Algae as a biomass source.
Ted Trainer writes to the EnergyResources mailing list:
Here is a summary of how biodiesel from algae situation seems to me to be in view of the evidence I have come across. Please let me have any new information.
In ideal conditions some species of algae grow at very high rates, up to 30 times the rate for land plants. Sheehan (1998) claims 50g/m/d, (which equates to 180t/ha/y although he does not say this growth rate can be kept up for a year.) Reference is made to a proposed scheme intended to harvest 67t/ha/y, more or less equivalent to sugar cane. The oil content can be 40%. Of special interest for energy production is the possibility of using sea water in large shallow desert ponds. 200,000 ha are claimed to be capable of producing 1 Quad, or 8.4 EJ of biodiesel. Presumably this is a gross output. The claim is puzzling; if a 50 t/ha yield is assumed and algae have the same energy content as wood, then the gross production would only be 160 - 200PJ, only 2% of the claimed amount. In any case that output corresponds to a photosynthesis rate of 7% p.a. When it is growing fast corn achieves c5%, but averages only .3% over a year. (Pimentel, 2004). Sorensen (2000, p 3.11) says algae on reefs average 2%, but this could be raised to 3.7%.
Sheehan points out that yields are more like 10g/m/d in field conditions, as distinct from the lab. A major problem is that constant high temperatures facilitate high yields, but large scale energy production would involve large open ponds in deserts, where temperatures fall at night. Siting ponds close to power plants would enable use of warm cooling water.
Cost estimates reported vary considerably, but the equivalent of oil at $(US)65-100 is quoted. Sheehan does not give energy costs of production.
One difficulty is that the conditions which increase growth rates reduce oil content. Starving the algae of nutrients raises their oil content. Another is that the sunlight conversion rate and therefore efficiency of the process is highest in low light levels, e.g., 10% of full sun.
Perhaps the major consideration is where would inputs come from for very large scale production of this biomass? Some advocates refer to use of nutrient rich waste water from agriculture, but far greater quantities would be needed to make a significant contribution to replacing fossil fuel dependence. Around 40% of the input material must be carbon dioxide and therefore the process could be coupled to coal-fired power stations, but it is not clear how far how much CO2 would have to be transported to hot regions for large scale production.
Mardon (2004), who has worked on various biomass input sources, including algae, for the Australian CSIRO, says they found that the energy cost of the process is so high that the energy return (ER) is negative. "The energy required to grow (and more particularly to harvest and process) the algae is considerably greater than what you can get out of it." Winter growth rates were found to be slow. "Filters are not an effective way of harvesting them, so a lot of energy is required for centrifugation. Even then, the cell mass is very wet, and some form of dewatering may be required" "Our field work showed that it was not a practical as a way of harnessing solar energy." Mardon also notes that ponds are prone to contamination, and require aeration.
However Briggs (2004) gives remarkably optimistic assessment of the potential of algae, claiming that its ER could be 10 and even 20. Little information is available in view of patent applications. Briggs figures indicate 13GJ gross output per tonne, higher than the yield from cellulosic inputs. Briggs says the energy costs are low because there is no planting or harvesting cost. The process is intended to use waste water inputs, and will enclose ponds to maintain temperatures, indicating that very large scale production is not intended. Again it is apparent that ER figures do not settle the viability of a renewable technology; a high figure for algae growing in costly structures might have quite limited application. On the other hand a low ER for steam generated by wood fuel can be quite viable, provided there is a sufficient quantity of input material.
Finally using power station CO2 would not affect the impact of that carbon on the atmosphere. It would end up in the atmosphere after the biodiesel was burnt. This factor alone would seem to disqualify large scale use of algae for the production of liquid fuels.
What do you think? Leave a comment below. See our commenting guidelines.
Sign up for regular Resilience bulletins direct to your email.