Refrigeration, an essential component of modern life, is at a critical juncture. Traditional vapor compression cycles, the backbone of current cooling technologies, are responsible for approximately 20% of global energy consumption. The growing reliance on these systems for heat pumps and industrial applications only exacerbates the environmental impact. The inefficiency of these systems, coupled with the global push towards decarbonization, underscores the urgency for alternative solutions.
Hydrofluorocarbons, commonly used in vapor compression systems, are potent greenhouse gases with a global warming potential vastly exceeding CO2. Their leakage is a significant contributor to global greenhouse emissions, with projections indicating a contribution of over 11% to global CO2 emissions by 2050. The Biden administration’s directive for a substantial phase-down of these substances in the US reflects a global recognition of the problem. However, finding ideal low-GWP alternatives remains a challenge due to trade-offs between environmental impact and safety, such as flammability.
Caloric cooling emerges as a promising alternative to traditional methods. This technique involves field-driven phase transitions in solids, offering high energy conversion efficiency. The diversity of caloric materials, including magnetocaloric, electrocaloric, and mechanocaloric, provides various avenues for exploration. Among these, elastocaloric materials, particularly those based on superelastic shape-memory alloys like NiMnTi and Nickel-Titanium (NiTi), demonstrate significant potential due to their large intrinsic temperature lifts. NiTi, also known as Nitinol, is a shape-memory alloy known for its unique properties, including the ability to return to a predetermined shape when heated. In the context of caloric cooling, particularly elastocaloric cooling, NiTi tubes can play a significant role. The elastocaloric effect is a phenomenon where a material will either absorb or release heat when it is subjected to mechanical stress or strain. NiTi, with its shape-memory characteristics, exhibits a significant elastocaloric effect. When it is deformed, it releases heat, and when it returns to its original shape, it absorbs heat. In a caloric cooling system, NiTi tubes can be mechanically compressed, causing them to release heat to the surrounding environment. This process increases the temperature of the surrounding area. When the mechanical stress is removed, the NiTi tubes return to their original shape, absorbing the released heat. This process cools the environment. The elastocaloric effect in NiTi alloys is particularly strong, making them efficient for use in cooling systems. Their ability to undergo many cycles of stress without degradation makes them suitable for long-term use. In practical applications, NiTi tubes could be arranged in a way that allows for efficient heat transfer. The system would need a mechanism to apply and release mechanical stress cyclically to the tubes, possibly through hydraulic or pneumatic means. To this account, a new research study published in the Science Journal by Suxin Qian, David Catalini, Jan Muehlbauer, Boyang Liu, Het Mevada, Huilong Hou, Yunho Hwang, Professor Reinhard Radermacher, and led by Professor Ichiro Takeuchi at the University of Maryland marks a significant stride in the field of refrigeration technology. They developed an innovative elastocaloric cooling system using nitinol (NiTi) tubes. This system is a major advancement in the quest for refrigeration technologies with zero global warming potential.
The research team designed a cooling system based on the elastocaloric effect of NiTi tubes. Elastocaloric materials generate or absorb heat when subjected to mechanical stress. They configured the NiTi tubes into a versatile architecture capable of operating in different modes—active regeneration for high temperature spans and maximum utilization for high cooling power. The design focused on ensuring the mechanical integrity of the NiTi tubes under cyclic compression and optimizing heat exchange. This consideration was crucial for the system’s longevity and performance. The system was thoroughly tested to evaluate its cooling power, temperature span, and overall efficiency.
The authors demonstrated that their designed system achieved a maximum cooling power of 260 watts and a temperature span of 22.5 kelvin. These impressive values are among the highest reported for caloric cooling systems, marking a significant improvement over existing technologies. Moreover, the ability to switch between active regeneration and maximum utilization modes allows the system to adapt to different cooling needs, balancing between temperature span and cooling power. They also demonstrated that elastocaloric cooling, especially using materials like NiTi, is a highly promising direction for commercializing caloric cooling as an environmentally friendly alternative to current refrigerants. Given its high performance and flexibility, the newly designed elastocaloric cooling system shows potential for applications ranging from air conditioning to industrial refrigeration.
In conclusion, Professor Takeuchi and colleagues successfully created an elastocaloric cooling system that not only pushes the boundaries of caloric cooling technology in terms of efficiency and performance but also presents a viable, environmentally friendly alternative to traditional refrigeration methods. Exploring different materials, such as Cu-based elastocaloric materials, and further optimizing the system can lead to even more efficient and effective cooling solutions. The potential for commercialization of this zero-GWP technology is vast, with implications for a wide range of applications from domestic refrigeration to large-scale industrial cooling. Their achievements in developing a system with high cooling power and a significant temperature span, all while using zero-GWP materials, mark a major leap forward in addressing global climate change concerns related to refrigeration and cooling technologies.
Qian S, Catalini D, Muehlbauer J, Liu B, Mevada H, Hou H, Hwang Y, Radermacher R, Takeuchi I. High-performance multimode elastocaloric cooling system. Science. 2023 ;380(6646):722-727. doi: 10.1126/science.adg7043.