In 1913 a solar plant in Egypt pumped Nile water to cotton fields
Earth absorbs more sunlight in about an hour than humanity uses in a year. People have tried to tap it for a long time: a solar steam engine ran at the 1878 Paris exposition, and by 1913 an American inventor's mirrored troughs near Cairo were pumping thousands of litres of river water a minute.
Sunlight arrives at the top of the atmosphere at about 174 petawatts. Roughly 30 percent bounces back into space, and the remaining 122 petawatts is soaked up by clouds, seas and land. That absorbed energy drives evaporation, convection, rain, wind and storms, holds the surface at an average of 14 °C and, through photosynthesis, builds the biomass from which even fossil fuels ultimately come. Across a year it adds up to around 3,850,000 exajoules. Almost every renewable source, apart from geothermal and tidal power, traces back to the Sun.
The pioneers ran into cheap fossil fuel. Augustin Mouchot showed off a solar steam engine in Paris in 1878 but abandoned it because coal was so inexpensive. Frank Shuman built a small engine in 1897 that boiled ether in black-piped boxes, then with A. S. E. Ackermann and Charles Vernon Boys used mirrors to reach temperatures high enough for water. Between 1912 and 1913 he built what is described as the world's first solar thermal power station, at Maadi in Egypt, where parabolic troughs drove an engine that lifted more than 22,000 litres a minute from the Nile. World War I and then cheap oil in the 1930s shelved the idea until interest revived in the 1970s.
Engineers split the technology into active and passive. Active systems such as photovoltaics, concentrated solar power and solar collectors add energy supply. Passive design cuts demand: thermal mass like stone or water that soaks up daytime heat, solar chimneys that pull air through a building as they warm, and deciduous trees that shade windows in summer and let light through in winter, though bare branches still block a third to a half of sunlight.
Solar hot water is already widespread. By 2015 about 436 thermal gigawatts were installed, with China holding 71 percent, and in Israel and Cyprus over 90 percent of homes use it. Even the humble box cooker, an insulated container with a transparent lid that Horace de Saussure devised in 1767, can reach 90 to 150 °C under partly cloudy skies.
Source: Solar energy
