The Salt Battery: Energy of the Future?

This morning Red Baron became electrified by the above headline. In the Netherlands, at the TU Eindhoven, a heat battery is being developed that could free millions of households in the country from gas dependency. Project leader Professor Adan said, "About 150 PetaJoule of residual heat from industry per year will enable us to take almost 3.5 million homes off the gas."

Red Baron thought himself well-informed about energy technologies. The current limitation on renewable energy is storage. Lithium batteries used in cars are marvels of technology but are still expensive and a potential fire hazard with deadly consequences in car crashes.

Solar electricity-producing households effectively use heavy lead batteries in the basement of their houses. By storing surplus electricity during sunny days, they arrive at a certain autonomy with their electricity consumption.

This is electricity, the noble form of energy. But what about heat available not only during sunny hours in summer but from the industry. Heat energy is used as process heat in the industry at high temperatures of up to 1000 ⁰C. It is released at lower temperatures of about 150 ⁰C into the environment as a waste product. Wouldn't it be nice to store this heat efficiently for future use?

Here some chemical reactions come in handy that can store common heat energy into chemical energy that can give off heat in a reverse reaction.

Indeed, the heart of the Eindhoven heat battery essentially revolves around a relatively old thermochemical principle: the reaction of a salt hydrate with water vapor. Professor Adan explained, "The salt crystals absorb the water, become larger, and, in the process, release heat."

But the reverse is also possible. "By adding heat, you evaporate the water and basically 'dry' the salt, thus reducing the size of the salt crystals. As long as no water gets to this dry salt powder, the heat is always stored in it. So, unlike with other types of heat storage, nothing is lost: the battery is completely loss-free", Professor Adan added.

In fact, you need a material that you can continue to use cyclically. Professor Adan and his team settled on potassium carbonate as a basis, an easily extracted salt that is part of many products such as soap or glass.

The closed-loop system (©TU Eindhoven)

Adan and his coworkers built the so-called "closed-loop system." This recirculating system consists of a heat exchanger, fan, evaporator/condenser, and boiler with salt particles. At 7 kWh, this system is small, although it could provide heating for a typical family of four for two days.

Thirty lockers (©TU Eindhoven)

In the meantime, some 30 'lockers' have been combined, presenting a total storage capacity of over 200 kWh. Adan puts it into perspective: "That's equivalent to two fully charged Teslas."

But there is more to it. While heat transport through pipes used in district heating systems always runs up losses, the thermal battery can store heat loss-free, and its "innocent" chemical can also be transported loss-free. Nothing will happen to the dry salt if no water is added.

Is good old chemistry coming to the rescue of the world's energy problem? There is no lack of potassium and carbon dioxide; we currently have too much and growing.

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