An exploration of the combined effects of NIR and VIS spectrally selective thermochromic materials on building performance

Thermochromic (TC) windows are able to adjust solar radiation transmitted into buildings in response to varying window surface temperature. Vanadium Dioxide (VO2) is the most common TC material used for TC windows, as it can reduce near infrared (NIR) solar transmittance to attenuate undesirable solar heat gains during hot days when window surface temperature rises above a particular transition temperature. However, there have been few studies of the effect of TC windows on the indoor luminous environment. In order to improve daylight control, an innovative Iron-liquid based TC window film, which can control the visible (VIS) spectrum was introduced and applied alongside a VO2 based TC material in this study. The combined performance of these two types of TC material was evaluated under three climatic conditions within China: Beijing, Shanghai and Guangzhou. The results show that increasing either the NIR or visible transmittance difference after switching, is beneficial for thermochromic performance, the maximum energy saving increasing from 11% to 18%, and UDI500–2000 lx increasing by up to 27%. Combined application of NIR and visible spectral selection results in improved balance between energy demand and daylight provision. It was found that the energy saving potential and daylighting regulating capability offered by the combined implementation is highly dependent on matching the climate conditions to the TC windows.