Environment-Friendly Cooling Using Nickel-Titanium Shape Memory Alloys

In today’s world, cooling is a very important process. The challenge for the future is to carry out cooling that is not harmful to the climate, and facilitates the conservation of natural resources. Professors Stefan Seelecke and Andreas Schütze from Saarland University have used cooling systems that use shape memory alloys, also referred to as ‘artificial muscles’ or ‘metal muscles.

In collaboration with researchers based in Bochum, they are developing a novel cooling process where nickel-titanium ‘muscles’ are used for the transfer of heat and cold. Tests have provided results that are being utilized to form a prototype cooling circuit, to be employed to augment the cooling process efficiency. For the past three years, the project has been funded by the German Research Foundation (DFG). The DFG has consented to release 500,000 euros as additional funds, raising the total project funding to 950,000 euros.

Cooling occurs all over the world. Refrigerators are run 24/7, offices are being cooled by air conditioners, and computers and motors run in a smooth manner because of cooling systems. Population growth and climate change increase the demand for cooling. However, a higher number of cooling systems means not only more expenditure, but also an increase in electricity consumption. This results in a greater emission of greenhouse gases into the environment, and an accelerated global warming effect.

An environment-friendly cooling approach has been developed by engineers Stefan Seelecke and Andreas Schütze and their research groups, in collaboration with material scientists Gunther Eggeler and Jan Frenzel at Ruhr University Bochum. The process being developed does not use environment-harming refrigerants, and consumes less energy as compared to the existing cooling technologies.

In our systems, shape memory alloys (SMAs) are used to remove heat. Shape memory means that wires or sheets made from a nickel-titanium alloy have a certain ability to remember their original shape: if they undergo deformation, they will return to their earlier shape. So they are able to tense and flex like muscles. The fact that they absorb and release heat when they do so is something we exploit to achieve cooling.

Stefan Seelecke, Professor for Intelligent Material Systems, Saarland University

The lattice structure of a nickel-titanium wire or sheet changes and generates strain inside the material when the wire or sheet is pulled in tension, or otherwise deformed. This altering of the crystal structure, referred to phase transition, heats up the shape memory alloy. The stressed sample experiences considerable cooling, a temperature 20 degrees less than the ambient temperature when the sample is permitted to relax after reaching the environment temperature level.

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. The heat taken up in this process is then released externally to the surroundings. The SMA is then re-stressed in the surroundings, thereby raising its temperature, before the cycle begins again.

Stefan Seelecke, Professor for Intelligent Material Systems, Saarland University

The researchers at Saarland University and the Center for Mechatronics and Automation Technology (ZeMA) in Saarbrücken have undertaken modeling and experimental studies. They have demonstrated that this cooling approach is workable, and can be practically used. They employed a model system to optimize the cooling process efficiency, by examining factors such as the amount of deformation the material has to undergo to realize a specific cooling temperature, or whether the effectiveness of the process depends on the cooling rate – slow cooling or rapid cooling. In order to precisely analyze how the heating stage and cooling stage go on, a thermal imaging camera was utilized.

We’re currently using these results to construct an optimized prototype for an air-cooling system. We are creating a cooling cycle in which hot air passes over one side of a rotating bundle of shape memory wires. Multiple wires are used in order to enhance cooling power. 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.

Professor Schütze, Measurement Technology Lab, ZeMA

At present, the group of engineers are fine tuning the method to optimize its efficiency.

Further optimization of the cooling process will involve modeling all component stages and then refining these models by comparing the predictions with experimental results. The data from the modeling and experimental work should allow us to determine the ideal number of shape memory wires for our rotating wire bundle as well as the optimum speed of rotation.

Stefan Seelecke, Professor for Intelligent Material Systems, Saarland University

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