Electrocaloric Devices Research Articles

Concept design and numerical evaluation of a highly efficient rotary electrocaloric refrigeration device

Abstract Enabled by the discovery of giant electrocaloric effect (ECE) in inorganic ceramics and organic polymers, Electrocaloric (EC) cooling technology has attracted considerable attention from both academia and industrial. However, there remains many challenges in the refrigeration efficiency and/or capacity of the currently reported electrocaloric refrigeration devices. In this work, we propose a concept design of EC refrigeration device, in which the working bodies are rotating in a plane perpendicular to the direction of fluid flow. The rotary EC device adopting heat transfer fluid as an intermediate medium allows for improved heat exchange between EC materials and outside environment, and efficiently delivers cooling effects to where it’s needed. Simulations have been performed to investigate the system cooling performance under different operating strategies. Meanwhile, comparative researches on different heat transfer fluid are conducted and the corresponding cooling effects are studied. When using deionized water as heat transfer medium, the system can achieve a cooling power density of 9.89 W·cm−3 and COP of 10.48 over 10 K temperature span. In general, this work provides a highly-efficient rotary EC refrigeration device concept that employs fluid–solid conjugated heat transfer, and guides the optimization of operating strategy and selection of heat transfer fluid.

Electrostatic Actuating Double‐Unit Electrocaloric Cooling Device with High Efficiency

AbstractCompact solid‐state refrigeration systems that offer a high specific cooling power and a high coefficient of performance (COP) are desirable in a wide range of applications where efficient and localized heat transfer is required. Here, a double‐unit electrocaloric (EC) polymer‐based refrigeration device with high intrinsic thermodynamic efficiency is demonstrated using a flexible EC polymer film with improved performance by doping plasticizer and an electrostatic actuation mechanism. The double‐unit refrigeration device achieves a large temperature span of 4.8 K, which is 71% higher than that of the single‐unit device. A specific cooling power of 3.6 W g−1 and a maximum COP of 8.3 for the cooling device are produced. The surface temperature of a central processing unit (CPU) cooled by an active EC device is 22.4 K lower than that of the CPU cooled in air. The highly efficient and compact EC cooling device demonstrated here not only leapfrogs the performance of existing solid‐state cooling technologies, but also brings solid state cooling closer to reality for a variety of practical applications that require compact or mechanically flexible refrigeration.

Electrocaloric cooling over high device temperature span

Summary Electrocaloric cooling demonstrates direct electricity utilization and high energy efficiency, and is often proposed as an environmentally benign, solid-state alternative to the conventional vapor compression cooling technology. As a result, the fabrication of effective electrocaloric materials and engineering of electrocaloric cooling devices have attracted increasing attention in the cooling community. However, the limited material adiabatic temperature change under safe operation field poses a major challenge to practical electrocaloric cooling device development, since real-world applications usually require device temperature span over 20 K. This perspective presents recent efforts devoted to design electrocaloric cooling devices based on active heat regeneration and cascading approaches; both strategies have achieved significant device temperature lift ~10 K with adiabatic temperature change of 2~3 K. The strategies discussed in this work are broadly applicable to the design of efficient and compact cooling systems.

Structural and electrical properties of K(Ta,Nb)O3 thin film prepared by sol-gel method for electrocaloric devices

Structural and electrical properties of K(Ta,Nb)O3 thin film prepared by sol-gel method for electrocaloric devices

Electrocaloric devices part I: Analytical solution of one-dimensional transient heat conduction in a multilayer electrocaloric system

The analytical solution is reported for one-dimensional (1D) dynamic conduction heat transfer within a multilayer system that is the typical structure of electrocaloric devices. Here, the multilayer structure of typical electrocaloric devices is simplified as four layers in which two layers of electrocaloric materials (ECMs) are sandwiched between two semi-infinite bodies representing the thermal sink and source. The temperature of electrocaloric layers can be instantaneously changed by external electric field to establish the initial temperature profile. The analytical solution includes the temperatures in four bodies as a function of both time and location and heat flux through each of the three interfaces as a function of time. Each of these analytical solutions includes five infinite series. It is proved that each of these series is convergent so that the sum of each series can be calculated using the first [Formula: see text] terms of the series. The formula for calculating the value of [Formula: see text] is presented so that the simulation of an electrocaloric device, such as the temperature distribution and heat transferred from one body to another can be performed. The value of [Formula: see text] is dependent on the thickness of electrocaloric material layers, the time of heat conduction, and thermal properties of the materials used. Based on a case study, it is concluded that the [Formula: see text] is mostly less than 20 and barely reaches more than 70. The application of the analytical solutions for the simulation of real electrocaloric devices is discussed.

