VO2:Ge based thermochromic solar absorber coatings

Flat plate solar collectors face the problem of overheating and the ensuing high thermal stresses and general collector damage lead to high maintenance costs. To address this challenge, absorber coatings with a passive optical switch at critically high operating temperatures are proposed. The remarkable near- and mid-infrared spectral modulation of thermochromic VO2 makes it a promising material for smart selective absorbers with temperature dependent optical selectivity. Single VO2 films readily achieve thermal emittance modulations of 40% points (p.p.) at a phase transition temperature of 66 °C. With 5 at.% Ge added to the VO2 thin film, the transition temperature is increased to 82 °C. When VO2 and VO2:Ge films are combined with a selective CuCoMnOx layer and an SiO2 top layer, a thermal emittance modulation of 31 p.p. and 33 p.p. is retained, respectively. However, a simultaneous increase of the solar absorptance αsol by 7 p.p. and 5 p.p. limits the drop in collector efficiency at elevated temperatures. Simulations show that adding an absorber layer between the Al substrate and thermochromic VO2, decreasing solar absorptance over the phase transition is achieved. In line with these findings, a TiAlSiN//VO2:Ge//SiO2 multilayer is prepared. Below the phase transition temperature, the efficiency of the thermochromic collector is similar to that of standard collectors. At the transition temperature, αsol decreases from 0.93 to 0.91 and ϵth increases from 0.08 to 0.24. Such a decrease in αsol accompanied by an increase in εth for increasing temperature has been achieved for the first time. The subsequent drop in selectivity, limits the maximum stagnation temperature of the thermochromic collector to 160 °C, 17 °C lower than for a standard collector in identical conditions. The absorber durability is confirmed through accelerated aging tests in dry and humid conditions alike.