A prototype ‘freeze-thaw’ battery that traps energy as it transitions from liquid to solid could lead to grid-scale storage of renewable energy, its developers have said.

The battery, aimed at storing energy for months without losing much capacity, was developed by a team at the US Department of Energy’s (DOE) Pacific Northwest National Laboratory (PNNL).

The prototype is only about the size of a hockey puck, but the researchers said it could eventually be used for storage of renewable sources such as solar and wind during times of high production, for times of high demand.

“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.”

In the Pacific Northwest spring, for example, rivers heavy with mountain runoff power hydroelectric dams to their maximum, just as fierce winds blow down the Columbia Gorge. Currently, all that power must be harnessed immediately or stored for a few days at most.

Grid operators hope to harness that springtime energy, store it in large batteries, then release it later in the year when the winds are slow, the rivers are low, and demand for electricity peaks.

The new batteries would also enhance operators’ ability to weather power cuts during severe storms, making large amounts of energy available to be fed into the grid after extreme weather events.

The prototype is charged by heating it up to 180ºC, allowing ions to flow through the liquid electrolyte to create chemical energy. The battery is then cooled to room temperature, essentially locking in the 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 process is possible because the battery’s electrolyte is molten salt, which is liquid at higher temperatures but solid at room temperature. The technique is aimed at avoiding the problem of self-discharge while batteries sit unused, which would make grid-scale storage impractical. According to the PNNL, the freeze-thaw battery retains 92% of its capacity over 12 weeks.

The team avoided rare, expensive and highly reactive materials. The battery has an anode of aluminium and a cathode of nickel, and the molten salt electrolyte includes sulphur to enhance its capacity. Stable chemistry means glass fibre can be used as a separator instead of ceramic, cutting costs and making the battery sturdier during freeze-thaw cycles.

“Reducing battery costs is critical. That is why we’ve chosen common, less expensive materials to work with, and why we focused on removing the ceramic separator,” said corresponding author Guosheng Li, who led the study.

Energy is stored at a material cost of about $23 per kilowatt hour, measured before a 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 per kilowatt hour, roughly 15-times less than the material cost of today’s lithium-ion batteries.

The battery’s theoretical energy density is 260 watt-hours per kilogram, the researchers said, higher than today’s lead-acid and flow batteries.

Batteries designed for seasonal storage would likely charge and discharge just once or twice a year, meaning they do not need to last hundreds or thousands of cycles.

The researchers said the battery would ideally be heated by renewables or recovered waste heat when energy is needed.

"The heating and thawing of the battery is location and application dependent," first author Minyuan 'Miller' Li told Professional Engineering.

"The heat to melt the salt for operations would ideally be coming from renewables, in particular excess generation... However, if renewables are unavailable, the system can operate on other energy sources, such as waste heat, additional battery systems, or fossil fuels to jump start.

"Our preliminary design for a larger scale device will include an internal heating element that would be kickstarted by a pilot to initiate battery operation. During discharge, the battery also produces some heat itself that will help to maintain operation.

"Technically, if the internal cell temperature is raised to about 180ºC, say in an oven, it would be ready for discharge. The molten salt melts at 157-159ºC."

Excess molten salt "significantly burdens" the efficacy of the system, Li added, and must be minimised for optimal output to allow upwards of 88% theoretical charge recovery. "The concept demonstration has not reached a practical level, so further research is needed to find the minimal viable model."

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

Extracted from 05 Apr 2022 Professional Engineering

The research was published in Cell Reports Physical Science.

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