Nighttime Radiative Cooling for Water Harvesting from Solar Panels

Photovoltaics has played a significant and increasingly important role in renewable energy harvesting. However, it only works during the daytime when the sun is accessible. In this paper, we propose to extend the functionality of solar panels into the nighttime for water harvesting, using nighttime radiative cooling. We first determine the suitable temperature and humidity range for nighttime water harvesting using solar panels and elucidate the water harvesting potential from solar panels. We further show that, through emissivity engineering, both the water generation rate and the suitable temperature and humidity range can be significantly improved. As a case study, we show that the average weekly water generation for solar panels in Dubai can reach 261 mL/m2, sufficient for dust cleaning of solar panels. Moreover, it can be significantly enhanced up to 681 mL/m2 with further emissivity engineering. The collected water can also be used for other applications including agrophotovoltaic and evaporative cooling of solar panels during the day, and can be extended to other solar energy harvesting systems. Our results point to new avenues to explore the nighttime utilization of a wide range of existing sky-facing solar energy harvesting systems and highlight the opportunities to use both the sun and outer space in existing energy systems.