Flat Panel
Evacuated Tube
Glass tube or flat panel mounted on roofs
There are two types of Solar panels that are used for heating water - Flat Panel and Evacuated tube.
Flat Panel
With Flat Panel collectors the water to be heated is circulated through a single copper pipe that winds its way up and down the panel and is covered by a sheet of glass. These work fine when the sun is shining however if it is overcast, windy or cold then they stop working. In extreme situations the heat loss can easily exceed the heat gain - turning your flat plate panel into a radiator! So essentially Flat Plate collectors only provide a useful amount of heat during the summer months in the UK. The main reason they are used is because they are cheap to produce and quick to install rather than being the best for the job!
Evacuated Tube
Vacuum tube collectors, however, provide useful heat all year round as long as the sun is shining, and can even produce heat in overcast weather. In April, an overcast day without any sunshine is still able to heat a 210 litre hot water cylinder to 40°C with a single 20 tube panel. A sunny day in April will warm the cylinder as high as 60°C!
The vacuum tube consist of a double wall glass tube (made from strong borosilicate glass - ie pyrex) with a space in the centre which contains the heat pipe. 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. This is in effect like a one-way mirror which has been optimised for infra-red radiation. In fact it is very efficient, of the sun light's energy hitting the tube's surface, 93% is absorbed, whereas only 7% is lost through reflection and re-emission. The presence of 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, unlike traditional flat plate collectors.
The suns radiation boils the liquid in the heat pipe and the vapour rises to the tip of the heat pipe where the heat is transferred to a copper manifold at which point the vapour condenses and runs back to the bottom of the tube. The manifold runs across the top of the panel in which water is circulated to heat the domestic hot water tank. If a tube is placed in direct sunlight on a summer day, the tip temperature can reach 250°C - so the system easily heats domestic hot water cylinders to 60°C even in cooler weather! The manifold is heavily insulated with a 2" thickness of pre-formed rockwool to keep the heat in.