Wanted: Materials that retain heat effectively without insulation

During the summer, there is enough heat from the sun available to heat a house for the whole year. But how do you store this heat until you need it six months later? No, an enormous insulated water tank is not enough. But it might be possible with a tank of hydrateable salt, uninsulated and ten times smaller. ECN Efficiency & Infrastructure is investigating the possibilities in its laboratory.

If a modern house wanted to store the heat it needed for the winter months in an insulated water tank, it would need about 70 m3 of water. So as a storage medium, hot water does not score particularly well as far as the amount of heat per cubic metre is concerned. “But water does score well on availability and safety in use,” Herbert Zondag of ECN Efficiency & Infrastructure observes. “We are looking for materials that are also cheap and safe to use, but that can store more heat per unit volume and do not require insulation. These materials do not store the heat in a tangible form (increased temperature) but as chemical energy.”
Fortunately, such materials exist: salt hydrates. These are salts that absorb water into their crystal lattice. If you bring these salts into contact with water vapour, a number of water molecules will become bound to one of these salt molecules. The reaction is accompanied by the release of a large amount of heat per mol and is completely reversible. In the summer, you use the heat of a solar boiler to separate the water molecules from the salt, and then the water and salt are stored separately. In the winter, you reverse the process and the recombining of the water and salt releases heat.

Compact heat storage
ECN reported as long ago as 2004 that salt hydrates were very suitable for thermochemical storage of heat, from the sun or any other source. The remarkable thing is that no heat is stored, only the dehydrated salt. This means no insulation is necessary, and the volume required is ten times less than for water. So that is why these ThermoChemical Materials (TCM) are so suitable for the seasonal storage of heat.
Starting from 1 January 2009, an international group of researchers led by ECN has been jointly investigating compact heat storage possibilities, including TCM. As well as this, a group within ECN has been working on the development of a prototype. “We hope to present a working scale model of the reactor at the end of the year,” Herbert Zondag reports cautiously. The research into TCM is still in its infancy, everywhere in the world. The materials have very promising heat properties, but also a few very inconvenient characteristics. Zondag knows that these will not be solved in the blink of an eye.
The conduction of heat by salt hydrates is a good example of what the ECN researchers have to deal with. “Liquid hydrates perform better than solid ones in this respect,” Zondag explains. “But these liquids are extremely corrosive and hazardous. Not something you want to have in your house. We are making things more difficult for ourselves by aiming to use a substance that is cheap and safe under all circumstances. This leaves us with a very limited choice, but these prerequisites are essential for use in the built environment.” ECN has therefore opted for salts in the form of powder, such as calcium chloride (a salt that is commonly used for salting icy roads). The researchers are not discouraged by the low peak outputs. “We set up a small water boiler that is heated slowly by the TCM,” says Zondag. “This boiler can transfer its heat quickly to the water used for taking a shower. And after the shower, absorption of water vapour by the salt ensures that the boiler gradually heats up again.”
At this stage, the researchers are primarily concerned with examining the behaviour of the material under the conditions in which it might be used. “This covers aspects such as the speed of the reaction, the temperature that can be achieved and the extent to which the powder sticks together in the tank as a result of its own weight. By making an inventory of all the important parameters, we will be able in the future to assess more quickly whether materials are suitable.”

Integrated or separate reactor
Two types of system are possible for thermochemical storage; a system in which the reaction takes place in the storage tank, or a system in which the reaction takes place in a separate reactor. The first system is simpler, but the second works more quickly and efficiently. “Both options are being studied,” Zondag reports. “Here at ECN, we are the only people in the world carrying out research into a separate powder reactor for thermochemical heat storage.”A report was published recently of a study into concepts for TCM reactors. ECN had experimented with a powder reactor and TNO with a reactor for a suspension of small powder particles in an inert fluid. The emphasis was primarily on the properties of the reactor: how can we input the most water vapour to and withdraw the most heat from the chemical reaction as efficiently as possible. This showed – as expected – that powder is a poorer heat conductor. For the suspensions, the vapour transport was more of a problem and problems can also arise due to the sinking of the suspended powder. Both options are currently being investigated further within the TNO and ECN partnership Building Future.

Contact
Herbert Zondag
ECN Efficiency & Infrastructure
Tel.: +31 (0)22 456 4941
E-mail: Herbert Zondag  

Info
Click here to read or download the report on concepts for TCM reactors.
Click here for a link to IEA SHC task 42 on compact heat storage
Building Future website (www.buildingfuture.org)

Principle of thermochemical storage of heat.