As any plant would happily tell you if it could talk, the Sun is the ultimate renewable energy source. The plant may then rightly remind you that its kind is the only truly green energy technology there is.
The feeble attempts by men to turn solar rays into electricity take 2 forms: solar concentrators and photovoltaic panels. Solar concentrators are thermal conversion systems which focus the Sun’s rays to heat a fluid that produces steam, thereby driving a turbine. Photovoltaic cells more directly convert solar energy to electricity, requiring only an electrical inverter to achieve their aim.
There are 3 main solar concentrating thermal conversion systems: parabolic troughs, parabolic dishes, and central receivers.
Parabolic troughs are the simplest of the solar thermal systems. A curved trough covered with reflectors rotates to track the sun. The mirrors focus sunlight onto a fluid-filled pipe at the center of the trough. The heat transfer fluid, typically a thermal oil, may reach 400 °C. The hot oil is then used to make steam for a standard turbine generator that produces electric power. Trough systems can concentrate solar radiation to 100 times the intensity of normal sunlight.
In 1866, French inventor Auguste Mouchout used a parabolic trough to power a steam engine. American inventor and engineer Frank Shuman built the first parabolic trough system in 1897. He patented the entire system in 1912.
Cheap oil subsequently supplanted all interest in solar energy until the 1970s, when Shuman’s basic design found favor, albeit updated with better materials.
Parabolic dishes are like troughs, except they focus light to a point instead of a line. Such systems can concentrate up to 10,000 times the intensity of sunlight.
Central receiver systems use an array of computer-controlled sun-tracking mirrors (heliostats). The heliostats focus sunlight onto a single central tower, termed a receiver. The receiver is filled with a mixture of molten thermal salts. Hot molten salt is pumped through heat exchangers which produce steam to turn a turbine that generates electricity.
Commercial deployment of concentrating solar power plants commenced in the US in 1984, lasting only to 1990. Then, from 1991 to 2005, no plants were built anywhere in the world.
Since then, plants have built in Spain, US, India, and the Middle East. Sunny countries have a promising future of decently clean energy technology with concentrating solar power. But concentrators have not got nearly the attention of their downright dirty solar cousin: photovoltaics.