Stack Designs 2 Planar SOFC Design

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. 

Despite their fundamental attraction in terms of power density and efficiency compared with tubnlar designs, several organisations have abandoned development of the planar SOFC design because of all the inherent technical problems. These have included Siemens in Europe who had built 20-kW stacks by 1999. However, planar technology is being... 

FIGURE 5.1 Schematics of edge sealing of planar cells (above) and external gas manifold seals (below) used for a simple cross-flow SOFC stack design. 

The CORE-SOFC Project was designed to improve the durability of planar SOFC systems to a level acceptable for commercial operation. The work focuses mainly on materials selection for interconnects, contact layers and protective coatings to minimise corrosion between metallic and ceramic parts to achieve reliable and thermally-cyclable SOFCs. In all work packages, cells and stacks will be analysed by advanced chemical and ceramographic methods. 

Planar SOFC, in particular, monolithic designs (MHI) are capable of high (volumetric) power densities most favoured by direct and short current paths across the stack components. The PEN is principally square, rectangular and circular (Ceramic Fuel Cells Limited (CFCL), Mitsubishi Materials Corp., SulzerHexis) in shape with active surface areas of 100-200 cm2 (15.5-31 in2). A drawback of this design is that it often necessitates the use of high temperature sealants for application at the in-... 

Figure 3.20 shows a cylindrical layout often used for high-temperature SOFCs. Alternatives are a stack of planar cells or a disk concept with feed tubes in the centre. A consideration of efficient heat exchange is the cormnon design strategy for the high-temperature fuel cell geometry. 

A joint development between Mitsubishi Heavy Industries (MHI) and Chubu Electric Power Company is the so called MOLB-Type (Mono-block Layer Built) planar SOFC. The cells are manufactured up to a size of 200 X 200 mm, based on a corrugated electrolyte. In this way the electrolyte also contains the gas channels, which simplifies the design of the interconnects, where plane ceramic plates are used (see Fig. 5). The biggest stack of this type was built of 40 layers, delivering 2.5 kW at 1000°C. 

The so-called E- and F-designs for stacks with planar anode substrate type cells and metallic interconnects constitute the work horses at Forschungszentrum Jiilich used for testing SOFC materials, cells and manufacturing processes in cell and stack development since its introduction in the year 2000 [1]. Ferritic interconnects were chosen since they offer a high electric conductivity and thus the potential for high power density in the stacks. On the other hand the ferritic material gives rise to chromium evaporation (which can poison the cathodes) and is prone to massive corrosion at temperatures above approximately 900°C [2],... 

Figure 9.14 (a) An expanded view of a stack of planar design solid-oxide fuel cells (SOFCs) (b) tubular design of an SOFC (c) a stack of tubular SOFCs... 

The requirements, material choices, and general sealing concepts are common to most planar SOFC stack designs. Fundamentally, two different types of seals are being developed for SOFC bonded and compressive seals. 

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. 

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