MINI-REVIEW

Developments and perspectives of photobioreactorsfor biofuel production

Michael Morweiser & Olaf Kruse & Ben Hankamer &

Clemens Posten

Received: 9 March 2010 /Revised: 22 May 2010 /Accepted: 24 May 2010 /Published online: 10 June 2010# Springer-Verlag 2010

Abstract The production of biofuels from microalgaerequires efficient photobioreactors in order to meet thetight constraints of energy efficiency and economic profit-ability. Current cultivation systems are designed for high-value products rather than for mass production of cheapenergy carriers. Future bioreactors will imply innovativesolutions in terms of energy efficiency, light and gastransfer or attainable biomass concentration to lower theenergy demand and cut down production costs. A newgeneration of highly developed reactor designs demon-strates the enormous potential of photobioreactors. However,a net energy production with microalgae remains challenging.Therefore, it is essential to review all aspects andproduction steps for optimization potential. This includesa custom process design according to production organism,desired product and production site. Moreover, the potentialof microalgae to synthesize valuable products additionallyto the energetic use can be integrated into a productionconcept as well as waste streams for carbon supply ortemperature control.

Keywords Biofuels . Light transfer . Mass transfer .

Microalgae . Photobioreactor . Renewable energy

Introduction

Biofuels from microalgae have a great potential to meetfuture challenges of carbon dioxide neutral energy supplyand storage. The climate change and shortage of resourcescall for a substantial change of global power supply fromfossil to regenerative energy sources. For electricitygeneration, powerful techniques such as wind, photovoltaicor geothermal energy exist already today. However,currently, electricity accounts only for a minor fraction ofthe global energy demand. Two thirds of the world’s finalenergy consumption are covered by oil, coal and gas (IEA2009), and even in the long term electricity will notdisplace fuels entirely. Thus, both biofuels and renewableelectricity generation are required in future energy supply.

While it is generally proven to be possible to producefuels from algae cultures, process development is still atearly stage. Several pilot-scale plants have been success-fully tested, but in large scale, there is to date no facilitygenerating microalgal biofuels effectively in terms of bothenergy and financial cost. The largest operating plants useopen pond systems to grow algae for food, feed andcosmetic purposes. The productivity of these facilities iscomparatively low, but the simple design and the high valueof the products in the range of up to several thousanddollars per kilogram make these processes profitable.Producing biofuel with microalgae requires cultivationfacilities with much higher productivities for two reasons:First, the product value of biofuel is dictated by thecompeting fossil-based fuels and therefore several ordersof magnitude below today’s algae products. Secondly, in

M. Morweiser :C. Posten (*)Division of Bioprocess Engineering,Institute of Engineering in Life Sciences,Karlsruhe Institute of Technology,76131 Karlsruhe, Germanye-mail: clemens.posten@kit.edu

O. KruseDepartment of Biology, AlgaeBioTech Group,University of Bielefeld,Bielefeld, NRW 33501, Germany

B. HankamerInstitute for Molecular Bioscience, The University of Queensland,St. Lucia, Qld 4072, Australia

Appl Microbiol Biotechnol (2010) 87:1291–1301DOI 10.1007/s00253-010-2697-x

order to have any ecological benefit, generating biofuelsmust involve a positive energy balance, other than for foodor value products. The performance of standard cultivationsystems cannot meet the requirements for an energy-efficient production. This article summarizes recent advan-ces in next-generation bioreactor design and points outfurther fields of development.

The term biofuel is often directly associated to biodiesel.However, microalgae offer the possibility to produce avariety of compounds that can serve as energy carriers(Fig. 1). The choice of the product is of high importance, as itdirectly influences the energy balance of the process. While inthe downstream process, gaseous products can be harvestedquite easily and cheaply, intracellular products have to beharvested by energetically costly solid/liquid separation andextraction. However, more consideration is needed for theefficiency of the cells as such. While some authors consideronly photosynthesis for calculation of theoretical yields, thedifferent products differ in terms of ATP used for theirsynthesis with a given efficiency of each metabolic step(Tredici 2010). Turnover of the macromolecules and thephysiological state are other issues influencing efficiency ofgrowth processes (Langner et al. 2009).

For biodiesel production, the lipid fraction is separatedfrom the residual biomass. Liquid solvent extraction (Chisti2008a; Lardon et al. 2009) as well as mechanical methods(Greenwell et al. 2010) are proposed. Since chemicalcomposition of oil generated from biomass is differentfrom crude oil properties, chemical refining must beperformed to obtain the final product (Huber et al. 2006).The residual biomass contains polysaccharides and proteinsthat may also be recovered.

