Development of scalable radiative cooling coatings and modules with self-adaptive properties for buildings

Abstract

Passive daytime radiative cooling (PDRC) is an innovative cooling technology that harnesses the atmospheric transparency window (8–13 µm) to release heat into the cold outer space (~3K) while preventing solar absorption. Unlike conventional cooling methods that rely on expensive equipment and consume substantial electricity, PDRC operates without any power consumption, making it an appealing and environmentally friendly solution to lower building energy consumption. Therefore, there is high potential to develop PDRC materials with multifunctional properties applicable in various scenarios. However, widespread adoption of PDRC faces several challenges. Particularly, for PDRC roof coatings, scalability is a fundamental requirement. Additionally, architectural coatings need to meet the enhanced cooling requirements during the daytime while adhering to architectural aesthetic criteria. Furthermore, addressing the problem of overcooling during cold nights is essential for optimizing building energy consumption. Firstly, an aqueous PDRC roof paint with low volatile organic compounds (VOCs) emissions (~0 g/L) that can be scaled up was developed. Along with the advancement of PDRC materials, fabrication methods by precision lithography or phase change chemistry are no longer a preferred option due to the concomitant hazardous organic solvents. Instead, it is desirable to develop radiative coolers by aqueous processing-based and cost-effective methods so as to promote their mass production and applications in buildings. Herein, a PDRC roof paint with an average infrared emissivity of 92% and a solar reflectivity of 94% was synthesized using a simple mechanical agitation-based method. Additionally, an in-depth evaluation of the cooling power of the paint in a variety of cities demonstrated its wide application potential on building roofs across varying climate regions. Secondly, to enhance the cooling performance of the PDRC during daytime, a novel metal-free PDRC roof coating with fluorescence was developed. The integration of Passive Daytime Radiative Cooling (PDRC) with photoluminescent materials has garnered significant attention due to its potential to enhance daytime cooling efficiency while preserving architectural aesthetics. However, the prevalent use of rare earth elements or leaded perovskite materials in current photoluminescent PDRC coatings raises substantial environmental concerns. To address this problem, we develop the novel eco-friendly photoluminescent materials, carbon dots, and combine it with PDRC coatings. By utilizing the fluorescent polymer particles containing carbon dots, we present a novel type of smart cooling beads (SCBs) that are created by coating hollow glass beads with these particles. The coated surface of the SCBs contain hydrophilic groups, enabling their easy incorporation into aqueous roof coatings on a large scale. The work provides a general strategy to introduces the carbon dots for enhancing the cooling performance of PDRC coating in buildings. Thirdly, a dual-mode self-adaptive radiative cooling module for thermal regulation has been proposed to address the overcooling issue caused by static PDRC coatings. This module combines paraffin wax with a radiative cooler, resulting in the development of a self-adaptive radiative cooler (SRC) with two distinct modes based on dynamic optical properties varying with temperature. Through calculation of the spectral properties using the Fresnel equation, it was found that the SRCs have the ability to optimally absorb solar energy (Δα = 0.322) and automatically adjust their thermal emittance (Δε = 0.552) in response to changes in ambient temperature, facilitated by the liquid-solid transition. Overall, this thesis develops scalable aqueous PDRC roof coatings and creates innovative PDRC coatings with enhanced daytime cooling performance by incorporating metal-free fluorescent materials. Furthermore, a dual-mode radiative cooling module for intelligent thermal regulation in buildings has been proposed to mitigate the overcooling issue.