Thermochromic solar absorber coatings with elevated transition temperature

Due to their simple design and operation, solar thermal collectors for domestic hot water gener-ation and space heating are one of the most common solar energy harvesting systems in use today. Dur-ing cold periods all the absorbed energy is useful. During hot periods, however, when solar radiation is abundant and demand is low, stagnation occurs. Storage is limited and excess heat cannot be diverted. The heat transfer fluid evaporates and temperature of the solar absorber can exceed 200°C even in cen-tral European latitudes. Glycols in the heat transfer fluid degrade, the frame, thermal insulation and selec-tive absorber coating deteriorate and become less efficient. A new generation of solar collectors is envisaged which can absorb and repel heat in a controlled manner by changing their optical properties in the infrared spectral region. Thermochromic VO2 based absorber coatings change their optical properties according to temperature. Through the perfectly reversible ther-mochromic transition, at a critical temperature TC = 68°C, the thermal emittance of the absorber changes markedly, from ≈0.05 emissivity below TC to ≈0.35 - 0.4 above TC. Increased transition temperatures, ben-eficial for solar absorber applications, are achieved by Ge doping of VO2 films. For 5.9 at.% Ge doping, a Tc ≈ 96°C is observed. Ge doping also leads to the increase of thermal emittance modulation Δε, especially due improved thermal emittance in the high-temperature state. Moreover, crystalline, switching films are sputtered at substrate temperature as low as ≈ 310°C. Advanced thermochromic concepts involving Fabry-Pérot inspired multilayers with greatly enhanced thermal emittance modulation, absorbers with light-trapping, black spinel nanoneedles or plasmonic W nanoparticles obtained by means of nanoimprint lithography are investigated. For the main, selective solar absorber application, the thermochromic function is successfully integrated into the multilayered coating design. First, absorbers based on VO2 and VO2:Ge and nanocrystalline Cu-CoMnOx black spinel layers are proposed. However, a large thermal emittance modulation of Δε > 0.3, is accompanied by an unwelcome increase of the solar absorptance with increasing temperature. Simula-tions indicate that a relatively high n and k material inserted between the substrate and the thermo-chromic layer, can revert the change in solar absorptance. The concept is adapted to industrial absorber designs and improved absorbers based on Al//TiAlSiN//VO2:Ge//SiO2, with decreasing αsol and increasing εth over the thermochromic phase transition, are reported for the first time. The positive emittance and negative absorptance modulation of the absorbers, limit the collector stagna-tion temperature by nearly 20°C, to a maximum Tstagnation ≈ 159°C. This leads to shorter duration of stagna-tion conditions and the overall reduction of thermal charges on the system. Glycol degradation is hin-dered, leading to important reductions in maintenance costs. At Tstagnation ≈ 150°C and at 3 bar pressure typically present in such systems, evaporation of the heat transfer fluid in the collector loops is avoided. Accelerated aging tests in dry and humid conditions reveal the environmental stability of the thermo-chromic absorber coatings and a minimum service lifetime of 25 years is guaranteed for multilayers with antireflective and oxidation barrier coatings.