ADVANCED MATERIALS & PROCESSES | APRIL 2024 51 FEATURE 5 work highlights the importance of architecture design and heat transfer fluid, paving the way for the large-scale commercialization of this ecofriendly refrigeration technology. PROTOTYPE 3---COMPRESSION AND LONG NiTi TUBES Over the years, Ichiro Takeuchi’s group at the University of Maryland has developed several elastocaloric systems using NiTi wires and tubes (Fig. 3). As an initial demonstration, the team built a low-power liquid-cooling device with a simple hand-crank mechanism and saw that it was possible to deliver cooling to a liquid medium (Fig. 3a). A larger system was built in 2012. It used a “rotating birdcage” design, where NiTi wires connect the outer perimeter of a pair of ring frames, which are slightly offset from the parallel configuration so that as the rings rotate together, wires experience cyclical stretching and unstretching (Fig. 3b). Two critical lessons learned from this device were that wires would break after just a few thousand cycles, and direct heat exchange between the air and metallic wires was inefficient. These results led the team to embrace the concept of tube compression, where the heat exchange fluid can flow inside or around the tube to exchange heat. The first compression-based elastocaloric system was built in 2015 (Fig. 3c)[8]. It incorporated a reciprocating work- recovery mechanism to extract cooling with high efficiency. Later, the team built a 400 W reciprocating four-bundle system with a self-aligned vertical configuration (Fig. 3d)[9]. In recent years, the team has been focusing on developing different system operation modes. In the high utilization mode, the cooling power reached 260 W, while in the active regenerator mode, the temperature lift reached 22.5 K[10]. CONCLUSION AND PATH FORWARD Researchers around the world are working synergistically to solve challenges arising from the extreme use of shape memory alloys, i.e., tens of millions of cycles with significant deformation, while trying to extract heat efficiently and cost effectively. Tension-based elastocaloric systems enable convenient loading and ease of heat exchange. However, due to limitations in the material’s intrinsic mechanical properties, the tension-based approach suffers from poor fatigue life. The compression-based approach has the advantage of long fatigue life but is limited in heat exchange configuration. This system must address buckling issues and typically requires a higher load than the tension- based approach. A combination of better materials and moderate loading may mitigate these issues. Cascading multiple working materials with different phase transition temperatures can create a large temperature range, while more efficient heat exchange fluid may significantly improve power density. Lastly, a system design allowing convenient replacement of the working material for cycling may relax the fatigue requirement on the working materials. With solutions to these technical challenges being developed, applications of elastocaloric heat pumps have appeared on the horizon. While the elastocaloric effect has many possible applications, ranging from dehumidifiers to wine coolers to air conditioners, the first commercial use will likely be the heat pump for residential water heaters. This is because the heating system is less concerned with heat leaks than having a reliable cooling system. Moreover, a shape memory alloy’s latent heat tends to be bigger for ones where transformation is finished at a higher temperature. Several worldwide initiatives are in progress, which should further mature and unlock the full potential of elasto- caloric materials for large-scale adoption. For example, in 2024, The National Aeronautics and Space Administration (NASA) established a new initiative for advancing the performance of refrigeration systems based on the elasto- caloric effect. This Early Stage Innovations (ESI) program was established to accelerate the development of groundbreaking, high-risk/high-payoff space technologies to support the future space science and exploration needs of NASA, other government agencies, and the commercial space sector [https://www.nasa.gov/general/early-stage- innovations-esi-2023/]. In Europe, research and development in alternative and efficient cooling and heating has been supported through programs by the European Research Council, European Innovation Council, and by the federal governments of different countries. In Germany, several initiatives are being driven by the automotive industry among others, and a consortium around Saarland University was awarded €18 million in funding to push the technology from research into the market. For global advancement and acceleration, the International Elastocalorics Society was founded in Saarland in 2023 with the goal of joining forces to tackle today’s challenges in an interdisciplinary manner. Further, the German Research Foundation’s Priority Program 1599, “Caloric Effects in Ferroic Materials: New Concepts for Cooling,” has been funding research on elastocaloric materials since 2012. In Asia, research and development efforts on SMA- based elastocaloric refrigeration technology are relatively small. Elastocaloric research projects not only started late, but also lagged far behind those of Europe and the U.S. in terms of progress, commercialization effort, and funding. Nonetheless, there are some promising efforts that are contributing to overall technology development and growth. ~SMST Acknowledgments The work carried out at Saarland University was realized within the DFG priority program 1599, “Caloric Effects in Ferroic Materials: New Concepts for Cooling.” Work at The Hong Kong University of Science and Technology was
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