift reactor (ALR) configuration (US Patent Application 20050239182). Tests have been carried out with a set of 30 ALRs each having a volume of 30 L at MIT. An assessment of Pulz [25] of the so called 3D matrix system (3DMS) with Greenfuels at the Redhawk Power Station, Arizona, revealed very high productivities of nearly 100 g/(m2d) on average of 19 days. As this design exhibits other advantages like effective suppression of fouling, reservations are discussed with respect to harvesting problems, energy demand for bubbling and negative effects of slow secondary light/dark cycles induced by circulation through the shaded downcomer (values not clear or not available). www.sciencedirect.com
Closed photo-bioreactors as tools for biofuel production Lehr and Posten 283
Another reactor type used for the installation of bigger plants during the past years is the Flat Panel Airliftreactor (FPA, patent WO 02/31102) from Subitec GmbH, Germany (URL: http://www.subitec.com/technik.html). The reactor consists of two deep-drawing film half-shells, which are welded together to form internal static mixers. Thus, the riser is subdivided into interconnected chambers by means of horizontal baffles. Therefore, the ascending bubbles induce vortices that lead to short light/dark cycles of the algae cells [30,31]. The downcomer is arranged at the same level in between to raiser units. From the points outlined above there are several starting points for reduction of auxiliary energy demand. To reduce pressure and volumetric energy demand the reactor design should change from quite elevated plates mounted in series on a given area to humble units located close to each other. That minimizes the hydrodynamic pressure and reduces the space between the units, which is usually necessary to avoid too much mutual shading between the single units. This idea has been followed by the G3 design of Solix (Figure 2, Bryan Willson, paper submitted for publication). Flat bags are floating in a water basin that acts as cheap mechanical support for the bags, temperature control and as a means for light distribution over the bags surfaces. Carbon dioxide enriched air is pumped through tubes along the reactor axis and is sparged only against a few centimetres of medium. The next version (G4) will avoid bubbles at all and provide gas exchange by membrane aeration. A second basic point to consider is light dilution. State of the art is distribution of light over large outer reactor surfaces. This principle has already been followed for
years. However, the combination with the idea of vertical thin films has the potential to lead to high biomass concentration [8]. This is besides high productivities another very important issue as the increase from 1 g/L to 10 g/L could cut the cost for solid/liquid separation in downstream processing to 10%.
Future perspectives Another promising idea is to supply light via conducting structures inside the reactor volume. This idea as such is not new [32–38] but has never been brought even to pilot scale. A recent example is given by Zijffers [39,40]. Besides optimum light distribution, this reactor type offers a remarkable saving of transparent material. This makes employment of glass reasonable with advantages in shelf life cycle and gas impermeability. The consequent end of this development trend could be a photobioreactor where light is collected over a large area and conducted to a central closed reactor. Inside the algal suspension it is distributed again to an appropriate intensity. Sterile and controlled operation could be achieved quite easily. Also for hydrogen production or operation under overpressure, this is a promising approach. However, this seems to be up to now too costly and too ineffective [11,37]. As only approximately 50% of the total solar irradiation can be used for photosynthesis, maybe the residual fractions of the sunlight can be used with other techniques. If for example the energy of the IR-fraction of the sunlight could be employed for mixing purposes, this could shift the energy balance significantly to the safe side. Not only mechanical engineering but also genetic engineering approaches can solve parts of the problems. New
Figure 2
G3 photo-bioreactor design of Solix Biofuels. Left: schematic drawing; right: picture of outdoor facility (Picture by courtesy of Bryan Willson, Solix Biofuels). www.sciencedirect.com
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284 Energy biotechnology
microalgae strains with reduced antenna size could show high efficiency even at high irradiance, making light dilution less important [41,43].
Conclusions A photo-bioreactor that is cheap enough, energy efficient enough and stable enough to be used for commercial biofuel production over large areas does not presently exist. Nevertheless, the specific problems are addressed and partially solved. In some pilot plants productivities have been reached which were thought some months ago to be possible only in lab scale. A perfect adjustment between algal physiology and sophisticated engineering has the potential to solve the problems in cost and performance. According to the current opinion, it is a question of a few years to show economical feasibility of biofuel production by algae on a whole hectare and during a whole year [42].
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