Saarland University researchers developing 3D printed elastocaloric cooling systems
Nickel-titanium shape-memory alloy heats and cools through mechanical deformation and is moving closer to commercial viability, with additive manufacturing now central to the next generation of heat-exchange structures
Researchers at Saarland University have demonstrated an elastocaloric cooling and heating system that uses no refrigerants and no fossil fuels, and which relies instead on the physical properties of nickel-titanium, a shape-memory alloy that releases and absorbs heat when mechanically stressed and then released.
The team is led by Paul Motzki, Professor of Smart Material Systems for Innovative Production at Saarland University and Scientific Director and CEO at the Center for Mechatronics and Automation Technology in Saarbrücken (ZeMA). The group has spent more than 15 years studying the elastocaloric effect with the long-term goal of cooling and heating cars, buildings, and industrial facilities without the environmental costs associated with conventional systems.
The European Commission has identified elastocaloric cooling as the most promising alternative to existing cooling technologies, and the World Economic Forum also lists it among its ‘Top Ten Emerging Technologies’.
How the technology works
The elastocaloric effect relies on a reversible phase transformation within nickel-titanium’s crystal lattice.
“At room temperature, the alloy is in its high-temperature phase. When we apply tensile or compressive stress to the material, we force it to adopt the low-temperature phase,” stated Professor Motzki.
“This is an exothermic process in which the material warms up and releases heat to the surroundings. Once the material has cooled back down to ambient temperature, we release the mechanical stress. This enables the alloy to transform back to its high-temperature phase and, as this is an endothermic process, the material cools down.
“Each deformation of the wires corresponds to a specific electrical resistance value. So the resistance measurements can tell us exactly how the material is deforming at any given moment. That means a position sensor is effectively built in.”
3D printed structures and the path to application
Working alongside manufacturing engineers in Professor Dirk Bähre’s team, Motzki’s group moved beyond wire bundles and thin sheets to produce porous, three-dimensional nickel-titanium lattices via additive manufacturing (see the photo above).
The geometry is designed to increase the surface area available for thermal exchange with a flowing medium — air or water — making heat transfer more efficient.
“This is the next stage in the development of elastocaloric technology,” said Motzki. “The research we are currently undertaking on these new structures is still in the realm of basic research — but we are already thinking about practical use and developing solutions for real-world applications.”
Durability is a key design requirement, as outlined by Motzki, who continued: “For example, in designs that use wire bundles, we want to achieve a lifetime of more than one million cycles. We are designing the relevant components so that they can be exchanged easily, because maintainability is a key factor in determining whether this new technology can translate into reliable day-to-day deployment.”
At Hannover Messe last month, the team exhibited a functional prototype of an elastocaloric mini fridge — a proof-of-concept device that cooled a drinks can using bundles of 200-micrometer-thin nickel-titanium wires rotating around a circular cooling chamber.
