Fridges, A/C systems, and datacenters consume enormous amounts of electricity and the cooling demand is constantly on the rise. A team of research from the Karlsruhe Institute of Technology (KIT) and the University of Tsukuba in Japan is now presenting a new approach substantiated by a prototype where heat assumes the role formerly taken by an electric motor. Two ultra-thin shape-memory films interact to convert thermal energy into mechanical work in a first step and finally into measurable cold. Their work opens up new possibilities for the future use of waste heat and solar energy for solid-state cooling. The results have been published in Nature Energy. (DOI: 10.1038/s41560-026-02122-6)
The basic cooling principle for fridges, A/C systems, or datacenters has been the same for more than a hundred years. An electricity-driven compressor transfers heat carried by a refrigerant from one location to another. Since the cooling demand is constantly on the rise, cooling and heating meanwhile account for almost half of the global energy consumption. This is aggravated by the fact that many common refrigerants contribute to global warming. Elastocaloric solid-state cooling is considered a promising alternative: Shape-memory alloys tend to cool down once a previously applied mechanical load is released. However, even elastocaloric systems have so far relied on an electrically driven actuator to generate the required force – which means that they could not directly use abundantly available heat sources such as waste heat or solar energy.
Driven by Heat instead of Electric Power
This is exactly where the new approach developed by the research team comes into play, coupling two ultra-thin nickel-titanium films that have complementary functions. The first film uses a shape-memory effect: Once it is heated up, it starts to shrink, thereby converting thermal energy directly into mechanical work – without the help of an electric motor. This motion immediately transfers to the second film where cyclic loading and unloading brings about reversible alterations in the crystal structure that generate cold. Thus, heat replaces the electrically driven actuator, which was previously used to drive elastocaloric cooling systems.
“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,” said Dr. Jingyuan Xu who leads the Young Investigator Group of the ZEco Thermal Lab at KIT’s Institute of Microstructure Technology (IMT). “This way, we’re establishing a new approach to drive solid-state cooling, thereby opening up exciting possibilities for the use of waste heat and solar energy.”
First Cooling Capacity Values Confirmed in the Lab
At an actuator temperature of 86° Celsius, the prototype of this system achieved a temperature difference of 4° Celsius on the component level, while the temperature change in the elastocaloric refrigerant amounted to nearly 13° Celsius. This way, the researchers were able to prove the feasibility of their concept in experiment for the first time. The setup also worked reliably with an external heat source that provided 130° Celsius, demonstrating that the system is capable to work 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 heat-driven system,” said Yi-Ting Hsiau, lead author of the study and doctoral researcher at the IMT. “This showed us that the principle doesn’t just work in theory.”
The current setup has been designed as a feasibility study and is therefore not optimized for a maximum cooling capacity yet. The team is already working on connecting multiple films in parallel to increase the cooling capacity. Potential applications range from cooling of processors in computers, which could use their own waste heat for this purpose, to cooling of sensitive electronics in automobiles using the heat from the drive train.
The study was conducted in collaboration with the University of Tsukuba in Japan, paving the way for heat-driven solid-state cooling that is fit for use in practice. “We believe that this is only the beginning,” said Xu. “By scaling up this technology we want to develop compact cooling systems that leverage abundantly available heat sources for sustainable cooling.”
Funds for this study came from the Carl Zeiss Foundation (CZS Nexus project), the Baden-Württemberg Foundation (elite postdocs program), and the Hector Fellow Academy.
In close partnership with society, KIT develops solutions for urgent challenges – from climate change, energy transition and sustainable use of natural resources to artificial intelligence, sovereignty and an aging population. As The University in the Helmholtz Association, KIT unites scientific excellence from insight to application-driven research under one roof – and is thus in a unique position to drive this transformation. As a University of Excellence, KIT offers its more than 10,000 employees and 23,000 students outstanding opportunities to shape a sustainable and resilient future. KIT – Science for Impact.