Bard van de Weijer / Article in Volkskrant, 25 April

Tapping residual heat in industry to heat offices and homes later. Dutch scientists have been successful with a so-called heat battery. Crucial ingredient of that battery? Ordinary salt.

Image Raymond Rutting / de Volkskrant

Halfway through the interview, Olaf Adan takes a glass tube out of his pocket with white balls the size of peppercorns. He pours in a little tap water and hands it to the reporter, who almost burns his fingers when he tackles it; the tube has become red-hot.

What Adan (Professor of Technical Physics at Eindhoven University of Technology, principal researcher at TNO) just handed over is a heat battery, a battery for heat. The principle is as follows: charge the battery in places with a lot of residual heat, such as a power station, and use that heat later in another place, in homes or offices.

Unlike lithium-ion batteries in electric cars, the battery that Adan and fellow researchers in Eindhoven are working on contains no toxic chemicals, no rare or precious materials. The heat that goes into it is stored indefinitely until you need it. What's the magic stuff in this battery? Salt. Potassium carbonate, to be precise - harmless, cheap and abundantly available.

Theory and practice

The thermochemical process underlying the heat battery has long been known. By evaporating water from salt, the crystal structure changes and it absorbs energy. That energy can be transported without loss. Then add water and heat is released. A lot of heat. One cubic metre of salt can store 1.3 gigajoules of energy. That is the equivalent of 361 kilowatt hours, as much as would fit in the batteries of about seven medium-sized e-cars.

Although the principle is generally known, the thermal battery is hardly ever used. This is, as so often, because theory and practice are not easily reconciled. For instance, the salt was often not stable (it disintegrates after a few charges and discharges) and there was no affordable device that could easily store the heat and extract it from the salt again.

'On these two points we have made a breakthrough in recent years,' says Adan in his office in Eindhoven. The researchers have succeeded in making the salt granules more stable and have built a device that can easily harvest the stored energy. The installation in the hall of his Eindhoven office looks a lot like the central heating system in the attic of many Dutch people. The components are also fairly simple: there is a fan, needed to blow air mixed with water vapour past the salt grains. There is a pressure vessel in which the salt pellets are bedded, a heat exchanger to transfer the heat, an evaporator and a condenser - all standard stuff. Comes right off the shelf.

The nice thing is that the power output of the installation is adjustable depending on the heat requirement. We work with a temperature of about 65 degrees Celsius,' says Adan. This means that the system can be connected to an existing central heating system. This offers great opportunities for greening the use of heat in the Netherlands, says the professor.

Three to six million households off the gas

Sustainable heat is one of the pillars of the energy transition. Now that Climate Minister Jetten has declared biomass for heating applications taboo, and the Netherlands was hardly able to get rid of gas anyway, this part of the transition to sustainable energy is also at risk of getting stuck. Adan has good news: his invention can switch three to six million households off the gas.

Residual heat is available in abundance in the Netherlands: about 1.5 petajoules per year. This is low-grade heat, below 150 degrees Celsius, which industry can do little with. It is therefore discharged into the surface water or into the air. Capture this heat in Adan's salts and transport it to where it is needed. For example, in a large office battery, or a large unit in the neighbourhood, where the installation can convert the salt back into hot water.

The advantage of storing and transporting residual heat is that there is no need for large heat networks, says Adan. They are expensive, take a long time to build, have high start-up losses (because the heat network often has to be built first and customers come later) and if the heat source is lost, the network is worthless. With the Method-Adan, you can collect and deliver heat anywhere, in a manner similar to how the coal farmer used to fill the coal box in the backyard. And if a heat source disappears, you get the heat elsewhere, says the professor.

Heat battery as air conditioning

A promising application', says professor of solid state chemistry Elias Vlieg of Radboud University. According to him, the chain is now being worked on in many places, but he sees that a lot of research work is still needed. For example, I am not yet sure whether potassium carbonate is the best candidate', he says. It's possible, but there may be salts with an even higher energy density.

To be able to commercially exploit their invention, the Eindhoven research group has started a start-up: Cellcius. Next year, a trial will start in Limburg, where heat will be extracted from the Chemelot industrial complex and transported to homes in Sittard-Geleen.

Professor Adan has one last trump card: in houses and offices, his heat battery can also work 'the other way round': in summer, heat from buildings can be used to extract water from the salt (and thus put energy into it). By extracting heat from buildings, the system acts as an air conditioner. The heat stored in the salt can be used again at a later time.

Fly is positive: 'I am convinced that we have not yet grazed the whole field of interesting opportunities.

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