Thermochromic coatings for overheating protection of solar thermal collectors - Towards industrial implementation - Annual Report 2019

With the advent of a new generation of switchable absorber coatings, overheating and the resulting stagnation of solar thermal collectors – until recently a common problem even in central European latitudes – can be successfully overcome. Based on thermochromic thin films, these absorbers exhibit a change in thermal emittance at a critical temperature TC. The collector temperature is passively regulated through this thermal emittance change from low thermal emittance below TC to high thermal emittance above TC. The transition temperature of the vanadium oxide based films is usually around 68°C, but in the framework of previous research at EPFL/LESO-PB a novel type of doping has been determined which allowed for the successful increase of the transition temperature to above ~95°C. Besides the considerable increase in transition temperature, Ge doped thermochromic films have been shown to have the additional benefit of an increased thermal emittance above the critical temperature. This allows for a better heat dissipation and lower stagnation temperatures in the collector. The thermal emittance modulation of such absorbers of above 40% is substantial, allowing a considerable reduction of the stagnation temperatures below the evaporation temperature of the heat transfer fluid and the degradation of glycols is mostly prevented. Such « smart » solar collectors allow for a better dimensioning of solar thermal systems. In this project, suitable designs of multi-layered coatings for maximized performance shall be developed, with an emphasis on industry-compatible deposition methods. The lifetime of the coatings shall be determined and optimized. In addition to this, new designs with a maximized emittance switch will be explored.