Artificial Muscles for a Future Air Conditioner

A prototype device has been developed that is able to transfer heat using ‘muscles’ made from nickel-titanium. Nickel-titanium (aka nitinol) is a shape-memory material that releases heat to its surroundings when it is mechanically loaded in its superelastic state and absorbs heat from its surroundings when it is unloaded.

Nitinol is a ‘smart alloy’ or as ‘muscle wire’. This effect has been exploited by the Saarbrücken researchers who have developed environmentally friendly heating and cooling system that is two to three times more efficient than conventional heating and cooling devices.

They have a patent-pending cam drive whose rotation ensures that bundles of 200 micron-thick nitinol wires are alternately loaded and unloaded in such a way that heat is transferred as efficiently as possible. Air is blown through the fiber bundles in two separate chambers: in one chamber the air is heated and in the other chamber is cooled. The device can be operated either as a heat pump or as a refrigerator.

They used a combination of experimental investigations and numerical modeling they were able to identify how to maximize the efficiency of the underlying mechanism, the wire loading level needed to achieve a specific degree of cooling, the ideal rotational speed and how many nitinol wires need to be included in a bundle. ‘The greater the surface area, the faster the heat transfer, that’s why bundles of wires provide the best cooling capabilities,’ explains Susanne-Marie Kirsch. ‘We use a thermal imaging camera to analyze precisely how the heating and cooling stages proceed.’

They can now adjust and tailor their system to meet different needs. They have a software program to precisely tune the heating and cooling technology for specific applications.

This basic research may well have interesting industrial applications because the novel heating and cooling technology developed in Saarbrücken is highly efficient. Depending on the alloy used, the heating or cooling power of the system is up to thirty times greater than the mechanical power required to load and unload the alloy wire bundles.

The new system is already twice as good as a conventional heat pump and three-times better than a conventional refrigerator. ‘Our new technology is also environmentally friendly and does not harm the climate, as the heat transfer mechanism does not use liquids or vapors. The air in an air-conditioning system can be cooled directly without the need for an intermediate heat exchanger and leak-free, high-pressure piping is not needed.

The team is optimizing heat transfer within the system in order to boost the efficiency of the new technology even more. The goal is to use almost all of the energy from the phase transition for heating or cooling.

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