Energy - Solar thermal

Solar water heating

Solar water heating panels

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.
There are two main types of heat transfer tube - the simpler direct-flow type and the heat-pipe type. In the direct-flow type, the heat transfer tube takes the form of a U-tube, or a tube separated internally into flow and return paths. A highly insulated manifold connects a number of collectors to a common flow and return through which the transfer fluid is pumped.
In the second type, the absorber plate is thermally attached to a heat-pipe - a highly efficient heat transfer device, which rapidly conducts the heat from the absorber to a heat exchanger at the end of the heat-pipe. A number of individual evacuated tube solar collectors tubes are connected to a highly insulated manifold. Thermal energy from the heat-pipes is transfered to the transfer fluid pumped through the manifold. The evacuated tube solar collector can be a very efficient device performing well even in diffuse solar energy conditions e.g. low sun or under cloudy skies.

FURTHER INFORMATION

Solar thermal organisations

  • Solar Trade Association
    An organisation for those with business interests covering solar thermal applications, such as the production of domestic or industrial hot water or the heating of swimming pools. The has a database of members who abide by the STA Code of Practice. More information on the STA website
  • Energy Saving Trust
    Solar water heating information page with directory of certified installers and products. www.energysavingtrust.org.uk

Solar water heating resources

  • Centre for Alternative Technology
    Provide an excellent free information service on solar water heating and other applications of renewable energy. www.cat.org.uk (home page). Solar water heating direct link.
  • Build-It-Solar
    A wealth of information on DIY approach to solar water heating and other renewable energy topics. This USA website also includes Solar Analysis tools www.builditsolar.com
  • Arizona Solar Center
    Another USA website strong on solar thermal collector information. www.azsolarcenter.org/
  • Low-Impact Living Initiative (LILI)
    LILI help people to understand the damage that human impact is causing to our planet, to change aspects of their lives to lower their own impact, and at the same time save money and improve their quality of life. They produce a factsheet on solar water heating. www.lowimpact.org

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