Innovative Smart Selective Coating to Avoid Overheating in Highly Efficient Thermal Solar Collectors

Highly efficient solar thermal systems generally undergo stagnation conditions with temperature inside the solar collectors as high as 190-200 °C, as soon as the domestic hot water demand is poor or when the system is off while the collectors are still submitted to a strong solar radiation (> 950 W/m2). These stagnation conditions are known to be one of the major problem of thermal solar systems and often lead to vaporization and glycol degradation, loss of performances, and the need for regular maintenance with associated costs for the end user. Thanks to a novel smart selective coating, characterized by a strong increase of its infrared emissivity (thermochromic effect) at a critical temperature, stagnation temperatures can be reduced to 150 °C for solar radiation and ambient temperature of 1000 W/m2 and 35 °C, respectively. As the novel smart selective coating presents a high solar absorption coefficient (>94%) and a low emissivity (∼6%) at low temperature, and because the thermochromic effect starts at a temperature around 70 °C, the high performance of the new thermochromic thermal solar systems is guaranteed for domestic hot water heating. The properties of this new generation of selective coatings, based on a mixture of vanadium and aluminum oxides (VO2/VnO2n±1/Al2O3/SiO2), are presented and discussed with regard to composition, structure and optical properties analysis. FTIR spectroscopy and infrared camera pictures clearly show the strong increase of emissivity for temperature higher than 70 °C. Aging performances (high temperature, humidity, thermal cycling) are also presented in order to guarantee a minimum life time of 25 years for the new generation of thermochromic solar collectors. Finally, stagnation temperatures recorded under the same natural sun radiation on scale one (2.3m2) standard and thermochromic collectors are compared.