Daytime radiative cooling (DRC) is a well-recognised passive cooling solution for future building envelopes to mitigate the intensity of urban overheating. Its superior cooling performance is based on two mechanisms that rely on a high reflection of incident light in the solar range to limit absorbance of solar heat and on a thermal emission of the absorbed heat in the atmospheric window to exploit the outer space as a heat sink. Unfortunately, this cooling technology has been recently shown in the literature to have the potential of compromising the outstanding cooling benefits exhibited in hot weather because of the undesired overcooling that it can produce under cold conditions and that may require additional heating in buildings. For this purpose, the ability for a building surface to switch between a DRC and a Solar Heater (SH) represents an attractive feature to mitigate, or even eliminate, the undesired overcooling while still aiming to minimise the cooling and heating demands of buildings. In this context, this paper presents the prototyping and testing of an active modulating radiative system that can operate as a DRC or SH depending on the weather conditions and building requirements. The system is based on a sandwich structure that encases a set of films with different optical properties. The active modulation between the two operational states of DCR and SR is performed through the actuation of temperature-controlled motors that modify the arrangements of the films to enable the desired performance to be deployed. The system is capable of accommodating different films because the design of the sandwich structure enables for an easy installation and replacement of the films. The sandwich structure provides the additional benefit of protecting the system from rain and wind. The paper also considers the performance of the proposed modulating system when compared to static cooling technologies. The proposed design is scalable and its programmable and responsive nature can support its applicability for a wide range of climatic conditions.
Kracic et al. (Sun,) studied this question.