A novel compact radiative sky cooler enabling unidirectional heat transfer: harnessing nighttime cooling to enhance daytime performance

Subtropical rooftops, especially tin shacks housing many elderly residents, are exposed to prolonged, intense solar radiation, causing surface temperatures to exceed 70°C during summer. This drives up cooling energy demand and poses significant health risks. Cooling such rooftops is vital for protecting vulnerable residents, lowering energy use, and mitigating Hong Kong’s urban-heat-island (UHI) effect. As a promising passive cooling technology, radiative sky coolers (RSCs) can generate over 200% more usable cooling at night than during daytime, offering a significant opportunity to harness nighttime-cooling to enhance daytime performance. However, conventional RSCs remain highly inefficient at utilizing nighttime-cooling. The primary reason is as follows. At night, heat transfer must be enhanced to efficiently capture and store cooling under narrow temperature differentials; however, during the day, this enhanced heat transfer causes the stored cooling to be rapidly depleted by solar heat gain, leaving no cooling available to enhance daytime performance. Thus, harnessing nighttime-cooling requires satisfying two opposing requirements simultaneously: HIGH heat transfer at night to capture and store cooling, but LOW heat transfer during the day to prevent stored-cooling loss from solar heat gain. For the first time, we propose a novel RSC that leverages the unidirectional-heattransfer capability of a heat pipe to meet the opposing requirements. Our preliminary experiments demonstrate a 320.75% enhancement in daytime cooling performance. Building on these promising results, this project aims to address three key limitations of current design and studies: (1) poor compactness, as the bulky 300 mm height severely restricts practical use; (2) absence of an accurate dynamic model, which hinders reliable performance assessment and design optimization; and (3) lack of an effective method to optimize its high-dimensional interacting parameters for maximum daytime cooling. To address the limitations, the project is organized into four tasks. Task-1 will design a novel RSC structure with substantially improved compactness and experimentally investigate its unidirectional heat transfer characteristic. Task-2 will develop a dynamic model to capture the complex multi-phase coupled heat transfer across three major units: cooling generation, delivery and storage. Task-3 will develop a hybrid optimization method to maximize daytime performance while accounting for complex interactions among high-dimensional influencing parameters. Task-4 will conduct realworld experiments to assess its daytime cooling enhancement, long-term durability and cost-effectiveness. By harnessing nighttime-cooling, the proposed RSC can be widely implemented on subtropical rooftops to improve daytime performance, enhancing elderly health, reducing cooling energy use, and mitigating the UHI effect.