Solar energy is the most important source of energy available to the Earth and its inhabitants. Without it there would be no life. It is the energy that drives the photosynthesis reaction. As such, it is responsible for all the biomass on Earth’s surface, which is the source of fossil fuels, the products of photosynthesis mil- lions of years ago that became buried beneath Earth’s surface. Solar energy cre- ates the world’s winds and is, therefore, responsible for wind power; it evaporates the water that is responsible for rain and, therefore, the source of hydropower; even waves and ocean thermal power are both a result of insolation. In fact, apart from nuclear energy, geothermal energy, and tidal power, the sun is wholly or partly responsible for all the forms of energy that are exploited by humans.

While all these different sources of energy, each derived from the sun, can be used to generate electricity, solar energy itself can be used to generate electricity too. This can be achieved most simply by exploiting the heat contained in the sun’s radiation. The use of solar thermal energy has a long history.

Solar thermal power exploitation—that is, the use of the sun as a source of heat—can be traced back at least to Archimedes, but its application as a means to generate power is more recent. Among the early examples were attempts made during the 19th century to use parabolic reflectors to concentrate the sun’s heat energy and raise steam for a steam engine. At the start of the 20th century solar energy was harnessed to drive an engine that pumped irrigation water for agricultural use in Egypt while the first solar thermal power-generating plant was built in Italy in the 1960s. However, the real driving force for the development of solar thermal power generation was the energy crises of the 1970s.

Electricity can also be generated directly from sunlight using an electronic device called the photovoltaic or solar cell. This second route for converting sunlight into electricity can also be traced to the 19th century to the work of the French scientist Antoine-Ce´sar Becquerel. He was the first to observe the photovoltaic effect during which a voltage is generated when light falls upon an electrode. Following this, the first true solar cell was built at the end of the 19th century by Charles Fritts who coated gold onto selenium to capture light energy. Fritts’ device was very inefficient and it was not until the devel- opment of the silicon solar cell by Russell Ohl in 1941 that efficient solar energy collection by this means appeared feasible. This was demonstrated in 1954 when three scientists at Bell Labs in the United States—Gerald Pearson, Calvin Fuller, and Daryl Chapin—produced a cell with an energy conversion efficiency of 6%, and not long afterwards solar cells were adopted as power sources in the U.S. space program.

Today the solar photovoltaic cell or solar cell is rapidly becoming one of the most important sources of renewably generated electricity, and its global installed capacity is third among renewable technologies after hydropower and wind power. Solar thermal power generation has developed less rapidly but it too is beginning to show stronger growth and has the potential to become a major source of power generation later in the century.

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