Flow batteries are a tested renewable energy technology that could make large-scale wind and solar power storage more reliable. The problem is, to work at the scale required for a power grid, flow batteries require an awfully big footprint. University of Virginia associate professor of chemical engineering Gary Koenig believes he and his research team can shrink that footprint by a factor of 10.
“I can justify that number relative to other flow batteries,” Koenig said.
He can’t say the same compared to the size of lithium-ion systems, a more compact alternative and perhaps the most common stationary battery technology in use today. But flow designs offer advantages that lithium-ion cannot.
Koenig and his collaborators received $550,000 to pursue their strategy from the National Science Foundation’s Partnerships for Innovation, a program supporting the commercialization of new technology to address societal needs through multi-organizational collaborations. The team includes associate professor Geoffrey M. Geise and Ph.D. student Patrick McCormack in chemical engineering at UVA; University of Kentucky associate chemistry professor Susan Odom and assistant chemical engineering professor James Landon; and Robert M. Darling, an engineer at Raytheon Technologies Corporation.
Batteries of all sizes and designs discharge stored electricity on demand. They do this through electrochemical processes — chemical reactions between materials that are either caused by the application of electric current or that result in the conversion of chemical energy to electrical current.
In a flow battery, energy is stored in two self-contained tanks holding electrically charged chemicals dissolved in a liquid, usually water, called an electrolyte. One is a catholyte solution, the other an anolyte. They act like the positive and negative electrodes on a car battery. Pumps circulate the fluids into a power stack with electrodes separated by a thin membrane through which ions are exchanged during production of electricity.
When the electrolytes are depleted of energy, the electrochemistry runs in reverse, moving electrons via the power stack to recharge the tanks.
The membrane is critical. It must allow the ions to flow through while preventing the reactants from crossing over — which, at best, would cause a loss of power but could have much worse outcomes.
A key advantage of all flow battery designs is the ability to add capacity to either storage, which equates to how long the discharge lasts, or wattage, which increases power output, independently of each other.
“To double how long a flow battery can discharge, you buy a bigger tank,” Koenig said. “With the lithium ion battery, you have to integrate a whole new battery system, which is a lot more expensive.”
Similarly, to boost output, you can build a bigger power stack without having to increase storage.
“This decoupling of storage and power means that after the initial expense of building the system, the cost of adding hours of power delivery becomes cheaper,” Koenig said. “Scaling factors with flow batteries make sense at grid scale. To depend on solar and wind, because of their intermittency, you need to have long battery duration.”
Flow batteries have their own limitations, which contribute to their size and prevent them from squeezing into urban settings or adapting to portability.
To fit more energy in smaller tanks, the UVA-led team proposes using solid particles in a non-aqueous electrolyte. The particles react with the solution, effectively recharging it until the solids lose their energy, lengthening the charge duration, Koenig said. The team is not alone in trying solids, or in using non water-based electrolytes, but it is pioneering new or improved materials and energy storage chemical reaction processes to make the system work.
“One thing that does not exist at all is a membrane for this application,” Koenig said. “We’re trying to come up with a material that works for non-aqueous flow battery systems, because you need it to have high conductivity, low crossover and be stable in what are some very unusual environments. The membrane is where we’ve made the most progress.”
McCormack, the lead Ph.D. student researcher on the project who works in both Geise’s polymer materials lab and Koenig’s group, is developing the membrane. He has made one previously, and published his findings.
“Other studies have used commercial aqueous membranes because there is relatively little research on non-aqueous membranes,” McCormack said. “They don’t do their job as well in the non-aqueous system. I am making changes to my membrane to investigate how those changes affect the overall battery performance. This should help us fill in some of the gaps in knowledge about how polymers and membranes behave in non-aqueous solvents and allow us to design even better membranes in the future.”
Odom’s chemistry lab at the University of Kentucky is responsible for discovering the right “redox shuttles,” a name given to the chemical reactants in the electrolyte that produce the electricity when flowed into the power stack. Koenig’s lab, including Ph.D. student Devanshi Gupta, is studying the solid particles that will interact with the redox shuttles to extend the electrolyte charge.
The grant also provides for student participants to train in advanced industrial research and entrepreneurial skills, including completing internships.
Over the project’s four-year timeline, the team — including Landon at Kentucky and Raytheon’s Darling, who serves in an advisory role — will integrate the components with an off-the-shelf power stack to develop a working prototype battery system.
Finally, the researchers will perform an economic analysis using their laboratory test data to evaluate the system’s potential in the market.
“Ultimately, we are pushing for an approach to fundamentally reconfigure flow batteries to improve the efficiency and safety while minimizing costs for longer duration energy storage,” Geise said.
Can flow batteries address one of the biggest impediments to adopting renewable energy sources?
“I think the biggest impact will be as evidence for the viability of this type of system as a grid-scale energy storage solution,” McCormack said. “There is still a lot of work to be done before the system can be created in full scale. My hope is that it shows the promise of this type of battery and helps to encourage more research in this area to allow that to happen faster.”