24-h Bidirectional Thermal Energy Harvesting: From Light to Darkness

Thermal energy, encompassing both heating and cooling demands, accounts for the largest share of global energy consumption. Harvesting thermal energy from the environment, including the Sun and darkness, holds promise for decarbonizing thermal sectors, but suffers from low efficiency and intermittency. Here, inspired by ginkgo leaves featuring wax-coated vertical palisade cells, a 24-h bidirectional thermal energy harvesting approach is developed by integrating spectrally selective aerogels with anisotropic composite phase change materials (CPCMs). During daytime, sunlight is captured, converted into heat, and stored in anisotropic CPCMs with high axial thermal conductivity (24.16 W·m^-1·K^-1) and an anisotropy ratio of 3.7. Under one-sun irradiation, a high solar thermal energy storage efficiency of 87.5% with a peak temperature of 382.3 K is achieved by leveraging spectrally selective aerogels exhibiting "greenhouse effects". At night, a maximum radiative cooling power of 118.8 W·m^-2 is attained, enabling cold energy storage at a temperature 4.0 K below ambient. The proposed leaf-inspired device operates continuously over 24 h, delivering annual thermal energy savings of 5321.4 MJ·m^-2·yr^-1, outperforming standalone solar thermal and radiative cooling systems by 44.8% and 223.3%, respectively. This bioinspired bidirectional energy harvesting strategy employing both solar and outer space resources, establishes a promising approach toward a carbon-neutral thermal energy supply.