Experimental Investigations on the Stagnation Behavior of Thermochromic Flat Plate Collectors
Abstract
A thermochromic absorber coating in a solar thermal flat plate collector switches its emissivity for long wave radiation depending on the absorber temperature. Thus, the stagnation temperature can be reduced by 30 K compared to a standard flat plate collector and an overheating of the heat transfer fluid can be prevented. On the basis of 58 selected stagnation events we analyze and compare the stagnation behavior of solar thermal systems with standard and thermochromic collectors. We have carried out well defined stagnation experiments to deter¬mine the steam expansion, steam volume and steam producing power for both solar thermal systems with unfa¬vorable system hydraulics. By the use of thermochromic collectors at a standard system overpressure (1.0 bar) the steam expansion can be halved, the overall steam volume can be lowered by 33 % and the stagnation time can be reduced by 20 %. The steam expansion and the vaporization of the heat transfer fluid can be prevented at system overpressures above 3.4 bar and 4.1 bar, respectively. The steam producing power can be limited to 40 W·m-2.