Melted salt for energy storage gets another chance, maybe

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A team from the Pacific Northwest National Laboratory (PNNL) has developed an improved molten salt scheme for energy storage. The team claims that its “freeze-thaw battery” is a step towards creating batteries suitable for seasonal storage.

Every engineer involved in the path from energy acquisition to end use knows that there are three broad aspects to this path: capturing / collecting energy, storing and transferring it to cargo. This is true regardless of the scale, whether it is an interrupted low power load for a small IoT device or a large network on a scale. Depending on the specifics of the application and its size, the energy path will have these three elements in different proportions, each with its own unique problems.

The storage part of the mixture is extremely challenging, of course, especially in the context of renewable sources such as solar and wind energy, where the source is intermittent while the user’s requirements are not. In addition to cost and reliability, an important attribute of a viable storage scheme is that it has a relatively high density of energy storage by volume and weight. But it also carries risks.

The search for better ways to store energy is pursued in many ways: electrochemical (batteries), gravitational (water and weights) and dynamic mechanical (flywheel) approaches, as shown in Figure 1, to cite a few.

The research by the PNNL team, funded by Imre Guk, director of the energy storage department in the Energy Office of the Ministry of Energy, has led to an improved molten salt scheme for energy storage. However, this is not the first use of molten slats for this purpose, as the idea and various implementations have been known for decades.

The authors argue that what is different here is that their “freeze-thaw battery” is a step towards batteries that can be easily used for seasonal storage: saving energy in one season, such as spring, and using it through another, like autumn. The battery is first charged by heating to 180⁰C, which allows ions to pass through the liquid electrolyte to create stored chemical energy.

The battery is then cooled to room temperature, which has the effect of “locking” the battery’s energy. The electrolyte becomes solid and the energy-carrying ions remain almost stationary. The material is liquid at higher temperatures, but solid at room temperature. When energy needs to be available, the battery heats up again – probably through natural seasonal warming – and the stored energy then becomes available.

I will not go into details about the salt material or the electrochemical process, as they are fully presented in the PNNL team document. “Battery for freezing and thawing molten salt for seasonal storagePublished in Science Direct (plus – shhh! – chemistry is not one of my strengths). Their project explores three somewhat related methods for activating the nickel cathode in their battery for a comparative purpose – an interesting perspective.

They provide some of the highest numbers based on their hockey-sized demo block, as shown in Figure 2. These storage units passively store energy without much loss, as the lack of mobility at ambient temperature eliminates the pathways for self-dilution.

The researchers said a respectable recovery of capacity over 90% after a period of one to eight weeks, adding that “cells can effectively retain energy with comparable or superior performance compared to modern lithium-ion batteries at room temperature, which have low speeds of self-dilution at 2% -5% per month. “

An important advantage of this design is that the battery module and electrolyte use widely available materials rather than rare earth elements. The anode and cathode are rigid aluminum and nickel plates, while the separator is made of fiberglass and not of a more expensive ceramic structure that is prone to cracking during freeze / thaw cycles. Finally, materials (especially electrolytes) do not pose the various risks of conventional batteries.

Reading their article (I admit that much of the chemistry is beyond me), I did not get a clear idea of ​​the traditional numbers of energy storage, such as energy density by volume and weight, open cell voltage, rated current and power of energy flow). This may be due to a lack of understanding on my part or perhaps for other reasons.

How do you feel about the viability of this type of energy storage scheme? Is the idea of ​​seasonal freezing / thawing practical or only in very limited circumstances – if at all? Do you think it can be increased in size and capacity – often the biggest challenge in any energy storage concept – even if it is proven to be viable in a very small test?

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