Letteratura scientifica selezionata sul tema "Electrocaloric refrigeration"

ABSTRACTThe electrocaloric effect holds promise for possible application in refrigeration technologies. There is much interest in this subject and experimental studies have shown the possibility for creating materials with a modest sized electrocaloric response. However, theoretical studies lag behind the experimental effort due to the lack of computational methods to accurately study the finite temperature response. Here the freely distributed feram, an effective Hamiltonian molecular dynamics method, is demonstrated for predicting the electrocaloric response of BaTiO3.

An electrocaloric material with a negative and positive electrocaloric effect (ECE) is identified to be a high potential candidate for solid-state refrigeration technology due to a changing dipolar entropy under a varying electric field.

The electrocaloric effect of ferroelectrics is promising for new solid-state refrigeration. However, the current research on the electrocaloric effect of bulk ferroelectrics mainly focuses on (001) orientation. Thus, we studied the electrocaloric effect of BaZr0.15Ti0.85O3 single crystals with different orientations through the nonlinear thermodynamic approach and entropy analysis. The results show that the dipolar entropy of (111)-oriented BaZr0.15Ti0.85O3 single crystals exhibits a greater change after applying an external electric field, compared with (001)- and (110)-orientations, and the (001)-oriented electrocaloric responses are consistent with experimental observations. The (111)-oriented BaZr0.15Ti0.85O3 single crystals have a more significant electrocaloric response, resulting in a broader work temperature range with a large electrocaloric effect. These insights offer an alternative way to enhance the electrocaloric response of ferroelectric single crystals.

An ultrahigh room temperature adiabatic temperature change (∼1.6 K) was realized in a BaTiO₃-based bulk ceramic prepared by the tape casting technique, which makes a giant step-forward for electrocaloric refrigeration.

Chip scale refrigeration system is critical for the development of electronics with the rapid increase of power consumption and substantial reduction of device size, resulting in an emergent demand on novel cooling technologies with a high efficiency for the thermal management. In this thesis, active refrigeration devices based on Stirling cycle and an electrocaloric material, are designed and investigated to achieve a high cooling performance. Firstly, a new Stirling micro-refrigeration system composed of arrays of silicon MEMS cooling elements is designed and evaluated. The cooling elements are fabricated in a stacked array on a silicon wafer. A regenerator is placed between the compression (hot side) and expansion (cold side) diaphragms, which are driven electrostatically. Under operating conditions, the hot and cold diaphragms oscillate sinusoidally and out of phase such that heat is extracted to the expansion space and released from the compression space. A first-order of thermodynamic analysis is performed to study the effect of geometric parameters. Losses due to regenerator non-idealities and chamber heat transfer limitation are estimated. A multiphysics computational approach for analyzing the system performance that considers compressible flow and heat transfer with a large deformable mesh is demonstrated. The optimal regenerator porosity for the best system COP (coefficient of performance) is identified. To overcome the computational complexity brought about by the fine pillar structure in the regenerator, a porous medium model is used to allow for modeling of a full element. The analysis indicates the work recovery of the system and the diaphragm actuation are main challenges for this cooler design.The pressure drop and friction factor of gas flow across circular silicon micro pillar arrays fabricated by deep reactive ion etch (DRIE) process are investigated. A new correlation that considers the coupled effect of pillar spacing and aspect ratio, is proposed to predict the friction factor in a Reynolds v number range of 1-100. Silicon pillars with large artificial roughness amplitudes is also fabricated, and the effect of the roughness is studied in the laminar flow region. The significant reduction of pressure drop and friction factor indicates that a large artificial roughness could be built for pillar arrays in the regenerator to enhance the micro-cooler efficiency. The second option is to develop a fluid-based refrigeration system using an electrocaloric material poly(vinylidene fluoride-trifluoroethylene-chlorofluoroethylene) [P(VDF-TrFE-CFE)] terpolymer. Each cooling element includes two diaphragm actuators fabricated in the plane of a silicon wafer, which drive a heat transfer fluid back and forth across terpolymer layers that are placed between them. Finite element simulations with an assumption of sinusoidal diaphrahm motions are conducted to explore the system performance detailedly, including the effects of the applied electric field, geometric dimensions, operating frequency and externally-applied temperature span. Multiphysics modeling coupled with solid-fluid interaction, heat transfer, electrostatics, porous medium and moving mesh technique is successfully performed to verify the thermal modeling feasibility. The electrocaloric effect in thin films of P(VDF-TrFE-CFE) terpolymer is directly measured by infrared imaging at ambient conditions. At an electric field of 90 V/μm, an adiabatic temperature change of 5.2 °C is obtained and the material performance is stable over a long testing period. These results suggest that application of this terpolymer is promising for micro-scale refrigeration.

