Development of Thermoelastic Cooling Systems

Thermoelastic cooling, or elastocaloric cooling, is a cutting-edge solid-state based alternative cooling technology to the state-of-the-art vapor compression cooling systems that dominate the world today. Being environment friendly without any global warming potential, these thermoelastic cooling systems could reduce energy expenses and carbon emission since they are potentially more efficient than the vapor compression cycle (VCC). Nevertheless, as a result of its immature nature, its realistic application potential requires comprehensive research in material fundamentals, cycle design, system simulation, the proof-of-concept prototype development and testing. Therefore, understanding the performance potential and limitations of this emerging new cooling technology, building the theoretical framework and guiding future research are the objectives of this dissertation. Thermodynamic fundamentals of elastocaloric materials are introduced first. Cycle designs and theoretical performance evaluation are presented together with a detailed physics-based dynamic model for a water-chiller application. The baseline system coefficient of performance (COP) is 1.7 under 10 K system temperature lift. To enhance the system performance, a novel thermo-wave heat recovery process is proposed based on the analogy from the highly efficient “counter-flow” heat exchanger. Both the theoretical limit of the “counter-flow” thermo-wave heat recovery and the practical limitations by experimentation have been investigated. The results indicated that 100% efficiency is possible in theory, 60% ~ 80% heat recovery efficiency can be achieved in practice. The world first of-its-kind proof-of-concept prototype was designed based on the dynamic model, fabricated and tested using the proposed heat recovery method. Maximum cooling capacity of 65 W and maximum water-water temperature lift of 4.2 K were measured separately from the prototype. Using the validated model, performance improvement potentials without manufacturing constraints in the prototype are investigated and discussed. The COP is 3.4 with the plastic insulation tube and tube-in-tube design, which can be further improved to 4.1 by optimizing the system operating parameters. A quantitative comparison is made for thermoelastic cooling and other not-in-kind cooling technologies in order to provide insights on its limitations, potential applications, and directions for future research. Though under current research status, the system efficiency is only 0.14 of Carnot efficiency, which is less than 0.21 for conventional VCC systems, the framework carried out in this dissertation shows a technically viable alternative cooling technology that may change the future of our lives

Dynamic Performance Of A Compression Thermoelastic Cooling Air-Conditioner Under Cyclic Operation Mode

Traditional vapor compression cooling refrigerants are considered as high global-warming-potential (GWP) gases which face more and more legislation pressure nowadays. As an alternative option other than using synthetic low GWP refrigerants and natural refrigerants, solid-state cooling technologies show their advantages of zero GWP, and therefore recently attract more attentions. Apart from those well-studied solid-state cooling technologies, such as thermoelectric cooling, thermoacoustic cooling and magnetic cooling, thermoelastic cooling, a.k.a. elastocaloric cooling, is still under development and shows potential of better thermal performance compared with its competitors. In fact, from material perspective, it was estimated by literatures that the COPs for elastocaloric materials are 20% - 120% higher than other solid-state cooling materials under the same operating conditions. This study introduces the thermoelastic cooling concept at the beginning, and then demonstrates one method to operate the compression thermoelastic cooling cycle for air-conditioning application based on the reverse Martensitic phase transition principle. A dynamic model is developed to measure the temperature within the cycle in cyclic operation mode. The cyclic operation is a Brayton cycle consisting of an adiabatic Martensite-Austenite phase transition process, a constant strain heat transfer process between the solid-state refrigerant and the heat sink/source, and a heat recovery process aiming to improve the overall performance. The model uses experimental curve-fitted data to predict the work required to drive the cycle. The coefficient of performance (COP) and cooling capacity are then evaluated based on the power prediction. Parametric studies are conducted to investigate the influence of several significant parameters on the COP and cooling capacity from the model. It is found that the cycle duration parameters, the solid state refrigerant thickness, and operating flow rates are major contributing factors to the performance indices. Based on the parametric study results, a design guideline is then provided for the future researches

