Applied Microbiology and Biotechnology

, Volume 87, Issue 4, pp 1291–1301 | Cite as

Developments and perspectives of photobioreactors for biofuel production

  • Michael Morweiser
  • Olaf Kruse
  • Ben Hankamer
  • Clemens Posten
Mini-Review

Abstract

The production of biofuels from microalgae requires efficient photobioreactors in order to meet the tight constraints of energy efficiency and economic profitability. Current cultivation systems are designed for high-value products rather than for mass production of cheap energy carriers. Future bioreactors will imply innovative solutions in terms of energy efficiency, light and gas transfer or attainable biomass concentration to lower the energy demand and cut down production costs. A new generation of highly developed reactor designs demonstrates the enormous potential of photobioreactors. However, a net energy production with microalgae remains challenging. Therefore, it is essential to review all aspects and production steps for optimization potential. This includes a custom process design according to production organism, desired product and production site. Moreover, the potential of microalgae to synthesize valuable products additionally to the energetic use can be integrated into a production concept as well as waste streams for carbon supply or temperature control.

Keywords

Biofuels Light transfer Mass transfer Microalgae Photobioreactor Renewable energy 

Introduction

Biofuels from microalgae have a great potential to meet future challenges of carbon dioxide neutral energy supply and storage. The climate change and shortage of resources call for a substantial change of global power supply from fossil to regenerative energy sources. For electricity generation, powerful techniques such as wind, photovoltaic or geothermal energy exist already today. However, currently, electricity accounts only for a minor fraction of the global energy demand. Two thirds of the world’s final energy consumption are covered by oil, coal and gas (IEA 2009), and even in the long term electricity will not displace fuels entirely. Thus, both biofuels and renewable electricity generation are required in future energy supply.

While it is generally proven to be possible to produce fuels from algae cultures, process development is still at early stage. Several pilot-scale plants have been successfully tested, but in large scale, there is to date no facility generating microalgal biofuels effectively in terms of both energy and financial cost. The largest operating plants use open pond systems to grow algae for food, feed and cosmetic purposes. The productivity of these facilities is comparatively low, but the simple design and the high value of the products in the range of up to several thousand dollars per kilogram make these processes profitable. Producing biofuel with microalgae requires cultivation facilities with much higher productivities for two reasons: First, the product value of biofuel is dictated by the competing fossil-based fuels and therefore several orders of magnitude below today’s algae products. Secondly, in order to have any ecological benefit, generating biofuels must involve a positive energy balance, other than for food or value products. The performance of standard cultivation systems cannot meet the requirements for an energy-efficient production. This article summarizes recent advances in next-generation bioreactor design and points out further fields of development.

The term biofuel is often directly associated to biodiesel. However, microalgae offer the possibility to produce a variety of compounds that can serve as energy carriers (Fig. 1). The choice of the product is of high importance, as it directly influences the energy balance of the process. While in the downstream process, gaseous products can be harvested quite easily and cheaply, intracellular products have to be harvested by energetically costly solid/liquid separation and extraction. However, more consideration is needed for the efficiency of the cells as such. While some authors consider only photosynthesis for calculation of theoretical yields, the different products differ in terms of ATP used for their synthesis with a given efficiency of each metabolic step (Tredici 2010). Turnover of the macromolecules and the physiological state are other issues influencing efficiency of growth processes (Langner et al. 2009).
Fig. 1

Possible routes to energy products

For biodiesel production, the lipid fraction is separated from the residual biomass. Liquid solvent extraction (Chisti 2008a; Lardon et al. 2009) as well as mechanical methods (Greenwell et al. 2010) are proposed. Since chemical composition of oil generated from biomass is different from crude oil properties, chemical refining must be performed to obtain the final product (Huber et al. 2006). The residual biomass contains polysaccharides and proteins that may also be recovered.

Biogas can be produced either chemically similarly to coal gasification (Huber et al. 2006) or biologically in an anaerobic fermentation process (Gallert and Winter 2002), where several steps of bacterial degradation of biomass result in methane and CO2 as final gaseous products. No cell disruption is required for product recovery, but the value of the remaining synthesized organic compounds is lost during gasification.

