MIT engineers develop culture solar-powered system to make salty groundwater drinkable, sans batteries

Desalination has become important as groundwater is getting saltier. An electrodialysis system created by the scientists earlier this year needed costly batteries to adjust to sunlight

Muskan Bhatia

Engineers from the Massachusetts Institute of Technology (MIT) have created a system that uses solar power to turn salty groundwater into safe drinking water, without a battery. Their research, published Tuesday in Nature Water, explains how the system adjusts to changing sunlight conditions throughout the day to purify water efficiently. “Being able to make drinking water with renewables, without requiring battery storage, is a massive grand challenge. And we’ve done it,” said Amos Winter, co-author of the study, and mechanical engineering professor and director of MIT’s K. Lisa Yang Global Engineering and Research (GEAR) Center in a statement. Desalination, the process of removing salts from seawater to make it drinkable, has gained significant importance over the past two decades.

Desalination is becoming more popular due to the increasing problem of water scarcity, which is expected to affect 60% of the global population by 2025. According to the United Nations University scientists, removing salts from seawater is not a practical solution. Many inland families rely heavily on groundwater for drinking, but this water is becoming saltier due to climate change. As groundwater becomes more saline, finding clean water is a growing challenge for communities that depend on desalinated water to meet their water needs. This is what makes the solar-powered system disruptive. “This technology could bring sustainable, affordable clean water to underreached places around the world,” said Jonathan Bessette, PhD student at MIT.

Simpler control system enables battery-free operation; To remove salt from brackish groundwater, MIT scientists used reverse osmosis and electrodialysis, both of which need energy. In reverse osmosis, salty water is forced through a filter that removes most of the salt, making it safe to drink. Electrodialysis, on the other hand, uses electric currents to pull charged salt particles out of the water. In order to reduce greenhouse gas emissions from energy-intensive desalination, scientists have explored using renewable energy sources for generating electricity. However, this is difficult with reverse osmosis systems, which need a steady power supply—something that’s hard to achieve with variable sources like solar power. In a study earlier this year, Winter and his team had focused on using electrodialysis and worked on making it adaptable, allowing the system to adjust to changes in sunlight. They managed to adjust water production based on sunlight variations, using a model-based control system. They predicted the optimal water flow rate and the voltage needed to remove salt from the water. These predictions were made using sensors connected to water pumps, ion exchange membranes and solar panels.

However, this electrodialysis system required further improvement due to a slow response time, which needed energy stored in costly batteries. “We could only calculate every three minutes, and in that time, a cloud could literally come by and block the sun,” said Winter. This time, Bessette, Winter and their colleague Shane Pratt designed a new system that reduces the response time and eliminates the need for batteries. They introduced a control strategy called “flow-commanded current control”. This senses how much solar power is being produced and quickly adjusts the water pumps and electric current to match it.