Biogas can be produced either chemically similarly tocoal gasification (Huber et al. 2006) or biologically in ananaerobic fermentation process (Gallert and Winter 2002),where several steps of bacterial degradation of biomassresult in methane and CO2 as final gaseous products. Nocell disruption is required for product recovery, but thevalue of the remaining synthesized organic compounds islost during gasification.

Some microalgae strains are able to produce hydrogenunder anaerobic conditions (Gaffron and Rubin 1942). In atwo-stage process (Melis et al. 2000), H2 can be synthe-sized photosynthetically directly in the photobioreactor.Main advantages are the fact that the energy for the productis coupled out at an early stage of the metabolism, that theproduct separation is simple and that cells remain intact forbiomass recovery or other purposes. Although considerableimprovements in H2 production have been achieved (Kruseet al. 2005), efficiencies are to be further enhanced.

Polysaccharides could be another basis for the productionof biofuels. Especially red microalgae produce a mixture ofsulphated extracellular polysaccharides (Geresh and Malis1991) with possible industrial applications (Gasljevic et al.2009). Since the product is excreted from the cells,harvesting does not require cell disruption and may becarried out continuously (Fleck-Schneider 2004).

This variety of products indicates that reactor design mustrespect the properties of different process options, probablyresulting in a further customization and different reactortypes according to their target product. This review reflectsthe current state of photobioreactor development with focuson the mentioned constraints of biofuel production.

Requirements for cell growth

Photoautotrophic organisms generally need two differentsources to satisfy their main requirements: light for energysupply and usually CO2 as a carbon source. The abundanceof both is a crucial factor in photobioreactor design. Sincemicroalgal cultivation for energy purposes implies sunlightas the only source of energy, discussions about efficiencyare mainly focused on light as the limiting factor.

Theoretical efficiency

Generally, only a fraction of the energy of sunlight can beused to build up biomass and derived products. A measureof efficiency in this respect is the photon conversionefficiency (PCE), reflecting the proportion of incident solarenergy usable for the organism to build biomass. Severalfactors limit the PCE, some of which are due to physicalnature of light, others inherent in the principle ofphotosynthesis. One part of the energy is lost just bypassing the atmosphere. The remaining portion containslight of a wide spectrum, ranging from infrared to UV, butphotosynthesis is limited to wavelengths between 400 and700 nm (photosynthetically active radiation). Reflection atthe surface and losses at the pigments as fluorescence andheat diminish the PCE to 12.6% (Zhu et al. 2008).Additionally, respiration must be taken into account, withvalues varying around 25% of photosynthetic oxygenFig. 1 Possible routes to energy products

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evolution (Falkowski and Owens 1978; Harris and Piccinin1983). The formation of cellular compounds from thephotosynthetic products diminishes the yield further,resulting in a maximum PCE of roughly 9%.

It is important to note that the PCE reflects theproportion of energy stored as chemical energy in the totalbiomass, not the energy fraction accessible via a certainmetabolite. Efficiency of biofuel production depends on thedesired product and the energy demand for its formationinside the cell. The more metabolic steps it takes to producea certain compound, the lower the efficiency in respect tosunlight utilization. The PCE is important for bioreactordesign because it is the measure for the ability of the reactorto support algae growth in an optimal way. It defines thelimit for energy harvesting, which has to be remarkablyunderrun by the auxiliary energy demand of the reactor.

Kinetics and light distribution

Depending on incident light intensity, microalgae may be ina light-limited, light-saturated or light-inhibited state. Underlight-limited conditions the specific growth rate µ usuallycorrelates linearly to light intensity. Light saturation lowersor even halts the increase of µ with higher light intensities,therefore resulting in lower PCE values. Light inhibitionrefers to a state where an increase of incident light leads toa decrease of the specific growth rate. Consequently, thebest operating point of a photobioreactor would be in thehigh light-limited region with comparatively high specificgrowth rates and a favorable PCE (for measurement data,see Fig. 2). However, solar irradiation is strongly dependenton geographical and climatic conditions; in any case, it isnever constant. Firstly, the angle of the incoming radiationdetermines the photon flux density at the earth’s surface.This angle is a function of latitude but also varies over timeof day as well as over the seasons of the year. Secondly,climatic conditions, mainly the formation of clouds,influence the total amount of photons available at a certain

site. Thus, even at the same spot, light intensity varies in abroad range governed by a mixture of cyclic and randomevents. An overview over the global conditions for algaegrowth can be found in a review by Tredici (2010). Inaddition, the optimum light intensity is quite variableamong described strains (Halldal and French 1958; Sorokinand Krauss 1958). In order to ensure an efficient operation,reactor design must therefore be tailor-made respecting notonly properties of the operation site, such as radiationvalues, but also the requirements of the microalgae; thereactor must be “built around the algae”.

