Kazuo Onda

Performance analysis of planar-type unit SOFC considering current and temperature distributions

The solid oxide fuel cell (SOFC) is expected to be a candidate for distributed power sources in the next generation, due to its high efficiency, high-temperature waste heat utilization and low emission of pollutants to the environment. In this study, a quasi-two- (co- and counter-flow) and three- (cross-flow) dimensional simulation program for planar-type SOFC was made considering mass, charge and heat balances along the flow directions and perpendicular to the electrolyte membrane, in order to obtain temperature and current density distributions along the flow direction. Numerical results from this simulation with adiabatic boundary conditions show that the temperature increases along the flow direction in the co-flow case and the temperature profile has a maximum near the fuel inlet in the counter-flow case. The effects of the gas re-circulation ratio, operating pressure and physical properties on current and temperature distributions were also studied. The temperature distribution is uniform irrespective of flow type under the boundary condition of radiative exchange between the outer interconnector and the electric furnace surface with a realistic view factor. Temperature and current density profiles are discussed considering the Nernst potential and overvoltage changes along the flow direction.

Experimental Study on Heat Generation Behavior of Small Lithium-Ion Secondary Batteries

The overpotential resistance and the entropy change of two small lithium-ion secondary batteries, which are the heat source terms to increase the battery temperature, have been measured by several methods, changing the battery temperature and the state of charge. The temperature increase and the total heat generation rate of the batteries were calculated during the discharge cycle by using the measured resistance and entropy, being compared with the experimental results of the temperature increase and the total heat generation rate. The overpotential resistance was estimated by four measurement methods, i.e., the battery voltage-current characteristics during the constant current discharge, the difference between the open-circuit voltage and the cell voltage, the voltage change during the intermittent discharge for 60 s, and the ac impedance measurement. The overpotential resistance by the voltage-current characteristics is almost the same as by the difference between the open-circuit voltage and the cell voltage. However, in some cases the resistances by the intermittent discharge and the ac impedance are smaller than the former two resistances. The entropy change AS measured by the temperature change of the open-circuit voltage agrees almost with the AS measured by the heat production difference between the charge and discharge cycle. The temperature increases and the total heat generation rates estimated from the overpotential resistances by the voltage-current characteristics and ΔS by the temperature change of open-circuit voltage agree well with the measured ones for the two batteries during the constant current discharge.

Performance Analysis of Polymer-Electrolyte Water Electrolysis Cell at a Small-Unit Test Cell and Performance Prediction of Large Stacked Cell

Recently the hydrogen energy system has been proposed as a countermeasure for the depletion of fossil fuel and global warming. The polymer electrolyte electrolysis cell (PEEC) can efficiently produce pure hydrogen under high current density. To design a PEEC properly and to optimize its operating conditions we have measured and analyzed the PEEC performance. Using measured overpotentials we have made a two-dimensional simulation code for PEEC. Calculated results show that the profile of current density and temperature are constant along the water flow direction, because the exothermic heat from overpotentials is almost canceled out by the endothermic heat of both entropy change and evaporation, and by heat transfer to the constant-temperature separators, resulting in a constant water-electrolyzing potential along the flow direction. The current densities measured at a segmented-electrode cell agreed well with the calculated values. By applying this simulation code to a large unit-cell with adiabatic boundary conditions, we have predicted the performance of a large stacked PEEL having an electrode length of 1 m. The predicted cell temperature and current density increase only a little along the flow direction. Under operating conditions with high pressure, the endothermic heat of water evaporation decreases greatly and the cell temperature is apt to increase downstream compared to the atmospheric operation.

OH radical generation by atmospheric pressure pulsed discharge plasma and its quantitative analysis by monitoring CO oxidation

OH radicals play a very important role in non-thermal plasma chemical reactions for decomposition of gaseous pollutants or synthesis of methanol from methane, etc. In this paper, the CO oxidation monitoring method, which has been used in atmospheric chemistry, was examined to measure the concentration of OH radicals produced by a pulsed discharge plasma. The concentration of OH radicals in the discharge plasma of H2O/Ar mixture gas was estimated by measuring the amount of CO2 produced through oxidation of CO by OH radicals. The experimental results and the calculated results showed that it is possible to measure OH radicals using this simple method. In this experimental work, the maximum concentration of OH radicals produced by the pulsed discharge plasma of H2O/Ar was measured to be 9.4×1014 molecules cm−3 pulse−1. Within the temperature range investigated in this study (50–150°C), the formation of OH radicals increased with increase in the specific input energy (discharge energy dissipated in unit volume of the gas; SIE) value and the content of H2O; in contrast, it decreased with increase in the gas temperature.

Electric discharge removal of SO2 and NOx from combustion flue gas by pulsed corona discharge

Based upon the reported studies on removal of SO2 and NOx from combustion flue gas by pulsed corona discharge, experimental studies on simulated combustion gas have been conducted changing parameters such as pulse width, polarity, wire electrode diameter, discharge gap, gas composition, gas temperature, etc. that are believed to be fundamental in the deSOx and deNOx process. Corona wind, which is expected to promote mixing of oxidizing radicals with SO2 and NOx in pulsed corona discharge, has been tried to be visualized. Future works concerning about measurements of oxidizing radical OH concentration and about simulation of electric discharge chemical process, which are now in progress, have been put in order to explain more details of deSOx and deNOx processes.

