Image: TU Vienna

TU Vienna invents chemical heat storage

Storing energy over the long term is probably the biggest unsolved problem of the energy transition so far. A new type of chemical heat storage system has now been invented at the Vienna University of Technology that can be used to store large amounts of energy in an environmentally friendly way for a virtually unlimited period of time.

Heat is used to trigger a chemical reaction. This process produces high-energy chemical compounds that can be stored for months without any problems or loss of energy. If necessary, the chemical reaction can then be reversed, releasing the energy again. For example, waste heat from industrial plants or solar heat can be stored in the summer and used to heat buildings throughout the winter. The chemical reaction and the suspension reactor specially developed for it have now been patented.

Store in summer, use in winter

There are many methods of storing energy, but they all have their drawbacks: You can charge batteries, but their capacity is limited. It is possible to produce hydrogen with electric power, but it is difficult to store in the long term. The new method developed by TU Wien is based on a completely different principle – the conversion of thermal energy into chemical energy and back again.

“There are different chemical reactions that can be used for this purpose. For example, we use boric acid, a solid material that we mix with oil,” explains Prof. Franz Winter from the Institute of Process Engineering, Environmental Engineering and Technical Biosciences at TU Wien. “This oily suspension goes into a reactor whose wall is heated to a temperature between 70°C and 200°C.” Many processes in industry take place in this temperature range, so this method is ideally suited to utilizing waste heat from industrial plants that would otherwise simply be lost. But you can also achieve such temperatures simply by focusing sunlight.

The heat causes a chemical reaction – for example, boric acid is converted into boric oxide, and water is released in the process. The oily boric oxide suspension can then be stored in tanks. If water is then added to this suspension again, the chemical reaction takes place in reverse and the stored heat is released again.

“This closes the cycle and the suspension can be used one more time,” explains Franz Winter. “In the laboratory, we have shown that many charging and discharging processes can be performed in this way without any problems.”

Many advantages at the same time

The technology has already been patented, but now it will be studied in more detail to determine how it can best and most efficiently be applied. “Different reactor sizes will be optimal for different applications,” says Franz Winter. “You always have to see these reactors as part of an overall system. Depending on what heat quantities occur at what temperatures in an industrial plant, for example, and what other energy technology equipment already exists there, you have to adapt the process optimally.”

In addition to boric acid, other chemicals can be used – salt hydrates have also been studied. Boric acid and hydrated salts combine several advantages: they are inexpensive and easily available, relatively harmless and stable over many cycles, and can be stored for any length of time. The reactor technology can be scaled up to industrial scale. The oil used allows optimal heat transfer while protecting the reactor during reaction and the solids during storage.

It is not possible to give an exact efficiency of the process at this time – it will depend heavily on how the storage system is coupled with other technologies. The great advantage is the long-term storage possibility of heat quantities that would otherwise simply be lost, and their demand-oriented use.
“We now want to continue intensive research on this technology, also together with industrial partners,” announces Franz Winter. “We are convinced that this invention represents an important step forward that will also find its way into industrial applications in the coming years.”