This paper presents a novel approach to characterizing the relevant mechanical, thermal and caloric properties of elastocalorics material in a single testing device. Usually, tensile experiments are performed to determine the rate- and process-depending stress/strain behavior of nickel-titanium-based shape memory alloys and potentially other elastocaloric materials made from metallic alloys. These tests are relevant for, e.g., characterization of hysteresis properties and subsequent calculation of mechanical work input. In addition, simultaneous observation with an infrared camera is useful to understand temperature evolution and maximum temperature changes achievable during the loading/unloading process. Characterization of the caloric properties of the materials determines latent heats and, together with the mechanical work, also the material coefficient of performance. It is typically carried out via differential scanning calorimetry (DSC), which is performed in a separate device and requires a second experiment with different types of samples. Furthermore, DSC measurements do not reflect the way mechanically induced phase transformations trigger the release and absorption of latent heats as it is the case for elastocalorics. In order to provide a more consistent understanding of the relevant elastocaloric material properties, we here present a novel method that (a) allows for a systematic determination of load-dependent latent heats and (b) introduces a comprehensive testing setup and suitable testing routine to determine the mechanical, thermal and caloric parameters in the same experimental device and with the same sample, thus greatly simplifying the overall procedure.

In this paper, a hybrid dynamical model for Shape Memory Alloy (SMA) wire actuators is used to develop a computationally efficient simulation framework for solid-state thermo-mechanical cooling devices. The hybrid model is grounded on a physical description of the SMA based on the Müller-Achenbach-Seelecke model, which consists of a set of ordinary differential equations (ODEs) describing the dyna...

The original article listed the manuscript authors in the following order: Nicolas Michaelis, Andreas Schütze, Felix Welsch, Susanne-Marie Kirsch, Stefan Seelecke

Elastocaloric cooling uses solid-state NiTi-based shape memory alloy (SMA) as a non-volatile cooling medium and enables a novel environment-friendly cooling technology. Due to the high specific latent heats activated by mechanical loading/unloading, substantial temperature changes are generated in the material. Accompanied by a small required work input, a high coefficient of performance is achievable. Recently, a fully-functional and illustrative continuous operating elastocaloric air cooling system based on SMA was developed and realized. To assist the design process of an optimized device with given performance and efficiency requirements, a fully coupled thermo-mechanical system-level model of the multi-wire cooling unit was developed and implemented in MATLAB. The resulting compact simulation tool is qualified for massively parallel computation on modern multi-core computers, which allows fast and comprehensive parameter scans. The comparison of first measurements and simulation results showed differences in the system performance. As the airflow rate influences the thermal power and the outlet temperature significantly, the demonstrator is extended with a spatial airflow measurement system to analyze the crossflow between the hot and cold side. Following, the fluid transport model is advanced by the effect of cross-flow losses, and first modeling results with the variation of airflow rate and rotation frequency are presented.

Elastocaloric cooling uses solid-state NiTi-based shape memory alloy (SMA) as a non-volatile cooling medium and enables a novel environment-friendly cooling technology without global warming potential. Due to the high specific latent heats activated by mechanical loading/unloading, large temperature changes can be generated in the material. Accompanied by a small required work input, a high coefficient of performance is achievable. Recently, a fully-functional and illustrative continuous operating elastocaloric fluid cooling system based on SMA is developed and realized, using a novel mechanical concept for individual loading and unloading of multiple SMA wire bundles. The fluid-based heat transfer system is designed for efficient heat exchange between the stationary heat source/sink and the SMA elements, operates without any additional heat transfer medium. Rotation frequency and fluid flow-rate are adjustable during operation, which allows adapting the operation point to power- or efficiency-optimized demands. The versatile placement of the in- and outlets allows different duct lengths and counter-flow or parallel-flow experiments. To investigate the air flow parameters at the in- and outlets, as well as the crossflow between the hot and cold side, a measurement system is developed and integrated. In this contribution, the first measurement results of the output temperatures for inlet air flow variation in combination with different rotation frequencies are presented.

Elastocaloric (EC) cooling uses solid-state NiTi-based shape memory alloy (SMA) as a non-volatile cooling medium and enables a novel environment-friendly cooling technology. Due to the high specific latent heats activated by mechanical loading/unloading, substantial temperature changes are generated in the material. Accompanied by a small required work input, a high coefficient of performance is achievable. Recently, a fully functional and illustrative continuous operating elastocaloric air cooling system based on SMA was developed and realized. To assist the design process of an optimized device with given performance and efficiency requirements, a fully coupled thermo-mechanical system-level model of the multi-wire cooling unit was developed and implemented in MATLAB. The resulting compact simulation tool is qualified for massively parallel computation, which allows fast and comprehensive parameter studies. In this work, the influence of different SMA diameters, rotation frequencies, and airflow rates is investigated. The results are analyzed to find the suited parameter for high efficiency (COP) and temperature span.

This contribution provides an experimental analysis of relevant parameters to determine the air cooling potential of elastocaloric shape memory alloys (SMAs) in the form of wires. An essential step consists in developing a scientific test setup, which offers the ability to determine these parameters under varying conditions like wire geometry, material composition, and process parameters.With the help of this test setup the influence of different airflow rates on the heat transfer from SMA wire to air and the coherent heat transfer coefficients as well as the approximate air temperature ΔT values are determined and presented in this contribution. Furthermore, the latent heat of the SMA material is experimentally identified with a novel approach based on comparing dynamic temperature changes achieved by elastocaloric phase transformation with direct caloric heating. Analysing these parameters will greatly support the simulation, development and construction of elastocaloric air cooling devices to provide environmentally friendly alternatives to the prevalent vapour compression based cooling principles.

