Illustration by Sara Levine | Pacific Northwest National Laboratory
A long-duration grid-scale battery could use energy stored during the spring to cool a house on a hot summer day.

Pacific Northwest National Lab Creates 'Freeze-Thaw Battery' for Seasonal Energy Storage

April 12, 2022
Molten-salt battery marks step toward seasonal storage of grid-scale energy.

Scientists at the Department of Energy’s Pacific Northwest National Laboratory (Richland, Wash.) have created a battery designed for the electric grid that locks in energy for months without losing much storage capacity.

The development of the “freeze-thaw battery,” which freezes its energy for use later, is a step toward batteries that can be used for seasonal storage: saving energy in one season, such as the spring, and spending it in another, like autumn.

The prototype is small, about the size of a hockey puck. But the potential usefulness of the science behind the device is vast, foretelling a time when energy from intermittent sources, such as sunshine and wind, can be stored for a long time. The work by scientists was published online March 23 in Cell Reports Physical Science.

“Longer-duration energy storage technologies are important for increasing the resilience of the grid when incorporating a large amount of renewable energy,” said Imre Gyuk, director of energy storage at DOE’s Office of Electricity, which funded the work. “This research marks an important step toward a seasonal battery storage solution that overcomes the self-discharge limitations of today’s battery technologies.”

Harnessing and packaging nature’s energy

Renewable sources ebb and flow with nature’s cycles. That makes it difficult to include them in a reliable, steady stream of electricity. In the Pacific Northwest in the spring, for instance, rivers heavy with runoff power hydroelectric dams to the max just as winds blow fiercely down the Columbia Gorge. All that power must be harnessed immediately or stored for a few days at most.

Grid operators would love to harness that springtime energy, store it in large batteries, 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 enhance utilities’ ability to weather a power outage during severe storms, making large amounts of energy available to be fed into the grid after a hurricane, a wildfire or other calamity.

“It’s a lot like growing food in your garden in the spring, putting the extra in a container in your freezer, and then thawing it out for dinner in the winter,” said first author Minyuan “Miller” Li.

The battery is first charged by heating it up to 180 ° C (356 ° F), allowing ions to flow through the liquid electrolyte to create chemical energy. Then, the battery is cooled to room temperature, essentially locking in the battery’s energy. The electrolyte becomes solid and the ions that shuttle energy stay nearly still. When the energy is needed, the battery is reheated and the energy flows.

The freeze-thaw phenomenon is possible because the battery’s electrolyte is molten salt — a molecular cousin of ordinary table salt. The material is liquid at higher temperatures but solid at room temperature.

The freeze-thaw concept dodges a problem familiar to anyone who has let their car sit unused for too long: a battery that self-discharges as it sits idle. A fast discharge rate, like that of batteries in most cars or laptops, would hamper a grid battery designed to store energy for months. Notably, the PNNL freeze-thaw battery has retained 92% of its capacity over 12 weeks.

Battelle, which operates PNNL, has filed for a patent on the technology. Other authors of the paper include PNNL researchers Evgueni Polikarpov, Nathan Canfield, Mark Engelhard, J. Mark Weller and David Reed, and former PNNL scientist Xiaowen Zhan.

Visit PNNL's site for more details on the molten-salt battery.

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