Electrocaloric devices part II: All-solid heat pump without moving parts

Various designs have been introduced to build heat pumps using the electrocaloric effect (ECE). Each of all the current designs uses at least one moving part, which significantly reduces the reliability of the pump and adds complexities. In this work, a new all-solid design is introduced, in which two layers of an electrocaloric material (ECM) are permanently sandwiched in the source and sink, which would significantly increase the device’s reliability since nothing moves and all are permanently bound together. More importantly, the electric fields applied on two ECM layers are independently controlled. A special sequence for the electric fields on two ECM layers is introduced. Numerical calculation was used to simulate the device’s performance by using the newly introduced analytical solutions for the heat conduction in the system. It is concluded that a continuous heat transformation from the source to sink at the same temperature can be achieved when the contacting coefficient, [Formula: see text], is very small, where [Formula: see text], [Formula: see text], and [Formula: see text] are thermal conductivity, density, and heat capacity, respectively, while the superscript [Formula: see text] and [Formula: see text] represent the ECM and source/sink, respectively.

A cascade electrocaloric cooling device for large temperature lift

Cooling technology that is both compact and flexible is increasingly vital for the thermal management of wearable electronics and personal comfort. Electrocaloric (EC) cooling provides a potential solution, but the low adiabatic temperature change of EC materials has been the bottleneck in its progress. We demonstrate a cascade EC cooling device that increases the temperature change, with enhanced cooling power and cooling efficiency at the same time. The device integrates multiple units of EC polymer elements and an electrostatic actuation mechanism, all operating in synergy. Every two adjacent EC elements function in antiphase (in terms of both actuation and EC effect) to allow heat flow to be continuously relayed from the heat source to the heat sink. The antiphase operation also enables internal charge recycling, which enhances the energy efficiency. Operating at the EC electric field at which the adiabatic temperature change of the material is 3.0 K, a four-layer cascade device achieves a maximum temperature lift of 8.7 K under no-load conditions. The coefficient of performance is estimated to be 9.0 at the temperature lift of 2.7 K and 10.4 at zero temperature lift. Solid-state caloric cooling is a promising alternative to vapour-compression refrigeration yet practical devices are not sufficiently efficient for applications. Meng et al. combine cascade device design with charge recovery and improve the cooling efficiency and temperature lift of an electrocaloric device.

Numerical evaluation of a kilowatt-level rotary electrocaloric refrigeration system

Abstract Electrocaloric (EC) cooling technology has rejuvenated rapidly in recent years and gained the great attention in both academia and industrial. Currently, there are only handful studies of EC cooling devices reported over the years, and most of them are focusing on small-scale cooling applications. For practical application, however, it is essential for an EC device to achieve a kilowatt level of cooling power. In this work, we develop a rotary EC device, in which the working bodies are rotating in-plane, and the heat transfer fluid flow through the working bodies in out-of-plane direction. The design allows the continuous fluid–solid conjugated heat transfer and exhibits kilowatt level of cooling power. Numerical study has been performed to explore the internal correlation between the key parameters of the working condition and performance of the device. For a given EC working body, the electric-induced heat generation/absorption is found to significantly affect the cooling power. Meanwhile, parametric studies on the temperature span and cyclic period of the system are conducted. By fine-tuning the operation parameters, the system achieves an optimized overall performance, i.e., a cooling power of 1729.90 W, when operated over a temperature span of 10 K and cyclic period of 10 s. This work provides a general topological design of a rotary device that employs fluid–solid conjugated heat transfer, and guides the optimization of operation parameters to achieve giant cooling power.

Numerical prediction of thermal performance of an electrocaloric device based on ceramic material

Numerical prediction of thermal performance of an electrocaloric device based on ceramic material

Antiferroelectric Bent-Core Liquid Crystal for Possible High-Power Capacitors and Electrocaloric Devices

We present small-angle X-ray scattering, polarized optical microscopy and electric current measurements of a sulfur-containing bent-core liquid crystal material for characterization of the layer and director structures, thermally and electrically driven transitions between antiferroelectric and ferroelectric structures and switching properties. It was found that the material has polarization-modulated homochiral synclinic ferroelectric (SmCsPFmod), homochiral anticlinic antiferroelectric (SmCaPA) and racemic synclininc antiferroelectric (SmCsPA) structures that can be reversibly switched between each other either thermally and/or electrically. High switching polarization combined with softness of the liquid crystalline structure makes this compound a good candidate for applications in high-power capacitors and electrocaloric devices.

