  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.
