ADVANCED MATERIALS & PROCESSES | APRIL 2024 49 Fig. 1 — Schematic of rotating cage prototype built at Saarland University. PROTOTYPE PROGRESS REPORT: ELASTOCALORIC HEAT PUMPS Researchers around the world are racing to build elastocaloric cooling prototypes. Jun Cui,* Ames National Laboratory and Iowa State University, Ames Stefan Seelecke* and Paul Motzki,* Saarland University, Saarbrücken, Germany Qingping Sun, The Hong Kong University of Science and Technology Ichiro Takeuchi, University of Maryland, College Park Both shape memory and elastocaloric effects are related to the reversible martensitic phase transformation[1,2]. For the shape memory effect, thermal energy is used to drive the material through the phase transformation and recover its original shape. For the elasto- caloric effect, mechanical energy is used to induce phase transformation, and the associated latent heat is used to pump heat for cooling or heating. Heat pumps based on the elastocaloric effect have a low environmental impact and are highly efficient[3]. However, designing a prototype to harvest the full potential of the elastocaloric effect has been challenging. The stress required for phase transformation may be too high for traditional motors, the cost of the working materials (typically NiTi-base alloys) may be too expensive and have a short fatigue life, and the heat exchange system may be too complicated and inefficient for this purpose. Since its first demonstration, significant progress has been made to address these challenges[4]. This article summarizes three representative prototyping efforts in Germany, China, and the U.S. PROTOTYPE 1---ROTATING CAGE WITH NiTi WIRES A compact elastocaloric air-to-air demonstrator was developed by the Stefan Seelecke and Paul Motzki group at Saarland University in Germany in 2018 (Fig. 1). The system uses 24 bundles of NiTi-base wires (ɸ=200 µm). Each bundle consists of 30 wires, resulting in a total mass of 50 g of effective elastocaloric material. The machine uses an electric motor to rotate these NiTi-bundles within two fixed air ducts. A patented cam drive induces tensile loading at a fixed position and unloads on the opposite side, generating stationary hot and cold sides within the fluid duct system while simultaneously enabling optimal mechanical work recovery. Due to the high surface-to-volume ratio of the thin wires, a direct airflow (counterflow to rotation) can be used for heat exchange and heat transport. With this demonstrator, it is possible to achieve a temperature change of +/-20 K, corresponding to the measured air temperatures at the fluid duct entrances. Although quantitative cooling power measurements were not conducted, the simulation suggests a cooling power of approximately 250 W using 50 g of NiTi. This corresponds to a specific thermal power of 5 kW/kg and a coefficient of performance (COP) of 9.5[5]. PROTOTYPE 2---LINEAR CASCADE OF NiTi TUBES A multi-material, cascading elastocaloric cooling prototype was built by Qingping Sun’s group at The Hong Kong University of Science and Technology in 2023 (Fig. 2). FEATURE 3 *Member of ASM International (a) (b) (c)
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