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the tropical zones have no use for extra heat. There, however, cooking by solar
energy is becoming more and more important in everyday life.
India has a very limited supply of fuel – its main source for the home is dried
cow dung, which of course would be much better employed in fertilizing the soil. But
India has an abundance of sunshine. As early as the 1880's, an Englishman working
in India suggested the introduction of a cheap solar cooker, but until fairly recently no
really efficient device suitable for mass production had been invented. The Indian
National Physical Laboratory and one of the United Nations agencies eventually
developed solar cookers, which are being used in creasingly in Indian homes. One
type uses a reflecting mirror and a pressure cooker, another has four flat mirrors and
an insulated heat-collecting box filled with Glauber's salt crystals, which continue to
release heat when the sun has already set.
In the Sudan and East Africa a simple type of solar cooker has become fairly
popular. It consists of a concave aluminium reflector 4,25 feet across, mounted on an
upright iron rod; the concentrated rays of the sun fall on the pot or pan placed on a
wire-mesh holder which is attached to the reflector.
Another very important device is the solar 'still' for the distillation of fresh
water from salt water, usually working on the principle of a salt-water container
covered by a sloping glass roof; as the heat of the sun evaporates the water, the
vapour condenses in droplets on the glass roof from where they trickle down into a
fresh-water collector. The equally valuable salt is left behind in the saltwater
container.
Solar furnaces are still very much in the experimental stage. French scientists
are operating them in their research station in the Pyrenees; they are very large – one
has a flat reflecting mirror made up of 516 panes and covering an area of 43 feet
square and a 31-foot by 33-foot parabolic mirror at a distance of 80 feet. The heat
produced by this arrangement is sufficient to melt 130 lb, of iron per hour. The
Russians have built an enormous 'helio-boiler', consisting of an 80-foot tower
surrounded by twenty-three concentric railway tracks; bogeys move around on these
tracks, each carrying a 10-foot by 16-foot reflector to concentrate the sun's rays on to
a boiler in the tower. It is claimed that this machine produces enough superheated
steam for a turbogenerator with 1,000 kW output.
The most efficient way of generating electricity from sunlight, however, seems
to be the 'solar battery'. The first of this type was demonstrated in 1954 by a team of
scientists from the American Bell Laboratories. It operated with semi-conductor
crystals similar to those used in transistors either of germanium or of silicon. When
sunlight strikes such a crystal, an electric current is generated. A Bell battery of 400
silicon cells was able to produce a 12-volt current. Since its first demonstration, the
solar battery has been extensively developed and has taken part in one of Man's
greatest adventures – the sending of satellites and rocket vehicles into space. Solar
batteries, as well as the already mentioned atomic batteries, are very suitable for
powering the transmitters in space vehicles because of their long life.
Eventually, solar batteries may be developed to provide all the low-voltage
current needed in a house. Their theoretical top efficiency is 22 per cent,
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