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Solar water heating

Solar radiation comprises not only visible light, but also comprises longer wave radiation whose energy can be absorbed by gases, liquids or solids and can then be converted into heat energy. Obvious examples are the earth’s atmosphere which absorbs heat as light passes through it and the earth’s surface. It is the balance between absorption and reflection that enables life to be sustained on this planet. With solar heaters, selective absorber materials are used to absorb the infra red rays to heat water or a ‘solar’ fluid passing through a set of tubes.

The basis of all solar heater systems is the ability of the collector to absorb the infra red portion of sunlight and transfer this heat to water flowing in a tube. The flat plate collector is the most commonly used system and consists of a rectangular box typically 1-2 metres long and 800- 1000 millimetres wide. Small tubes which are attached to a black absorber plate run through the box with water flowing through these tubes which are heated by the absorption of the sunlight’s infra red rays.


  • the only external energy required is to pump the water through the solar collector
  • captures energy from sunlight which otherwise would not be utilised
  • negligible environmental emissions and impact
  • depending upon location. can produce up to 60% of hot water needs in the UK


  • possible visual impact on surroundings
  • less hot water in winter than summer
  • output dependent on weather conditions

Conversion process

conversion process

The ray diagram is illustrated in the Figure. Sunlight passes through the glass plate and onto an absorber plate. Most of the sunlight is absorbed; any that is reflected back towards the glass is in turn reflected by a heat reflective layer on the inside of the glass plate.

There are many flat-plate collector designs but generally all consist of: a flat-plate absorber, which intercepts and absorbs the solar energy; a transparent cover(s) that allows solar energy to pass through but reduces heat loss from the absorber; a heat-transport fluid flowing through tubes to remove heat from the absorber; and a heat insulating backing.


As a typical panel is 2 metres long by 1 metre wide, such panels are easiest mounted on the roof of a building if it is favourably orientated to receive sunlight during most of the day. The collector should ideally be mounted to face south and the optimum angle of mounting is related to the degree of latitude. For a flat roof this is easily achieved using a suitable frame. For a pitched roof, the collector is generally mounted parallel to the roof or for a new build can form part of the roof. If it is possible to adjust the collector angle, then an angle of 30º in summer and an angle of 70º in winter as the sun is lower in the sky are optimal in the UK.



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