Long-term energy storage is arguably the biggest unsolved problem in the energy transition. A new type of chemical heat storage system has now been invented at TU Wien (Vienna) which can store large amounts of energy in an environmentally friendly way for a virtually unlimited period of time.
Heat is used to initiate a chemical reaction. This produces energy-rich chemical compounds that can be stored for months without any loss of energy. If necessary, the chemical reaction can be reversed and the energy released again. In this way, waste heat from industrial plants or solar heat can be stored in summer and used to heat buildings in winter. The chemical reaction and the suspension reactor specially developed for it are now patented.
Use chemistry to store heat for winterr
There are many ways to store energy, but they all have their drawbacks: Batteries can be charged, but their capacity is limited. Hydrogen can be produced with electricity, but it is difficult to store it for long periods. TU Wien’s new method is based on a completely different principle – the conversion of thermal energy into chemical energy – and vice versa.
“There are different chemical reactions that can be used for this purpose. For example, we use boric acid, a solid material, which we mix with oil,” says Professor Franz Winter from the Institute of Engineering. chemical, environmental and bioscience of the TU. Vienna. “This oily suspension passes through a reactor whose wall is heated to a temperature between 70°C and 200°C.” Many industrial processes take place in this temperature range, so this method is ideally suited for utilizing waste heat from industrial plants that would otherwise simply be wasted. But it is also possible to reach such temperatures simply by concentrating sunlight.
The heat causes a chemical reaction – for example, boric acid is converted to boric oxide and water is released in the process. The oily suspension of boric oxide can then be stored in tanks. If water is added to this suspension, the chemical reaction takes place in the opposite direction and the stored heat is released again.
“This closes the cycle and the suspension can be used again,” says Franz Winter. “In the lab, we have shown that many cycles are possible in this way without any problems.”
The technology has already been patented, but scientists at TU Wien are still working on optimizing the process. “Different reactor sizes will be optimal for different applications,” says Franz Winter. “You always have to look at these reactors as part of an overall system. Depending on the amounts of heat generated at what temperatures in an industrial facility, for example, and other energy technology facilities that already exist there, you have to adapt the process optimally.”
Besides boric acid, other chemicals can also be used – hydrated salts have also been investigated. Boric acid and salt hydrates combine several advantages: they are inexpensive and readily available, relatively harmless and stable over many cycles, and they can be stored for any length of time. Reactor technology can be adapted to industry standards. The oil used allows for optimal heat transfer while protecting the reactor during the reaction and the solids during storage.
It is not yet possible to determine the exact overall efficiency of the process – it highly depends on how the storage is coupled with other technologies. The big advantage is the possibility of long-term storage of quantities of heat that would otherwise simply be wasted and their use according to demand.
“We now want to continue intensive research into this technology, also with industrial partners,” says Franz Winter. “We are convinced that an important step forward has been made with this invention, which will also find its way into industrial application in the coming years.”