Thermochromic films of VO2:W for smart solar energy applications
Overheating is a common problem both with the use of active and passive solar energy in thermal solar 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. A solution to such problems might be provided by "smart" thermochromic coatings. A durable inorganic thermochromic material is vanadium dioxide. At 68°C, VO2 undergoes a reversible crystal structural phase transition accompanied by a strong variation in optical properties. By doping the material with tungsten, it is possible to lower the transition temperature making it suitable as a window coating. In order to simulate the optical behaviour of multilayered solar coatings, precise knowledge on the optical material properties is necessary. Experimental data reported in the literature are rare and controversial. We determined the complex dielectric function for VO2:W by spectroscopic UV-VIS-NIR ellipsometry above and below the transition temperature and subsequent point-by-point analysis of the ellipsometric psi/delta data. For a validation, the solar reflectance, absorptance and transmittance were measured by spectrophotometry in the visible range and in the near infrared range up to 2500 nm. The experimental reflectance spectra have been compared with the computer simulations based on the determined optical material properties. Finally, we collected optical data in a more extended wavelength range by digital infrared imaging to detect the switch in thermal emissivity of VO2:W at around 45°C.