This paper reports on the research and development of a cooling device with an active electrocaloric regenerator (AER) based on the bulk ceramic material (1-x)Pb(Mg1/3Nb2/3)O3–xPbTiO3 (PMN-100xPT). For the purposes of the study a 2D transient numerical model of the AER was developed. This model makes it possible to investigate the cooling characteristics of a device with an AER while considering the effect of the hysteresis of the electrocaloric material and the effect of the electric-energy recovery related to the polarization/depolarization process. The results of the numerical analyses show that the degree of electric energy recovery has a major impact on the efficiency of the device. By considering an idealised system for electric-energy recovery the energy efficiency (expressed by the coefficient of performance) of the device could be increased by up to ten times. A validation of the numerical model was performed through the design, construction and experiments on an improved AER cooling device. The results revealed a maximum specific cooling power of 16?W kg-1 and a maximum temperature span of 3.1?K for the new device.
COBISS.SI-ID: 31863079
A study was made of the electrocaloric (EC) effect’s stability in relaxor Pb(Mg1/3Nb2/3)O3 multilayer elements. The sample was subjected to million unipolar cycles at an electric field amplitude of 110 kV cm-1. The dielectric and ferroelectric properties of the material change only slightly, while the microstructure does not reveal any detrimental evidence of the cycling. The initially measured EC temperature change of 1.45 K decreases by only 0.01 K upon cycling, exhibiting a fatigue-less behavior. The results justify the choice of relaxor multilayers as the working bodies in EC cooling devices, where the material should withstand numerous electric field cycles with high amplitudes, sometimes exceeding 100 kV cm-1.
COBISS.SI-ID: 30569511
This article reports on the novel resistive electromagnetic field source with the magnetic energy recovery, which enables the use of the static magnetocaloric regenerator. Most of the existing prototype magnetocaloric devices that operate near room temperature, use magnetic field sources consisting of permanent magnets. The alternating of the magnetic field that is required for the thermodynamic cycle often comes from the rotation of magnets over the refrigerant, that is, an active magnetocaloric regenerator (AMR). Such systems require moving parts and a motor drive, both of which cause additional costs and reduced energy efficiency. Further restrictions in existing devices result from the speed of the magnetisation/ demagnetisation process, which is, in addition to efficient heat transfer, crucial for the compactness of the device. Another drawback is that the instant change of the magnetic field is not feasible, regardless of the principle of movement. Permanent-magnet assemblies based on neodymium are also constrained by the use of this rare-earth-material. Therefore, a number of global research activities relate to the optimization of permanent-magnet-based magnetic field sources. However, ohmic loss, the active cooling of magnets, and considerable energy consumption are the reasons why another type of magnetic field source, that is, the electromagnet, was generally avoided by the magnetocaloric community. This article presents a novel and unique approach that enables substantially improved energy efficiency and applicable operation of rare-earth-free and static electromagnetic field sources, by implementing for the first time the magnetic energy recovery for magnetic refrigeration and heat pumping. To prove the advantages of such a system, a large number of numerical simulations, as well as an experimental proof, were conducted. A comparative analysis was made for the evaluation of the energy efficiency of the proposed novel system vs an example of the existing rotating-magnet assembly. The results of this study reveal that this new type of electromagnetic field sources provides a number of different and important advantages that can lead to new frontiers in research. However, the energy efficiency is still lower than that of the comparable rotating-magnet assembly.
COBISS.SI-ID: 16410139
Electrocaloric (EC) cooling elements in the form of multilayers (MLs) were prepared. The elements consist of five layers of the relaxor-ferroelectric 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3, about 60 µm thick, with internal platinum electrodes and exhibiting a dense, uniform microstructure with a grain size of 1.7 µm. The largest temperature change ?TEC of 2.26 K was achieved at an electric field (E) of 100 kV cm-1 and at 105 °C, measured by a high-resolution calorimeter. These results agree well with the indirect measurements. The EC coefficient, ?TEC/?E, obtained for the MLs, is similar to the value obtained for bulk ceramics of the same composition. The ?TEC values above 2 K over a broad temperature range from 75 to 105 °C make the ML elements suitable candidates for EC cooling devices at significantly lower voltages than bulk ceramic plates with comparable dimensions and mass.
COBISS.SI-ID: 29796903