Energy Storage: Hope in Hot Temperatures
Heating furnaces with renewable energy would be a quantum leap for steel and glass industries. It is one of the goals of Dr. Klarissa Niedermeier. She develops a heat storage system for temperatures above 700 °C and uses liquid metal for this purpose. The researcher and her team will present the only prototype of this kind at this year’s Hannover Messe.
If there was a supreme discipline in the development of heat storage systems, it would be high-temperature heat storage systems. These systems designed for use in industry store energy in the form of heat and reach temperatures of more than 500 °C. Liquid salts or solids are the media of choice for storing energy. But similar to sports competitions, there is an ultimate goal in the development of heat storage systems, something like the “ironman” of all projects: Storage of heat above 700 °C, which is as hot as lava.
To date, gases heated with electrical power have been used in this temperature range. They transfer their heat to a storage material that absorbs it. This storage material may be steel, volcanic rock, or slag. Dr. Klarissa Niedermeier from KIT’s Institute for Thermal Energy Technology and Safety, however, is pursuing an entirely new approach.
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Together with her team, the process engineer has developed a heat storage system based on lead-bismuth. She is among the first researchers worldwide using liquid metals in heat storage systems. “Thermal conductivity of liquid metals is 100 times higher than that of other materials,” Niedermeier says. “That is why liquid metals are very well suited for transporting and transferring heat.”
The researcher has been working on the technology for six years now. The 35-year-old engineer wants to help branches with a high resource consumption to better use the weather-dependent renewable energy sources. Industry processes in Germany consume 400 terawatt hours of heat per year, corresponding to 20 percent of the country’s total energy consumption. Steel, glass, and concrete are processed, molten, and dried at temperatures of up to 3000 °C every day. And these temperatures must be kept on a stable level. “90 percent of the fuels used for these processes are of fossil nature,” the engineer explains. “This must be changed.”
First approaches have been tested already, such as the electrification of processes or the use of hydrogen as energy carrier. With her liquid metal-based heat storage system, Niedermeier wants to mitigate fluctuations of power supply from renewable sources and to enable simple, inexpensive, and rapid energy storage at temperatures that are as close as possible to those of industrial processes.
Ceramic Beads as Storage Material
The new type of systems works as follows: Circulating liquid lead-bismuth is heated to more than 700 °C using electrical power. Then, the liquid metal passes through small, white ceramic beads in a steel tank and releases its heat to them. When the heat is needed again, the cooled liquid metal is returned through the beads and, thus, heated to more than 700 °C. The simulations run by Klarissa Niedermeier and her team at KIT’s liquid-metal laboratory KALLA (KArlsruhe Liquid Metal LAboratory) confirm that lead-bismuth allows for a more rapid heating of the heat storage system. Moreover, its packing can be denser than in case of gas. Smaller tubes, less space, less costs, and less time are needed.
Why did nobody think of using liquid metal in heat storage systems before? The first reason is a logistic one, Niedermeier points out. There are only few closed loop systems in the world, in which such a heat storage system can be tested. KALLA has a big lead-bismuth cycle that was built to study the cooling of fuel rods in times of nuclear research. Meanwhile, it is used for new projects in the area of renewable energy sources. “The cycle has been in operation for more than 20 years now. During this time, the team has gained vast know-how,” Niedermeier explains. “Moreover, liquid metals are highly corrosive, especially at high temperatures. KIT is developing special steel alloys for the pipelines and loops of the system.”
The other reason why only few researchers work with liquid metals in heat storage systems is of physical nature: In principle, these metals cannot store heat well. “First, you must have the idea to use liquid metal as a transport means only and not as a storage material in the tank,” the engineer explains.
Partnerships Wanted for 700 °Celsius and More
In spite of all advantages, Niedermeier points out: “There are still many open questions.” So far, the heat storage system has been tested at only up to 400 °C and the system has not yet been optimized. The team is looking for a cheaper storage material and is trying to further improve energy density. Moreover, pumps and valves must be tested for use with molten lead-bismuth at temperatures above 500 °C.
At Hannover Messe, Niedermeier’s team hopes to establish contacts to companies running energy-intensive high-temperature processes or producing waste heat at high temperatures and wishing to store it. At the world’s biggest trade show, the team will present a model of the heat storage system. Its size will be about half that of the real system at KIT that is designed for the storage of 100 kilowatt hours of heat. Niedermeier explains: “This is the first liquid-metal heat storage system of this kind worldwide having such a capacity. We want to show that the principle works and that its potential for the defossilization of industry is great.”
Isabelle Hartmann, March 25, 2024
Translated by Dipl.-Übers. Maike Schröder
