How humans are learning to harness solar power

The simplest way to tap the Sun's power is to collect its heat. In the 18th century, the Swiss scientist Horace-Benedict de Saussure (1740 - 99) designed the first solar heating panel. The panel a simple wooden box with a glass top and a black base - worked on the principle that sunlight penetrating glass will be absorbed by a dark surface on the other side, and trapped as heat. This is the reason why greenhouses become so much hotter than the air outside on a sunny day. De Saussure's panel reached temperatures of up to 88'C hotter than the world record open-air temperature.

Another way to collect solar energy is to build 'solar ponds'. At their simplest, these are lakes of salty water, which gradually collect the Sun's heat in the deepest, most salty layers. In a freshwater pond, convection currents keep lifting the warm water so that it mixes with cooler water and loses its heat. But in a saltwater pond, the saltiest layers are the densest, so they sink to the bottom and stay there, counteracting the process of convection.

Insulated in this way by the cooler upper layers, the bottom layers of the pond go on absorbing heat from the Sun, and their temperature can reach boiling point. To prove it. researchers in New Mexico, USA, have boiled eggs in five minutes by suspending them in a solar pond. Israel already generates some electricity from the heat that solar ponds accumulate. And by the year 2020, Israel plans to generate 30 per cent of its electricity in this way.

Some countries are trying to tap the heat of the Earth-what scientists call geothermal energy. In most parts of the world, the temperature of the Earth's crust rises only 25'C (45'F) with each kilometre in depth, which means that only very deep holes can tap high temperatures. In some volcanic areas such as Iceland, however, temperatures as high as 360'C (680'f) exist close to the surface, where they can be tapped relatively easily. In Denmark and Iceland, engineers use this heat by piping hot water from underground to warm nearby homes, offices and factories. An outdoor swimming pool in the Icelandic capital of Reykjavik is heated so efficiently by this method that it remains open and in use all year round.

On December 19. 1982, a solar-powered car-The Quiet Achiever-set out on the 4130km (2S66 miles) journey from the western Australian city of Perth to Sydney. The car, with an average cruising speed of about 25km/h (15mph), had a fibreglass body, bicycle racing brakes and tyres, an electric motor, batteries, chain drive - and a flat roof containing 720 solar cells. The car was co-driven by two Australians: Larry Perkins, a racing-car driver who, with his brother Gary, designed and built the machine: and Hans Tholstrup, whose idea it was. The Quiet Achiever coasted into Sydney on January 7, 1983, 20 days after starting off-becoming the first machine to cross a continent on sunshine alone.

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