Improving Efficiency of Elastocaloric Cycles by Applying Novel Thermodynamic Cycles and Proper Training Regime

Vapor-compression technology emerged nearly a century ago by utilizing natural refrigerants, which had inherent problems, i.e., low efficiency, inflammability and toxicity. Consequently, stable and more efficient synthesized refrigerants such as chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs) were developed, nevertheless, CFCs depleted the ozone layer and HFCs had high global warming potential. Therefore, due to significant environmental impact, vapor-compression technology should soon revert to natural refrigerants. As a result, caloric cooling/heating technologies that use environmentally benign solid-state refrigerants have been considered as serious alternatives to vapor-compression technology, among which, elastocaloric technology, which is based on the latent heat of phase transformation of shape memory alloys (SMAs), has been nominated as the most promising one by the US Department of Energy and the EU Commission. Accordingly, elastocaloric effect of different SMAs has been investigated, and several elastocaloric devices using different geometries and thermodynamic cycles have been designed and tested with promising results. Developing durable elastocaloric devices that are more efficient than their vapor compression counterparts is still the main focus of the ongoing research. However, some of the basics of the elastocaloric cycle have been either overlooked or have not been investigated thoroughly. This study investigates the effects of training (stabilization of thermomechanical response) and the parameters of the elastocaloric cycle on the elastocaloric response of NiTi wires under tensile loading. Multiple training regimes with different stress amplitudes and strain rates have been applied to NiTi thin wires and the elastocaloric response of the samples has been evaluated subsequently. A typical elastocaloric cycle consists of an adiabatic loading and an adiabatic unloading segment each of which is followed by a holding period throughout which the heat can be transferred to/absorbed from the heat transfer medium or a contact heat sink/source. The parameters of the holding period have been systematically investigated by applying constant stress and constant strain throughout the holding period resulting in isostress and isostrain thermodynamic cycles respectively¹. In addition to the applied training regime, the parameters of the holding period can significantly affect the generated elastocaloric effect and alter the nature of the applied thermodynamic cycle. While the typically applied isostrain cycle results in a Brayton thermodynamic cycle, applying the isostress cycle with partial phase transformation changes the thermodynamic cycle and shifts it toward the Carnot cycle, which has the maximum theoretical efficiency. In addition to the efficiency of the cycle, the isostress cycle generates larger adiabatic temperature changes under lower applied stress levels that can enhance the tensile fatigue life of the wires and reduce the size of the actuators that are needed in practical devices. In multielement configurations like elastocaloric regenerators², lower stress levels needed in isostress cycles allow for increasing the number of elastocaloric elements that improves the performance of the regenerator by increasing its heating/cooling power.<br/><br/>¹ P. Kabirifar et. al, J. Phys. Energy 4, 044009 (2022).<br/>² Z. Ahčin et. al, Joule 6, 2338 (2022).<br/><br/>Acknowledgments: J. Tušek and P. Kabirifar acknowledge support of European Research Council (ERC Starting Grant No. 803669).