Passive Freezing Desalination driven by Radiative Cooling

With global temperature rises posing fundamental economic, health, and security risks to human civilization, maintaining habitable built environments has emerged as one of our times' most pressing challenge. Passive radiative cooling in particular has emerged as a promising technology, driven by a net radiative heat loss to cold outer space via the atmospheric transmission window in the long-wave infrared (LWIR) wavelengths. However, its potential for generating and remediating water remains an open frontier for research. Indeed, with increasing water scarcity there is now significant interest in finding lower-cost ways to generate fresh water. At the same time, many industrial processes, including oil and gas production, produce high-salinity wastewater that requires remediation.<br/>Conventional desalination processes, including membrane-based and thermal desalination approaches, require large energy inputs which can become prohibitive as salinity increases. Alternatively, the most commonly used passive thermal desalination method is solar desalination which uses the sun as the heat source in an evaporation-condensation cycle. However, weather conditions and solar insolation limit its applicability throughout the year.<br/>Here, we propose a passive approach to a more thermodynamically attractive phase change that can also enable desalination: freezing. In particular, we use radiative cooling to enable passive freezing desalination for the first time. We experimentally demonstrate passive desalination of 37.3 g/L salt water to 1.88 g/L after two radiative cooling-driven freezing desalination stages, with 50% recovery and 17.5 g/L salt water to 0.7 g/L after two radiative cooling-driven freezing desalination stages, with 65% recovery. We develop and validate a thermal model that accurately predicts the performance of the system and extend it to probe the theoretical limits of performance. These results demonstrate that passive freezing desalination driven by radiative cooling could fundamentally enable new technological possibilities for desalination. Further, this could be a complementary method to solar desalination to enable 24-hour, year-round passive thermal desalination.