Energy modelling and saving potential of polymeric solar-responsive thermochromic window films

Windows are often considered the weakest components in the thermal envelope driven by their low thermal resistances and static transmittances to solar gains. While low-E coatings improve the former, climate-responsive coatings that adjust their thermo-optical properties in response to changing boundary conditions are promising to address the latter. In particular, thermochromic films are passive technologies that rely on intrinsic material properties to adapt to varying ambient conditions and are more accessible with simpler structures than their active counterparts; hence, they prevail for solar control applications. However, the nature of current building energy simulation tools imposes limitations on evaluating their performance. In this article, a 1-D transient heat transfer model was developed in MATLAB/Simulink to evaluate the annual heating and cooling energy performance of thermochromic glazing while overcoming several limitations of current building energy simulation tools by accounting for spectral-selectivity, gradual transmittance change, hysteresis behaviour, and delayed switching time. The model was benchmarked against EnergyPlus, and the annual energy performance of a representative room was then evaluated for several double-glazing configurations, window-wall ratios, and exposures in the cold and hot climates of Toronto, ON and Abu Dhabi, UAE, respectively, while quantifying the effects of varying the hysteresis width and switching time. The results showed that commercial thermochromic glazing outperformed clear and low-E windows for both climate conditions. In particular, for a window-wall ratio of 80 % and compared to clear reference glazing, annual energy use intensity reductions up to 6.3 kWh/m2 and 12 kWh/m2 were realized in Toronto and Abu Dhabi, respectively, utilizing a glazing configuration that combined exterior thermochromic and interior low-E coated panes, where the latter helped in increasing the former's temperature causing it to switch at lower ambient temperatures. While the typical 30-minute switching time of thermochromic coatings was found to increase the annual energy use intensity by up to 0.5 kWh/m2, a 5 °C hysteresis reduced the annual cooling energy use intensity by up to 0.2 kWh/m2 in Abu Dhabi. Less significant effects and savings were found for lower window-wall ratios, particularly in Toronto, where the coating rarely reached temperatures higher than 45 °C.