A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

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Caloric materials near ferroic phase transitions

Recent progress on caloric effects are reviewed. The application of external stimuli such as magnetic field, hydrostatic pressure, uniaxial stress and electric field give rise respectively to magnetocaloric, barocaloric, elastocaloric and electrocaloric effects. The values of the relevant quantities such as isothermal entropy and adiabatic temperature-changes are compiled for selected materials. Large values for these quantities are found when the material is in the vicinity of a phase transition. Quite often there is coupling between different degrees of freedom, and the material can exhibit cross-response to different external fields. In this case, the material can exhibit either conventional or inverse caloric effects when a field is applied. The values reported for the many caloric effects at moderate fields are large enough to envisage future application of these materials in efficient and environmental friendly refrigeration.

Advanced materials for solid-state refrigeration

The search for materials with large caloric effects has become a major challenge in material science due to their potential in developing near room-temperature solid-state cooling devices, which are both efficient and clean, and that can successfully replace present refrigeration technologies. There are three main families of caloric materials: magnetocaloric, electrocaloric, and mechanocaloric. While magnetocaloric and electrocaloric materials have been studied intensively in the last few decades, mechanocaloric materials are only very recently receiving a great deal of attention. The mechanocaloric effect refers to the reversible thermal response of a solid when subjected to an external mechanical field, and encompasses both the elastocaloric effect, corresponding to a uniaxial force, and the barocaloric effect, which corresponds to the response to hydrostatic pressure. Here, the state of the art in giant mechanocaloric effects is reviewed and a critical analysis of the thermodynamic quantities that characterize the major families of barocaloric and elastocaloric materials is provided. Finally perspectives for further development in this area are given.

Materials with Giant Mechanocaloric Effects: Cooling by Strength.

A large fraction of global energy use is for refrigeration and air-conditioning, which could be decarbonized if efficient renewable energy technologies could be found. Vapour-compression technology remains the most widely used system to move heat up the temperature scale after more than 100 years; however, caloric-based technologies (those using the magnetocaloric, electrocaloric, barocaloric or elastocaloric effect) have recently shown a significant potential as alternatives to replace this technology due to high efficiency and the use of green solid-state refrigerants. Here, we report a regenerative elastocaloric heat pump that exhibits a temperature span of 15.3 K on the water side with a corresponding specific heating power up to 800 W kg−1 and maximum COP (coefficient-of-performance) values of up to 7. The efficiency and specific heating power of this device exceeds those of other devices based on caloric effects. These results open up the possibility of using the elastocaloric effect in various cooling and heat-pumping applications. Although heating and cooling consume a large fraction of global energy, current technologies are not energy efficient. Tusek et al. report an elastocaloric heat pump with active regeneration that can outperform other caloric-based cooling and heat-pumping devices.

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A regenerative elastocaloric heat pump

General information Publication status: Published Organisations: Department of Energy Conversion and Storage, Electrofunctional materials, Department of Wind Energy, Composites Mechanics and Materials Mechanics, University of Barcelona Contributors: Tusek, J., Engelbrecht, K., Millán-Solsona, R., Mañosa, L., Vives, E., Mikkelsen, L. P., Pryds, N. Number of pages: 5 Publication date: 2015 Peer-reviewed: Yes

The Elastocaloric Effect: A Way to Cool Efficiently

Vapor compression (VC) is by far the most dominant technology for meeting all cooling and refrigeration needs around the world. It is a mature technology with the efficiency of modern compressors approaching the theoretical limit, but its environmental footprint remains a global problem. VC refrigerants such as hydrochloroflurocarbons (HCFCs) and hydrofluorocarbons (HFCs) are a significant source of green house gas emissions, and their global warming potential (GWP) is as high as 1000 times that of CO2 [Buildings Energy Data Book (Building Technologies Program, Department of Energy, 2009)]. There is an urgent need to develop an alternative high-efficiency cooling technology that is affordable and environmentally friendly [A. D. Little, Report For Office of Building Technology State and Community Programs, Department of Energy, 2001]. Here, we demonstrate that elastocaloric cooling (EC), a type of solid-state cooling mechanism based on the latent heat of reversible martensitic transformation, can have the coefficient of performance as high as ≈11, with a directly measured ΔT of 17 °C. The solid-state refrigerant of EC completely eliminates the use of any GWP refrigerants including HCFCs/HFCs.

Demonstration of high efficiency elastocaloric cooling with large ΔT using NiTi wires

A system for heating/cooling includes a plurality of thermoelastic modules. Each of the modules includes one or more structures formed of shape memory alloy, which converts from austenite to martensite upon application of a first stress and release latent heat from the conversion. During a first part of a heating/cooling cycle, a first module is stressed to cause conversion. The latent heat released from the first module is rejected to a heat sink while a second unstressed module absorbs heat from a heat source. During a second part of the heating/cooling cycle, the first and second modules are connected together to transfer heat therebetween for heat recovery. The cycle can be repeated indefinitely with the first and second modules alternating roles. Structures of the thermoelastic cooling material and specific applications thereof are also disclosed.

Yiming Wu

Papers

Demonstration of high efficiency elastocaloric cooling with large ΔT using NiTi wires

Solid-state heating or cooling systems, devices, and methods

Composition Design and Tensile Properties of Additive Manufactured Hf-Nb-Ta-Ti-Zr High Entropy Alloys Based on Atomic Simulations