A research team led by Professors Stefan Seelecke and Andreas Schütze at Saarland University, Denmark has developed a prototype device, which is able to transfer heat using ‘muscles’ made from nickel-titanium. Nickel-titanium or 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. This unusual property is the reason why nitinol is also referred to as a ‘smart alloy’ or as ‘muscle wire’. The phase transitions that occur in the alloy’s crystal lattice release or absorb latent heat, depending on which part of the cycle the material is in. When pre-stressed nitinol wires are unloaded at room temperature, they cool down by as much as 20 degrees. This phenomenon makes it possible to remove heat from the system. When the wires are mechanically loaded they heat up by a similar amount, so that the process can also be used as a heat pump.

This effect has been exploited by the Saarbrücken researchers who have developed an environmentally friendly heating and cooling system that is two to three times more efficient than conventional heating and cooling devices. The EU Commission and the US Department of Energy have both assessed the new process and consider it to be the most promising alternative technology to existing vapour compression refrigeration systems. The prototype is the first continuously operating machine that cools air using this process. The team has designed and developed 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 fibre bundles in two separate chambers: in one chamber the air is heated, in the other it is cooled. The device can therefore be operated either as a heat pump or as a refrigerator.

(Source: HVAC&R News, March 21, 2019)