Some microalgae strains are able to produce hydrogen under anaerobic conditions (Gaffron and Rubin 1942). In a two-stage process (Melis et al. 2000), H2 can be synthesized photosynthetically directly in the photobioreactor. Main advantages are the fact that the energy for the product is coupled out at an early stage of the metabolism, that the product separation is simple and that cells remain intact for biomass recovery or other purposes. Although considerable improvements in H2 production have been achieved (Kruse et al. 2005), efficiencies are to be further enhanced.

Polysaccharides could be another basis for the production of biofuels. Especially red microalgae produce a mixture of sulphated extracellular polysaccharides (Geresh and Malis 1991) with possible industrial applications (Gasljevic et al. 2009). Since the product is excreted from the cells, harvesting does not require cell disruption and may be carried out continuously (Fleck-Schneider 2004).

This variety of products indicates that reactor design must respect the properties of different process options, probably resulting in a further customization and different reactor types according to their target product. This review reflects the current state of photobioreactor development with focus on the mentioned constraints of biofuel production.

Requirements for cell growth

Photoautotrophic organisms generally need two different sources to satisfy their main requirements: light for energy supply and usually CO2 as a carbon source. The abundance of both is a crucial factor in photobioreactor design. Since microalgal cultivation for energy purposes implies sunlight as the only source of energy, discussions about efficiency are mainly focused on light as the limiting factor.

Theoretical efficiency

Generally, only a fraction of the energy of sunlight can be used to build up biomass and derived products. A measure of efficiency in this respect is the photon conversion efficiency (PCE), reflecting the proportion of incident solar energy usable for the organism to build biomass. Several factors limit the PCE, some of which are due to physical nature of light, others inherent in the principle of photosynthesis. One part of the energy is lost just by passing the atmosphere. The remaining portion contains light of a wide spectrum, ranging from infrared to UV, but photosynthesis is limited to wavelengths between 400 and 700 nm (photosynthetically active radiation). Reflection at the surface and losses at the pigments as fluorescence and heat diminish the PCE to 12.6% (Zhu et al. 2008). Additionally, respiration must be taken into account, with values varying around 25% of photosynthetic oxygen evolution (Falkowski and Owens 1978; Harris and Piccinin 1983). The formation of cellular compounds from the photosynthetic products diminishes the yield further, resulting in a maximum PCE of roughly 9%.

It is important to note that the PCE reflects the proportion of energy stored as chemical energy in the total biomass, not the energy fraction accessible via a certain metabolite. Efficiency of biofuel production depends on the desired product and the energy demand for its formation inside the cell. The more metabolic steps it takes to produce a certain compound, the lower the efficiency in respect to sunlight utilization. The PCE is important for bioreactor design because it is the measure for the ability of the reactor to support algae growth in an optimal way. It defines the limit for energy harvesting, which has to be remarkably underrun by the auxiliary energy demand of the reactor.

Kinetics and light distribution

Depending on incident light intensity, microalgae may be in a light-limited, light-saturated or light-inhibited state. Under light-limited conditions the specific growth rate µ usually correlates linearly to light intensity. Light saturation lowers or even halts the increase of µ with higher light intensities, therefore resulting in lower PCE values. Light inhibition refers to a state where an increase of incident light leads to a decrease of the specific growth rate. Consequently, the best operating point of a photobioreactor would be in the high light-limited region with comparatively high specific growth rates and a favorable PCE (for measurement data, see Fig. 2). However, solar irradiation is strongly dependent on geographical and climatic conditions; in any case, it is never constant. Firstly, the angle of the incoming radiation determines the photon flux density at the earth’s surface. This angle is a function of latitude but also varies over time of 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 a broad range governed by a mixture of cyclic and random events. An overview over the global conditions for algae growth can be found in a review by Tredici (2010). In addition, the optimum light intensity is quite variable among described strains (Halldal and French 1958; Sorokin and Krauss 1958). In order to ensure an efficient operation, reactor design must therefore be tailor-made respecting not only properties of the operation site, such as radiation values, but also the requirements of the microalgae; the reactor must be “built around the algae”.
Fig. 2

