People have been using solar water heating systems since the 1970s. Solar hot water panels – otherwise known as solar thermal – work by absorbing radiant heat from the sun. This warms liquid in the panel, which is then transferred with directly or indirectly to a hot water tank inside the house. They do not rely on direct sunlight alone but they will produce more hot water when it is sunny.
Many properties are suitable for solar thermal panels. They work best when facing south. If there is not enough room to have the panel on the roof it is possible to mount them on a frame on the side of a building, or on the garage. However, the further the panels are from the hot water tank, the more heat will be lost when water is transferred to the hot water tank from the panel.
It is possible to install two smaller panels on the roof if sunlight doesn’t fall on the same area for most of the day, although this may increase installation costs.
Different technologies
There are three types of solar water heaters: evacuated tube, flat plate and unglazed collectors. The hot water can either be supplied to the home through a twin coil tanks, or by adding a second tank.
Flat plate
Consist of a dark, flat plate that absorbs heat from the sun.
The absorber plate contains thin copper pipes. Heat absorbed is transferred to water in these pipes by conduction, which is then carried to the hot water tank.
These are slightly less efficient compared to evacuated tubes – so the collector area may be slightly bigger to gain the same performance.
Evacuated tube
Made up of an array of glass collector tubes, within each of which is another glass tube – the space between the tubes is evacuated to minimise heat loss.
Radiation from the sun can pass through this evacuated space where heat is absorbed to a liquid contained in the inner tube.
There are two main types:
Direct flow: Water flowing to the hot water tank passes directly through the inner pipe of the solar collector.
Indirect flow: The inner pipe contains a condensing liquid, usually alcohol. When heated, this evaporates and rises thorough the tube, condensing when it passes water flowing to the hot water tank and transferring the heat. These can have an inbuilt thermostat.
Evacuated tubes are often lighter and smaller than flat plates, which can increase ease of installation. Evacuated tubes will give a better efficiency at heating water in the winter months compared to a flat plate collector.
However, the glass tubes will make them a little more fragile than a flat plate.
Unglazed collectors
A series of small black pipes in which water is pumped and collects heat directly from radiation from the sun. These are only really suitable for low temperature applications such as swimming pools.
Water tanks
Twin coil tanks contain two heating coils. One coil is fed by the boiler, the other is fed by the solar collector. The tank will be larger than most standard tanks, although only one tank is required.
A second tank will require extra space. It will be of a similar size to the original tank and only store water heated by the solar thermal panel.
Temperature control
Modern solar collectors can allow water to reach very high temperatures that could potentially cause a risk of scalding. Thermostatic mixing valves can be fitted to the system which allows cold water to be mixed in with the hot water to maintain a constant temperature.
Finding the product
Solar thermal units can be purchased from a number of manufacturers. We strongly recommend that these should always be fitted by an accredited installer. For a list of solar thermal units which are accredited and eligible for a grant under LCBP or SCHRI visit; www.redbooklive.com.
Compliance
Roof mounted s
olar thermal is now permitted under planning regulations unless the panels protrude more than 200mm on the roof, or if they were fitted on the principal elevation facing onto or visable from a highway in a Building Conservation Areas and World Heritage Sites.
Stand alone solar thermal systems have different restrictions where permission is permitted unless: it is more than 4 metres in height, installed less than 5 metres from any boundary, above a maximum array of 9m2 or situated within any part of the curtilage of the dwelling house or would be visible from the highway in Conservation Areas or World Heritage Sites.