Inside the biosuspension, mutual shading of the cells leadsto an exponential decline of light intensity from the surface tothe center of the reactor. With increasing cell densities, thisproblem becomes more fundamental, resulting in dark zoneswith low PCE values. A high surface-to-volume ratio (SVR)and short light path length reduce this negative effect.

Flashing light effect

Furthermore, adequate mixing can counterbalance the lightgradient problem. If cells undergo alternating bright and darkcycles, the net irradiance for the cell population may bemaintained at an equal level below the incident illumination.However, the frequency of these cycles strongly influencesthe specific growth rate. Slow cycles in the range of secondsdiminish µ severely (Fig. 2), even below the value expectedfor the same net irradiance applied as continuous light. Onthe other hand, data of lab-scale experiments show that fastcycles in the range of milliseconds can lead to higherspecific growth rates than under continuous illumination atthe same net intensity. Of course, these very short timeconstants, known as the “flashing light effect”, can only beestablished at the expense of more mixing energy, so bothaspects should be traded off against each other thoroughly.

Carbon dioxide

The second main factor in microalgal cultivations besideslight is carbon supply. The photosynthetic activity definesthe CO2 demand of the cell population. With an intracel-lular carbon fraction of at least 0.45 and CO2 being the onlycarbon source, a minimum of 1.65 g CO2 per gram ofbiomass must be provided; for oil-rich algae, this value cango up to 3 g CO2/g biomass. Besides this stoichiometricaspect, a kinetic aspect has to be considered. Most algaestrains can take up enough CO2 only at a minimum partialpressure of 0.1–0.2 kPa in the fluid phase (Doucha et al.2005). Higher values can be necessary at high lightintensities or to support product formation (Yoo et al.2010). The partial pressure of CO2 in the atmosphere is at0.04 kPa, which indicates that pure air is not sufficient forCO2 supply; an enriched gas mixture is required. Also, the

Fig. 2 Growth kinetics of Porphyridium purpureum under continuouslight (●) and light–dark cycles (■)

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mass transfer from the CO2-enriched gas phase to themedium must be ensured. A large gas/liquid interface, i.e.small gas bubbles, good mixing and high CO2 concen-trations favor the CO2 transport into and the O2 transportout of the liquid phase.

Designs of photobioreactors

General principles

For many algae strains, peak sunlight intensities as presentat the reactor surface are too high and lead to lightinhibition. In vertical reactor installations, these highirradiations can be reduced by an adapted distance betweenadjacent reactors with a certain degree of mutual shading(Chini Zittelli et al. 2006; Rodolfi et al. 2009). Additionally,vertical reactor arrangements can be installed in north–southorientation, instead of facing sunlight directly. This way thelow-intensity light in the morning and evening hours hits thereactor surface in a favorable angle, whereas at noon the partfacing direct sunlight is reduced, therefore diminishing thepeak intensity applied to the algae. Another way to overcomethis problem is “diluting” the excess irradiation by increasingthe inner transparent surface of the reactor. Simple con-structions use curved or edged wall panels for this purpose,whereas more elaborate designs employ complex light-conducting structures from the surface to deeper regions ofthe reactor. Typical values for light dilution factors are 5–10,but in some thin film designs even higher. An extreme designis the concept to capture all light for cultivation with a lenssystem and use light-conducting structures to illuminate aremote reactor (Zijffers et al. 2008), which combines goodillumination with flexibility in reactor geometry, while thelight conduction system accounts for additional cost andcomplexity of the system.

The main design principle for efficient photobioreactorsis a favorable SVR as a requirement for high PCE values.This includes short light path lengths, which can be attainedby various geometries and a low water coverage (amount ofmedium per foot print area) to minimize mixing energy.Three most important geometric designs are given in Fig. 3.

For achievement of high areal productivities, often verticalor stacked arrangements are applied. A major differenceconsists in the way the energy for mixing is provided.

Bubble column and airlift reactors

In these types of reactors, mixing energy is provided by thegas intake, thus combining aeration and dispersion. Gener-ally, the reaction volume is sparged from the bottom.Integrating a designated downcomer region may result in acircular plug flow regime, which enhances axial dispersion.Geometry options are flat plate reactors, columns, dome-shaped or annular reactors. To reach a high SVR, thesereactors have a small footprint area, combined with avertical or inclined setup. Dome-shaped and annularreactors have a reduced volume with an additional internalsurface, thereby avoiding dark zones with low productivity.

