Cell-Stack Designs

Other important parts of the cell are 1) the structure for distributing the reactant gases across the electrode surface and which serves as mechanical support, shown as ribs in Figure 1-4, 2) electrolyte reservoirs for liquid electrolyte cells to replenish electrolyte lost over life, and 3) current collectors (not shown) that provide a path for the current between the electrodes and the separator of flat plate cells. Other arrangements of gas flow and current flow are used in fuel cell stack designs, and are mentioned in Sections 3 through 8 for the various type cells. 

Despite the advanced technology of the AQUATECH System, its installation is not complex. In fact, it is simpler and less costly than conventional electrolysis processing. AQUATECH Systems need only two electrodes for an entire 100-150 cell unit stack, avoiding complicated busbar arrangements. Furthermore, scale-up and installation of the unit are facilitated by the modular skid mounted cell stack design. 

With relatively high cost, low capacity, poor efficiency and short lifetimes, PEM electrolysers currently available are not as mature as alkaline electrolysers. It is expected that the performance of PEM electrolysers can be improved significantly by additional work in materials development and cell-stack design (Grigoriev et al., 2006). 

Fig. 15.4 Advancements in the development of the fuel cell stacks designed at General Motors from 1997 to 2004 is depicted [22]...

The most common fuel cell stack design is the so-called planar-bipolar arrangement (Figure 1-2 depicts a PAFC). Individual unit cells are electrically connected with interconnects. Because of the configuration of a flat plate cell, the interconnect becomes a separator plate with two functions ... 

A number of planar cell stack designs have been developed based on planar anode-supported SOFC with metal interconnects. Typically, cells for full-scale stacks are about 10 to 20 cm mostly square or rectangular (though some are round). Stacks with between 30 and 80 cells are the state-of-the-art. Figure 7-26 shows examples of state-of-the art planar anode-supported SOFC stacks and selected performance data (68,78, 79). The stacks shown are the result of three to seven generations of full-scale stack designs by each of the developers. The capacities of these stacks (2 to 12 kW operated on reformate and at 0.7 V cell voltage) is sufficient for certain small-scale stationary and mobile (APU) applications. 

A general problem of such cell-stack designs are shunt losses, caused by connection of the electrodes at different voltages via the electrolyte. Special design of the electrolyte channels can reduce the shunt loss to about 3% of the discharged energy. 

Estimate how much weight savings in terms of fuel and oxidizer would be reahzed by replacing a 100 W, 20 A fuel cell stack designed for 4000 h service with a reversible fuel cell recharged by a solar panel for a space application. Because fuel and oxidizer are recycled, you can assume an effective stoichiometry of 1.0 for the anode and cathode in both cases. 

Several different radial fuel cell stack designs have been developed. The primary reason to develop a radial system is to have the same form factor as a common battery. The radial design is typically fed fuel from an inner hollow core and air from the ambient around the unsealed cathode edges of the cell, as shown in Figure 6.48. An advantage of the radial design in this instance is that the diffusion path length is minimized. 

Stack Manifold Flow One of the most difficult engineering challenges in fuel cell stack design is the proper manifold design for fuel, oxidizer, and coolant flow. The manifold design challenge centers around three main consttaints ... 

The fabrication processes selected for each planar SOFC cell/stack design depend on the configuration of the cells in the stack. The key step in any selected process is the fabrication of dense electrolytes. In general, ceramic fabrication processes for planar SOFCs can be classified into two groups, based on the fabrication approach for the electrolyte the particulate approach and the deposition approach. The particulate approach involves compaction of ceramic powder into cell components and densification at elevated temperatures. Examples of the particulate approach are tape casting and tape calendering. The deposition approach involves formation of cell components on a support by a chemical or physical process. Examples of the deposition approach are chemical vapour deposition, plasma spraying, and spray pyrolysis. 

Table 4.1 Microtubular cells stack designs by various firms...

The following are the key aspects of a fuel cell stack design ... 

Barbir, F., PEM Fuel Cell Stack Design Considerations, in Proc. Fuel Cell Technology Opportunities and Challenges (AlChE Spring National Meeting, New Orleans, LA, March 2002) pp. 520-530. 

From the parameters in Equations (10-5) and (10-6), a new parameter may be derived, the so-called design factor, Df (kg/m ), which is simply the stack mass divided by the active area. This factor is useful in comparisons of various stack designs. A good fuel cell stack design, which means a stack... 

---