Designing high-performance nighttime thermoradiative systems for harvesting energy from outer space.

Energy harvesting using thermoradiative systems has been extensively explored in recent years as a novel strategy for further reducing our energy footprint. However, the nighttime application, thermodynamic limit, and optimal design of such a system remain largely unaddressed so far. Here we propose an improved nighttime thermoradiative system (NTS) for electrical power generation by optically coupling Earth's surface with outer space. Our theoretical model predicts that the NTS operating with Earth (deep space) at 300 K (3 K) yields a maximum power density of 12.3Wm-2 with an efficiency limit of 18.5%, which is potentially more advantageous than previous nighttime energy harvesting systems, such as a nighttime thermoelectric generator. We find that optimizing the thickness of the active layer, enhancing thermal infrared emission, and employing a silver backreflector for photon recycling are crucially important in improving system performance. This Letter provides new insights for the optimal designs of NTSs and paves the way toward practical nighttime power generation.