Solar water heating can be used in the home or for larger applications, such as swimming pools.

For domestic hot water there are three main components:

• solar panels - are fitted to your roof. They collect heat from the sun's radiation.

• heat transfer system - uses the collected heat to heat water.

• hot water cylinder - stores the hot water that is heated during the day and supplies it for use later.
  

Solar Panels

There are basically two types of solar panels (collectors).

• Flat plate collectors
• Evacuated tube collectors

Flat plate collectors look like large windows. A flat collector is basically a piece of copper tube, bent to form a serpentine shape and then soldered to a sheet of copper. The tube and sheet are painted black and are mounted in an insulated box with a suitable glass or plastic lid.


Evacuated tube collectors consist of glass tubes in which the solar radiation is converted into heat. Mirroring behind the tubes concentrates the suns rays. The vacuum in the tubes allows almost no heat to escape and, because of this, they deliver more heat in spring, autumn and winter than flat plates.

The collector (solar panel) should face south or close to south and lie on a pitched roof that will provide the natural angle to face the sun.

The table below shows suggested collector area and cylinder capacity needed for hot water provision for various sized households. This can only be a rough guide as it depends on whether residents are in or out most of the day, prefer a shower to a bath, use a washing machine regularly and so on.

Household size     Total collector area  (m2)     Cylinder Capacity(litres)
1-2                                 2.5                                  160
3-4                                 3.5                                  195
4-5                                 4.5                                  245
5-6                                 5.5                                  294

Heat Transfer System

Transfer of solar heat to the thermal store water is usually achieved using what is known as a heat-exchanger. A conventional central heating pump forces water through a coiled pipe in the solar panel where it is heated by the sun. The heated water then flows down and through a second (lower) coil in your hot water cylinder (boiler). The hot water passing through this coil heats the water in the cylinder.

The slightly cooled water is then returned back to the solar panel via the pump. The controller box continuously compares the temperature in the panel against that in the hot water cylinder . It switches the pump on when the water temperature in the panel is hotter than that in the cylinder and switches it off when the reverse conditions apply. As long as the water in the hot water cylinder is at the required temperature, your existing boiler will not switch on.

The water flowing around the solar system is used to heat the water in the cylinder indirectly. This means that no water in the SWH system will come into contact with water in your hot water cylinder. The heat is transferred, not the water.

This is beneficial because:

   1. it keeps the total amount of water flowing in the solar system to a minimum, making the system more efficient at bringing heat down from the panel
   2. anti-freeze needs to be added to the water in the solar system circuit as it is partly outside the house and could freeze in winter.

(As an alternative to using anti-freeze, a drain back system can be used. In this the panel water drains back into a special bottle when the pump switches off.)

Hot Water Cylinder

You can add solar panels to most existing hot water systems, though you will usually have to add an additional 'pre-heat' water cylinder or change your existing one to a twin coil cylinder. You may need a pump to circulate the water, and some regulating equipment to prevent overheating.

It is more difficult to use solar water heating with a 'combi' boiler because they are designed to take cold mains-pressure water, and solar systems supply hot or warm low-pressure water (although new modulating combi boilers will accept pre-heated water). Check with the boiler manufacturer or a solar engineer to see if your boiler is suitable.

If your boiler needs replacing, a condensing boiler is the most efficient type. Condensing boilers are more expensive but the savings on fuel should compensate for the extra cost within a few years - and from then on you're saving money and fuel! Do also make sure that your cylinder is well-insulated, with a good thick cosy jacket.

You can also buy an electronic display board which can be mounted in a convenient inside location which gives both roof panel and cylinder water temperatures. This is fun when showing guests how marvellous it is to heat your water from the sun!