The technology behind solar water heating is fundamentally quite simple. In a nutshell, the energy from the sun's rays is collected by an array and converted into heat. This energy is then used to heat liquid in a closed solar circuit which in turn heats your hot water cylinder. This provides your taps and applicances with hot water from the free energy of the sun.

The diagram (left) shows how it works. At the top the sunlight hits the solar array. When the panel is hotter than the bottom of the water cylinder, liquid in the closed solar circuit is circulated, transfering heat from the panel to the cylinder.

If you have a conventional hot water system, in which the water is fed from a header tank in the attic and heated in a tank by a boiler and/or immersion heater, then we will replace that water tank with a special solar one. These come in different shapes and sizes, to fit the space available, but ideally are tall and thin. The solar-heated water goes round in a coil in the bottom half of the tank, whereas your boiler will use a coil in the upper half of the tank. Our tanks also include an immersion heater in the top, powered by mains electricity, as an extra form of heating. This is often the best option in summer, when the boiler can be switched off entirely.

Solar water heating always requires a secondary heating source – because there are some days when the sun does not shine!

If you have a mains pressure system or a combi boiler (one that heats the water as you need it rather than in a tank), then we will use slightly different approaches – please ask for details.

Sensors in the panel and in the water tank monitor their temperatures all the time. The controller displays the temperatures, so you can see what is happening at any time. It also tells you the temperature at the top of the tank – this is the temperature of the water from the taps.

The use of electricity is small. The pump only uses 30 watts – a third that of a conventional light bulb – and is only on when the panel is hotter than the tank.

The panel can be one of two types – a flat plate or an array of evacuated tubes.


The evacuated tubes explained

On the roof, we'll place 20, 30 or 40 glass tubes, pointing upwards. Inside each glass tube is another glass tube, this time painted black, and the whole is sealed so there is a vacuum between the two tubes – like in a thermos flask. The sun heats the black inner tube, which gets very hot, up to 250°C in summer!

Why so hot? Because heat cannot escape by the normal route of convection (when a hot body heats the air and air movements carry away the heat). This heating depends on radiant energy from the sun, and is not reduced when the outside weather is cold.

A tube is shown right, with the heat pipe and silver-coloured fins drawn out to show its internal construction. Inside the inner tube, connected to it by metal fins, is a thin metal pipe, inside which is a liquid. This is a heat pipe. As the tube heats, the liquid vapourises, and rises rapidly to the top of the pipe, where it conveys its heat to a small copper cylinder, like a finger protruding from the tube. Thus the heat energy in the inner glass tube is conveyed to the "finger" at the top.

Tubes inserted into the manifold. After a cold night, the early morning sun is melting the frost on the outside of the tubes. In half an hour the tubes will be hot enough to start heating the water.

The tube is so efficient that 93% of the sun's radiant energy reaching the tube is passed up to the heat exchanger. Even on cloudy days the tubes generate considerable amounts of heat.