That shot of hot air coming from the bottom of the
refrigerator may soon serve a greater purpose than just warming
your feet. A new saltwater distillation technique that uses solar
energy and waste heat from appliances could provide remote
Southwestern communities with clean drinking water.
Researchers Nirmala Khandan and Veera Gude of New Mexico State
University in Las Cruces have created a desalination system that
could provide almost a million gallons of fresh water annually
– enough to supply 25 homes with up to 100 gallons of water
each per day.
Salty water has long plagued states like
New Mexico, Arizona and Nevada, where high levels of naturally
occurring minerals, as well as industrial waste and agricultural
runoff, have made most groundwater too salty for drinking or
agriculture. The U.S. Geological Survey estimates that 75 percent
of New Mexico’s groundwater has salt levels above 1,000
micrograms per liter, the point at which water becomes saline.
But that briny reserve has a lot of potential, if
technology can convert it to freshwater. “We live in a pretty
dry area, and more people are looking to live here every
year,” says Thomas Mayer, project manager for Sandia National
Laboratories’ geochemical department. “We’re just
plain out of (fresh)water. But desalination is relatively
drought-proof.”
So far, a major challenge of
desalination has been the enormous amount of energy it requires,
something this new method addresses.
Traditional
distillation technologies, which require boiling saltwater, take as
much as 11,000 kilowatt-hours of electricity to produce 36,500
gallons of freshwater (the amount of water per house per year
produced by Khandan and Gude’s project). Their prototype
creates a natural vacuum, which makes water boil at lower
temperatures. The new technology saves about half of the energy
used by traditional methods; it’s roughly equivalent to
running a household water heater.
Unlike other
desalination methods, the prototype runs almost entirely on
renewable energy: solar power and waste heat, like that emitted by
air conditioners. Other desalination methods — such as reverse
osmosis, a method that filters water through special membranes —
use even less energy, but that efficiency is offset by their
reliance on fossil fuels.
The prototype uses a more
sophisticated version of a 3,000-year-old method. A vacuum sucks
water from a saltwater tank into a depressurized container. This
evaporation tank acts much like a glass jar left out in the sun,
turning water into steam and leaving a more concentrated salt
solution behind. Those salty leftovers collect in a waste bin,
while the freshwater steam travels through tubes and condenses in a
final tank. From there, pure water can then be pumped away to fill
empty glasses in nearby communities. Because it produces water more
slowly than other methods, solar desalination works best for small
communities.
Regardless of the desalination method used,
researchers must contend with the waste that’s left behind.
“There’s salts left over, and there are not very good
options for disposing of it,” says Mayer. One technique for
treating waste is to evaporate leftover water in ponds, package the
salts left behind and ship it to industrial waste facilities.
However, doing this exposes wildlife and the public to potential
environmental hazards.
Another uncertainty is the cost of
building a solar desalinator. “Even if (a solar desalinator)
was cheap to run, the capital cost of building it would be
astronomical,” says Mayer. But Gude contends that
“desalination is like it’s almost free” because
the energy is free, and Khandan estimates that building one of the
plants would cost about $50,000.
Meanwhile, Khandan and
Gude are testing their prototype, have already applied for a patent
license, and are fielding offers from water utilities and private
contractors interested in their invention. They hope to have a
final design available next year.
Despite some
skepticism, competing researchers admire the possibilities. Mayer
says, “The New Mexico prototype is a very ingenious
device.”
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