German, US scientists work to commercialize energy storage systems designed to offset fluctuations in renewable power production.

Researchers at Karlsruhe Institute of Technology (KIT) in Germany are developing a high-temperature heat storage system that employs liquid-metal technology to enhance the use of renewable energy sources.

Highly conductive liquid metals can be heated to more than 700 degrees Celsius (1,290 degrees Fahrenheit) using green electricity to flexibly store industrial heat.

The KIT scientists showcased a model of the energy storage system in late April at Hannover Messe, one of the world's largest trade fairs dedicated to industry development.

Worldwide, high-temperature heat storage systems using metals are being developed to supply resource-intensive production companies with heat independent of fluctuating renewable energy production. These storage systems can convert electricity into heat, which can be sequestered until needed.

The heat is then used during periods when electricity is expensive, and production processes cannot be interrupted. The higher the temperature stored, the better. This can reduce the amount of additional energy that would be needed to reach the desired production temperature.

Some pilot plants, including a joint venture brought online earlier this year in Colorado, use liquid salts as well as metals to store temperatures of around 550 C (1,022 F).

Even higher temperatures have been achieved with gases so far. When electrically heated to about 700 C (1,290 F), they transfer their heat to storage materials such as steel, volcanic rock or slag.

"However, the heat transfer from the hot gas to the storage material is far from being efficient," said Klarissa Niedermeier, Ph.D., a researcher at KIT's Institute for Thermal Energy Technology and Safety.

Niedermeier leads a team of scientists working on a novel heat storage system based on lead and bismuth for the high-temperature range.

"The thermal conductivity of this mix of liquid metals is 100 times higher than that of other materials used in storage systems," Niedermeier said.

The high-temperature heat storage system is being tested in a loop. In a steel tank, the heated lead-bismuth mix seeps through ceramic beads about 2 millimeters (about 0.08 in) in size and releases its heat to them. When the heat is needed, the "cold" liquid metal is returned through the beads, which heat it up again.

Simulations at KALLA, the institute's Karlsruhe Liquid Metal Laboratory, have confirmed that the use of liquid metal increases the efficiency of heat storage, especially when a very compact package is used.

"When the liquid metal is heated with power from renewable energy sources, companies (will) have an efficient solution to mitigate fluctuations of power supply and to enable simple, inexpensive, and rapid energy storage at temperatures that are as close as possible to those used in industrial processes," observed Niedermeier, who is among the first researchers in the world working with liquid metals in heat storage systems.

KALLA

Ceramic beads store the heat in a lab-scale prototype of the bismuth-lead liquid metals battery being developed by Klarissa Niedermeier and her team at Karlsruhe Institute of Technology.

Powerful potential

The process could potentially help de-fossilize fuel sources for industry.

"Thermal conductivity of liquid metals is 100 times higher than that of other materials," Niedermeier said. "That is why liquid metals are very well-suited for transporting and transferring heat."

The 35-year-old engineer has been working on the technology for six years, hoping to help sectors of the German economy with high resource consumption to better use their weather-dependent renewable energy sources.

Industry processes in Germany consume 400 terawatt hours of heat per year, corresponding to 20% of the country's total energy consumption. Steel, glass, and concrete are processed, melted, and dried at temperatures of up to 3000 C every day. And these temperatures must be kept stable.

"90% of the fuels used for these processes are of fossil nature," she said. "This must be changed."

Industrial processes in the United States currently consume energy sourced primarily from fossil fuels, with nuclear (10% and renewables (12.7%) accounting for less than one-quarter of the mix.

Other approaches to energy storage have been tested, such as the electrification of processes or the use of hydrogen as the energy carrier.

With the liquid metal-based heat storage system, Niedermeier aims to mitigate the inherent 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.

So far, liquid metals have hardly been used in heat storage systems. According to Niedermeier, this is mainly due to logistical obstacles. There are only a few closed-loop systems in the world where such a heat storage system can be tested. KALLA has a large lead-bismuth cycle, which is used for new projects in the field of renewable energy sources, among others.

"Thermal conductivity of liquid metals is 100 times higher than that of other materials," She said. "That is why liquid metals are very well-suited for transporting and transferring heat."

An engineer-by-training, the 35-year-old has been working on the technology for six years. She wants to help industries with high natural resource consumption better use weather-dependent renewable energy sources.

Tests at KALLA confirm that the lead-bismuth design allows for more rapid heating of the energy storage system and denser packing than the use of gas. Smaller tubes, less space, less costs, and less time are needed.

Major challenges ahead

Despite its powerful potential, significant obstacles remain to commercializing the process.

For example, liquid metals are highly corrosive, especially at high temperatures, according to Neidermeyer.

KIT is developing special steel alloys for the pipelines and loops of the system to overcome this problem.

The other reason why only a few researchers are working with liquid metals in heat storage systems is that these materials 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," Niedermeier explained.

Also, many open questions remain. So far, the heat storage system has been tested only up to 400 C, and the system has not yet been optimized.

The KIT 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, she said.

At Hannover Messe, Niedermeier's team planned to establish contacts with companies running energy-intensive high-temperature processes or producing waste heat at high temperatures and wishing to store it.

The researchers also planned to showcase a model of the heat storage system – roughly half the size of the actual system at KIT that is designed to store 100-kilowatt hours of heat.

"This is the first liquid-metal heat storage system of this kind worldwide having such a capacity," added Niedermeier. "We want to show that the principle works and that its potential for the de-fossilization of industry is great."