Squeeze to remove heat: Elastocaloric materials may hold the key to more efficient refrigeration

Friday 27 Mar 15

Journal of Applied Physics

Journal of Applied Physics is an influential international journal publishing significant new experimental and theoretical results of applied physics research. http://jap.aip.org

Article title: Elastocaloric effect of Ni-Ti wire for application in a cooling device. Authors: J. Tušek, K. Engelbrecht, L.P. Mikkelsen and N. Pryds, all DTU Energy at the Technical University of Denmark
Researchers from the Technocal University of Denmark: Elastocaloric effect opens the door to alternative forms of solid-state refrigeration that are direct replacements for vapor compression technology.

Traditional cooling technology is based on vapor compression, a proven and mature technology but also a technology with inherent inefficiencies and relatively high environmental impact. For several decades researchers around the world have been investigating alternatives to vapor compression. One of the latest and most promising is elastocaloric cooling. It is potentially more efficient and relies on environmentally friendly refrigerants.

Elastocaloric refrigeration is based on the fact that certain solids change their temperature in response to a mechanical stress. A team of researchers from DTU Energy reports in their recent paper Elastocaloric effect of Ni-Ti wire for application in a cooling device in Journal of Applied Physics on a promising material for use as a refrigerant in an elastocaloric refrigerator.

The elastocaloric effect is only one of several “caloric” effects in which a sudden change of an external field induces a change in the temperature of a solid material. DTU Energy has for many years been researching the magnetocaloric effect and its application for refrigeration. Now, researchers have also decided to explore the potential of elastocaloric cooling.

"This is an important step toward the use of elastocaloric materials in cooling devices such as household refrigerators and air conditioners, or even heat pumps"
Jaka Tušek, postdoc at DTU Energy

Jaka Tušek, lead author and a postdoc at DTU Energy, explains its advantages: “The large amount of latent heat released during the elastocaloric effect, as well as its potentially high power densities that can be significantly higher compared to the magnetocaloric effect.”

Elastocaloric effect on Ni-Ti metalThe elastocaloric effect is associated with a structural phase transition in which the atomic lattice of the material changes its structure from a so-called austenitic to a martensitic phase. This can be induced by reducing the temperature or by applying an external stress.

Jaka Tušek explains the principle behind the elastocaloric cooling as follows: “When an elastocaloric material in the austenitic phase is axially stressed, an exothermic (heat-evolving) austenitic to martensitic transformation occurs. If this happens fast enough, the material heats up. It then expels heat to its surroundings and cools down to the ambient temperature.” After the stress is removed, the crystal structure reverts back to its austenitic phase, which causes the material to cool down and further absorb heat from its surroundings.

The team has for the first time shown that an elastocaloric material, in this case a nickel-titanium (Ni-Ti) alloy, can be cyclically loaded and unloaded with a reproducible elastocaloric effect over a wide temperature range.

“This is an important step toward the use of elastocaloric materials in cooling devices such as household refrigerators and air conditioners, or even heat pumps, for which the required temperature between the heat source and its heat sink is approximately 30 kelvin or more,” Tušek adds.

Tušek and colleagues also stabilized the Ni-Ti alloy to ensure a reproducible effect, which is crucial for practical applications, and created a uniform elastocaloric effect for the alloy.

The potential of the technology is significant. “Elastocaloric cooling can be viewed as a direct substitute for vapor compression technology -- one that’s more efficient and environmentally friendlier -- to be used in a wide range of applications,” Tušek points out. The technology also is gravity independent and potentially highly reliable, so it may find use in the thermal management of space systems as well.

The next big step is “to build the first prototype to demonstrate the technology’s potential,” says Tušek. “Afterward, we hope to build more complex, high-performance machines that can compete with commercial technologies.”

For the immediate future, the team will focus on ways to load the material to increase its resistance to fatigue, which is considered to be the technology’s main limitation.

(Based in part on a press release from AIP, avaible here)