Solar Hack Opens Channel to ​Practical Desalination

Nearly a third of the world’s population don’t have access to a regular supply of clean drinking water. A promising solution is to use the heat of the sun to purify salty or contaminated water, but creating a scalable, cheap, and dependable way of doing that has proved elusive. A new solar-powered device, which could provide a family with a reliable source of clean drinking water for as little as US $4, may have made a substantial step toward solving the problem.

Desalination plants powered by electricity are already widely used in countries with limited freshwater resources, such as Saudi Arabia, the United Arab Emirates, and Australia. A study this month in the journal Desalination, for instance, notes that “Desalination technologies are a classic example of the interlink between energy, water, and the environment. By integrating renewable energy with desalination, it is possible to mitigate the environmental impacts of energy consumption.”

Most desalination technologies today, however, require considerable investments to set up as well as reliable sources of power, says Xiangyu Li, a postdoctoral researcher at MIT. And neither, Li says, are typically found in abundance in regions of developing countries where clean water shortages are most acute.

This nexus of needs and technological potential has prompted many researchers (including the authors of the new Desalination paper, above) to look at ways to purify or desalinate water with little more than sunlight. These approaches use the heat of the sun to evaporate water, which then condenses on a collection surface. As the water is vaporized it leaves behind any contaminants, providing a source of clean drinking water.

“This could be a disruptive technology towards practical solar desalination. Its overall cost is extremely low, and scalability of this technology will not be a problem.”
—Swee Ching Tan, National University of Singapore

This process can’t produce as much water as powered desalination, but recent breakthroughs have significantly increased the efficiency of systems that take this approach. Question marks over their practicality remain, though, says Li, due to the use of expensive materials or the tendency for devices to get clogged up with salt crystals.

Now, Li and colleagues have devised a new approach that both matches the efficiency of the best solar evaporation systems and eliminates the problem of salt accumulation, all while using cheap and easy-to-find materials. “The materials we use are almost household items,” says Li. “This is a simple and low-cost device that can help people in developing countries, or those struggling after natural disasters, get the freshwater that they need.”

A confined water layer above a floating thermal insulation, according to new research from MIT, yields a water-desalination and purification system that can inexpensively provide 6.5 liters of clean water per day for a personal-size device. MIT

One of the main challenges with solar evaporation is generating high enough temperatures to efficiently generate vapor. If you leave a bucket out in the sun, the bulk of the heat will simply diffuse evenly throughout the mass of water. To get around this roadblock, researchers have in recent years devised ways to concentrate heat from the sun on smaller amounts of water, boosting the evaporation efficiency.

Typically this is done using a porous “wicking” structure, which floats on top of a water reservoir and naturally draws liquid up in much the same way water will climb up a piece of tissue paper. These materials are also designed to absorb solar energy and to be insulating, so the resulting heat doesn’t diffuse into the reservoir below. This results in the thin layer of water at the top of the structure receiving a concentrated dose of heat, generating vapor much more efficiently.

The problem, says Li, is that as the water evaporates it leaves behind salts and other gunk, which can quickly coat the top of the wicking structure and reduce its ability to absorb sunlight. That’s because the water moves relatively slowly through the very fine pores of the material, which leaves plenty of time for crystals to form, he says. The MIT team’s solution was to rely on convection—the tendency of fluids to flow from areas of high density to areas of low density—to naturally circulate the concentrated brine back down into the reservoir below.

To do this, the researchers created a floating structure made of a thick layer of insulating polystyrene foam coated with light-absorbing black paint on the top and a copper weight at the bottom to keep the structure submerged just below the surface. They then drilled 2.5-millimeter channels through the structure to allow water to travel between the top layer and the main reservoir.

Unlike the narrow pores of a wicking structure, these channels are wide enough to allow a convective current to form, carrying salty water from the top of the device down through the channels. It’s a careful balancing act, though, Li says, because making the channels overly wide could lead to too much circulation that would transport all the concentrated heat down into the cooler water below.

In a paper in Nature Communications, the team showed that with careful optimization of the size and configuration of the channels they were able to match the efficiency of the best wicking-based solar evaporators, and could continue to extract clean water from brine so concentrated that it was 20 percent salt by weight.

Swee Ching Tan, an assistant professor of materials science and engineering at the National University of Singapore, who's unaffiliated with the current research, says the most intriguing part of the research is this almost ideal trade-off between heat concentration and circulation. “This could be a disruptive technology towards practical solar desalination,” he says. “Its overall cost is extremely low, and scalability of this technology will not be a problem.”

So far the team has tested the setup only in a beaker of water a few inches tall, but Li says it should be fairly simple to build larger versions. The most expensive part is the copper weight, but in low-resource settings this could be replaced with concrete. The team estimates that a 1-square-meter device would cost around US $4 and could provide 6.5 liters of water a day in good conditions, which should be sufficient to provide drinking water for a small family.