A lithium-ion battery used to power an automobile degrades over time (although not all that quickly if you believe Elon Musk’s claim that Tesla batteries will soon last for 1 million miles of driving). There comes a point when it can no longer charge rapidly or deliver instantaneous power to keep you ahead of the pack during the stoplight grand prix.
But when no longer suitable for use in a vehicle, those batteries typically have 60 to 70% of their useful life remaining. They are still fully capable of storing electricity in a residential or business environment, but you can’t just wheel a battery up to your house on a hand truck, plug it in to a standard wall outlet, and send out for pizza. There are a whole host of control mechanisms in between your electric meter and your repurposed battery.
The folks at Oak Ridge National Lab have dedicated themselves to solving all those interface issues and they think they have come up with ways to make second-use batteries suitable for residential use. Now they want to take what they have learned and scale it up to make second-use batteries satisfactory for use in grid scale storage applications.
The supply of used batteries is going to skyrocket in coming years as millions of electric cars take to the streets. The Edison Foundation predicts there will be about 19 million EVs in service by 2030 — about 7% of the total. (Those of us who spend our days gazing at our navels at CleanTechnica Tower think the actual number will be much, much higher.)
“Developing new technologies for energy storage is essential to a well-balanced, modern power grid increasingly reliant on renewable, distributed energy,” says Imre Gyuk, director of Energy Storage Research at DOE’s Office of Electricity. “This project also supports a circular economy by reusing a valuable commodity, minimizing waste, and ensuring reliable, secure delivery of electricity for a robust economy.”
Oak Ridge researcher Michael Starke has been leading a project supported by the Office of Electricity’s Energy Storage Program to create and enhance controls for second-use battery energy storage, including testing of a 15 kilowatt system at a Habitat for Humanity home site in North Carolina. Testing is also taking place at an Oak Ridge research house in a neighborhood near the lab.
Buy Low, Sell High
Using the software and power electronics hardware developed by Starke and his colleagues, the second-use battery system is designed to reduce a home’s purchases from the utility to zero during critical peak demand periods when the cost of electricity is at its highest. The home instead uses electricity generated or stored from on-site renewables. “The net benefit is a much lower electricity bill taking advantage of real time rate structures,” Starke said.
The process also frees up some of the demand from the utility’s service area during peak periods, helping to balance the grid. “The big takeaway is the showing that secondary-use energy storage is economically viable in these applications,” Starke adds. “Secondary use is not just taking used car batteries and throwing [them] into something. Getting to a utility grade system is a challenge. Utilities want something reliable and efficient that they can depend on for a long period. And they want just one device to communicate with that can manage a huge resource.”
The system developed by the Oak Ridge researchers relies on cloud-based communications that allow for remote control. Included in the system is a flexible inverter that converts power for either household use or utility transmission together with controls that synchronize the system with the grid and ensure safe startup and shutdown.
The Non-Homogeneous Battery Conundrum
When it comes to grid scale systems, lots of recycled batteries may be linked together, batteries from different manufacturers with different voltages, cell chemistries, and amounts of degradation. Getting them all to work together harmoniously and reliably requires highly complex control systems.
The researchers have developed a system that presupposes a desired amount of charging or discharging and then responds with the status and ability of each battery cell to meet the request, resulting in efficient dispatch of resources. “This is all behind what the utility sees,” Starke said. “Utilities just want a block of energy, but there are a lot of subsystems that have to coordinate and control these resources in order to make a 1 megawatt system.”
Starke and his colleagues are currently developing a 100-kilowatt second-use battery system to further test their controls and hardware solutions. It will be the first project of its kind to repurpose EV batteries for a grid-scale system, on the scale of a neighborhood with multiple homes.
Don’t look for any of this on the shelves at your local Lowe’s or Home Depot any time soon, but it is encouraging that ORNL is thinking about what to do with EV batteries that have exceeded their useful life but still have many years of service as energy storage devices remaining. In the old days, we used to take old tires and discard them in the woods. It’s good to know that EV batteries will not suffer a similar fate.