As vital as they are, refrigeration and air conditioning are known to consume huge amounts of electricity, inspiring engineers to come up with new ways to improve efficiency. To help with that, researchers have developed an unusual new technique that could lead to "twist fridges", which cool by unravelling fibers that are tensely twisted.
The new design is based on a principle called elastocaloric cooling. In simple terms, a rubber band will heat up when it’s stretched, and then cool down again when it’s allowed to relax. The same seems to apply to twisting and untwisting the material, which the researchers call “twistocaloric cooling.”
“This elastocaloric behavior of natural rubber has been known since the early 1800s,” says Ray Baughman, corresponding author of the study. “But to get high cooling from a rubber band, you have to release a very large stretch. With twistocaloric cooling, we found that all you have to do is release twist.”
In tests, the researchers stretched out rubber fibers, then twisted them so much they became “supercoiled.” They found that by quickly releasing the twist, they could cool the surface of the rubber by as much as 15.5° C (27.9° F). When they released both the twist and stretch at the same time, that cooling effect was bumped up to 16.4° C (29.5° F).
But it’s not just rubber – the team found that the twistocaloric cooling effect also worked on materials like fishing line and nickel titanium wire. Releasing the stretch of a coiled piece of fishing line cooled the surface by 5.1° C (9.2° F). The nickel titanium performed better, achieving a maximum cooling of 17° C (30.6° F) by untwisting a wire. This could be pushed even further by unplying bundles of wires – a bundle of four nickel titanium wires, for instance, cooled the surface by 20.8° C (37.4° F).
Next, the team demonstrated how the effect could cool neighboring substances, not just the surface of the material itself. Unplying a bundle of three nickel titanium wires inside a device cooled a stream of water by up to 7.7° C (13.9° F).
The team says that devices that cool using this technology – which they dub “twist fridges” – could be a possibility in the future, but there are plenty more hurdles to leap over before then. And by coating fibers in thermochromic paint – which changes color as the temperature changes – sensors could be made that monitor strain and twist in materials.
“Many challenges and opportunities exist on the path from these initial discoveries to the commercialization of twist fridges for diverse large- and small-scale applications,” says Baughman. “Among the challenges are the need to demonstrate refined devices and materials that provide application-targeted cycle lifetimes and efficiencies by recovering part of the inputted mechanical energy. The opportunities include using performance-optimized twistocaloric materials, rather than the few presently studied commercially available candidates.”
The research was published in the journal Science.
Source: University of Texas at Dallas
