Evaluation of the thermal and optical performance of thermochromic windows for office buildings in China

Thermochromic (TC) windows were developed as a passive building component to improve indoor comfort and building energy conservation in place of traditional clear glazing systems. Thermochromic materials have the ability to regulate daylight and solar heat gains through windows stimulated by heat. This means that when the temperature of a thermochromic window becomes higher than its transition temperature, less solar radiation, primarily in the near infrared, will be admitted inside the building, reducing over-heating on hot days. The aim of this research is to explore the potential of thermochromic glazing under various climatic conditions by modelling the energy and daylight performance of a typical office room with five different thermochromic glazing types (with varying transition temperatures ranging from 20 °C to 41.3 °C and solar transmittances ranging from 0.412 to 0.690) simulated under five climatic conditions in China, representative of different climate zones. A comprehensive analysis was conducted, including a study of the thermal and optical behaviours of the selected thermochromic glazed windows; energy use for heating, cooling and artificial lighting of the selected office; and effects of window-to-wall ratios on office performance under the selected climatic conditions. The objectives are: to thoroughly understand the characteristics of the selected types of thermochromic glazing, to find the appropriate window-to-wall ratio for the thermochromic windows, and to investigate the suitability of thermochromic glazing for particular climatic conditions to realise which could achieve energy conservation and desired daylighting simultaneously. The results showed the following: (1) Low transition temperature (i.e. 20 °C) is not essential for building energy conservation, larger modulation of solar transmittance is more desirable for most of the climatic conditions. (2) Higher solar absorptance could increase thermochromic layer temperatures, improving glazing tinting capability, but it may increase window heat gains in the form of secondary heat gains and further induce cooling energy consumption on hot days. (3) All the studied thermochromic windows led to building energy savings (up to 19.9%) and better daylighting performance (i.e. increase of the desired range of illumination, UDI_500-2000lux, is up to 15.52%) when compared with traditional clear double glazing. (4) Under climates such as Harbin and Beijing with more cold days, limited types of thermochromic glazing could achieve both energy conservation and desired daylighting under a particular window size, while almost all types of thermochromic glazing are suitable to be used in climates with more hot days.