Growth kinetics of Porphyridium purpureum under continuous light (●) and light–dark cycles (■)

Inside the biosuspension, mutual shading of the cells leads to an exponential decline of light intensity from the surface to the center of the reactor. With increasing cell densities, this problem becomes more fundamental, resulting in dark zones with 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 light gradient problem. If cells undergo alternating bright and dark cycles, the net irradiance for the cell population may be maintained at an equal level below the incident illumination. However, the frequency of these cycles strongly influences the specific growth rate. Slow cycles in the range of seconds diminish µ severely (Fig. 2), even below the value expected for the same net irradiance applied as continuous light. On the other hand, data of lab-scale experiments show that fast cycles in the range of milliseconds can lead to higher specific growth rates than under continuous illumination at the same net intensity. Of course, these very short time constants, known as the “flashing light effect”, can only be established at the expense of more mixing energy, so both aspects should be traded off against each other thoroughly.

Carbon dioxide

The second main factor in microalgal cultivations besides light is carbon supply. The photosynthetic activity defines the CO2 demand of the cell population. With an intracellular carbon fraction of at least 0.45 and CO2 being the only carbon source, a minimum of 1.65 g CO2 per gram of biomass must be provided; for oil-rich algae, this value can go up to 3 g CO2/g biomass. Besides this stoichiometric aspect, a kinetic aspect has to be considered. Most algae strains can take up enough CO2 only at a minimum partial pressure of 0.1–0.2 kPa in the fluid phase (Doucha et al. 2005). Higher values can be necessary at high light intensities or to support product formation (Yoo et al. 2010). The partial pressure of CO2 in the atmosphere is at 0.04 kPa, which indicates that pure air is not sufficient for CO2 supply; an enriched gas mixture is required. Also, the mass transfer from the CO2-enriched gas phase to the medium must be ensured. A large gas/liquid interface, i.e. small gas bubbles, good mixing and high CO2 concentrations favor the CO2 transport into and the O2 transport out of the liquid phase.

Designs of photobioreactors

General principles

For many algae strains, peak sunlight intensities as present at the reactor surface are too high and lead to light inhibition. In vertical reactor installations, these high irradiations can be reduced by an adapted distance between adjacent 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–south orientation, instead of facing sunlight directly. This way the low-intensity light in the morning and evening hours hits the reactor surface in a favorable angle, whereas at noon the part facing direct sunlight is reduced, therefore diminishing the peak intensity applied to the algae. Another way to overcome this problem is “diluting” the excess irradiation by increasing the inner transparent surface of the reactor. Simple constructions use curved or edged wall panels for this purpose, whereas more elaborate designs employ complex light-conducting structures from the surface to deeper regions of the reactor. Typical values for light dilution factors are 5–10, but in some thin film designs even higher. An extreme design is the concept to capture all light for cultivation with a lens system and use light-conducting structures to illuminate a remote reactor (Zijffers et al. 2008), which combines good illumination with flexibility in reactor geometry, while the light conduction system accounts for additional cost and complexity of the system.

The main design principle for efficient photobioreactors is a favorable SVR as a requirement for high PCE values. This includes short light path lengths, which can be attained by various geometries and a low water coverage (amount of medium 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 vertical or stacked arrangements are applied. A major difference consists in the way the energy for mixing is provided.
Fig. 3

Basic reactor designs: (a) flat plate reactor; (b) annular reactor; (c) tubular reactor

Bubble column and airlift reactors

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

The flat plate reactor is the most common design as it combines high SVR with a simple setup. An example is the so-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 or by up-numbering in parallel fences. Although CO2 supply can be obtained with moderate aeration, bubbling is a quite expensive way of mixing, so the amount of auxiliary energy for aeration contributes to the overall energy balance remarkably as can be seen from Table 1.
Fig. 4

Green Wall Panel, Almeria, Spain

Table 1

Operating data for bubbled and airlift reactors

Reactor type and reference

Reactor volume (m3)

Width/height/ length (m)

Gassing rate (v/v/m)

Liquid velocity (m/s)

Mixing time (s)

kLa value (1/s)

Lateral dispersion coefficient (m2/s)

