Full-scale assessment of a liquid CsWO3-SnO2 near-infrared shielding coating and a nanoceramic WO3 photochromic window film for in-situ window retrofits

Abstract

Windows play a pivotal role in building design by providing daylight, solar heat gains, and natural ventilation. However, older buildings with outdated windows and inadequate solar control suffer from overheating, discomfort glare, and high cooling costs. While replacing existing windows with high-performance products can significantly boost energy efficiency and occupant comfort, the associated costs can be prohibitive. Consequently, retrofit technologies that utilize existing glass panes and frames offer a compelling solution. This paper investigates the thermal, energy, and daylight performance of two innovative window retrofit technologies relative to a clear reference tested simultaneously at the BeTOP outdoor test cells in Toronto, ON-a vicinity with distinct warm summers and cold winters. The retrofits included a near-infrared absorbing CsWO₃-SnO₂ nanosuspension and a nanoceramic WO₃-based photochromic window film with dynamic absorptance to visible radiation. The low-cost technologies were applied directly to interior glass surfaces, allowing for swift installation with minimal occupancy interruption. The results show that retrofitting existing glass did not significantly influence the equivalent thermal transmittance but significant reductions in the equivalent solar factor and solar transmittance were observed. Although the retrofits did not significantly influence the energy consumption in this particular setting due to Toronto's cold climate and the adopted low window-wall ratio (10%), they contributed to the mitigation of overheating indicated by up to 50% reduction in the percentage of people dissatisfied. Furthermore, the photochromic film significantly improved visual comfort indicated by a 10%–17% improvement in the useful daylight illuminance and a 9% reduction in peak discomfort glare probability.