Algae-based biofuel has long been one of the dark horses in the race for sustainable transportation fuels, and the US National Research Council (NRC) suggests that it may remain in the shadows for a time.
One of the selling points has been that algae can be grown on non-productive land, in laboratories or even offshore, thus eliminating any direct conflict with food resources. Producers can also get more out of the final product: whereas ethanol contains roughly one-third less energy than petrol, algae-based biodiesel contains slightly more energy than petrol (but still a bit less than normal diesel fuel). This has spurred considerable interest in recent years, but the budding industry has also been plagued by questions about resource use and whether it could be realistically be ramped up.
At the request of the US Department of Energy, the NRC scanned the literature and determined that scaling up algae-based biofuels to even 5% of the market for US transportation fuels “would place unsustainable demands on energy, water and nutrients.” The NRC report was released Wednesday.
Estimates regarding the size of that strain vary to an extraordinary degree. For instance, every litre of algal biofuel requires anywhere from around 3 litres of freshwater — which is in the range of a typical litre of petrol — to 3,650 litres of fresh water. The difference arises from the sheer variety of methods, including whether the algae is grown in saltwater or freshwater, whether it is grown in ponds (above) or reactors — and, if in ponds, where those ponds are located (evaporation rates are higher in Arizona than in Michigan, for instance).
Even more troubling, however, are the fertilizer requirements. The amount of nitrogen necessary to keep algae happy and produce 39 billion litres of fuel, which is 5% of the US market, ranges from 44% to 107% of the country’s total nitrogen use today. Phosphorus requirements ring in at 20–51% of the total US phosphorus market. The land-use requirements could also be prohibitive, the NRC says, and despite all of these potential downsides, the greenhouse-gas benefits are questionable and depend highly on what kind of process is chosen.
None of this necessarily means that algae-based fuel is dead in the water. A smart operator could design a facility that minimizes water use and recycles nutrients, or even one that taps a waste stream that provides both water and nutrients. And science has a part to play in improving algal strains and industrial processes.
It is also true that many, if not all, of the clean energy solutions that we have at our disposal fail to pass the scalability test at some level, but there may well be room for niche technologies to play out where they make the most sense. To that end, the NRC is proposing a stepwise framework for conducting comprehensive sustainability assessments for each phase of the development, from the selection of technologies to the siting and specification of production processes.
“The committee does not consider any one of these sustainability concerns a definitive barrier to sustainable development of algal biofuels because mitigation strategies for each of those concerns have been proposed and are being developed,” the NRC said. “However, all of the key sustainability concerns have to be addressed to some extent and in an integrative manner … if the promise of sustainable development of algal biofuels has any chance of being realized.”