by
Paul
BenhaimToday, those problems are behind us. The UK alone has
over 44,000 solar water heating systems. A survey in 1995 found
that most were saving up to S200 per year, and around 75% of
customers were Very satisfied' with their systems. Solar water
heating is commercially mature technology, competing very well
with conventional energy sources - although, obviously, the
payback time relates directly to the available solar resource of any
particular site. Typical installation costs vary from about S800 for a
Do It Yourself system, to S4500 for a commercial system which
can provide approximately 60% of typical domestic hot water
needs in the UK.
The
'Photovoltaic
Effect'
Generating electricity from sunlight. Exploration of the photovoltaic
effect began a surprisingly long time ago. In 1839, the French
scientist Bequerel noticed that when light was directed on to one
side of a simple battery cell, the generated current could be
increased. Thirty-four years later, the British scientist Willoughby
Smith discovered that selenium was sensitive to light. He found
that selenium's ability to conduct electricity increased if it was
exposed to more light. This discovery inspired scientists to carry
out further experiments with this rare element. In 1880, Charles
Fritts developed the first selenium-based solar cell. Research
continued on the selenium solar cell throughout the first half of the
20th century, despite its very low efficiencies and expensive
production costs. The real breakthrough came in the 1950s when
Bell Laboratories discovered that silicon -the second most
abundant element on earth - was also sensitive to light and
generated a substantial voltage when treated with certain
impurities. By 1954, Bell had developed a solar cell using silicon