Over the next fifty years, mankind is going to have to find large-scale primary energy sources that aren’t based on fossil fuels. Even if stocks of oil and gas don’t start to run out, the effects of man-made global warming are likely to become so pressing that the most die-hard climate-change sceptics will begin to change their tune. Meanwhile, the inhabitants of the rapidly developing countries of Asia will demand western-style standards of living, which in turn will demand western levels of energy use. Can nanotechnology help deliver the energy needed for all the world to have a decent standard of living on a sustainable basis?
Although wind and hydroelectric energy can make significant dents in total energy requirements, it seems that only two non-fossil primary energy sources really have the potential to replace fossil fuels completely. These are nuclear fission and photovoltaics (solar cells). Nuclear power has well known problems, though there have been recent signs of a change of heart by some environmentalists, notably James Lovelock, about this. Solar power is viable, in the sense that enough sunlight falls on the earth to meet all our needs, but the capital expense of current solar cell technology is too great for it to be economically viable, except in areas remote from the electricity grid.
To make a dent in the world’s total power needs we’re talking about bringing in many gigawatts (GW) of capacity per year (total electricity generating capacity in the UK was around 70 GW in 2002, in the USA it was 905 GW). Roughly speaking 65 million square meters (i.e. 65 square kilometers) of a moderately efficient photovoltaic gives you a GW of power. Here we see the problem of conventional silicon solar cells: a silicon wafer production plant with a 30 cm wafer process produces only 88,000 square meters a year; the cost is high and so is the energy intensity of the process, to the extent that it takes about 4 years to pay back the energy used in manufacture. We need to be able to make solar cells on a continuous basis, using a roll-to-roll process, more like a high volume printing press. A typical printing press takes just a few hours to process the same area of material as a silicon plant does in a year; at this rate we’re approaching the possibility of being able to make a GW’s worth of solar cells (roughly comparable to the output of a nuclear power station) from a year’s output from one production line. Several new technologies based on incremental nanotechnology promise to give us solar cells made by just this sort of cheap, large scale, low energy manufacturing process.
The most famous, and probably best developed technology is the Graetzel cell, invented by Michael Graetzel of the EPFL, Lausanne. This relies on nanostructured titanium dioxide whose surfaces are coated by a dye; the nanoparticles are then embedded in a polymer electrolyte to make a thin film which can be coated onto a plastic sheet. This process is being commercialised by a number of companies, including Konarka and Sustainable Technologies International. Other technologies use nanostructured forms of different kinds of semiconductors; companies involved include Nanosys, Nanosolar, and Solaris. A third class of non-conventional photovoltaics uses semiconducting polymers of the kind used in polymer light emitting diode displays, sometimes in conjunction with fullerenes. These technologies still need to make improvements to their efficiencies and lifetimes to be fully viable, but progress is rapid, and all offer the crucial benefit of low energy, large scale manufacturability.
It’s not at all clear which of these technologies will be the first to deliver the promised benefits. We shouldn’t forget that more conventional technologies, like thin film amorphous silicon, are also advancing fast – Unisolar has a commercial reel-to-reel process for producing this type of solar cell in quantity, with a projected annual production of 30 MW (i.e. 3% of a nuclear power station) coming soon. But it does seem as though this is one area where incremental nanotechnology could have a transformational and positive effect on the economy and the environment.
This discussion draws on two recent articles: Manufacturing and commercialization issues in organic electronics, by J.R. Sheats, Journal of Materials Research 19 1974 (2004), and Organic photovoltaics: technology and market”, by C.J. Brabec, Solar Energy Materials and Solar Cells, 83 273 (2004).