The cooling system powered directly by heat rather than an electric motor has worked in laboratory tests, offering a possible way to turn waste heat or solar energy into cooling. The prototype, developed by researchers at Karlsruhe Institute of Technology (KIT) Germany, and the University of Tsukuba, Japan, uses two ultrathin nickel-titanium films that convert heat first into mechanical motion and then into cold. The concept addresses a limitation of elastocaloric cooling, an emerging solid-state alternative to conventional refrigeration.
Shape-memory alloys cool when a mechanical load applied to them is released, but existing elastocaloric systems still need electrically powered actuators to supply that force. The new design instead uses heat itself to drive the process. That distinction matters because cooling and heating account for almost half of global energy consumption as demand continues to grow. Conventional refrigerators, air conditioners, and data centers have relied for more than a century on electricity-driven compressors that move heat with refrigerants, many of which also contribute to global warming.
The system pairs two nickeltitanium films with different jobs. When heated, the first film shrinks through a shapememory effect, converting thermal energy directly into mechanical work without an electric motor. That movement acts on the second film. Repeated loading and unloading cause reversible changes in its crystal structure that produce cooling. In effect, the first film replaces the electrically driven actuator that elastocaloric systems have previously required. “The crucial innovation is that we combine two complementary functions of shape memory alloys, with one film converting heat into mechanical work and the other film converting this work into cold.
“This way, we’re establishing a new approach to drive solidstate cooling, thereby opening up exciting possibilities for the use of waste heat and solar energy,” said Dr. Jingyuan Xu, who leads the Young Investigator Group of the ZEco Thermal Lab at KIT’s Institute of Microstructure Technology (IMT). In experiments, the prototype produced measurable cooling. At an actuator temperature of 86° Celsius (187° Fahrenheit), it achieved a temperature difference of 4° Celsius (7.2° Fahrenheit) at the component level, while the elastocaloric refrigerant underwent a temperature change of nearly 13° Celsius (23.4° Fahrenheit).
The system also operated reliably using an external heat source at 130° Celsius (266° Fahrenheit), demonstrating that the concept can function with real-world heat sources. “The decisive moment for us was when we were able to measure the cold that had indeed been generated by a heatdriven system. This showed us that the principle doesn’t just work in theory,” said Yi-Ting Hsiau, lead author of the study and doctoral researcher at the IMT.
