Hisataka Yakabe

3-D model calculation for planar SOFC

Abstract A three-dimensional mathematical model for a planar SOFC was constructed. The concentrations of the chemical species, the temperature distribution, the potential distribution, and the current density were calculated using a single-unit model with double channels of co-flow or counter-flow pattern. The finite volume method was employed for the calculation, which is based on the fundamental conservation laws of mass, energy, and electrical charge. The internal or external steam-reforming, the water-shift reaction, and the diffusion of gases in the porous electrodes were taken into the model. The effects of the cell size, the operating voltage and the thermal conductivity of the cell components on the calculated results were investigated. From the simulated temperature distributions in the electrolyte and the inter-connector, the stress distributions were calculated using the finite element method. The results demonstrated that the steam reforming would generate internal stresses of several tens MPa in an electrolyte.

Evaluation and modeling of performance of anode-supported solid oxide fuel cell

For an anode-supported planar SOFC, a single-unit with double channels was modeled for a counter-flow pattern, and the concentration polarization at the anode was estimated. The flow phenomena were simulated using the finite volume method and the distribution of the gaseous species was calculated. In the model, it was assumed that the gas flow in the porous anode is governed by Darcys Law, and the reactant species are transported to the electrolyte/anode interface mainly by diffusion in a multicomponent mixture system. For binary H2–H2O and CO–CO2 systems, the calculated concentration polarization was found comparable to the experimental results. As an example for a multicomponent system, a model using steam-reformed methane as a fuel was employed to simulate the concentration polarization at a high fuel utilization. From the simulated results, it was evident that the shift reaction effectively reduces the concentration polarization when the fuel utilization is high.

Correlation between Hall Numbers and Tc in Y1-xCaxBa2Cu3O7-δ ( 0≤x≤0.5) Thin Films Prepared by RF Thermal Plasma Deposition Method

The temperature dependence of the resistivity and the Hall coefficient R H was measured for Y1-x Cax Ba2Cu3O7-δ thin films prepared on SrTiO3 substrates using the RF thermal plasma deposition method. Transition temperature T c and resistivity of the as-grown film decrease with increasing Ca concentration. However, oxygen treatment affects the transport properties of the films and a slight increase in oxygen deficiency δ increases T c in heavily Ca-doped samples through changes in the hole density. There is a maximum T c of about 90 K with variation of the carrier density and further reduction of the oxygen content lowers T c again. The value of the maximum T c is independent of Ca concentration and a parabolic relationship between Hall number and T c exists in the Y1-x Cax Ba2Cu3O7-δ system.

Solid Oxide Fuel Cells as Promising Candidates for Distributed Generators

SOFCs have a potential of showing extraordinarily high electric conversion efficiency. Thus they are considered as promising candidates for future distributed generators. However operations of SOFCs are not simple compared with the other generators such as gas engines or gas turbine since SOFCs have to be operated at a high temperature. It is difficult to start up SOFCs rapidly and thus they are not suitable for situations where there is a large fluctuation of demand. This limits the application of SOFCs and the introduction is restricted within a narrow market. In order to promote the introduction of SOFCs to a market, how to make use of SOFCs is a very significant factor. Techniques to operate and control SOFCs are key factors to draw the potential of SOFCs. Therefore information technologies are expected as a powerful tool to improve the total performance of SOFCs.

Growth Mechanisms of YBa2Cu3O\ssmbix Films Prepared by RF Thermal Plasma Evaporation

YBa2Cu3Ox films prepared by rf thermal plasma evaporation exhibited spiral growth in a wide range of deposition conditions from observations by atomic force microscopy (AFM). However, surface morphology, for example, step width (λ), grain size, and grain shape, varied with deposition conditions. Because surface morphology is affected by growth mechanisms, growth mechanisms can be elucidated by analyzing the change in morphology with deposition conditions. In this study, we investigated the relationship between λ and deposition conditions from a viewpoint of supersaturation. As a result, λ increased with increasing substrate temperature and decreasing raw powder feeding rate, decreasing supersaturation in both cases. In addition, the change in λ and growth rates was explained well by the change in supersaturation using a model based on the BCF theory.

