A solar collector is a device for extracting the energy of the sun directly into a more usable or storable form. The energy in sunlight is in the form of electromagnetic radiation from the infrared to the ultraviolet wavelengths. The solar energy striking the earths surface at any one time depends on conditions and your location on the surface, but overall averages about 200 Watts per square metre.
A typical solar collector uses water as the storage medium, because it has a high thermal capacity and is convenient to handle. The direct radiation is captured using a black-painted surface which absorbs the radiation and conducts it to the storage medium. Metal is usually used because it conducts heat well, and copper or aluminium is best. By coating the surface with black paint the surface has the properties of a black-body radiator, which will readily absorb radiation.
As it heats up, the collector will itself start to radiate heat back into space, which reduces its efficiency. This is countered in two ways. First, a glass plate is placed above the collector plate which will trap the radiated heat within the airspace below it. This exploits the so-called greenhouse effect, which is in this case a property of the glass - it readily transmits solar radiation in the visible and ultraviolet spectrum, but does not transmit the lower frequency infra-red re-radiation very well. The plate is also insulated below to prevent losses by radiation to whatever is below the collector. The second way efficiency is improved is by cooling the collector plate - this is readily done by ensuring that the water is circulated sufficiently quickly through it - the water carries away the absorbed heat so cooling the plate. The warmer water is either directly stored, or else pasess through a heat exchanger to warm another tank of water, or used to heat a building directly. The temperature differential across an efficient solar collector is usually only 10 or 20 degrees C - a large differential may seem impressive, but is in fact an indication of a less efficient design.
The two main types of solar collector system are Thermosyphon and pumped. In the thermosyphon system, the storage tank is placed above the collector. As the water in the collector is heated, it will rise and naturally start to circulate to the tank. This draws in colder water from the bottom of the tank. This system is self regulating and requires no moving parts or external energy, so is very attractive. Its main drawback is the need for the tank to be above the collector, which may prove to be physically difficult. A pumped system uses a pump to circulate the water, so the tank can be below the collector. This system requires external energy to run the pump (though thus can be solar, since the water should only be circulated when there is incident sunlight). It also requires control electronics to measure the temperature gradient across the collector and modulate the pump accordingly.
Solar collectors can be mounted on a roof but need to face the sun, so a north facing roof in the southern hemisphere, and a south-facing roof in the northern hemisphere are required. Collectors are usually also angled to suit the latitude of the location, biased in favour of the winter position of the sun. Where sunshine is readily available, a 6-10 square metre array will provide all the hot water heating required for a typical family house. Such systems are a key feature of sustainable housing, since water and space heating is usually the largest single consumer of energy in households.