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Researchers develop low-cost, low-energy desalination process

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 could be powered by the waste heat of an air conditioning system.



Civil engineering professor Nirmala Khandan, right, and doctoral student Veera Gnaneswar (John) Gude check the control center of their prototype water desalination project. Operational data can be downloaded onto a laptop computer. (NMSU photo by Ben LaMarca)

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 - 45 to 50 degrees Celsius (113 to 122 degrees Fahrenheit) rather than the 60 to 100 C (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 heat."

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.

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.