Advancing passive radiative cooling technology for green buildings: The potential and challenges of hydrogels

Passive radiative cooling (PRC) technology is gaining attention for its potential to enhance energy efficiency, reduce cooling costs, and support sustainability goals. Recent advancements have focused on materials that improve cooling efficiency, durability and scalability. Hydrogels are promising due to their high water content, three-dimensional (3D) porous network, eco-friendliness, and effective heat absorption and dissipation capabilities. They can significantly reduce air conditioning energy demand, improve thermal comfort and lower carbon emissions. Research on hydrogels for PRC is still in its early stages, with limited reviews available on their cooling principles and mechanisms. This review explores the fundamental principles and mechanisms of hydrogel cooling, including evaporation, water absorption, radiation, and thermal insulation. It also proposes mathematical correlations for theoretical modelling and designing hydrogel porous structures to enhance cooling efficiency. The review highlights the core concepts of passive radiative cooling, which are essential for developing advanced cooling hydrogels. It also discusses recent innovations in hydrogel-based PRC for green buildings, addresses challenges in scaling up and improving durability, and offers recommendations for future research. By showcasing these advancements, this review aims to contribute to developing energy-efficient and sustainable cooling solutions, ultimately supporting global decarbonization efforts.