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Made available in DSpace on 2014-10-09T12:37:50Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:08:31Z (GMT). No. of bitstreams: 1 05380.pdf: 5707781 bytes, checksum: f7a7b65bad72a837f9123ca6deee3226 (MD5) / Dissertacao (Mestrado) / IPEN/D / Instituto de Pesquisas Energeticas e Nucleares - IPEN/CNEN-SP

During operation of the solid polymer fuel cell (SPFC). its performance is limited by the concentrations of hydrogen and oxygen at the reaction interfaces and in most SPFC designs, the hydration state of the membrane. Since in general, the concentrations of water and the gaseous species vary along the flow channel, the performance is also likely to change along the flow channel. In order to study this phenomenon, a measurement system was developed to map the current distribution across the electrode surface. The current distribution has been measured by dividing one of the current collectors into a number of electrically isolated segments. The current flowing through each of the segments was measured while maintaining a constant potential across the surface of the gas diffusion layer. Two separate segmented current collectors were developed. The first was used to measure the current distribution for an 80 cm2 single cell, and the second was used to investigate the local current densities around a single flow channel. The effects of the feed gas humidities on the spatial current density in the 80 cm2 fuel cell were investigated for two different membrane-electrode configurations. With Nafion 117 as electrolyte and at a cell temperature of 80°C. the membrane was found to dehydrate in the initial portion of the gas flow channel when the relative humidity of both the hydrogen and oxygen feed gases was less than 50%. With a Gore-Select membrane electrode assembly (hydrogen and air. temperature- 60°C). the membrane was sufficiently hydrated at all feed gas humidification conditions. The performance of the cell was found to deteriorate at higher feed gas humidities as a result of the lower partial pressures of the reactant gases. Measurements of the effects of gas pressures, stoichiometries and humidities on the length-wise and width-wise perfonnance around a single flow channel (Gore-Select membrane electrode assembly) are discussed in relation to a gas flow model.

Since only little is known in the field of Solid Oxide Fuel Cell (SOFC) operation about internal reforming of methane at present, the aim of this thesis study is to conduct a detailed investigation delivering the basis for further experimental and theoretical work. Also information is required if the concept of internal reforming has technical development potential. The thesis is arranged into two major parts being a thermodynamic investigation and an application of a suitable kinetic model. Pure methane tends to decompose at the high operation temperatures of the SOFC (about 950°C) thus forming solid carbon. Therefore it is necessary to include a fuel preparation process delivering H2 and CO that can be utilized by the SOFC. The fuel processing can either be carried out by steam reforming or partial oxidation. It is shown by a thermodynamic investigation that fuel processing by partial oxidation yields a fuel gas of inferior quality than fuel processing by steam reforming. The kinetic part contains the application of a model describing the chemical and electrochemical conversion occuring in the SOFC as detailed as possible at present. This model is used to investigate the thermal behaviour of an SOFC process referring to technical operation parameters. It is shown that internal reforming has technical development potential although a lot of care must be paid to the heat management. Particular operation conditions might exist where the highly endothermic steam reforming process could cause a breakdown of the complete fuel cell process due to its enormous local cooling effect.

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No description available.

Many wave energy convertors are designed to use either vertical (heave) or horizontal (surge) movements of waves. But the frequency response of small heaving buoys and oscillating water column devices shows that they are too stiff and so their resonance is at too short a period. A device moving in the horizontal (surge) direction has less restoring spring and so its resonance is at too long a period. It follows that a device that moved at some intermediate slope angle could have an intermediate value of hydrodynamic stiffness and so be resonant at a variable and desirable part of the wave spectrum. There have been two series of model tests in this work. The first used a simple free-floating model with no power take-off apparatus and with constraint achieved by means of a large inertia plate lying in the slope plane. The second used a rig that constrained the slope movement of the buoy head by means of hydrostatic bearings running on a guide rod set to the chosen slope angle. An external power take-off system was used to simulate a linear damper for absorbing the incident wave energy and control the motion of the model. This thesis firstly studies the potential of varying the slope angle as a way of tuning the natural period of the device to suit useful wave periods. Secondly, it studies the experimental and theoretical power capture ability of models with different slope angles in regular waves in the frequency domain. The hydrodynamic coefficients of the model were determined both experimentally and numerically based on linear hydrodynamic concepts. The power absorption of the models was calculated using the experimental data of the hydrodynamic coefficients and also measured directly. Some control of power take-off was also investigated. Some irregular wave tests were carried out for the 45 degrees slope angle case. The results show that it is feasible to alter the slope angle of the device as a way of tuning its natural period. However, in further studies of the power capture ability for different slope angles, the device shows a very wide bandwidth and high efficiency performance when it is set to 45 degrees slope angle. This suggests that to constrain the device to a 45 degrees slope angle is suitable for most of the sea states.

No description available.

No description available.

No description available.