It sounds crazy: a refrigerator made from a rubber band. But if you stretch one and hold it against your lips, it will be noticeably warmer. Release it, and it cools. This simple “elastocaloric” effect can transfer heat in much the same way as compressing and expanding a fluid refrigerant in a fridge or air conditioner. Now, scientists have created a version that not only stretches the rubber band, but also twists it. It may one day lead to greener cooling technology.
To find out how twisting might enable a new kind of fridge, engineering graduate student Run Wang at Nankai University in Tianjin, China, and colleagues compared the cooling power of rubber fibers, nylon and polyethylene fishing lines, and nickel-titanium wires. For each material, they pulled a 3-centimeter length taut in a vise and began to wind it with a rotary tool. The fibers not only twisted, but also began to coil around themselves—and coil around the coils (a process known as “supercoiling”). The different fibers warmed up by as much as 15°C. When allowed to unwind, the fibers cooled by the same amount.
To understand why the materials warmed when twisted, researchers peered into the molecular structure of each fiber using bright x-ray beams. The mechanical stresses of twisting rearranged molecules into a more ordered state. The total order in the system does not change, so the trade-off is an increase in the molecular vibrations, which means a higher temperature.
By twisting and untwisting the fibers in a water bath, the researchers could measure their performance as coolants. For the rubber fiber, they measured a heat exchange of about 20 joules of heat energy per gram—up to eight times more energy than the rotary tool expended. The other fibers performed about as well. That level of efficiency is comparable to that of standard refrigerants and twice as high as stretching the same materials without twisting, the researchers report today in Science. “That would definitely be a high-performance system,” says Kurt Engelbrecht, an elastocaloric cooling expert at the Technical University of Denmark in Roskilde who was not involved in the study.
The setup would avoid the need for fluid refrigerants that can leak and contribute to global warming. Although manufacturers have phased out ozone-destroying chlorofluorocarbons, the replacement chemicals used in most systems today are still greenhouse gases, many times more powerful than carbon dioxide.
A twisty cooling system would also be physically more compact than a pure-stretch system. To get a high degree of cooling in rubber without twisting, for example, it typically has to be stretched to seven times its length, says Ray Baughman, a physicist at the University of Texas in Dallas, and an author on the paper.
As a demonstration, the researchers built a tiny fridge about the size of a ballpoint pen cartridge powered by twisted nickel titanium wires. Using this “twistocaloric” method, they cooled a small volume of water by 8°C in a few seconds. Next, the team plans to run the device on a repeating cycle, alternately heating the water (and moving that heat to the outside world) and cooling it (so that it can absorb heat from the interior volume). Coated with temperature-sensitive dyes, the fibers could also serve as strain gauges or mood rings.
If researchers can scale up the technology, it may give new meaning to unwinding with a cold beer.