A solar desalination system does not require additional batteries
MIT engineers have built a new desalination system that works with the rhythm of the sun.
The solar-powered system removes salt from water at a rate that closely follows changes in solar energy. As sunlight increases throughout the day, the system ramps up the desalination process and automatically adjusts to sudden variations in sunlight, for example by turning down in response to a passing cloud or turning up as the sky clears .
Because the system can respond quickly to subtle changes in sunlight, it maximizes the utility of solar energy and produces large quantities of clean water despite variations in sunlight throughout the day. Unlike other solar-powered desalination designs, the MIT system does not require additional batteries for energy storage, nor an additional power supply, such as from the electrical grid.
The 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 used an average of more than 94 percent of the electrical energy generated by the system’s solar panels, producing up to 5,000 liters of water per day, despite major weather fluctuations and available sunlight.
“Conventional desalination technologies require constant power and require battery storage to utilize a variable energy source such as solar energy. 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. “It is a huge challenge to produce drinking water with renewable energy sources, without the need for battery storage. And we have succeeded.”
The system aims to desalinate brackish groundwater – a saline water source found in underground reservoirs that is more common than fresh groundwater. The researchers see brackish groundwater as a huge untapped source of potential drinking water, especially as freshwater reserves are under pressure in parts of the world. They envision that the new renewable, battery-free system can provide much-needed drinking water at a low cost, especially for inland communities where access to seawater and electricity grids is limited.
“The majority of the population actually lives far enough from the coast that seawater desalination would never reach them. They are therefore highly dependent on groundwater, especially in remote, low-income areas. And unfortunately, this groundwater is becoming increasingly salty due to climate change,” says Jonathan Bessette, a PhD student in mechanical engineering at MIT. “This technology could bring sustainable, affordable clean water to underserved places around the world.”
The researchers report details about the new system in an article appearing in Nature Water. The study’s co-authors are Bessette, Winter and staff engineer Shane Pratt.
Pump and power
The new system builds on an earlier design, which Winter and his colleagues, including former MIT postdoc Wei He, reported on earlier this year. That system was intended 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, salt water is pumped through a membrane using pressure and salts are filtered out. Electrodialysis uses an electric field to extract salt ions while pumping water through a stack of ion exchange membranes.
Scientists have tried to power both methods with renewable sources. But this has been especially challenging for reverse osmosis systems, which traditionally operate at a stable energy level that is incompatible with naturally variable energy sources such as the sun.
Winter, He and their colleagues focused on electrodialysis and looked for ways to create a more flexible, “time-variant” system that would respond to variations in renewable solar energy.
In their previous design, the team built an electrodialysis system consisting of water pumps, an ion exchange membrane stack and an array of solar panels. The innovation in this system was a model-based control system that used sensor measurements from each part of the system to predict the optimal rate at which water should be pumped through the stack, and the voltage to apply to the stack to maximize the amount of water. salt removed from the water.
When the team tested this system in the field, it was able to adjust water production according to the sun’s natural variations. On average, the system directly used 77 percent of the available electrical energy produced by the solar panels, which the team found was 91 percent more than traditionally designed solar electrodialysis systems.
Still, the researchers believed they could do better.
“We could only calculate every three minutes, and in that time a cloud could literally come by and block out the sun,” says Winter. “The system might say, ‘I need to operate at this high power.’ But some of that power has suddenly decreased because there is now less sunlight, so we had to supplement that power with extra batteries.”
Solar assignments
In their latest work, the researchers sought to eliminate the need for batteries by reducing the system’s response time to a fraction of a second. The new system can update 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 compensate for any power lag with additional power supplies.
The key to smoother desalination is a simpler control strategy, devised by Bessette and Pratt. The new strategy is one of ‘flow-commanded current control’, where the system first senses 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 “instructs” the system to push more water through the electrodialysis stacks. At the same time, the system diverts some of the extra solar energy by increasing the electrical current supplied to the chimney, to expel 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 our flow rate and power a little bit.’ If we look again and see that there is even more excess energy, we increase it again. As we do that, we can very precisely match our consumed power to the available solar energy, during the day the less battery buffering we need. “
The engineers integrated the new control strategy into a fully automated system they tailored to desalinate brackish groundwater at a daily volume that would be 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 Research Facility in Alamogordo, New Mexico. Throughout the trial, the prototype operated under a wide range of solar conditions, utilizing on average more than 94 percent of the solar panel’s electrical energy for direct desalination.
“Compared to the way you would traditionally design a solar desalination system, we have reduced the required battery capacity by almost 100 percent,” says Winter.
The engineers plan to further test and scale up the system in the hopes of providing larger communities, and even entire municipalities, with cheap, completely solar-powered drinking water.
“While this is a big step forward, we are still working hard to develop cheaper, more sustainable desalination methods,” says Bessette.
“Our focus now is on testing, maximizing reliability and building a product line that can deliver desalinated water using renewable energy sources to multiple markets around the world,” adds Pratt.
The team will launch a company based on their technology in the coming months.
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.
Research report:Directly driven photovoltaic electrodialysis via current-driven current control
Research report:Flexible batch electrodialysis for low-cost solar desalination of brackish water