A solar collector generates heat whenever sunlight falls on the absorber – even at times when no heat is required. This may, for example, be the case in summer when residents are on holiday. If heat transfer, through the DHW cylinder or heating water buffer cylinder, is no longer possible because either is already fully heated, the circulation pump switches off and the solar thermal system goes into stagnation.
If further insolation falls on the collector, its temperature will rise until the heat transfer medium evaporates, causing high thermal stresses on system components such as seals, pumps, valves and the heat transfer medium itself. In systems with ThermProtect temperature-dependent shutdown, the formation of steam is reliably prevented.
Flat-plate collector with switching absorber layer
For the first time, a flat-plate collector has been developed and patented that prevents further energy absorption once a certain temperature has been reached. The absorber coating of the Vitosol 200-FM is based upon the principle of ‘switching layers’. The crystalline structure, and therefore the collector's output, change depending on the collector temperature, thereby reducing the stagnation temperature. At absorber temperatures of 75 °C and above, the crystalline structure of the coating changes, increasing the rate of heat radiation many times over. This reduces the collector output as the collector temperature rises, the stagnation temperature drops significantly and prevents the formation of steam.
Once the temperature in the collector falls, the crystalline structure returns to its original state. More than 95 percent of the incoming solar energy can now be absorbed and converted into heat; only a tiny proportion (less than 5 percent) is irradiated back. This means that the yield of the new collector is higher than that of conventional flat-plate collectors, as the collector never enters the stagnation phase and can supply heat again at all times. There is no limit to the number of times the change in crystalline structure can be activated, meaning that this function is always available.