Les enjeux énergétiques ont pris une importance considérable dans notre vie quotidienne. Ils doivent répondre en effet au double défi du besoin croissant d'énergie et des considérations environnementales et écologiques. Bien que l'énergie fossile présente des avantages indéniables, ses défauts poussent constamment la recherche vers des solutions alternatives. Dans cette course, des matériaux écologiques et performants sont toujours recherchés en vue du développement des condensateurs de stockage d'énergie et des dispositifs de refroidissement électrocalorique (EC). On outre, le contexte de la miniaturisation nécessite la fabrication des matériaux de taille de plus en plus réduite allant de céramiques aux films minces nanostructurés (1D) ayant des structures hiérarchiques en passant par les films minces (2D) denses et continus. La présente thèse a pour objectif de mieux cerner l'apport de la dimensionnalité, l'architecture, la géométrie et l'orientation des grains au sein des céramiques et des films minces élaborés afin d'augmenter la densité d'énergie stockée et l'effet électrocalorique de ces matériaux. Nous avons développé dans un premier temps des céramiques de Ba0.85Ca0.15Zr0.1Ti0.9O3 (BCZT) par deux méthodes de synthèse (sol-gel : BCZT-SG et électrofilage : BCZT-ES). Une optimisation de temps de frittage a été menée sur ces céramiques. Les analyses effectuées nous ont permis de caractériser l'effet de la méthode de synthèse et le temps de frittage sur les propriétés diélectriques, électrocaloriques et de stockage d'énergie des céramiques de BCZT. Les caractérisations physico-chimiques ont révélé d'une part que les céramiques de BCZT frittées pendant 6h présentent des propriétés microstructurales, diélectriques, électrocaloriques et de stockage d'énergie améliorées. D'autre part, les céramiques préparées par la méthode d'électrofilage (BCZT-ES) sont plus performantes que celles préparées par le procédés sol-gel (BCZT-ES). Cela pourrait être attribué à la finesse des grains des poudres de BCZT-ES et au rapport important des proportions des phases tétragonale (T) et orthorhombique (O) coexistantes dans ces céramiques. En effet, la céramique de BCZT-ES-6h présente une densité d'énergie récupérée intéressante de 233,69 mJ/cm3 et une efficacité énergétique élevé de 72,17% à E = 40 kV/cm, au voisinage de la transition ferroélectrique-paraélectrique. Ainsi, cette céramique montre un coefficient électrocalorique atteignant une valeur élevée de ζ ≈ 0,523 K.mm/kV. L'utilisation de BCZT sous forme de couche mince au lieu de la céramique nous a permis d'appliquer des champs électriques plus forts favorisant des propriétés de stockage d'énergie intéressantes. Une optimisation des paramètres expérimentaux (liant utilisé, mode et température de recuit de cristallisation) a été faite sur les films minces de BCZT déposés par la méthode sol-gel combinée à la technique de revêtement par centrifugation. Le film BCZT-850 cristallise dans une structure pérovskite pure présentant une microstructure granulaire relativement dense. Il présente une valeur élevée de Wr = 1,211 J/cm2 avec une efficacité énergétique intéressante de 72,31% à un champ appliqué de 372 kV/cm. Afin de comprendre l'effet de la forme et de l'orientation des grains sur les propriétés diélectriques des films minces, une synthèse hydrothermale des films nanostructurés (1D) verticalement alignés de BaTiO3/TiO2 a été menée. Une étude systématique des différents paramètres de synthèse a permis de déterminer les conditions optimales d'élaboration de ce type de films présentant une structure hiérarchique 1D avec une orientation préférentielle des nanotiges suivant l'axe c, un pourcentage élevé de la phase de BaTiO3 (94%), et une permittivité diélectrique (εr′ ) très élevée de ≈ 16085. À la lumière de ces résultats, ce travail de thèse peut contribuer à la conception de matériaux écologiques ayant un effet électrocalorique et des performances de stockage d'énergie électrique