Experimental Evaluation of Compressive Elastocaloric Cooling System

Elastocaloric cooling, or thermoelastic cooling, has the potential to substitute the state-of-the-art vapor compression cooling systems. The elastocaloric effect refers to the latent heat associated with the stress-induced martensitic phase transformation process in shape memory alloys. In this study, we demonstrated our latest testing results of world’s first of-its-kind prototype based on elastocaloric effect. Two beds with Ni-Ti alloy tubes were implemented in the system to generate cooling and heating. Water was used as the heat transfer fluid to reject heat to an air-cooled heat sink and deliver cooling to an electric heater. The system was driven by a linear actuator under compression mode. Operating under a single stage reverse Brayton cycle, the two beds design also enabled the heat recovery and work recovery feature to enhance the system’s performance. The initial effort led to a successful elastocaloric cooling system prototype with useful cooling capacity of 38 W at a water-to-water temperature lift of 1.5 K. To enhance the system performance, a series of modifications were applied to the system. By better aligning the linear actuator and the two Ni-Ti tube beds, the system temperature lift was increased up to 1.8 K due to increased strain and latent heat. The plastic insulation tube design significantly reduced the heat loss from heat transfer fluid to the metal parts, which successfully increased the system temperature lift to 2.8 K. Furthermore, by modifying the motor layout and making it compress more Ni-Ti tubes per bed the system achieved a 4.2 K temperature lift with 65 W cooling capacity. Plastic insertions to block partial heat transfer fluid inside each Ni-Ti tube reduced the cyclic loss associated with periodic heating and cooling of the heat transfer fluid, which boosted the system temperature lift to 4.7 K.  Unfortunately, the active thermal mass of Ni-Ti tubes was too small when compared to the big losses in the prototype, which indicated that the system mass should be reduced significantly. The system temperature lift of 6.1 K was predicted based on the test results, when assuming no pump parasitic heat generation and no heat conduction loss to the metal supporting frame in each bed. This study built the step stone for elastocaloric cooling technology by successfully achieving an effective cooling for the first time in the world. However, the elastocaloric cooling technology needs a substantial following research to enhance its performance

Elastocaloric Cooling of Additive Manufactured Shape Memory Alloys with Large Latent Heat

The stress-induced martensitic phase transformation of shape memory alloys (SMAs) is the basis for elastocaloric cooling. Here we employ additive manufacturing to fabricate TiNi SMAs, and demonstrate compressive elastocaloric cooling in the TiNi rods with transformation latent heat as large as 20 J g−1. Adiabatic compression on as-fabricated TiNi displays cooling DT as high as −7.5 °C with recoverable superelastic strain up to 5 %. Unlike conventional SMAs, additive manufactured TiNi SMAs exhibit linear superelasticity with narrow hysteresis in stress-strain curves under both adiabatic and isothermal conditions. Microstructurally, we find that there are Ti2Ni precipitates typically one micron in size with a large aspect ratio enclosing the TiNi matrix. A stress transfer mechanism between reversible phase transformation in the TiNi matrix and mechanical deformation in Ti2Ni precipitates is believed to be the origin of the unique superelasticity behavior

Fatigue-resistant high-performance elastocaloric materials made by additive manufacturing

Elastocaloric cooling, a solid-state cooling technology, exploits the latent heat released and absorbed by stress-induced phase transformations. Hysteresis associated with transformation, however, is detrimental to efficient energy conversion and functional durability. We have created thermodynamically efficient, low-hysteresis elastocaloric cooling materials by means of additive manufacturing of nickel-titanium. The use of a localized molten environment and near-eutectic mixing of elemental powders has led to the formation of nanocomposite microstructures composed of a nickel-rich intermetallic compound interspersed among a binary alloy matrix. The microstructure allowed extremely small hysteresis in quasi-linear stress-strain behaviors—enhancing the materials efficiency by a factor of four to seven—and repeatable elastocaloric performance over 1 million cycles. Implementing additive manufacturing to elastocaloric cooling materials enables distinct microstructure control of high-performance metallic refrigerants with long fatigue life

Effectiveness of Entransy Dissipation Metric and Entropy Generation Units in The Design of Fin-Tube Heat Exchangers

