Materials

Shape memory alloys the basis for more efficient refrigerant-free cooling

Shape memory alloys the basis ...
Engineers Marvin Schmidt and Johannes Ullrich Own with their prototype cooler
Engineers Marvin Schmidt and Johannes Ullrich Own with their prototype cooler
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Diagram of the shape-memory cooler
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Diagram of the shape-memory cooler
Engineers Marvin Schmidt and Johannes Ullrich Own with their prototype cooler
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Engineers Marvin Schmidt and Johannes Ullrich Own with their prototype cooler

By preserving our food and keeping our buildings comfortable in hot weather, mechanical cooling systems have been a boon, but with their refrigerant gases and high power consumption they're not exactly environmentally friendly. In an effort to make a greener, more energy efficient cooling system, a team of engineers from Germany's Saarland University is turning to shape memory materials to replace the refrigerant gases used in conventional cooling technologies.

Shape memory materials, also known as "metal muscles" or "artificial muscles," have the ability to snap back into their original shape after being stretched, squashed, bent, or otherwise deformed. If the material is a metal alloy, such as nickel-titanium, the deformation changes the material's crystal lattice structure in what's known as phase transition, which causes the material to become hotter. Allowing the material to relax and return to its original form cools it by about 20° C (36° F) below the ambient temperature.

"In our systems, shape memory alloys (SMAs) are used to remove heat," says Stefan Seelecke, Professor for Intelligent Material Systems at Saarland University. "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."

Diagram of the shape-memory cooler
Diagram of the shape-memory cooler

A prototype air cooling system using nickel-titanium alloy is under development by teams from Saarland University, the Center for Mechatronics and Automation Technology (ZeMA), and Ruhr University Bochum led by engineers Seelecke and Andreas Schütze. According to Seelecke, the basic idea is to cool a space by prestressing a super-elastic shape memory material, then letting it relax and release the heat to the external environment. The material is then re-stressed to repeat the cycle.

The purpose of the prototype is to demonstrate the principle and to help figure out how to make the process as efficient as possible. In experimental and modeling studies conducted to determine the system's optimum efficiency, the team examined various factors while monitoring the device using a thermal imaging camera, such as how much to deform the material and how fast the process should cycle.

"We're currently using these results to construct an optimized prototype for an air-cooling system," says Schütze. "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. Further optimization of the cooling process will involve modelling all component stages and then refining these models by comparing the predictions with experimental results. The data from the modelling 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."

With refrigerators a household staple and the popularity of air conditioners in homes, businesses and commercial properties, the potential benefits of a cooling technology that is more energy efficient and doesn't rely on refrigerants that are harmful to the environment appear obvious.

The project has been granted an additional €500,000 (US$560,000) by the German Research Foundation, which has been funding the project for the past three years.

Source: Saarland University

8 comments
Mr T
I must be missing something here. The energy required to deform the material would be greater than the heat energy moved by the device, given that there's also energy required to rotate the bundle. This would give the system an efficiency below 100%. Given that most refrigeration systems range from 200 to 500% or greater (a CoP somewhere between 2 and 5), how is this an improvement?
Perhaps the system is just not well explained, but I can't see the value in throwing yet another half a million at something that can never approach the efficiency of existing systems.
Mel Tisdale
It is a long time since I studied thermodynamics so will take Mr T's comment at face value. As far as I can see the main benefit to this system in not to be found with regard to energy efficiency, but in the absence of a refrigerant.
On reflection, if this system could take its energy input from a vehicle's suspension, it might usefully provide a novel climate control system, being able to provide both warmth as well as cooling depending on air-flow management.
Bob Stuart
I don't see a theoretical limit to the efficiency of heat separators. For comparison I'd sure like to see the numbers on a conventional one with an air motor instead of an orifice for expansion. Turbulence makes heat.
Sergius
I believe compression refrigeration systems may be replaced by absorption systems if they find a less toxic chemical pair and aggressive than the former and popular chemical pair ammonia / water. This, to me, would be the greatest ecological and energy-saving step that scientists of "smart" materials should give.
ADVENTUREMUFFINffin
Yes, I am mystified as well as to how this may work. The latest refrigerant that is being used successfully, getting 4 to 5 times the energy that is put in by using the vapor compression cycle is a CO2 heat pump. Sanden is one such manufacturer.
Bob Flint
Seems to me the effort is to remove any need for chemicals in cooling. Through the study of expansion & contraction of this type of device as it is rotating. The lack of explanation from were does the mechanical stress originate from is the missing link...
For example a vertical cylinder sitting in the sun, one side will heats up and expands while the shade side contracts as it rotates around slowly either mechanically driven with solar electrical or wind power. Internally the air flow controlled container keeps the inner contents chilled with the differing air flow.
Still not convinced that the mechanical advantage will ever surpass the chemical evaporative efficiencies we have today, but interesting non the less.
Picture a large metal thermos with outer skin comprised of vertical memory material. Under the skin air flows through in alternating waves, thermal transfer to the inner chamber lowers the temperature. By the way Herr Schütze for more feed back bobflint@excite.com
Douglas Bennett Rogers
This will have a very demanding high cycle fatigue requirement.
MadMaxx
Shades of the old nichrome wire heat energy engines. Their problem was durability. Wonder if that is true here.