There is currently a strong need for a compact and affordable heat battery for storing heat in homes. TNO is working on a range of technologies to make this happen. In Eindhoven, for example, a team is working on thermochemical storage based on potassium carbonate. To get the technology on the market, TNO and Eindhoven University of Technology have set up a spinoff – Cellcius.

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Get in contact with Olaf Adan


Storing heat in salt and then using a thermochemical reaction to release it again without any loss of energy is one thing. But developing a technology and a device with which that process can be applied in an actual home in an efficient manner is quite another, especially on a larger scale, such as in streets or even entire districts. Olaf Adan, a professor at Eindhoven University of Technology and principal scientist at TNO, has been working with his research team on such a total solution for more than ten years.  

How a compact heat battery works

The biggest challenge is the stability of the salt particles. Adding vapour to salt crystals causes them to enlarge and releases heat. To recharge the system, the salt crystals have to be separated from the water, and then regain their original structure. This is precisely what happens if you put energy in a heat battery. The only thing is, it is not possible to do this very often with most types of salt. The salt particles are liable to clump together or disintegrate. 

Compact heat battery

High storage capacity for in the home

After many years of experiments using different types of salt and configurations, the breakthrough came in 2017. It appeared that a specifically developed salt composite – based on K2CO3 (potassium carbonate) – possesses the required properties. Even with monthly charging and discharging, this salt composite remains stable and can be used for more than twenty years. And the storage capacity? This is many times greater than that of heat storage in water. As a result, it should in principle be possible to apply it in a compact device.

Bonus: extra reduction in CO2

The heat battery can be charged with both heat and electricity, including the power from solar panels. And meanwhile, no energy is lost. That makes this type of energy storage much more sustainable than using an electrical home battery, for example. But that’s not all – CO2 is needed to produce potassium carbonate, so that means an extra reduction in CO2.

A spinoff – Cellcius

The next challenge is to develop a compact, robust, and affordable device for storing energy in the home, and to launch it on the market. Such a device would have to give a high yield in different situations, and be easy to install and to use. To hasten the development process, TNO and Eindhoven University of Technology have set up a spinoff, Cellcius, which works closely with industrial companies.

This partnership is not just essential for the purpose of introducing a device that meets the needs of users to the market, but also – in due course – for rapidly scaling up the production of the devices and the necessary potassium carbonate. Another apparent benefit is that the solution is suitable not only for use in individual homes, but also for storing heat at street and district level.

International demonstration project

Cellcius will be fitting a prototype in a number of homes in Eindhoven, as well as in several residences in the south of France and in Poland. This international demonstration project, which is being financed partly by the European Commission, will also give greater clarity about the effect of different types of climate and dwelling on the performance of heat batteries. And that in turn will help Cellcius to develop devices for a larger market – in other words, outside the Netherlands as well.

Relieving networks

“We have big ambitions,” confirms Adan. “We believe that our heat storage solution can be particularly useful in relieving the peak periods on heating and electricity networks. This is currently a key aspect of the energy transition. It therefore feels very gratifying to be able to contribute towards one of the possible solutions to that problem."

Meanwhile in Delft...

In Delft, TNO is helping to develop a device for storing energy using salt and water, with which energy can be released as heat at a later time.

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And elsewhere at TNO...

In addition to thermochemistry-based heat batteries, we at TNO are working on other heat storage technologies. 

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Prof.Dr. Olaf Adan MSc

  • thermochemical heat storage
  • energy storage
  • energy efficiency in the built environment
  • materials technology
  • Horizon2020