Innovative solar desalination system could provide low-cost drinking water for millions

MIT researchers say technology does not need to rely on expensive battery as it adjusts output when sunlight levels change

MIT engineers tested a community-scale prototype on groundwater wells in New Mexico in the US over six months. Photo: Shane Pratt

Scientists have built a solar-powered desalination system that does not need expensive batteries and which could produce vast amounts of low-cost drinking water for millions around the world.

The innovative plant, which was designed by MIT researchers, responds quickly to subtle changes in the level of sunlight – such as a passing cloud – to make the most of the available solar energy.

As a result, it requires no extra batteries for energy storage, nor a supplemental power supply, and therefore can produce large amounts of drinking water with exclusively renewable energy.

The MIT engineers behind the project tested a community-scale prototype on groundwater wells in New Mexico in the US over six months.

The system produced up to 5,000 litres of water per day, despite large swings in weather and available sunlight, and used more than 94 per cent of the electrical energy generated from the system's solar panels.

“Conventional desalination technologies require steady power and need battery storage to smooth out a variable power source like solar,” said Amos Winter, a professor of mechanical engineering at MIT. “By continually varying power consumption in synch with the sun, our technology directly and efficiently uses solar power to make water.”

“Being able to make drinking water with renewables, without requiring battery storage, is a massive grand challenge. And we’ve done it.”

MIT's Jon Bessette, Shane Pratt and Muriel McWhinnie stand in front of the electrodialysis desalination system during an installation in July. Photo: Shane Pratt

Climate change means there is an increasing reliance on desalination around the world. Some estimates suggest that, by 2030, there will be a 40 per cent gap between water supply and demand. The UAE, similar to its Gulf neighbours, has come to depend on desalination, which provides about half of the country's water needs.

Scientists say the technology will be particularly useful in areas far from the coast that only have access to salty, brackish groundwater that is difficult to convert into clean drinking water.

“The majority of the population actually lives far enough from the coast, that seawater desalination could never reach them,” said Jonathan Bessette, MIT PhD student in mechanical engineering. “They consequently rely heavily on groundwater, especially in remote, low-income regions. And unfortunately, this groundwater is becoming more and more saline due to climate change.”

“This technology could bring sustainable, affordable clean water to under-reached places around the world.”

The plant relies on a process known as electrodialysis, which uses an electric field to draw out salt ions as water is pumped through a stack of ion-exchange membranes.

The researchers felt that the process would be more effective than the alternative, known as reverse osmosis, which pumps salty water through a membrane and filters out salts. That method traditionally runs at a steady power level that is incompatible with naturally variable energy sources such as the sun, researchers say.

The team had hoped to use electrodialysis to create a “time-variant” system that would be responsive to variations in renewable, solar power. In their latest work, the researchers looked to eliminate the need for batteries, by shaving response time to a fraction of a second.

The new system is able to update its desalination rate, three to five times per second. This enables the system to adjust to changes in sunlight throughout the day, without having to make up any lag in power with additional power supplies.

“Compared to how you would traditionally design a solar desalination system, we cut our required battery capacity by almost 100 per cent,” Prof Winter said.

The researchers report details of the new system in a paper published in Nature Water. They plan to further test and scale up the system in hopes of supplying larger communities, and even whole municipalities, with low-cost, fully sun-driven drinking water.

“While this is a major step forward, we’re still working diligently to continue developing lower cost, more sustainable desalination methods,” Mr Bessette said.