|A low-cost water
desalination system developed by New Mexico State University
engineers can convert saltwater to pure drinking water on a
round-the-clock basis and its energy needs are so low it can be
powered by the waste heat of an air conditioning system.
A prototype built on the NMSU campus
in Las Cruces can produce enough pure water continuously to supply a
four-person household, said Nirmala Khandan, an environmental
engineering professor in NMSU's Department of Civil Engineering.
New Mexico and other parts of the
world have extensive brackish groundwater resources that could be
tapped and purified to augment limited freshwater supplies, but
traditional desalination processes such as reverse osmosis and
electrodialysis consume significant amounts of energy.
This research project, funded by the
NMSU-based New Mexico Water Resources Research Institute, explores
the feasibility of using low-grade heat — such as solar energy or
waste heat from a process such as refrigeration or air conditioning
— to run a desalination process.
Khandan said the project builds on a
process, first developed by researchers in Florida, that makes
distillation of saline water possible at relatively low temperatures
— 113 to 122 degrees Fahrenheit rather than the 140 to 212 F
required by most distillation processes.
The system utilizes the natural
effects of gravity and atmospheric pressure to create a vacuum in
which water can evaporate and condense at near-ambient temperatures.
Two 30-foot vertical tubes — one rising from a tank of saline
water and the other from a tank of pure water — are connected by a
horizontal tube. The barometric pressure of the tall water columns
creates a vacuum in the headspace.
At normal temperatures, Khandan said,
evaporation from the pure-water side will travel to the saline side
and condense as the system seeks equilibrium. "That's
nature," he said. "We want it to go the other way."
Raising the temperature of the water
in the headspace over the saline column slightly more than that of
the freshwater column causes the flow to go in the other direction,
so that pure, distilled water collects on one side and the brine
concentrate is left behind in a separate container. A temperature
increase of only 10 to 15 degrees is needed, Khandan said.
"That's the trick of this
vacuum," he said. "We don't have to boil the water like
normal distillation, so you can use low-grade heat like solar energy
or waste heat from a diesel engine or some other source of waste
Potentially a desalination system
using this method could be coupled to a home's refrigerated air
conditioning system, Khandan said.
"When you air condition a house,
you are pumping the heat outside the house, and the heat is wasted
into the atmosphere," he said. "We want to capture that
heat and use it to power this desalination system."
The 30-foot-tall NMSU prototype is
powered by a solar panel. Khandan and his research assistant, civil
engineering doctoral student Veera Gnaneswar Gude, have modified the
process originally developed by Florida researchers to incorporate a
thermal energy storage device that allows the system to operate
around-the-clock, using stored energy at night. The Institute of
Energy and Environment housed in the NMSU College of Engineering
helped them instrument the system.
Their research on the system's
capabilities has been presented at national and international
conferences and their research continues.
As with any desalination process, the
system leaves behind a brine concentrate that must be disposed of,
and some potential users may be put off by the unit's height,
"but this technology could go to commercial scale pretty
quickly," Khandan said. "The overall cost of desalination
by this process can be very competitive."
The project is one of many research
initiatives at NMSU aimed at addressing the critical needs of New
Mexico and the nation.
"Eye on Research" is
provided by New Mexico State University. This week's feature was
written by Karl Hill of University Communications.