Sky cooling-driven radiant-capacitive hydronic system for all-day building cooling

Daytime Radiative Cooling (DRC) surfaces enable heat rejection by emitting infrared radiation to the sky while reflecting solar radiation, allowing for sub-ambient cooling even under direct sunlight. This study develops and validates a transient numerical model of a DRC-based hydronic cooling system designed for building applications. The system integrates ceiling-mounted radiant capacitive modules (RCMs) with outdoor sky radiators (SRs) that dissipate indoor heat to outer space, cooling down a heat transfer fluid. The model is validated using experimental data from a full-scale demonstrator using a commercially available DRC emitter and is employed to assess system performance for a single-family building during a typical cooling season in the cities of Madrid and Rome. Compared to a system limited to nighttime radiative cooling, the DRC-enhanced setup delivers seasonal energy performance improvements of +6.2 % with a commercial DRC material and +10.3 % with an ideal broadband emitter. The study further investigates the effects of varying the surface area ratio between SRs and RCMs and alternative SR placements (rooftop vs. external surface). A fully passive building model with a DRC roof is also considered for comparison. Results show that the DRC-hydronic system can consistently maintain indoor thermal comfort throughout the cooling season, achieving seasonal energy efficiency ratios (SEER) up to 35 times higher than those of conventional air conditioning systems for the case studies analyzed, although the two systems differ in controllability and application scenarios. These findings highlight the strong potential of DRC-integrated hydronic cooling as a highly energy-efficient and sustainable alternative for the climate control of residential buildings.