Energy issues have taken on considerable importance in our daily lives. They have to address the dual issue of the expanding energy demand as well as environmental and ecological concerns. Although, fossil fuels have undeniable advantages, their drawbacks are constantly driving the search for alternative solutions. In this regard, eco-friendly and effective materials are always being investigated for the development of energy storage capacitors and electrocaloric (EC) refrigeration devices. Additionally, the context of miniaturization requires the fabrication of smaller and smaller material sizes, ranging from ceramics to dense and continuous (2D) thin films to nanostructured (1D) thin films exhibiting hierarchical structures. The goal of this thesis is to understand how the dimensionality, architecture, geometry, and grain orientation of the ceramics and thin films intend to enhance the electrocaloric effect and stored energy density of these materials. By using two synthesis techniques: sol-gel (SG) and electrospinning (ES), we first synthesized BCZT ceramics (BCZT-SG and BCZT-ES) which underwent a sintering time optimization process. Then, the bulk ceramics were subjected to studies in order to evaluate how the synthesis technique and sintering time influenced the dielectric, electrocaloric, and energy-storage properties of BCZT ceramics. The physico-chemical characterisations revealed that on the one hand, BCZT ceramics sintered for 6h had improved microstructural, dielectric, electrocaloric and energy storage properties, on the other hand, the ceramics prepared by the electrospinning method (BCZT-ES) are more efficient than those prepared by the sol-gel process (BCZT-ES) which could be attributed to the fineness of the grains of the BCZT-ES powders and to the significant ratio of the coexisting tetragonal (T) and orthorhombic (O) phases in ceramics.. Indeed, the BCZT-ES-6h ceramic showed an interesting recovered energy density of 233.69 mJ/cm3 and a high energy efficiency of 72.17% at E = 40 kV/cm, around the ferroelectric-paraelectric transition. Thus, this ceramic shows an electrocaloric coefficient reaching high value of ζ ≈ 0.523 K.mm/kV. Instead of using ceramic, using BCZT thin layers produced using the sol-gel process coupled with the spin coating technique, allowed us to apply higher electric fields, which promoted interesting energy storage properties. For these BCZT thin films, experimental parameters (binder, mode and temperature annealing) were optimized. the BCZT-850 film crystallises in a pure perovskite phase with relatively dense granular microstructure. It exhibits a high value of Wr = 1.211 J/cm3 with an interesting energy efficiency of 72.31% at an applied field of 372 kV/cm. In addition, a hydrothermal synthesis of vertically aligned (1D) nanostructured BaTiO3 (BT) films was conducted to better understand the effect of grain shape and orientation on the dielectric properties of thin films. The optimal conditions for the synthesis of these types of films were established by a methodical analysis of the various hydrothermal synthesis parameters. A 1D BaTiO3 lattice with nanorods preferentially aligned along the c-axis exhibiting a high aspect ratio of ≈ 9.27, and a very high dielectric constant of ≈ 16539 were produced by using the optimal parameters

