Warming up to the Sun
Solar power has started replacing conventional power in some areas even as researchers persevere in developing more efficient solar cells to tap this clean source of almost limitless energy (New Scientist, Vols 145, 146, Nos 1969, 1974).
In Germany, bus stops, shelters, parking ticket machines and speed warning signs powered by photovoltaic cells are beginning to appear all over. Explains Alexander Mayr-Benz of Mabeg, which manufactures solar photovoltaic cells, "Photovoltaic systems are much cheaper and pay for themselves within 2 years." Installing conventional power lines for such street structures, on the other hand, works out to be expensive.
The solar panels store the solar energy during the day and deliver light at night. Light-emitting diodes and low-energy lamps are used which sparingly use the power stored in the panels. For instance, the background lighting for a half-metre square information panel in a bus stop uses only 1.3 watts.
Meanwhile, scientists from London's Imperial College report the development of a device to enhance the efficiency of conventional solar photovoltaic cells. Called the "quantum well", the device is an extremely thin layer of a semiconductor sandwiched between 2 layers of another. Conventional solar cells are junctions between 2 layers of a semiconductor, each layer having different qualities because of being doped with different impurities.
The new device, its developers Keith Barnham, Jenny Nelson and Paul Griffin, hope would ultimately increase the eff iciency of a solar cell to the theoretical limit of 31 per cent calculated by physicists- Solar cells available commercially show an efficiency of about 20 per cent.
"We have all the elements to turn a 20 per cent efficient cell into one with 22 per cent efficiency," says Barnham, adding that there's still "an awful lot of optimisation to do."
In a 1993 experiment, Barnham and his team were able to raise the efficiency of an aluminium gallium arsenide solar cell from the 9 per cent then attainable to 14 per cent by incorporating 50 quantum wells of gallium arsenide.
Development Organisation and the Central Research Institute of Electric Power industries, are exploring drill holes to depth "s of 2 lcm, where temperatures reach around 200
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