An Exploration of the Combined Effects of NIR and VIS Spectrally Selective Thermochromic Materials on Building Performance

Abstract 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 block undesirable solar heat gains during hot days when window surface temperature rises above a particular transition temperature. However, few have studied the effect of TC windows on the indoor luminous environment. In order to improve the daylighting 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 materials was discussed under three climatic conditions within China: Beijing, Shanghai and Guangzhou. The results show that enlarging either NIR or visible change, which is the transmittance difference before or after switching, is beneficial for thermochromic performance, the maximum energy saving increased from 11% to 18%, and UDI500-2000lux is increased by up to 27%. Combined application of NIR and visible spectral selection results in more significant balance between energy and daylighting in demand. While their energy saving potential and daylighting regulating capability affected by the combined implementation is highly dependent on the climate conditions of the TC windows.