Ohmic Heat Generation Research Articles

Thermo-electrochemical evaluation of lithium-ion batteries for space applications

Advanced energy storage and power management systems designed through rigorous materials selection, testing and analysis processes are essential to ensuring mission longevity and success for space exploration applications. Comprehensive testing of Boston Power Swing 5300 lithium-ion (Li-ion) cells utilized by the National Aeronautics and Space Administration (NASA) to power humanoid robot Robonaut 2 (R2) is conducted to support the development of a test-correlated Thermal Desktop (TD) Systems Improved Numerical Differencing Analyzer (SINDA) (TD-S) model for evaluation of power system thermal performance. Temperature, current, working voltage and open circuit voltage measurements are taken during nominal charge–discharge operations to provide necessary characterization of the Swing 5300 cells for TD-S model correlation. Building from test data, embedded FORTRAN statements directly simulate Ohmic heat generation of the cells during charge–discharge as a function of surrounding temperature, local cell temperature and state of charge. The unique capability gained by using TD-S is demonstrated by simulating R2 battery thermal performance in example orbital environments for hypothetical extra-vehicular activities (EVA) exterior to a small satellite. Results provide necessary demonstration of this TD-S technique for thermo-electrochemical analysis of Li-ion cells operating in space environments.

Minimum quench energies of uncooled low temperature superconductors with temperature dependent thermophysical parameters

An analytical method for calculation of minimum quench energies (MQEs) of uncooled composite low temperature superconductors is presented. The method takes into account transient heat transfer in the conductor as well as temperature dependent ohmic heat generation and temperature dependent thermophysical properties of the conductor. MQE of the conductor is calculated based on the analysis of evolution of peak temperature in the normal zone. The method is validated by comparison of the obtained results with the experimental data as well as with analytical and numerical results taken from literature.

Application of thermochromic colorants on textiles: temperature dependence of colorimetric properties

Commercial thermochromic colorants were applied to a conductive cotton fabric prepared by using nichrome/cotton core‐spun yarns in weft and 100% cotton in warp. The fabrics were pretreated and coloured with thermochromic pigments in isolation, in combination with each other, or in a mixture with a non‐thermochromic pigment. The weft yarns were joined to allow passage of current through the fabric to enable ohmic heat generation to increase fabric temperature. The heat generation and temperature rise could be controlled by monitoring the voltage applied. The colour of the samples changed gradually with an increase in temperature, and significantly so when the temperature of the fabric rose beyond the activation temperature of the thermochromic colorant. The temperature‐dependent properties of commercial thermochromic colorants were determined using a spectrophotometer. Wash fastness of the samples was found to be fair in all cases except with the yellow colorant. Predetermined colour effects, such as camouflage or novel design, can be produced by combining thermochromic colorants with conventional pigments or thermochromic colorants with different activation temperatures.

Optimization of Current Leads for HTS Elements Immersed in Liquid Nitrogen

The dimension of metallic current leads is optimized for minimizing the cooling load, taking into account the Ohmic heat generation of conductor part immersed in liquid nitrogen. Since HTS elements in liquid nitrogen are located well below the liquid level for thermal and electrical safety, considerable length of leads is in contact with liquid at constant temperature. In theory, the conductor size needs to be optimized for the upper part above the liquid surface, but a large cross-sectional area is favored for the lower part immersed in liquid in order to reduce the cooling load. On the other hand, this current-lead design aims at a simple metallic conductor with uniform cross-section for the entire length. It is rigorously shown that there exists a unique optimum for the total length-to-area ratio to minimize the overall cooling load, if the length fraction of liquid part is given. A new and useful design formula is derived upon the Wiedemann-Franz law for the temperature-dependent properties of conductor.

