Angular solar absorption and thermal energy storage seasonally modulate radiative cooling

Passive daytime radiative cooling materials (RCs) maintain low surface temperatures under sunlight and can reduce building cooling loads and urban overheating. However, RCs with static optical properties overcool in cold seasons, and adaptive RCs proposed to date rely on temperature/humidity to switch their optical/radiative properties. Further, they address either daytime or nighttime response and are subject to ageing and to false switching on mild sunlit winter days. Here, we propose a system to seasonally modulate RCs with angular solar absorption, achieved with L-shaped fins, and with thermal energy storage (TES), implemented with phase change materials (PCMs) and concrete mass. The L-shaped fins have solar-absorptive, equator-facing walls shaded by horizontal eaves, enabling high direct solar absorption only at the low solar elevations of winter, with RC film on all other surfaces. Coupled with TES, the system is an angular-dependent thermal-mass-enhanced radiative cooler (ATMRC). In field campaigns in Western Sydney, the finned RC samples reached winter daytime surface temperatures 6–9 °C above the static RC and up to 11 °C above the unfinned RC + TES, resolving the daytime overcooling that TES alone introduces. The ATMRC with the 8 °C PCM reduced winter dew and frost durations by 56% and 73%, respectively, relative to the static RC. The summer daytime penalty against the static RC was 2 °C. Ray-tracing yields seasonal absorbed fractions of 37% in winter and 21% in summer in Sydney. Effective adaptive RCs therefore require both TES against night overcooling and angul…