Solar-powered desalination system requires no additional batteries and could provide low-cost drinking water

MIT engineers have built a new desalination system that runs at the pace of the sun. The researchers report details of the new system in an article appearing in Natural water.

The solar-powered system removes salt from the water at a rate that closely follows changes in solar energy. As sunlight increases throughout the day, the system speeds up its desalination process and automatically adapts to any sudden changes in sunlight, for example by decreasing the volume in response to a passing cloud or by accelerating when the sky clears.

Because the system can respond quickly to subtle changes in sunlight, it maximizes the utility of solar energy, producing large quantities of clean water despite variations in sunlight throughout the day. Unlike other solar-powered desalination models, the MIT system does not require any additional batteries for energy storage, nor an additional power supply, such as from the grid.

Engineers tested a community-scale prototype on groundwater wells in New Mexico for six months, working in varying weather conditions and water types. The system harnessed on average more than 94% of the electrical energy generated by the system’s solar panels to produce up to 5,000 liters of water per day despite wide variations in weather and available sunlight.

“Conventional desalination technologies require constant power and battery storage to mitigate a variable energy source like solar power. By continuously varying energy consumption in sync with the sun, our technology directly and efficiently uses solar energy to produce water,” says Amos Winter, the Germeshausen Professor of Mechanical Engineering and director of the K. Lisa Yang Global Engineering and Research (GEAR) Center at MIT.

“Being able to produce drinking water from renewable energy, without requiring battery storage, is a big challenge. And we’ve done it.”

The system aims to desalinate brackish groundwater, a source of saline water found in underground reservoirs and which is more prevalent than fresh groundwater resources. Researchers view brackish groundwater as a huge untapped source of potential drinking water, especially as freshwater supplies are threatened in some parts of the world.

They envision that the new renewable, battery-free system could provide much-needed drinking water at low cost, particularly for inland communities where access to seawater and the electricity grid is limited.

“The majority of the population actually lives far enough from the coast that seawater desalination can never reach them. It is therefore heavily dependent on groundwater, particularly in remote and low-income regions. And unfortunately, this groundwater is becoming increasingly salty due to climate change,” says Jonathan Bessette, Ph.D. of MIT. student in mechanical engineering.

“This technology could bring clean, sustainable and affordable water to disadvantaged regions around the world.”

Pump and flow

The new system builds on a previous design, which Winter and colleagues, including former MIT postdoc Wei He, reported earlier this year. This system aimed to desalinate water using “flexible batch electrodialysis”.

Electrodialysis and reverse osmosis are two of the main methods used to desalinate brackish groundwater. With reverse osmosis, pressure is used to pump salt water through a membrane and filter out the salts. Electrodialysis uses an electric field to extract salt ions when water is pumped through a stack of ion exchange membranes.

Scientists sought to power both methods with renewable sources. But this has proven particularly difficult for reverse osmosis systems, which traditionally operate at a stable power level incompatible with naturally variable energy sources such as the sun.

Winter, He and their colleagues focused on electrodialysis, looking for ways to create a more flexible, “time-varying” system that would respond to variations in renewable solar energy.

In their previous design, the team built an electrodialysis system consisting of water pumps, a stack of ion exchange membranes, and an array of solar panels.

The innovation of this system was a model-based control system that used sensor readings from each part of the system to predict the optimal rate at which to pump water through the chimney and the voltage that should be applied to the chimney to maximize the amount of water. salt extracted from water.

When the team tested this system in the field, they were able to vary their water production based on natural variations in the sun. On average, the system directly used 77% of the available electrical energy produced by the solar panels, which the team estimated to be 91% more than traditionally designed solar-powered electrodialysis systems.

Still, the researchers thought they could do better.

“We could only calculate every three minutes, and in that time a cloud could literally pass by and block the sun,” Winter says. “The system might say, ‘I need to run at this high power.’ But some of that energy suddenly dropped because there is now less sunlight. So we had to compensate for this energy with additional batteries. »

Solar controls

In their latest work, the researchers sought to eliminate the need for batteries, reducing the system’s response time to a fraction of a second. The new system is capable of updating its desalination rate three to five times per second. The faster response time allows the system to adapt to changes in sunlight throughout the day, without having to make up any power lags with additional power supplies.

The key to more agile desalination lies in a simpler control strategy, designed by Bessette and Pratt. The new strategy is “flux-driven current control”, in which the system first detects the amount of solar energy produced by the system’s solar panels.

If the panels generate more power than the system uses, the controller automatically “commands” the system to increase its pumping, pushing more water through the electrodialysis columns. At the same time, the system diverts some of the extra solar energy by increasing the electrical current supplied to the chimney, in order to extract more salt from the faster flowing water.

“Let’s say the sun rises every few seconds,” Winter explains.

“So three times a second we look at the solar panels and say, ‘Oh, we have more energy, let’s increase our flow and current a little bit.’ When we look again and see that there is even more excess energy, we increase it again, allowing us to very accurately match our consumed energy with the available solar energy, throughout of the day this, the less battery buffering we need.

The engineers integrated the new control strategy into a fully automated system that they sized to desalinate brackish groundwater at a daily volume sufficient to supply a small community of about 3,000 people. They operated the system for six months on several wells at the National Brackish Water Research Center in Alamogordo, New Mexico.

Throughout the trial, the prototype operated in a wide range of solar conditions, on average harnessing more than 94% of the solar panel’s electrical energy to directly power desalination.

“Compared to traditional solar desalination system design, we have reduced our required battery capacity by almost 100%,” says Winter.

Engineers plan to further test and expand the system in hopes of providing low-cost, completely solar-powered drinking water to larger communities and even entire municipalities.

“While this is a big step forward, we are still working diligently to continue to develop less expensive and more sustainable desalination methods,” Bessette said.

“We are now focused on testing, optimizing reliability and creating a range of products capable of providing desalinated water using renewable energy to several markets around the world,” adds Pratt.

The team will launch a company based on their technology in the coming months. Co-authors of the study are Bessette, Winter and engineer Shane Pratt.

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news in MIT research, innovation and education.