“More than one billion people lack access to clean drinking water, and the situation is only expected to get worse,” says John Lienhard, a professor of mechanical engineering and director of the Center for Clean Water and Clean Energy at MIT and King Fahd University of Petroleum and Minerals. Desalination — or the removal of salt from seawater, resulting in potable water — will be essential to solving the world’s growing water crisis, he says.
For the last two years, Lienhard and colleagues have immersed themselves in studying desalination. And already they’ve made significant contributions to the field using a classic MIT approach: the application of basic science to address a human need.
Today’s commercial desalination systems can’t meet the water needs of many people, especially those in developing countries. Among other drawbacks, they are expensive, energy-intensive, use fossil fuels, and require infrastructure to distribute the resulting water that is often not available in rural or poorer areas.
One promising alternative mimics Nature’s approach: the evaporation of seawater, leaving salts behind, followed by condensation of that water vapor into fresh water — rain. Known as humidification-dehumidification (HD) desalination, the system separates these basic natural processes into distinct components, such as a solar collector and a humidifier. Among other advantages, says Lienhard, HD can use an energy source readily available to many third-world countries — the sun.
HD is not, however, very energy efficient. Could it be made more so?
To find out, Lienhard and colleagues analyzed the thermodynamics behind different HD systems. “It turns out that no one had done this carefully before,” he notes. “My background is in thermal science, so my ‘bread and butter’ is in a discipline that applies very directly to this problem.”
Specifically, he says, “we developed a set of tools that allowed us to assess the thermodynamic efficiencies of these systems, so we were able to systematically compare competing designs.” Tools in hand, the team went on to see if they could optimize HD, for example by operating the components under different pressures. 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.
Lienhard and colleagues are also interested in whether HD systems could be made inexpensively in poor or rural areas by using local materials. One of his students, for example, 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. Loofah appears to be promising, but work continues.
The challenge of providing potable water for all is daunting. But Lienhard concludes on an optimistic note. “We’ve found that some desalination problems are very amenable to attack from the classical methods of thermodynamics.”