Unconventional smart windows: Materials, structures and designs

As populations grow, energy demands gradually increase, so more efficient energy use is essential. Building energy consumption comprises more than 40% of global energy consumption; heating, ventilation, and air conditioning comprise approximately 50% of all such consumption. Improvements in building energy efficiency are important. Window energy efficiency of windows is one of the lowest in the building structure. In summer, more than 70% of heat is transmitted into the room through windows; in winter, windows cause 30% of heat loss. Therefore, adjustment of sunlight and heat radiation transmitted through windows could reduce heating, ventilation, and air conditioning energy consumption, while enhancing energy efficiency. Progress in the implementation of thermochromic and electrochromic smart windows has been extensively reviewed. However, mechanochromic windows, which react to humidity, and smart magnetochromic windows have received less attention. In this review, we summarize recent developments in humidity-triggered, mechanochromic, and smart magnetochromic windows with a focus on the materials, structures, and designs. We also discuss emerging technologies, including dual-stimulus-triggered smart windows and multifunctional integrated devices. We provide practical recommendations and describe the future of mechanochromic, humidity-triggered, and smart magnetochromic window development.