Stacked Cell Design

Stacking an assembly of individual EFCs is the most direct system-level approach to increasing power generation and is more flexible and universal than simply increasing the electrode size of a single-cell EFC. 

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Capillary gap cell — The undivided capillary gap (or disc-stack) cell design is frequently used in industrial-scale electroorganic syntheses, but is also applicable for laboratory-scale experiments when a large space-time yield is required. Only the top and bottom electrodes of c.g.c. (see Figure) are electrically connected to - anode and cathode, respectively, whereas the other electrodes are polarized in the electrical field and act as -> bipolar electrodes. This makes c.g.c. s appropriate for dual electrosynthesis, i.e., pro duct-generating on both electrodes. 

Figure 2.56 shows a variety of stacked cell designs employed by SOFCs. Since an individual fuel cell produces a low voltage (typically < 1V), a number of cells are connected in series forming a fuel cell stack. An interconnect comprising a high-density material is used between the repeating anode-electrolyte-cathode units of... 

A dual-stack cell design that enclosed two stacks within the same pressure vessel was developed to reduce the number of pressure vessels required in some power systems. This conflguration was termed a common pressure vessel. These powered the Iridium constellation of 88 satellites (98 satellites, including orbital spares). While these batteries have functioned reasonably well, there are some difficulties in maintaining all the cells at a similar state of charge. However, these single pressure vessel batteries continue to perform well in orbit. 

The requirements for a battery separator can best be understood in the context of how the separator is used. Currently there are two major designs spirally wound or stacked plate. There are two types of stacked-plate designs, one relying on stack pressure to maintain good interfacial contact between the electrodes and separators, and a second which uses an adhesive to bond the electrodes and separators. The manufacturing processes for both spiral and stack cell designs are reviewed by Brodd and Tagawa [7]. Both processes put stress on the separator. 

Although cathode-supported tubular SOFCs in large-scale stacks are the type of SOFC stack most widely commercialized, recent alternative tubular cell designs have been developed with anode-supported designs for smaller-power applications. Cells in these stacks have diameters on the order of several millimeters rather than centimeters,... 

Two distinct classes of cell design exist the monopolar and the bipolar. Most commercial stacks have the bipolar design, which means that the single cells are connected in series both electrically and geometrically. The bipolar cell design has the advantages of compactness and shorter current paths with lower voltage losses. 

A conceptual design of an improved technology stack operating at 120 psi (8.2 atm) and 405°F (207°C) was produced based on cell and stack development and tests. The stack was designed for 355 10 ft (approximately 1 m ) cells to produce over 1 MW dc power in the same physical envelope as the 670 kW stack used in the 11 MW PAFC plant built for Tokyo Electric Power. The improvements made to the design were tested in single cells, and in subscale and full size short stacks. 

The parameters in the above equations were calibrated from 400 sets of FCE s laboratory-scale test data and were further verified by several large-scale stack experiments. These parameter values may be dependent on the FCE cell design and characteristics and may not be directly applicable to other carbonate technologies. Figure 6-16 is a comparison of the measured data match with the model prediction. 

Japan Fuji Electric has developed a 100 kWe on-site system. To date, they have tested a 50 kW power plant using innovative cell design that improves electrolyte management. They tested this stack (154 cells) for about 2,000. They have tested 65, 50 kWe units for a total cumulative operating tome of over 1 million hours. They have tested 3, 500 kWe units for a total of 43,437 hours. Their latest design, FPIOOE, has been shown to have a net AC efficiency of 40.2% (LHV). 

Australia Ceramic Fuel Cells Limited was demonstrated a 5 kWe laboratory prototype fuel cell system in 1997. Their system has thin sheet steel components as interconnects in a planer fuel cell design. They are currently scaling up to a 25 kWe pre-commercial stack module. 

Research and development efforts have been directed toward improved cell designs, theoretical electrochemical studies of magnesium cells, and improved cathode conditions. A stacked-type bipolar electrode cell has been operated on a lab scale (112). Electrochemical studies of the mechanism of magnesium ion reduction have determined that it is a two-electron reversible process that is mass-transfer controlled (113). A review of magnesium production is found in Reference 114. 

The gas supply to the stack is accomplished using either external manifolds, usually made of stainless steel, or internal manifolds (Versa Power Systems, HT-ceramix). In the latter case, the manifolds are integrated into the cell design and interconnect (Delphi). 

To model a complete stack, which may be constituted of more than 1000 cells, it is necessary to adopt a different approach. In this chapter a finite difference model is presented. Only energy equation and current conservation are solved. This allows one to examine possible improvements in the stack configuration design that can be achieved by taking advantage of the relation between temperature and elec-tronic/ionic resistivity, heat transfer and chemical reactions, etc. In addition, this model can be used for analyzing the effects of possible anomalies and performance degradation. 

In Part Two, the reader is provided with practical examples of how the general equations defined in part one can be simplified/adapted to specific cases. The examples cover the most widely employed single cell designs, for steady-state and dynamic conditions, and evolve towards stack and system modehng. Finally, Chapter 10 introduces the reader to the problem of mechanical stresses in SOFC, and shows an approach for modeling mechanical stresses induced by the operating conditions. 

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