Whether in a refrigerator, an air conditioner, or a data center: cooling has been based on the same basic principle for more than a hundred years. An electric-powered compressor uses a refrigerant to transfer heat from one location to another. As the demand for cooling continues to grow, cooling and heating now account for nearly half of global energy consumption. Many common refrigerants also contribute to global warming. Elasto-caloric cooling is considered a promising solid-state alternative: shape-memory alloys cool down as soon as a previously applied mechanical stress is released. Until now, however, even elasto-caloric systems required an electrically powered actuator to generate the necessary force—which meant they could not directly utilize abundant heat sources such as waste heat or solar energy.

Heat Instead of Electricity as a Power Source

This is precisely where the new concept developed by a research team at the Karlsruhe Institute of Technology (KIT) and the University of Tsukuba in Japan comes in: It couples two ultra-thin nickel-titanium films with complementary functions. The first film utilizes a shape-memory effect: When heated, it contracts and converts thermal energy directly into mechanical work—without any electric motor at all. This movement is immediately transferred to the second foil, where repeated loading and unloading trigger reversible changes in the crystal structure that generate cold. In this way, heat replaces the electrically driven actuator that previously powered elasto-caloric cooling systems.

“The key innovation is that we combine two complementary functions of shape-memory alloys, so that one film converts heat into mechanical work, while the other converts that work into cooling,” says Dr. Jingyuan Xu, Young Investigator Group Leader at the ZEco Thermal Lab at the Institute for Microstructure Technology (IMT) at KIT. “With this, we are establishing a new way to drive solid-state cooling and opening up exciting possibilities for utilizing waste heat and solar energy.”

Initial cooling performance confirmed in the lab

In the prototype, the system achieved a temperature difference of four degrees Celsius at the component level at an actuator temperature of 86 degrees Celsius, while the elasto-caloric refrigerant exhibited a temperature change of just under 13 degrees Celsius. This provided the first experimental proof of the concept’s feasibility. The setup also functioned reliably with an external heat source of 130 degrees Celsius—proof that the system can operate with real-world heat sources. “For us, the decisive moment was seeing measurable cooling for the first time that was actually generated by a heat-driven system,” says Yi-Ting Hsiau, first author of the study and a doctoral student at IMT. “This showed us that the principle works not only on paper.”

The current setup is intended as a feasibility study and has not yet been optimized for maximum cooling performance. The team is already working on connecting multiple films in parallel to increase cooling performance. Potential applications range from cooling computer processors—which could use their own waste heat for this purpose—to cooling sensitive electronics in vehicles using heat from the powertrain.

The work was conducted in collaboration with the University of Tsukuba in Japan and paves the way for practical, heat-driven solid-state cooling. “We believe this is just the beginning,” says Xu. “By scaling up this technology, we aim to develop compact cooling systems that utilize abundant heat sources for sustainable cooling.”

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