Interest in algae as a potential biofuel feedstock stems from two features observed in naturally occurring strains. Firstly, many algae are remarkably efficient at photosynthesis, converting up to 6% of incident solar energy into biomass. The theoretical limit for photosynthesis is about 11% of incident sunlight, whereas most crops achieve less than 1%, and even sugar cane achieves only around 2%. Algae are so productive that for the most part, the challenge is preventing them from growing – as testified by the recent experience of algal growth in the Chinese Olympic boating lake, as well as in open air swimming pools across the world.
Secondly, for a handful of these algae, much of this captured energy is stored in the form of plant oils that can be extracted and processed into biodiesel in the same way that that soy or palm oil is today. Unlike soy and palm, algae do not require fertile farmland to grow and so offer the enticing possibility of large amounts of biofuel from relatively small areas of otherwise non-productive land – or water – and a number of companies such as Petrosun and BioFields are considering large scale algae farms in desert regions.
However, these advantages are at present negated by two related drawbacks.
The first is the high capital cost of enclosed algae production systems, or ‘bioreactors’. The highest rates of photosynthesis are only possible if other growth constraints are absent. This means that algae must be cultivated in a soup of nutrients (as might be provided by a sewage plant), and in the presence of elevated levels of carbon dioxide (as might be provided by a power station). Enclosing algae production in a bioreactor located close to a good source of both nutrients and CO2 is technically feasible and has been demonstrated in pilot scale plants, but the capital cost of the bioreactor’s pipes, pumps and so on creates a very significant obstacle to the economic feasibility of this route. In addition, there is only so much land located between power stations and sewage works, and it is often more expensive than farmland. Some estimates put the cost of biofuel production by this route at several hundred dollars per barrel of oil equivalent.
The alternative approach is to grow algae extensively in open ponds resembling paddy fields. Proponents of this method argue that producing enough algae to meet a significant proportion of the world’s transport fuel requires an agricultural solution, on a similar scale to sugar cane in Brazil or maize in the US. This removes the possibility of growing algae in elevated CO2 concentrations, but whilst growth rates are consequently lower, the reduced cost of ponds compared to bioreactors should more than make up for this penalty. However, the second drawback becomes apparent here. Naturally occurring algae soon arrive in the open ponds, and begin to compete for nutrients, sunlight and space. Since the biofuel algae are selected for their ability to create oils rather than their raw ability to multiply, they are easily out-competed and oil yields fall dramatically.
There are a number of possible solutions to these two problems. One interesting approach under development by Shell in Hawaii is a hybrid route. Large quantities of biofuel algae are grown in closed systems, and then released into open seawater ponds to multiply up. The hope is that by introducing cultivated biofuel algae at a sufficiently high rate, naturally occurring invasive algae will not have time to overtake their less prolific relatives before the resulting biomass is harvested for oil.
The widespread adoption of algae as a biofuel feedstock will ultimately depend upon its economic performance compared to other low-carbon alternatives – for example, the continuing expansion of sugar and starch-based ethanol, thermochemical and other ‘second generation’ routes using abundant cellulosic feedstocks, and the increasing use of electrical power for transport.
Growing biofuel feedstock in the desert sounds like a perfect answer to the debate about food prices and agricultural land use, but electricity generated by concentrated solar power stations may ultimately prove a more economic ‘well to wheel’ use of desert areas in North America and North Africa purely on logistical grounds – power cables are considerably cheaper than pipelines, and mirrors don’t need constant feeding.