Solid-state cooler uses heat to make things cold

You can’t brighten a room with darkness, you can’t dry a towel by soaking it, and you surely can’t cool things with heat. Right? Well, maybe. Scientists have just built a solid-state cooling device that can use heat to generate cold, potentially opening a way for processors and other hot-running electronics to recycle some of their own waste heat for cooling.Researchers at Germany's Karlsruhe Institute of Technology (KIT) and Japan's University of Tsukuba created the experimental cooler by coupling two incredibly thin shape-memory alloy films. One converts heat into mechanical movement, while the other converts that movement into cooling. In effect, heat handles the job normally performed by the electrically powered motor or actuator that drives the cooling cycle. The work was published in the journal Nature Energy.Most refrigerators and air conditioners use vapor-compression cooling. An electrically powered compressor squeezes a refrigerant gas, increasing its pressure and temperature. The hot refrigerant then sheds heat to its surroundings, before passing through an expansion device that drops its pressure and temperature. It can then absorb heat from the fridge, room, or other space being cooled before returning to the compressor to repeat the process.Solid-state cooling takes a different route. Rather than relying on a compressor to circulate a refrigerant through repeated gas-liquid phase changes, these technologies exploit materials whose temperature changes in response to some external stimulus. Thermoelectric Peltier coolers, for example, use an electric current to move heat across semiconductor junctions. Magnetocaloric materials respond to magnetic fields, while electrocaloric materials respond to electric fields.Then there is elastocaloric cooling, where the 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 transformation from one solid crystal phase to another, releasing heat in the process. Allowing the heat to escape, then removing the load, causes a reverse phase transition that absorbs heat, making the material colder. Repeating the cycle turns the alloy into a solid-state refrigerant.However, there is a rather inconvenient catch. Something has to keep stretching and releasing the refrigerant. Current elastocaloric prototypes typically use electromechanical motors or hydraulic actuators to generate the substantial forces required to repeatedly deform their shape-memory alloys. So while the refrigerant itself is solid, the cooling system still needs an electrically powered drivetrain to make it work.The KIT and Tsukuba team’s solution is wonderfully simple: use another shape-memory alloy to actuate the first one.Their prototype contains a 22-micrometer-thick titanium-nickel (TiNi) film that acts as a thermal actuator, mechanically coupled to a 26.5-micrometer titanium-nickel-iron (TiNiFe) film that acts as the refrigerant.The TiNi actuator contracts when heated. When the heat is removed, it returns to its preset shape, stretching the TiNiFe refrigerant. That stretching triggers a stress-induced phase transition in the refrigerant, causing it to release heat. After the heat is rejected to a heat sink, the actuator cools and relaxes. The load on the refrigerant falls, its crystal structure reverts, and it absorbs heat and cools.Reheating the actuator again restarts the cycle.In other words, the researchers have taken an elastocaloric refrigerant that needs to be repeatedly stretched and released, paired it with another smart metal that can provide that movement when heated and cooled, and mechanically bound the two materials together.“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 Jingyuan Xu, who leads the Young Investigator Group of KIT’s ZEco Thermal Lab.The researchers first tested the complete system by electrically resistance-heating the TiNi actuator. At an average peak actuator temperature of 86 °C (187 °F), the TiNiFe refrigerant itself produced a temperature span of 12.9 K. Once the heat exchangers and the rest of the prototype were included, the complete device settled at a temperature span of 4.0 K after 20 cycles.They then replaced the Joule heating with an external solid heat source maintained at 130 °C (266 °F), transferring heat directly into the actuator through contact. The cooler continued to work, producing a steady device-level temperature span of 2.2 K, with the hot side sitting about 1.2 °C above its starting temperature and the cold side about 1.0 °C below it. This configuration produced 2.09 milliwatts of cooling power, equivalent to 3.32 W per gram of active refrigerant.Those figures are a reality check on the project's current state. The external-heat prototype’s absolute cooling output is measured in milliwatts, and its 2.2-degree temperature span is tiny compared to the thermal loads handled by modern computer cooling systems. Therefore, nobody will be strapping one of these onto a 500-W GPU anytime soon.The achievement is more of a demonstration of principle. The researchers have experimentally demonstrated that heat alone can provide the mechanical actuation needed to run an elastocaloric cooling cycle. KIT describes the current hardware as a feasibility study that has yet to be optimized for maximum cooling capacity.Source: Karlsruhe Institute of Technology