Compressible metal offers a greener approach to refrigeration

Physics 16, 91

A system built around a metal that gains and loses heat when it is compressed could pave the way for commercializing chemical-free refrigeration technology.

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Wine coolers made from materials that cool when compressed could be a reality by next summer.

Refrigeration systems that use materials that cool in response to an applied electrical, magnetic, or mechanical force offer environmentally friendly alternatives to those that leak greenhouse gases found in most homes and factories (see article: In Hot Pursuit of 21st Century Cooling). Now researchers have demonstrated a cheap and scalable version of one of these so-called “calorie coolers” systems. [1]. Their approach, which uses a material that responds to a mechanical force, breaks performance records set by methods that rely on magnetic fields. The researchers say their technology is also much less expensive to make and operate, and could be ready for commercial use within a year.

Magnetic-based cooling systems first gained fame in 1998 with the demonstration of a near-ambient temperature system that kept its contents cold for 1500 hours [2]. In that system, a magnetic field was applied to a magnetocaloric material, resulting in a rise in temperature, as atomic vibrations compensated for the entropy lost when the unpaired spins in the material aligned. Deactivating the field reversed that increase, allowing the material to act as a coolant that could be used in the cooling coils of a home refrigerator. But inducing the magnetocaloric effect requires strong magnetic fields (>1 tesla), which can only be provided by expensive permanent magnets that contain rare earth alloys.

An alternative approach is to use an elastocaloric material. Such a material experiences an entropy-induced temperature change when subjected to a mechanical force large enough to partially change the material’s phase. In 2012 Ichiro Takeuchi of the University of Maryland found that, when stretched, a commercially available wire made of nickel titanium (NiTi) undergoes such a change, with a rise in temperature large enough to be felt by hand. He later discovered that a decrease in temperature occurs when compressing NiTi tubes and then used the effect in 2016 to develop an early electrocaloric cooling system. “We started producing [low-power cooling] systems that used NiTi tubes in compression mode about ten years ago,” says Takeuchi.

In the system developed by Takeuchi and colleagues, a steel actuator compresses a bundle of NiTi tubes at 700 megapascals (bundle 1). The water is pumped from the cold end of that bundle towards the hot end. At the same time, the actuator discharges a second bundle of NiTi tubing (bundle 2), and water flows from the hot end of that bundle to the cold end.In the system developed by Takeuchi and colleagues, a steel actuator compresses a bundle of NiTi tubes at 700 megapascals (bundle 1). The water is pumped from the cold end of that bundle towards the hot end. At the same time, the actuator unloads a second… Show more

Now a team led by Takeuchi and Reinhard Radermacher of the University of Maryland has brought elastocaloric cooling to the forefront of the race for greenhouse gas-free refrigeration. Several engineering challenges arose between the 2016 demo and the new one, which improved fluid recovery, reduced frictional heat loss, and provided denser tube bundles. In the new device, water, the heat transfer fluid, flows through two bundles of commercially available NiTi tubes. The two bundles connect via an actuator, which applies a load to one bundle while unloading the other, thus creating compression cycles that drive the refrigeration. The system can operate in two different modes, depending on how much water flows through the system during one cycle. One mode optimizes the cooling power, the other the temperature range. The team demonstrated that they could cool the system by 22.5K, compared to 4.7K in their 2016 scheme.

However, the team’s calculations indicate that overall system efficiency could be improved by a factor of 6 by using more efficient actuators. Furthermore, the researchers think they can improve efficiency by swapping NiTi for a known copper-based material that exhibits a similar elastocaloric temperature change under less stress. Such materials aren’t currently commercially available, but Takeuchi says he’s excited about implementing them in low-stress cooling systems.

The data from Takeuchi and his team “are really impressive,” says Kilian Bartholomé, who researches thermal energy converters at the Fraunhofer Institute for Physical Measurement Techniques in Germany. He points out that nearly all of the elastocaloric systems demonstrated use NiTi that has neither been manufactured nor optimized for use in refrigeration devices, meaning there is still “great potential” to increase systems performance. Takeuchi believes he and his colleagues will be able to improve their system’s performance enough to make the technology commercially viable within a year. The first application he imagines: a compact wine cellar.

Rachel Berkowitz

Rachel Berkowitz is correspondent editor forPhysics magazine based in Vancouver, Canada.

References

  1. S.Qian et al.High performance multi-mode elastocaloric cooling system, Science 380722 (2023).
  2. KA Gschneidner, Jr. and VK Pecharsky, Magnetic Refrigeration Materials (invited), J.Appl. Phys. 855365 (1999).

Thematic areas

Materials science Energy research

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