Harnessing direct solar energy – a progress report
Box 3 | Light to electricity
Martin Green and Stuart Wenham, from the University of New South Wales, have invented or co-invented a number of distinct solar cell technologies. These cells have held the world efficiency record for converting sunlight into electricity.
In the1999 Australia Prize in Energy Science and Technology was awarded to Martin Green and Stuart Wenham, an indication of the pair’s dominance in the world of photovoltaic research.
Photovoltaic cells convert light directly into electricity. It has been known for more than 150 years that light can have an effect on the electrical properties of some materials. This is known as the photoelectric effect. In 1921, Einstein received the Nobel prize for his work explaining this. Photovoltaic cells are based on a related phenomenon called the photovoltaic effect, and interest in this has increased greatly during this century.
The generation of electricity from light relies upon the separation of positive and negative electric charges – electrons and positively charged 'holes', both generated by the light – at the junction between two parts of a semiconductor crystal. Silicon is a semiconductor that can be mixed with tiny quantities of impurities (such as phosphorus and boron) in a process called doping. Doping can produce P (for positive) and N (for negative) materials. A photovoltaic cell is simply a wafer of semiconductor in which there is a junction between N and P materials. On exposure to light, a photovoltaic cell produces a voltage of about 1 volt, comparable with that of a torch battery. The silicon, which is expensive to make in pure form, is in the form of a thin wafer, to catch as much light as possible.
The ultimate efficiency of a silicon photovoltaic cell in converting sunlight to electrical energy is less than 30 per cent, and the School of Photovoltaic and Renewable Energy Engineering at the University of New South Wales holds the present world record with a cell efficiency of about 25 per cent. But cells like this, made from single-crystal silicon, are expensive, and large areas of them are needed to produce useful amounts of power. The search is therefore on for much cheaper cells without too much sacrifice in efficiency.
Several promising lines have been pursued by the University of New South Wales. Instead of cutting slices from specially grown silicon single crystals, one possibility involves growing thin films of silicon on much cheaper polycrystalline silicon wafers. Another technique deposits thin films of silicon onto glass plates.This process uses 99 percent less silicon than conventional techniques and is now being use commercially in Europe.
While expensive photovoltaic cells can be used in solar car races, and solar photovoltaic modules on the roofs of houses may also become common, the ultimate aim of photovoltaic technology is to produce large amounts of electrical power from cheap photovoltaic cells, connected together to give a high voltage, and to convert this to alternating current to feed into the power grid. This has already been done on an experimental scale in Sydney, and Australia is a leader in the race.
Boxes
Box 1. Eliminating the zeroes
Box 2. Driving on a sunbeam
Box 4. The Big Dish
Box 5. Chemical fuels from the sun
Related sites
Photovoltaics (Energy Efficiency and Renewable Energy Network, US Department of Energy)
Solar electricity basics (Florida Solar Energy Center, University of Central Florida, USA)
School of Photovoltaic and Renewable Energy Engineering (University of New South Wales)
Page updated November 2009.