Before elastocaloric shape memory alloys (SMAs) can be used as heat transfer medium in cooling applications, the material needs to be stabilized in its mechanical and thermal behaviour. This process consists in tensile loading and unloading with low strain rates for up to 100 cycles and is currently observed with the help of the sample’s stress–strain diagram as well as infrared images of the sample to illustrate the phase transformation. Afterwards the sample can be used in cooling applications by applying high strain rates for loading and unloading to achieve relevant temperature changes and high cooling efficiency. This contribution discusses a new approach for monitoring the material stabilization by analysing the self-sensing properties of the SMA. With the help of a scientific test setup different self-sensing parameters have been investigated, with the result that even a relatively simple resistance measurement of the SMA during the elastocaloric training process reflects the stress behaviour and therefore the material stabilization. These results allow an implementation of the monitoring approach directly in SMA-based cooling devices without expensive components such as force and thermographic sensors.

The following contribution provides analyses of the air cooling potential of elastocaloric shape memory alloys (SMAs) in form of NiTiCo wires. An essential step consists in developing a scientific test setup, which offers the ability to determine the air-cooling potential under varying conditions like wire geometry, material composition, and process parameters. With the help of this test setup the influence of different airflow rates on the heat transfer from SMA wire to air and the coherent heat transfer coefficients as well as the expectable air temperature Delta T values are determined and presented in this contribution. Analysing these parameters will greatly support the simulation, development and construction of elastocaloric air cooling devices to provide environmentally friendly alternatives to the prevalent vapour compression based cooling principles.

Elastocaloric cooling based on NiTi exhibits an excellent cooling capability, due to the high specific latent heats activated by mechanical loading/unloading and the small required work input. The current funding period of the DFG Priority Program SPP 1599 focuses on the development and realization of a continuously operating elastocaloric cooling device. A novel mechanical loading concept and a novel bundle concept of thin SMA wires enable a thermodynamically optimized cooling process by utilizing a minimal amount of SMA material in a compact design space and high cooling capability. The versatile realization of the device allows the independent variation of process parameters, thermal boundary conditions, and material parameters. This work presents the design process from the development of the novel loading and heat exchange concept, the fluid management and the wire arrangement up to the fabrication and validation of an efficient continuously operating cooling device based on SMA.

Elastocaloric cooling is a novel environment-friendly alternative to vapor compression-based cooling systems. This solid-state cooling technology uses NiTi shape memory alloys (SMAs) as cooling medium. SMAs are well known for lightweight actuator systems and biomedical applications, but in addition, these alloys exhibit excellent cooling properties. Due to the high latent heats activated by mechanical loading/unloading, large temperature changes can be generated in the material. Accompanied by a small required work input, this also leads to a high coefficient of performance superior to vapor compression-based systems. In order to access the potential of these alloys, the development of suitable thermodynamic cooling cycles and an efficient system design are required. This paper presents a model-based design process of an elastocaloric air-cooling device. The device is divided into two parts, a mechanical system for continuously loading and unloading of multiple SMA wire bundles by a rotary motor and a heat transfer system. The heat transfer system enables an efficient heat exchange between the heat source and the SMA wires as well as between the SMA wires and the environment. The device operates without any additional heat transfer medium and cools the heat source directly, which is an advantage in comparison to conventional cooling systems. The design of this complex device in an efficient manner requires a model approach, capable of predicting the system parameters cooling power, mechanical work and coefficient of performance under various operating conditions. The developed model consists of a computationally efficient, thermo-mechanically coupled and energy based SMA model, a model of the system kinematics and a heat transfer model. With this approach, the complete cooling system can be simulated, and the required number of SMA wires as well as the mechanical power can be predicted in order to meet the system requirements. Based on the simulation results a first prototype of the elastocaloric cooling system is realized.

Solid-state cooling is an environmentally friendly, no global warming potential alternative to vapor compression-based systems. Elastocaloric cooling based on NiTi shape memory alloys exhibits excellent cooling capabilities. Due to the high specific latent heats activated by mechanical loading/unloading, large temperature changes can be generated in the material. The small required work input enables a high coefficient of performance. An overview of elastocaloric cooling from basic principles, such as elastocaloric cooling cycles, material characterization, modeling, and optimization, to the design of elastocaloric cooling devices is presented. Current work performed within the DFG (Deutsche Forschungsgemeinschaft) Priority Program SPP1599 Ferroic Cooling, which is focused on the development and realization of a continuously operating elastocaloric cooling device, is highlighted. The cooling device operates in a rotatory mode with wires under tensile loading. The design allows maximization of cooling power by suitable wire diameter scaling as well as efficiency optimization by implementing a novel drive concept. Finally, computer-aided design (CAD) models of the discussed solid-state air cooling device are presented.

The following contribution presents a new concept of an air conditioning device based on the elastocaloric cooling effect of shape memory alloys (SMA’s). This technology provides an energy efficient and environment friendly alternative to conventional vapor compression based cooling principles. Starting from the thermodynamic investigation of the elastocaloric cooling process, a continuous operating elastocaloric air cooling device is developed. The device enables an optimized thermodynamic process control under various operating conditions as well as large temperature spans. This work presents the design process of such a system starting from SMA based heat engines to a thermodynamically optimized design of an elastocaloric air conditioning device.