ABSTRACT
When sunlight hits a dark object, that object heats up. This well known effect is behind solar thermal systems. Solar collectors convert sunlight into usable heat. The absorber – a black plate made of copper, aluminium or sometimes even plastic in simple systems – is exposed to the sun, which heats the plate up. The heated plate then passes the heat on to a fluid (heat carrier) flowing through tubes embedded in the plate, and this fluid flows to the consumer device (to provide heated service water). Various technologies are implemented to reduce heat losses: 1
Flat-plate collectors, the most common type, have an absorber within a frame. The cover facing the sun is transparent, and the cover is well insulated to the sides and on the back. Such solar collectors are 50–60 per cent efficient at a temperature of 50° Celsius. They can, however, reach temperatures of up to 80°C under direct sunlight.
Evacuated tube collectors have their absorbers inside a sort of thermos bottle that is transparent on the side facing the sun. The vacuum reduces heat losses greatly. These evacuated collectors therefore have greater efficiencies, especially when the temperature difference is great between the absorber and the ambient air. If the difference is 70K (= 70 degrees), evacuated tube collectors are around 15 per cent more efficient than flat-plate collectors. But that figure drops to only 5 per cent if the difference is 40K. The benefits of evacuated tube collectors thus make themselves felt especially during the cold season. It therefore makes sense to use them to support heating systems.
When outdoor swimming pools are heated, heat loss is not that great an issue, so no insulation is generally used. The water is pumped through black absorber hoses to absorb the solar heat. Such systems are, however, only useful in applications where only a few degrees of heat is required, such as in outdoor pools.
Solar collectors also have to be properly oriented to be efficient. In Germany, roof angles are optimally 45 degrees and facing due south. Fortunately, efficiencies drop only slightly if these ideal values are missed; collectors still produce around 90 per cent of their rated output if the collectors have an angle between 0 degrees and 50 degrees and face anywhere from southeast to southwest. It is therefore possible to use a slightly larger collector surface to compensate for suboptimal orientation. Some evacuated tube collectors even allow you to compensate by slightly turning the absorber within the tube.
