Experimental Methods for Investigation of Shape Memory Based Elastocaloric Cooling Processes and Model Validation

Abstract

Shape Memory Alloys (SMA) using elastocaloric cooling processes have the potential to be an environmentally friendly alternative to the conventional vapor compression based cooling process. Nickel-Titanium (Ni-Ti) based alloy systems, especially, show large elastocaloric effects. Furthermore, exhibit large latent heats which is a necessary material property for the development of an efficient solid-state based cooling process. A scientific test rig has been designed to investigate these processes and the elastocaloric effects in SMAs. The realized test rig enables independent control of an SMA's mechanical loading and unloading cycles, as well as conductive heat transfer between SMA cooling elements and a heat source/sink. The test rig is equipped with a comprehensive monitoring system capable of synchronized measurements of mechanical and thermal parameters. In addition to determining the process-dependent mechanical work, the system also enables measurement of thermal caloric aspects of the elastocaloric cooling effect through use of a high-performance infrared camera. This combination is of particular interest, because it allows illustrations of localization and rate effects -both important for efficient heat transfer from the medium to be cooled.The work presented describes an experimental method to identify elastocaloric material properties in different materials and sample geometries. Furthermore, the test rig is used to investigate different cooling process variations. The introduced analysis methods enable a differentiated consideration of material, process and related boundary condition influences on the process efficiency. The comparison of the experimental data with the simulation results (of a thermomechanically coupled finite element model) allows for better understanding of the underlying physics of the elastocaloric effect. In addition, the experimental results, as well as the findings based on the simulation results, are used to improve the material properties.

Elastocaloric effect vs fatigue life: Exploring the durability limits of Ni-Ti plates under pre-strain conditions for elastocaloric cooling

“…[7,56,57]. In addition to mechanical stabilization, training also results in spatial homogenization of the eCE (adiabatic temperature changes) over the sample as shown and explained in [7,55], which is highly desirable for elastocaloric cooling. Supplementary video 1 and Supplementary video 2 show the temperature changes during loading and unloading at the first five and the last five training cycles, respectively.…”

“…At each of the evaluated conditions, three cycles were repeated. Since the temperature changes during the loading and unloading can be highly inhomogeneous over the sample [3,7,55], the adiabatic temperature changes presented in this work are obtained by averaging the temperature values from the thermal images over the entire gauge section of the sample. Figure 2(a.)…”

NiTi‐Based Elastocaloric Cooling on the Macroscale: From Basic Concepts to Realization

“…Different thermodynamic cycles with various mechanical and thermal boundary conditions were investigated. Therefore, a scientific test setup, as shown in Figure , was developed enabling an independent investigation of the influence of each control parameter on the process variables . Designed for optimal observability, these tests were executed using a discontinuous heat transfer principle with conductive heat exchange between ribbon‐shaped NiTi specimens and metal heat sink and source blocks, respectively.…”

“…Therefore,as cientific test setup,a ss hown in Figure 3, was developed enabling an independent investigation of the influence of each control parameter on the process variables. [63,64] Designed for optimal observability,t hese tests were executed using ad iscontinuous heat transfer principle with conductive heat exchange between ribbon-shaped NiTis pecimens and metal heat sink and source blocks,r espectively. This arrangementa llowsf or simultaneous temperature field measurements for thermal power determination along with force and displacement measurements to calculatem echanical work.…”

Mesoscopic free energy as a framework for modeling shape memory alloys

“…Here, due to the complexity of the energy landscape, the barrier peaks had to be approximated from the driving forces (the relative minimum energies between the phases). The driving force technique was also used in several 1D SMA cases, even though the approximation of the energy barriers was unnecessary (Ballew and Seelecke, 2009; Furst et al, 2012; Schmidt et al, 2016c; Ullrich et al, 2014; Welsch et al, 2018); this is mainly a reflection of the authors using the most recent version of the model at the time of publication.…”