Specific power input (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

Tubular reactors

Tubular reactors consist of long transparent tubes of small diameter, often mounted as parallel loops on a rigid scaffold. Pumps provide for a circulating longitudinal plug flow along the tube loop. The circular profile of the tubes leads to a light concentration effect counterbalancing mutual shading; SVRs can exceed 100 m−1. This results in very high biomass concentrations of up to 6 g/L. Conversely, the large inner surface leads to a high energy demand by friction of more than 500 up to 2000 W/m3 pumping energy (Table 2). Although this reactor type is quite effective—as the world’s largest closed photobioreactors (1.2 ha) located in Klötze near Wolfsburg, Germany—it is too expensive and needs too much auxiliary energy for pure biofuel production. So it is not further considered here.
Table 2

Operating data for tubular reactors

Reactor type and reference

Reactor volume (m3)

Length/ inner diameter (m)

Liquid velocity (m/s)

Residence time (s)

Specific power input (W/m3)

Lateral/radial dispersion coefficient (m2/s)

kLa value (1/s)

Maximum oxygen concentration (%)

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

Advanced designs

New developments seek to combine high productivity and low auxiliary energy demand with low cost criteria for large-scale application. An improved airlift design is presented by Subitec, Stuttgart, Germany. The Flat Panel Airlift reactor (FPA, patent no. EP 1 169 428 B1 and EP 1 326959 B1) has been developed for large-scale outdoor purposes with a reactor volume of 180 L. Two synthetic film shells, structured by deep drawing, are welded together therefore making an industrial large-scale production possible (Fig. 5). Gas spargers lead to an upward movement in several parallel chambers, while baffles included in the panel wall induce defined vortices and lead to improved light penetration of the fluid phase (Degen et al. 2001). Downcomers of small diameter in the same plane ensure short dwell periods outside the baffled compartments. The advantages of this system are clearly the low-cost design, good mixing, utilization of the “flashing light effect” and short light paths without any unexposed zones. The energy demand is stated to be reduced below 200 W/m3 (Subitec, 2010). A pilot plant is already in operation. Further cost reduction should be approached by saving costly stillages of the single modules. Furthermore, the specific cost for CO2 supply by bubbling can be cut by reducing the height of the single plates leading to low-ceilinged designs.
Fig. 5

Flat panel airlift reactor (Subitec): (a) outdoor pilot plant; (b) induction of vortices by inbuilt baffles; (c) new reactor module

Solix Biofuels (Fort Collins, CO, USA) have developed a series of water-embedded reactor systems (Fig. 6). The latest operating version consists of synthetic bags floating partially submerged in an artificial pond, with the surrounding water acting as a scaffold, temperature regulation and light diffuser at the same time. Currently, the gassing system consists of an integrated sparger tube running along the lower seam of the bag. In the future, it is due to be replaced by a membrane gassing system. Solix claims peak oil production rates of 2,000 gallons/(ac ∙ year) (18,700 L/[ha ∙ year)) and expects a 2.5- to 4-fold increase for production sites (Solix-Biofuels 2009).
Fig. 6

New pilot reactor from Solix Biofuels: (a) low-ceilinged design; (b) water as support

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 without any additional supports. The company claims to reach biomass concentrations up to 10 g/L due to this small light path length. The investment is stated to be 200 k€/ha (=20 €/m2) and the need for auxiliary energy 20 kW/ha (=2 W/m2). This would be approximately 50% of the cost and energy of the theoretical limits of economic viability in Central Europe. Even lower values are prospected. A pilot plant started operation, and a larger facility is planned. Productivity data are not yet available.
Fig. 7

New pilot reactor from Proviron, Belgium

Process management

As the reactor is the hardware part of the process, only sophisticated operation makes it viable at the end. Process management offers several options to further improve the performance of the system and lower energy demands.

Temperature control

The impact of temperature control on the energy balance of the process is highly dependent on the applied reactor system, algae strain, but most of all the operating region of the plant. At warm, highly irradiated sites like southern USA or Australia, cooling of the cultures is likely to become a critical parameter of the process. Whether this problem is tackled by direct evaporation or a closed cooling system, excess heat must be actively taken out of the system, adding to the energy demand of the process. Spraying the outer wall of the reactor with water is a means but requires the availability of cooling water.

