Passive freezing desalination driven by radiative cooling

Increasing water scarcity in the face of climate change has driven significant interest in finding low-carbon-intensity ways of generating fresh water from saline sources. Conventional membrane and thermal desalination techniques require large energy inputs that can become prohibitive as salinity increases. Alternatively, solar desalination is a well-developed passive evaporative approach but is limited seasonally and geographically by solar insolation. Here, we propose and demonstrate a passive approach to a more thermodynamically attractive phase change that can also enable desalination: freezing. We develop a system that uses passive radiative cooling to the ultimate heat sink, outer space, to freeze and desalinate salt water. We experimentally demonstrate the 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 with 65% recovery. Our results highlight the potential of harnessing untapped thermodynamic resources for water technologies.