Les matériaux caloriques à l'état solide, qui subissent un changement de température adiabatique ou un changement d'entropie isothermal lorsque certains stimuli externes (champ électrique, champ magnétique, contrainte ou pression mécanique) est appliquée ou retirée, sont prometteurs pour la réfrigération à l'état solide, comme alternative aux dispositifs de refroidissement conventionnels inventé il y a cent ans qui utilisent des gaz dangereux. Compte tenu des améliorations des systèmes de réfrigération à compression de vapeur approchant très vite de leur limite d'efficacité théorique, en plus des préoccupations environnementales accrues, il y a eu récemment une recrudescence de la recherche mondiale pour de nouvelles solutions de réfrigération plus économiques et respectueuses de l'environnement. Les caloriques les plus importants sont les matériaux "ferroiquement" ordonnés (ferroélectriques, ferroélastiques et ferromagnétique / antiferromagnétique) qui présentent souvent des effets caloriques géants près de leurs transitions ferroïques. Dans cette thèse, nous présentons nos résultats théoriques et expérimentaux sur l'effet électrocalorique, élastocalorique, barocalorique et magnétocalorique dans différents matériaux ferroïques. Nos résultats montrent que tous ces effets caloriques peuvent donner des solutions de réfrigération prometteuses avec un faible impact environnemental. Nous abordons les ferroélectriques qui apparaissent comme matériaux idéaux permettant à la fois des réponses électrocaloriques, élastocaloriques et barocaloriques géantes près de la température ambiante. Pour la première fois, nous mettons en évidence un effet électrocalorique négatif dans des films minces antiferroélectriques et nous proposons un nouveau mécanisme pour comprendre la réponse calorique dans antiferroiques en général incluant antiferroélectrique et antiferromagentique. Par ailleurs, pour la première fois en utilisant une caméra infra-rouge, nous effectuons la mesure résolue spatialement sur l'effet électrocalorique dans des condensateurs multicouches, l'un des systèmes les plus étudiés considérés comme le prototype électrocalorique le plus prometteur. Nos résultats fournissent la première preuve expérimentale directe sur le flux de chaleur électrocalorique à la fois temporellement et spatialement dans un dispositif électrocalorique spécifique. En outre, pour la première fois, nous concevons un cycle de réfrigération multicalorique combinant effet électrocalorique avec des effets élastocaloriques / magnétocaloriques via des matériaux ferroélectriques. Nous avons réalisé ce cycle mutlicalorique pour résoudre un problème réel et de longue date, à savoir une grande hystérésis magnétique qui a empêché l'utilisation pourtant prometteuse de FeRh découvert il y a près de 26 ans en tant que matériau magnétocalorique. Nous espérons que cette thèse fournira non seulement des connaissances utiles pour comprendre fondamentalement l'effet calorique à l'état solide dans les matériaux ferroïques et ce qui est véritablement mesuré, mais pourra aussi servir de guide pratique pour exploiter et développer les ferrocalorics vers la conception de dispositifs appropriés
Solid-state caloric materials, which undergo an adiabatic temperature change or isothermal entropy change when some external stimulus (electric field, magnetic field, stress and pressure) is applied or withdrawn, are promising for solid-state refrigeration, as an alternative to hazardous gases used in conventional cooling devices invented a hundred years ago. Given that the highly refined vapor-compression refrigeration systems asymptotically approach their theoretical efficiency limit in addition to the concern on environment, there has been a recent upsurge in worldwide search for new refrigeration solution which is economical and environmentally friendly. The most prominent calorics are ferroically ordered materials (ferroelectric, ferroelastic and ferromagnetic/antiferromagentic) that often exhibit giant caloric effects near their ferroic transitions. In this thesis, we present our theoretical and experimental results on electrocaloric effect, elastocaloric effect, barocaloric effect and magnetocaloric effect in different ferroic materials. Our findings show that all these caloric effects may appear promising with low environmental impact. We address ferroelectrics emerging as ideal materials which permit both giant elastocaloric, electrocaloric and barocaloric responses near room temperature. For the first time, we find a large negative electrocaloric effect in antiferroelectric thin films and we propose a new mechanism to understand the caloric response in antiferroics including antiferroelectric and antiferromagentic. In addition, for the first time using Infra-red camera we carry out spatially-resolved measurement on electrocaloric effect in multilayer capacitors, one of the most studied systems which are regarded as the most promising electrocaloric prototype. Our findings provide the first direct experimental evidence on the electrocaloric heat flux both temporally and spatially in a specific electrocaloric device. Moreover, for the first time, we design a multicaloric refrigeration cycle combining electrocaloric effect with elastocaloric/magentocaloric effects bridged by ferroelectric materials. We realized such mutlicaloric cycle to solve a real and longstanding problem, i.e., a large hysteresis that impeded reversibility in an otherwise promising magnetocaloric material FeRh discovered almost 26 years ago. We hope that this thesis will not only provide a useful background to fundamentally understand the solid-state caloric effect in ferroics and what we are really measuring, but also may act as a practical guide to exploit and develop ferrocalorics towards design of suitable devices

