In India, most of the groundwater is brackish, necessitating some form of desalination to make it potable in water-scarce regions. Rural areas with no or unreliable access to electricity face an added burden beyond simply the cost to design and operate the necessary groundwater treatment systems.
Looking to find a more efficient process, a team of researchers developed a novel solar-powered approach to the desalination method: flexible electrodialysis reversal. Comprising a form of electrodialysis reversal that operates in sync with available solar power, the flexible treatment system requires a much smaller battery and less operator attention than conventional EDR systems. These differences could reduce treatment costs associated with EDR, helping to facilitate greater use of the system in remote areas in India and in other countries that rely on brackish groundwater.
Flexible approach
EDR modules comprise a stack of ion-exchange membranes through which two streams of water — one dilute and the other more brine-laden — are pumped while an electric field is added. The process causes ions to move from the dilute stream to the brine stream, producing drinkable water. The name of the process includes the word “reversal” because the electric field occasionally is reversed to prevent scale buildup on the membranes.
Conventional batch EDR systems typically operate with a fixed voltage and flow rate. When run on solar power, such systems cannot ramp up and down to keep up with the intermittent nature of solar power. As a result, the systems require storage batteries to capture and save solar energy when sunlight is available, usually during midday, to generate power when solar energy is low or unavailable, typically during mornings and evenings.
In the article “Flexible batch electrodialysis for low-cost solar-powered brackish water desalination,” which was published online on March 26 by the journal Nature Water, researchers from King’s College London, the Massachusetts Institute of Technology, and the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy describe their efforts to develop what they call a “flexible EDR technology” that adapts to the variable nature of solar energy.
The flexible batch EDR system “incorporates a time-variant voltage and flow rate adjustment,” according to the article. “A model-based control method enables the EDR system to align its power consumption with available solar power at each time step while optimizing water production under varying solar conditions,” the article states. In other words, the flexible system can rapidly ramp up and down as solar power is available, reducing the need for battery storage.
In addition to the model-based main controller, the flexible EDR system includes a programmable logic controller, an electrodialysis stack, two pumps with variable-frequency drives, several sensors, a diluate tank, a brine tank, solar panels, and an inverter module.
Although the flexible system consists of “off-the-shelf components,” its “novelty lies in how they are used with (the) novel model-based controller,” says Wei He, Ph.D., a senior lecturer in the Department of Engineering in the Faculty of Natural, Mathematical, and Engineering Sciences at King’s College London and the lead author of the Nature Water article.
Performance advantages
At the Brackish Groundwater National Desalination Research Facility in Alamogordo, New Mexico, the researchers tested a flexible EDR system capable of producing fresh water at a rate of 6 cu m/day. For comparison, the batch system operated in flexible and constant operation modes.
Despite variable solar conditions during its six-day test, the flexible EDR system met the target production rate of 6 cu m/day of fresh water. However, a key difference in performance between the two modes of operation was the extent to which they directly used available solar energy. Whereas the flexible system could use 77% of the available solar energy on average, the conventional constant-mode system could use only about 40% because it was unable to take advantage of peak periods of solar energy.
Since it could use more solar energy, the flexible system required vastly less battery use, needing only 0.27 kWh of battery power on average. By comparison, the conventional system required 3.3 kWh of battery power, a difference of 92%, the article states.
The flexible system also performed better in terms of operation time. During testing, the flexible system produced the same amount of treated water as the constant-mode system, but in 33% less time, according to the article.
Cost-competitive system
Such reductions in the need for batteries and operator attention could lower the operating and maintenance costs of an off-grid EDR system, making it more affordable for remote areas. In fact, as part of a case study in a rural village in India, the researchers found that the flexible system could produce water that is 22% less expensive than that generated by a state-of-the-art solar-powered EDR system and 46% less expensive than water generated by a conventional solar-powered EDR system.
Meanwhile, the flexible solar-powered EDR system was found to be “cost-competitive” with on-grid reverse-osmosis systems that are often used to treat brackish groundwater in Indian villages, according to the article. Despite its higher capital costs, the flexible solar-powered EDR system offers certain advantages compared to on-grid RO. After installation, “... there are no (additional) energy costs during (its) lifetime, so the overall lifetime costs (of flexible solar-powered EDR) will be lower,” He notes.
Other benefits include higher recovery rates and, therefore, less brine requiring management.
Ahead of efforts to commercialize the technology, additional testing and further research of the flexible solar-powered EDR system will be conducted at the Alamogordo facility, He says.
Jay Landers is the editor in chief of Parking Today.
This article first appeared in the July/August 2024 issue of Civil Engineering as “Solar Power Boost.”
