SOFC, design

Fig. 5.1 Commonly used SOFC designs (Celik, 2006). (a) Tubular SOFC, (b) 24 cell tubular SOFC stack, (c) a tubular SOFC module with 48 stacks, (d) 28 cell internally manifolded stack design by Versa Power Systems.

Discuss the relative merits of the tubular and planar SOFC designs. With special reference to the electroceramics involved and to their processing, sketch a feasible design for a planar SOFC stack running at 900 °C. 

Matching the coefficients of expansion of electrode structure, electrolyte and interconnect is a development problem for all SOFC designers pursuing good resistance to thermal cycling or thermal shock. 

Figure 2.56. Types of solid oxide fuel cell (SOFC) designs. Reproduced with periuission from Electronic Materials Chemistry, Bernhard Pogge, H. ed., Marcel Dekker New York. Copyright 1996 Taylor ...

Windes, W.E. and Lessing, P.A., Fabrication Methods of a Leaky SOFC Design, paper presented at the Eighth International Symposium of Solid Oxide Fuel Cells (SOFC VIII), 203rd Meeting of the Electrochemical Society, Paris, April 27-May 2, 2003. 

The bipolar plates (Fig. 3.20) providing electric contact between cells and forming impermeable channels for gas transport are in most SOFC designs made of ferritic stainless steel (low thermal expansion coefficient), although at the highest operating temperatures, more advanced (and expensive) metal structures must be used. 

Fig. 7. Solid oxide fuel cell configurations. A Siemens-Westinghouse tubular cell B Tubular integrated interconnector concept. Similar interconnected systems exist in planar geometry C Planar SOFC designs, differing only in gas flow manifolding.

The mathematical models of such processes in SOFC as charge, heat and mass transfer, diffusion, viscous flow of gases in channels were elaborated. The models gave necessary information for optimizing the SOFC design. 

A different type of SOFC design is under development by Hexis [9]. The HEXIS (heat exchanger integrated stack) stack concept can be used for small co-generation plants. The metallic intercoimect in this case serves as a heat exchanger as well as a bipolar plate. 

Nakajo et al. [12] in 2006 carried out a numerical analysis, studying the effect of the temperature profile characteristics on the stress field of the cell components in a tubular SOFC design. Radial thermal gradients were of particular interest. A simplified WeibuU analysis was performed to evaluate the evolution of the probabiHty of survival of the cell components in steady and transient state. Sensitivity analysis... 

In the planar design, a series of ceU components are configured as thin, flat plates, then electrically connected to build up desirable electrochemical performance. A schematic of a generic planar SOFC design is shown in Fig. 2. The planar ceUs can be either electrolyte supported, electrode supported, or metal supported. Each of these designs can also have a number of interesting variants for example, the planar SOFC may be in the form of a circular disk fed with fuel from the central axis, or it may be in the form of... 

The SOFC design differs from that of low-temperature fuel cells. Figure 1.13 shows the so-called planar anode-supported SOFC developed... 

Figure 1.13 SOFC design of the Forschungszentrum Jiilich (Blum et al.,...

A feature of the anode-supported SOFC design is its large anode thickness It — 1 mm). Experiments show that the reaction penetration depth into the anode is in the order of 10 [xm (Mogensen and Hendriksen, 2003). Thus, for the anode-supported SOFC, is a small parameter s 0.01. 

A major problem with almost all SOFC designs is the requirement to seal the structure in order to prevent the fuel and oxidant gases from coming into direct contact and causing direct combustion of the fuel. One possible way around this is to operate the cell at a temperature below which no direct combustion will take place and to allow the fuel and oxidant to mix. The cell then requires very selective electro-catalytic electrodes in order to generate electrochemical power. Ideally, the anode should only catalyze the fuel-electrochemical oxidation and the cathode the electrochemical reduction of the oxidant. Neither electrode should catalyze direct chemical reaction of fuel and oxidant. 

Since automotive applications require a rather robust SOFC design with rapid starting capability, good thermal cycling properties and high tolerance of the anode to oxidizing atmospheres, the replacement of the nickel cermet anode substrate by a material less sensitive to oxygen and with improved thermomechanical stability appears advisable. 

Fig. 21.20 Single repeating units and 15 kW class stack of the stationary SOFC design...

Table 21.5 Electricity intensive steps in stationary SOFC design production Anode-substrat...

Each of the three functional layers of the cell, anode, electrolyte, and cathode, are deposited onto the substrate using screen printing. In practice, each of the functional components requires multiple layers to provide the necessary functionality and manufacturability. Unlike other SOFC designs and similar to the metal-supported cell, the IP-SOFC uses neither the electrolyte nor electrodes to provide stmctural support. This allows very thin cells with consequently very small material quantities (cf. previous section). The cell functional layers again are made of conventional SOFC materials yttria-stabilized zirconia electrolyte, nickel cermet anode, and rare earth... 

Like other concepts, the IP-SOFC design uses yttria-stabihzed zirconia as electrolyte material. By screen printing, the electrolyte is fabricated with a layer thickness of about 10 pm, which leads to a minimal ionic resistance so that the electrolyte resistance has only a small contribution to the total losses in the cell [76]. 

The most common SOFC designs are planar and tubular, and their many variants. 

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