| With ever increasing social-economics development, a wide gap between the demand and supply for energy production has been more and more prominent. At the same time, ecological and environmental deterioration caused by fossil fuel gives rise to tremendous challenge for the sustaining development of human society. Because the chemical energy in fossil fuels has to be converted into heat energy before it is converted into useful mechanical or electrical energy, there is obvious drawback in the energy industry. Further, the efficiency of this conversion is low only at 33%-35% due to limiting of the Carnot cycle. In the process, a mass of waste water, exhaust gas, residue and noises are also produced and the issues of extensive environmental pollution have been not solved effectively.The emergence of Fuel Cell (FC) technology, however, presents new and alternative opportunities for energy utilization. Fuel cell is a kind of energy device that directly converts energy stored in fuel to electrical energy via chemical reactions. The conversion process does not depend on the Carnot cycle and could achieve a comparatively higher energy conversion efficiency of 40%-60%. In addition, fuel cell technology is environmentally friendly as utilizing the hydrogen fuel produces only water as by-product. So long as there are adequate hydrogen fuel and oxygen, the fuel cell could operate steadily in long duration. Hence the fuel cell is of a strong potential developing prospect for future energy production. Among the various types of fuel cells, the Proton Membrane Exchange Fuel Cell (PEMFC) is of virtues of low working temperature, energy efficiency as high as 60%, no harmful by-product and easy to operate. Those attractive characteristics make that the PEMFC is got increasing attention by governments, research organizations and enterprises. The usages of PEMFC have been extending progressively to many domains such as transportation, military, aeronautics as well as portable and mobile power device.The thesis focuses on a series of researches on PEMFC developed by Temasek Polytechnic in Singapore. The two paramount factors (purge cycle and temperature control) are researched through experiments and the factors may affect PEMFC efficiency. As shown experimentally, a selection of purge cycle for the PEMFC has to depend upon working load. If purge frequency is less than frequency required, the water accumulated at the PEMFC cathode cannot be removed in time and will lead to a phenomenon called "flooding", in which the parallelism of single cell voltage and distribution of surface temperature will be affected. On the other hand, excessive purging will lead to under-utilization of anode hydrogen and fuel wastage. Experiments also show that an excellent temperature control will stabilizes effectively the PEMFC against disturbances in the load. By keeping the surface temperature variation low during changing load, the working lifespan of the fuel cell could be greatly increased. However, temperature control could not markedly increase the output power of PEMFC. The PEMFC experiments aforementioned could provide scientific infrastructure for future developments of portable fuel cell devices as well as guidelines for control system and auxiliary system design.The designs and experimentations of thesis based upon the PEMFC as a portable energy source include both hardware and software design. Hardware design includes PEMFC control circuit, DC-DC circuit and charging circuit with charging chip MAX1870A. Software design consists of software control with PIC microcontroller, which achieves the functions of PEMFC start-stop system, protection and double lithium battery inter-switch. Experimental results show that the PEMFC portable system designed could realize high reliably and steady electrical energy output. |
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