Olaf Adan, professor of applied physics at TU/e, chief researcher at TNO and now chief technical officer of the company Cellcius. Image by Koen Verheijden

Maarten van Gestel / Article in Trouw, 25 April 2022

Moving factory heat to homes, not via hot water in pipelines but via salt and trucks. It sounds cumbersome, but researchers from Eindhoven think that this could be the key to the energy transition.

Some people always carry a bottle of perfume or a box of TicTacs. Olaf Adan (62), professor of applied physics at Eindhoven University of Technology and principal researcher at TNO, always carries two other bottles. One with water, one with salt. Not to quench thirst or to perk up a bland lunch, but to be able to show people what he has been working on for the past twelve years. Something that, according to him, could potentially provide the solution for the 'too slow pace' of the transition to sustainable energy.

In a meeting room on the Brabant university campus, Adan places the two bottles on the table in front of him. "I'm just going to show you what happens," he says, using a pipette to take drops of water from the water bottle and add them to the bottle of salt. It is silent for a few seconds, then the grains of salt begin to smoke and hiss. When Adan wants to pick up the bottle of salt grains a little later, he burns his hands because it is so hot. "Because of the reaction, it gets hotter than sixty degrees," he says.

Adan adds water to the salt grains, and they become "damn hot."
Image Koen Verheijden

No more dependence on Russian gas and fossil fuels

The reaction in the vials is seemingly simple, and gives the feeling of a chemistry teacher astonishing his class on day one with a chemical trick. But according to Adan, the heating of this specific type of salt - potassium carbonate - by water or water vapour could potentially play a key role in the energy transition. It could make a startling difference to the Netherlands' and Europe's desire to get rid of Russian gas first and, in the longer term, to no longer depend on fossil fuels because of the disastrous consequences of climate change.

It's like this. Dutch industry - factories, data centres, chemicals - generates a huge amount of residual heat at relatively low temperatures, below 150 degrees. Heat that, in theory, is enough to heat between three and six and a half million homes. "But that heat is mostly thrown away into the atmosphere, while people heat their homes with gas." District heating, where waste heat from industry heats water that is brought to homes via pipelines, has been used for some time. Or biomass burning, where residual material from industry ends up in a furnace that heats water that flows into homes. Crucial developments, according to Adan, but each also has disadvantages. "Laying those pipelines is expensive. The water loses temperature over time. And the biggest problem: it takes a long time to lay these pipelines between factories and homes, partly because of the need to obtain permits, for example in nature reserves." Time that is not there, says Adan. "We're in a damned hurry at the moment."

The principle of the two salt bottles offers a new solution, according to Adan, who is now also one of the bosses of the company Cellcius, set up by the TU/e and TNO. "The beauty of it is that you can also reverse the reaction I just showed you." The water droplets released the heat from the salt. But if you extract water from the hot salt mass - if you dry it - you store the heat in dry grains. Those grains can be taken anywhere without losing heat. "I can take this bottle to the South Pole, and it won't have lost anything in energy." Add water vapour there again, and it becomes warm again. "We can repeat that at least two hundred and fifty times, with no loss of energy."

The hot air in a factory hall can therefore be stored in grains of salt. A lorry transports the salt pellets to a residential area, where they can generate enough heat from a sort of transformer house with tap water for the surrounding houses, via thin tubes under the ground. In two years' time, the first fifty houses must actually receive heat in this way: near Sittard-Geleen, from Limburg's Chemelot site.

Image Anne Blaak

"We are the first in the world with this," says the Brabant professor with some pride. If everything goes as planned, this new technique could be used everywhere in the Netherlands from 2025 or 2026. "The government's ambition is now to save three and a half megatonCO2 in the built environment by 2030. With this technology, we could potentially double that."

The principle of the salt battery sounds simple, says Adan, but it is not (see box). "That simplicity is deceptive. The Brabant professor had the advantage of being able to do twelve years of fundamental research from TU/e and TNO. In that time, he had two breakthroughs. After testing more than a thousand types of salt, he found a salt that, when combined with other substances, could be recharged over and over again, just like an electric battery. A type of salt that is also widely available in Europe, unlike, for example, rare battery metals such as lithium and cobalt. The second breakthrough was to design a device that could actually charge this material and allow it to give off heat in an efficient and economically viable way.

Initially, the plan was to produce an individual salt battery for homes, the size of a refrigerator, which should produce enough heat as an individual system. Partly due to a European subsidy of 7 million euros , a trial will start later this year with four households, each with a battery. It is still an interesting project, Adan believes, but he now believes that the technology will only be truly effective if it transports residual heat from industry to homes using salt granules. "That is much more powerful than those individual batteries. So that's what we're aiming for now."

Adan in front of the salt battery, which consists of separate cabinets with salt that can each be 'recharged'.
Image by Koen Verheijden

Comments on the system with lorries

It remains to be seen whether the system of lorries that bring 'charged' salt to neighbourhoods will actually succeed, or whether this is just a story that works on paper for the time being. The current world of heat networks reacts to the Eindhoven innovation with some reservation. For example, about one of the most important advantages that Adan mentions in his innovation: the 'loss-free' transport of heat stored in salt, as opposed to water, which cools down over time. A spokesman for Vattenfall - one of the largest heat network operators in the Netherlands - says that only 20 per cent of heat loss currently occurs in the large pipelines between industry and residential areas, and that eighty per cent of loss occurs in the fine network under homes. "So this innovation can at most do something about that twenty per cent."

Vattenfall also has reservations about the system of transporting the salt to residential areas in trucks. "It is questionable whether the costs would outweigh those of a transport pipeline." But according to Adan, the lorry idea will indeed be profitable, both from a cost and an environmental point of view. "We have calculated how often a lorry has to go to the neighbourhood. That frequency is much lower than supplying supermarkets. In our situation, it is once a month in summer, up to twice a week in the most extreme winter week."

According to Adan, the truck system also has the advantage that sparsely populated areas in the Netherlands can easily make use of the residual heat from industry. "Hot water cools down the further you transport it, anyway. In the salt, the heat is stored without loss."

Not all the money together yet

It will be a few years before it becomes apparent whether the Eindhoven salt battery can really contribute as much to the energy transition as the Brabant professor claims. The first investment round is as good as completed, with the Province of Brabant as lender, among others. A fortnight ago, money was also allocated from the National Growth Fund to the consortium of which Cellcius is also a member. Nevertheless, not all the money has been gathered yet for the first Cellcius trial, with Chemelot and the houses in Sittard-Geleen. "It still depends on a few subsidies. It is sometimes difficult to find funds quickly, especially because we do not fall within the traditional funding categories of either industry or buildings."

But if the find really breaks through, Adan himself, as a stakeholder in Cellcius, could also become rich from it. "Maybe, I don't know. But at my age it is not so important any more.

Original article on trouw.nl