World's First Heat-Powered Cooling System Turns Waste Heat Into Cold

Researchers at Germany's Karlsruhe Institute of Technology (KIT) and Japan's University of Tsukuba have built what they call the world's first heat-driven elastocaloric cooling system, a prototype that uses waste heat or solar energy rather than an electric motor to produce cooling. The work was published in the journal Nature Energy.

Elastocaloric cooling is an emerging solid-state alternative to conventional refrigeration: certain shape-memory alloys cool down when a mechanical load applied to them is released. Until now, elastocaloric systems still needed an electrically powered actuator to supply that load. Cooling and heating together account for almost half of global energy consumption, and conventional refrigerators, air conditioners and data centers still rely on electricity-driven compressors that use refrigerants, many of which also contribute to global warming.

The new system pairs two ultrathin nickel-titanium films with different jobs. When heated, the first film shrinks through a shape-memory effect, converting thermal energy directly into mechanical work without an electric motor; that motion acts on the second film, and repeated loading and unloading there produces cooling through reversible changes in its crystal structure. "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 of KIT's Institute of Microstructure Technology.

In tests, the prototype produced a temperature difference of 4 degrees Celsius at the component level when its actuator was heated to 86 degrees Celsius, while the elastocaloric refrigerant itself underwent a temperature change of nearly 13 degrees Celsius. The system also worked reliably with an external heat source at 130 degrees Celsius. The current device was built to demonstrate feasibility rather than maximum cooling capacity, and the researchers are now working to connect multiple films in parallel to increase how much cooling the system can provide.

Possible applications include computer processors that use their own waste heat to help cool themselves, and automotive electronics cooled using heat from a vehicle's drivetrain.