One way to reduce the heating problem is the avoidance of IR radiation. This part of the sunlight spectrum makes up 40% of the total energy without being used by the algae. IR-reflecting glass or plastic is already available (Holland and Siddall 1958) and is used to reduce heat in parked cars or to reduce heat radiation from lightbulbs.

Heating in spring is another option discussed especially in Central Europe, where the irradiance of the sun is already at a remarkable level, but outdoor temperatures are too low for sufficient cell growth. Here, it has been stated that low-temperature heat like cooling water from power plants is in principle available and could be used for heating of the cultivations. In the “water bed reactors” mentioned above, at least temperature fluctuations between day and night can be compensated by the amount of water around the growth chambers, which exceeds the usual values in open ponds. This concept could be even sharpened by employing the so-called phase changing materials. These materials are commercially produced in wallpapers for flats to control the room temperature at the given value of phase transition. For photobioreactors, that means temperature control not only at the day/night average but also at an adjustable value.

Feeding strategy

Photobioreactors usually are operated as batch or sequential batch (semi-continuous), where harvesting is done preferably at the afternoon. Maximum biomass concentration with highest mutual shading is reached with highest irradiation during daytime, while lowest biomass concentration in the night leads to lowest biomass loss by respiration. It is an underestimated fact that during continuous cultivation, the total productivity is given by the PCE value and the local irradiation, leading to low dilution rates for high cell concentrations (PCE ∙ I ∼ DcX). This has an impact on strain selection because not only high maximum growth rates are required but strains with high PCE at low growth rates could also be successfully cultivated.

Sunlight is only available at daytime, at broadly varying intensities. An option to reduce the energy intake is to couple gassing and consequently mixing to the photosynthesis rate (Buehner et al. 2009), since the CO2 demand is proportional to the photosynthetic activity. At night, however, there is no photosynthesis at all, and consequently the energy input for gassing can be reduced to the absolute minimum necessary for oxygen supply necessary for respiration.

Minimum requirements for the medium composition arise from the elemental balances, e.g. for nitrogen or phosphate. An intentional low nitrogen level prohibits the formation of proteins and nucleic acids forcing some microalgal strains to store CO2 and light energy as lipids. It must further be stated that the usual mineral media for lab use are not suitable to reach high biomass concentrations. This leads to very high salt concentrations that are subjected to precipitation or even growth inhibition. In these cases, a fed-batch-like additional dosing of single medium compounds during the growth process is required. However, to these points, not much quantitative scientific data or practical experiences are published.

Measurement and control

The photobioreactor has to provide ideal conditions for the microalgal cells with respect to a desired physiological state under the constraints of incoming light or other given external parameters. This can be done by measurement of physical conditions inside the medium and controlling technical variables like gas supply. Two pO2, pH and pCO2 sensors along the main reactor axis—this means along the strongest mass transfer gradient—should be mandatory. However, this topic is a bit neglected in current installations. As the cells are the only reasons for maintaining the process, online measurement of optical density and fluorescence pulse amplitude modulated fluometry (PAM) can help to assess the physiological state and react with online optimization of mixing, gassing or diluting.

Process integration and assessment

Process integration

While the photobioreactor has to maintain optimum conditions for the cells on the one hand, it has to cope with mass and energy flows available at a given site on the other hand. Cost savings can be obtained, e.g. by using nutrients from local waste water streams or CO2 from chemical plants, while the produced O2 is possibly a marketable product. In any case, reactor design and operation strategy depend on these circumstances. In the sense of an integrated process, development problems in one stage of the process could be tackled in another stage. Examples for algal cultivations, where technical problems can be solved on the biological side, are the reduction of the cellular antenna size to reduce the problem of self-shading and light inhibition (Mussgnug et al. 2007). Conversely, reactor design contributes to the biological problem of light saturation as shown above. Similarly, problems in downstream processing can be addressed by a better choice of cell strains and physiological state of the cells and by an improved reactor design. Furthermore, the whole process has to fit into an ecological, economical and social scenario.