Effects of uniaxial compressive stress on the electrocaloric effect of ferroelectric ceramics

The effects of uniaxial compressive stress on the electrocaloric effect (ECE) of ferroelectric ceramics are studied by Landau–Ginzburg–Devonshire (LGD) thermodynamic approach, direct method, and indirect techniques. Soft lead zirconate titanate ceramics is used as a model material. The direct measurement results are given by an infrared camera combined with a set of specially designed testing setup. It is demonstrated that ECE can be significantly tuned by uniaxial compressive stress. The direct measurement results are essentially in agreement with the LGD theory calculated results, while significant discrepancies between direct and indirect methods are observed. These results are explained by the complex domain switching and possible phase transition behavior under the coupled thermo-electro-mechanical field. In addition, with compressive stress of 50 MPa, direct measurement shows that an improvement of ~ 66.7% in cooling capacity can be achieved at 375 K, which demonstrates that the application of compressive stress is an effective approach for enhancing ECE in ferroelectric ceramics. Our results not only provide insights into the effects of uniaxial compressive stress on ECE, but also offer more opportunities for the design of electrocaloric materials and devices.

Correlation between phase compatibility and efficient energy conversion in Zr-doped Barium Titanate

Recent demonstrations of both heat-to-electricity energy conversion devices and electrocaloric devices based on first-order ferroelectric phase transformations identify the lowering of hysteresis and cyclic reversibility of the transformation as enabling criteria for the advancement of this technology. These demonstrations, and recent studies of the hysteresis of phase transformations in oxides, show that satisfying conditions of supercompatibility can be useful for lowering hysteresis, but with limitations for systems with only a few variants of the lower symmetry phase. In particular, it is widely accepted that in a classic cubic-to-tetragonal phase transformation, with only three tetragonal variants having only six twin systems, tuning for improved crystallographic compatibility will be of limited value. This work shows that, on the contrary, the tuning of lattice parameters in Ba(Ti1-xZrx)O3 for improved crystallographic compatibility, even at low doping levels of Zr (x ≤ 0.027), give significant improvement of transformation and ferroelectric energy conversion properties. Specifically, the transformation hysteresis is lowered by 25%, and the maximum value of the polarization/temperature ratio dP/dT at the phase transformation is increased by 10%.

Structural and electrical properties of Ba(Sr,Ti)O3/ K(Ta,Nb)O3 multilayer thin film for the application of electro-caloric devices

Structural and electrical properties of Ba(Sr,Ti)O3/ K(Ta,Nb)O3 multilayer thin film for the application of electro-caloric devices

Structural and Electrical Properties of K(Ta,Nb)O3 Thin Films for the Application of Electrocaloric Devices

In this study, K(Ta0.6Nb0.4)O3 thin films were fabricated by sol–gel method and their structural and electrical properties were measured to investigate the applicability for electrocaloric devices. As the thickness of thin films increased, crystallinity was improved. The average thickness of the thin films with a single coating was about 130–140 nm. Dielectric constant, dielectric loss, remanent polarization, and coercive field of the 6 times coated thin KTN films at 30 °C were 4920, 0.492, 18.49 μC/cm2 and 54.7 kV/cm, respectively. As the number of coatings increased, the temperature at which the decrease in remanent polarization began also subsided. Likewise, the rate of change in remanent polarization increased as the temperature became higher. When a voltage of 220 kV/cm was applied to the 3 times coated KTN films, the electrocaloric property was 2.95 °C. When the 3 times coated thin KTN films were at 75 °C, electrocaloric property for a unit electric field was about 1.47 °C.