The flat plate reactor is the most common design as itcombines high SVR with a simple setup. An example is theso-called green wall panel (Tredici and Rodolfi 2004),which is in principle a plastic bag shaped by a wire netting(Fig. 4). It can easily be scaled up in horizontal direction orby up-numbering in parallel fences. Although CO2 supplycan be obtained with moderate aeration, bubbling is a quiteexpensive way of mixing, so the amount of auxiliary energyfor aeration contributes to the overall energy balanceremarkably as can be seen from Table 1.

Tubular reactors

Tubular reactors consist of long transparent tubes of smalldiameter, often mounted as parallel loops on a rigidscaffold. Pumps provide for a circulating longitudinal plugflow along the tube loop. The circular profile of the tubesleads to a light concentration effect counterbalancing

Fig. 3 Basic reactor designs: (a) flat plate reactor; (b) annular reactor;(c) tubular reactor Fig. 4 Green Wall Panel, Almeria, Spain

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mutual shading; SVRs can exceed 100 m−1. This results invery high biomass concentrations of up to 6 g/L. Conversely,the large inner surface leads to a high energy demand byfriction of more than 500 up to 2000 W/m3 pumping energy(Table 2). Although this reactor type is quite effective—asthe world’s largest closed photobioreactors (1.2 ha) locatedin Klötze near Wolfsburg, Germany—it is too expensive andneeds too much auxiliary energy for pure biofuel production.So it is not further considered here.

Advanced designs

New developments seek to combine high productivity andlow auxiliary energy demand with low cost criteria forlarge-scale application. An improved airlift design ispresented by Subitec, Stuttgart, Germany. The Flat PanelAirlift reactor (FPA, patent no. EP 1 169 428 B1 and EP 1326959 B1) has been developed for large-scale outdoorpurposes with a reactor volume of 180 L. Two syntheticfilm shells, structured by deep drawing, are welded togethertherefore making an industrial large-scale productionpossible (Fig. 5). Gas spargers lead to an upward movement

in several parallel chambers, while baffles included in thepanel wall induce defined vortices and lead to improvedlight penetration of the fluid phase (Degen et al. 2001).Downcomers of small diameter in the same plane ensureshort dwell periods outside the baffled compartments. Theadvantages of this system are clearly the low-cost design,good mixing, utilization of the “flashing light effect” andshort light paths without any unexposed zones. The energydemand is stated to be reduced below 200 W/m3 (Subitec,2010). A pilot plant is already in operation. Further costreduction should be approached by saving costly stillagesof the single modules. Furthermore, the specific cost forCO2 supply by bubbling can be cut by reducing the heightof the single plates leading to low-ceilinged designs.

Solix Biofuels (Fort Collins, CO, USA) have developed aseries of water-embedded reactor systems (Fig. 6). The latestoperating version consists of synthetic bags floating partiallysubmerged in an artificial pond, with the surrounding wateracting as a scaffold, temperature regulation and light diffuserat the same time. Currently, the gassing system consists of anintegrated sparger tube running along the lower seam of thebag. In the future, it is due to be replaced by a membrane

Table 1 Operating data for bubbled and airlift reactors

Reactor type and reference

Reactorvolume (m3)

Width/height/length (m)

Gassing rate(v/v/m)

Liquid velocity(m/s)

Mixingtime (s)

kLa value (1/s) Lateral dispersioncoefficient (m2/s)

Specific powerinput (W/m3)

Flat plate (low values) (Sierra et al. 2008)

0.25 0.07/1.5/2.5 0.15 0.00357 105 0.0025 0.026 15

Flat plate (high values) (Sierra et al. 2008)

0.25 0.007/1.5/2.5 0.32 0.0076 150 0.0063 0.012 53

Bubble column (Camacho Rubio et al. 2004)

0.06 0.193/2.3 – 0.03 60 – 0.025/0.0003 –

Inclined airlift (Merchuk et al. 2007)

– 0.14/2.12 – 0.01 – 0.004 – –

Annular reactor (Chini Zittelli et al. 2006)

0.12 0.045/0.5/1.9 0.23 0.007 – 0.010 – –

Table 2 Operating data for tubular reactors

Reactor type and reference

Reactorvolume (m3)

Length/ innerdiameter (m)

Liquid velocity(m/s)

Residencetime (s)

Specific powerinput (W/m3)

Lateral/radialdispersioncoefficient (m2/s)

kLa value (1/s) Maximum oxygenconcentration (%)

Horizontal tubular, airlift (Acien Fernandez et al. 2001)

0.200 80/ 0.06 0.5 – – 0.005/ ? 0.007 300

Helical airlift (Hall et al. 2003)

0.075 106/ 0.03 0.28 – 2000 0.006 0.003 280

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gassing system. Solix claims peak oil production rates of2,000 gallons/(ac ∙ year) (18,700 L/[ha ∙ year)) and expectsa 2.5- to 4-fold increase for production sites (Solix-Biofuels2009).

