Solar collectors transform solar radiation into heat and then transfer that heat to a medium (water, solar fluid, or air). The term is applied to solar hot water panels, but may also be used to denote more complex installations such as solar parabolic, solar trough and solar towers.
Solar hot water systems use sunlight to heat water. In low geographical latitudes (below 40 degrees) from 60 to 70% of the domestic hot water use with temperatures up to 60 °C can be provided by solar heating systems.[
For residential use the three most common types of hot water solar collectors are the flat plate solar collector, the evacuated tube collector, and the concentrating collector. They each work a little differently to heat water.
Flat plate solar collectors are the most common for heating water and air in the home. The flat plate solar collector is an insulated box with a plastic or glass “lid” on top. Water is heated when sunlight passes through the top plastic or glass glazing of the collector, strikes a dark-colored absorber plate underneath this “lid”, which then heats the air or water inside the collector.
An evacuated (vacuum) tube solar collector is an array of tubes. The vacuum tube solar is a second generation collector and it is more expensive then flat plate solar collector. According to some tests, the evacuated tube collector is approximately 1.6 to 4 times more effective than a flat plate collector (4 times more effective in January and only 1.6 in August).
Concentrating solar collectors use mirrored surfaces to concentrate the sun's energy on an absorber called a receiver. Concentrating collectors also achieve high temperatures, but unlike evacuated-tube collectors, they can do so only when direct sunlight is available.
In China currently is installed 114 million square meters of rooftop solar collectors for heating water and they plan to increase that to 300 million by 2020. Europe goal is to reach 500 million square meters of installed solar collectors by 2020, and US goal is 300 million square meters by 2020.
With appropriate assumptions for developing countries other than China, the global total of rooftop solar collectors in 2020 could exceed 1.5 billion square meters. This would give the world a solar thermal capacity by 2020 of 1,100 thermal gigawatts, the equivalent of 690 coal-fired power plants.
Flat plate solar systems were perfected and first used on a very large scale in Israel. In the 1950s there was a fuel shortage in the new Israeli state, and the government forbade heating water between 10 p.m. and 6 a.m. Levi Yissar built the first prototype Israeli solar water heater and in 1953 he launched the NerYah Company, Israel's first commercial manufacturer of solar water heating. Despite the abundance of sunlight in Israel, solar water heaters were used by only 20% of the population by 1967. Following the energy crisis in the 1970s, in 1980 the Israeli Knesset passed a law requiring the installation of solar water heaters in all new homes.
Solar thermal collectors for nonpotable pool water use are often made of plastic. Pool water, mildly corrosive due to chlorine, is circulated through the panels using the existing pool filter or supplemental pump.
The amount of heat delivered by a solar water heating system depends primarily on the amount of heat delivered by the sun at a particular place. In tropical places the insolation can be relatively high, e.g. 7 kWh per day, whereas the insolation can be much lower in temperate areas where the days are shorter in winter, e.g. 3.2 kWh per day.
The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850,000 exajoules (EJ) per year. In 2002, this was more energy in one hour than the world used in one year.