by Brittany Linn
figures by Swathy Karamchedu

To escape the oppressive summer heat, many of us seek the cool retreat of air-conditioned shopping centers, movie theaters, and public buildings; however, traditional air-conditioning systems are not without environmental consequences. Climate change awareness has inspired numerous laws and regulations to phase out chemicals produced by air-conditioning systems that contribute to climate change. Liquid refrigerants, such as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), are examples of such chemicals, which deplete the earth’s ozone layer (Figure 1).  As a result, CFCs and HCFCs were replaced by hydrofluorocarbons (HFCs) in the 1990s. Although HFCs do not damage the ozone layer, they, along with CFCs and HCFCs, are potent greenhouse gases capable of trapping heat hundreds to thousands of times more efficiently than CO2.

In search of an environmentally-friendly cooling solution, Dr. Ichiro Takeuchi, a materials scientist at the University of Maryland, and his research lab have developed an innovative “elastocaloric” cooling device made of nitinol—a nickel-titanium alloy—that does not rely on the use of harmful liquid refrigerants. Instead, nitinol tubes possess a unique physical property that allows the alloy to absorb heat, thereby cooling the area around it. This new device cools so effectively that it could be used in commercial refrigerators or air conditioners in the future. Widespread adoption of such a “caloric” cooling device would significantly diminish our dependence on environmentally harmful chemicals for cooling. 

Why ‘Elastocaloric’ Cooling?

Caloric cooling devices are a class of newly developed materials that change temperature under certain conditions. For example, ‘magnetocaloric’ materials change temperature when placed in a magnetic field, and ‘electrocaloric’ materials change temperature when exposed to an electric field. Elastocaloric materials, on the other hand, respond to mechanical force, such as stretching or compressing, and absorb heat after the mechanical strain is released. These materials are non-volatile, meaning they do not easily evaporate into a gas. Therefore, their potential to contribute to global warming is, operationally, zero, making them extremely attractive candidates for alternative refrigeration. Elastocaloric devices produce cooling through four steps as described in Figure 2: (1) the application of an external force increases the material’s internal temperature; (2) this generated heat is expelled into the environment; (3) the removal of the external force causes the material’s temperature to decrease; and (4) as its internal temperature drops, the material absorbs heat from the hot room, creating the desired cooling effect.

Although magnetocaloric devices are currently the most developed of these types of caloric cooling devices, they are not the most efficient. Electrocaloric and elastocaloric devices have higher cooling powers, meaning they can cool down a given space faster. This is because they release less heat to the environment during phases one and three of the four-step cycle. However, the application of the appropriate electric fields required for electrocaloric cooling devices is expensive, as is the application of the magnetic field for magnetocaloric cooling devices. Therefore, elastocaloric materials are the most attractive alternative. The most promising elastocaloric materials are shape-memory alloys which, like an elastic rubber band, recover their original shape upon removal of applied strain. Although other nitinol-based elastocaloric prototypes exist, Takeuchi’s work stands out for implementing a unique multimode configuration that improves the device’s cooling efficiency. 

How Does the New Elastocaloric Cooling Device Work?

Dr. Takeuchi’s new cooling device is made of two cylinders. Each cylinder consists of bundled nitinol tubes surrounded by stainless-steel tubes. The nitinol tubes are compressed from the top and bottom of the cylinders. As shown in Figure 3, the internal temperature of the nitinol tubes increases upon compression. That heat then needs to be expelled from the system. For that purpose, the cylinders housing the nitinol tubes also contain an evenly dispersed heat exchange fluid, which transfers the generated heat away from the nitinol tubes. The compressed tubes then decompress, lowering the material’s temperature and allowing the tubes to absorb heat from the environment. As one set of nitinol tubes decompress, a set of hydraulic cylinders converts the energy coming from the decompression into a compressive force to be applied to the other set of nitinol tubes. The two sets of tubes are therefore coupled so that as one bundle is being compressed, the second is allowed to expand back to its original shape. The paired nitinol tubes therefore cycle through the four steps of caloric cooling out of phase with each other.

How Does the Elastocaloric Cooling Device Stack Up?

Compared to other caloric devices, Dr. Takeuchi’s new device “is among the top 15% [in consideration of the capable temperature range and cooling power] among all reported caloric cooling systems in the past four decades” according to the scientific publication. The device can lead to temperature changes of up to 22.5 ℃ (40.5 ℉).  In comparison, electrocaloric devices have only achieved temperature spans as large as 13 ℃. Furthermore, even though magnetocaloric devices can currently outcompete Dr. Takeuchi’s elastocaloric device in achieving impressive temperature spans, they require magnetic fields and/or operate at very cold temperatures that are not practical for room-temperature applications.  

Dr.Takeuchi and his team’s research in elastocaloric cooling technology presents a promising avenue for addressing the environmental challenges associated with traditional cooling systems. By harnessing the unique properties of the nitinol shape memory alloy and implementing a multimode configuration, their device offers unprecedented efficiency in environmentally friendly cooling applications. The commercialization of this technology could transform industries reliant on cooling, from food preservation to climate control in buildings, and reduce energy consumption on a global scale. Beyond reducing greenhouse gas emissions and mitigating climate change, the widespread adoption of elastocaloric cooling devices could lead to energy savings, improved air quality, and enhanced resilience to climate-related challenges. As we confront the urgent need for solutions to combat climate change, Dr. Takeuchi’s work represents a significant step towards a more sustainable future.

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Brittany Linn is a third-year PhD student studying chemistry at MIT. 

Swathy Karamchedu is a graduate student in the Media, Medicine, and Health program at Harvard Medical School who is developing visual narratives for sleep health education.

Cover image by RoadLight from pixabay.

For More Information:

• To learn more about global warming potentials and which gases have the highest values, visit this website by the EPA.
• For more information about the greenhouse gas emissions from the refrigeration sector, check out this page by the International Institute of Refrigeration.