A similar idea has been followed by Proviron (2010).Each reactor module is one big translucent plastic bag,which contains multiple vertical panes of 1 cm thickness(Fig. 7). The reactor can be unrolled from a big coil withoutany additional supports. The company claims to reach biomassconcentrations up to 10 g/L due to this small light path length.The investment is stated to be 200 k€/ha (=20 €/m2) and theneed for auxiliary energy 20 kW/ha (=2 W/m2). This wouldbe approximately 50% of the cost and energy of thetheoretical limits of economic viability in Central Europe.Even lower values are prospected. A pilot plant startedoperation, and a larger facility is planned. Productivity dataare not yet available.

Process management

As the reactor is the hardware part of the process, onlysophisticated operation makes it viable at the end. Processmanagement offers several options to further improve theperformance of the system and lower energy demands.

Temperature control

The impact of temperature control on the energy balance ofthe process is highly dependent on the applied reactorsystem, algae strain, but most of all the operating region ofthe plant. At warm, highly irradiated sites like southernUSA or Australia, cooling of the cultures is likely tobecome a critical parameter of the process. Whether thisproblem is tackled by direct evaporation or a closed cooling

system, excess heat must be actively taken out of thesystem, adding to the energy demand of the process.Spraying the outer wall of the reactor with water is ameans but requires the availability of cooling water.

One way to reduce the heating problem is the avoidanceof IR radiation. This part of the sunlight spectrum makes up40% of the total energy without being used by the algae.IR-reflecting glass or plastic is already available (Hollandand Siddall 1958) and is used to reduce heat in parked carsor to reduce heat radiation from lightbulbs.

Heating in spring is another option discussed especiallyin Central Europe, where the irradiance of the sun is alreadyat a remarkable level, but outdoor temperatures are too lowfor sufficient cell growth. Here, it has been stated that low-temperature heat like cooling water from power plants is inprinciple available and could be used for heating of thecultivations. In the “water bed reactors” mentioned above,at least temperature fluctuations between day and night canbe compensated by the amount of water around the growthchambers, which exceeds the usual values in open ponds.This concept could be even sharpened by employing the so-called phase changing materials. These materials arecommercially produced in wallpapers for flats to controlthe room temperature at the given value of phase transition.For photobioreactors, that means temperature control notonly at the day/night average but also at an adjustablevalue.

Feeding strategy

Photobioreactors usually are operated as batch or sequentialbatch (semi-continuous), where harvesting is done prefer-ably at the afternoon. Maximum biomass concentrationwith highest mutual shading is reached with highestirradiation during daytime, while lowest biomass concen-

Fig. 5 Flat panel airlift reactor(Subitec): (a) outdoor pilotplant; (b) induction of vorticesby inbuilt baffles; (c) newreactor module

Fig. 6 New pilot reactor fromSolix Biofuels: (a) low-ceilingeddesign; (b) water as support

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tration in the night leads to lowest biomass loss byrespiration. It is an underestimated fact that duringcontinuous cultivation, the total productivity is given by thePCE value and the local irradiation, leading to low dilutionrates for high cell concentrations (PCE ∙ I∼D ∙ cX). This has animpact on strain selection because not only high maximumgrowth rates are required but strains with high PCE at lowgrowth rates could also be successfully cultivated.

Sunlight is only available at daytime, at broadly varyingintensities. An option to reduce the energy intake is to couplegassing and consequently mixing to the photosynthesis rate(Buehner et al. 2009), since the CO2 demand is proportionalto the photosynthetic activity. At night, however, there is nophotosynthesis at all, and consequently the energy input forgassing can be reduced to the absolute minimum necessaryfor oxygen supply necessary for respiration.

Minimum requirements for the medium compositionarise from the elemental balances, e.g. for nitrogen orphosphate. An intentional low nitrogen level prohibits theformation of proteins and nucleic acids forcing somemicroalgal strains to store CO2 and light energy as lipids.It must further be stated that the usual mineral media for labuse are not suitable to reach high biomass concentrations.This leads to very high salt concentrations that aresubjected to precipitation or even growth inhibition. Inthese cases, a fed-batch-like additional dosing of singlemedium compounds during the growth process is required.However, to these points, not much quantitative scientificdata or practical experiences are published.

Measurement and control

The photobioreactor has to provide ideal conditions for themicroalgal cells with respect to a desired physiological stateunder the constraints of incoming light or other givenexternal parameters. This can be done by measurement ofphysical conditions inside the medium and controllingtechnical variables like gas supply. Two pO2, pH andpCO2 sensors along the main reactor axis—this meansalong the strongest mass transfer gradient—should bemandatory. However, this topic is a bit neglected in current

installations. As the cells are the only reasons formaintaining the process, online measurement of opticaldensity and fluorescence pulse amplitude modulated flu-ometry (PAM) can help to assess the physiological state andreact with online optimization of mixing, gassing ordiluting.

