Global demand for energy from the industrialised, fossil fuel-based economy continues to grow. This is at a time when enormous progress is being made in the development and use of new energy sources and energy-saving technologies. But this progress has problems of its own: what if the weather doesn't cooperate and you can't produce enough energy? Think of producing solar energy on drizzly days or wind energy on windless days. And where do you put an excess of energy production to use later for days like these?

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Heat storage in salt

This energy, which is produced on favourable days, can be stored in two ways: converted into electricity or stored as heat, for which the heat battery uses a salt hydrate. This method enables each house or block of houses to efficiently store heat for, close to the user.
TNO and TU/e have developed a brand new device principle and a breakthrough material, in which the heat is stored. Together they form the heat battery. It is so small that it fits into the limited space available in most homes. The breakthrough material is a salt composite, with K2CO3 (potassium carbonate) as the base material.

This is the first real heat battery for the home: compact, loss-free, stable and affordable.

Sustainable solution and the benefits at a glance

In a society that demands energy-saving technology, the heat battery for the home is a product for a sustainable future. Here are the benefits:
1. The performance of the material is stable; the material has a minimum life expectancy of >20 years, when charged/discharged monthly.
2. It is a compact solution; due to the refrigerator size storage it offers enough warmth to an average family to shower for 2 weeks. The energy density of the system is therefore at least 10 times that of water storage, and it also far exceeds the energy density of state-of-the-art electric household batteries.
3. It is an affordable solution, with a cost price of many factors below that of electrical storage, and it fits in the home.


The heat battery for the home has a market potential of 7 million homes in the Netherlands alone and even 60 million if we look at the whole of the EU. With the TNO and TU/e consortium, we have an internationally leading knowledge position in the Netherlands, which offers industrial opportunities:
1. Salt hydrates: Completely new market of a substantial size for salt hydrates (i.e. chemical industry). This salt can already be found in many other applications, including in food. The application in compact heat storage is new.
2. Manufacturing: the Dutch installation sector could benefit from the new device, and has an internationally strong position.

With regard to point 1, we are now working with strong industrial partners, including Caldic.

We must and want to accelerate the process of getting this product to market. We would like to get in touch with interested investors and companies about this. Mail Olaf Adan.

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More insights on
Thermochemical principle

The heat battery uses a so-called thermochemical principle. The heat battery for the home is based on two components: water and a salt hydrate. As soon as water vapour and salt are added, the water binds to the salt and the salt transforms into a new crystalline form. This reaction that releases heat is reversible. When heat is added to separate the water from the new crystal, the two original components are obtained again. In fact, it is this heat that is stored, and as long as these two components are separated, the stored heat is retained. This makes it a process without heat loss, which in turn is a prerequisite for long-term storage of heat. In this way a lot of heat can be stored in a small volume. Significantly more than water, and considerably more than in so-called phase transition materials.

That doesn't make it immediately practicable as a storage material. Incoming and outgoing heat, and thus incoming and outgoing water vapour, cause the crystal to change considerably in terms of volume. The salt particle then threatens to lose its cohesion, tear apart, disintegrate or otherwise. We solved that by developing a stable salt composite that can continue to perform its function.

The newly developed device principle fully exploits the potential of the salt composite; the so-called closed-loop principle. The device itself remains compact in size and is affordable. A unique feature of the heat battery for the home is that it is an ingenious piece of technology that performs using only four components. This simplicity is an important starting point to keep it cheap, but also to enable rapid development towards a product and thus get to market in the short term.

Service life and use

The reaction is now stable. Tests show that the battery can be charged and discharged for a long time without loss of energy. This means that the heat battery has a service life of 20 years, assuming it is fully charged and discharged once a month. The system can be charged either via heat or electrically, whether or not in combination with a heat pump. We envision electric heat pumps functioning better at low-energy moments and solar collectors generating higher yields . All in all, there is significant potential for absorbing peak loads from the energy grid.


Prof.Dr. Olaf Adan MSc

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