Connecting large batteries to the electric grid is an idea that has multiple benefits. Most fundamentally, an electricity system powered by an increasingly large share of solar and wind will almost surely require storing energy in some fashion, given the variability of the wind and sunshine at any particular moment in the day coupled with our expectation that power will always be available. Indeed, large battery storage systems are already being deployed across the world in sophisticated ways. For example, a Massachusetts town of about 27,000 people is that can help power the town during power outages, stock up on less-expensive energy during off-peak hours and feed electricity back to the grid during times of peak demand, when utilities generally have to fire up their most carbon-intensive diesel-powered backup systems. Grid-tied battery storage systems, however, come with a few caveats. For one, it takes a lot of batteries to store enough energy to actually make a significant difference, which means battery storage systems are expensive. Moreover, if you consider the climate emissions involved in manufacturing the batteries and the mining for the lithium and cobalt needed to make them, .
Throw at this predicament a seemingly too-good-to-be-true solution: Right now, electric vehicles represent the largest use of large batteries, and EVs are steadily gaining market share across the globe. However, manufacturers estimate that batteries in new EVs will likely need to be replaced after eight to 12 years, or when they can hold about three quarters of their original charge. These slightly diminished batteries, however, could still be useful for other things, like — you guessed it — grid-tied storage. Now add into the mix the fact that EV sales have been strong for the past few years and we could be a decade or so away from having millions of used batteries ready for a “second life” in the electric grid. Importantly, this approach could reduce the costs and climate impacts of building large storage systems compared to those that use new batteries.
Undoubtedly, it’s an idea with tons of promise. The catch, at least for now, is that we don’t really know how well it will work, according to Associate Professor of Electrical and Computer Engineering Xuan Zhou. Zhou says our understanding of the performance of new EV batteries is pretty solid and getting better all the time. However, we simply don’t have the same level of data about what’s really going on inside used EV batteries to say for sure they’ll be as reliable an option for grid storage as we hope they’ll be. The reason is that charging and discharging a battery hundreds of times — and subjecting it to thousands of hours of road vibrations — changes its underlying chemical and physical properties. That’s ultimately why it loses capacity over time. And many details about a battery’s potential second life inside a grid storage system — like how many years it would last, how safe it would be and how cost-effective it would be — are still largely unknown.
Zhou, along with his Electrical and Computer Engineering Department collaborators Associate Professor Mengqi Wang and Professor Wencong Su, hope they’re about to provide the world with some of the best data-backed answers to date. With a new , the largest state grant in the university's history, the team of researchers is set to build a 500 kW grid-tied storage system that will use actual used EV batteries.
Su and Wang say some of the best work in this field is currently being done at Stanford University. But those experiments involve taking new EV batteries and subjecting them to hundreds of charging and discharging cycles in order to intentionally deplete them in a way that mimics what happens in an EV under certain driving conditions. “For the emulation, however, they might not be able to emulate the true environment the batteries would be operating in,” Wang says. “We can imagine, in an EV, there will be vibrations. There will be accelerations and decelerations. The devices around the pack might also generate heat. And there will be variations in the temperature and all these environmental conditions for the battery pack — they will not be constant at all. This all impacts the battery. So by using real used EV batteries, we hope our data will end up being more practical.”
Zhou says they are still working out many details for the design and build of the project. One of the unexpected challenges was simply locating used EV batteries. “Initially, it looked like we were going to have to get them from Europe, where EVs have been more popular,” Zhou says. “But now it looks likely that we will be able to get about eight to 20 battery packs from used GM vehicles.” After they secure the batteries, Zhou says they’ll perform extensive tests to get a sense of their chemical and physical properties. Meanwhile, Wang and Su will work out how the batteries can be connected and tied to the grid. Because the team is planning to test several types of use cases, this will require a carefully planned system of custom power converters, controllers and control algorithms. For example, they want to see how well the batteries perform in a “peak shaving” role. That’s when the batteries would feed electricity back to the grid during times of peak demand, which helps utilities avoid using their expensive, carbon-intensive backup systems. They also want to test “load shifting,” which is where the batteries would charge during non-peak hours and then release that energy during peak hours, when electricity is more expensive. Another variable: the partner site that will host the storage system has a solar array. Wang says connecting all those components and systems in a way that doesn’t produce harmonics on the grid, damage equipment or compromise power quality takes careful planning. Many of the components will have to be custom built, too, since this kind of system is not an off-the-shelf technology.
Another big hurdle the team might encounter is testing the system. At 500 kWh, this project is so big that the current testing facilities at -Dearborn — or any other university the team knows of — isn’t equipped to handle a test of the full system. Zhou says the new battery lab in Ann Arbor may be able to provide single pack-level testing, and they may need an industry partner for tests beyond that. Wang says the size and testing requirements also make it more challenging to design other aspects of the system. “I think the piece of equipment that has the highest power rating is our battery simulator in our IAVS lab, which has 100 kW testing capacity. So we’ll start with a smaller system as a proof-of-concept design. And then we’ll scale up to the real-world test. But this makes it much more challenging. The components, like the semiconductors or power converters, can’t just be scaled up. They’ll have to be swapped out with new hardware with higher power ratings. So we’re going to be busy!”
One other noteworthy feature of the project: After the study of the storage system is complete, the research team’s Genesee County-based industry partner on the project, ReCharge ReCycling, which is helping design and build the storage system, will help prepare the battery pack for a third life. “That’s why we’re calling this project ‘Closing the loop,’” Zhou says. “They’ll be dismantling the battery down to the cell level and then trying to recover the precious metals so they can be used to make a new battery. So we’ll really get a full picture of what it’s going to take to take a battery through this full cycle.”
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Story by Lou Blouin. Photos by Annie Barker.
