The solar-powered desalination system does not require additional batteries

MIT engineers have built a new desalination system that works according to the rhythm of the sun.

The solar-powered system removes salt from the water at a rate that closely matches changes in solar energy. As the amount of sunlight increases throughout the day, the system speeds up the desalination process and automatically adjusts to any sudden changes in sunlight, such as reducing intensity in response to a passing cloud or increasing rpm as the sky clears.

Because the system can respond quickly to subtle changes in sunlight, it maximizes the utility of solar energy by producing large volumes of clean water despite changes in sunlight throughout the day. Unlike other solar-powered desalination projects, the MIT system does not require additional batteries for energy storage or additional power, such as from the grid.

Engineers tested the prototype on a community scale in groundwater wells in New Mexico for six months, working in varying weather and water conditions. On average, the system used more than 94 percent of the electricity generated from the solar panels to produce up to 5,000 liters of water per day despite wide fluctuations in weather and available sunlight.

“Conventional desalination technologies require constant power and battery storage to provide a variable power source such as solar power. By constantly changing energy consumption in sync with the sun, our technology directly and efficiently uses solar energy to produce water,” says Amos Winter, professor of mechanical engineering at Germeshausen and director of the K. Lisa Yang Global Engineering and Research Center (GEAR) at MIT. “The ability to produce drinking water from renewable sources without having to store batteries is a huge challenge. And we did it.”

The system is aimed at desalination of brackish groundwater – a salty water source found in underground reservoirs and more common than fresh groundwater resources. Scientists see brackish groundwater as a huge untapped source of potential drinking water, especially as freshwater supplies are depleted in some parts of the world. They predict that the new, renewable and battery-free system could provide much-needed drinking water at low cost, especially for inland communities where access to seawater and grid power is limited.

“Most of the population actually lives far enough from the coast that seawater desalination could never reach them. They therefore rely heavily on groundwater, especially in remote, low-income regions. Unfortunately, due to climate change, groundwater is becoming more and more saline,” says Jonathan Bessette, an MIT graduate student in mechanical engineering. “This technology can provide sustainable and affordable clean water in hard-to-reach places around the world.”

The researchers describe the new system in detail in a paper published today in the journal Water Nature. Co-authors of the study are Bessette, Winter and staff engineer Shane Pratt.

Pump and flow

The new system builds on a previous project that Winter and his colleagues, including former MIT postdoc Wei He, reported earlier this year. The purpose of this system was to desalinate water through “flexible batch electrodialysis”.

Electrodialysis and reverse osmosis are the two main methods used to desalinate brackish groundwater. In reverse osmosis, pressure is used to pump salty water through a membrane and filter out the salt. Electrodialysis uses an electric field to pull out salt ions while pumping water through a stack of ion-exchange membranes.

Scientists tried to power both methods with renewable sources. However, this has been particularly difficult for reverse osmosis systems, which traditionally operate at a constant power level that is 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 changes in renewable solar energy.

In their previous project, the team built an electrodialysis system consisting of water pumps, a stack of ion-exchange membranes, and an array of solar panels. The innovation in 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 water would be pumped through the stack and the voltage that should be applied to the stack to maximize the amount of salt pulled from the water.

When the team tested this system in the field, they were able to adjust water production according to natural changes in sunlight. On average, the system directly used 77 percent of the available electricity generated by the solar panels, which the team estimated was 91 percent more than traditionally designed solar-powered electrodialysis systems.

Still, the researchers thought they could do better.

“We could only make calculations every three minutes, and in that time a cloud could literally come in and block the sun,” Winter says. “The system might say, ‘I have to run at this much power.’ But some of that power has suddenly dropped because there is now less sunlight. So we had to supplement this power with additional batteries.”

Solar commands

In their latest work, researchers sought to eliminate the need for batteries by reducing the system’s response time to a fraction of a second. The new system is capable of updating the 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 supplement power delays with additional power supplies.

The key to more efficient desalination is a simpler control strategy developed by Bessette and Pratt. The new strategy is “flow-driven current control,” in which the system first detects the amount of solar energy produced by the system’s solar panels. If the panels produce more energy than the system uses, the controller automatically “tells” the system to pump more, pushing more water through the electrodialysis stacks. At the same time, the system reverses some of the extra solar energy, increasing the electrical current delivered to the chimney to remove 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 power – let’s increase the flow rate and current a little bit.’ When we look again and see that there is even more excess power, we will increase it again. Thanks to this, we are able to very precisely match the energy we consume to the available solar energy during the day. The faster we loop this, the less battery buffering we will need.

Engineers incorporated the new control strategy into a fully automated system sized to desalinate brackish groundwater in sufficient daily volumes to supply a small community of approximately 3,000 people. The system operated for six months on several wells at the National Salt Ground Water Desalination Research Facility in Alamogordo, New Mexico. Throughout the testing period, the prototype operated in a wide range of solar conditions, using an average of more than 94 percent of the solar panel’s electricity to directly power the desalination plant.

“Compared to a traditional solar desalination plant design, we reduced the required battery capacity by almost 100 percent,” says Winter.

Engineers plan to continue testing and scaling up the system, hoping to supply larger communities, or even entire municipalities, with cheap drinking water powered entirely by solar energy.

“While this is a big step forward, we continue to work diligently to continue to develop cheaper and more sustainable desalination methods,” Bessette says.

“We are currently focused on testing, maximizing reliability and building product lines that can deliver desalinated water using renewable energy sources to multiple markets around the world,” adds Pratt.

In the coming months, the team will launch a company based on its technology.

This research was supported in part by the National Science Foundation, the Julia Burke Foundation, and the MIT Morningside Academy of Design. This work was additionally supported in kind by Veolia Water Technologies and Solutions and Xylem Goulds.