Power plants that run on fossil fuels are able to adapt their production levels according to demand. But in the case of generators of renewable energy, such as solar panels, solar collectors, or wind turbines, things are rather more difficult. They tend to generate large amounts of energy when there is hardly any demand for it, or fall short when demand is high. Thermochemical heat storage can be a major help in combatting that loss of energy. TNO is therefore fully committed to developing compact heat storage with a high energy density.

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Of course, there are already battery solutions with which homeowners can store power that has been generated by their own solar panels. One example is the Tesla Powerwall, a large rechargeable ‘home battery’, as it were. However, they have a fairly low energy density.

Anyone seeking to store the overcapacity from their solar panels will find that the current range of home-based solutions do not come up to scratch. This makes them less suitable for bridging the time between storing and using energy. Moreover, they are only capable of storing electric energy. An unfortunate consequence of this is that any excess heat from thermal solar collectors is lost.

The problem of storing heat in water

There are also boiler-type solutions for storing heat. They are fine for covering shorter periods between storing and using. But what about storing heat generated from solar panels in the summer for a longer period of time, and then using it for heat during the winter? Not possible. By the time winter has arrived, the water will have cooled down too much. And in any case, the solution is not a particularly compact one. For larger quantities, this type of storage requires a sizeable water container.

The current situation – supplying back energy

Now that more and more people in the Netherlands have solar panels on their roofs, it is obvious that more solar power is being generated. An offsetting scheme is currently in place in the country – anyone producing power with their solar panels is able to supply it back to their energy company. The number of kWh is then deducted from their consumption.

And suppose you (or your solar panels) produce more than you actual consume? In that case, you receive a supply-back payment. The amount can vary from one energy provider to another, but is generally below the price per kWh payable by consumers for the energy that they use. 

Ever-larger peaks on the electricity network

The offsetting scheme makes the use of solar panels an attractive proposition. But now that so many people are taking advantage of the scheme, it is becoming increasingly difficult for network operators to cope with the peaks on the electricity network. This is especially true with the increasing size of wind farms in the North Sea, which on windy days can generate enormous amounts of electricity. So, a solution is needed, and quickly. After all, it is this renewable energy that is the leading player in the energy transition.

Smart grids

Smart grids are being developed in the Netherlands. These are energy networks in which processes that do not have to be carried out at any particular time can take place during periods when there is excess energy. Examples include running washing machines or charging electric cars while the sun is shining. This certainly helps, and TNO is fully involved with making further improvements to smart grids of this kind.

Local heat storage

But more remains to be done. Much more. It is now clear that there will always be a mismatch between supply and demand when it comes to energy. We therefore now need to fully commit to storing energy, with local storage of heat being a key priority. Applying this in homes and other buildings will help achieve a better balance between demand and locally produced energy.

As a result, there will be fewer peaks for the electricity network to cope with. The technological developments relating to local heat storage are very encouraging. At TNO, for example, there are two teams working on a compact heat battery based on thermochemistry.

The advantages of thermochemical heat storage 

The fact that you can create a chemical reaction using salt and water that releases heat is nothing new. It has also long been known in the world of science that the process can be reversed, and that the salt hydrates can be separated again by adding energy to them. What you then have is salt and water again. They can again be induced to have a thermochemical reaction, causing the stored heat to be released.

The great thing here is that no energy is lost while the heat is stored. This makes heat batteries of this kind highly suitable for bridging the time between storing and using energy. The excess solar energy from the summer can therefore be used in winters to heat homes. Another benefit is that such a thermochemical system can be charged with heat and electricity.

Heat batteries: long-term high volume heat storage for the home

Advanced technology

Research into heat storage based on thermochemistry has been going on for many decades now. But developing a device that is compact, robust, and affordable has proved difficult, especially one that is also suitable for charging and recharging time after time. Nonetheless, there have been sizeable advances in this area during the past ten years.

In Eindhoven (in collaboration with Eindhoven University of Technology) and Delft alike, TNO has developed technologies that have brought thermochemical heat storage that much closer. There are still several issues that need to be resolved, however. Despite this, both technologies are at an advanced stage of development and it looks as though the first compact thermochemical heat batteries will be on the market in a few years’ time – probably in 2024, and possibly earlier. 

Looking ahead

“How successful the introduction is will depend largely on government policy at the time,” stresses Sten de Wit, whose work as a programme manager at TNO is in the field of energy in the built-up environment.

“It has already been acknowledged that the offsetting scheme for solar panels cannot continue in its present form. There is no real indication as yet of what the new scheme will be, but what is clear is that supplying solar power back to the grid will be wound back in the years up to 2030. This will lead to people who have had solar panels fitted or who are planning to buy them wondering whether they made the right decision. And if, at the time, there are heat batteries on the market that can be used to solve that problem... well, that would be ideal.”


Dr. ir. Sten de Wit

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