A comparison between different materials with elastocaloric effect for a rotary cooling prototype

Solid-state refrigeration based on the caloric effects has recently received widespread consideration because of its high efficiency and environmental protection. Compared with other solid-state refrigeration technologies, the elastocaloric effect-based one has great application potential. The elastoCaloric Effect (eCE) is defined as the reversible adiabatic temperature change (ΔTad) and the reversible isothermal entropy change (ΔST) in Shape Memory Alloys (SMA), following a variation in the mechanical field applied. To date, the elastocaloric materials, considered as benchmark, since offering the best compromise between thermal and mechanical performances are represented by the NiTi binary alloys. In this paper, a comparison among elastocaloric materials was carried out to identify the best one to be used in an experimental device for air conditioning based on the elastocaloric effect. The device is a rotary that ensures continuous flows of hot or cold air and it lodges the elastocaloric material in the form of 600 wires (diameter 0.5 mm and length 300 mm) for a total mass of 230 g. The model can reproduce the thermofluido-dynamic behavior of the air flowing in the device exchanging heat with the wires of elastocaloric material. For a correct dimensioning of the prototype, a two-dimensional model was built to simulate the real rotation of the device. Before building the prototype, an energy performance analysis was conducted for various elastocaloric materials to identify the most promising one. The materials used for the comparison are: Ni50.8Ti49.2, Ni55.9Ti44.1, Ni45Ti47.2Cu5V2.75, (Ni50Mn31.5Ti18.5)99.8B0.2 and PbTiO3. The innovations contained in this article are: i) the design of a device for environmental conditioning that can have competitive performance respect to traditional systems; ii) the presentation of a 2D model able to simulate the real rotation of the device; iii) the construction of an energy performance map for various elastocaloric materials to identify the most promising one. The numerical test campaign has been carried out at a fixed cycle frequency (0.5 Hz) varying the air velocity in the range 3.0-20.0 m s-1. The results reveal very encouraging energy performances, attributing the prototype as suitable for the macro-scale application: (Ni50Mn31.5Ti18.5)99.8B0.2 and Ni45Ti47.2Cu5V2.75 ensure the achievement of a greater than 30 K temperature span, with a peak of 38.6 K for the former material while air flows at 3 m s-1. Both the materials allow the overcoming of the 1KW thresholds for cooling power, even if (Ni50Mn31.5Ti18.5)99.8B0.2 carries the device up to 6.66 as Coefficient Of Performance (COP) at 12 m s-1, corresponding to 86% as a second-law efficiency.