Sodium sulphide. This is the base material being used by TNO for the development of a technology that in the near future will be used for compact heat storage in homes. Although there are a few important aspects that still need ironing out, the technology has already convincingly proved its worth in demonstration projects.

Want to know more about compact heat storage?

Get in contact with Ruud Cuypers


With funds and support from the European Commission, a TNO sea container found itself in Spain in 2015 (as did another later, in Poland). Inside was a kind of student’s room and an experimental construction designed to demonstrate an innovative heat storage system. It involved a closed system where sodium sulphide and water were allowed to react thermochemically in a vacuum container, thereby releasing heat that had been stored in the system earlier.

This was the first demonstration of a heat battery based on thermochemistry. At the time, it was quite a sizeable set up which, all told, was able to store and release just a quarter of a gigajoule of heat – not even enough to heat a home for a week. 

Heat battery with highest energy density

Since that time, a great deal of work on the technology has been carried out, in Delft. TNO was also invited to join the EU ‘Sustainable Self-support Urban Smart Grid’ project. That in turn resulted in a new demonstration project for the heat storage method involving sodium sulphide. In 2020, the improved system was able to achieve an energy density of 0.35 gigajoules per cubic metre of heat battery. That is around five times greater than the storage density of the 2015 experiment.

With the current set up, a density of 0.5 – 0.75 GJ/m3 can be achieved – the highest-known energy density for any heat battery technology. This is a solid starting point for the development of a compact device that could be used in existing homes, many of which do not have much space.

Why a closed vacuum system?

The thermochemical process in which vapour binds itself to salt is quicker if there are no other molecules in the way. A closed vacuum system therefore ensures that water molecules are absorbed into the salt quickly and efficiently. It also means that the water can be separated more easily from the salt when it dries. 

Extra focus on safety

A closed vacuum system has another significant function. When sodium sulphide reacts chemically with water, hydrogen sulphide may be released as a by-product. This toxic gas smells of rotten eggs, and may not in any circumstances be released into a residential dwelling. The vacuum container has an important role to play here. In the event of a leak, any hydrogen sulphide is sucked into the system to prevent it entering the surroundings.

“There is of course an extra focus on safety with this system,” emphasises TNO researcher Ruud Cuypers, who has been working on the development of new heat battery technologies since 2009, and with the sodium sulphide variant in particular.

“That is why we are in the process of building up a portfolio of alternative materials, in addition to sodium sulphide. The materials in question lend themselves well to use in homes and are also suitable for this type of reactor. Obviously, any heat battery of this kind will only come onto the market after we have taken all the necessary safety precautions and have demonstrated that it is safe to apply the system in built-up environments.”

Contributing to the energy transition through compact heat storage

TNO is concentrating only on the development of the technology, so will not itself be launching the heat battery on the market. This will be done by a commercial party when the time comes, and it is they who will determine the relevant pricing levels. It is not yet known when the market introduction will be taking place.

What is clear, however, is that the technology has great potential. The contribution to the energy transition that the vacuum reactor-based heat battery will make cannot be overstated. One example is that of storing heat generated from solar panels in the summer and then using it for heat during the winter.

Promising business case

“Now that the offsetting scheme for locally generated solar power is gradually being wound down, more and more solar panels owners will start looking for ways of benefitting from the large amount of solar energy that they generate in the summer,” expects Cuypers. “So, although we will not be commercialising the technology ourselves, we do know that the elements for a promising business case are there.”

Meanwhile in Eindhoven...

TNO is also working in partnership with TU Eindhoven on a device for storing energy using salt and water, with which energy can be released as heat at a later time.

Read more

And elsewhere at TNO...

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

Read more
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