Climate-friendly refrigeration with a twist

18 October 2019

ORANGE-FRIDGE

Orange freezer-fridge with the upper freezer door half open

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Twisting and untwisting fibres heats them up and cools them down, and this may hold the key to a new generation of fridges...

We’ve known since 1805 that stretching and then releasing a rubber band causes it to heat up and then cool down. Now, a new study shows that twisting and untwisting materials, including rubber bands, fishing lines, and nickel-titanium wires, produces this effect as well.

Stretch-based refrigerators, also known as “elastocaloric” refrigerators, have been proposed previously as a more environmentally friendly alternative to the currently widespread vapour-compression refrigerators, but the team behind the new twist-based study suggest that their concept of a "twistocaloric refrigerator", is an even more effective solution.

Ray Baughman, one of the lead authors of the paper published this week in Science, explains that “by using twist we could make refrigerators that are more efficient than the refrigerators we can make that utilize just stretch and stretch release. There is an enormous need for humankind in improved methods of refrigeration since so much of global electrical energy is consumed by refrigeration.”

A fridge that relies on twist wouldn’t need to be very big either: a prototype built by the team uses twisted fibres immersed in water; a compact simple motor rotates one end of the fibres in alternating directions. Elastocaloric fridges, in contrast, would need a larger amount of space to fit the stretch and release cycle.

Moreover, the twistocaloric fridge was able to achieve about 67% efficiency, which, when compared to that of existing commercial fridges - usually around 60% - is a significant improvement.

In another experiment, the researchers varied the direction in which the fibres were twisted and then coiled into a spring. The direction in which they’re twisted is called chirality, or handedness. By twisting a fibre to maximal twist, and then continuing to coil it in the same direction, we can get a “homochiral” fibre. And if we stretch a homochiral fishing line fibre for example, the cooling that the researchers measured was 50 times higher than if they had just stretched an untwisted fibre.

An even stranger effect happens if we twist in one direction, say clockwise, and then coil the twisted fibre in the opposite direction, or counterclockwise. In this case, the cooling and heating effects of the stretching cycle are now reversed! Stretching cools it down, and releasing heats it up. For these kinds of heterochiral fibres, stretching actually causes an untwisting of the initial fibre, which then has a cooling effect.

“The materials that we investigated here can cool down by as much as 20 degrees just by releasing stretch from them when they’re coiled,” says Ray Baughman. And the materials that they used were off the shelf rubber bands, polyethylene fishing line and NiTi wires. Other materials could have even higher cooling potential, or new materials could be purposely developed to increase this cooling effect.

Behind the cooling and heating effects of stretching or twisting fibres is entropy. In a stretched or twisted state, the long molecules in the materials being studied were more orderly aligned than in a relaxed state, and so they were at lower entropy. And it’s this switching from low entropy to high entropy that causes cooling, and then heating when done in reverse.

This research provides an alternative refrigeration technology that has the potential to be more efficient and also more environmentally friendly. With the increased concern for climate change and the problems caused by refrigerant gases being released into the atmosphere from devices such as fridges and air conditioning, more sustainable options are needed soon.

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