Tri-band electrochromic smart window for energy savings in buildings

Electrochromic windows provide a sustainable solution for use in energy-efficient buildings as their varying optical properties in changing weather conditions allow the optimization of solar radiation heat gain and indoor thermal comfort. However, as the wavelength range of sunlight that can be used remains limited, broader implementation of this technology is hampered. Here we present an electrochromic design that combines radiative cooling of mid-infrared light and maximized utilization of both visible and near-infrared light. Our electrochromic window takes advantage of a WO₃/VO₂ film structure that, with a controllable lithium-ion intercalation depth, affords three active optical states to control visible and near-infrared transmittance independently. Moreover, the use of electrodes with preferred emissivity at both surfaces serves to optimize the radiative heat exchange between the indoor and outdoor environments. Field experiments and simulations show that our device exhibits higher energy savings than a commercial low-emissivity glass in most climate zones around the world. Our findings suggest ample opportunities for energy-saving window designs that can help achieve global carbon neutrality and sustainability.