The electrocaloric effect (ECE) is a phenomenon in which reversible temperature and entropy changes of a material due to polarization appear under the application and removal of an electric field. Materials with a giant ECE have recently been reported, suggesting practical application in cooling devices. In this paper, a refrigeration system composed of silicon MEMS cooling elements is designed based on the ECE in a terpolymer. Finite element simulations are performed to explore the system performance. The effect of the form of the applied electric field is studied. The time lag between the electric field and the diaphragm motion is found to affect the cooling power significantly. A parametric study of the operating frequency is also conducted. The results indicate that when the system is operated at a temperature difference of 5 K, a cooling power density of 2 W/cm2 is achieved for one element.

Electrocaloric (EC) cooling technology, which has reversible temperature change of a polarizable material in an adiabatic condition with the application and/or removal of an electric field, exhibits some great advantages for efficient solid-state refrigeration. However, many challenges still exist in EC cooling technology. One of the main challenges is how to control the heat transfer direction. Some of the reported device types require movement of EC material by step motor or fluid media by pump back and forth between heat source and heat sink for controlling heat transfer direction. The other device designs utilize thermal diodes by adjusting their thermal conductivity to control heat transfer direction. Here we report a solid-state electrocaloric refrigeration using unimorph beam structure which has temperature change due to EC effect and bending behavior due to converse piezoelectric effect. The new device design can eliminate problems of fluid medium loss, friction, high thermal conductivity ratio requirement and external system assistance, etc., existed in the previously reported EC cooling device types. An analytical model is also derived by considering multi-physical phenomenon. The model shows that the temperature change is a combinatorial result from the couplings of thermal, electric and mechanical field in the device.

Solid state refrigeration processes, such as magnetocaloric and electrocaloric refrigeration, have recently shown to be a promising alternative to conventional compression refrigeration. A new solid state elastocaloric refrigeration process using the latent heats within Shape Memory Alloys (SMA) could also hold potential in this field. This work investigates the elastocaloric effects in Ni-Ti-based superelastic Shape Memory Alloy (SMA) systems for use in an elastocaloric cooling processes. Ni-Ti alloys exhibits large latent heats and a small mechanical hysteresis, which may potentially lead to the development of an efficient environmentally friendly solid-state cooling system, without the need for ozone-depleting refrigerants. A systematic investigation of the SMA is conducted using a novel custom-built scientific testing platform specifically designed to measure cooling process related phenomena. This testing system is capable of performing tensile tests at high rates as well as measuring and controlling the solid-state heat transfer between SMA and heat source/heat sink. Tests are conducted following a cooling process related training cycle where the material has achieved stabilized behavior. First, a characterization of the elastocaloric material properties is performed followed by an investigation of the material under cooling process conditions. A comprehensive monitoring of the mechanical and thermal parameters enables the observation of temperature changes during mechanical cycling of the SMA at high strain rates. These observations can be used to study the rate dependent efficiency of the elastocaloric material. The measurement of the temperature of both the heat source/heat sink and the SMA itself, as well as the required mechanical work during a running cooling process, reveals the influence of the operating conditions on the elastocaloric effect of the material. Furthermore investigations of the process efficiency at different thermal boundary conditions (temperature of heat source/heat sink), indicates that the process is dependent on the boundary conditions which have to be controlled in order to optimize the efficiency.

