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Cooling with “muscle”

Doctoral research students Felix Welsch and Susanne Marie Kirsch with the prototype “muscle” machine. Photo: Oliver Dietze

GERMANY: Engineers at Saarland University are to exhibit a prototype heating and cooling system using shape memory materials at next month’s Hannover Fair.

The machine is able to transfer heat using “muscles” made from nickel-titanium. Also known as nitinol, nickel-titanium 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. This effect has been exploited by the research team led by Professors Stefan Seelecke and Andreas Schütze who have developed an environmentally friendly heating and cooling system that is said to be two to three times more efficient than conventional heating and cooling devices.

The EU Commission and the US Department of Energy are both said to have assessed the new process and consider it to be the most promising alternative technology to existing vapour-compression refrigeration systems.

Principle

The underlying principle is simple and essentially involves subjecting a particular shape-memory alloy – 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,” explained Professor Stefan Seelecke. The effect is said to be particularly pronounced in wires made from nickel-titanium. 

The research has found that when pre-stressed nitinol wires are unloaded at room temperature, they cool down by as much as 20º. 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.

The prototype is claimed to be 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 200micron-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.

Efficient

The system is said to be highly efficient. Depending on the alloy used, the heating or cooling power of the system is up to 30x greater than the mechanical power required to load and unload the alloy wire bundles. According to researchers, this already makes the new system at least twice as good as a conventional heat pump and 3x 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 vapours. So the air in an air conditioning system can be cooled directly without the need for an intermediate heat exchanger, and we don’t have to use leak-free, high-pressure piping,” explained Professor Seelecke.

Optimising

The team is currently working on further optimising heat transfer within the system in order to boost the efficiency of the new technology even more. The objective is to get to a stage where almost all of the energy from the phase transition is being used for heating or cooling.

The team of Saarbrücken engineers will be exhibiting the technology at this year’s Hannover Fair from April 1-5 on the Saarland Research and Innovation stand

Related stories:

Developing refrigeration with “muscle”3 February 2016
GERMANY: Refrigeration using shape memory materials could be the energy efficient, refrigerant-less means of cooling in the future, according to scientists. Read more…

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