Salt-rejecting continuous passive solar thermal desalination via convective flow and thin-film condensation

Passive solar desalination is an emerging low-cost technology for fresh water production. State of the art desalinators typically evaporate water using wicking structures to achieve high solar-to-vapor efficiency by minimizing heat loss. However, wicking structures cannot reject salt continuously which limits the operating duration of the desalinators to several hours before the devices are turned off to reject salt. While significant research has focused on developing efficient evaporators to achieve high solar-to-vapor efficiency, inefficient condensers have become the bottleneck for the overall solar-to-water efficiency. To overcome these challenges, we designed a passive inverted single stage solar membrane desalinator that achieves continuous desalination and salt rejection. By flowing salt water on a radiative absorbing, porous, hydrophobic evaporator membrane using gravity, salt continuously diffuses away from the membrane while allowing heated water vapor to transport to and condense on a cooler microporous membrane below. Our design utilizes thin-film condensation on a microporous membrane which offers ample three-phase contact region to enhance condensation phase change heat transfer. By condensing within the microporous membrane, we reduce the gap distance between the condenser and evaporator membranes, which reduces the vapor transport resistance. We experimentally demonstrated a record-high continuous desalination and salt rejection test duration of 7 days under one-sun. Despite an increased convection heat loss necessary for salt rejection on the evaporator, our desalinator still achieved a water-collection rate of 0.487 $kg$ $m^{-2}h^{-1}$, which corresponds to a 32.2% solar-to-water efficiency. This work signifies an improvement in the robustness of current state of the art desalinators and presents a new architecture to further optimize passive solar desalinators.