A battery based on salt should conquer the market. TNO-TU/e-spinoff Cellcius is working on the further development of the thermal battery technology into a first fully-fledged product.
As a company, Cellcius BV has only been around for a few months. But the people behind this spin-off of TNO and TU Eindhoven have been working on the development of the heat battery technology to a first fully-fledged product and its commercialisation in various markets for much longer. Olaf Adan began his scientific search for solutions to heat storage as one of the ways to make the built environment energy neutral back in 2008. "Heat determines three quarters of the demand for energy. And that demand cannot be solved electrically alone."
Adan co-wrote the framework for the European Commission within which innovation must take place in order to achieveCO2-free heating. But how do you do that? With fellow researchers Henk Huinink (TU/e) and Pim Donkers (TNO) he went looking for solutions.
It was clear that the energy itself would have to come from so-called renewable sources such as sun and wind. But the sun does not shine enough every day, and there are periods when it is not windy. For those days, perhaps even weeks, households must be able to store wind and solar energy in a battery so that they can use it on dark, windless days and nights. An important condition was that the system had to be affordable for the average household, otherwise no one would buy it. In addition, the installation must be so compact that it can be accommodated in a normal home. The heat battery can be charged by a heat source such as a solar collector or a heat network, with or without geothermal energy. It can also be charged by electricity, for example from solar panels or other green electricity, but that has to be converted back into heat first. "It is important to realise that our battery supplies heat as an energy source, not electricity," says Olaf Adan.
The solution was found by the team of Adan, Donkers and Huinink in the application of potassium carbonate. When water binds to this salt, heat is generated. This heat can be used to heat a boiler to the desired temperature for space heating or hot tap water via a heat exchanger. Recharging (i.e. storing heat) takes place via the reverse process. By adding heat to the salt, the water is driven out of the salt.
The process itself is simple. Water enters the evaporator through a tap water pipe. The water vapour is transported to the salt in the reactor vessel with a closed and recirculating air stream. The part of the vessel containing the salt is closed. It remains in there after the reaction. The heat released during the reaction is transported with the dried air to the heat exchanger, which directly heats the water in the boiler.
The raw material is potassium carbonate. Adan: "This raw material is the basis for a salt composite that we have developed, particles with a certain shape and size. A composite particle therefore contains even more substances, which ensure that the particle continues to perform its function well and that we can continue to charge and discharge it without it breaking down. Only then can you make a rechargeable battery." Which substances are added for that optimal, permanent and stable result, he does not tell. That's the secret of the blacksmith with which Cellcius wants to conquer the market for heat.
Tests in residential areas
In mid-2022, tests with the salt battery will start in residential areas in Eindhoven, in France (in Provence) and in Poland. "We are testing in three different countries because you have different climates there and there are big differences in use," says Huinink. "We want to be able to take into account the local culture, within which some devices in the energy system at home can differ greatly. For example, solar collectors will be more common in some people's homes, while in others a heat network will be important."
Poland has a relatively cold continental climate, France in Provence a warm Mediterranean climate and the Netherlands has a temperate maritime climate. "We are figuring out how the heat battery can best function in all these different conditions."
The next question is how Adan envisages the application of the salt battery in practice. For a house or building with solar panels or small wind turbines on the roof, that's obvious. You would place the battery somewhere where you would also place a washing machine, for example. It could be about the same size, but that depends on the need. "It all depends on how much energy you want to store," says Adan. "That in turn depends on how big the demand for energy is and what period you want to bridge. For a typical 4-person family in a standard terraced house, a washing machine-sized battery has enough heat in it to provide for 2 weeks. Those 2 weeks typically represent the worst-case situation where the sun isn't shining and the wind isn't blowing."
Adan lists the advantages again: "We can put a lot of energy into a small volume. More than the best electric home storage, and much more than in water. This allows you to make storage small, which is important if you want to fit it into the limited space available. We are the only storage facility that is truly lossless. That allows you to span long periods of time, unlike water that cools, and unlike even electric storage in home batteries."
The Eindhoven researchers are not worried about scalability either. "It can be small, in homes; larger in collective storage in flats, even larger at the district level, or largest in industrial applications for low-temperature residual heat. It's simply a matter of more salt for more power and more capacity."
The salt battery is one of four iconic projects of the Eindhoven Institute for Renewable Energy Systems. EIRES was founded in 2020 by TU/e. Other projects include the development of metal fuel, the development of cheap electrolyzers. Digital models are also being developed (digital twins) in order to gain insight into the best choice for new technologies that will be developed during the energy transition. The choice of these must be made before 2050.