For a family of four, a well designed Solar Heating system should contribute between 1,500kWh and 2,000kWh, which will be equivalent to 40 to 50% of the household's water heating energy needs.
How it works
A solar water heating system comprises 3 main components:
- solar panels fitted to the roof which retain heat from the sun's rays and transfer the heat to a fluid.
- hot water cylinder to store the water that is heated during the day for use when required.
- plumbing system of piping and a pump to move the fluid round the system.
The illustration shows a typical Solar Heating system layout. An electronic controller constantly compares the temperature of the solar collectors with the temperature of the water in the cylinder. Whenever the collectors are hotter than the cylinder, the controller switches on the system's circulating pump. A mixture of antifreeze and water is then circulated through the collectors and the cylinder's heat exchanger, heating the cylinder in just the same way as a central heating boiler.
Types of solar collectors
There are two main types of solar collector, which can be used in Solar Water Heating systems. These are:
Flat plate collectors
Flat plate systems comprise a copper grid absorber plate painted with a dark coating to absorb and retain heat, with a transparent solar glass cover. The sun’s radiation is transferred to fluid passing through pipes attached to the absorber plate. Insulation at the sides and bottom of the collector minimises heat loss.
Evacuated tubes
The vacuum tubes comprise a row of glass tubes that each contain an absorber plate feeding into a manifold which transports the heated fluid. Each tube consists of a double wall glass tube. The sun's radiation is absorbed by the selective coating on the inner glass surface, but prevented from re-radiating by the silvered innermost lining, like a one-way mirror. Of the sun energy hitting the tube's surface, 93% is absorbed and only 7% is lost through reflection and re-emission. The vacuum wall prevents any losses by conduction or convection - just like a thermos flask. Because of this, the system will work even in very low temperatures.
Evacuated tubes are smaller and more efficient, but tend to be more expensive (although they are now becoming competitive with flat-plate collectors). Their higher efficiency means they are better able to take advantage of any winter sunshine, and are therefore well suited to the UK climate. However a Government study found that over the course of the year they do not provide significantly more energy than flat plate collectors.
Advantages and disadvantages of Flat plate collectors and evacuated tubes
Flat Plate Collectors |
Evacuated Tubes |
Cheap and simple construction |
Generally more expensive but costs coming down |
Peak energy output at mid-day when sun is perpendicular to the collector |
Cylindrical shape means they absorb sun’s energy throughout the day |
Loses heat more rapidly in wind and cool temperatures |
The vacuum reduces heat loss |
Often requires antifreeze to protect from cold weather |
Can be used in low temperatures without sustaining damage |
If dmamged the whole plate needs to be replaced |
If damaged it is easy to replace one tube |
Very efficient in hot sunny conditions |
Can heat water all year round |
Larger collector area |
Smaller collector area |
Large collector can make installation more difficult |
Easy installation |
Flat glazing more aesthetically pleasing and easier to compliment existing building materials |
Roof top collector more visible |