Process integration and assessment

Process integration

While the photobioreactor has to maintain optimum con-ditions for the cells on the one hand, it has to cope withmass and energy flows available at a given site on the otherhand. Cost savings can be obtained, e.g. by using nutrientsfrom local waste water streams or CO2 from chemicalplants, while the produced O2 is possibly a marketableproduct. In any case, reactor design and operation strategydepend on these circumstances. In the sense of anintegrated process, development problems in one stage ofthe process could be tackled in another stage. Examples foralgal cultivations, where technical problems can be solvedon the biological side, are the reduction of the cellularantenna size to reduce the problem of self-shading and lightinhibition (Mussgnug et al. 2007). Conversely, reactordesign contributes to the biological problem of lightsaturation as shown above. Similarly, problems in down-stream processing can be addressed by a better choice ofcell strains and physiological state of the cells and by animproved reactor design. Furthermore, the whole processhas to fit into an ecological, economical and social scenario.

CO2 supply

As stated before, the concentration of atmospheric CO2 isfar too low to serve as carbon source in efficient algaecultivation. Nevertheless, there are plenty sources forhigher concentrated waste CO2 streams; practically anycombustion plant emission contains the required concen-tration, and usually exhaust gas CO2 concentrations exceed

Fig. 7 New pilot reactor fromProviron, Belgium

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5% (Negoro et al. 1991, 1993; Doucha et al. 2005). NOx

from fuel gases are reported to be used by the algae asnitrogen source. Here the quality of the exhaust gas is ofmajor importance. While the combustion of natural gas isreported to be suitable for algae CO2 supply, elevated levelsof sulfur and nitrogen oxides as from coal-fired powerplants could be damaging (Maeda et al. 1995) and thereforerequire elaborate gas purification, which would add to theenergy balance of the algal process. Additionally, althoughhighly abundant, CO2 from fossil energy sources preventsthe biofuel process from being CO2 neutral, which could beavoided by the use of CO2 from combined heat and powerunits fueled by spare wood or biogas. In both cases,suitable sites for such plants may be a question: Largepower plants usually exist in densely populated areas withexpensive land prices, whereas most wood- or biogas-firedplants are located in rural areas with fertile soil which mayrather be used for agriculture than for algae production.

Membrane gassing

Membrane gassing is one concept to reduce auxiliaryenergy similarly as it is done in animal cell cultures, inthat case to reduce shear effects of the bubbles to the cells.Dissolving of CO2 is no longer effected at the boundarylayer between gas bubbles and the culture medium andgassing and mixing are decoupled, which gives moredegrees of freedom in process operation. Instead, a gas-permeable membrane creates the surface for CO2 dissolvingand O2 removal directly at the gas intake. This saves theenergy for the formation of gas bubbles, which is lost whenthe bubble collapses at the top of the reactor. It also lowersthe loss of CO2 via the off-gas, resulting from thehomogenous gas supply over large parts of the reactorsurface. However, by using membrane gassing, gas disper-sion from the membrane to the opposite side of the reactorwill still require soft agitation. So far, no general designprinciple for membrane gassing is established. Employmentof modern CFD tools can lead to further optimization ofenergy demand (Perner-Nochta and Posten 2007).

Solid–liquid separation

For further processing, the algae suspension has to beseparated from the medium in most cases. For solid–liquidseparation, two general principles exist: filtration- andsedimentation-based methods. Sedimentation separates par-ticles according to a difference in specific density, while forfiltration, particle size and surface play an important role.

Solid–liquid separation of organic matter is alreadystandard in large scale. Baker’s yeast [world annual produc-tion 2 Mio. t (Fischer and Rahn 2004)] is concentrated withseparators and filter units to dry mass contents up to 35%.

Also, green algae for food are harvested and dewatered byscreens and spray-drying (Earthrise-Nutritionals 2009). Butother than for food or high-value products, for biofuelproduction, constraints are much tighter in respect to energydemand for the separation process, while standard techniqueshave been highly developed in a way that it is unlikely toachieve a major drop in the energy demand of the separationprinciple as such.

Employing centrifugation, the rotational speeds suffi-cient for algae separation in a small-scale disk stackcentrifuge consumes approximately 5 kWh/m3 at a flowof 1 m3/h (manufacturer communication), scale up maylower the energy demand to 1–3 kWh/m3 (Molina Grima etal. 2003; Schenk et al. 2008). Less energy input cannot beexpected; however, an increase in biomass concentration inthe culture medium can improve the efficiency for the algaeprocess substantially.

