Elastocaloric cooling is a new alternative solid-state cooling technology undergoing early stage research and development. This study presents a comprehensive review of key issues related to achieving a successful elastocaloric cooling system. Fundamentals in elastocaloric materials are reviewed. The basic and advanced thermodynamic cycles are presented based on analogy from other solid-state cooling technologies. System integration issues are discussed to characterize the next generation elastocaloric cooling prototype. Knowledge acquired from the elastocaloric heat engines is provided as the basis for the design of cooling system configuration. Commercially available drivers enabling proper compression and tension are also presented. A few performance assessment indices are proposed and discussed as guidelines for design and evaluation of future elastocaloric cooling system. A brief summary of the up-to-date elastocaloric cooling prototypes is presented as well.

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A review of elastocaloric cooling: materials, cycles and system integrations.

The entropy change ΔS between the high-temperature cubic phase and the low-temperature tetragonally based martensitic phase of Ni2+xMn1−xGa (x=0–020) alloys was studied The experimental results obtained indicate that ΔS in the Ni2+xMn1−xGa alloys increases with the Ni excess x The increase of ΔS is presumably accounted for by an increase of magnetic contribution to the entropy change It is suggested that the change in modulation of the martensitic phase of Ni2+xMn1−xGa results in discontinuity of the composition dependence of ΔS

Entropy change at the martensitic transformation in ferromagnetic shape memory alloys Ni2+xMn1−xGa

Recent findings on Heusler-type alloys in NiMnIn- and NiMnSn-based systems show a specific martensitic (i.e., diffusionless) transformation from a ferromagnetic parent (P) phase to an antiferromagnetic-like martensite (M) phase. In this paper, the magnetic and martensitic properties and the details of the magnetic field-induced shape memory effect (SME) are introduced in NiMnIn-based alloys. The martensitic transformation temperatures of these alloys significantly decrease with the application of a magnetic field, and a metamagnetic phase transformation occurs from the M phase to the P phase. By using this transition, a magnetic field-induced strain of approximately 3%, namely, a metamagnetic SME, is confirmed.

Metamagnetic shape memory effect in NiMn-based Heusler-type alloys

Grain size and pseudoelastic behaviour of a Cu–Al–Be alloy

Thermoelastic cooling is a recently proposed, novel solid-state cooling technology. It has the benefit of not using high global warming potential (GWP) refrigerants which are used in vapor compression cycles (VCCs). Performance enhancements on a thermoelastic cooling prototype were investigated. A few novel design options aiming to reduce the cyclic loss were proposed. It was found that the maximum temperature lift increased from 6.6 K to 27.8 K when applying the proposed novel designs, corresponding to 0–152 W cooling capacity enhancement evaluated under 10 K water–water system temperature lift. In addition, a multi-objective optimization problem was formulated and solved using the genetic algorithm to maximize the system capacity and coefficient of performance (COP). With all the novel designs, the optimization could further enhance 31% COP, or 21% cooling capacity, corresponding to COP of 4.1 or 184 W maximum cooling capacity.

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Performance enhancement of a compressive thermoelastic cooling system using multi-objective optimization and novel designs

Calorimetric measurements have been carefully performed on four kinds of Cu–Zn–Al samples, arising from the combination of different martensitic structures and from the presence or not of grain boundaries. The differences found in the transformation behaviour, characteristic transformation temperatures and in the corresponding thermodynamic properties (enthalpy and entropy changes) have been analyzed. The remarkable finding was that the transformation to 2H in the polycrystalline samples couldn’t go on completion. On the other hand, this behaviour was not observed for the transition to 18R, within the experimental scatter.

Calorimetry in Cu–Zn–Al alloys under different structural and microstructural conditions

A study of the entropy change Δ S between the β phase and the martensite in Cu-based shape memory alloys is presented. From a compilation of available experimental data, the composition dependence of Δ S was studied. The experimental data were analyzed within the frame of a simple model, which is based on the specific heats of the phases. It was demonstrated that the dependence of Δ S with composition comes only through the lattice parameter and the effective mass of the alloy. For the studied composition range, the greater vibrational entropy of β phase is mainly controlled by the high-mass Cu atoms.

Change of entropy in the martensitic transformation and its dependence in Cu-based shape memory alloys

An analysis of the influence of the friction, which arises between the compression sample and the test machine, on the martensitic transformation of Cu Zn Al alloys was performed. It is shown that the error introduced by neglecting the friction term is greater than the typical error of tensile experiments. A method to take the friction into account is presented.

J.L. Pelegrina

Papers

Calorimetry in Cu–Zn–Al alloys under different structural and microstructural conditions

Change of entropy in the martensitic transformation and its dependence in Cu-based shape memory alloys

On the stress induced martensitic transformation compression test