As wind and solar become a bigger share of our electricity mix, the reliability of our power grid faces some fundamental challenges. Because the wind isn't always blowing and the sun isn't always shining, supply from these sources can vary greatly throughout the day. Large-scale energy storage, which could bank renewable energy for later use, is seen as a fundamental way of dealing with this unpredictability challenge, and energy engineers are working on a variety of interesting storage technologies. In fact, one of the most promising solutions may be lurking in our driveways. Electric vehicles, after all, are basically huge batteries on wheels: The 98 kWh standard range battery in the Ford F-150 Lightning, for example, can power an average American home for three days. And since a typical car spends the vast majority of its life parked, connecting our EVs to the grid could, one day, create a massive, reliable backup power system — absorbing excess renewable energy during peak production and sending it back to the grid when we need it most.
Researchers are also looking for ways to harness the power of gently used EV batteries. Due to performance issues, batteries in electric vehicles are typically retired when they reach 80% of their original capacity. But the used batteries are still valuable. Right now, the market for used batteries is strongest among recyclers, who covet them for a variety of their component materials, especially nickel and cobalt. There’s also a lot of interest in keeping the batteries intact, since a slightly diminished battery can still hold a lot of energy. By stringing together thousands of them, we could potentially create a whole “second life” for EV batteries as a storage system for the electric grid. This application is also considered one of the most environmentally friendly grid storage options, as it cuts out the mining and energy-intensive manufacturing required for creating new batteries.
A recent project by UM-Dearborn doctoral students Ali Hassan and Shahid Aziz Khan explored just how much potential this second life approach holds. Supervised by Electrical and Computer Engineering Professor Wencong Su, their work was part of an international CO2 reduction competition sponsored by OPAL-RT Technologies. The simulation technology company hosted the challenge to discover new ways that its advanced “hardware-in-the-loop” simulation systems could be used to reduce climate-warming emissions. For their project, Hassan and Khan modeled a microgrid that integrated a large proportion of wind and solar energy and used 80%-degraded Nissan Leaf batteries as backup storage. They then simulated several real-world scenarios, including using the batteries to cover sudden surges in demand, which grid operators typically respond to with fossil fuel-based “peaker” power plants. The potential for carbon savings looks promising indeed. According to their models, they estimate a single used 80 kWh battery, if used until it can hold 40% of its capacity, could save 46 metric tons of carbon dioxide over five years. That’s the equivalent of removing 10 average gasoline-powered cars from the road for an entire year — for each battery.
Hassan and Khan took home the top prize in the competition, and they’re excited their work is helping demonstrate that used batteries have real potential as a storage system for the grid. They say one of the advantages of using an OPAL-RT hardware-in-the-loop simulator is that it gives you a much clearer picture of how a complicated system like a microgrid, which uses high-voltage hardware, will function in real life. “We have many ways of building simulations, but for the most part, they aren’t going to give you something that’s implementable in the real world without any hiccups,” Hassan explains. “But with the hardware-in-the-loop simulator, you can connect your software models of, say, an EV battery, and you’re going to get real-life voltage and current levels that you’d expect in a real grid or microgrid. So it’s much more accurate.”
Hassan and Khan say it could still be a few years before used EV batteries are playing this kind of role. That’s mainly because EVs are simply still too new to have yielded enough used batteries to create a market for this kind of application. But with global EV sales growing at about 30 percent a year, and batteries lasting eight to 10 years before they need to be replaced, Hassan and Khan say this market dynamic could change relatively quickly. In the meantime, they and other energy engineers will be busy figuring out the details of how to make our cars a vital link in a cleaner grid.
Story by Lou Blouin