Filtration requires less power than centrifugation, but thesuccess of filtration of microalgae depends strongly on theproperties of the strain (Molina Grima et al. 2003; Schenket al. 2008) and the type of membrane (Rossignol et al.1999). Membrane fouling, pressure drop and the requiredfilter surface must be considered when applying filtrationfor microalgal separation.

From these facts it is obvious that the solution for theenergy problem cannot be found in developing newseparation techniques but in increasing the biomassconcentration in the photobioreactor. Maximum biomassconcentrations in an open pond system reach 1–2 g/L. Evenin best cases, this means 20% of the energy content of theseparated algae is used for the first separation step in thecase of centrifugation. In closed bioreactor systems, 10 g/Lbiomass are already attained or at least projected. Besidesthe energetic advantages during cultivation, this wouldreduce the amount of water going to the separation unitremarkably with the corresponding energy savings.

Process evaluation

Considerable effort has been put into economic andenvironmental evaluation of algae biofuel processes. Sincedata from large-scale production facilities are unavailableyet, a fundamental assessment on the viability of an algaebiofuel process cannot be expected from current studies.While it is indispensable for bioprocess design to definecertain benchmarks and to identify possible bottlenecks, thevariance of basis data leads to significantly differentoutcomes of recent analyses. This has led to livelydiscussions whether algae biofuel will ever be competitive,either economically or ecologically.

Critics argue that investment and operating costs exceedbenefits from energy production by far (Steiner 2008). Evenafter rather optimistic estimations, algae biofuel production

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will require a value co-product in order to be economicallyfeasible, at least for the near future (Stephens et al. 2010).However, economic constraints are subject to substantialfluctuations, as crude oil price, material and labor cost have adirect influence on the process; therefore, the current marketsituation does not exclude a viable technique permanently.

The fundamental criterion whether an algal biofuelproduction is a reasonable option for future energy supplyis the ecological impact of the process. If a substantial netenergy gain should remain unattainable, the basis for algalbiofuel production would be missing. Thus, net energyproduction potential is a major focus of process assessments,but also the ecological benefit compared to crop-basedbiofuel processes.

In respect to the latter aspect analyses, results are verydiverse (Chisti 2008a, b; Reijnders 2008; Clarens et al.2010). While the required energy input of algal cultivationshould not be underestimated, the general problem withcrop-based biofuel production is the demand for arableland, which may displace terrestrial plant biofuel regardlessof its energy balance.

Regarding areal productivity, Weyer et al. (2010) havepresented a maximum range for algal oil production basedon irradiation and cultivation data, combined with severalassumptions concerning photosynthetic rates. Their best-case scenario reveals areal oil productivities of approxi-mately 50 m3/(ha ∙ year) at irradiation levels in the range of6,000 MJ/(m2 ∙ year) and 50% oil content. Jorquera et al.(2010) projects 32 m3/(ha ∙ year) with a 30% oil contentusing flat plate bioreactors. Stephens et al. (2010) predict60–100 m3/(ha ∙ year) at 8000 MJ/(m2 ∙ year) and an oilcontent of 25–50%, a factor 1.5 compared to the reasonablemaximum according to Weyer, respecting the difference inirradiation. Even though each study has its applied datawell documented, the multiplication of assumed terms andconstants originating from different sources leads to a majordifference in projected areal productivity, although this is afactor where large-scale data are available, at least for openpond systems.

An assessment of downstream processing is even morecomplicated since a common standard procedure fordewatering, oil extraction and residual biomass processingis neither established nor have reliable data been publishedwith real algal biomass from a large-scale facility withrespect to biofuel production. Separation techniques cur-rently applied in algae biomass production are very energyconsuming, contributing mainly to the fact that today’smicroalgae production facilities are net energy negative.Alternative strategies like flocculation or sedimentation(Lardon et al. 2009) have not been evaluated yet in largescale, at least not in the context of biofuel production,repeatedly and with designated production strains. Forproduct recovery (Lardon et al. 2009) and refining (Greenwell

et al. 2010), existing methods from terrestrial crops may betransferrable, but for an estimation in respect to energyconsumption, there are too little data accessible. Neverthe-less, balancing only the biomass growth without downstreamprocessing (Jorquera et al. 2010) for production systemevaluation may be a misleading approach since variationsin biomass concentration resulting from different bioreactortypes directly influence energy expenses for dewatering.With downstream processing probably accounting for amajor fraction of the overall energy demand, a generalassumption regarding the net energy ratio remains specu-lative. The most valuable information at this stage ofprocess development may therefore be outcomes ofsensitivity analyses for a defined hypothetical process.Stephens et al. (2010) have found biomass productivity tobe the major influence on process viability, while expensesfor labor, power and maintenance do not play a dominantrole in their assessment.

