From the humble refrigerator and air conditioner to the complex refrigeration systems used in industrial processes, staying cool is an important part of modern life. Global warming and a growing world population are driving the increasing need for energy-efficient cooling systems. But low temperatures often mean high electricity consumption, which in turn often results in a large carbon footprint and the risk of emissions of refrigerants that typically have a high global warming potential. A research team of academic and industrial partners, led by Professor Stefan Seelecke of Saarland University and the Centre for Mechatronics and Automation Technology (ZeMA), is currently developing an environmentally friendly cooling system.
Our process is energy-efficient and does not require the use of climate-damaging refrigerants. “In fact, our technology is up to 15 times more efficient than systems based on conventional refrigerants,” explained Stefan Seelecke. Both the European Commission and the U.S. Department of Energy have evaluated the new process and consider it the most promising alternative to the vapor-compression refrigeration technology currently in use.

Seelecke’s team has developed the world’s first continuously operating prototype that cools air using this new process. The refrigeration technology that the team is presenting at this year’s Hannover trade fair uses artificial muscle fibers composed of bundles of ultra-thin, shape-memory threads made from the nickel-titanium alloy “nitinol.” These threads have the special property of returning to their original shape after being stretched or otherwise deformed. Thus, they are able to tense and relax like human muscles.

The reason for this behavior lies in the structure of the metal alloy. The atoms in the alloy are arranged in a crystalline lattice structure. If the nickel-titanium wire is deformed or pulled under tension, the layers of atoms in the crystalline lattice move relative to one another, creating stress in the material. This stress is released when the wire returns to its original shape. These changes in the material's crystalline structure are known as phase transitions, and they cause the wires to absorb or release heat. This is the effect that Seelecke and his team harness in their novel cooling system. The shape-memory material releases heat when mechanically loaded in its superelastic state and absorbs heat from its surroundings when unloaded. And this effect is particularly pronounced in the case of nitinol. “When the pre-tensioned nitinol yarns are unloaded at room temperature, they cool down by up to 20 degrees,” explains Stefan Seelecke, Chair of Intelligent Materials Systems at Saarland University. “

We take advantage of this property to dissipate heat,” explains Susanne-Marie Kirsch, who helped develop the cooling system as part of her doctoral research project. “The basic idea is to allow the superelastic shape-memory yarns to relax and thus cool the space by removing heat,” Kirsch explained. The heat absorbed by the shape-memory yarns is released to the outside when the yarns are re-tensioned in the environment.

However, the cooling system in Saarbrücken is considerably more complex. The team has designed and developed a cooling circuit in which a patent-pending cam drive rotates, alternately stretching and relaxing bundles of 200-micron-thick nitinol yarn to transfer heat as efficiently as possible. Air is blown through the yarn bundles in two separate chambers: one where the air is heated, and the other where it is cooled. This allows the machine to both cool and heat. “When the wires are mechanically loaded, they heat up by about 20 degrees, so the process can also be used as a heat pump,” explains Felix Welsch, who also worked on the prototype system as part of his PhD. Depending on the alloy used, the heating or cooling power of this new technology is up to thirty times greater than the mechanical power required to load and unload the bundles of alloy wires. This makes the new system significantly better than currently available conventional heat pumps and refrigerators.

The cooling system is the result of many years of research in different projects, numerous award-winning doctoral theses, and close collaboration with the group led by Professor Andreas Schütze at Saarland University. Funding was provided, in part, by the German Research Foundation (DFG) through its priority program “Ferric Refrigeration.” Through a combination of experimental studies and numerical models, the researchers were able to optimize the underlying mechanisms and determine how many nitinol wires should be included in a bundle or what level of wire loading is needed to achieve a specific degree of cooling. Based on these results, the research team can now customize the system. They have developed a software package that allows them to simulate, plan, and adjust cooling systems for specific applications.

The Saarbrücken team is currently working on several projects to further optimize the heat transfer process and thus increase the efficiency of the new technology even more. The goal is to reach a stage where virtually all the energy from the phase transition is used for heating or cooling.

Researchers in the Seelecke team are currently developing their elastocaloric technology for use as a cooling system in electric vehicles. Since January 2022, the team has been working with academic and industrial partners on the government-funded NEKKA collaborative project, which aims to develop a novel elastomeric cooling system. The project will receive a total of €6 million in funding, of which approximately €1 million will be allocated to the Saarbrücken team for their electric vehicle cooling project. Our goal is to develop, mathematically model, and validate an alternative air conditioning system that can be used in all vehicle classes. The system will be able to heat and cool the vehicle and, at the same time, will be more efficient, more environmentally friendly, and more sustainable than other systems currently available. “Our technology will also be more compact, lighter, and more cost-effective,” explains Paul Motzki, research engineer and managing director of the Seelecke team.
To develop this and other smart material systems for commercial and industrial applications, researchers at the Intelligent Material Systems Laboratory have founded the spin-off company “mateligent GmbH.”