Fuel cell stack design

A fuel cell stack is formed by connecting a number of tri-layer single fuel cell units or MEA units separated by interconnect or bipolar plates in series to meet the required power output. The MEAs are placed in good contact on both anode and cathode sides with the electrically conducting plates, often referred to as thefluid flow-field plates or bipolar plates or interconnect plates, which have integrated flow channels. Liquid- or gas-phase fuel and oxidant streams are fed through external and internal manifolds and distributed into the 

Gas flow-field plate, (a) Parallel flow-field plate, (b) Counter flow-field plate, (c) Cross flow-field plate. 

Gas flow-field plate and bipolar plate with fuel and oxidant flow channels, (a) Gas flow-field 

Fuel cell stack with the series of MEAs and gas flow-field plates, (a) Fuel cell stack with anode and cathode, (b) Fuel cell stack with gas flow-field plates and cooling plates. 

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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. 

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 ... 

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 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... 

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. 

Global has also designed and built a dual-stage, low-temperature adsorbent desulfurizer. Sulfur in propane can exceed as much as 300-ppm compared to natural gas, which ranges from 2 to 15-ppm sulfur and it must be removed to block any poisoning of the fuel cell. The test results indicated that no sulfur compounds were present in the outlet gas of the desulfurizer. The system design uses a modular assembly and layout, including a circular hot box where the fuel cell stacks and the fuel processor are located and easily accessed. 

The primary focus of ongoing research is to improve the performance of the cell and lower its cost. The principal areas of development are improving cell membranes, handling the CO in the fuel stream, and refining electrode design. There has been an effort to incorporate system requirements into the fuel cell stack in order to simplify the overall system. This work has included a move toward operation with zero humidification at ambient pressure and direct fuel use. 

The cell and stacks that compose the power section have been discussed extensively in the previous sections of this handbook. Section 9.1 addresses system processes such as fuel processors, rejected heat utilization, the power conditioner, and equipment performance guidelines. System optimization issues are addressed in Section 9.2. System design examples for present day and future applications are presented in Sections 9.3 and 9.4 respectively. Section 9.5 discusses research and development areas that are required for the future system designs to be developed. Section 9.5 presents some advanced fuel cell network designs, and Section 9.6 introduces hybrid systems that combine fuel cells with other generating technologies in integrated systems. 

NETL looked at improving upon conventional MCFC system designs, in which multiple stacks are typically arranged in parallel with regard to the flow of reactant streams. As illustrated in Figure 9-19a, the initial oxidant and fuel feeds are divided into equal streams which flow in parallel through the fuel cell stacks. 

In an improved design, called an MCFC network, reactant streams are ducted such that they are fed and recycled among multiple MCFC stacks in series. Figure 9-19b illustrates how the reactant streams in a fuel cell network flow in series from stack to stack. By networking fuel cell stacks, increased efficiency, improved thermal balance, and higher total reactant utilizations can be achieved. Networking also allows reactant streams to be conditioned at different stages of utilization. Between stacks, heat can be removed, streams can be mixed, and additional streams can be injected. 

Key to the concept of networking is the arrangement of multiple fuel cell stacks relative to the flow of reactant streams. Conventional fuel cells systems have been designed such that reactant streams flow in parallel through fuel cell stacks. In a fuel cell network, however, reactant streams are ducted such that they are fed and recycled through stacks in series. 

In another study, Chen and Zhao [55] demonstrated that by using a Ni-Cr alloy metal foam as the cathode DL (and current collector), instead of a CFP or CC, the performance of a DMFC can be enhanced significantly due to the improvement of the mass transfer of oxygen and overall water removal on the cathode side. Fly and Brady [56] designed a fuel cell stack in which the distribution layers were made out of metal foams (open cell foams). In addition, more than one foam (with different porosity) could be sandwiched together in order to form a DL with variable porosity. 

A fuel stack is a series connection of fuel cells. Strictly speaking, it is fuel cell battery (cf. footnote 1). The composition and design of the fuel cell stack differ for the implementation of each type of cell. 

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