• Energy Research

Abstract In order to develop a vertically arranged passive DMFC with a porous carbon plate, PCP, the effect of the head height of the methanol solution in contact with the porous carbon plate on the power generation was investigated for a 55 mm height using a single cell. The single cell was operated at several methanol concentrations greater than 70 wt%. By filling the reservoir with 90 and 100 wt% methanol solutions, power densities greater than 30 mW cm −2 for over 10 h were demonstrated. Based on the result of the single cell study, a passive DMFC stack consisting of 8 unit cells with the PCP was designed and fabricated. The power generation characteristics were then experimentally measured. The maximum power output of 1.8 W, which was almost 10% lower than that expected from the single cell performance, was obtained with 100% methanol. At the same time, a nonuniform cell voltage among the 8 unit cells was found as a reason for the decreasing power output with the increasing current.

Abstract Based on an energy utilization diagram, energy and exergy analysis of a pervaporation system is performed by taking three subsystems into account, supply of heat, pervaporation, and suction. By this single diagram, the energy balance, the exergy loss in each subsystem, and the strength of forces driving processes can be disclosed. Energy transformation in the pervaporation system and the effects of manipulating variables are clarified by comparison with a distillation system. The energy utilization diagram is also applied to examine a system in which a pervaporator and a distillation column are combined. It is shown that separation by pervaporation is quite effective from the viewpoint of exergy, especially when the pervaporation system can proceed by the difference between the temperature of the feed liquid and that of the cooling water only. This is because in this case low-grade energy is utilized completely.

Abstract Vertical operation of a passive DMFC employing a porous carbon plate, PCP, with different resistances for fluid flow and bubble point pressure was investigated to clarify the properties required for the PCP for vertical operation. Moreover, the cell performance was investigated under different solution head pressures within 22 mm height and was discussed based on the methanol transport through the PCP. In contrast to the horizontal orientation, the static pressure of the liquid as a function of its height on the vertical axis h along the PCP surface, in the vertical orientation, enhanced the convective methanol flux through the PCP and affected the DMFC performance depending on the properties of the PCP and the methanol concentration used. The effect of the solution head pressure on the DMFC performance in the vertical orientation could be controlled by using PCP with high bubble point pressures. The DMFC could attain stable performance under both horizontal and vertical cell orientations and different solution head pressures even with 100% methanol by using a PCP with the proper resistance for the methanol transport and bubble point pressure. A thin PCP of 0.5 mm thickness with the proper resistance enabled the vertical operation producing a constant power density over 40 mW/cm 2 using 100% methanol at 0.25 V.

Abstract The effect of the pore structure and thickness of the porous carbon plate, PCP, as well as the gas barrier thickness on the methanol transport and the performance of a passive DMFC under the different cell voltages of 0.1, 0.2 and 0.3 V using different methanol concentrations was investigated. As a result of the mass transfer restrictions by employing the PCP, high methanol concentrations over 20 M could be efficiently used to produce the relatively high power density of 30 mW cm −2 for more than 10 h. The DMFC was operated under limiting current conditions in all the PCPs at 0.1 and 0.2 V to more than 20 M. The main factors for controlling the methanol transport were the barrier of the gas layer with CO 2 , which was formed between the anode surface and the PCP and the properties of the PCP. At the low current densities of less than 60 mA cm −2 , when no CO 2 bubbles are emitted, both the pore structure and thickness of the PCP did not affect the methanol transport and the current voltage relationship. At the higher current densities, CO 2 bubbles were evolved through the PCP and different resistances to the methanol transport were observed depending on the PCP pore structure and thickness. The CO 2 gas layer between the MEA and the PCP caused a major resistivity for the methanol transport, and its resistivity increased with its thickness increasing. By using the PCP at 0.1 V, the energy density of the passive DMFC was significantly increased, e.g., more than seven times.

著者らが考案した粉・粒流動層において, 層内に供給した微粒子の滞留時間分布を調べた.残余濃度曲線法を用いて微粒子の滞留時間分布を測定し, 滞留時間に対する微粒子供給速度, ガス速度, 微粒子径, 媒体粒子径, 流動層高さの影響を調べた.実験で得られた平均滞留時間は, 微粒子が付着, 凝集せずにガスに同伴され層内を通過すると考えた理論平均滞留時間の150～500倍も大きな値であった.このことは, 層内において微粒子の媒体粒子への付着がかなり強く起きていることを示唆している.平均滞留時間は微粒子径が小さく, ガス速度が小さいほど, そして流動層高さが高いほど長くなった.