Characterization and Quantification of Electronic and Ionic Ohmic Overpotential and Heat Generation in a Solid Oxide Fuel Cell Anode

The development of a solid oxide fuel cell (SOFC) with a higher efficiency and power density requires an improved understanding and treatment of the irreversibilities. Losses due to the electronic and ionic resistances, which are also known as ohmic losses in the form of Joule heating, can hinder the SOFC’s performance. Ohmic losses can result from the bulk material resistivities as well as the complexities introduced by the cell’s microstructure. In this work, two-dimensional (2D), electronic and ionic transport models are used to develop a method of quantification of the ohmic losses within the SOFC anode microstructure. This quantification is completed as a function of properties determined from a detailed microstructure characterization, namely, the tortuosity of the electronic and ionic phases, phase volume fraction, contiguity, and mean free path. A direct modeling approach at the level of the pore-scale microstructure is achieved through the use of a representative volume element (RVE) method. The correlation of these ohmic losses with the quantification of the SOFC anode microstructure are examined. It is found with this analysis that the contributions of the SOFC anode microstructure on ohmic losses can be correlated with the volume fraction, contiguity, and mean free path.

Analytical Prediction of Quench Energies of Cooled Superconductors Based on the Hyperbolic Heat Conduction Model

The critical energy characteristics of cooled composite superconductors is analytically predicted based on the one-dimensional hyperbolic heat conduction model. The temperature dependence of the Ohmic heat generation, the finite speed of heat transfer, and the finite duration and finite length of the thermal disturbances are taken into account in the present model. The critical energies are calculated using a model based on the analytical solution of the hyperbolic heat conduction equation by the Laplace transformation method. The computational model results show that the critical energy depends on the relaxation time and disturbance duration. It is found that the hyperbolic conduction model predicts a lower-critical energy as compared to the predictions of the parabolic heat conduction model.

Numerical Modeling and Performance Study of a Tubular SOFC

A quasi-2D model was proposed and made available for numerical studies on the performance of a single tubular solid oxide fuel cell (SOFC) under practical operating conditions. The model takes account of the air and fuel flow velocity fields, ohmic and thermodynamic heat generation, convective heat-transfer, mass transfer of participating chemical species including the electrochemical processes, and the electric potential and electric current in the electrodes and electrolyte. Numerical computation was carried out to test the proposed model for a single unit cell having a specific geometry being operated at a few different thermal and composition conditions for the inlet fuel and air flows. Obtained numerical results show that the quasi-two-dimensional approximation adopted in the model to mitigate the computational cost effectively can work reasonably well. At low electric current density, the cell terminal voltage was overpredicted. In order to improve the model on this point, the simple treatment adopted for the activation and concentration polarization in the model must be replaced by a more sophisticated approach in future studies. Discussions were further given concerning the obtained results for the overall cell performance and the detailed features of the velocity, thermal, and mass-transfer fields in the cell in addition to the local electrochemical characteristics. It is suggested that the air flow convective heat-transfer is important as a cooling means and that overpotential due to concentration polarization is more serious for the cathode side than for the anode side. All the presented results including the electricity conversion efficiency were observed to agree reasonably well with the popularly accepted cell performance. © 2004 The Electrochemical Society. All rights reserved.

Numerical Study of Superconducting-Tape Thermal Stability under the Effect of a Two-Dimensional Hyperbolic Heat Conduction Model

Thermal stability of a superconducting tape subjected to thermal disturbances under the effect of a two-dimensional hyperbolic heat conduction model is numerically investigated. The thermal inertia of the material, the finite speed of heat transfer, the temperature dependence of the Ohmic heat generation, and the finite duration and finite length of the thermal disturbances are taken into consideration in the mathematical model. Two types of superconductor tapes are considered, Types II and I. It is found that hyperbolic model predicts a wider stable region as compared to the predictions of its parabolic counterpart, the hyperbolic model shows a higher peak at early stages and the difference between the two models is significant at higher values of disturbance intensity and smaller values of time. The effects of different design, geometrical and operating conditions on superconducting tape thermal stability are also studied.

Analytical method for determining critical energies of uncooled superconductors based on the hyperbolic model of heat conduction

The paper presents an analytical method for calculation of critical energies of uncooled composite superconductors based on the hyperbolic equation of heat conduction. The mathematical model developed takes into account: the finite speed of heat transport, the temperature dependence of the ohmic heat generation, the finite duration and the finite length of thermal disturbances. The thermophysical parameters of the conductor are assumed constant. Critical energies are determined by the analysis of variation of the maximum temperature in the normal zone with time. The results obtained are compared with those predicted by the related model based on the classical parabolic equation of heat conduction. The differences are not large but they can be substantial especially in the case of high current density devices.

