VO2:Ge based thermochromic solar absorber coatings

Abstract

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 VO 2 makes it a promising material for smart selective absorbers with temperature dependent optical selectivity. Single VO 2 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 VO 2 thin film, the transition temperature is increased to 82 °C. When VO 2 and VO 2 :Ge films are combined with a selective CuCoMnO x layer and an SiO 2 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 α sol 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 VO 2 , decreasing solar absorptance over the phase transition is achieved. In line with these findings, a TiAlSiN//VO 2 :Ge//SiO 2 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, α sol decreases from 0.93 to 0.91 and ϵ th increases from 0.08 to 0.24. Such a decrease in α sol accompanied by an increase in ε th 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. • Thermochromic VO 2 and VO 2 :Ge based selective absorber coatings are deposited. • ε th increases at 66 °C for VO 2 and 82 °C for VO 2 :Ge with 5 at.% Ge. • First absorber designs with both increasing ε th and decreasing α sol are reported. • High collector efficiencies and reduced stagnation temperatures are achieved. • Accelerated aging test in dry and humid conditions are successfully completed.