The first objective was addressed within WPs 1-4. In WP1, we first conducted a comprehensive review of elastocaloric (eC) materials and different ways of forming them into various porous structures to be loaded in compression. Among the available geometries of shape memory alloys - SMAs(i.e. Ni-Ti alloys), the thin-walled tubes were selected as ideal candidates for elastocalorics as they offer fatigue resistant operation, high reproducible elastocaloric effect (eCE) and the best compromise between buckling stability and heat transfer efficiency. Work in WP1 further focused on evaluating some alternative methods to improve the efficiency of the eC device, namely analyzing different thermodynamic cycles and different training regimes. We discovered a novel eC thermodynamic cycle that approximates the most efficient Carnot thermodynamic cycle. We also selected an optimal training regime (i.e. training stress and strain rate) that leads to the most efficient and/or the most powerful eCE.
As part of WP2, we developed a novel tube-based geometry for compression loaded elastocaloric regenerators that resembles a shell-and-tube heat exchanger. Different shell-and-tube-like regenerators were tested for the thermo-hydraulic properties. On this basis, new thermo-hydraulic correlations were defined for shell and tube-like thermal regenerators. The results showed that these geometries represent an excellent compromise between heat transfer and pressure drop characteristics. In WP2, we developed a new numerical model of eC regenerators and performed a parametric study of their geometrical and operational properties.
In WP3, we developed a new shell finite element model as a simulation tool for predicting the mechanical stability of superelastic structures and conducted extensive experiments on the buckling behavior of various Ni-Ti tubes, based on which a phase diagram of buckling stability was generated.
In the first part of WP 4, we focused on the wear and tribological behavior of NiTi in contact with various materials that act as supporting elements and prevent buckling of the eC material. The results should serve as guidelines for the selection of the most suitable materials that would act as supporting elements for NiTi alloys under compressive loading. The main goal of WP4 was a comprehensive analysis of the fatigue behavior of eC materials at different mean strains and strain amplitudes, ranging from compressive to mixed and tensile loads, while simultaneously monitoring the eCE. Based on this, we have created the first Haig-like (in)finite life diagram of a NiTi alloy in 3D space, consisting of the mean strain and strain amplitude together with the associated adiabatic eC temperature changes.
Finally, we have constructed several different active shell-and-tube-like eC regenerators (integrating the findings from WP1-4) and tested them for their cooling and het-pumping characteristics in our in-house built modular experimental setup. The best of them showed record-breaking cooling and heating performance and fatigue-resistant operation. This was the world's first eC device to demonstrate breakthrough performance with fatigue-resistant operation and record-breaking cooling and heating characteristics with a maximum temperature span of more than 30 K and a maximum specific cooling/heating power of 4,400 W per kg of eC material. In terms of specific cooling/heating properties, our eC regenerator outperforms all caloric cooling devices developed to date. With that, the objective 1 was delivered.
Objective 2 was addressed in WP 5, where we created a theoretical and numerical model to study the torque behaviour of different loading and unloading paths of the drive mechanism. On this basis, we designed the most suitable cam-disc-based drive mechanism for the shell-and-tube elastocaloric regenerators developed in the project. Finally, the drive mechanism was built and tested in the rotating universal testing machine, where we showed that the system can provide a complete work recovery with relatively constant torque during operation. With that the objective 2 was delivered. A patent application on an innovative design of a cam disc was recently submitted to the Knowledge transfer office at the University of Ljubljana.
Objective 3 was addressed in WP6, where the goal was to integrate the eC regenerators (objective 1) and the cam-disc-based drive system (objective 2) into the eC prototype device. The device was successfully integrated and tested from a mechanical point of view.
