Energy - Solar thermal
Solar water heating
Technologies harnessing solar energy for heating water (solar thermal) range from solar collectors for heating water for domestic and commercial use, to very large concentrating solar power systems which generate steam to drive multi mega-watt power stations.
A typical solar water heating system consists of a roof-mounted solar collector, a circulating pump, control unit and hot water storage cyclinder. A black-coated metal absorber plate within the solar collector housing, is heated by the sun. This heat is conducted to the transfer fluid circulating within tubing integrated with, or thermally attached to, the solar absorber plate. The pump circulates the fluid from the solar collector around a heat exchanger in the hot water cylinder and back to the collector. The controller ensures that the circulation pump is running only when the temperature of the fluid in the solar collector is higher than the temperature of the water in the hot water cylinder.
A larger array of solar collectors i.e. above that required for hot water requirements, together with an efficient thermal store, may also be used to provide a contribution to space heating.
Types of solar collector
The main types of solar collector are the Flat plate solar collector and the Evacuated tube solar collector. There are also concentrating type solar collectors which employ curved mirrors to focus the sun's energy on a tube containing water or other heat transfer fluid. As concentrating types must be able to track the sun throughout the day, these are normally used in large-scale solar thermal systems.
Flat plate collector
The flat plate solar collector is essentially of simple construction: an enclosure, with a glass or plastic cover, housing an absorber plate mounted on a bed of insulation. The glass cover helps reduce radiated heat loss from within the enclosure; the glass is largely transparent to short-wave solar radiation but opaque to the long-wave radiation emitted by the heated black absorber plate. The cover also prevents heat loss by convection - e.g. cooling effect of wind, and offers protection from the elements. The insulation behind the absorber plate and the sides of the enclosure, reduce conduction heat losses. The absorber plate (copper or aluminium), is given a special (wavelength selective) coating which readily absorbs short wave solar radiation (high absorptivity) but also has low losses due to long-wave thermal radiation (low emissivity).
Evacuated tube collector
Evacuated tube / vacuum tube solar collectors are constructed from a number of evacuated glass tubes. The vacuum created within the tube is a very efficient insulator. Within each tube is a narrow absorber plate thermally attached to a heat transfer tube. As with the flat plate collector, the absorber plate has a wavelength selective coating.
Solar thermal organisations
Solar water heating resources
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