A completely passive solar-powered desalination system developed by researchers at MIT and
in China could provide more than 1.5 gallons of fresh drinking water per hour for every
square meter of solar collecting area.
Such systems could potentially serve off-grid arid coastal areas to provide an efficient,
low-cost water source.
The system uses multiple layers of flat solar evaporators and condensers, lined up in a
vertical array and topped with transparent aerogel insulation.
It is described in a paper appearing in the journal Energy and Environmental Science.
The key to the system’s efficiency lies in the way it uses each of the multiple stages
to desalinate the water.
At each stage, heat released by the previous stage is harnessed instead of wasted.
In this way, the team’s demonstration device can achieve an overall efficiency of 385 percent
in converting the energy of sunlight into the energy of water evaporation.
The device is essentially a multilayer solar still, with a set of evaporating and condensing
components like those used to distill liquor.
It uses flat panels to absorb heat and then transfer that heat to a layer of water so
that it begins to evaporate.
The vapor then condenses on the next panel.
That water gets collected, while the heat from the vapor condensation gets passed to
the next layer.
Whenever vapor condenses on a surface, it releases heat; in typical condenser systems,
that heat is simply lost to the environment.
But in this multilayer evaporator the released heat flows to the next evaporating layer,
recycling the solar heat and boosting the overall efficiency.
Adding more layers increases the conversion efficiency for producing potable water, but
each layer also adds cost and bulk to the system.
The team settled on a 10-stage system for their proof-of-concept device, which was tested
on an MIT building rooftop.
The system delivered pure water that exceeded city drinking water standards, at a rate of
5.78 liters per square meter (about 1.52 gallons per 11 square feet) of solar collecting area.
This is more than two times as much as the record amount previously produced by any such
passive solar-powered desalination system.
Theoretically, with more desalination stages and further optimization, such systems could
reach overall efficiency levels as high as 700 or 800 percent.
Unlike some desalination systems, there is no accumulation of salt or concentrated brines
to be disposed of.
In a free-floating configuration, any salt that accumulates during the day would simply
be carried back out at night through the wicking material and back into the seawater.
Their demonstration unit was built mostly from inexpensive, readily available materials
such as a commercial black solar absorber and paper towels for a capillary wick to carry
the water into contact with the solar absorber
The most expensive component of the prototype is a layer of transparent aerogel used as
an insulator at the top of the stack, but the team suggests other less expensive insulators
could be used as an alternative.
The team’s key contribution is a framework for understanding how to optimize such multistage
passive systems, which they call thermally localized multistage desalination.
The formulas they developed could likely be applied to a variety of materials and device
architectures, allowing for further optimization of systems based on different scales of operation
or local conditions and materials.
The researchers plan further experiments to continue to optimize the choice of materials
and configurations, and to test the durability of the system under realistic conditions.
They also will work on translating the design of their lab-scale device into a something
that would be suitable for use by consumers.
The hope is that it could ultimately play a role in alleviating water scarcity in parts
of the developing world where reliable electricity is scarce but seawater and sunlight are abundant.
Water scarcity has become an increasingly severe global challenge as more than one-third
of the world's population is affected by water shortage.
Seawater desalination is regarded as one of the most effective solutions to address water
scarcity.
In the last decade, commercial large-scale desalination facilities including the newly
built reverse osmosis plants and legacy installations with multi-stage flash evaporation and multi-effect
distillation have partly relieved the water-shortage stress.
However, these technologies require well-developed infrastructure and centralized installations,
which are not available in many developing regions.
Passive solar desalination systems with simple configuration and off-grid operation are attractive
for several developing regions with abundant solar energy and seawater, but the high cost
of water production and large area requirements from low conversion efficiency have limited
their adoption thus far.
So, this invention can boost the usage of Passive solar desalination systems.