Researchers from the Karlsruhe Institute of Technology and Tsukuba University have developed a solid-state cooling system that directly uses waste heat to power its cooling cycle.
Cooling using heat sounds quite counterintuitive at first glance. However, scientists from the Karlsruhe Institute of Technology (KIT) in Germany and Tsukuba University in Japan have developed an experimental system that does exactly this. Researchers enabled one material to convert heat into mechanical motion, and another material to convert this motion into cooling.
Based on the elastocaloric cooling method
At the heart of the study is a method called elastocaloric cooling. While traditional refrigerators and air conditioners perform cooling through electrically powered compressors and refrigerants, elastocaloric systems utilize the temperature change of special materials that are mechanically stretched or compressed.
For this, researchers utilized shape memory alloys. When these materials are stretched or compressed, a change occurs in their crystal structures. When the mechanical load on the material is removed, the structure reverses, and heat absorption takes place during this process. Thus, the material can act as a solid-state coolant.
However, this method has a significant problem: the alloy needs to be continuously stretched and released. In existing elastocaloric cooling systems, this is typically performed by electric motors or hydraulic actuators. Therefore, even if the refrigerant material in the system is solid, electrical energy is required to operate the cycle.
Heat powers the cooling cycle on its own
Researchers from KIT and Tsukuba University introduced a different solution to this problem. The system uses another shape memory alloy to move the refrigerant alloy. In the experimental device, a 22-micrometer-thick titanium-nickel (TiNi) film acts as a thermal actuator. A 26.5-micrometer-thick titanium-nickel-iron (TiNiFe) film, mechanically connected to it, serves as the coolant.
When the TiNi film is heated, it contracts. When the heat is removed, it returns to its predetermined shape, stretching the TiNiFe film. This stretching initiates a stress-induced phase transformation in the refrigerant alloy, causing the material to release heat. After the released heat is transferred to a heat sink, the actuator cools down and relaxes. As a result, the mechanical load on the TiNiFe decreases, the alloy returns to its original crystal structure, and during this process, it absorbs heat from its surroundings, thus cooling down. When the actuator is reheated, the cycle restarts.
Jingyuan Xu, who leads the research, explains the system's fundamental innovation as the combination of the complementary properties of two different shape memory alloys. One film converts heat into mechanical work, while the other film converts this mechanical work into cold.
2.2 K temperature difference achieved in experiments
In initial tests, researchers heated the TiNi actuator with electrical resistance. When the actuator's average maximum temperature reached 86 °C, the TiNiFe coolant alone created a 12.9 K temperature difference. Considering heat exchangers and other system components, a total temperature difference of 4.0 K was achieved in the entire device after 20 cycles.
In a more notable experiment, researchers completely eliminated electrical heating and heated the actuator with an external solid heat source at 130 °C. The system continued to operate under these conditions, creating a 2.2 K temperature difference. In this test, the hot side of the device became approximately 1.2 °C warmer than the initial temperature, while the cold side became approximately 1.0 °C cooler. The achieved cooling power was 2.09 mW, reaching a value of 3.32 W/g per active refrigerant material.
Aiming to increase cooling capacity
The prototype, currently a feasibility study, is not optimized for maximum cooling capacity. In the next phase, researchers aim to increase the system's cooling capacity by connecting multiple films in parallel.
The technology's potential application areas are quite broad. For instance, waste heat generated by computer processors could be used to cool the processor itself. Similarly, heat generated by automotive powertrains could be utilized for cooling sensitive electronic components.
Furthermore, it might be possible to transfer heat obtained from solar energy to the system. This could open the door to developing more sustainable and compact cooling systems that operate without the need for electrical energy.
Researchers aim to develop compact solid-state cooling systems that can utilize widely available waste heat sources as the technology scales up.
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