  Although seawater is abundant, desalination is energy-intensive and
expensive. Using the sun as an energy source is attractive for desalinating
seawater; however, the performance of state-of-the-art passive devices is
unsatisfactory when operated at less than one sun ($<$ $1$ $kW$ $m^{-2}$).
Here, we present a completely passive, modular, and low-cost solar thermal
distiller for seawater desalination. Each distillation stage is made of two
opposed hydrophilic layers separated by a hydrophobic microporous membrane, and
it does not require further mechanical ancillaries. Under realistic laboratory
and outdoor conditions, we obtained a distillate flow rate of almost $3$ $L$
$m^{-2}$ $h^{-1}$ from seawater at less than one sun - twice the yield of
recent passive device reported in the literature. In perspective, theoretical
modelling suggests that the distiller has the potential to further doubling the
peak flow rate observed in the current experiments. This layout can satisfy
freshwater needs in isolated and impoverished communities, as well as realize
self-sufficient floating installations or provide freshwater in emergency
conditions.
