Desal System Optimized for Sun Power
Brackish groundwater is a huge untapped source for potential drinking water, especially in water-stressed regions in the developing world. Traditional renewable desalination technologies (including reverse osmosis) typically require batteries for energy storage and a supplemental power supply connected to a grid. This complicated infrastructure drives up costs.
To make a more efficient and affordable desalination process that is powered by renewable energy, Massachusetts Institute of Technology engineers developed a new type of desalination system that runs with the rhythms of the sun—extracting salt from water using only sunlight at a rate that closely follows changes in solar energy. As sunlight increases through the day, the system ramps up salt removal and automatically adjusts to any sudden variation in sunlight, such as dialing down in response to a passing cloud or revving up as the skies clear.
The research team, led by Amos Winter, professor of mechanical engineering at MIT, designed and built an electrodialysis (ED) system consisting of water pumps, an ion-exchange membrane stack, and a solar panel array.
ED flows a feed solution through a stack of spacers, each separated by selectively charged cation and anion exchange membranes. An applied electric field drives ions across the membranes. The process is operated with either continuous flow of water through the stack, or via batches of water recirculating through the stack. “This electrically driven—rather than pressure-driven—process reduces energy consumption and cost in brackish water and partial desalination scenarios,” Winter said. “The energy efficiency of ED also reduces solar or wind array sizes and thus also capital cost and system footprint.”
The electrodialysis system flows a feed solution through a stack of spacers, each separated by selectively charged cation and anion exchange membranes. The process is operated with either continuous flow of water through the stack, or via batches of water recirculating through the stack. Image: MIT
For example, if the solar panels are generating more power than the system is using, the controller automatically “commands” the system to dial up its pumping, pushing more water through the electrodialysis stacks. Simultaneously, the system diverts some of the additional solar power by increasing the electrical current delivered to the stack, to drive more salt out of the faster-flowing water.
“We are using photovoltaic electrodialysis to enable direct-drive [little to no energy storage], optimally controlled desalination at high production rates,” Winter said. “The control scheme employs cascade feedback control, which increases response speed compared to other control strategies that use computationally expensive numerical solvers.”
More Like This: Wave-powered desalination system
The team constructed a community-scale pilot system to test controller robustness and reduction in energy storage in a six-month field trial on real groundwater wells in New Mexico, working in variable weather conditions and water types.
On average, the system harnessed over 94 percent of the electrical energy generated from the system’s solar panels to produce up to 5,000 liters of water per day, despite large swings in weather and available sunlight.
“The prototype also featured an energy storage to water productivity ratio of over 99 percent less than the median PV desalination systems in the literature,” Winter said. “The high water efficiency of ED is especially beneficial in water stressed regions and reduces brine volume, which is often challenging to manage and a relatively large source of operational costs.”
For You: Battery-Based Desalination Made Possible
The engineers plan to further test and scale up the system in hopes of supplying larger communities with low-cost, fully sun-driven drinking water. Much of the R&D will be conducted through a company they plan to launch in the coming months that will continue to develop lower cost, more sustainable desalination methods.
“Our focus now is on testing, maximizing reliability, and building out a product line that can provide desalinated water using renewables to multiple markets around the world,” Winter said. “Our technology provides a simple strategy to desalinate water for resource-constrained communities and also has implications for decarbonizing larger, energy-intensive desalination industries.”
Mark Crawford is a technology writer in Corrales, N.M.
To make a more efficient and affordable desalination process that is powered by renewable energy, Massachusetts Institute of Technology engineers developed a new type of desalination system that runs with the rhythms of the sun—extracting salt from water using only sunlight at a rate that closely follows changes in solar energy. As sunlight increases through the day, the system ramps up salt removal and automatically adjusts to any sudden variation in sunlight, such as dialing down in response to a passing cloud or revving up as the skies clear.
The research team, led by Amos Winter, professor of mechanical engineering at MIT, designed and built an electrodialysis (ED) system consisting of water pumps, an ion-exchange membrane stack, and a solar panel array.
ED flows a feed solution through a stack of spacers, each separated by selectively charged cation and anion exchange membranes. An applied electric field drives ions across the membranes. The process is operated with either continuous flow of water through the stack, or via batches of water recirculating through the stack. “This electrically driven—rather than pressure-driven—process reduces energy consumption and cost in brackish water and partial desalination scenarios,” Winter said. “The energy efficiency of ED also reduces solar or wind array sizes and thus also capital cost and system footprint.”
Automatic Control
One of the most innovative features of the control system is that it uses 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 maximize the amount of salt drawn out of the water.
The electrodialysis system flows a feed solution through a stack of spacers, each separated by selectively charged cation and anion exchange membranes. The process is operated with either continuous flow of water through the stack, or via batches of water recirculating through the stack. Image: MIT
“We are using photovoltaic electrodialysis to enable direct-drive [little to no energy storage], optimally controlled desalination at high production rates,” Winter said. “The control scheme employs cascade feedback control, which increases response speed compared to other control strategies that use computationally expensive numerical solvers.”
More Like This: Wave-powered desalination system
The team constructed a community-scale pilot system to test controller robustness and reduction in energy storage in a six-month field trial on real groundwater wells in New Mexico, working in variable weather conditions and water types.
On average, the system harnessed over 94 percent of the electrical energy generated from the system’s solar panels to produce up to 5,000 liters of water per day, despite large swings in weather and available sunlight.
“The prototype also featured an energy storage to water productivity ratio of over 99 percent less than the median PV desalination systems in the literature,” Winter said. “The high water efficiency of ED is especially beneficial in water stressed regions and reduces brine volume, which is often challenging to manage and a relatively large source of operational costs.”
Maximizing Reliability
“Conventional desalination technologies require steady power and battery storage to smooth out a variable power source like solar,” Winter said. “By continually varying power consumption in sync with the sun, our technology directly and efficiently uses solar power to make water.”For You: Battery-Based Desalination Made Possible
The engineers plan to further test and scale up the system in hopes of supplying larger communities with low-cost, fully sun-driven drinking water. Much of the R&D will be conducted through a company they plan to launch in the coming months that will continue to develop lower cost, more sustainable desalination methods.
“Our focus now is on testing, maximizing reliability, and building out a product line that can provide desalinated water using renewables to multiple markets around the world,” Winter said. “Our technology provides a simple strategy to desalinate water for resource-constrained communities and also has implications for decarbonizing larger, energy-intensive desalination industries.”
Mark Crawford is a technology writer in Corrales, N.M.
ME is All Digital
ASME members can access Mechanical Engineering magazine in its new, enhanced digital format. Everything you love about ME, now everywhere you go.
As renewable energy gains momentum, oil and natural gas continue to remain primary global energy sources. This collection delves into latest digital trends enabling breakthroughs in energy transformation.