Mateligent, a spin-off of the Department of Intelligent Materials (iMSL) at Saarland University, has set out to close the gap between research and the market by developing customised mechatronic systems based on shape memory alloys (FGL) and dielectric elastomers (DE). The "elastocaloric" cooling process now being presented at HANNOVER MESSE 2023 has already been declared the most promising alternative to current compression refrigeration machines by the EU Commission and the United States Department of Energy. Which is not surprising when you hear the assessment of Stefan Seelecke, Professor of Intelligent Material Systems at Saarland University: "Our process does not require any climate-damaging refrigerants at all and has a significantly high energy efficiency: it is far more efficient than current air-conditioning techniques and up to fifteen times more efficient than conventional refrigerants".

Relaxation leads to cooling

The world's first continuously running cooling demonstrator that cools air with the new process was developed over many years at Saarland University. The exhibit shown in Hanover cools with "artificial muscles", so-called shape memory wires of the alloy nickel-titanium, called Nitinol. These wires have the special property of taking on their old shape again when they are deformed or pulled: In other words, they tense and relax again in a similar way to muscles. The reason for this lies hidden deep inside the metal: its atoms sit in a crystal lattice. If the wire is deformed or pulled, the atomic layers shift and tension arises in the lattice. These tensions are released when the wire subsequently returns to its old shape. During these processes - the researchers call them phase transformations - the wires absorb heat and release it again. Seelecke and his team use this effect in their cooling machine: "The shape memory material releases heat when it is pulled in the so-called superelastic state and absorbs heat when it is relieved. In the case of Nitinol, this effect is particularly strong: when previously tensioned wires are relieved at room temperature, they cool down by up to about 20 degrees below the ambient level," says Seelecke.

A simple basic idea...

"We use these properties to transport heat away," explains Susanne-Marie Kirsch, who helped develop the cooling machine in her doctoral thesis. "The basic idea is to bring pre-stretched, super-elastic shape memory wires into a room where they relieve themselves, cooling down considerably in the process and thereby removing heat from the room," the engineer explains. Outside the room, the scientists load the wires again, where the heat is released into the environment.

...and their complex implementation

In the Saarbrücken cooling machine, however, things are a lot more complicated: in a cooling circuit, a specially designed cam drive (patent pending) ensures that bundles of 200-micrometre-thick nitinol wires are constantly pulled and relieved during rotation. Air is blown through the bundles in two separate chambers, heating them in one and cooling them in the other. This allows the machine to cool, but also to heat. "When loaded, an equally large heating of about 20 degrees takes place, so the process can also be used as a heat pump," explains Felix Welsch, who also worked on the prototype as part of his doctoral thesis. Depending on the alloy, the new cooling technology can deliver up to thirty times more heat or cooling power than it requires mechanical power for pulling and releasing. This makes the system considerably better than current heat pumps and conventional refrigerators.

Approaches to optimisation

The engineers from Saarbrücken are currently working on several projects to further optimise the heat transfer of the machine in order to increase its efficiency even more: the entire energy from the phase transformations is to be used completely for cooling or heating without losses.

Alternative air-conditioning system for electromobility

The latest goal of Seelecke's team is to further develop elastocaloric technology for cooling in electric vehicles. To this end, it has been working with partners from industry and science since January 2022. This joint project, called "NEKKA - Development of a Novel Elastocaloric Climate System", funded by the Federal Ministry of Economics and Technology, has a total budget of six million euros. The sub-project of Seelecke's team is funded with about one million euros. "The aim is to develop, simulate and validate an alternative air conditioning system for vehicles of all vehicle classes: It should continuously provide heat and cold and at the same time be more efficient, environmentally friendly and sustainable than previous methods. In addition, our technology should be smaller, lighter and more cost-effective in comparison," explains engineer Paul Motzki, also a researcher in Seelecke's team.

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