As the pressure to decarbonize electricity grids mounts, so does the need to have long-term storage options for power generated from renewables–especially for sources like wind and solar, which have discontinuous availability. Rechargeable batteries are the best option for consumer-level usage, but they are not practical for grid-scale consideration. Scientists are looking for ways to store gravity, thermal, or geothermal energy, as well as molten salt batteries.
The Pacific Northwest National Laboratory (PNNL) has recently studied molten salt battery technology that can “freeze” or keep its charge for up to six months. The researchers showed that the battery retained 92% of its capacity after three months. Minyan Miller Li, the first author of this study, said they have several test cases running for six months. He anticipates that the battery will retain more than 80 per cent of its charge over this period.
As the name suggests, a Molten-salt battery uses a liquid, the molten salt electrolyte. This freezes at room temperatures, so the batteries can be kept in an inactive condition. The anode and cathodes are separated when the batteries are activated. The molten electrolyte acts as a highly conductive medium for ionic exchange.
Because they discharge when unutilized, batteries are unreliable for long-term or seasonal storage. The PNNL team demonstrated that the freeze-thaw mechanism in the molten salt could be used to solve this problem. The cathode was made of nickel and aluminium, and the anode was made from sodium aluminium chloride (NaAlCl4). These materials are inexpensive and abundant in earth-abundant minerals. The electrolyte melts at around 157°C and stays solid for a wide range of room temperatures.
Guosheng Li is the lead researcher. He says we want to charge the battery when there are plenty of renewable energy sources. Then, we will keep it at ambient temperature to freeze it and shut off the self-discharge for long storage. You can heat the battery with waste heat or activate some of it at the beginning. Then, you can use the electricity to self-heat .”
They built a small, hockey-puck-sized battery to demonstrate their idea. Li believes there are no obstacles to practical scaling-up, as the materials are readily available. He says that even though these are not the best materials, “we know they aren’t the best, so [our next goal is] to replace nickel with a cheaper material span>
They are also looking into other electrolytes, and iron is one of their options. Researchers are not happy with the current melting point for NaAlCl4 at 157°C. Li states that future research will be focused on low-cost materials and operating temperatures while still being above the ambient temperature. Right? Ideal temperatures would be around 70-80 degrees [freezing point], so we don’t have to heat the battery as high as 180 degrees [as shown in the study span].
Vincent L. Sprenkle, a co-author, says that the operating temperatures should be kept as low as possible to maximize their potential applications. He says that the resilience of critical infrastructures is one of the possible applications. “If you can freeze that self-discharge mechanism out, these battery systems could sit around for years and …[when necessary] be heated by a readily accessible means ?”
The PNNL team is trying to reach a 2030-2035 timeframe for commercial application, especially for operational and deployed systems. It is a long road ahead. This proof of concept was only to test the technology, Li says. “If we can do a more elegant experiment, we may be able to [bring down the] temperature, possibly to just above room temperature…and save lots of energy span>
Researchers believe that other than reducing operating temperatures and replacing cores with cheaper materials, other challenges will become more apparent as they scale up. Sprenkle says that understanding the value of stored energy is an important challenge. “A large portion of our value is just resilience …[ and] we have difficulty accurately pricing resiliency…So there are some larger policy questions that come into play [but] still need .”