The minimum fluidization velocity was measured for various binary particle mixtures of coarse particles of group A or B and fine particles including group C particles of Geldart classification in the range where the weight fraction of the fine particles was less than 15 wt%. The minimum fluidization velocity decreased with the increase in the fraction of the fine particles in all cases; especially in the systems with group C particles, a remarkable decrease in the minimum fluidization velocity was observed with the increase in the fine particle fraction. Such a decrease in the minimum fluidization velocity observed in the systems with group C particles was expressed by the formation of aggregated particles consisted of the coarse and the fine particles. Mixing/segregation appearance of the system was also observed, revealing that group C particles can be fluidized and mixed homogeneously with coarse particles in a certain rage of the fine particle fraction. The range of the fine particle fraction for homogeneous mixing was dependent on the systems.

Abstract Utilizing highly concentrated methanol solution in the direct methanol fuel cells (DMFCs) is strongly preferable because it distinctly diminishes the cells size and simultaneously improves the power density. In this regard, water back-diffusion from cathode to anode is an important process as water is a reactant in the anode reactions. Beside the low cell efficiency, lack of water at the anode leads to form toxic intermediates which environmentally constrain the commercial applications. This study investigates exploiting hydrophobic cathode air filter as simple and effective strategy to enhance water back-diffusion. Typically, a passive vapor-feed DMFC using a porous carbon plate (PCP) on the anode surface was operated with different methanol concentrations up to 100% with and without the use of a hydrophobic cathode air filter. In situ mass spectrometry using a capillary probe was conducted for the anode gas layer to investigate the effect of the addition of the filter on the formation of reaction products at the anode surface. In normal DMFC, analysis of anode gas-layer indicated that formation methylformate (HCOOCH 3 ) significantly increases upon utilizing high methanol concentrations. However, addition of the cathode air filter led to distinct decrease of the methanol crossover and increase of the water back-diffusion from the cathode to the anode especially at high methanol concentrations. The in situ gas analysis of the cell operated at 100 wt% (24.7 M) methanol revealed that the addition of the cathode filter significantly decreased the formation of the undesirable intermediates; 65% reduction in the amount of formed methylformate was observed. This effect was related to the increase in the ratio between the methanol and water vapor pressure (P CH3OH /P H2O ) after addition of the filter and hence increased the performance of the passive DMFC when highly concentrated methanol was used as a fuel.

Abstract The effect of employing a porous carbon plate on the performance of a passive direct methanol fuel cell (DMFC) under closed circuit conditions was investigated. The porous carbon plate and a CO2 gas layer that formed between the anode and the porous plate stably controlled the mass transfer of the methanol and water from the reservoir to the anode, which made operation with very high concentrations of methanol, even neat methanol, possible. The i–V and i–t performances of the DMFC with and without the porous plate were measured at different methanol concentrations, and the performances were compared. The maximum power density, 24 mW cm−2 at room temperature, obtained at 2 M without the porous plate was reproduced at 16 M with the porous plate. Also, the methanol crossover flux and water flux through the MEA was evaluated, and the Faraday efficiencies of the DMFC with and without the porous carbon plate were analyzed. When high concentrations of methanol were used with the porous plate, it was confirmed that the Faraday efficiency remained high, and the back diffusion of water from the cathode to the anode through the membrane occurred which resulted in no flooding at the cathode, contrary to the case without the porous plate. By increasing the distance between the anode and the porous plate, the power density decreased, suggesting that the distance of the CO2 layer played an important role in obstructing the mass transport.

微粒子とガスとの接触操作のために考案した粉粒流動層を用いて, 転炉ダスト (サブミクロン～数十μm径の酸化鉄) の直接還元を行い, 還元鉄微粒子を連続的に得た.転炉ダストの還元率に対する反応温度, 媒体粒子層高, ガス流速, 水素濃度の影響を調べた.本実験における最適運転条件では, 98%の還元率を得ることができた.また, 運転中に焼結が起こる反応条件を明らかにし, 定常な連続操作ができる範囲を確立した.得られた粉鉄は, 空気中で極めて酸化され易い.再酸化防止のための粉鉄の高温処理および低濃度酸素処理の効果を調べた.