MIT’s motto “Mind and Hand” is more than a slogan to mechanical engineers at MIT. It’s a credo they live every day. John Lienhard, for example. The MechE professor and his colleagues have spent the last two years working on the critical problem of how to get clean drinking water to people across the world who need it.
“More than one billion people lack access to clean drinking water, and the situation is only expected to get worse,” says Lienhard, director of the Center for Clean Water and Clean Energy at MIT and at King Fahd University of Petroleum and Minerals. His solution: desalination—removing salt from seawater to produce potable water. Lienhard and a team of colleagues have immersed themselves in the challenges of desalination, and they’re making significant strides.
One of the problems with commercial desalination systems is that they can’t meet the needs of enough people, especially those in developing countries. They are also expensive, energy-intensive, rely on fossil fuels, and require a distribution infrastructure. As a result, desalinated water is often not available in poor or rural areas.
Lienhard and his team turned to nature for inspiration: the evaporation of seawater, leaving salts behind, followed by the condensation of that water vapor into fresh water is basically the atmospheric process that results in rain. Known as humidification-dehumidification (HD) desalination, the system assigns these basic natural processes to distinct components, such as a solar collector and a humidifier. Among other advantages, says Lienhard, HD can use an energy source readily available in many third-world countries—the sun.
Attacking the problem with thermodynamics
HD does have a downside, however. It’s not energy efficient. Lienhard and his team wondered if the process could be made greener and analyzed the thermodynamics behind different HD systems. “It turns out that no one had done this carefully before,” he says. “My background is in thermal science, so my ‘bread and butter’ is in a discipline that applies very directly to this problem.”
Lienhard and his colleagues developed a set of tools that made it possible to assess the thermodynamic efficiencies of these systems, so they were able to systematically compare competing designs. Tools in hand, they investigated whether they could optimize HD—by operating the components under different pressures, for example.
The results are promising.
“We’ve found very substantial improvements over the efficiency of existing HD systems,” Lienhard says, adding that one of the team’s proposed systems could outperform a leading commercial technique with respect to the amount of energy needed to produce a liter of drinking water.
But is this a process that works for every population? Lienhard and colleagues are investigating whether HD systems can be produced inexpensively in poor or rural areas by using local materials. One of his students has explored whether the packing material key to one component—the humidifier—could be made of materials like loofah or bamboo that are native to an area. In fact, early research shows that loofah may indeed work in this application. Lienhard’s team will continue to explore this and other options.
Providing potable water for everyone on the planet is daunting, but Lienhard is optimistic. “We’ve found that some desalination problems are very amenable to attack from the classical methods of thermodynamics.”
He will be addressing the International Desalination Association Energy Conference in November in Huntington Beach, California.
A version of this article, written by Elizabeth Thomson, appeared in the Summer 2010 issue of MIT Spectrum.