A wide range of valuable substances have so far been produced with microalgae. Commercial applications partially aim at high value products, e. g., carotenoids or poly-unsaturated fatty acids (PFUAs) for the pharmaceutical and cosmetics indus­tries. Microalgae biomass, rich in unsaturated fatty acids, is also a valuable source for food supplements and suitable for feed in aquacultures. Recent efforts explore the production of fine chemicals and energetic utilization of microalgae biomass and their products, e. g., biodiesel, ethanol, biogas, and hydrogen.

R. Dillschneider • C. Posten (H)

Institute of Life Science Engineering, Division III: Bioprocess Engineering, Fritz-Haber-Weg 2, 76131 Karlsruhe, Germany e-mail: clemens. posten@kit. edu

J. W. Lee (ed.), Advanced Biofuels and Bioproducts, DOI 10.1007/978-1-4614-3348-4_26, © Springer Science+Business Media New York 2013

One of the most persuasive benefits for the energetic utilization of microalgae is their capability of efficiently converting incident solar radiation to biomass. In terms of efficiency phototrophic organisms and also entire cultivation systems can be evaluated by their photoconversion efficiency (PCE). This value represents the percentage of incident solar radiation which is ultimately stored as chemical energy of the biomass. In theory, microalgae can attain PCE values of 12.6% [51]. However, in practice a PCE value of only 5% is achievable [30]. The difference can be traced to physiological and physical causes. With regard to physiology, high oxygen concentrations induce respiration and therewith loss of biomass. Moreover, excess incident light energy is dissipated as fluorescence and heat. Physical causes com­prise, amongst others, mutual shading of cells and reflectance of radiation at the surface of reactors. However, photobioreactor improvement aims at optimizing conditions, such as gas concentrations and illumination in order to minimize losses and to approach a generally assumed technical upper limit of 9% PCE [26, 30]. In temperate climates, PCE values for terrestrial plants are reported to be in the range of or even below 1% [6, 29].

For each individual application and valorization of products a certain price limit for the biomass is given and fundamentally influences the process design itself. Prices for pharmaceutical products are certainly higher than for biodiesel and there­fore justify expensive processes. In this case, energy balances and process costs are not crucial for the overall profitability. In the aquaculture sector, production costs of dry microalgae biomass range from 50 to 150 US$/kg. Maximal values were even specified to reach 1,000 US$/kg [32]. Prices for biomass targeting the animal feed market need to decline to less than 10€ (circa 13 US$). Production cost for biomass targeting the energy market need to be even below these values [35] .

Cultivation of microalgae for the energy market imposes challenging restraints for bioreactor design. Even though prices on the energy market are expected to continually increase in the next years and even decades, the continuing exploitation of fossil resources confines the upper price limit for alternative and sustainable energy sources. Gross margins earned with low-price products, such as hydrogen or biodiesel, are very small. Learning curve effects in microalgae cultivation and cost reduction of large scale implementation (economies of scale) are not only expected to reduce costs but are also necessary for price-competitive applications [5] . Moreover, an integrated utilization of products serving the energy market and the simultaneous valorization of side-products might be a promising approach to meet the challenge and increase the overall added value.

Nevertheless, price-competitive bioprocesses must be focused on and engineer­ing must aim at providing low-cost bioreactors which attain high productivities. Moreover, a positive net energy balance is crucial for a competitive bioprocess and also fundamentally determines the ecologic benefit of the process. In order to meet both economic and energetic demands, development of novel photobioreactors requires the consideration and permanent assessment of the three indicators produc­tivity, cost, and net energy gain.