A Modular Fuel Cell, Modular DC-DC Converter Concept for High Performance and Enhanced Reliability

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 mal-performing 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 PEM fuel cell stack powered by a modular DC-DC converter are discussed.