Industrial energy storage – hopes are high at the Karlsruhe Institute of Technology

Liquid metal has a huge potential for decarbonising industry: At the Liquid Metal Laboratory Kalla in Karlsruhe, Klarissa Niedermeier is researching high-temperature heat accumulators.
Image: Markus Breig, KIT

A publication issued by the German Society for Chemical Engineering and Biotechnology (Dechema) about carbon sources and their integration into power-to-X added value chains focuses on options for separating and using carbon dioxide for sustainable production pathways. The technology is an important part of decarbonising the metal industries.

For the steel or glass industry, this would be a quantum leap: powering furnaces with renewable energy. This is one of the goals of Dr Klarissa Niedermeier. She is developing a heat accumulator for temperatures above 700 degrees Celsius. The highlight of this device is that it works with liquid metal. If there ever should be a supreme discipline in the development of heat stores, it would be high temperature heat accumulators, according to KIT. These systems, which are to be used for industrial purposes, save energy as heat and reach temperatures of more than 500 degrees Celsius. Liquid salts or solids are the medium of choice for storing this energy. But as with other competitions, there is an even higher goal for heat stores, something akin to the Ironman of endeavours: storing more than 700 degrees Celsius of heat – the temperature of lava. In this area, gases are currently used as a medium and heated using electricity. They then transfer their warmth to a heat-absorbing storage medium, for example steel, volcanic stone, or slag. But Dr Klarissa Niedermeier from the Institute for Thermal Energy Technology and Safety at KIT wants to head in a completely different direction within this category.

The heat accumulator uses liquid lead-bismuth as a heat conductor.
Image: KALLA

Conducting heat 100 times better
The doctor of process engineering and her research team have developed a heat store based on lead and bismuth. This makes her one of the first persons worldwide to use liquid metals for storing heat: “The conductivity of liquid metals for heat is 100 times more than that of other materials”, says Niedermeier. “They are therefore extremely good at transporting and transferring energy.”
The scientist has been working on this technology for around six years. The 35-year-old wants to help industrial sectors that need a lot of resources to better utilise renewable sources of energy which depend on the weather. The reason for this is that industrial processes in Germany eat up 400 terawatt hours of heat each year; 20 percent of the Federal Republic of Germany’s overall power requirements. Whether steel, glass, cement or concrete: Day in, day out, materials are tempered, smelted and dried at up to 3,000 degrees Celsius. And the temperatures must remain stable throughout. “To achieve this, fossil fuels are currently used at a rate of 90 percent”, the engineer explains. “That has to change.”
There are already approaches to solving this problem, for example electrification of processes or the use of hydrogen as a source of energy. With her heat accumulator based on liquid metals, Niedermeier wants to offer companies a solution to compensate for the fluctuating availability of electricity from renewable sources and to allow storing energy in a way that is simple, cheap, quick and happens at temperatures which are as close as possible to those of the industrial processes.

Prototype at laboratory scale: The ceramic pellets store the heat.
Image: KALLA

Ceramic pellets as storage material
The principle of the novel system: Electricity heats up the circulating liquid lead-bismuth compound to more than 700 degrees Celsius. Inside a steel tank, the liquid metal then oozes past tiny, white, round ceramic pellets. The lead-bismuth transfers its heat to the little pellets, which act as a storage medium. When the heat is needed, the cooled-down liquid metal then flows back past the little pellets and is heated to more than 700 degrees Celsius. As the simulations carried out by Klarissa Niedermeier and her team in the liquid metal laboratory Kalla (Karlsruhe Liquid Metal Laboratory) at KIT demonstrate: With combined lead-bismuth, the heat accumulator can be heated faster and packed tighter than with gas. You need smaller tubes and less space, which saves costs and time.
Why did nobody think about using liquid metal in heat accumulators before, if it seems to have so many advantages? According to Niedermeier, the number one reason is logistics. There are only few circulation systems in the world where such a heat accumulator could be tested. Kalla, however, had a large circulation system for lead-bismuth dating back to the nuclear research era, which was originally intended to research cooling technology for nuclear rods and which is now also used for new renewable energy projects. “The circulation system has been running for more than 20 years. The team therefore has gained a lot of know-how”, Niedermeier explains. “And another thing is that liquid metals are corrosive, especially at high temperatures. For this reason, KIT develops specialised steel coatings for tubes and components of the circulation system.”
Another reason why only few researchers focus on liquid metals in heat accumulators is physical: These special metals are not good at storing heat per se. “You first have to think of using liquid metal only as a ‘means of transport’ and not as a storage medium in the tank itself”, the engineer explains.

Still many scientific questions to be answered
In spite of all the advantages, Niedermeier points out: “There are still many scientific questions to be answered.” The heat accumulator has until now only been tested up to 400 degrees Celsius, and the system has not yet been optimised. The team of researchers is, for example, looking for a cheaper storage material and, at the same time, trying to further improve energy density. Also, pumps and valves for liquid lead-bismuth must be tested at temperatures above 500 degrees Celsius.
At the industrial exhibition at Hannover Messe 2024, the scientist presented a prototype. The model of a heat accumulator, about half the size of the actual test store at KIT, which is designed to store 100 kilowatt hours of heat. Niedermeier explains: “This is the first liquid metal heat accumulator of its kind worldwide that has such a capacity. We want to prove that the principle works and has a huge potential for defossilising industry.”
Source: KIT