First Demonstration of a Bending-Mode Elastocaloric Cooling ‘Loop’

Abstract

This paper describes the first demonstration of solid-state elastocaloric cooling using Nitinol (NiTi) wire in a continuous `flow loop' configuration. Elastocaloric material performance was determined by infrared thermography uniaxial- or bending-mode mechanical stress. The effects of strain rate (from 0.001 to 0.025 s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and strain mode on the temperature evolution and coefficient of performance (COP) were determined. Benchmark uniaxial results show a strain-induced adiabatic temperature reduction of 16°C, calculated material endothermic latent heat of 7.52 J/g, and a maximum material COP of 3.5. Bending-mode experiments, performed using the same alloy, show a temperature reduction of 9°C with a 6× reduction in required actuation force and equivalent material COP values to uniaxial testing. Based on bending-mode strain, a first-of-a-kind elastocaloric cooling `flow loop' using NiTi wire tracking around a designed radius of curvature was demonstrated. Continuous cooling of 1.85°C with a maximum cooling power of 49 mW was obtained. Future studies, which aim to improve the material latent heat, mechanical efficiency, and hot-side cooling efficiency, are expected to provide significant improvements in cooling power and temperature drop for equivalent operating conditions.