Improved electrocaloric properties in La doped (Bi0.5Na0.5)0.92Ba0.08TiO3 lead-free ceramics

The electrocaloric effect of (Bi0.5Na0.5)0.92Ba0.08–3x/2LaxTiO3 lead free ceramics with x = 0, 0.01, 0.02, and 0.03 was investigated by an indirect method. A high electrocaloric response was achieved in a (Bi0.5Na0.5)0.92Ba0.05La0.02TiO3 composition at 70 °C with an adiabatic temperature change ΔT around 2.11 °C and an electrocaloric strength ΔT/ΔE of 0.42 K mm kV−1, under a relatively low electric field of 50 kV/cm. For this material, a maximum entropy change ΔS of 2.46 J kg−1 K−1 was also obtained under the same conditions. These results are comparable with the best values reported in lead-free materials, suggesting this system as a potential candidate for practical electrocaloric device applications.

Enhanced electrocaloric effect in La-based PZT antiferroelectric ceramics

The electrocaloric effect (ECE) has been investigated in (Pb0.98La0.02)(Zr0.95Ti0.05)0.995O3 antiferroelectric ceramics obtained via the solid-state reaction method. The results from indirect measurements across the temperature range considered reveal a large electrocaloric temperature change (ΔT) of approximately 5 K at 373 K. The enhanced ECE, which is significantly higher than those reported for some lead-free and lead-based electro-ceramics, was obtained by applying an electric field of 60 kV/cm lower than what has been reported for commonly studied ceramic systems. This result suggests that this system is a potential candidate for practical electrocaloric device applications.

Polarizing Oxygen Vacancies in Insulating Metal Oxides under a High Electric Field.

We demonstrate a thermodynamic formulation to quantify defect formation energetics in an insulator under a high electric field. As a model system, we analyzed neutral oxygen vacancies (color centers) in alkaline-earth-metal binary oxides using density functional theory, Berry phase calculations, and maximally localized Wannier functions. The work of polarization lowers the field-dependent electric Gibbs energy of formation of this defect. This is attributed mainly to the ease of polarizing the two electrons trapped in the vacant site, and secondarily to the defect induced reduction in bond stiffness and softening of phonon modes. The formulation and analysis have implications for understanding the behavior of insulating oxides in electronic, magnetic, catalytic, and electrocaloric devices under a high electric field.

Highly efficient electrocaloric cooling with electrostatic actuation.

Solid-state refrigeration offers potential advantages over traditional cooling systems, but few devices offer high specific cooling power with a high coefficient of performance (COP) and the ability to be applied directly to surfaces. We developed a cooling device with a high intrinsic thermodynamic efficiency using a flexible electrocaloric (EC) polymer film and an electrostatic actuation mechanism. Reversible electrostatic forces reduce parasitic power consumption and allow efficient heat transfer through good thermal contacts with the heat source or heat sink. The EC device produced a specific cooling power of 2.8 watts per gram and a COP of 13. The new cooling device is more efficient and compact than existing surface-conformable solid-state cooling technologies, opening a path to using the technology for a variety of practical applications.

An electrocaloric refrigerator with direct solid to solid regeneration

Regenerative cooling has been used to improve the efficiency of refrigerators. Recently, exploiting the easy controlling of the electric fields, a regenerative electrocaloric (EC) cooling device with direct solid to solid regeneration has been proposed. Here, we report an experimental study of such an EC cooling device. For the EC device to be operated at a household voltage, the EC multilayer ceramic elements were first developed employing the multilayer ceramic capacitor fabrication technique. Under a voltage of 200 V, the EC multilayers with the commercial Y5V ceramic composition exhibit an electric field induced temperature change ΔT of 0.86 K. Testing results of a prototype EC device based on these EC multilayer elements show that even with only two EC rings, which is the minimum unit required to demonstrate the operation, such an EC device can generate a temperature span between hot and cold ends about three times of ΔT of the EC multilayers.

Spatially Resolved Imaging of Electrocaloric Effect and the Resultant Heat Flux in Multilayer Capacitors

Multilayer capacitors (MLCs) are one of the most studied electrocaloric (EC) systems because they are regarded as the most promising configuration for developing electrocaloric prototype devices. Indeed, (1) they combine thin films prone to exhibit the so-called giant EC effect; (2) they are made of several hundred active layers; and (3) their internal interdigitated structure alternating highly thermally conductive electrodes with EC layers should dramatically increase their propensity to extract heat. Although the latter has been foreseen by modeling, it has never been observed experimentally. Here we use an infrared camera to observe for the first time the heat flux crossing standard EC MLCs during operation. Interestingly, we observed that heat flux perpendicular to the terminal–terminal direction accounts for only 20% of the total heat flux. This observation strongly suggests that all future electrocaloric cooling devices should rely on a design favoring heat exchange through the MLC terminals.