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."

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.

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