‘Heat Battery’ A heat battery based on salt and water – Eindhoven University of Technology

The heat battery is based on an old thermochemical principle, which is that when water is added to salt it produces heat. The reverse is also possible, whereby heat can be used to evaporate the water, thus storing the heat energy inside the salt.

Storing heat within dry salt makes the battery completely loss-free, providing an incredibly efficient way to store energy for future use. This is particularly useful when energy supply is coming from renewable sources, such as wind and solar, which tend to fluctuate significantly and therefore require gas or other sources to supplement them.

A heat battery with salt and water as simple components could provide a quick and large-scale solution for over three million households in the Netherlands – twice the target set by the Dutch government. This heat battery, being developed by a consortium of Eindhoven University of Technology, TNO, spin-off Cellcius and industrial partners, is cheap, compact, loss-free and now ready for the first real-world tests.

With heat storage in homes and by harnessing the vast amounts of industrial waste heat that would otherwise be thrown away, this battery is a potential game-changer for the energy transition. Here are four reasons to get charged up for the arrival of this innovative battery.

The basis of the battery is amazingly simple
A simple experiment immediately reveals the essence of the heat battery. Fill a small bottle with white salt grains, add a little water and it starts to sizzle. Moreover, as if by magic, the bottle instantly feels incredibly hot. Olaf Adan has demonstrated the experiment countless times, amazing onlookers time and again.

Adan, TU/e professor and principal investigator at TNO, is at the heart of the Eindhoven heat battery, which essentially revolves around a relatively old thermochemical principle: the reaction of a salt hydrate with water vapor. “The salt crystals absorb the water, become larger and, in the process, release heat,” says Adan. Hence the rapidly warming bottle.

But the reverse is also possible. “By adding heat, you evaporate the water off and basically ‘dry’ the salt, thus reducing the size of the salt crystals,” Adan explains. 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.

You can repeat this process endlessly, one way or the other, thereby providing the basis for a heat battery that can store heat and use it at a later time and in a different place. This is a solution for the fluctuating supply of renewable energy in homes and buildings, and for the expedient reuse of ‘heat waste’ in another place.

“It still looked pretty basic, with existing, mature technology, but it allowed us to demonstrate that our concept, simple as it was, worked.”

While the principle of the battery may be simple, applying it in a battery certainly is not. Witness the fact that Adan has been working on this for over twelve years. For example, the choice of the specific salt material is not self-evident. There are thousands of known reactions of salt hydrates with water. Adan studied them all in great detail, ultimately discovering that only a very limited number have the right properties for use in a battery.

“A salt crystal like that gets bigger and smaller, heat goes in and out all the time. So something happens to such a particle. As a result, it can quickly disintegrate or clump together with other particles. So you need a material that you can continue to use cyclically,” says Adan. In the end, he and his team settled on potassium carbonate as a basis, an easily extracted salt that can be found in many products, such as food, soap or glass.

Then you also need a device that will make full use of the potential of this material. If it has to fit in a house, it has to be compact, and preferably also affordable as well as highly efficient. “So you start looking at all kinds of reactor concepts, such as in a vacuum or with open air, but without success to date,” says Adan.  

The ‘closed-loop system’ as the basis for the heat battery. Air circulates in it, thanks to a fan (bottom center). Cold, moist air enters the boiler (white, top left) which contains the salt particles. The reaction with salt makes the air dry and warm. The heat exchanger (bottom left) extracts the heat. The cold air enters the condenser to humidify it again and so it can go back to the boiler. This process can also take place in reverse, whereby the dry air is heated (with heat exchanger), the salt is dried, becomes moist and cold and is dried again by evaporator. Photo: Bart van Overbeeke.

Eventually, Adan arrived at the so-called “closed-loop system” (see image), a demonstrator of which he built in 2019. This recirculating system consists of components including a heat exchanger, fan, evaporator/condenser and a boiler with salt particles. At 7 kWh, the thing was still pretty minimal – in theory, this could provide heating for a typical family of four for two days.

“It still looked pretty basic, with existing, mature technology, but it allowed us to demonstrate that our concept, simple as it was, worked. Evidence that allowed Adan within the European consortium HEAT-INSYDE (including TU/e, TNO, Caldic and parties from France, Belgium, Poland and Switzerland) to win a European subsidy of seven million euros for further development. The team then set to work to ‘upgrade’ the demonstrator to a prototype ready for use in practice. This has now been achieved.

2. The technology is optimized for real-world use

In terms of dimensions, the prototype that has now been realized is probably comparable to the demonstrator, but that is where the visible similarities end. The prototype looks like a sort of large cabinet with dozens of lockers, with all sorts of loose cables sticking out of the side.

The prototype with the “lockers” that each form a separate module of the thermal battery. Photo: Vincent van den Hoogen

Amazingly, each duo of small ‘lockers’ represents a heat battery that matches the entire, original demonstrator in terms of storage capacity. The whole device contains some 30 ‘lockers’, with a total storage capacity of over 200 kWh. Adan puts it into perspective: “That’s equivalent to two fully charged Tesla’s.”

“We have optimized the earlier version in countless ways,” Adan proudly explains. “We redesigned the individual components, such as the evaporator and heat exchanger, made better use of space and used other materials.” Meanwhile, the unit also includes a measurement and control system, for example, so that you know when to charge and how much heat is left in the system.

Most applications do not require such a large battery. That’s why we deliberately chose those multiple, small units that you can combine at will; a modular system, in other words. “If you have one big container of salt, you have to start using it all at once. That’s really inefficient,” says Adan. So you can use ‘bits’ of the battery, separate from the rest.

In addition, the separate units offer all kinds of design possibilities, making different shapes and sizes possible, depending on the desired practical situation. Adan speaks of a user-oriented prototype. “It is not yet a product, but everything is now ready to be tested for the first time in a real-world situation.”

Photo: Vincent van den Hoogen

And that testing will start later this year, with the first pilots of the heat battery in homes. A battery of about 70 kWh will be installed – enough to last a few days without sun or wind – in four homes, two in Eindhoven, one in Poland and one in France.

Even though it is ‘only’ four homes, Adan expects that they will “learn an awful lot from this.” For example, testing is going to provide valuable input as to what else is needed in practice to apply the battery on a large scale as well as what the user thinks of it. For example, should there be an app to operate the battery?