The cost of the extraction and transesterification stages was based on one biomass production process technology, with the results shown in Fig. 13. The extraction stage was found to be significantly more expensive than the transesterification stage. The major contributors to the extraction costs were the large fixed capital costs and the cost of large quantities of solvents, respectively. The main components of the fixed capital costs were the costs of large mixing tanks and the pumping capacity required in filling and emptying the tanks. The small cost incurred during transesterification resulted from the significantly reduced volume of materials that required processing, with only 7.8 tonnes of saponifiable lipid estimated to pass to

■ Cultivation □ Dewatering О Extraction ■ Esterification

Fig. 13 Extraction and transesterification costs for algal lipids the transesterification stage daily from the original 151 tonnes of biomass processed in the extraction stage. The electricity costs for both processes were minor, as shown in Fig. 13.

1.7.2 Overall Production Costs

Overall, considering the economic outcome, a raceway pond coupled with a dual­stage dewatering process would be the preferred method to produce biodiesel. Considering biodiesel as the only saleable product, production costs were estimated as approximately $74/L of biodiesel. The calculations are based on the assumptions that glycerol is allowed to be sold, residue from the process also sold as animal feed and carbon credits received as discussed in Sect. 7. However, including these in the model only reduced biodiesel production costs to $72.60/L. With petroleum-based diesel currently retailing at ~ $1.10/L, though this analysis incurred a -50% error, biodiesel from microalgae still remains far too expensive, as compared with tradi­tional fuel.