Researchers from UTD’s NanoTech Institute and Nankai University in China discovered a way to increase the efficiency of traditional refrigeration methods through the use of rubber fibers.
Chemistry professor Ray Baughman, the director of the NanoTech institute, and Zunfeng Liu of Nankai University were corresponding authors for the paper, which was published in the Oct. 11 issue of Science. The scientists discovered that rather than compressing gas, as traditional refrigerators do, a more efficient method could be to twist rubber fibers. Untwisting the wires produced a more effective cooling method and didn’t release greenhouse gases like traditional refrigerators.
Baughman said that while this discovery could potentially combat the efficiency issues found with traditional refrigeration, which has an efficiency of less than 60%, it hasn’t been tested in a fully functional fridge.
“Our goal was to get higher efficiency than conventional compressional refrigeration,” Baughman said. “We wanted a cheap, smaller refrigerator, and a smaller volume refrigerator, which this tech allows.”
This project was based off the knowledge gained from a previous study conducted by Baughman and his associates. For over 15 years, Baughman and the Nanotech Institute have been researching the concepts that eventually allowed for this technology to be developed.
“About a year ago, we had a paper in which we took several kinds of fibers like nanotube yarns and regular fishing line,” Baughman said. “We were able to make a mechanical artificial muscle just using those materials. Each discovery is based on the one before that.”
Most air conditioners, heat pumps and refrigerators use vapor compression of refrigerants to provide their temperature altering effects. Air conditioning and refrigerators take up around 20% of the world’s electrical energy according to the International Institute of Refrigeration. Refrigerators also release gases that can pollute the atmosphere and contribute to climate change. Other types of alternative refrigeration techniques such as thermoelectric cooling and hydrostatic pressure haven’t been used on a large scale.
The technology used in this particular study is simple, Baughman said. If you take rubber and stretch it, that decreases entropy, or disorder. Releasing that stretch results in the conversion from low entropy to high entropy. The universe prefers more disorder, or higher entropy, so the release results in cooling.
“The problem with this is you have to stretch the rubber almost 600% to get cooling. We use twist,” Baughman said. “We maintain only a little stretch, we just have to release twist and you get cooling.”
Baughman said that by using coiled or even supercoiled fibers, one can provide the same amount of cooling as a stretched fiber at at approximately 30% the amount of stretch. Releasing the twist and stretch from a supercoiled rubber fiber produces cooling of about 16 degrees Celsius.
“Efficiency is key,” Baughman said. “With stretch you only get half efficiency. The efficiency of our method is at 67%.”
The researchers testing the cooling ability of other materials as well. One of them was nickel titanium wires. These wires were able to reduce the temperature of running water by almost 8 degrees Celsius.
“Think of a metal spring. When you stretch it, it’ll get longer but not because its length is changing,” Baughman said. “It’s because the twist in the coiling is being converted to the twist in the wire.”
Baughman said that there has been industry interest in technology. This material performs better than competing ideas in circulation. The concept of twist-based cooling could also potentially be used to make clothing that keeps the wearer cool.
“We know now that we can take artificial electrochemical energy and harvest mechanical energy from it,” Baughman said. “Who knows what else we can find out from this discovery?”