Energy-efficient thermally smart windows with tunable properties across the near- and mid-infrared ranges.

Space heating and cooling account for approximately 15 % of the world's energy consumption, underscoring the pressing need for improved thermal management. Macroscopic temperature regulation can be significantly optimized by improving radiative heat control, in particular through radiative cooling in summer and sunlight capture for heating in winter. These processes are typically tailored independently and thereby remain passive. In this article, we propose thermally smart windows with radiative properties that adapt to a building's heating and cooling demands in a tunable manner. To achieve this, one ought to control, simultaneously, the window's response to near- and mid-infrared (IR) radiation for solar heating and radiative cooling, respectively. We propose device architectures to realize such operations using phase-change materials and liquid crystals. Compared to conventional silica glass, the proposed architectures may reduce the energy demand of buildings at the latitude of Barcelona by more than 40 %, showcasing the potential of tunable materials for radiative thermal management in the energy transition. We discuss that significant promise lies in the development of materials that can warrant near-unity modulation of NIR reflectance, which should be the key property to reach as much as 64 % reduction in energy consumption.