Takuto Araki

AN EXPERIMENTAL INVESTIGATION OF GASEOUS FLOW CHARACTERISTICS IN MICROCHANNELS

The microchannel is one of the essential components for the construction of various micro systems. However, it has been reported that the flow and heat transfer behavior in a microchannel deviates from predictions based on the conventional assumption generally accepted in macro-scale channels. Frictional characteristics of nitrogen and helium flows in three different microchannels (hydraulic diameter range 3-10 w m) have been investigated experimentally. The frictional resistance of gaseous flow in a trapezoidalcross-section microchannel was observed to be smaller than that in the conventional-sized channel. The reduced frictional resistance in microchannels is caused by the rarefaction effect due to extremely small dimensions of flow passages. By using Maxwells first-order slip boundary condition, we can well predict the mass flow rate through microchannels and the friction constant.

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.

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.

Simulation Study for the Series Connected Bundles of Microtubular SOFCs

Solid oxide fuel cells (SOFCs) have the highest energy conversion efficiency among various power generators and expected to be earlier commercialization. Our study aims to develop fabrication techniques of microtubular SOFC bundles and establish realistic bundle structure for kilowatt class module. So far, we have succeeded to establish fabrication technology of the microtubular SOFC bundles using porous supporting matrices. In this study, the simulation study of the microtubular SOFC bundle was carried out to understand Joule heat and temperature distribution in the microtubular SOFC bundle during operation. The results indicated that the method of current collection had to be carefully considered, since the total output power loss of the bundle was estimated to be 27.8%. The temperature distribution of the bundle using porous MgO matrices turned out to be moderate compared with that in the previous bundle using porous (La, Sr) x(Co, Fe)O 3 matrices due to the difference in the thermal conductivity of each matrix constitute.

Polymer Electrolyte Dehumidifying Cell and Its Application to Air Conditioners

Large amounts of chlorofluorocarbons (CFCs) have been released into the atmosphere, resulting in the destruction of the ozone layer in the stratosphere. There is thus a need to develop air conditioners that do not use CFCs. As one such air conditioner, we propose a combined system of dehumidifying cells that use proton exchange membranes (PEMs) with a water evaporation air cooler. PEMs are the solid electrolytes for polymer electrolyte fuel cells and can transfer water molecules with protons. The performance of existing dehumidifying cells using PEMs is not well understood. As important factors to determine the dehumidifying performance, in this study we first measured the transmissibility and the electro-osmotic coefficient n d of water molecules through the membrane and electrode assembly (MEA), water vapor diffusivity through the gas diffusion layer (GDL), and the mass-transfer coefficient between the channel flow and the GDL. These measured factors were then adopted in a performance analysis of our dehumidifying cell. Our simulation code solves simultaneously the conservation equations of mass and energy with an equivalent electric circuit of the cell. The calculated results describe well the experimental dehumidifying performance. By extending this simulation code we predicted the coefficient of performance (COP) of our novel air-conditioning system. The calculated COP for our test cells is small at 0.10 or 0.21, but can be made as large as 4 if n d of PEM can be increased to 5.

Numerical Analysis of Current Distribution at Proton Exchange Membrane Fuel Cell Compared by Segmented Current Collector Cell

In order to grasp properly proton exchange membrane fuel cell (PEMFC) power generation performances, it is necessary to know factors for water management such as diffusivity and electro-osmotic coefficient of water vapor through the membrane and factors for power loss such as active and resistive overpotentials. In this study, we have measured these factors to analyze our experimental results of PEMFC power generation tests by using our pseudo-two-dimensional simulation code. It considers simultaneously the mass, charge and energy conservation equations, and the equivalent electric circuit for PEMFC to give numerical distributions of hydrogen/oxygen concentrations, current density, and gas/cell-component temperatures. Various experimental conditions such as fuel and oxygen utilization rates, inlet dew-point temperature, averaged current density, and flow configuration (co- or counterflow) were changed, and all of the numerical distributions of current density agreed well with the measured distributions by segmented current collector. The current distributions were also obtained from hydrogen/oxygen concentration changes along the gas flow measured by gas chromatography. The current distributions measured by the two different methods coincided with each other, showing reliability of our measurement methods.

Numerical simulation of de-NOx performance by repetitive pulsed discharge when added with hydrocarbons such as ethylene

Emission regulations are gradually being tightened recently to prevent further air pollution. Cost-effective and efficient technologies must be developed to process the NOx generated in the combustion of fossil fuels. One of the candidate technologies to process NOx is the denitrification of flue gas by pulsed corona discharge, which has been demonstrated experimentally to show high de-NOx performance. However, the optimization of operation conditions and the appropriate understanding of the de-NOx process still remain to be clarified. Therefore, following our previous study on ammonia injection, we have simulated in the present study the de-NOx process to which hydrocarbons such as ethylene have been added to provide guidelines on its proper operation conditions and its main reaction paths to remove NOx. The simulated results show that the removal efficiency in a case of ethylene addition becomes lower than for ammonia addition, but the de-NOx energy consumption rate becomes lower than for ammonia inject...

Developments of mems-based thermocouple array for sensing effects of a flow channel on pemfc local temperature distribution

Measuring temperature inside PEMFCs is necessary for proper water management, because water vapor pressure is a strong function of temperature. In this paper, we have developed thin film thermocouple (TFTC) array to measure temperature distributions near the Cathode Catalyst layer (CCL) with a resolution smaller than the rib-channel scale.The sensor array was placed between CL and gas diffusion layer (GDL) at the cathode. No performance decrement was observed with the insertion of TFTC array. The measurements of temperature distributions showed that the temperature rises at the cathode CL is about 9 °C at 1.2 A/cm2 with supplying 98.5 % RH Hydrogen / Air.Copyright