The intense energy of the sun has long been used to heat liquids. Among the first mechanical uses of the sun was a 20-square-meter, parabolic concentrating reflector that boiled water and produced steam. This steam was used in a steam-driven printing press at the 1878 World's Fair in Paris.
In the late 1800s, relying upon the sun to heat water was common practice in the southwestern United States. Photos can be found showing pioneer families proudly showing off new homes equipped with solar water heaters. At one point, almost a quarter of the residents of Los Angeles relied upon the sun to heat their water with rooftop solar thermal systems.
The sun's heat can be used in two ways with homes and businesses. The sun is used to heat water for domestic hot water systems, or the sun's light can be concentrated and water temperatures increased to make steam and electricity.
Solar energy can also generate electricity. Over the past 20 years, solar electricity generation technologies have grown by leaps and bounds, registering annual growth rates between 25 and 41 percent. Costs have also fallen by 80 percent. Global solar electric generation technologies contribute roughly 2,000 MW of electricity today. That figure is less than a tenth of the world's global electricity supply.
While solar photovoltaics (PV) are better known, California actually gets far more of its electricity from a solar thermal power plants. Nine distinct solar thermal power plants located in the Mojave Desert (Shown in picture above) total 360 megawatts, by far the largest central solar power station in the world. (That's enough electricity to power about 360,000 homes.)
These solar thermal power plants rely upon curved mirrored troughs that concentrate sunlight. The sun heats a liquid that creates steam to turn a traditional turbine. A more efficient technology is called the "stirling dish," which is powered by an entirely new kind of engine. Instead of the internal combustion engine, which relies upon an explosion inside the engine walls to turn pistons, the dish stirling engine relies upon the sun to heat tubes filled with hydrogen that turn the crankshaft.
Solar PV panels register efficiencies ranging from 9 to 15 percent. The solar thermal trough rankine cycle facilities are approximately 22 percent. Stirling solar dishes have been measured at efficiencies as high as 30 percent. (These efficiency numbers are based on calculations that convert the sun's energy into the equivalent of British Thermal Units, a universally recognized measuring unit of energy commonly referred to as "BTU's". One BTU is the same quantity of heat required to raise the temperature of one pound of water one degree Fahrenheit.)
Solar thermal electric capacity is predicted to increase worldwide. The cost of building, operating, and maintaining solar thermal electric systems has decreased dramatically -- in some cases by a factor of 10 -- during the 1980s and 90s and is expected to continue dropping. Solar-thermal designs may be economically competitive with some conventional electricity-generating technologies. By 2010, some solar thermal electric technologies could be producing electricity at $0.06 to $0.07 per kilowatt hour (kWh).