Solar water heating (or solar thermal) systems capture the free heat from the sun and use it to heat up water for use in the home.
How does solar thermal work?
- panels on your roof absorb heat from the sun – they are known as the collector
- the water in the panels heats up
- this hot water is pumped through a coil in your hot water cylinder
- in doing so, the heat is transferred from the water in the coil to the water in the cylinder (most systems do not involve the direct use of the hot water that has come from your roof)
The advantages
- reduce your carbon footprint
- reduce energy bills
- be more self sufficient
- some financial return
The ideal situation for solar thermal panels is facing due south, although they are effective facing anywhere between south east and south west. As a rule of thumb you need between 1 and 2m2 of collector (solar panels) per person living in the house. Shade on the panels at any time of day will reduce the performance.
Most panels are mounted on a roof, but they can also be mounted at ground level. It is important that they get direct sunlight. To get the best results they should be at an angle between 20 and 50 degrees from horizontal (most pitched roofs fall within this bracket).
If you have an electric shower it won’t use your solar hot water. Similarly, cold-fill dishwashers and / or washing machines heat the water they use. In these situations, a solar thermal system would not be suitable unless you or other occupants of your home use the bath for most of your personal washing and bathing, as you won’t be able to use much of the solar hot water you generate.
Solar thermal panels are compatible with most existing hot water systems. However, you will need a new cylinder with two coils. Ideally it should be big enough to hold two days’ worth of hot water.
Solar thermal with a combi boiler is more difficult, but still possible. If you have a combi boiler it is important to check with the manufacturer that it will accept pre-heated water.
In the summer your system should meet most of your hot water needs, in the winter you may need a boiler or immersion to top your water up to the right temperature.
The Energy Saving Trust field trials found that solar thermal panels will provide about 60 per cent of a household’s hot water needs, if well-installed and properly used.
How much you benefit will depend on a variety of factors:
- How much hot water your household or business uses. The higher the usage, the more benefit you get from a solar thermal system.
- How much interest you take in how the system works and adapt to make the most of the free hot water (for example ie having showers in the evening rather than the morning). The sun isn’t as reliable as a timer clock.
- The size of your cylinder. Many cylinders only hold enough water for a day’s supply of hot water, so a day or two of cloud and rain will mean you have to turn on the boiler or immersion heater.
- How you programme your back up heating. If your control panel does not allow you to programme the hot water and central heating separately, you may not get the maximum benefit from the solar thermal when the heating is turned on. By only boosting the hot water once the sun has gone down, you maximise the opportunity for solar heating.
- Adequate insulation of both cylinder and pipes carrying hot water.
- Allowing hot water temperature to vary. If you do not need high temperatures all the time, you will have less need for back-up heating. You will also reduce heat loss. However, it is important to make sure your cylinder reaches more than 60 degrees centigrade at least once a week to avoid the risk of Legionella.
There are two types of solar thermal panel:
Flat plate panels consist of an absorber plate in an insulated metal box. The top of the box is glass or plastic, to let the sun’s energy through, while the insulation minimises heat loss. Lots of thin tubes carry water through the absorber plate heating it up as it passes through.
Evacuated tubes are glass tubes containing metal absorber tubes, through which water is pumped. Each tube is a vacuum which minimises heat loss.
The Energy Saving Trust reports that a 3.6m2 system Solar thermal system will cost about £4,000 – £5,000.
Solar thermal systems are eligible for the Renewable Heat Incentive.
The installation of solar thermal on residential properties is likely to be classed as ‘permitted development’, meaning there is no need to apply to the local planning department for planning permission however, there are certain exemptions. You should contact your local planning department to check conditions will be met.
