Engineers from Saarland University, in Germany, are using shape memory materials made from a nickel-titanium alloy to develop a cooling process that does not require the use of environmentally harmful refrigerants. Researchers led by Stefan Seelecke, professor of Mechatronics and Materials Science, and Professor Andreas Schütze, from the University's Measurement Technology Lab, have experimented with stretching nickel-titanium wires to change the crystal lattice structure and create strain within the material. This change in the crystal structure, known as a phase transition, causes the shape memory alloy (SMA) to become hotter. The researchers found that, if the stressed sample is allowed to relax after temperature equalization with the environment, it undergoes substantial cooling to about 20 degrees below ambient temperature.

"The basic idea was to remove heat from a space—like the interior of a refrigerator—by allowing a pre-stressed, super-elastic shape memory material to relax and thus cool significantly," says Seelecke. The heat taken up in this process is released externally to the surroundings. The SMA is then re-stressed in the surroundings, raising its temperature before the cycle begins again.

Marvin Schmidt and Johannes Ullrich are developing the cooling process. Image credit: Oliver Dietze.The researchers used a model to determine how to optimize the cooling process efficiency, examining factors such as how the material has to be elongated or bent in order to achieve a certain cooling performance, and whether the process is more effective when carried out slowly or rapidly. A thermal imaging camera analyzed how the heating and cooling stages proceed.

Seelecke and Schütze are using these results to construct an optimized prototype for an air-cooling system. The prototype involves creating a cooling cycle in which hot air passes over one side of a rotating bundle of shape memory wires. The bundle is mechanically stressed on one side as it rotates, thus heating up the SMA wires. As it rotates further, the SMA relaxes and cools. The air to be cooled is guided past the cold wire bundle, thus cooling an adjacent space.

The engineers are fine-tuning the process to optimize its efficiency by determining the ideal number of shape memory wires for the rotating wire bundle, as well as the optimum speed of rotation, says Schütze.