Switchable Selective Absorber Coatings for Overheating Protection of Solar Thermal Collectors

Overheating is a common problem both with the use of active and passive solar energy in solar thermal energy systems and in highly glazed buildings. In solar thermal collectors, the elevated temperatures occurring during stagnation result in reduced lifetime of the collector materials. Highly glazed building facades provide high solar gains in winter, but imply in most cases high energy needs for air conditioning in summer. The aim of this thesis is to propose ”smart” thermochromic coatings as a solution to such problems. A durable inorganic thermochromic material is vanadium dioxide. At 68°C, VO2 undergoes a reversible crystal structural phase transition accompanied by a strong change in optical properties. By doping the material with tungsten, it is possible to lower the transition temperature, making it suitable as a window coating. Controlled doping of VO2 based films was possible using a co-sputtering installation. This doping lowered the transition temperature from 67.9 ± 0.5°C to 49.5 ± 0.5°C. In order to simulate the optical behavior of multilayered solar coatings, precise knowledge of the optical properties of the material is required. In this study the complex dielectric functions of VO2 and VO2 : W were determined by spectroscopic UV-VIS-NIR-MIR ellipsometry above and below the transition temperature. For this purpose, a point-by-point analysis of the ellipsometric angles Ψ/∆ was performed. A validation of our results was obtained comparing the optical constants determined by point-by-point fitting with those determined by the Lorentz - Drude and the empirical Lorentz - Cauchy dispersion formula. The optical constants of VO2 show a considerable change in the near/middle infrared range. The maximum k (extinction coefficient) change of a factor 7.4 between the semiconducting state and the metallic state occurs at 13490 nm. Reflectance and absorptance were measured by spectrophotometry in the near infrared range up to 20 μm in order to be compared with the computer simulations based on the determined optical properties of the material. A solar absorptance of 0.96 below the transition temperature was reported for a VO2 based absorber. Structural and electrical analyses were performed as well. The thermal emittance of new nanocomposite materials based on VO2 was also investigated applying the Bruggeman effective medium approximation. A thermal emittance switch from 0.08 to 0.32 was simulated for a 350 nm thick VO2 : W film mixed with a 40% volume fraction of SiO2. The glycols used in solar thermal collectors start to degrade above 170°C, the use of this coating as solar absorber lowers the stagnation temperature below this critical point. As alternative solution to VO2, gas chromic MgyTi1−y switchable mirrors were studied by XRD and ellipsometry. In this thesis, the characterization of the optical properties of VO2 and VO2 : W is reported. This characterization shows that these coatings are efficient absorbers of thermochromic solar thermal panels.