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A MODULAR FUEL CELL, MODULAR DC–DC CONVERTER CONCEPT FOR HIGH PERFORMANCE AND ENHANCED RELIABILITY
ABSTRACT
Fuel cells are electrochemical devices that process H2 and oxygen to generate electric power, having water vapor as their only by-product. The voltage resulting from the reaction of the fuel and oxygen varies with the load, and ranges from 0.8 V at no-load to about 0.4 V for full-load. Due to their low output voltage, it becomes necessary to stack many cells in series to realize a practical system. The proposed modular concept electrically divides the fuel cell stack into various sections, each powered by a dc– dc converter. This modular fuel cell powered by modular dc–dc converter eliminates disadvantages, resulting in a fault tolerant system. Fuel cell stacks are constructed by stacking several individual cells, which is equivalent to connecting many voltage sources in series, each with its own internal impedance. The membrane humidity may vary from cell to cell depending on the heat distribution within the fuel cell. Cells with a drier membrane will produce less voltage than cells with a more moisturized membrane, and this will produce a voltage closer to its nominal. If the load current exceeds the limit given by for a long period of time, the stack overheats due to the additional internal losses in the malperforming cells, and operation of the fuel cell has to be discontinued. Compare both approaches, traditional and modular; the maximum power that can be produced by each is evaluated. If a traditional approach is used, to avoid overheating, the current in the stack should be limited to a value given by its weakest section.
To take advantage of the modular fuel cell stack, an appropriate dc–dc converter and control scheme are required. Advantage of constructing a fuel cell stack with several sections is that faulty portions of the stack can be bypassed, while the rest of the stack can continue operation. In order to optimize the power extraction from each of the sections in the fuel cell, an appropriate control scheme needs to be devised. The implementation of this control scheme can be carried out by combining digital and analog controllers. The proposed system has been shown to be fault tolerant and can continue to operate at a reduced power level under fuel cell or power module faults.

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A MODULAR FUEL CELL, MODULAR DC-DC CONVERTER CONCEPT FOR HIGH PERFORMANCE AND ENHANCED RELIABILITY
ABSTRACT :
Fuel cell stacks produce a dc output with a 2:1 variation in output voltage from no-load to full-load. The output voltage of each fuel cell is about 0.4 V at full-load, and several of them are connected in series to construct a stack.
An example 100 V fuel cell stack consists of 250 cells in series and to produce 300 V at full load requires 750 cells stacked in series.
Since fuel cells actively convert the supplied fuel to electricity, each cell requires proper distribution of fuel, humidification, coupled with water/thermal management needs. With this added complexity, stacking more cells in series decreases the reliability of the system.
For example, in the presence of bad or malperforming cell/cells in a stack, uneven heating coupled with variations in cell voltages may occur. Continuous operation under these conditions may not be possible or the overall stack output power is severely limited.
In this paper, a modular fuel cell powered by a modular dc–dc converter is proposed. The proposed concept electrically divides the fuel cell stack into various sections, each powered by a dc–dc converter. The proposed modular fuel cell powered by modular dc–dc converter eliminates many of these disadvantages, resulting in a fault tolerant system. A design example is presented for a 150-W, three-section fuel cell stack and dc–dc converter topology. Experimental results obtained on a 150-W, three-section proton exchange membrane (PEM) fuel cell stack powered by a modular dc–dc converter are discussed.