Do biofuels from microalgae beat biofuels from terrestrial plants?

Energy in algal oil 37.90 Energy in biogas from residual biomass 50.00 Estimated as 22.85 MJ kg 1 of urea and 2.94 MJ kg 1 of diammonium phosphate. Using sedimentation followed by continuous vacuum belt filters. Approximate only in view of the developmental nature of algal oil recovery processes. Estimated as 80.4 MJ m 2 of facility area divided by a 20 year working life and the mass of oil produced annually. Estimated as fossil energy requirement of 27.2 MJ t 1 of machinery [18] (including equipment for biogas production) divided by the 20 year working life of equipment Reijnders [1] claims that microalgae are inferior to terrestrial oil crops as net producers of renewable energy. Algae-derived fuels are claimed to provide barely as much energy as the fossil energy that is consumed in producing them [1]. This claim is based exclusively on two simplistic analyses by Hirano et al. [2] and Sawayama et al. [3]. However, both these studies show little understanding of large-scale algae culture and grossly overestimate the fossil energy required in producing algal biofuels. Hirano et al. [2] estimated the total energy requirements for Spirulina biomass production in raceway ponds. Biomass recovery was assumed to occur by a two-step process involving gravity sedimentation followed by centrifugation. The latter is an energy-intensive operation that is entirely impractical for recovering large quantities of biomass inexpensively, particularly if the product being produced is of a low value, such as biodiesel. Sawayama et al. [3] based their analysis on raceway production of an alga with 20.5% (w/w) oil. The assumed biomass productivity was 15 000 kg ha 1 year , which amounts to 16% of the average annual productivity that is commonly attained in raceway ponds operated in the tropics [4–6]. Furthermore, the assumed biomass concentration in the broth was 0.5 kg m , or only 50% of the typical concentration in a well-operated raceway [7]. This assumption alone was sufficient to double the cost of harvesting the algal biomass compared with amore typical case. In addition, harvesting was assumed to occur via a two-step sedimentation–centrifugation process, which is in practice an unrealistic choice, as pointed out above. Oil was recovered by an energy-intensive thermochemical liquefaction of the biomass. Based on these questionable assumptions, Sawayama et al. [3] and Hirano et al. [2] concluded thatmicroalgae were unsatisfactory for producing biofuels. In contrast with this, I and others have argued that microalgae are better than the other existing options for sustainably providing biofuels such as biodiesel [8,9]. In view of extensive past experience with microalgal culture [10–15], the data in Table 1 can be estimated for raceway production of microalgal biomass with 20% (w/w) oil, a biomass productivity of 0.025 kg m 2 d 1 [4–7,16] and concentration of 1 kg m 3 in the ponds. The energy ratio (i.e. the renewable energy produced per unit of fossil energy input) for the case in Table 1 is 2.8. The total energy yield for this case is 1,444 GJ ha 1 year , assuming a 90% operational factor for the production facility (Box 1). The case in Table 1 is conservative in assuming a relatively low