Improving Characteristics of Ozone Water Production with Multilayer Electrodes and Operating Conditions in a Polymer Electrolyte Water Electrolysis Cell

Recently, ozone is used for many purposes as an environmentally friendly oxidant. An ozone production device with high ozone concentration and low production energy is therefore desired. One candidate for such a device isozone water production in a water electrolysis cell using a solid polymer electrolyte with PbO 2 anode catalyst. Such a device would have the advantages of being compact and producing high-concentration ozone water directly through deionized water electrolysis. We have studied ozone water production with different electrode and electrolyte compositions and operation conditions, with the aim of improving ozone production performance. The two electrolytes tested were Nafion 117 and a membrane electrode assembly (MEA) with Pt catalyst on the cathode side of Nafion 117. The two electrodes tested were a single layer of Ti expanded metal and four layers of Ti-expanded metal with different meshes. Ozone water production tests were performed for many hours with changes in temperature, water flow rate, current density, current interruption time, and other factors to optimize experimental conditions. The voltage-current characteristics of water electrolysis cell were improved significantly when the electrode was four layers of Ti-expanded metal and the electrolyte was MEA with Pt catalyst on the cathode. Stable ozone water concentration was obtained after the cell had been operated for about 8 h. The optimum temperature, water flow rate, and current density for ozone water production are 25-30°C, 33 L/h, and 1.0 A/cm 2 , respectively. Further, the noninterrupted supply of small current suppressed the performance deterioration of ozone water production by current interruption, and the ozone production energy was reduced by supply of oxygen to the cathode.

Cycle Analysis of Combined Power Generation by Planar SOFC and Gas Turbine Considering Cell Temperature and Current Density Distributions

The planar solid oxide fuel cell (SOFC) with a Y 2 O 3 -stabilized ZrO 2 electrolyte is expected to be a candidate for distributed power sources in the next generation due to its high efficiency of power generation. In this study, we analyzed the system performance of a SOFC/gas turbine combined cycle of about 500 kW electrical output, using our two-dimensional simulation code for the planar SOFC with internal reformer. The effects of cell temperature, cell pressure, recirculation rations of fuel and air, utilization ratios of fuel and air, and average current density of SOFC on both the system efficiency and the cell temperature and current density distributions, were calculated under typical operating conditions taking account of realistic efficiencies and heat losses for auxiliary equipment. The addition of a Cheng cycle to the SOFC/gas turbine combined cycle improved system efficiency by 1-3%. The combined SOFC/gas turbine/Cheng cycle gave a high efficiency of 61.2% (based on a higher heating value) even under a small power generation scale of 500 kW class at 2.0 MPa SOFC pressure. We also discussed the possibility of carbon deposition at both external and internal reformers by calculating the chemical equilibrium carbon activities for estimated carbon deposition reactions.

Thermal Behavior of Small Nickel/Metal Hydride Battery during Rapid Charge and Discharge Cycles

To improve a nickel/metal hydride (Ni/MH) batterys rapid charge/discharge performance and enlarge it, a precise understanding of the thermal behavior is required. This report examines numerically and experimentally the thermal behavior of the Ni/MH cell during rapid charge and discharge cycles by considering entropy changes for electrochemical reactions, the exothermic reaction for hydrogen adsorption as MH, the exothermic heat from the side reaction, the heat generation by overpotential, and the heat transfer to ambient air. The overpotential resistance and the current efficiency, the ratio of main reaction current to charge current, have been measured during rapid charge and discharge cycles with constant current. Our proposed model works well to estimate the cell temperatures during charge and discharge cycles not only at smaller currents than the rated one, but also at rapid charge (∼2C) and discharge (∼3C) currents.

Cell Impedance Measurement by Laplace Transformation of Charge or Discharge Current–Voltage

In our previous study overpotential resistances of nickel/metal-hydride and lithium-ion batteries were measured to estimate the battery temperature rises during rapid charge and discharge cycles by using our battery thermal model. However, the cell impedance Z(ω) measured by ac impedance meter did not agree with those induced by charge/discharge characteristics. Therefore, Z(ω) values were again measured by the method of Takano et al. [J. Electrochem. Soc., 147, 922 (2000)], who measured Z(ω) of lithium-ion batteries by Laplace transformation of both signals of the voltage-step input and its current response. This method has been extended here to Laplace transformation of current-step or current-pulse input and its voltage response to measure Z(ω) for any charge/discharge current of nickel/metal-hydride or lithium-ion battery. Nearly the same Z(ω) was obtained by the three different methods (voltage-step, current-step, and current-pulse inputs), and the measured Z(w) did not depend on either the charge/discharge current or the state of charge/charge input. Moreover, the Z(ω) measured by the current-pulse method, which includes the Warburg impedance at low frequency, approaches the overpotential resistance that can provide a good estimate of the battery temoerature rise in our batterv thermal model.

Analysis of Current Distribution at PEFCs Using Measured Membrane Properties and Comparison with Measured Current Distribution

In order to properly understand the power generation performance of polymer electrolyte fuel cells (PEFCs), it is necessary to have accurate data on water management, such as the diffusion coefficient of water through the membrane electrode assembly (MEA) and gas diffusion layer (GDL), electro-osmotic coefficient through MEA, and power loss data such as the activation and resistance overpotentials. In this study we measured these data with the aim of analyzing our experimental results from PEFC power generation tests done using our two-dimensional simulation code. Our code simultaneously solves mass, charge, and energy conservation equations, and the equivalent electric-circuit for PEFC to obtain numerical distributions of hydrogen/oxygen concentrations, cell potential, current density, and gas/cell-component temperatures. The current density distributions calculated with our simulation code were compared with the distribution measured using a segmented electrode cell. The distributions measured under various operating conditions agreed well with the calculated ones, demonstrating that our code is reliable. The concentration overpotential through GDL was also estimated with the parallel fine-pore model, but the estimated concentration overpotential was very small. Also, the cathode flooding is discussed with the calculated distribution of saturation degree along the channel flow, in comparison with experimental stability.