The coupling of the electrocaloric effect in thin films with thermal switches has the potential to be used for efficient refrigeration. We studied the unsteady heat transfer performance and periodic thermal-switching behavior of a flat heat pipe to transfer cold energy from a changing heat source. The condenser of the flat heat pipe was the changing heat source and changed from −20 W to +20 W every 5 s. The temperature of the condenser surface changed in accordance with the heat generation of the heat source. The evaporator was a plate with a mesh wick attached to a water-flow pipe. Cold energy transferred from the condenser surface to the evaporator surface only when the temperature of the condenser surface was lower than that of the evaporator surface. We analyzed the unsteady temperature change and heat transfer performance of the flat heat pipe by numerical simulation. The analytical results showed that it was necessary to have two thermal switches to separate the heat energy and cold energy of the changing heat source. Also, it was important to reduce the thermal resistance and heat capacity of the evaporator surface to improve the unsteady heat transfer performance of the heat pipe. Next, we measured the unsteady heat transfer performance of the flat heat pipe experimentally. The experimental results showed that the thermal-switching behavior was observed when the heat generation of the heat source changed every 5 s.

Abstract It has been shown that about 20% of the global electricity supply is used for refrigeration and cooling. Thus, the substantial global warming impact of refrigerants remains a crucial concern. In an attempt to solve this issue, caloric cooling technology has shown significant potential as an alternative to vapor-compression technology. Caloric cooling technology utilizes the latent heat of a solid-state phase transformation of a caloric material by applying an external trigger. The caloric cooling technologies can be broken down into four categories including magnetocaloric, electrocaloric, barocaloric, and elastocaloric. Among these four materials, the considerable elastocaloric effect and easy actuation of these materials make them an excellent alternative to the usual approach. The elastocaloric effect is defined by the adiabatic temperature change (△ Tad) in elastocaloric materials when uniaxial stress (load) is applied or removed in the adiabatic state. This phenomenon can be found in all shape memory alloy (SMAs) materials. To elaborate further, SMAs undergo a stress-induced transformation between austenite and martensite phases upon adiabatic loading and unloading. These phase transitions result in △ Tad (heating or cooling) within the materials. For example, during unloading, the endothermic reverse transformation from martensite to austenite leads to decreased temperature (cooling) within the sample. It has been well-defined in the literature that NiTi materials, a well- known type of SMA materials, are the best candidate in the elastocaloric industry due to their large latent heat and excellent mechanical properties and result in a large elastocaloric effect. There has been a limited number of studies that have explored the elastocaloric effect of NiTi. Additionally, it has been shown in the literature that the elastocaloric effect of this material can be optimized by adjusting the process parameters, providing a new route to investigate the elastocaloric effect of additively manufactured NiTi. It should be known that any change in the thermomechanical and mechanical behavior of NiTi materials results in a difference of the elastocaloric effect. To elaborate further, any process or building orientation controls that enhance the strength of the martensitic and austenitic phases will reduce slipping and enable recoverable strains to reach higher levels, resulting in a larger △Tad. In this comprehensive study, the effect of building orientation on the elastocaloric response was investigated and followed by an investigation of the effect of porous structures on the elastocaloric response.

The electrocaloric effect (ECE) refers to the change in temperature and/or entropy of a dielectric material due to the electric field induced change of dipolar states. Giant ECE is discovered in P(VDF-TrFE) ferroelectric copolymers near ferroelectric-paraelectric (F-P) transition temperature which is normally much higher than room temperature. This paper presents the two defect-inducing methods to lower and broaden working temperature range of P(VDF-TrFE) based copolymers for ECE, and thus make it preferable for practical cooling device. Giant ECE is experimentally demonstrated in large temperature range (0–55°C). In addition, an electrocaloric oscillatory refrigerator (ECOR) was proposed and simulated by finite volume method and its high performance was theoretically demonstrated. Temperature gradient larger than 30 °C can be maintained across the two sides of a 1 cm device. For ΔT = 20 °C cooling condition, a high cooling power (5.4 W/cm2) and significantly higher coefficient of performance (COP) can be achieved (50% of Carnot efficiency).