future view

Storing summer heat for use in winter

16 September 2016 • 4 min reading time

TNO is currently developing a heat battery which will store heat in the summer for use in the winter. In a sustainable world, the battery is an essential technology for energy storage. We put six questions to Huub Keizers, manager of the Energy in the Built Environment programme.

1. What is a heat battery?

“A standard battery stores electrical energy. A heat battery stores heat. It becomes possible to capture heat during the summer and use it months later during the cold, dark days of winter. TNO is part of the international MERITS consortium, which is working to develop a prototype heat battery for use in the home.”

2. How does it work?

“The heat battery is based on a vat which is filled with salt. For those who like to know these things, the salt in our system is sodium sulfide, which is both inexpensive and in plentiful supply. The charging and discharging technology is simple hydration. When heat is stored, the salt dries. Later, the addition of water releases the heat. This is known as thermochemical storage. Heat can be collected using solar panels of the type that many people already have installed on their rooftops. The heat that is released from the battery can be used to heat water in a boiler, which can in turn is used for the domestic hot water supply and central heating.”

3. Why is the heat battery needed?

“We are working towards a future in which all homes are energy neutral or actually produce more energy than they use. They will be well insulated, comfortable, healthy homes which have their own means of energy production. For example, solar panels which both generate electricity and collect heat for later use. If a house is to be truly energy neutral, it must have some form of long-term energy storage system. At present, the problem with sustainable energy is that there is a significant mismatch between supply and demand. On a sunny summer’s day, production is high but consumption is low. In the depths of winter, it is the other way around. A heat battery will enable householders to use surplus summer heat at other times of the year.”

“We want to arrive at a battery design with a volume of no more than three cubic metres. This would fit easily in a cellar or crawlspace”

4. What is TNO’s contribution?

“TNO is a member of the European MERITS consortium responsible for the proof of concept. Scientists and researchers from organizations such as TNO have been working alongside private sector partners, including several SME companies. The project team has produced a prototype which will now be further developed in the Netherlands. We know that the technology works. The next stage is to prepare for a market launch. Over the next few years, TNO staff will work to refine the design. It is essential to make the system more compact. This may entail increasing the energy density of the salt and devoting attention to the input and output systems for both heat and water. Greater efficiency will result in a smaller system. Of course, stability and service life are also important: it must be possible to charge and discharge the battery module hundreds of times without any problems. So, the main aim now is to make the heat battery smaller. Ideally, it should be a tenth of the size of the prototype. The system we have at the moment would be far too large for the domestic setting. We want to arrive at a volume of no more than three cubic metres, which will fit easily in a cellar or crawlspace.”

5. How does the heat battery compare with other forms of energy storage?

“Compactness is an advantage. A relatively small quantity of salt can store a large amount of energy. The horticultural sector currently uses large water basins to store residual heat. The system works, but you need large volumes. Another advantage of the heat battery is that, once charged, the heat will not ‘leak out’ as is the case with heat from water basins or power from an ordinary battery. As a result, the heat can continue to be stored for months. There is a market for various types of energy storage device. The heat battery is just one option among many. Energy storage always demands a tailor-made solution: what works in one house may not work in another, or will only be efficient when scaled up to district level. A solution that is effective in one country may not be appropriate in another. For example, the Scandinavian countries use surplus electricity to raise water into huge mountainside reservoirs. Later, it is released to power hydroelectric turbines. We can’t do that in the Netherlands.”

6. When will the heat battery come onto the market?

“We hope to have a product available for early adopters within six years. However, it is still too early to say anything about economic viability. That will depend on several factors, but obviously the price must be low enough to appeal to homeowners. The further development of TNO’s heat battery will take place within various programmes, including the Multiannual Plan for Compact Conversion and Storage and the European CREATE project. In short, we are putting a lot of our own energy into the heat battery which, appropriately enough, will be recovered later in the form of an innovative technology for the construction and energy market.”


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