Energy-efficient cooling elements from a 3D printer: Elastocaloric cooling systems at Hannover Messe
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The elastocaloric technology offers a cleaner, greener alternative to traditional cooling and heating systems. Professor Paul Motzki and his team at Saarland University are key players in the field. Working with 3D-printing specialists led by Professor Dirk Bähre, they are developing novel, energy-efficient geometries for the cooling elements. Doctoral research students Thorben Trodler (left) and Michael Fries (right) are involved in the optimization of these delicate heat-exchange structures made from nickel-titanium alloy, through which air and water can flow. The three-dimensional alloy structures are produced layer by layer using additive manufacturing in a 3D printer. The team is showcasing their technology at Hannover Messe from 20 to 24 April.
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Visitors to this year’s Hannover Messe can experience a sudden drop in temperature at first hand – all brought about by simply stretching a metal alloy and then releasing it again. The underlying elastocaloric technology offers a cleaner, greener alternative to traditional cooling and heating systems. Professor Paul Motzki and his team at Saarland University are key players in the field and are driving developments ever closer towards real-world applications. Working with 3D-printing specialists led by Professor Dirk Bähre, they are also developing novel, energy-efficient geometries for the cooling elements. The team is showcasing their technology at Hannover Messe from 20 to 24 April (Hall 11, Stand D41).
The shiny cubes, each with a striking geometry, could easily be taken for stylish decorative items. For the researchers who work with these 3D-printed structures, however, their appeal lies in their functionality rather than their aesthetics. The manufacturing engineers in Professor Dirk Bähre’s team and the smart materials specialists led by Professor Paul Motzki are interested in how these metal structures behave in the innovative cooling and heating systems currently being developed in Saarbrücken. ‘This is the next stage in the development of elastocaloric technology. 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,’ explains Paul Motzki. The novel geometries of these new cooling and heating elements are designed to boost heat transfer efficiency by maximizing the surface area over which thermal energy is exchanged.
Instead of cooling with refrigerants that are harmful to our climate, or heating with fossil fuels like oil or gas, elastocaloric systems use components manufactured from the shape-memory alloy nickel-titanium. Until now, Paul Motzki’s team at Saarland University has been researching the elastocaloric properties of bundles of ultrathin wires and thin sheets made from this alloy. These components release heat when pulled or compressed, and they absorb heat when the mechanical load is removed. The Saarbrücken engineers are using the elastocaloric effect to transport heat from one location to another – for example, to transfer heat out of a cooling chamber. The research teams at Saarland University and at the Saarbrücken Center for Mechatronics and Automation Technology (ZeMA) have been investigating the elastocaloric effect for more than 15 years, with the long-term aim of cooling and heating cars, buildings and industrial facilities in an environmentally friendly and energy-efficient way. At this year’s Hannover Messe, the team is demonstrating that their technology has moved beyond pure fundamental research and is already well on its way towards real-world applications.
Cool new materials
Enormous quantities of energy are consumed worldwide for cooling and heating – and as the climate changes, demand is set to rise further. Unlike conventional cooling and heating methods, elastocaloric technology promises significantly higher efficiency. Powered solely by electricity, elastocaloric systems are as clean as the electricity that is used to power them. The European Commission has identified elastocaloric cooling as the most promising alternative to conventional cooling technologies, and the World Economic Forum listed it among the ‘Top Ten Emerging Technologies’. The technology is based on the special properties of nickel-titanium – an alloy that, when deformed, behaves very differently from conventional metals.
Nickel-titanium is what is known as a ‘shape memory alloy’, i.e. the material can be deformed and then return to its original shape, due to a reversible phase transformation between two solid crystal lattice structures. This phase transformation is accompanied by heat transfer. ‘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. 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,’ explains Paul Motzki. Put simply: when a nickel-titanium wire is stretched, it releases heat to the air or liquid flowing past it; when the stress is removed, it cools down and is able to absorb heat from its surroundings. This mechanical deformation cycle of repeated tensile loading and unloading is the key principle behind the new technology. No additional sensors are required, as the material itself has its own intrinsic sensing properties. ‘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,’ Motzki explains.
