Solid-state cooling without electricity

GERMANY/JAPAN: Researchers in Germany and Japan have developed a new approach to solid-state cooling that uses heat instead of electricity.

A prototype developed by teams from the Karlsruhe Institute of Technology (KIT) and the University of Tsukuba in Japan employs two ultra-thin shape-memory films interacting 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 first film of the two, coupled, ultra-thin nickel-titanium films 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. 

In this way, the heat replaces the electrically driven actuator, which was previously used to drive elastocaloric cooling systems.

At an actuator temperature of 86°C, the prototype of this system is said to have achieved a temperature difference of 4°C on the component level, while the temperature change in the elastocaloric refrigerant amounted to nearly 13°C. The setup also worked reliably with an external heat source that provided 130°C, demonstrating that the system is capable to work with real-world heat sources. 

The current system is not yet optimised for a maximum cooling capacity, but 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 Dr Jingyuan Xu, who leads the Young Investigator Group of the ZEco Thermal Lab at KIT’s Institute of Microstructure Technology (IMT). “By scaling up this technology we want to develop compact cooling systems that leverage abundantly available heat sources for sustainable cooling.”