History. The Earth receives an incredible supply of solar energy

The Earth receives an incredible supply of solar energy. The sun, an average star, is a fusion reactor that has been burning over 4 billion years. It provides enough energy in one minute to supply the world's energy needs for one year. In one day, it provides more energy than our current population would consume in 27 years.

Solar energy is a free, inexhaustible resource, yet harnessing itis a relatively new idea. The ability to use solar power for heat was the first discovery. A Swiss scientist, Horace de Saussure, built the first thermal solar collector in 1767, which was later used to heat water and cook food. The first commercial patent for a solar water heater went to Clarence Kemp of the US in 1891. This system was bought by two California executives and installed in one-third of the homes in Pasadena by 1897.

Producing electricity from solar energy was the second discovery. In 1839 a French physicist named Edmund Becquerel realized that the sun's energy could produce a "photovoltaic effect" (photo = light, voltaic = electrical potential). In the 1880s, selenium photovoltaic (PV) cells were developed that could convert light into electricity with 1-2% efficiency (the efficiency of a solar cell is the percentage of available sunlight converted by the photovoltaic cell into electricity), but how the conversion happened was not understood. Albert Einstein proposed an explanation for the "photoelectric effect" in the early 1900s, for which he won a Nobel Prize.

"Solar technology advanced to roughly its present design in 1908 when William J. Bailey of the Carnegie Steel Company invented a collector with an insulated box and copper coils." By the mid-1950s Bell Telephone Labs had achieved 4% efficiency, and later 11% efficiency, with silicon PV cells. From then on, interest in solar power intensified. During the late 1950s and 1960s, the space program took an active role in the development of photovoltaics. Unfortunately, the cells were not practical for use on earth due to the high cost of making them efficient and lightweight, so further research was necessary.

"Only in the last few decades when growing energy demands, increasing environmental problems and declining fossil fuel resourcesmade us look to alternative energy options havewefocused our attention on truly exploiting this tremendous resource." For instance, the US Department of Energy funded the installation and testing of over 3,000 PV systems during the 1973-1974. Solar energy improvements were again sought during the Gulf War in the 1990s.

Considering that "the first practical solar cells were made less than 30 years ago," we have come a long way. The biggest jumps in efficiency came "with the advent of the transistor and semiconductor technology." The production cost has fallen to nearly 1/300 of what it was during the space program of the mid-century and the purchase cost has gone from $200 per watt in the 1950s to a mere $5 per watt today. The efficiency has increased dramatically to 18.8% (February 1999).