Remote area power supply (RAPS) is a potential early market for renewable energy - hydrogen systems because of the relatively high costs of conventional energy sources in remote regions. Solar-hydrogen RAPS systems commonly employ photovoltaic panels, a Proton Exchange Membrane (PEM) electrolyser, a storage for hydrogen gas, and a PEM fuel cell. Unitised Regenerative Fuel Cells (URFCs) use the same hardware for both electrolyser and fuel cell functions. Since both of these functions are not required simultaneously in a solar hydrogen RAPS system, URFCs based on PEM technology provide a promising opportunity for reducing the cost of the hydrogen subsystem used in renewable-energy hydrogen systems for RAPS. URFCs also have potential applications in the areas of aerospace, submarines, energy storage for central grids, and hydrogen cars. In this thesis, a general theoretical relationship between cell potential and current density of a single-cell PEM URFC operating in both fuel-cell (FC) and electrolyser (E) modes is developed using modified Butler-Volmer equations for both oxygen- and hydrogen-electrodes, and accounting for mass transport losses and saturation behaviour in both modes, membrane resistance to proton current, and membrane and electrode resistances to electron current. This theoretical relationship is used to construct a computer model based on Excel and Visual Basic to generate voltage-current (V-I) polarisation curves in both E and FC modes for URFCs with a range of membrane electrode assembly characteristics. The model is used to investigate the influence on polarisation curves of varying key parameters such charge transfer coefficients, exchange current densities, saturation currents, and membrane conductivity. A method for using the model to obtain best-fit values for electrode characteristics corresponding to an experime ntally-measured polarisation curve of a URFC is presented. The experimental component of the thesis has involved the design and construction of single PEM URFCs with an active area of 5 cm2 with a number of different catalyst types and loadings. V-I curves for all these cells have been measured and the performance of the cells compared. The computer model has then been used to obtain best-fit values for the electrode characteristics for the URFCs with single catalyst materials active in each mode on each electrode for the corresponding experimentally-measured V-I curves. Generally values have been found for exchange current densities, charge transfer coefficients, and saturation current densities that give a close fit between the empirical and theoretically-generated curves. The values found conform well to expectations based on the catalyst loadings, in partial confirmation of the validity of the modelling approach. The model thus promises to be a useful tool in identifying electrodes with materials and structures, together with optimal catalyst types and loadings that will improve URFC performance. Finally the role URFCs can play in developing cost-competitive solar- hydrogen RAPS systems is discussed, and some future directions for future URFC research and development are identified.

Thesis (M. Tech. - (Engineering: Electrical, Department: Electronic Engineering, Faculty of Engineering and Technology)) -- Vaal University of Technology, 2011. / “Never before in peacetime have we faced such serious and widespread shortage of energy” according to John Emerson, an economist and power expert for Chase Manhattan Bank. Many analysts believe that the problem will be temporary, but others believe the energy gap will limit economic growth for years to come. A possible solution to this problem can be fuel cell technology. Fuel cells (FCs) are energy conversion devices that generate electricity from a fuel like hydrogen. The FC however, could also be used in the reverse or regenerative mode to produce hydrogen. The reversible fuel cell (RFC) can produce hydrogen and oxygen by introducing water to the anode electrode chamber, and applying a potential across the anode and cathode. This will cause the decomposition of the water to produce oxygen at the anode side and hydrogen at the cathode side. In order to make this process as efficient as possible several aspects need to be optimised, for example, the operation temperature of the RFC, water management inside the RFC and supply voltage to the RFC. A three cell RFC and its components were constructed. The three cell RFC was chosen owing to technical reasons. The design factors that were taken into consideration were the different types of membranes, electrocatalysts, bipolar plates and flow topologies. A water trap was also designed and constructed to eliminate the water from the hydrogen water mixture due to water crossover within the MEA. In order to optimise the operation of the RFC a number of experiments were done on the RFC. These experiments included the optimal operating voltage, the effect that the temperature has on the production rate of hydrogen, and the effect that the water flow through the RFC has on the production rate of hydrogen. It was found that there is no need to control the water flow through the RFC because it had no effect on the production rate of hydrogen. The results also showed that if the operating temperature of the RFC were increased, the energy it consumes to warm the RFC significantly decreases the efficiency of the whole system. Thus the RFC need not be heated because it consumes significantly more energy to heat the RFC compared to the energy available from the hydrogen produced for later use. The optimised operating voltage for the three cell RFC was found to be 5.05 V. If the voltage were to be increased or decreased the RFC efficiency would decrease.

Thesis (M. Tech. - (Engineering: Electrical, Department: Electronic Engineering, Faculty of Engineering and Technology))--Vaal University of Technology. / Industrial countries, such as South Africa, rely heavily on energy sources to function profitably in today’s economy. Based on the 2008 fossil fuel CO2 emissions South Africa was rated the 13th largest emitting country and also the largest emitting country on the continent of Africa, and is still increasing. It was found that fuel cells can be used to generate electricity and that hydrogen is a promising fuel source. A fuel cell is an energy generation device that uses pure hydrogen (99.999%) and oxygen as a fuel to produce electric power. A regenerative fuel cell is a fuel cell that runs in reverse mode, which consumes electricity and water to produce hydrogen. This research was aimed at designing and constructing an optimised control system to control the hydrogen pressure in a proton exchange membrane regenerative fuel cell. The hydrogen generated by the fuel cell must be stored in order to be used at a later stage to produce electricity. A control system has been designed and constructed to optimise the hydrogen pressure control in a regenerative proton exchange membrane fuel cell. An experiment that was done to optimise the hydrogen system included the effects that the cathode chamber pressure has on the production of hydrogen and the most effective method of supplying hydrogen to a storage tank. The experiment also included the effects of a hydrogen buffer tank on the output hydrogen pressure and if the system can accommodate different output pressures. It was found that the cathode chamber pressure doesn’t need to be controlled because it has no effect on the rate of hydrogen produced. The results also showed that the flow of hydrogen need not to be controlled to be stored in a hydrogen storage tank, the best method is to let the produced hydrogen flow freely into the tank. The hydrogen produced was also confirmed to be 99.999% pure. The system was also tested at different output pressures; the control system successfully regulated these different output pressures.

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Made available in DSpace on 2014-10-09T12:49:30Z (GMT). No. of bitstreams: 0 / Made available in DSpace on 2014-10-09T14:02:27Z (GMT). No. of bitstreams: 1 10381.pdf: 10221895 bytes, checksum: 882d02701e24d30dd8869849c8502249 (MD5) / Tese (Doutoramento) / IPEN/T / Intituto de Pesquisas Energeticas e Nucleares, IPEN/CNEN-SP