CO2 supply

As stated before, the concentration of atmospheric CO2 is far too low to serve as carbon source in efficient algae cultivation. Nevertheless, there are plenty sources for higher concentrated waste CO2 streams; practically any combustion plant emission contains the required concentration, and usually exhaust gas CO2 concentrations exceed 5% (Negoro et al. 1991, 1993; Doucha et al. 2005). NOx from fuel gases are reported to be used by the algae as nitrogen source. Here the quality of the exhaust gas is of major importance. While the combustion of natural gas is reported to be suitable for algae CO2 supply, elevated levels of sulfur and nitrogen oxides as from coal-fired power plants could be damaging (Maeda et al. 1995) and therefore require elaborate gas purification, which would add to the energy balance of the algal process. Additionally, although highly abundant, CO2 from fossil energy sources prevents the biofuel process from being CO2 neutral, which could be avoided by the use of CO2 from combined heat and power units fueled by spare wood or biogas. In both cases, suitable sites for such plants may be a question: Large power plants usually exist in densely populated areas with expensive land prices, whereas most wood- or biogas-fired plants are located in rural areas with fertile soil which may rather be used for agriculture than for algae production.

Membrane gassing

Membrane gassing is one concept to reduce auxiliary energy similarly as it is done in animal cell cultures, in that case to reduce shear effects of the bubbles to the cells. Dissolving of CO2 is no longer effected at the boundary layer between gas bubbles and the culture medium and gassing and mixing are decoupled, which gives more degrees of freedom in process operation. Instead, a gas-permeable membrane creates the surface for CO2 dissolving and O2 removal directly at the gas intake. This saves the energy for the formation of gas bubbles, which is lost when the bubble collapses at the top of the reactor. It also lowers the loss of CO2 via the off-gas, resulting from the homogenous gas supply over large parts of the reactor surface. However, by using membrane gassing, gas dispersion from the membrane to the opposite side of the reactor will still require soft agitation. So far, no general design principle for membrane gassing is established. Employment of modern CFD tools can lead to further optimization of energy demand (Perner-Nochta and Posten 2007).

Solid–liquid separation

For further processing, the algae suspension has to be separated from the medium in most cases. For solid–liquid separation, two general principles exist: filtration- and sedimentation-based methods. Sedimentation separates particles according to a difference in specific density, while for filtration, particle size and surface play an important role.

Solid–liquid separation of organic matter is already standard in large scale. Baker’s yeast [world annual production 2 Mio. t (Fischer and Rahn 2004)] is concentrated with separators and filter units to dry mass contents up to 35%. Also, green algae for food are harvested and dewatered by screens and spray-drying (Earthrise-Nutritionals 2009). But other than for food or high-value products, for biofuel production, constraints are much tighter in respect to energy demand for the separation process, while standard techniques have been highly developed in a way that it is unlikely to achieve a major drop in the energy demand of the separation principle as such.

Employing centrifugation, the rotational speeds sufficient for algae separation in a small-scale disk stack centrifuge consumes approximately 5 kWh/m3 at a flow of 1 m3/h (manufacturer communication), scale up may lower the energy demand to 1–3 kWh/m3 (Molina Grima et al. 2003; Schenk et al. 2008). Less energy input cannot be expected; however, an increase in biomass concentration in the culture medium can improve the efficiency for the algae process substantially.

Filtration requires less power than centrifugation, but the success of filtration of microalgae depends strongly on the properties of the strain (Molina Grima et al. 2003; Schenk et al. 2008) and the type of membrane (Rossignol et al. 1999). Membrane fouling, pressure drop and the required filter surface must be considered when applying filtration for microalgal separation.

From these facts it is obvious that the solution for the energy problem cannot be found in developing new separation techniques but in increasing the biomass concentration in the photobioreactor. Maximum biomass concentrations in an open pond system reach 1–2 g/L. Even in best cases, this means 20% of the energy content of the separated algae is used for the first separation step in the case of centrifugation. In closed bioreactor systems, 10 g/L biomass are already attained or at least projected. Besides the energetic advantages during cultivation, this would reduce the amount of water going to the separation unit remarkably with the corresponding energy savings.

