Molten Salt Battery Marks Step Towards Seasonal Grid-Scale Energy Storage
Scientists have created a battery designed for the power grid that locks in energy for months without losing much storage capacity.
The development of the “freeze-thaw battery”, which freezes its energy for later use, is a step towards batteries that can be used for seasonal storage: saving energy at one season, such as spring, and spending it to another, like fall.
The prototype is small, about the size of a hockey puck. But the potential usefulness of the science behind the device is vast, predicting a time when energy from intermittent sources, like the sun and wind, can be stored for a long time. The work of scientists at the Department of Energy’s Pacific Northwest National Laboratory was published online March 23 in Physical Sciences Cell Reports.
“Longer-lasting energy storage technologies are important for increasing grid resilience when incorporating a large amount of renewable energy,” said Imre Gyuk, director of energy storage at the Bureau of electricity from the DOE, which financed the work. “This research marks an important step towards a seasonal battery storage solution that overcomes the self-discharge limitations of today’s battery technologies.”
Harnessing and conditioning the energy of nature
Renewable sources come and go with the cycles of nature. It is therefore difficult to include them in a reliable and regular flow of electricity. In the Pacific Northwest in the spring, for example, rivers laden with hydroelectric dams feed runoff to the max, just as winds blow violently through the Columbia Gorge. All this power must be harnessed immediately or stored for a few days at most.
Grid operators would like to harness this springtime energy, store it in large batteries, and then release it later in the year when the region’s winds are slow, rivers are low and electricity demand peaks.
The batteries would also improve the ability of utilities to withstand a power outage during severe storms, making large amounts of power available for injection into the grid after a hurricane, wildfire or other calamity.
“It’s kind of like growing food in your garden in the spring, putting the excess in a container in your freezer, and then thawing it for dinner in the winter,” said first author Minyuan “Miller” Li.
The battery is first charged by heating it up to 180 degrees Celsius, allowing ions to flow through the liquid electrolyte to create chemical energy. Then the battery is cooled to room temperature, essentially locking the battery’s energy. The electrolyte becomes solid and the energy-carrying ions remain almost motionless. When energy is needed, the battery is warmed up and the energy flows.
The freeze-thaw phenomenon is possible because the battery electrolyte is molten salt – a molecular cousin of regular table salt. The material is liquid at higher temperatures but solid at room temperature.
The freeze-thaw concept avoids a problem familiar to anyone who has left their car unused for too long: a battery that self-discharges when idle. A rapid rate of discharge, like most car or laptop batteries, would hamper a grid battery designed to store energy for months. In particular, the PNNL freeze-thaw battery retained 92% of its capacity over 12 weeks.
In other words, the energy doesn’t degrade much; it is preserved, just like food in a freezer.
Regular ingredients a plus
The team avoided rare, expensive and highly reactive materials. Instead, the aluminum-nickel-molten salt battery is full of common materials that are abundant on Earth. The anode and cathode are solid plates of aluminum and nickel, respectively. They are immersed in a sea of molten salt electrolyte which is solid at room temperature but sinks in liquid form when heated. The team added sulfur – another common and inexpensive element – to the electrolyte to improve the battery’s energy capacity.
One of the biggest advantages of the battery is the composition of a component, called separator, placed between the anode and the cathode. Most high temperature molten salt batteries require a ceramic separator, which can be more expensive to manufacture and susceptible to breakage during the freeze-thaw cycle. The PNNL battery uses simple fiberglass, possible due to the stable chemistry of the battery. This reduces costs and makes the battery more robust during freeze-thaw cycles.
“Reducing battery costs is key. That’s why we chose common and cheaper materials to work with, and why we focused on eliminating the ceramic separator,” said corresponding author Guosheng Li. , who led the study.
Battery energy is stored at a material cost of about $23 per kilowatt-hour, measured before the recent nickel price spike. The team is exploring the use of iron, which is cheaper, in hopes of reducing the material cost to around $6 per kilowatt-hour, about 15 times less than the material cost of current lithium-ion batteries.
The battery’s theoretical energy density is 260 watt-hours per kilogram, more than today’s lead-acid and flux batteries.
The researchers point out that batteries designed for seasonal storage would likely only charge and discharge once or twice a year. Unlike batteries designed to power electric cars, laptops, or other consumer devices, they don’t need to last hundreds or thousands of cycles.
“You can start to imagine something like a big battery on a 40-foot tractor-trailer parked at a wind farm,” said co-author Vince Sprenkle, senior strategic adviser at PNNL. “The battery is charged in the spring, 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 a patent on the technology.