Most proposals for microalgal biofuels from open ponds or bioreactors focus on biodiesel made from algal oil (Scragg et al. 2002; Chisti 2007; Huntley and Redalje 2007; Wijffels 2008; www. oilgae. com; Liu et al. 2008). However, there have, for instance, also been proposals to convert algal biomass into methanol via synthesis gas or into bio-oil via pyrolysis (Hirano et al. 1998; Sawayama et al. 1999). The alga Botryococcus braunii has been looked into, in view of its ability to produce substantial amounts of hydrocarbons, which may be turned into transport biofuels by catalytic cracking (Bachofen 1982; Banerjee et al. 2002). As pointed out in Chap. 1, current strains of this microalga are slow growing, which has not been conducive to its application (Banerjee et al. 2002).

Of the microalgae commercially grown in open ponds, Spirulina apparently has the best yields per hectare per year in commercial cultivation (Belasco 1997). Max­imum productivities in open ponds are achieved under tropical or subtropical con­ditions (Jimenez et al. 2003). Yields currently obtained in industrial facilities for the cultivation of Spirulina located in these regions range from 10 to 30 Mg dry biomass per hectare per year (Vonshak and Richmond 1988; Jimenez et al. 2003). Low yields of, for example, Spirulina may however occur due to, for example, phage infections and rainfall conducive to the growth of unfavourable organisms (Shima — matsu 2004). For instance, Li and Qi (1997) reported that the 80 Chinese Spirulina production plants had production on average of 3.5 Mg ha-1 year-1.

It may be that in the future, microalgal yields from raceway ponds may be in­creased over current levels, for instance through improving photosynthetic activ­ity by minimizing light harvesting chlorophyll antenna size (Neidhardt et al. 1998; Mussgnug et al. 2007). On the other hand, a focus on algal lipids for transport bio­fuel production may well lead to biomass yield limitations, because nutrient lim­itations are conducive to high lipid contents but not to maximizing biomass yield (Wijffels 2008; Liu et al. 2008).

Hirano et al. (1998) studied Spirulina production and processing to supply methanol (via synthesis gas) and assumed a yield of approximately 110Mgha-1 year-1. When both fossil fuel inputs in infrastructure and operation are considered, this would correspond with an overall solar energy to biofuel conversion efficiency of about 0.12%.

Actual yearly yields much exceeding 30Mgha-1year-1 have been claimed for microalgae growing in water that has been saturated in CO2 (Kheshgi et al. 2000; Wang et al. 2008). Algal ponds that are to be saturated in CO2 have been proposed to capture the CO2 of power plants (Kheshgi et al. 2000). Also, closed bioreactors have been proposed for algal capture of CO2 from power plants (Skjanes et al. 2007). The efficiency of algal CO2 capture in open ponds has been estimated to be in the order of 30% (Benemann 1993; Kadam 2002), whereas an efficiency of 40% has been suggested for algae in photobioreactors (Ono and Cuello 2006). Whether such per­centages can be achieved is not certain. Yields from open ponds saturated with CO2 have proved disappointing, and maintaining desired algal cultures in such ponds has turned out to be difficult (Benemann et al. 2003). There is also the matter of the efficiency of CO2 sequestration by algae. The suggested efficiency for photobiore­actors of 40% is, for instance, higher than efficiencies so far reported by Hsueh et al. (2007) and Jacob-Lopes et al. (2008) for flue gases with high concentrations of CO2 handled by photobioreactors. Moreover, the latter efficiencies were achieved under good irradiation, whereas the CO2 emission of power plants may also occur at night and when solar irradiation is poor. CO2 capture and sequestration (CCS) in aquifers or abandoned natural gas or oil fields would be able to reduce the emission of power plants with an efficiency of about 90% (Odeh and Cockerill 2008). Thus, whether the application of CO2 capture by algae will be important in the future depends to a large extent on the emission requirements for such plants.

Microalgal yields from closed bioreactors subject to solar irradiation may be much higher than from current commercial open ponds (Eriksen 2008). For the production of algal oil, a value of about 16Mgha-1year-1, has been suggested as ‘possible with state of the art technology’ in closed systems (Wijffels 2008). However, growing algae aiming at high outputs in bioreactors requires large inputs of energy for building the reactors and for nutrients and intensive mixing. It has been estimated that this could lead to a negative energy balance for flat panel bioreactors and an even more negative energy balance for tubular bioreactors (Wijffels 2008).