Explicit expression for critical energy of uncooled superconductors considering transient heat conduction

An analytical method for calculation of critical energies of uncooled composite superconductors is presented based on the transient model for heat conduction. In the model, the dependence of the ohmic heat generation on temperature as well as the finite duration and the finite length of thermal disturbances are taken into account while the thermophysical parameters of the conductor are assumed to be constant. Critical energies, determined on the basis of analysis of the time-dependent maximum temperature in the normal zone, are used to formulate a simple explicit expression for the critical energy by the method of non-linear regression.

Thermal analysis of oxide-isolated stripe diode lasers

The finite-element thermal model of the oxide-isolated stripe geometry diode laser is presented in this work. The calculation procedure was carried out with a standard IBM PC/AT microcomputer. The ohmic contact heat generation and the thickness dox of the SiO2 layer, as two parameters of OIS lasers crucial from a thermal aspect, were taken into account. The system of isotherms obtained for this laser allowed a discussion of the heat spreading process within the laser structure and a comparison of the relative contributions of all heat sources.

Stability of large current aluminium stabilized conductors

A complete stability analysis is given for a highly stabilized large current (50–100 kA) conductor cooled by a He II bath. The analysis concentrates on the minimum normalizing energy pulse, the minimum quench energy pulse (transient stability) and the maximum non-propagating current for constant length travelling normal zones (dynamic stability). The minimum normalizing energy pulse is defined as the energy required to raise the strand temperature above the current sharing level. The quench energy pulse is the pulse over which the conductor cannot recover. The dynamic stability is defined as two constant length normal zones travelling at constant velocity in opposite directions and is related to the change in ohmic heat generation following a normalization pulse. The computational model includes thermal conduction in Nb-Ti/Cu strands and the aluminium stabilizer, the thermal and electrical properties of both, heat transfer to helium at the conductor surface and current diffusion.

Comparison of measured value and calculated value of multiple stability in forced-flow cooled superconductors

The multiple stability observed exclusively in forced-flow cooled superconductors is numerically calculated, and the result is quantitatively compared with the value measured by J.W. Lue et al. (1980). The calculated and measured values agreed well in certain cases, and did not in others. Based on this comparison, the effects of the transient heat transfer coefficient and ohmic heat generation on the quantitative prediction of stability are discussed. From this comparison, it is learned that a precise understanding of the transient heat transfer coefficient is essential for reliable predictions, and also that the ordinary evaluation method of ohmic heat generation, which considers the flux-flow resistance, tends to overevaluate the situation. >

The Hall effect in superconductor stability

The resistive transition process in superconductors stabilized with high-purity normal metal results in a localized transient level of ohmic heat generation well in excess of the steady level. These transient losses result from the redistribution of the transport current by diffusion into the normal metal stabilizer. Recently, a previously unrecognized source of heat generation has been identified: Hall-effect-induced secondary currents in high-purity conductors having non-uniform current distributions. In this paper, the phenomenon of Hall-effect-induced transient currents during transport current redistribution is treated. Approximate calculations show the transient Hall effect heating to be of the same order as the heating associated with the transport current redistribution in a superconductor undergoing a resistive transition, i.e., a propagating normal zone. The phenomenon will also be active in the a.c. current skin effect with high-purity conductors in background fields, increasing the losses above those predicted by ‘skin depth’ computations.

Stability of superconductors against localized disturbances of limited magnitude

Steady state equilibrium theory has been used to predict the size of the minimum energy disturbance needed to quench a cryostable superconductor; experimental results are in reasonable agreement. The size of the disturbance from which a superconductor can recover decreases rapidly with increasing normal state ohmic heat generation in the conductor. Nevertheless, for disturbances of moderate size, stable recovery can occur in conductors with normal state generation higher than the normally accepted values of ∼ 3 kW m −2. It should therefore be possible to design cryostable magnets to operate at higher current densities than hitherto, provided the size of the maximum disturbance can be predicted.