The Heat-battery

The Heat Battery: local & lossless storage

Storing energy in the home. In a device that is affordable, not too big and efficient. This desire has long been the subject of many studies. For a long time, it remained a pipe dream. But Professor Olaf Adan and his TNO - TU/e team have succeeded. We spoke to him about the result: the thermal battery, a compact device, based on thermochemistry, that stores energy without loss.

Water and salt as a basis

The basis for the heat battery is a salt composite based on K2CO3 (potassium carbonate) that was designed especially for this project. "With a bit of luck, we came up with this," Olaf says himself, "but mainly through long and focused development." The composite is suitable for multiple cyclic use. Or, in other words: loading and unloading repeatedly. The team screened all possible salts to find suitable candidates. This was followed by much research to arrive at this version.

The battery is a combination of the composite and a reactor technology. Developing a suitable reactor also proved to be a challenge. Olaf: "Reactor technology is a very mature field, with many applications. But no single variant was suitable for making the best use of the composite."  Olaf explains how they found the solution by chance: "We had an ‘Eureka moment’ . That put us on the track towards the final reactor. And so now we have a concept based on the two most common elements on earth: water and salt. Sounds deceptively simple, but of course it is incredibly complex.".

Here's how the thermal battery works

The basis is a thermochemical principle based on two components: water vapor and a salt hydrate. As soon as water vapor is introduced to the salt hydrate, the water binds to the salt. This reaction creates a new crystal form and releases heat. And you can turn that around. With heat, you can split that new crystal back into its original ingredients. You dry the new crystal. And as long as you keep those two original components separate, that heat is stored loss-free. The amount of heat that this battery can supply is more than enough to make a significant contribution to the energy demand of an average family in an average home.

"Of course, several factors play a role in that," says Olaf. “Of how many people does the family exist? How well is the house insulated? And how big is their demand for hot water? But when they ask us how much energy you can store, our standard answer is that, with the size of the current prototype, an average family can enjoy two weeks of hot showers with it.”

While the storage, thanks to the thermochemical principle, is completely lossless. The conversion of electricity or heat to stored energy in the battery - the conversion - does result in some energy loss.

Multicyclic use for maximum efficiency

Throughout the year, the user charges and discharges the system continuously. The choice of multiple cycles per year is driven by practical limitations and cost. "Suppose you wanted to make a battery that you would charge once in the summer and then use throughout the winter, you would need a lot of salt. And therefore, a lot of space to store that salt. Think of several cubic meters. And that, of course, is very impractical. Because that kind of space simply isn't available in most Dutch homes. Certainly not in our typical terraced house or an apartment. Moreover, you only use that entire volume once a year and that is economically inefficient, so very expensive."

Nor is it necessary, because during a year there is enough energy available to dry the salt to recharge the battery. "At most in the winter there will be periods when that is not the case for a while. But in extreme situations - when neither the sun is shining, nor the wind is blowing - we're talking about a maximum of two weeks. And that's the period you can easily bridge with this battery", explains Olaf. That's why the battery needs only a few hundred kilos of salt. And thus, also a modest storage capacity.

Ideal companion for solar panels and heat pump

According to Olaf, the heat battery offers even more advantages: "You will soon get more out of your solar panels and heat pump thanks to this battery. The energy from the solar panels can be stored as heat. And it remains available when the sun goes down or is not visible during the day." For heat pumps, the battery is an excellent companion. The heat battery then takes care of part of the temperature increase by the heat pump, especially if it is very cold outside. This makes the heat pump much more efficient, and we do not need to use the electricity grid or use it much less.

In the future: a refrigerator for heat

In terms of size, a typical heat battery for standalone use in a home will soon be comparable to a refrigerator. Or with the unit of an average hybrid heat pump, for example. The energy density is much higher than that of the most modern, currently available electric house batteries.  But the most important feature, according to Olaf, is the lossless storage: "Because this allows you to use the energy independent of time and place. So, you can also transport it to where it is needed more."  And despite all these impressive specifications, Olaf and his team expect that such a unit is going to be a good affordable investment for a lot of households.

Getting heat from where it already is

A standalone unit in residential homes is a longer-term elaboration. For the time being, the focus is on the transportability of the stored heat. Olaf: "That application will be the first to be marketed. In this way, we are introducing a completely new concept as an alternative to heat networks. The great advantage of our technology is that we can get the heat from anywhere where there is a surplus, or where heat is a by-product. Think of industry and data centers. We store the heat and transport it to collection points in neighborhoods. That way we can start supplying homes with heat."

This way, the heat battery makes energy supply flexible and accessible to everyone. But what is even more important according to Olaf: "We don't need to build new heat sources, such as geothermal installations or power plants running on traditional fuels. The heat we use is already there. It is residual heat. We transport it via existing infrastructure. By rail and by road. So: no digging, no open streets, and no complex permit procedures. That makes this solution much cheaper, faster and above all greener."

The battery will not only prove its added value in residential applications. Because the larger the unit, the more energy it can hold. That also makes this technology suitable for industrial applications. With the thermal battery, the team kills two birds with one stone: the technology is both feasible and scalable.

Aiming for 2024

Olaf and his team are working with several parties in the Eindhoven region to get the battery into production as quickly as possible. They didn't have to think about that for long: "After all, we have a great manufacturing industry, supplemented with the necessary technical knowledge, here in the area." says Olaf. The salt will soon come from a producer in Germany. Olaf: "We will soon be running pilots in France and Poland, and we expect the first application in homes as early as 2024." For the heat network application-our current focus-we will start a pilot together with the chemical industry at the Chemelot campus in Limburg in 2023.

Support projects like this. Donate to the fund and help researchers and students find answers to the questions of today and tomorrow.