Thermochromic coatings for overheating protection of solar thermal collectors - novel type of doping (TARGET95)

Overheating and the resulting stagnation of solar thermal collectors is a common problem even in central European latitudes. During stagnation, high temperatures lead to water evaporation, glycol degradation and stresses in the collector with increasing vapour pressure. Special precautions are necessary to release this pressure; only mechanical solutions exist nowadays. Additionally, the occurring elevated temperatures lead to degradation of the materials that compose the collector, such as sealing, thermal insulation and the selective absorber coating. A protection of solar thermal systems without any mechanical device (e.g. for shading or for pressure release) might be provided by thermochromic coatings which exhibit a change in optical properties at a critical temperature Tc. Desired are a high solar absorptance and a low thermal emittance below the Tc, and a low solar absorptance and a high thermal emittance above Tc. Such "smart" solar collectors will allow a better dimensioning of solar thermal systems. In a previous project, thermochromic films have been developed at EPFL/LESO-PB. The transition temperature of the vanadium oxide based films is usually around 68°C and can be lowered by tungsten doping. Preliminary experiments have shown that by a novel type of doping of the thermochromic films, the transition temperature can also be raised considerably (e.g. to ~95°C). In this project, the effect of doping on the transition temperature is confirmed, studied and understood in more detail. It is investigated precisely to which extent the transition temperature can be raised. Suitable designs of multi-layered coatings for maximised performance are developed. An alternative thermochromic oxide is proposed and investigated. Finally, a variety of applications are explored, and promising fields for market introduction are identified.