UCLA materials scientists have developed a compact cooling technology that can pump away heat continuously using layers of flexing thin films. The design is based on the electrocaloric effect, in which an electric field causes a temporary change in a material’s temperature.
In lab experiments, the researchers found that the prototype could lower ambient temperatures of its immediate surroundings by 16 degrees Fahrenheit continuously and up to 25 degrees at the source of the heat after about 30 seconds.
Detailed in a paper published in the journal Science, the approach could be incorporated into wearable cooling technology or portable cooling devices.
“Our long-term goal is to develop this technology for wearable cooling accessories that are comfortable, affordable, reliable and energy-efficient — especially for people who work in very hot environments over long hours,” said principal investigator Qibing Pei, a professor of materials science and engineering at the UCLA Samueli School of Engineering. “As average temperatures continue to rise with climate change, coping with heat is becoming a critical health issue. We need a variety of solutions to the problem and this could be the basis for one.”
The experimental material is composed of a circular stack of six thin polymer films, just under an inch in diameter and one-quarter of an inch thick for the entire stack. Each layer is coated with carbon nanotubes on both sides. The resulting material is ferroelectric, which means it changes shape when an electric field is applied.
When the device’s electric field is switched on, the stacked layers compress against each other in pairs. When the electricity switches off, the stacked pairs come apart to then press against the other neighboring layers. As this alternating process repeats itself, the self-regenerative, accordion-like cascading action continually pumps heat away, layer by layer.
“The polymer films use a circuit to shuttle charges between pairs of stacked layers, which makes the flexible cooling device more efficient than air conditioners,” said Hanxiang Wu, one of the study’s co-lead authors and a postdoctoral scholar working in Pei’s lab.
“This cooling device integrates advanced materials with an elegant mechanical architecture to deliver energy-efficient cooling by embedding functionality directly into its structure, reducing complexity, energy use and computational demands,” said the study’s co-lead author Wenzhong Yan, a postdoctoral scholar in mechanical engineering.Traditional cooling technology relies on air conditioning and refrigeration, which require vapor compression that not only consumes a great deal of energy but also uses carbon dioxide as a coolant. The new device is a simpler design that does not require greenhouse-gas-generating coolants or liquids. It operates solely with electricity, which can be sustainable when generated through renewable energy sources such as solar panels.
Pei holds a joint faculty appointment in the Department of Mechanical and Aerospace Engineering and runs the Soft Materials Research Laboratory at UCLA. He and his team have been researching electrocaloric cooling technologies designed to drop enough temperatures for real-world applications.
“Because we can use thin flexible films, electrocaloric cooling would be most ideal for next-generation wearables that can keep us cool under strenuous conditions,” Pei said. “It could also be used to cool electronics with flexible components.”
Sumanjeet Kaur, a materials staff scientist at Lawrence Berkeley National Laboratory and leader of its Thermal Energy Group, is another author of the study and a co-inventor on the patent application UCLA has filed for this invention. “The potential of efficient wearable cooling in driving energy savings and mitigating climate change cannot be overstated,” Kaur said.
In addition to Wu and Yan, UCLA Samueli graduate student and a member of Pei’s research group Yuan Zhu is another co-lead author. Other authors are materials science graduate students Siyu Zhang, William Budiman, Kede Liu, Jianghan Wu and former materials science postdoctoral scholar Yuan Meng — all members of Pei’s research group; bioengineering graduate student Xun Zhao; and Ankur Mehta, a UCLA associate professor of electrical and computer engineering.
The research was funded by the U.S. Office of Naval Research, the Laboratory Directed Research and Development Program at Lawrence Berkeley National Laboratory, the California NanoSystems Institute (CNSI) at UCLA and the Defense Advanced Research Projects Agency. The study used instruments at the CNSI cleanroom and the Electron Imaging Center for NanoMachines at UCLA.