Nighttime electric power generation at a density of 350 mW/m2 via radiative cooling

<h2>Summary</h2> The coldness of the universe is a thermodynamic resource that can be harvested for renewable energy generation. Theoretically, on the Earth's surface, the maximum power density that can be harvested from the earth's thermal radiation is around 6,000 mW/m². However, most experiments conducted so far have demonstrated a much lower power density in the tens of mW/m². Here, we maximize the power density of such an energy-harvesting system by selecting efficient components with optimal sizing, minimizing heat leakage, and optimizing the thermal radiation spectrum. Our prototype achieves a sustained high power density of 350 mW/m² at night, and we also demonstrate a power density at the 1,000 mW/m² level using an above-ambient-temperature heat source. Our work here shows that the coldness of the universe can be harvested to generate renewable energy at the power density level that approaches the established theoretical bound.