Hot water production in the UK accounts for some 8% of all our total energy use. In houses it is typically produced by heat from gas or oil fired central heating boilers or from electric immersion heaters (or a combination of the two). This results in millions of tonnes of carbon dioxide being released into the atmosphere each year. The use of solar energy reduces these emissions thus helping to combat climate change.
Solar thermal systems are a simple and well-proven technology producing low-carbon energy. A small amount of electrical energy is consumed to run a pump circulating a fluid, which is heated in the solar collector by free energy from the sun. Tests have shown that solar water heating systems produce between ten and twenty times more heat energy than they consume in electricity, a figure which compares very favourably with other renewable heating technologies.
Demand for hot water is fairly constant year round which means that there is a significant demand for hot water when solar energy is at its peak in the summer months. Solar water heating systems can provide most of the hot water during this period as well as making a useful contribution in the less sunny winter months. Solar water heating can also make a contribution to space heating but this usually involves much larger systems, not least because the availability of solar energy is much less in the winter when heating is most required. Typically, in the UK, solar water heating is most economic when used to heat water for baths, showers, hand washing and other domestic hot water requirements. It is also very effective at heating swimming pools and has been used in industry, in hotels and for agricultural applications such as dairy farms.
A solar water heating system uses solar collectors, normally mounted on a roof, to capture the energy released by the sun to heat water. There are many variations in design of the system, but they all make use of the same principles.
The two main systems are either:
Another type of system, primarily used in the hotter areas of Southern Europe is the ‘Thermosyphon system’. This is a roof mounted system which not commonly found in Northern Europe as it is suited to hotter climates.
As far as we are aware, all installed systems in the UK are either a Drainback or Fully Filled system. Both these systems can be direct or indirect, with a direct system being one where the water used at the taps is circulated through the system, and an indirect system is one where a heat transfer fluid is used instead.
The diagram shows an indirect system where the heat transfer fluid is circulated through the system. This is a very basic module of a system, but shows how the main components fit together within a solar hot water heating system.
The systems work with an electronic controller constantly comparing the temperature of the solar collectors with the temperature of the water in the cylinder. Whenever the collectors are hotter than the cylinder (unusually by around 4-6ºC( the controller switches on the systems circulated through the collectors and the cylinder’s heat exchanger, heating the cylinder in just the same way as a central heating boiler.