Solid-State Cooling When Heat Turns to Cold

Researchers at KIT and the University of Tsukuba have developed a cooling system that uses heat as a source of power for refrigeration. The elasto-caloric prototype could harness waste heat and solar energy for more energy-efficient cooling.

Researchers at the Karlsruhe Institute of Technology (KIT) (Germany) and the University of Tsukuba in Japan have developed a heat-driven elasto-caloric cooling system. The prototype uses two ultra-thin films made of a nickel-titanium shape-memory alloy to convert thermal energy first into mechanical work and then into cooling. This allows waste heat or solar energy to be used directly for solid-state cooling. 

Heat Instead of Electricity as a Power Source

Refrigerators, air conditioners, and data centers consume large amounts of energy. Conventional cooling systems typically use an electrically powered compressor that transfers heat from one place to another using a refrigerant. Many of the refrigerants used can also contribute to global warming.

One possible alternative is elastocaloric cooling. In this process, shape-memory alloys change their temperature when they are subjected to mechanical stress and then released. Until now, however, even such systems required an electrically powered actuator to generate the necessary force. As a result, heat sources such as industrial waste heat or solar energy could not be used directly as a power source.

The new design combines two films with different functions. When the first nickel-titanium film is heated, it contracts due to the shape memory effect. In doing so, it converts thermal energy directly into mechanical work. This movement is transferred to the second film.

Two Films, Two Functions

The second film acts as an elasto-caloric refrigerant. Repeated mechanical loading and unloading causes its crystal structure to change reversibly. This results in a measurable temperature change that can be used for cooling.

“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 cold,” says Dr. Jingyuan Xu, head of the Young Investigator Group at the ZEco Thermal Lab at the Institute for Microstructure Technology (IMT) at KIT.

In this way, heat replaces the electrically powered actuator that has previously driven elastocaloric cooling systems. The basic principle could thus also be applied in situations where waste heat or solar-generated heat is available.

Initial Cooling Capacity in the Laboratory

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 s (186.8 degrees Fahrenheit). The elasto-caloric refrigerant itself exhibited a temperature change of just under 13 degrees Celsius (55.4 degrees Fahrenheit). The researchers thus experimentally demonstrated the fundamental feasibility of the concept.

The setup also functioned reliably when using an external heat source with a temperature of 130 degrees Celsius. “For us, the decisive moment was seeing measurable cold 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.

The current setup serves primarily as a proof of concept and is not yet designed for maximum cooling capacity. The research team is therefore investigating how multiple films can be connected in parallel. This is expected to increase the cooling capacity.

Prospects for Waste Heat and Solar Energy

Possible applications include, among other things, the cooling of processors and other electronic components. In the future, computers could use at least some of their own waste heat for the cooling process. Vehicles could also benefit from this approach if heat from the powertrain is used to cool sensitive components.

Further development is being carried out in collaboration with the University of Tsukuba. The goal is to scale up the system and develop compact cooling solutions for various heat sources. The work was funded by, among others, the Carl Zeiss Foundation, the Baden-Württemberg Foundation (Germany), and the Hector Fellow Academy.

The original paper, “Heat-driven elastocaloric cooling with shape memory films,” was published in Nature Energy. DOI: 10.1038/s41560-026-02122-6