Can the fossil fuels we use in internal combustion engines be practicably replaced by fuels derived from plant materials – biofuels? This question has, in these times of high oil prices and climate change worries, risen quickly up the agenda. Plants use the sun’s energy to convert carbon dioxide into chemically stored energy in the form of sugar, starch, vegetable oil or cellulose, so if one can economically convert these molecules into convenient fuels like ethanol, one has a route for the sustainable production of fuels for transportation. The sense of excitement and timeliness has even reached academia; my friends in Cambridge University and Imperial College are, as I write, frantically finalising their rival pitches to the oil giant BP, which is planning to spend $500 million on biofuels research over the next 10 years. Today’s issue of Nature has some helpful features (here, this claims to be free access but it doesn’t work for me without a subscription) overviewing the pros and cons.
The advantages of biofuels are obvious. They exploit the energy of the sun, the only renewable and carbon-neutral energy source available, in principle, in sufficient quantities to power our energy-intensive way of life on a worldwide basis. Unlike alternative methods of harnessing the sun’s energy, such as using photovoltaics to generate electricity or to make hydrogen, biofuels are completely compatible with our current transportation infrastructure. Cars and trucks will run on them with little modification, and existing networks of tankers, storage facilities and petrol stations can be used unaltered. It’s easy to see their attractions to those oil companies which, like BP and Shell, have seen that they are going to have to change their ways if they are going to stay in business.
Up to now, I’ve been somewhat sceptical. Plants are, by the standards of photovoltaic cells, very inefficient at converting sunlight into energy; they require inputs of water and fertilizer, and need to be converted into usable biofuels by energy intensive processes. The world has plenty of land, but the fraction of it available for agriculture is not large, and while this is probably sufficient to provide enough food for the world’s population the margin is not very comfortable, and is likely to get less so as climate change intensifies. One of the highest profile examples of large scale biofuel production is provided by the US program to make ethanol from corn, which is only kept afloat by huge subsidies and high protective tariff barriers. In energetic terms, it isn’t even completely clear that the corn-alcohol process produces more energy than it consumes (even advocates of the program claim only that it produces a two-fold return on energy input).
The Nature article does make clear, though, that there is a much more positive example of a biofuel program, in ethanol produced from Brazilian sugar-cane. Estimates are that it produces an eightfold return on the energy input, and it’s clear that this product, at around 27 cents a litre, is economic at current oil prices. The environmental costs of farming the stuff seem, if not negligible, less extreme than, for example, the destruction of rain-forest for palm oil plantations to produce biodiesel. The problem, as always, is scaling-up, finding enough suitable land to make a dent on the world’s huge thirst for transport fuels. Brazil is a big country, but even optimists only predict a doubling of output in the near future, which would still leave it accounting for less than one percent of the world’s demand for petrol.
Can there be a technical fix for these problems? This, of course, is the hope behind BP’s investment in research. One key advance would be to find more economical ways of breaking down the tough molecules that make up the woody matter of many plants, cellulose and lignin, into their component sugars, and then into alcohol. This brings the prospect of being able to use, not only agricultural waste like corn husks and wheat straw, but new crops like switch-grass and willow. There seems to be a choice of two methods here – using the same technology that Germany developed in the 1930’s and 40’s to convert coal into oil, using high temperature and special catalysts, or developing new enzymes based on the ones that fungi that live on tree stumps use. The former is expensive and as yet unproven on large scales.
What has all this got to do with nanotechnology? It is very easy to get excited by the prospect of a nano-enabled hydrogen economy powered by cheap, large area unconventional photovotaics. But we mustn’t forget that our techno-systems have a huge amount of inertia built into them. According to Vaclav Smil, there are more internal combustion engines than people in the USA, so potential solutions to our energy problems which promise less disruption to existing ways of doing things will be more attractive to many people than more technologically sophisticated but disruptive rival approaches.