The researchers in Saarbrücken aim to maximize thermal energy transfer by maximizing surface area. The larger the surface area, the more efficiently heat can be transferred to the working medium – air or water. Up until now, the team has increased surface area by creating bundles containing many ultrathin shape-memory wires. In the next generation of these devices, the cooling and heating elements will provide even more contact area by incorporating a porous geometric nickel-titanium structure. To achieve this goal, Paul Motzki’s research group is working with Dirk Bähre’s team to develop an intricate nickel-titanium structure through which the heat-transfer medium (air or water) can flow. The researchers are refining and optimizing the design of these delicate alloy lattices. A variety of complex geometries are undergoing experimental testing to determine which structures yield the most efficient heat transfer. The three-dimensional alloy structures are produced layer by layer using additive manufacturing in a 3D printer.
Preparing the technology for real-world applications
While laboratory experiments and testing are ongoing, Motzki and his team are also working to develop the emergent field of elastocalorics for real-world deployment. The materials that will be used in future elastocaloric cooling systems will need to be suitable for continuous operation in refrigerators and cooling units. ‘We are working to develop materials and designs that are robust enough for continuous use and for ease of maintenance. We build questions about potential future applications into the development process right from the outset; it’s a core principle of our research and it also shapes the curricula of our degree programmes such as Systems Engineering and Sustainable Materials and Engineering,’ says Paul Motzki, who, like Dirk Bähre, involves numerous doctoral researchers as well as undergraduate students in this work.
One of the questions being addressed experimentally is how to mechanically load the materials in ways that ensure a long service life. This involves matching the properties of the alloy to the tensile and compressive cycling regimes. ’For example, in designs that use wire bundles, we want to achieve a lifetime of more than one million cycles,’ says Paul Motzki. At some point, however, even the best material will fatigue. ‘That’s why we are also developing a simple and fast replacement concept. 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,’ explains Motzki.
Funding and current projects in elastocalorics
The German Federal Ministry of Research, Technology and Space is funding the project ‘DEPART!Saar’ with up to €18 million under its ‘T!Raum’ programme. The aim of this project is to strengthen Saarland’s economy by developing regional innovation and transfer structures that will accelerate the transfer of elastocaloric technology into real-world applications. In the SmartCool project, which is funded by the Federal Ministry for Economic Affairs and Energy, the Saarbrücken engineers are working with Volkswagen AG, Fraunhofer IPM and the company Ingpuls to develop lightweight, energy-efficient cooling systems for electric vehicles. In a further research project, the team is working with European partners to develop an elastocaloric air conditioning system that can be used to cool and heat individual rooms of residential buildings. The project consortium led by Paul Motzki will receive a total of €4 million in funding under the ‘EIC Pathfinder Challenge’ from the European Innovation Council. With additional funding from an ‘ERC Starting Grant’ from the European Research Council, Paul Motzki and his team are advancing elastocaloric technology using a globally unique combination of shape-memory materials and smart-film actuators. Dielectric elastomers are the second smart materials field in which Paul Motzki is a recognized expert.
At Hannover Messe, the researchers are on hand to explain the technology and are also looking for partners from academia and industry to develop elastocaloric systems further and create applications ranging from household appliances to industrial cooling systems. One of the exhibits being showcased is a functional prototype of the first elastocaloric mini fridge, which demonstrates proof of concept by cooling a drinks can. At the heart of the mini fridge are bundles of 200-micrometre-thin nickel-titanium wires that rotate around a circular cooling chamber. The wire bundles are stretched on one side of the chamber, and the tension is released on the other. Air that flows past the wires, carries heat out of the chamber, cooling the chamber and the can of drink it contains.
Joint exhibition stand ‘Germany’s Saarland’ (Hall 11, Stand D41).