Several techniques and metrics based on the Second Law of Thermodynamics have been used in the past for the analysis of heat exchangers. The terms used for these techniques include irreversibility analysis, entropy generation minimization, exergy analysis and thermodynamic efficiency. Entransy is a recently developed concept reflecting the heat transfer potential, rather than the ability to convert heat to work. Entransy is transferred along with heat flux in the heat transfer process, and subsequently dissipates. The entransy dissipation extremum principle is applicable to heat transfer enhancement. Entropy on the other hand is a thermodynamic state-based quantity. This study focuses on the comparison of entransy dissipation and entropy generation units in the context of optimizing the widely used fin-tube heat exchanger. Local entransy balance equations are established and implemented in a finite-volume based fin-tube heat exchanger model. The model can then calculate the entransy dissipation in each control volume, as well as the total dissipation for the entire heat exchanger. Parametric study about two heat exchangers, one undergoing only single phase heat transfer (water coil) and the other undergoing both single phase and two-phase heat transfer (R134a evaporator) are conducted without water condensation on the air side

Overcoming fatigue through compression for advanced elastocaloric cooling

Elastocaloric materials exhibit extraordinary cooling potential, but the repetition of cyclic mechanical loadings during long-term operation of cooling systems requires the refrigerant material to have long fatigue life. This article reviews the fundamental cause of fatigue from aspects of initiation and propagation of fatigue cracks in shape-memory alloys (SMAs) that are used as elastocaloric materials, and highlights recent advances in using compression to overcome fatigue by curtailing the generation of surfaces associated with crack propagation. Compression is identified as a key means to extend fatigue lifetime in engineering design of elastocaloric cooling drive mechanisms. We summarize the state-of-the-art performance of different SMAs as elastocaloric materials and discuss the influence of low cyclic strains and high resistance to transformation. We present integration of compression-based material assemblies into a cooling system prototype and optimization of the system efficiency using work recovery and related measures.</p

Overcoming fatigue through compression for advanced elastocaloric cooling

Elastocaloric materials exhibit extraordinary cooling potential, but the repetition of cyclic mechanical loadings during long-term operation of cooling systems requires the refrigerant material to have long fatigue life. This article reviews the fundamental cause of fatigue from aspects of initiation and propagation of fatigue cracks in shape-memory alloys (SMAs) that are used as elastocaloric materials, and highlights recent advances in using compression to overcome fatigue by curtailing the generation of surfaces associated with crack propagation. Compression is identified as a key means to extend fatigue lifetime in engineering design of elastocaloric cooling drive mechanisms. We summarize the state-of-the-art performance of different SMAs as elastocaloric materials and discuss the influence of low cyclic strains and high resistance to transformation. We present integration of compression-based material assemblies into a cooling system prototype and optimization of the system efficiency using work recovery and related measures.</p

Energy-Efficient Elastocaloric Cooling by Flexibly and Reversibly Transferring Interface in Magnetic Shape-Memory Alloys

Elastocaloric cooling is currently under extensive study owing to its great potential to replace the conventional vapor-compression technique. In this work, by employing multiscale characterization approaches, including in situ neutron diffraction in a loading frame, in situ transmission electron microscopy observation at different temperatures, in situ synchrotron X-ray Laue micro-diffraction, and high-resolution infrared thermal imaging, we have investigated the thermal and stress-induced martensitic transformation, the stability of superelastic behavior and the associated elastocaloric effect for a Heusler-type Ni50.0Fe19.0Ga27.1Co3.9 single crystal. On the basis of transformation from cubic austenite into monoclinic martensite with a flexibly and reversibly transferring interface, this unique single crystal exhibits a giant elastocaloric effect of 11 K and ultralow fatigue behavior during above 12 000 mechanical cycles. The numerical simulation shows that the Ni50.0Fe19.0Ga27.1Co3.9 alloy offers 18% energy saving potential and 70% cooling capacity enhancement potential compared to the conventional shape-memory nitinol alloy in a single-stage elastocaloric cooling system, making it a great candidate for energy-efficient air conditioner applications