Fuel Cell Stacking

Fig. 3. Schematics of gas manifolds for MCFC stacks (a) internally manifolded fuel cell stack (b) externally manifolded fuel cell stack.

A subsidiary of lEC and Toshiba Corp. called ONSI Corp. was formed for the commercial development, production, and marketing of packaged PAEC power plants of up to 1-MW capacities. ONSI is commercially manufacturing 200-kW PAEC systems for use in a PC25 power plant. The power plants are manufactured in a highly automated faciHty, using robotic techniques to assemble the repeating electrode, bipolar separator, etc, units into the fuel cell stack. 

Like M( F(7s, S()F(7s can integrate fuel reforming within the fuel cell stack, A prereformer converts a substantial amount of the natural gas using waste heat from the fuel cell, (iornpoiinds containing sulfur (e,g, thiophene, which is cornrnonlv added to natural gas as an odorant) must be removed before the reformer. Typically, a hvdrodesiilfii-rizer combined with a zinc oxide absorber is used. 

As of 2000, it also looks as though more and more electric utilities are becoming interested in fuel cell stacks as local microgenerators to top up power from large power stations, without the need for long-distance transmission of electricity and its attendant expense and power losses. 

Fuel cell is an ambiguous term because, although the conversion occurs inside a fuel cell, these cells need to be stacked together, in a fuel cell stack, to produce useful output. In addition, various ancillai y devices are required to operate the stack properly, and these components make up the rest of the fuel cell system. In this article, fuel cell will be taken to mean fuel cell system (i.e., a complete standalone device that generates net power). 

Fuel cell stack voltage varies with external load. During low current operation, the cathode s activation overpotential slows the reaction, and this reduces the voltage. At high power, there is a limitation on how quickly the various fluids can enter and... 

Fuel Cell Stack with end-plates and connections... 

A complete fuel cell system, even when operating on pure hydrogen, is quite complex because, like most engines, a fuel cell stack cannot produce power without functioning air, fuel, thermal, and electrical systems. Figure 3 illustrates the major elements of a complete system. It is important to understand that the sub-systems are not only critical from an operational standpoint, but also have a major effect on system economics since they account for the majority of the fuel cell system cost. 

Alternatively, the fuel cell stack can he operated at ambient pressure. Although this simplifies the system considerably and raises overall efficiency, it docs reduce stack power and increase thermal management challenges. 

Heat rejection is only one aspect of thermal management. Thermal integration is vital for optimizing fuel cell system efficiency, cost, volume and weight. Other critical tasks, depending on the fuel cell, are water recovery (from fuel cell stack to fuel processor) and freeze-thaw management. 

Electrical management, or power conditioning, of fuel cell output is often essential because the fuel cell voltage is always dc and may not be at a suitable level. For stationai y applications, an inverter is needed for conversion to ac, while in cases where dc voltage is acceptable, a dc-dc converter maybe needed to adjust to the load voltage. In electric vehicles, for example, a combination of dc-dc conversion followed by inversion may be necessary to interface the fuel cell stack to a, 100 V ac motor. 

Fuel cells can run on fuels other than hydrogen. In the direct methanol fuel cell (DMFC), a dilute methanol solution ( 3%) is fed directly into the anode, and a multistep process causes the liberation of protons and electrons together with conversion to water and carbon dioxide. Because no fuel processor is required, the system is conceptually vei"y attractive. However, the multistep process is understandably less rapid than the simpler hydrogen reaction, and this causes the direct methanol fuel cell stack to produce less power and to need more catalyst. 

Fuel cell technology probably offers a new emerging area for polyheterocyclic polymers as membranes. Fuel cells are interesting in transport applications and are now being evaluated in Chicago in transit buses with a 275-hp engine working with three 13 kW Ballard fuel cell stacks. 

The transient response of DMFC is inherently slower and consequently the performance is worse than that of the hydrogen fuel cell, since the electrochemical oxidation kinetics of methanol are inherently slower due to intermediates formed during methanol oxidation [3]. Since the methanol solution should penetrate a diffusion layer toward the anode catalyst layer for oxidation, it is inevitable for the DMFC to experience the hi mass transport resistance. The carbon dioxide produced as the result of the oxidation reaction of methanol could also partly block the narrow flow path to be more difScult for the methanol to diflhise toward the catalyst. All these resistances and limitations can alter the cell characteristics and the power output when the cell is operated under variable load conditions. Especially when the DMFC stack is considered, the fluid dynamics inside the fuel cell stack is more complicated and so the transient stack performance could be more dependent of the variable load conditions. 

In the design of membrane-type fuel cell stacks (batteries), membrane-electrode assemblies (MEAs) are used, which consist of a sheet of membrane and of the two electrodes (positive and negative) pressed onto it from either side. 

Fig. 23. (a) Experimental IR-free overpotentials in MCFC-based separator. Cell performance 0.25% C02 Feed. All curves calculated [32] (b) C02 production scheme using molten carbonate fuel cell stack. 

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