MIT engineers have developed an innovative desalination system that operates in sync with solar energy. This solar-powered system efficiently removes salt from water, adjusting its desalination rate in response to fluctuations in sunlight. As sunlight increases throughout the day, the system speeds up, and it automatically adapts to sudden changes, such as cloud cover, by reducing its output or ramping up when the skies clear.
The system’s ability to quickly respond to subtle changes in sunlight allows it to maximize solar energy use, producing significant amounts of clean water despite varying sunlight levels. Unlike other solar-powered desalination models, the MIT system does not require additional batteries for energy storage or extra power sources like the electrical grid.
Engineers tested a community-scale prototype on groundwater wells in New Mexico over six months, working under varying weather conditions and water types. On average, the system utilized over 94% of the electrical energy generated by its solar panels, producing up to 5,000 liters of water per day despite significant weather and sunlight variations.
"Traditional desalination technologies need constant power and battery storage to mitigate variable energy sources like solar power. By continuously adjusting power consumption in sync with the sun, our technology directly and efficiently uses solar energy to produce water," says Amos Winter, a professor of mechanical engineering at MIT. "Being able to produce potable water from renewable energy without needing battery storage is a significant challenge. And we’ve achieved it."
The system is designed to desalinate brackish groundwater—a salty water source found in underground reservoirs more common than fresh groundwater resources. Researchers see brackish groundwater as a vast, untapped source of potential drinking water, especially as freshwater reserves are threatened in parts of the world. They envision that this renewable, battery-free system could provide essential, low-cost drinking water, particularly for inland communities with limited access to seawater and the electrical grid.
"Most people live far enough from the coast that seawater desalination could never reach them. They rely heavily on groundwater, especially in remote and low-income regions. Unfortunately, this groundwater is becoming saltier due to climate change," explains Jonathan Bessette, a mechanical engineering PhD student at MIT. "This technology could bring clean, sustainable, and affordable water to disadvantaged regions worldwide."
The researchers’ report detailing the new system is published in a paper in Nature Water. Co-authors of the study include Bessette, Winter, and engineer Shane Pratt.
Pump and Flow
The new system builds on a previous design by Winter and colleagues, including former MIT postdoc Wei He. This earlier system aimed to desalinate water using "batch electrodialysis."
Electrodialysis and reverse osmosis are two primary methods for desalinating brackish groundwater. Reverse osmosis uses pressure to push salty water through a membrane, filtering out salts. Electrodialysis uses an electric field to extract salt ions as water is pumped through a stack of ion-exchange membranes.
Scientists have sought to power both methods with renewable sources, but this has been particularly challenging for reverse osmosis systems, which traditionally operate at a stable power level incompatible with naturally variable energy sources like the sun.
Winter, He, and their colleagues focused on electrodialysis, seeking 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 comprising water pumps, a stack of ion-exchange membranes, and a solar panel array. The innovation was a model-based control system that used sensor readings from each part of the system to predict the optimal flow rate for pumping water through the stack and the voltage to apply to maximize salt extraction.
When the team tested this system in the field, they could vary water production based on natural sunlight variations. 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.
However, the researchers believed they could improve further.
"We could only calculate every three minutes, and during that time, a cloud could literally pass and block the sun," explains Winter. "The system might say, ‘I need to operate at this high power.’ But some of that energy suddenly dropped because there’s now less sunlight. So, we had to compensate for that energy with additional batteries."
Solar Controls
In their latest work, the researchers aimed to eliminate the need for batteries by reducing the system’s response time to a fraction of a second. The new system can update 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 needing to make up for any power lag with additional supplies.
The key to more agile desalination lies in a simpler control strategy designed by Bessette and Pratt. The new strategy is "flow-driven current control," where the system first detects the amount of solar energy produced by its solar panels. If the panels generate more energy than the system uses, the controller automatically "commands" the system to increase its pumping, pushing more water through the electrodialysis columns. Simultaneously, the system diverts some of the extra solar energy by increasing the electrical current supplied to the stack, extracting more salt from the faster-flowing water.
"Let’s say the sun comes out every few seconds," explains Winter. "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 bit.’ When we look again and see there’s even more excess power, we increase it again. By doing this, we’re able to very precisely match our consumed energy with the available solar energy throughout the day. And the faster we loop this, the less battery buffering we need."
The engineers integrated the new control strategy into a fully automated system they scaled 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 Brackish Groundwater National Desalination Research Facility in Alamogordo, New Mexico. Throughout the trial, the prototype operated under a wide range of solar conditions, utilizing an average of more than 94% of the solar panel’s electrical energy to directly power desalination.
"Compared to the traditional design of a solar desalination system, we’ve reduced our required battery capacity by nearly 100%," says Winter.
The engineers plan to further test and scale the system in hopes of providing low-cost, fully solar-powered drinking water to larger communities and even entire municipalities.
"While this is a big step forward, we are still diligently working to continue developing less expensive and more sustainable desalination methods," says Bessette.
"We are now focusing on testing, optimizing reliability, and creating a range of products capable of providing desalinated water using renewable energy to multiple markets worldwide," adds Pratt.
The team plans to launch a company based on their technology in the coming months.
This research was partially funded by the National Science Foundation, the Julia Burke Foundation, and the MIT Morningside Academy of Design. The work was also supported in kind by Veolia Water Technologies and Solutions and Xylem Goulds.
