Renewable energy, namely wind and solar, has been rapidly replacing fossil fuels for electricity generation in recent years. As various renewable resources continue to integrate into the electric grid, their inherent intermittency becomes a more significant challenge to match the consumer appetite. Grid-level storage of seasonal excess can be an important asset to renewable electricity.
A team of scientists at Pacific Northwest National Laboratory (PNNL) has created an advanced new freeze-thaw molten salt battery designed for the electric grid that locks in energy for months without losing much storage capacity.
The new “freeze-thaw battery” consists of an aluminum anode and nickel cathode, both immersed in a sea of molten-salt electrolyte that is solid at room temperature but flows like a liquid when heated. The team added sulfur – another common, low-cost element – to the electrolyte to enhance the battery‘s energy capacity.
The battery is first charged by heating it up to 180 degrees Celsius, allowing ions to flow through the liquid electrolyte to generate 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.
“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.
Although the prototype is small, about the size of a hockey puck, the researchers are optimistic about its potential to be scaled up. The battery’s theoretical energy density is 260 watt-hours per kilogram – higher than today’s lead-acid and flow batteries – and its energy is stored at a materials cost of about $23 per kilowatt-hour. 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 materials cost of today’s lithium-ion batteries. In tests, the PNNL freeze-thaw battery has retained 92% of its capacity over 12 weeks.
Researchers say the development of a “freeze-thaw battery” that freezes its energy for use later is a step toward batteries that can be used for seasonal energy storage. Also, because the battery can sit idle while maintaining most of its stored energy, it would only need to be charged and discharged a few times a year.
“You can start to envision something like a large battery on a 40-foot tractor-trailer parked at a wind farm,” said co-author 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.”