Another possible source of biodiesel are the microalgae, which will not grow on agri­cultural land. Microalgae are more photosynthetically efficient than land plants and the consensus for microalgal production and carbon dioxide fixation are 1.7 g/l/day and 25.7 g CO2/m2/day, respectively. For example a 10,000 l microalgal bioreactor

193 I

with a depth of 0.15 m would cover an area of 6670 m2 or 0.67 ha. The bioreactor would yield 51,000 kg biomass within a 300-day year. If the oil yield from the micro­algae was 30%, this would yield 15,300 kg oil per year which is equivalent to 22,930 kg oil per hectare or 22.93 t/ha which is considerably better than rapeseed at 1 t/ha. In the data put forward by Chisti (2007), the yield of oil was 30,000 kg from

100.0 kg of biomass from an area of 7828 m2. This is a yield of 38.32 t/ha which is somewhat higher than the figures used above. Therefore, to replace 100% diesel,

23.989.0 t, using the lower yield figure would require 1,046,184 ha, which repre­sents 5.8% of the agricultural land (Fig. 8.26).

There has been some discussion about the provision of biofuels from microalgae. Chisti (2007) has proposed that biodiesel from microalgae is considerably better than the use of terrestrial crops. In contrast Reijnders (2008) has taken three studies on the life cycle of microalgal fuels and has concluded that the net energy yield of oil palm and sugarcane was better than microalgae (Table 8.5). The table contains raw energy data and net energy yields where the net energy gains for sugarcane and oil palm are better than microalgae, but in terms of raw energy they are very similar.

However, the study does not take into consideration the environmental effects of increasing both sugarcane and oil palm at the expense of rainforest, whereas micro­algae can be grown on non-agricultural land, in marine environments and in temper­ate climates. In a study of oil extraction, sunflower seeds were compared with oil extraction from microalgae (Bastianoni et al., 2007). The results show that sunflower oil extraction was more energy-efficient, but if extraction from microalgae can be improved it should be considered as a fuel source.