Green shorts - Solar hot water systems

 

 

 

Solar hot water
A domestic solar hot water system is one which absorbs the sun’s energy and transfers it to a storage cylinder. It is different from photovoltaics in that it does not produce electricity, the solar heating panels heat water directly.
In the UK it is unlikely that a solar hot water system will be the sole provider of hot water; it will likely complement a conventional system using gas, oil, electricity or solid fuel; however, it will pre-heat water so that bills are drastically reduced. During summer months the system may well provide all of the hot water needed by a household.
Solar heating panels can either transfer heat to a separate pre-heat cylinder, or heat a twin-coil cylinder via the bottom coil.

Direct and indirect systems
In a direct system, the water that passes through the panels is the water that eventually comes out of the hot tap; in an indirect system, the water in the panels passes through a heat exchanger (coil) in the cylinder and then back to the panels in a continuous loop. Anti-freeze can be added to this. This system has the added benefit of having no problem with lime-scale build-up.
The two main types of collectors are flat-plate and evacuated tube. Flat-plate collectors heat the water directly, evacuated tubes contain a fluid which evaporates at low temperatures, and the resulting gas rises and condenses on a manifold, transferring its heat as it does so; their extra complexity is reflected in their price.
Flat Plate Collectors consist of a black absorber – dark colours attract most heat - contained in a weatherproof box (housing), insulated at the back and glazed at the front (transparent cover), with a series of pipes, containing the heat transfer medium, running through the box to the hot water tank. Both the insulation below the absorber and the glazing at the front reduce heat losses that occur due to convection. A flat plate collector uses the well-known ‘greenhouse effect’. The energy from the sun reaching the earth is mainly in wavelength that we cannot see. The glazing material (glass or plastic) does not absorb these wavelengths and when the sun shines the heat is allowed to pass to the blackened absorber. As the sun heat radiation passes through the collector cover it warms the absorber plate and air in the collector and is prevented from escaping directly back to the atmosphere. This rise in temperature is passed to heat transfer fluid in pipes, which then passes through the hot water tank and transfers heat to the water in the tank, heating water for washing, showers and baths.
Evacuated Tube Collectors create a vacuum between the glazing and the absorber plate, thus reducing the losses in the collector, through convection and conduction. In addition, this protects the absorber plate in the long term against corrosion or other environmental influences. However, the improvements in solar collection achieved by these techniques have to be weighed against the extra costs incurred.
In this type of vacuum collector, rather than one large absorber plate there are individual absorber strips each located in an evacuated and pressure proof glass tube. The heat transfer fluid flows through the absorber directly rather than being heated by it. The heat pipe collector incorporates a special fluid, which begins to vaporize even at low temperatures. The steam in the individual heat pipes rises to warms up the carrier fluid in the collection tube by means of a heat exchanger. The condensed liquid then flows back into the base of the heat pipe. A typical system consists of several single tubes connected together in a row.

What are the benefits?
Solar hot water is probably the most cost-effective renewable energy technology that can be installed in a domestic situation in this country, coupled with the shortest payback time.
A DTI investigation into solar hot water systems in the UK from 1970-2000 found that a typical system will provide 72% of a household’s hot water over the course of a year (around 15% in winter and 100% in summer). This is assuming that the roof is south-facing – although facing south-east or south-west amount to only a 5% loss of efficiency.

 

Background information
Solar power facts
As part of an energy efficient system, a solar water heater will save fuel and help to achieve reductions in emissions.
Active solar power