Illustration of freeze/thaw battery in action

Freeze/Thaw Battery Stores Energy for Months

April 13, 2022
The molten-salt battery could hold renewable energy generated in spring to be used in the fall or winter when it is in high demand.

Engineers at DOE’s Pacific Northwest National Laboratory (PNNL) have developed a battery that is well suited for supporting the electric grid by storing energy for months without losing much to self-discharge.

The battery technology seems ideal for seasonal storage in which energy generated in one season is then used in another. In the Pacific Northwest, for example, rivers heavy swollen with melted snow power a string of hydroelectric dams to maximum output, while strong winds blow down the Columbia Gorge that can push wind turbines to their maximum as well. But all that power must be used quickly and can only be stored for a few days at most. Much of it gets wasted.

Grid operators would prefer to store that energy, then release it late in the year when the region’s winds are slow, the rivers are low, and demand for electricity peaks. The batteries would also give utilities the ability to supply electricity during power outages due to severe storms, natural disasters, accidents or vandalism.

The key to the so-called “freeze-thaw” phenomenon is its electrolyte, a form of salt that is liquid at higher temperatures but solid at room temperature.

The battery is charged by heating it up to 356°F, which lets ions flow through the liquid electrolyte to create chemical energy. The battery is then cooled to room temperature, essentially locking in the battery’s energy when the molten salt solidifies and the ions that transfer energy remain nearly still. When energy is needed, the battery is reheated and the electricity flows.

This approach sidesteps a scenario familiar to anyone who has let their car sit unused for too long: The battery self-discharges as it sits idle. Fast discharge rates, like those of batteries in most cars and laptops, would plague a grid battery designed to store energy for months.

The PPNL team has built a prototype of the freeze/thaw battery which is about the size of a hockey puck. In tests, it retained 92% of its capacity over 12 weeks.

The team also avoided rare and highly reactive materials to ensure larger batteries would be relatively inexpensive. The anode and cathode, for example, are simply solid plates of aluminum and nickel. The team added sulfur—another common, low-cost element—to the electrolyte to enhance the battery’s energy capacity. And the separator, the barrier between the anode and cathode, is simple fiberglass which can be used because of the battery’s stable chemistry. Fiberglass cuts costs and makes the battery sturdy enough to survive the freeze-thaw cycles. Most higher-temperature molten-salt batteries need expensive ceramic separators which are susceptible to breaking due to temperature changes.

The battery’s stores energy at a materials cost of about $23/kWhr, a figure calculated prior to the recent jump in the cost of nickel. The team is exploring the use of iron, which is less expensive, in hopes of bringing the materials cost down to around $6/kWhr, roughly 1/15th the cost of materials in lithium-ion batteries. And the freeze/thaw battery’s theoretical energy density is 260 watt-hours per kilogram, higher than that of lead-acid and flow batteries.  

Researchers point out that batteries used for seasonal storage would likely charge and discharge just once or twice a year. Unlike batteries designed to power electric cars, laptops, and other consumer devices, they don’t need to withstand hundreds or thousands of cycles.

“We envision something like a large battery on a 40-ft tractor-trailer parked at a wind farm,” says Vince Sprenkle, senior strategic advisor at PNNL. “The battery is charged in the spring and then the truck is driven down the road to a substation where the battery is available if needed during the summer heat.”

Battelle, which operates PNNL, has filed for a patent on the technology.

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