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Heat battery: a breakthrough in the storage of renewable energy

A battery that stores the warmth of the summer sun and uses it to heat homes and buildings in the winter. Together with a group of European scientific partners and companies, TNO has achieved a world first by successfully delivering a working prototype. The idea is that, a few years from now, this sustainable solution will be making an important contribution to energy conservation and to reduced CO2 emissions in the built environment.

The generation of renewable energy using solar panels or wind turbines suffers from one major drawback. At times when there is a lot of demand for energy, clouds may hide the sun or the wind may stop blowing. Conversely, sun and wind may occasionally produce more energy than is needed. It would be ideal if you could store the heat of the summer sun in a battery, without losing any energy, and use it to heat your home in winter. Heat storage is an important step in creating zero-energy homes and in promoting the widespread use of renewable energy. It also represents a financial windfall for consumers, because heating is a really major expense.

Breakthrough: fine-tuning the properties of salts

‘This battery is now a reality, and we have shown that it works under everyday conditions’, says TNO project leader Christophe Hoegaerts. ‘We achieved a breakthrough by fine-tuning the properties of specific salts to make them suitable for heat storage. To date, no-one else has managed to do this. We owe this success to our cooperation with universities and knowledge institutions in the Netherlands and elsewhere in Europe, with major industrial partners and with various innovative SMEs. The company that built the actual battery was a Dutch SME.’

Thermochemical storage

The technology that underpins this solution is thermochemical storage. Solar thermal collectors on the roofs of houses convert sunlight into heat, in much the same way that photovoltaic solar panels do when generating electricity. A chemical reaction transfers the absorbed heat into a special system, where it is stored. This system consists of a container that is mostly filled with salt, and a little water. The heat is held in the salt, and the chemical reaction that trapped it there is reversed to release the heat again when it is needed. ‘It’s all about hydration and dehydration, or wetting and drying salt. This may not sound like anything out of the ordinary, but the secret lies in the use of a complex combination of materials that we process in a special way. We have already applied for a patent on this process’, Christophe adds.


What is special about this system is its proven ability to sustain many cycles of hydration and dehydration, which means that it has real practical value for everyday use. The salts involved are inexpensive and widely available. This will make it possible to create a cheap and compact commercial heat battery, once the development work is complete and the system is being manufactured on a large scale.

Follow-up project

Following the delivery of a working system by TNO and its partners at the end of 2015, in the context of the European MERITS project, a follow-up project by the name of CREATE has now been launched. ‘MERITS involved a major scientific component, but industry is the overriding consideration in CREATE. We and our partners will now be focusing on optimizing the heat battery, and on installing a working system in a demo house. After all, that’s why we got into this in the first place: zero-energy homes for a better environment, and savings for consumers.’

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