Saarland University researchers have developed an air conditioning system that transfers and absorbs heat using ‘muscles’ made from nickel-titanium.
Nickel-titanium or Nitinol is a shape-memory material that releases heat when it is mechanically loaded in its superelastic state, and absorbs heat from its surroundings when it is unloaded.
It is due to this unusual property that nitinol is often referred to as a ‘smart alloy’ or ‘muscle wire.’
Saarland Professors Stefan Seelecke and Andreas Schütze have now exploited this effect to develop an air conditioning system that is 2-3 times more efficient than conventional heating and cooling devices.
Both the EU Commission and the US Department of Energy have assessed the invention and consider it to be the most promising alternative to existing vapor-compression refrigeration systems.
In order to cool something down, you need to remove heat from it. And in order to warm something up, thermal energy has to be supplied to it.
The Saarland University prototype system does both these things effortlessly.
The system transports heat using a novel method that avoids the problems and disadvantages associated with conventional heating and cooling systems.
It subjects a particular shape-memory alloy (SMA) – in this case nickel-titanium – to controlled loading/unloading cycles.
“The resulting 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,” explains Seelecke.
This effect is particularly pronounced in wires made from nickel-titanium.
“When pre-stressed nitinol wires are unloaded at room temperature, they cool down by as much as 20 degrees,” says Felix Welsch who has been working on the prototype with his team colleague Susanne-Marie Kirsch.
This phenomenon makes it possible to remove heat from the system.
The prototype is also the first continuously operating machine that cools air using this process.
The team designed and developed a 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, in the other it is cooled.
The highly efficient device can therefore be operated either as a heat pump or as a refrigerator.
Image and content: Oliver Dietze/Saarland University