Whenever the energy transition is mentioned, many people immediately think of technologies that use renewable energy sources, such as wind turbines and photovoltaic systems. However, one resource is often overlooked: industrial heat energy that could be stored and reused. Particularly energy-intensive industries and power plants generate enormous amounts of process and waste heat – often at such high temperatures that it is difficult to reuse. What has been missing so far? One thing is heat storage systems for temperatures above 500 degrees Celsius. The LIMELISA (Liquid Metal and Liquid Salt Heat Storage System) project, in which KIT, the German Aerospace Center (DLR), and KSB SE & Co. KGaA have been collaborating since 2021, addresses this issue. Their common goal is to develop key components for the energy storage of the future in order to enable the safe and efficient handling of liquid metal or salt as a heat transfer medium. The collaboration at KIT focuses on the development of high-temperature-resistant pumps and valves for a novel liquid metal heat storage system, which is currently being built at KIT as a demonstration experiment.

700 degrees Celsius liquid metal – which material can withstand it?

Dr. Klarissa Niedermeier from the Institute for Thermal Energy Technology and Safety (ITES) at KIT has been researching liquid metal-based heat storage systems for high-temperature applications since 2019: “Liquid metals allow us to work in a very wide temperature range, from 100 to 1000 degrees Celsius. At the same time, liquid metals have extremely high thermal conductivity, up to 100 times higher than conventional heat transfer fluids such as oil.” These impressive properties make liquid metals, such as the lead bismuth used at KIT, ideal heat carriers for high-temperature applications. “The dilemma for us is that there are hardly any materials or standard components available on the market that can transport liquid metal under extreme conditions above 500 degrees Celsius without problems,” the researcher sums up. In particular, working with lead as a medium posed challenges for the engineers. Conventional high-temperature steels have a high nickel content. At such high temperatures, the liquid lead attacks the nickel contained in the steel pipes and fittings. This means that conventional materials reach their limits during prolonged operation.

A bit like alchemy

The lack of experience with dynamic behavior under these extreme conditions makes development an open-ended project. The search for materials was an important part of the project work at KIT. Dr. Alfons Weisenburger and his team at the Institute for Pulsed Power and Microwave Technology (IHM) at KIT conducted extensive material tests with many different materials available on the market in contact with liquid metal at very high temperatures. Aluminum coatings that are diffused into the base material proved to be particularly promising. “These are exposed to an oxygen-containing atmosphere either after production or during operation and then form aluminum oxide layers only a few micrometers thick, which provide effective protection against the liquid metal. With the help of full-surface coatings, we achieve a longer service life for the components,” says Weisenburger.

From components to the test stand at KIT

Parallel to component development, a challenging test stand was set up at ITES. This enables the pumps and valves manufactured by KSB to be put to the test. The test cycle contains two main tanks—a sump tank as a collection and storage tank and a pump tank into which the KSB pump to be tested is immersed. Martin Lux, the test engineer responsible for the high-temperature cycle, reports: “Finding materials and measurement technology for our purposes was especially challenging here, because the measurement tasks in the test stand are complex: flow, pressure, fill levels, and temperatures have to be precisely recorded at various points in the 700-degree liquid metal cycle.” The final construction work on the test bench is currently ongoing, the KSB components have been installed and instrumented, and the first tests can then begin: “We plan to operate the cycle at a constant temperature of 700 degrees Celsius for a certain number of hours and cycle times, switching the valves hundreds of times and then analyzing the material changes. The long-term tests will show whether our developments can withstand the required loads over many operating hours,” explains Niedermeier, adding: ”These tests will only be carried out isothermally, at a constant temperature. In the real-world application of a heat storage unit, however, dynamic start-up and shutdown processes raise further questions that still need to be clarified.” “If we achieve this, we will take the next big step towards industrial application,” Bosbach affirms.

Future energy

“We have already increased the practical applicability of our research in this industry-oriented project. Without this partnership, we would not be where we are today,” summarizes Niedermeier. The chances are good that marketable products will result from the research project – even if greater strides are still needed in the field of energy storage. Demand is growing in high-temperature industries such as steel and cement because waste heat utilization is becoming an increasingly important additional pillar of the energy industry. The high-temperature components developed could make an important contribution to the more efficient use of renewable energies in the future by enabling longer-term storage of thermal energy. In addition, the high-temperature media liquid salt and liquid lead offer new possibilities in the development of inherently safe Generation IV nuclear reactors.

After successful laboratory tests, KSB plans to further develop the technology for industrial applications. “With these robust pumps, companies can now implement systems that previously seemed impossible. We can offer innovative solutions in both high-temperature heat storage and new nuclear technologies, thereby contributing to the transformation of the energy system,” says Harsch. “The research project serves as an important reference to show that our pumps and valves are ready for new applications. Products for the nuclear sector are currently being developed based on the same materials,” adds Bosbach.