Process evaluation

Considerable effort has been put into economic and environmental evaluation of algae biofuel processes. Since data from large-scale production facilities are unavailable yet, a fundamental assessment on the viability of an algae biofuel process cannot be expected from current studies. While it is indispensable for bioprocess design to define certain benchmarks and to identify possible bottlenecks, the variance of basis data leads to significantly different outcomes of recent analyses. This has led to lively discussions whether algae biofuel will ever be competitive, either economically or ecologically.

Critics argue that investment and operating costs exceed benefits from energy production by far (Steiner 2008). Even after rather optimistic estimations, algae biofuel production will require a value co-product in order to be economically feasible, at least for the near future (Stephens et al. 2010). However, economic constraints are subject to substantial fluctuations, as crude oil price, material and labor cost have a direct influence on the process; therefore, the current market situation does not exclude a viable technique permanently.

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

In respect to the latter aspect analyses, results are very diverse (Chisti 2008a, b; Reijnders 2008; Clarens et al. 2010). While the required energy input of algal cultivation should not be underestimated, the general problem with crop-based biofuel production is the demand for arable land, which may displace terrestrial plant biofuel regardless of its energy balance.

Regarding areal productivity, Weyer et al. (2010) have presented a maximum range for algal oil production based on irradiation and cultivation data, combined with several assumptions concerning photosynthetic rates. Their best-case scenario reveals areal oil productivities of approximately 50 m3/(ha ∙ year) at irradiation levels in the range of 6,000 MJ/(m2 ∙ year) and 50% oil content. Jorquera et al. (2010) projects 32 m3/(ha ∙ year) with a 30% oil content using flat plate bioreactors. Stephens et al. (2010) predict 60–100 m3/(ha ∙ year) at 8000 MJ/(m2 ∙ year) and an oil content of 25–50%, a factor 1.5 compared to the reasonable maximum according to Weyer, respecting the difference in irradiation. Even though each study has its applied data well documented, the multiplication of assumed terms and constants originating from different sources leads to a major difference in projected areal productivity, although this is a factor where large-scale data are available, at least for open pond systems.

An assessment of downstream processing is even more complicated since a common standard procedure for dewatering, oil extraction and residual biomass processing is neither established nor have reliable data been published with real algal biomass from a large-scale facility with respect to biofuel production. Separation techniques currently applied in algae biomass production are very energy consuming, contributing mainly to the fact that today’s microalgae production facilities are net energy negative. Alternative strategies like flocculation or sedimentation (Lardon et al. 2009) have not been evaluated yet in large scale, at least not in the context of biofuel production, repeatedly and with designated production strains. For product recovery (Lardon et al. 2009) and refining (Greenwell et al. 2010), existing methods from terrestrial crops may be transferrable, but for an estimation in respect to energy consumption, there are too little data accessible. Nevertheless, balancing only the biomass growth without downstream processing (Jorquera et al. 2010) for production system evaluation may be a misleading approach since variations in biomass concentration resulting from different bioreactor types directly influence energy expenses for dewatering. With downstream processing probably accounting for a major fraction of the overall energy demand, a general assumption regarding the net energy ratio remains speculative. The most valuable information at this stage of process development may therefore be outcomes of sensitivity analyses for a defined hypothetical process. Stephens et al. (2010) have found biomass productivity to be the major influence on process viability, while expenses for labor, power and maintenance do not play a dominant role in their assessment.

Energy balance

Unless combined with the production of substantial masses of high-value products, microalgal culture for an entirely CO2 neutral biofuel generation imperatively requires a net energy production, including all steps of the production process. The gross energy must be provided entirely by incident solar irradiation, which in central Europe accounts for 1000 kWh/(m2 ∙ year) = 114 W/m2; world peak values reach 2500 kWh/(m2 ∙ year) = 285 W/m2. Even with the maximum PCE of 9%, the best areal continuous power output of an algae plant would be 10.3 W/m2 for central Europe and 25.7 W/m2 best site values, subject to the condition that all the energy chemically stored in algae biomass is accessible for an energetic use. For biodiesel production, these values must be multiplied by the fraction of processable oil content. However, assuming constant PCE, a higher oil content means less dry weight for thermodynamic reasons (calorific value of oil-rich algae is 30 MJ/kg, whereas oil-poor algae have 20 MJ/kg). Furthermore, a long residence time can push up the oil content but on cost of the energy balance.

