Temperature limiting absorber coating
Once the absorber reaches about 80°C, the crystalline structure of the selective coating begins to change. The result is that more heat is emitted, limiting the stagnation temperature to 145°C. (Graphic: Viessmann)
The flat-plate collector that KBB developed jointly with the German Institute for Solar Energy Research Hameln (ISFH), is equipped with self-limiting aluminium heat pipes that independently limit the temperature in the solar circuit. (Photo: KBB)
This year, Viessmann of Germany made another concept ready for the market. The idea is simple: what would happen if during stagnation phases the collector never got hot enough to create steam? Temperature loads would be lower and safety measures and pressurisation systems would be much simpler. Implementing the idea, which was done jointly with the solar research institute ISFH surely cost a good deal of effort, but now it works. After all, says Viessmann product manager Michael Beckmann, "The Vitosol 100-FM and Vitosol 200-FM flat-plate collectors have a special absorber coated with the patented Thermprotect selective layer, which changes its physical characteristics at high temperatures." From approximately 80°C, emissivity slowly increases, limiting the stagnation temperature to about 145 °C. At an operating pressure of 3 bar, this eliminates the possibility of steam development in the system. That protects the expansion tank and reduces its expansion volume. Auxiliary vessels and stagnation coolers are also unnecessary. "Of course this reduces stress on all of the other components as well," says Beckmann. One possible disadvantage, he says, is that the higher operating pressure reduces the working volume of the expansion tank. "But in reality, that is not a problem because for systems with Vitosol flat-plate collectors that have safety valves with an actuation pressure of 6 bar, the valve can be swapped out for one that actuates at 8 bar."
Solar thermal systems that are also used for space heating go into stagnation more frequently when the hot water tank is too small in relation to the collector area. With the new absorber coating, Viessmann now allows for generously designed collector fields.
Two circuits in one collector
Tigi, an Israeli company, has also developed a new method of limiting the stagnation temperature to about 150°C. In its transparent-insulated honeycomb collectors, the company installs not only the primary circuit, which draws heat from the collector but also a second one in the form of a closed heat-pipe circuit. As soon as the temperature of the absorber rises above 105 °C, the heat transfer fluid in the heat pipe produces steam which travels upward to the condenser integrated into the collector. The condenser then dissipates the thermal energy into the surrounding air, preventing the collector from heating up over 150°C. Like the Viessmann solution, the temperature limitation always works and, more importantly, so does the restarting of the system. After all, a defective sensor or improperly adjusted regulator can have no influence on plant shut-down. The disadvantage is that the honeycomb collector with its costly materials does not come cheap.