Solar-powered desalination system requires no additional power

Engineers from the Massachusetts Institute of Technology (MIT) have built a solar-powered system that removes salt from water at a pace that closely follows changes in solar energy. As sunlight increases through the day, the system ramps up its desalting process and automatically adjusts to any sudden variation in sunlight, for example by dialling down in response to a passing cloud or revving up as the skies clear.

Because the system can quickly react to subtle changes in sunlight, it maximises the utility of solar energy, producing large quantities of clean water despite variations in sunlight throughout the day. In contrast to other solar-driven desalination designs, the MIT system requires no extra batteries for energy storage, nor a supplemental power supply, such as from the grid.

The engineers tested a community-scale prototype on groundwater wells in New Mexico over six months, working in variable weather conditions and water types. The system harnessed on average over 94% of the electrical energy generated from the system’s solar panels to produce up to 5000 litres of water per day despite large swings in weather and available sunlight.

“Conventional desalination technologies require steady power and need battery storage to smooth out a variable power source like solar. By continually varying power consumption in sync with the sun, our technology directly and efficiently uses solar power to make water,” said 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 make drinking water with renewables, without requiring battery storage, is a massive challenge — and we’ve done it.”

The system is geared towards desalinating brackish groundwater — a salty source of water that is found in underground reservoirs and is more prevalent than fresh groundwater resources. The researchers see groundwater as a huge untapped source of potential drinking water, particularly as reserves of fresh water are stressed in parts of the world. They envision that the new renewable, battery-free system could provide much-needed drinking water at low costs, especially for inland communities where access to seawater and grid power are limited.

“The majority of the population actually lives far enough from the coast that seawater desalination could never reach them. 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,” said Jonathan Bessette, MIT PhD student in mechanical engineering. “This technology could bring sustainable, affordable clean water to under-reached places around the world.”

Pump and flow

Electrodialysis and reverse osmosis are two of the main methods used to desalinate groundwater. With reverse osmosis, pressure is used to pump salty water through a membrane and filter out salts. In contrast, electrodialysis uses an electric field to draw out salt ions as water is pumped through a stack of ion-exchange membranes.

Scientists have looked to power both methods with renewable sources. But this has been especially challenging for reverse osmosis systems, which traditionally run at a steady power level that’s incompatible with naturally variable energy sources such as the sun.

The MIT engineers and their colleagues focused on electrodialysis, seeking ways to make a more flexible, ‘time-variant’ system that would be responsive to variations in renewable, solar power.

In their previous design, the team built an electrodialysis system consisting of water pumps, an ion-exchange membrane stack and a solar panel array. The innovation in this system was a model-based control system that used sensor readings from every part of the system to predict the optimal rate at which to pump water through the stack and the voltage that should be applied to the stack to maximise the amount of salt drawn out of the water.

When the team tested this system in the field, it was able to vary its water production with the sun’s natural variations. On average, the system directly used 77% of the available electrical energy produced by the solar panels, which the team estimated was 91% more than traditionally designed solar-powered electrodialysis systems.

Still, the researchers felt they could do better.

“We could only calculate every three minutes, and in that time, a cloud could literally come by and block the sun,” Winter said. “The system could be saying, ‘I need to run at this high power’. But some of that power has suddenly dropped because there’s now less sunlight. So, we had to make up that power with extra batteries.”

In a direct-drive electrodialysis desalination system, solar panels optimally allocate energy to the pump and electrodialysis stack. Feed water flows through the pump into the electrodialysis stack, where it is desalinated and split into drinking water (light blue) and concentrated brine (dark blue). Credit: Jonathan Bessette.

Solar commands

In their latest work, the researchers looked to eliminate the need for batteries, by shaving the system’s response time to a fraction of a second. The new system is able to update its desalination rate three to five times per second. The faster response time enables the system to adjust to changes in sunlight throughout the day, without having to make up any lag in power with additional power sources.

The key to more nimble desalting is a simpler control strategy, devised by Bessette and Pratt. The new strategy is one of “flow-commanded current control”, in which the system first senses the amount of solar power that is being produced by the solar panels. If the panels are generating more power than the system is using, the controller automatically commands the system to increase its pumping, pushing more water through the electrodialysis stacks. Simultaneously, the system diverts some of the additional solar power by increasing the electric current delivered to the stack, to drive more salt out of the faster-flowing water.

“We’re able to closely match our consumed power with available solar power really accurately, throughout the day. And the quicker we loop this, the less battery buffering we need,” Winter explained.

The engineers incorporated the new control strategy into a fully automated system that they sized to desalinate brackish groundwater at a daily volume that would be enough to supply a small community of about 3000 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, harnessing over 94% of the solar panels’ electrical energy, on average, to directly power desalination.

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

The engineers 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,” Bessette said.

The team will be launching a company based on their technology in the coming months.

The researchers’ report details the new system in a paper appearing in Nature Water.

Top image caption: Jon Bessette sits atop a trailer housing the electrodialysis desalination system at the Brackish Groundwater National Desalination Research Facility in Alamogordo, New Mexico. Image credit: Shane Pratt