Dr. ir. Sten de Wit
Compact heat storage is a necessary building block in the energy transition. The government and energy experts are now convinced of this. TNO already has numerous irons in that particular fire and is working on a range of heat storage systems. An overview.
Everyone knows the principle behind the boiler. For some time now, there have been solutions available on the market with which heat can be stored in a water tank. In many cases, the hot-water storage forms part of a heat pump system. The disadvantage is the low energy density per volume. You need a water tank containing hundreds of litres in order to be able to store enough heat. There are even 2,000-litre tanks available, for truly larger dwellings.
TNO has long been working on improving this rather traditional method of storage. And successfully, too. Storing heat in water may involve some loss of heat, but with the help of new technology it is possible to significantly restrict that level of loss. So that’s good news. It also looks as though a commercial party will be able to start manufacturing a product in the short term that uses the improvements developed by TNO.
In terms of energy density and efficiency, storing heat in metals could also be a useful option. However, that is a much bigger challenge. The reason is that with this type of heat storage, temperatures of roughly 400 to 1000 degrees Celsius are involved. Because of this greater range of temperatures, the storage density could be far higher than is the case with water up to 100 degrees. This technology is currently at the research stage.
There is a new generation of heat batteries based on thermochemistry. For the batteries to work, it is essential that the process is reversible – in other words, that water and salt can be reseparated through the addition of heat.
TNO is currently working on two thermochemical heat batteries. A team in Delft is developing a device based on sodium sulphide. Meanwhile, a team working in partnership with Eindhoven University of Technology is seeking to develop a heat battery based on potassium carbonate.
Both teams have demonstrated very convincingly that it is technically possible to store large amounts of energy in a small volume with the help of thermochemistry and to release that energy later as heat, without any energy being lost during the period of storage.
Metal oxidises when it comes into contact with oxygen. In certain circumstances, that process of oxidisation can be reversed by adding hydrogen. That, crudely speaking, is the principle behind the REDOX heat battery. TNO has now mastered the process, but it is very much the question as to whether the principle can actually be applied in a heat battery. It is also not clear yet whether such a battery would be fully effective in a built-up environment or an industrial application. So, the research is still at an early stage.
It is not clear at present which heat-storage technology is going to prevail. However, because each solution differs in terms of its usability and application, it is also quite conceivable that there is a market for multiple systems. And if, in the meantime, a new technology emerges that could very well lend itself to use in a heat battery, then TNO would be keen to investigate that in order to explore its potential.
The same is true of other technologies that could be used with existing solutions. Can heat storage be made less costly? Easier? Quicker? With a higher energy density? When scanning new possibilities, these are types of question that TNO constantly asks.
“At TNO, we are not only developing different heat-storage technologies, but we are also looking carefully at the environments in which such a solution could be applied,” emphasises TNO programme manager Sten de Wit, who is involved with energy in the built-up environment.
“When we are developing a renovation concept in partnership with other parties, we look at the possible options for heat storage as an integral part of that. For example, that could involve a device that is able to operate effectively in combination with, say, a heat pump, one that can be properly regulated, and one that engineers will know exactly how to connect and set up.
We don’t simply say, ‘Here’s the technology – good luck with it!’. Heat storage is not a stand-alone entity. It’s about the operation of a total system, and here at TNO we focus a great deal on that. And that’s as it should be, because that system-based approach is a prerequisite for achieving solutions that actually work in practice.”
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