Stacking of fuel cells

The bipolar plate with multiple functions, also called a flow field plate or separation plate (separator), is one of fhe core components in fuel cells. In reality, like serially linked batteries, fuel cells are a serial connection or stacking of fuel cell unifs, or so-called unif cells fhis is why fuel cells are normally also called sfacks (Figure 5.1) [2]. The complicated large fuel cells or module can consist of a couple of serially connecfed simple fuel cells or cell rows. Excepf for the special unit cells at two ends of a simple stack or cell row, all the other unit cells have the same structure, shape, and functions. 

The intercoimecting material used in building a stack of fuel cells must have a high electrical conductivity and a coefficient of expansion similar to that of the electrolyte. The perovskite oxide LaCrOs meets these requirements nicely and is widely used although ferritic stainless steels have been used successfully in certain fuel cell geometries. 

A proton exchange membrane (PEM) design is to be utilized. A stack of fuel cells will be provided, each of which oxidizes hydrogen at its platinum-ruthenium anode ... 

The current produced by a cell or stack of fuel cells depends on the average current density and on the surface area of the cell. In a power generator, the nominal current density varies in the interval 0.5 A/cm -0.8 A/cm with maximum current... 

SOLUTION (a) This example is quite similar to the previous one, but with an additional complication that this is now a stack of fuel cells. Since they are in series, each fuel cell... 

The number of fuel cells that are stacked is determined bv the desired electrical potential. For svsterns it can be about 200... 

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. 

In general, technical developments will lead to a decrease in overall costs of this technology per unit of installed generating capacity (kWe). Some types of fuel cells need to achieve higher power densities per kg weight or m3 most need to increase lifetimes of stacks or other plant components. For smaller applications, the technology must reach a reliable level sufficient to allow the plants to operate unattended. 

While the mass production of fuel cell cars is some time away, if cost-competitive fuel cell stacks are available soon, it can change the competitive mix of transportation options. 

In 1998 a report prepared for the California Air Resources Board (CARB) called Status and Prospects of Fuel Cells as Automotive Engines favored methanol fuel cell stacks in cars over a direct-hydrogen infrastructure. Hydrogen is not as ready for private automobiles because of the difficulties and costs of storing hydrogen on board and the large investments that would be required to make hydrogen more available. 

GM also announced the expansion of fuel cell development activity with Giner, Inc., to include applications beyond the transportation field, including hydrogen generation for refueling systems and regenerative fuel cells for stationary power. GM s fuel cell stack set a new world standard for power density that packed 60% more power. The new stack generated 1.75 kilowatts (kW) per liter. 

FuelCell Energy is a partner with Versa Power Systems, Nexant, and Gas Technology Institute to develop more affordable fuel-cell-based technology that uses synthesis gas from a coal gasifier. The key objectives include the development of fuel cell technologies, fabrication processes, and manufacturing capabilities for solid oxide fuel cell stacks for multi-mega-watt power plants. 

Figure 14.16 the shows fuel cell stack performance of a 1 kWe atmospheric PEMFC stack using PtRu anodes, operating on various gas compositions. As can be clearly seen, already small concentrations of CO lead to a large decrease of fuel cell performance. An air-bleed of 1.5% air in hydrogen is able to mitigate this ef-... 

S. Lasher, J. Sinha, and Y. Yang. Cost analyses of fuel cell stack/systems. Annual progress report, DoE Hydrogen Program, U.S. Department of Energy, Washington, D.C. (2007) 695-699. 

Although it is difficult to determine the quantitative requirements of plate and plate materials appropriately for various fuel cells and different applications in a development phase, such a target would be helpful to direct the development effort and make necessary trade-offs. The cascaded performance requirement targets in 2010 and 2015 for bipolar plates of fuel cells in transportation applications were set by the U.S. DoE (Department of Energy) according to functions of the plate mentioned before and overall requirements of performance, reliability, manufacturability, and cost of a stack, as shown in Table 5.1 [7]. The technical target in the DoE s multiyear research, development, and demonstration plan has been popularly and worldwide... 

Numerous demonstrations in recent years have shown that the level of performance of present-day polymer electrolyte fuel cells can compete with current energy conversion technologies in power densities and energy efficiencies. However, for large-scale commercialization in automobile and portable applications, the merit function of fuel cell systems—namely, the ratio of power density to cost—must be improved by a factor of 10 or more. Clever engineering and empirical optimization of cells and stacks alone cannot achieve such ambitious performance and cost targets. 

Figure 25. (A) Comparison of the energy storage capability of fuel cells and batteries. Only after several refueling operations are fuel cells more efficient energy storage devices on a Wh/L and Wh/kg basis. (B) Fuel cells have a set volume and weight for the fuel cell stack and peripherals to supply the reactants to the stack. The small incremental fuel volume to continue operation supplying energy makes them more efficient for longer operations.

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