The sun is humanity's oldest energy source, and scientists and engineers have been trying to harness the power of sunlight for a wide range of heating, lighting, and industrial tasks for a long time. Every child knows that focused sunlight is hot enough to set things afire; engineers and scientists know that every square meter of the Earth receives about 1 kilowatt of thermal energy when the sun is overhead. Gathering and converting this energy into usable form has been explored since burning mirrors were first used in China in about 700 BC for ignition of firewood.
Solar thermal collectors are devices that take solar energy in the form of light and convert it into heat. Two types of solar thermal collectors are being used, evacuated tube collectors and flat-plate collectors.
Click for more specific information on solar hot water heating or solar thermal systems for space heating and cooling.
Glazed flat-plate collectors are insulated, weatherproofed boxes that contain a dark absorber plate under one or more glass or plastic (polymer) covers. Water is heated when it is pumped through tubing inside the collector. This is a relatively simple and robust design which has been in use since William J. Baily patented the "Day and Night" water heater in 1909.
The inherent weakness of the flat plate design is the loss of efficiency as the fluid temperature increases relative to the ambient temperature. This makes them less suitable in systems where a high fluid temperature is needed or where the ambient temperature gets very cold. However, modern manufacturing methods and improved insulation materials have vastly increased the efficiency of these panels over the early prototypes.
There are also unglazed flat-plate collectors which have a dark absorber plate, made of metal or polymer, without a cover or enclosure. These are typically used for pool heating applications in warm climates.
Evacuated tube collectors are more efficient at higher fluid temperatures because they use a vacuum instead of traditional insulation so almost all heat loss is eliminated. This is accomplished by removing the air between two cylinders of glass and fusing them together to make something like a thermos. Anti-reflective coatings are applied to the inner cylinder and a heat pipe is placed in the center.
The heat pipe is an evacuated copper tube with a small amount of fluid inside. All heat collected is concentrated into the heat pipe and moved to the top of it by evaporation and condensation of the fluid in the pipe. The top of the heat pipe fits into a dry socket in a copper header at the top of the panel. A water/glycol fluid is pumped through the header to absorb the heat from the heat pipes and then into a building to be used.
Solar cells, also called photovoltaic (PV) cells by scientists, convert sunlight directly into electricity. PV gets its name from the process of converting light (photons) to electricity (voltage), which is called the PV effect. The photovoltaic effect was first noted by a French physicist, Edmund Bequerel, in 1839, who found that certain materials would produce small amounts of electric current when exposed to light.
The first photovoltaic module was built by Bell Laboratories in 1954, but it was too expensive for practical applications. Today, solar PV systems are found everywhere from wrist watches to airplanes and thousands of people power their homes and businesses with them. Utility companies are also using PV technology for large power stations. Click for more specific information on solar PV systems.