Flat Plate Collectors with Thermochromic Absorber Coating Under Dynamic System Tests
Abstract
Thermochromic absorber coatings switch their emissivity for long wave radiation depending on the absorber temperature. Below the specific switching temperature of 68 °C the coating’s emittance is quite similar to a commercial, highly selective absorber coating (ε = 5 %). At higher absorber temperatures the emittance reaches ε ≈ 35 %, whereby the collector heat losses increase. As a result, the stagnation temperature is reduced by 30 K and an overheating or vaporization of the heat transfer fluid can be entirely prevented. We have carried out collector performance measurements according to ISO 9806. By means of dynamic system tests (ISO 9459 5) we predict the performance of a domestic hot water system with standard and thermochromic collectors. At typical daily tapping rates the marginally lower absorptance of the thermochromic coating raises the auxiliary energy demand up to 2.5 % and reduces the solar fraction by about 1 %-absolute. In additional system stagnation tests, we analyzed the stagnation behavior of thermochromic collectors, especially the vaporization of the heat transfer fluid, the steam expansion into the solar pipes and the thermal stagnation load on sensitive solar loop components. A reduction of the stagnation period by 60 % and a limitation of the stagnation temperature to 145 °C were investigated.