Solar power: thermal and electrical

     Harnessing energy from the Sun is, of course, an environmentally acceptable method of producing power. Conversion via heat exchange units (often referred to as solar panels) provides thermal energy to raise the temperature of swimming pools or to provide domestic hot water. Conversion via photovoltaic systems (often termed solar cells) produces electricity and involves the use of semiconductors. Initially, NASA’s space programme was the driving force behind the development of solar cells, and applications in satellites and other space vessels remain at the cutting edge of design technology. However, we all now feel the benefits of solar cells which are used in items such as solar-powered calculators. Silicon has been the workhorse of this commercial operation. The thickness of a typical cell is 200–350 mm, and is constructed of an n-doped layer (which faces the sun), a p-doped layer and a metal-contact grid on the top and bottom surfaces. The latter are connected by a conducting wire. At the n–p junction, electrons move from the p-type to the n-type silicon, and ‘holes’  move in the opposite direction; this leads to a flow of electricity around the circuit. Power output per cell is small, and a large number of cells must operate together to produce a viable voltage supply. Weather conditions and the number of daylight hours are key factors that have to be accommodated if adequate solar power is to be generated for domestic or similar uses. Other semiconductors in use in solar cells include GaAs (e.g. in space satellites), CdTe (a promising newcomer to solar cell development) and TiO₂ (used in the Gratzel cell which involves a novel design in which a TiO₂ film is coated with an organic dye).