Any energy term necessary for running the plant must be subtracted from these values. This includes auxiliary energy like pumping and gassing, temperature regulation, nutrient supply, product extraction and refinement as well as water and nutrient recycling. One essential field of work will be a further reduction of the auxiliary energy demand. As most of the auxiliary energy input is volume dependent, it becomes clear that the photobioreactor should have low water coverage, as little medium as possible per square meter footprint. The best reactors in this concern show values at about 40 L/m2. Integrating the above-mentioned reactor improvements can reduce the total energy demand of the process, while the maximum output is limited by maximum PCE value. The best reactors with respect to energy efficiency (e.g. the different “water bed” designs) have now come into the range of 50% need of auxiliary energy in relation to the collected sun energy by reduction of the auxiliary energy to 50 W/m3 (or 2 W/m2 with 0.04 m3/m2) as postulated by Posten (Posten 2009).

Discussion

Today, algae biofuel production as a single product is not at the same time energetically and economically profitable. The crucial step to be taken in the direction of a large-scale biofuel production is the development of cheap and efficient bioreactors. Several promising concepts have been evaluated and will give further insight whether higher productivities are possible. However, irradiation and photosynthetic efficiency set an upper limit to the energy yield, which necessitates the whole process to be reviewed in terms of energy savings potential. Actually promising starting points on the reactor and on the cell level have been proposed. As already mentioned, membrane gassing could be a solution for the bubbling problem, which implies short path lengths from the membrane surface to the rest of the reactor volume. Combined with an ultra-low-ceilinged design, the requirements for light dilution result in a horizontal plate with structured upper surface and a gas-permeable membrane at the bottom side. Mixing must be provided by a moderate induced horizontal convection to ensure equal light and nutrient conditions and prevent settling of the culture. Although employing a horizontal plate configuration, the general principles are similar to those of the latest reactor generation, as light distribution is also effected by vertical light guidance to deeper reactor regions, however by the reactor surface instead of the water-filled clear spacing between reaction compartments. This design would allow extremely short light paths with well-distributed incident light, no additional support structures and an uncoupling of mixing and gas supply. However, the approach would require rigid surfaces and an additional energy input for mixing, so it remains to be determined whether such a reactor would perform better than the existing designs.

With the latest generation of photobioreactors, we have seen substantial improvement concerning efficient light utilization, area footprint and installation cost reduction. Light path lengths of only a few centimeters enable these reactors to produce high total biomass concentrations while the use of plastic material led to higher flexibility and lower material cost. Interestingly, even though many geometry options are possible, the vertical plate-like layout is a common feature in these late developments. The Solix and Proviron setups employ a low-ceilinged design, which helps to reduce gas pressure drop together with water as support and for temperature balancing. Whether it is more favorable to take advantage of evaporation for cooling and accept a higher water demand as in Solix’ concept or whether the enclosed water bed in the Proviron design is more practical and sufficient for tempering remains to be shown and may essentially depend on the operating site. The fundamental problem, however, is still the demand for auxiliary energy, which requires the entire process from upstream of the bioreaction stage to the downstream processes to be critically reviewed and checked for energy savings potential. Valuable byproducts and energy waste streams may be necessary to make microalgae biofuel production viable. However, combining all of the recent results shows the great potential of algae biofuel for future energy supply.

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Copyright information

© Springer-Verlag 2010

Authors and Affiliations

  • Michael Morweiser
    • 1
  • Olaf Kruse
    • 2
  • Ben Hankamer
    • 3
  • Clemens Posten
    • 1
  1. 1.Division of Bioprocess Engineering, Institute of Engineering in Life SciencesKarlsruhe Institute of TechnologyKarlsruheGermany
  2. 2.Department of Biology, AlgaeBioTech GroupUniversity of BielefeldBielefeldGermany
  3. 3.Institute for Molecular BioscienceThe University of QueenslandSt. LuciaAustralia

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