Flat plate collectors

These consist of a black painted metal sheet which is directly heated by the sun. This heat is transferred to a fluid in tubes attached to, or embedded in, the sheet. In some systems the water which is to be heated is used directly, in others a separate fluid flows through the collector and transfers heat to the water via an exchanger. The latter system is preferred as the fluid's properties can be selected, for instance one can ensure that it does not freeze in cold weather etc. To decrease heat losses the metal sheet is placed in a shallow insulated box with a glass or perspex lid. A well designed system will collect about 40-50% of the sun's energy, although system efficiencies, taking into account losses in pipework etc, are more likely to be about 30-40%.

A more sophisticated systems involves a metal collector plate contained in an evacuated glass tube and placed at an angle. This avoids any convective losses from the collector plate (the same principle as a vacuum flask). A tube connected to the plate contains a fluid with a low boiling point which evaporates when heated by the sun. The vapour rises to the top of the tube where it exchanges its heat with a pipe containing water, condenses and returns to the bottom of the tube to be reheated. Vacuum collectors are more efficient than flat plate ones, converting 60-70% of the sun's energy to useful heat, and heating water to very high temperatures. System efficiencies, taking into account losses in pipework etc, are usually more like 50%.

System design

Provision must be made for a hot water tank, preferably close to the collectors to minimise pipework and heat losses. In most systems the circulant fluid is pumped through the tank and the collector, controlled by  a sensor which detects that the collector fluid is at a higher temperature than that in the tank. If the hot water tank is higher than the collector then a thermosyphon system can be used in which the buoyancy of the heated circulant in the collector drives the fluid around.

Pumped system	Thermosyphon system

Partly because the principal market is for retrofitting to houses, quoted prices for full installation are high, from £750-900 per square metre for flat plate collectors and £1300-1500 per square metre for vacuum tubes. However for larger installations, or multiple small installations, costs can be reduced to £500 per square metre or less. A well maintained system will have a lifetime of 20 years or more.

Further information can be obtained from the UK Solar Energy Society , the National Energy Foundation and Energy Efficiency Advice Centre and the National Energy Foundation's Renewable Energy Website