The solar thermal collector is a device that uses solar radiation to heat up water to a high enough temperature that it can be used as ‘hot water’.
The thermal system as we know it was invented in 1909 by William. J Bailey, who used a thermosyphon system with an insulated storage tank so that the water would stay hot at night – hence the (rather odd) name ‘The Day and Night Solar Water Heater’. Despite the odd name, he did rather well for himself selling a good few thousand models. However, the discovery of cheap natural gas in the 1920’s saw a virtual end to his business and it wasn’t until the 1970’s oil price hikes that a resurgence of the solar water heater was seen.
A modern version of Bailey’s thermosyphon arrangement is still very much in use today, but only in counties where the climate allows. In frost-free parts of the world such as Africa, the Mediterranean and the Middle East, the thermosyphon is widely used to heat water for domestic use. This type of arrangement dispenses with a circulation pump and relies on the natural convection of rising hot water to pump the system. The water passes directly through the solar panel and is then stored in the tank set atop the system. Because of the amounts of solar radiation available in these areas, the collector panel is generally about half the size of a similar one in Northern Europe.
The collectors used in Northern Europe are typically one of two types: a flat plate collector or an evacuated tube collector. Both of these come under the title of ‘pumped’ collectors.
A pumped collector will allow water to heat up in the absobtion panel. When hot enough a pump will be activated via a heat sensor, or control box, and the hot water will be transferred down to a hot water storage tank that contains a heat exchanger. The heat from this water is then radiated out into the water in the tank, thus heating it up. The reason that the water doesn’t flow directly through the collector and into the tank, as with the thermosyphon system, is because it has to contain an anti freeze solution allowing it to run through the cold European winter months without rupturing a pipe.
Flat plate collector
The flat plate collector comes in two varieties: glazed and unglazed. The glazed version consists of three main components: and insulated back plate; an absorber plate; and the glazing. The back plate forms the back (funnily enough) of the unit. It is insulated to reduce heat loss. Attached to this is the absorber plate, which is simply a set of copper pipes attached to a metal plate (usually steel). The water to be heated will circulate through these pipes in the direct path of solar radiation, thus picking up the heat present in the sunlight. The absorber plate is painted with a high absorbency black paint that maximises absorption. Put simply, the reason that the black paint is used is because black will absorb all of the colours/wavelengths with the spectrum, while conversely, white will absorb none. Once the water is hot enough, it is pumped to the heat exchanger in the storage tank.
The unglazed version is used to heat water to a lower temperature than would be used for domestic hot water, for instance the water for an indoor swimming pool. This consists of exactly the same as the collector described above, but without the glazing element… obviously.
Evacuated tube collector
The other commonly used type of solar thermal collector is the evacuated tube collector. This is a more efficient – and more expensive – system. It consists of a number of modular tubes arranged next to each other vertically, with the water pipe attached along the top. Each tube consists of a heat pipe, an absorber plate and evacuated glazing.
This time the absorber plate is a metal strip that runs down the centre of each tube, again painted with high absorption black paint. In most cases the tube are about 1.5 – 2 metres long and about 10 – 15 centimetres wide. It uses a heat pipe to transfer the absorbed solar energy to the water running horizontally along the top of the collector in the pipe joined to each tube and then down to the storage tank. The heat pipe is a device that contains a liquid, which is pressurised so as to boil at the top – thus heating the water – and condense at the bottom. This means that it has a high level of thermal conductivity and transfers large amounts of energy. This absorber strip and heat pipe assembly is placed inside an evacuated glass tube. The evacuated part of the name stems from the fact that the air in the space between the absorber tube and the glass is ‘evacuated’ to leave a vacuum, in much the same way as double-glazing, greatly reducing convective heat losses. The tubes are capped at both ends, mounted side by side to a frame and inserted into the water-carrying pipe at the top to form the collector. An evacuated tube collector will need a little maintenance from time to time, but should still last 25 years’ plus.
A very simple rule of solar thermal collectors is; a flat plate collector will cost you less, but won’t be as efficient as an evacuated tube collector, which will do the job better but cost you more. A little research into what best suits your budget and needs is essential.
