Mechanically Tunable Transmittance Convection Shield for Dynamic Radiative Cooling

Radiative cooling is the process to dissipate heat to the outer space through an atmospheric window (8-13 μm), which has great potential for energy savings in buildings. However, the traditional "static" spectral characteristics of radiative cooling materials may result in overcooling during the cold season or at night, necessitating the development of dynamic spectral radiative cooling for enhanced energy saving potential. In this study, we showcase the realization of dynamic radiative cooling by modulating the heat transfer process using a tunable transmittance convection shield (TTCS). The transmittance of the TTCS in both solar spectrum and atmospheric window can be dynamically adjusted within ranges of 28.8-72.9 and 27.0-80.5%, with modulation capabilities of ΔTsolar = 44.1% and ΔT8-13 μm = 53.5%, respectively. Field measurements demonstrate that through the modulation, the steady-state temperature of the TTCS architecture is 0.3 °C lower than that of a traditional radiative cooling architecture during the daytime and 3.3 °C higher at nighttime, indicating that the modulation strategy can effectively address the overcooling issue, offering an efficient way of energy saving through dynamic radiative cooling.

Passive and Dynamic Phase-Change-Based Radiative Cooling in Outdoor Weather

Radiative cooling has attracted considerable attention due to its tremendous potential in exploiting the cold reservoir of deep sky. However, overcooling always occurs in the conventional static radiative coolers because they operate only in the cooling mode in both hot and cold. Therefore, a dynamic radiative cooler based on phase change materials is highly desired. Nevertheless, the practical outdoor phase-change-based dynamic radiative cooling has not yet been experimentally demonstrated. To satisfy the stringent requirement of the phase-change-based radiative cooler in outdoor weather conditions, we engineered the phase-change material (VO₂) to possess the room-temperature phase-transition capability for typical weather conditions. Second, the reconfigurable cavity consists of the lossless spacer to ensure the magnitude of thermal modulation and suppress the solar absorption simultaneously. Third, the practical selective-filtering method is devised to shield the solar irradiance while permitting the thermal emission. Our experiment demonstrates that these materials and photonic measures can work together to realize the dynamic radiative cooling in actual weather conditions, which shows a self-adaptive switch between the ON-cooling state in hot daytime and the OFF-cooling state in cold nighttime. The study pushes the radiative cooler toward multifunctionality and provides beneficial guidance for the phase-change-based intelligent thermal control.