Energy balance

Unless combined with the production of substantial massesof high-value products, microalgal culture for an entirelyCO2 neutral biofuel generation imperatively requires a netenergy production, including all steps of the productionprocess. The gross energy must be provided entirely byincident solar irradiation, which in central Europe accountsfor 1000 kWh/(m2 ∙ year)=114 W/m2; world peak valuesreach 2500 kWh/(m2 ∙ year)=285 W/m2. Even with themaximum PCE of 9%, the best areal continuous poweroutput of an algae plant would be 10.3 W/m2 for centralEurope and 25.7 W/m2 best site values, subject to thecondition that all the energy chemically stored in algaebiomass is accessible for an energetic use. For biodieselproduction, these values must be multiplied by the fractionof processable oil content. However, assuming constantPCE, a higher oil content means less dry weight forthermodynamic reasons (calorific value of oil-rich algae is30 MJ/kg, whereas oil-poor algae have 20 MJ/kg).Furthermore, a long residence time can push up the oilcontent but on cost of the energy balance.

Any energy term necessary for running the plant must besubtracted from these values. This includes auxiliary energylike pumping and gassing, temperature regulation, nutrientsupply, product extraction and refinement as well as waterand nutrient recycling. One essential field of work will be afurther reduction of the auxiliary energy demand. As mostof the auxiliary energy input is volume dependent, itbecomes clear that the photobioreactor should have lowwater coverage, as little medium as possible per squaremeter footprint. The best reactors in this concern showvalues at about 40 L/m2. Integrating the above-mentionedreactor improvements can reduce the total energy demand

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of the process, while the maximum output is limited bymaximum PCE value. The best reactors with respect toenergy efficiency (e.g. the different “water bed” designs)have now come into the range of 50% need of auxiliaryenergy in relation to the collected sun energy by reductionof the auxiliary energy to 50 W/m3 (or 2 W/m2 with0.04 m3/m2) as postulated by Posten (Posten 2009).

Discussion

Today, algae biofuel production as a single product is not atthe same time energetically and economically profitable.The crucial step to be taken in the direction of a large-scalebiofuel production is the development of cheap andefficient bioreactors. Several promising concepts have beenevaluated and will give further insight whether higherproductivities are possible. However, irradiation and pho-tosynthetic efficiency set an upper limit to the energy yield,which necessitates the whole process to be reviewed interms of energy savings potential. Actually promisingstarting points on the reactor and on the cell level havebeen proposed. As already mentioned, membrane gassingcould be a solution for the bubbling problem, which impliesshort path lengths from the membrane surface to the rest ofthe reactor volume. Combined with an ultra-low-ceilingeddesign, the requirements for light dilution result in ahorizontal plate with structured upper surface and a gas-permeable membrane at the bottom side. Mixing must beprovided by a moderate induced horizontal convection toensure equal light and nutrient conditions and preventsettling of the culture. Although employing a horizontalplate configuration, the general principles are similar tothose of the latest reactor generation, as light distribution isalso effected by vertical light guidance to deeper reactorregions, however by the reactor surface instead of thewater-filled clear spacing between reaction compartments.This design would allow extremely short light paths withwell-distributed incident light, no additional support struc-tures and an uncoupling of mixing and gas supply.However, the approach would require rigid surfaces andan additional energy input for mixing, so it remains to bedetermined whether such a reactor would perform betterthan the existing designs.

With the latest generation of photobioreactors, we haveseen substantial improvement concerning efficient lightutilization, area footprint and installation cost reduction.Light path lengths of only a few centimeters enable thesereactors to produce high total biomass concentrations whilethe use of plastic material led to higher flexibility and lowermaterial cost. Interestingly, even though many geometryoptions are possible, the vertical plate-like layout is acommon feature in these late developments. The Solix and

Proviron setups employ a low-ceilinged design, whichhelps to reduce gas pressure drop together with water assupport and for temperature balancing. Whether it is morefavorable to take advantage of evaporation for cooling andaccept a higher water demand as in Solix’ concept orwhether the enclosed water bed in the Proviron design ismore practical and sufficient for tempering remains to beshown and may essentially depend on the operating site.The fundamental problem, however, is still the demand forauxiliary energy, which requires the entire process fromupstream of the bioreaction stage to the downstreamprocesses to be critically reviewed and checked for energysavings potential. Valuable byproducts and energy wastestreams may be necessary to make microalgae biofuelproduction viable. However, combining all of the recentresults shows the great potential of algae biofuel for futureenergy supply.

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