Renewable energy requires long-term storage options. A chemical heat storage system has been invented at the Vienna University of Technology that could bring surplus energy from summer to winter.
Rising global energy demand and climate change make the phase-out of fossil fuels inevitable – and current geopolitical developments call for rapid solutions. One building block still missing is methods for long-term storage of surplus renewable energy.
Existing methods to store surplus energy have serious drawbacks: In the case of batteries, the storage capacity is too small – and when solar energy is converted to hydrogen, it generally cannot be stored for long. Now, at the Institute of Process Engineering, Environmental Engineering and Technical Biosciences at the Vienna University of Technology, Austria, a novel chemical heat storage system has been invented that could change that.
At the heart of the chemical heat energy storage system is a scalable suspension reactor in which excess heat is used to activate solid heat storage material. Comparable to storing electricity in a battery, the stored heat in the suspension reactor can be released when needed.
Utilization of waste heat
A chemical heat storage system of this type requires temperatures between 70 and 200 degrees Celsius. This makes the method particularly suitable for energy-intensive industries and especially for industrial plants where waste heat is generated. The temperature range is common in almost all industries from cement to pulp and paper to heating plants, waste incineration and power plants, explains Professor Franz Winter of the Institute of Process Engineering, Environmental Engineering and Technical Biosciences at the Vienna University of Technology. He adds, “When higher temperatures are generated in industrial plants, they are usually used. But waste heat from a temperature range of 70 to 200 degrees Celsius is mostly lost.”
If heat is used to trigger chemical reactions, then high-energy chemical compounds are formed that can be reversed to release the energy again. This function enables a chemical heat storage device of this type to store large amounts of energy for a virtually unlimited period of time. As a result, energy stored in summer can be used in winter to heat the building.
The chemical reaction
Different chemical reactions can be used for this purpose. At the Vienna University of Technology, for example, boric acid, a solid material, is mixed with oil. The heterogeneous oily mixture is placed in a reactor and its wall is heated to between 70 and 200 degrees Celsius.
The heat causes a chemical reaction and the boric acid is converted to boric oxide. Water is released in the process. What remains is an oily boric oxide suspension that can be stored in tanks. To reverse the chemical reaction and release the stored heat, water is returned to the suspension.
This closes the thermochemical cycle and can be restarted by adding heat. “The suspension can be used repeatedly,” explains Professor Winter. “In the laboratory, we have shown that many charging and discharging processes are possible in this way without any problems.”