Growth mechanisms of YBa2Cu3Ox films prepared by RF thermal plasma evaporation

We prepared YBa2Cu3Ox films on (100)MgO and (100)SrTiO3 substrates by rf thermal plasma evaporation, and investigated growth mechanisms of these films by observation of the surfaces using an AFM technique. As a result, 2-D nucleation and further coalescence between vicinal grains were observed in the initial stage of growth. In the films prepared for deposition time of 3 min, the different complex growth modes including spiral. growth, “birth & spread” growth, and 2-D growth were observed, which might be due to high growth rate over 55 nm/min of this process.

Development of Membrane Reformer System for Hydrogen Production from Natural Gas

An advanced membrane reformer system for hydrogen production from natural gas has been developed and demonstrated its long-term durability. A pilot-scale membrane reformer system, of which capacity is 40 Nm3/h, has been developed with membrane reactors consist of Pd-based thin hydrogen separation membranes and reforming catalyst beds. The system, which has already achieved a significant hydrogen production efficiency of 81.4% (HHV), has been operated for 8000 h and demonstrated its durability. A scale up analysis was also carried out for the purpose of estimating the efficiency of a commercial-scale system with a capacity of 300 Nm3/h using a process simulator and actual operation data obtained from the pilot system. The hydrogen production efficiency of the 300 Nm3/h system was estimated to be 82.9% (HHV).

Hybrid Systems Using Solid Oxide Fuel Cell and Polymer Electrolyte Fuel Cell

In this paper, the concept of an SOFC (Solid Oxide Fuel Cell) and PEFC (Polymer Electrolyte Fuel Cell) hybrid system is presented. Large-scale SOFC systems operated in a thermally self-sustainable state produce excess heat. The excess heat can be used for producing hydrogen. Several variations of hydrogen production systems are presented here. One way is to produce the hydrogen by using an extra reformer. Another way is purifying the off-fuel of SOFCs. The produced hydrogen can be used as the fuel for PEFCs. The overall electrical efficiency of a combination of an SOFC and PEFCs is higher than that of a standalone SOFC. When the hydrogen produced by purifying the off-fuel of the SOFC is used as the fuel for PEFCs, the overall electrical conversion efficiency increases by around 20%.Copyright

Thermoelectric properties of Na/sub x/CoO/sub 2-/spl delta// (x/spl ap/0.5) system; focusing on partially substituting effects

Polycrystalline samples of a 3d-transition-metal oxide Na/sub x/CoO/sub 2-/spl delta// (x/spl ap/0.5) system have been synthesized using a standard solid-state reaction method, and the thermoelectric properties have been measured. Although it has been, in general, believed that oxides are not suitable for thermoelectric materials, Na/sub x/CoO/sub 2-/spl delta// system shows a good thermoelectric performance, i.e. a high electrical conductivity, a large thermoelectric power and a low thermal conductivity in a wide temperature range from room temperature to 1073 K. The unexpected large thermoelectric power is likely to come from a strong electron-electron correlation. In this study, we have tried to partially substitute alkali-metal, alkaline-earth metal, transition-metal, and rare-earth element for Na or Co site in order to investigate the transport properties of Na/sub x/CoO/sub 2-/spl delta//. Due to the alkaline-earth-metal-doping, the thermoelectric power was enhanced while the conductivity was reduced. This indicates that the effective carriers are electron holes. On the other hand, the Ag-doping improves both the conductivity and the thermoelectric power, suggesting that the transport properties of Na/sub x/CoO/sub 2-/spl delta// are not classical metallic behaviour.

Hall Effect and Magneto Resistance of Y0.5Ca0.5Ba2Cu3O7-δ Thin Film in The Mixed State

The longitudinal (ρxx) and Hall (ρxy) resistivity of Y0.5Ca0.5Ba2Cu3O7-δ thin films in the mixed state have been measured under a static field up to 7 T and a pulsed magnetic field up to 30 T. With increasing the magnetic field, Tconset is suppressed and the superconducting transition is slightly broadened in the ρxx-T curve for the sample with small 8. On the other hand, heavily oxygen depleted sample shows the invariant Tconset and broadening of the superconducting transition with increasing the magnetic field. In ρxy-T curve, a sign change of ρxy has been observed for the samples with a certain carrier concentration level. The measurements of ρxx and ρxy under a static and a pulsed magnetic field have revealed that the simple expression of Hall conductivity σxy as σxy=C1B-C3/B is not applicable to the present films.