You can't brighten a room with darkness or dry a towel by soaking it. But it turns out you can use heat to power a cooler. Researchers have built a solid-state device that uses heat to drive its cooling cycle. The technology could eventually allow processors and other electronics to use some of their waste heat for cooling.
Researchers at Germany's Karlsruhe Institute of Technology (KIT) and Japan's University of Tsukuba created the experimental cooler by connecting two extremely thin shape-memory alloy films. One converts heat into mechanical movement, and the other uses that movement to transfer heat away from the area being cooled. The work was published in Nature Energy.
Most refrigerators and air conditioners use vapor-compression cooling. An electrically powered compressor squeezes a refrigerant gas, raising its pressure and temperature. The hot refrigerant sheds heat to its surroundings before passing through an expansion device that lowers its pressure and temperature. It can then absorb heat from the space being cooled before returning to the compressor.
Solid-state cooling takes a different approach. Instead of using a compressor to circulate a refrigerant through repeated changes between gas and liquid, it relies on solid materials to move heat. Thermoelectric coolers use an electric current to move heat across semiconductor junctions. Magnetocaloric materials respond to magnetic fields, while electrocaloric materials respond to electric fields.
Elastocaloric cooling is driven by mechanical stress. Certain shape-memory alloys, including nickel-titanium-based materials, change their crystal structure when stretched or compressed. Loading the material can trigger a shift from one solid crystal structure to another, releasing heat. After that heat escapes, removing the load reverses the change and cools the material. The cooled material can then absorb heat from its surroundings.
Something must keep stretching and releasing the refrigerant. Current prototypes typically use electric motors or hydraulic actuators to generate the forces required. The researchers' solution was to use a second shape-memory alloy film as the actuator, the component that produces movement. Their prototype connects a 22-micrometer-thick titanium-nickel actuator film to a 26.5-micrometer-thick titanium-nickel-iron refrigerant film.
The actuator contracts when heated, pulling on the refrigerant film. The stretched refrigerant releases heat, which is carried away by a heat sink. As the actuator is cooled, the pull on the refrigerant weakens. The refrigerant returns to its previous length and cools, then absorbs heat from the part of the device being cooled. Repeating these steps moves heat from the cold side of the cooler to the hot side.
The researchers first tested the system by passing an electric current through the actuator to heat it. At an average peak actuator temperature of 86 degrees Celsius, the refrigerant film reached a temperature span of 12.9 kelvins. The temperature difference between the hot and cold sides of the complete device was 4.0 kelvins.
The team then used an external solid heat source held at 130 degrees Celsius to heat the actuator through direct contact. After 20 cycles, the cooler reached a steady temperature difference of 2.2 kelvins across the device. The hot side rose about 1.2 degrees Celsius, while the cold side dropped about 1.0 degree. In a separate measurement at zero temperature difference between the two sides, the device produced 2.09 milliwatts of cooling power, or 3.32 watts per gram of refrigerant.
The external heat supplied the force needed to stretch the film, while the lab setup still used electrically powered equipment to move the heat exchangers. The prototype's milliwatt-scale output is far below what's needed to cool a powerful computer processor. The experiment shows that a heat-driven film can do the mechanical work required for elastocaloric cooling. Putting that principle to work in electronics will require a device that can move much more heat.