solar energy

Energy derived from the light and heat from the Sun. The amount of energy falling on just 1 sq km/0.4 sq mi is about 4,000 megawatts, enough to heat and light a small town. Solar heaters usually consist of concave mirrors or reflective parabolic surfaces that concentrate the Sun's rays onto a black (heat-absorbing) panel containing pipes through which air or water is circulated, either by thermal convection or by a pump. The heat energy of the air or water is converted into electrical energy via a turbine and a generator. Hot water for industrial and domestic use can be produced by circulating water through panels, the water absorbing heat from the Sun as it passes through the panels.

Solar energy may also be harnessed indirectly using solar cells (photovoltaic cells) made of panels of semiconductor material (usually silicon), which generate electricity when illuminated by sunlight. Although it is difficult to generate a high output from solar energy compared to sources such as nuclear or fossil fuels, it is a major non-polluting and renewable energy source used as far north as Scandinavia as well as in the southwestern USA and in Mediterranean countries.

A solar furnace built in 1970 at Odeillo in the French Pyrenees had thousands of mirrors to focus the Sun's rays; it produced uncontaminated intensive heat (up to 3,000°C/5,400°F) for industrial and scientific or experimental purposes. The world's first solar power station connected to a national grid opened in 1991 at Adrano in Sicily, Italy. Scores of giant mirrors move to follow the Sun throughout the day, focusing the rays into a boiler. Steam from the boiler drives a conventional turbine. The plant generates up to 1 megawatt. A similar system, called Solar 1, has been built in the Mojave Desert near Daggett, California, USA. It consists of 1,818 computer-controlled mirrors arranged in circles around a central boiler tower 91 m/300 ft high. Advanced schemes have been proposed that would use giant solar reflectors in space to harness solar energy and beam it down to Earth in the form of microwaves.

In March 1996, the first solar power plant capable of storing heat was switched on in California's Mojave Desert. Solar 2, part of a three-year government-sponsored project, consists of 2,000 motorized mirrors that will focus the Sun's rays on to a 91 m-/300 ft-high metal tower containing molten nitrate salt. When the salt reaches 565°C/1049°F it boils water to drive a 10 megawatt steam turbine. The molten salt retains its heat for up to 12 hours.

Despite their low running costs, their high installation cost and low power output have meant that solar cells have limited applications outside space probes and artificial satellites. However, increasing environmental pressure is pushing forward the need for the development of more efficient solar panels. Solar heating is now widely used for domestic purposes in many parts of the world, and is an important renewable source of energy. Large-scale power generation schemes in countries with suitable desert sites, such as Australia and parts of the USA, are constantly proposed and are becoming increasingly economically viable.