Planar designs

In order to improve the performance of SOFC, a thirmer yttria-stabilized zirconia (YSZ) electrolyte is considered for lower ohmic resistance and for operation in the intermediate temperature range of 500°C-800°C. Ionic conductivity decreases with decrease in temperature and hence the area-specific resistance (ASR) of an electrolyte increases with lower operating temperature. Fabricating the electrolyte in a dense and thinner film reduces the ASR or the resistance to ionic transport, allowing a lower operating temperature. For this purpose, efforts are being made in fabricating SOFC cell on the basis of either a thicker anode-supported or a thicker cathode-supported SOFC 

Different configurations of planar SOFC designs, (a) Electrolyte-supported cell, (b) Anode-supported cell, (c) Cathode-supported cell. 

Operating Temperatures and Thicknesses of Different Planar SOFC Configurations 

Electrolyte-Supported Cell Anode-Supported Cell Cathode-Supported Cell 

Temperature 1000°C Typical thicknesses Anode 50 jm Electrolyte 100 rm Cathode -50 jm Temperature 600°C-800°C Typical thicknesses Anode -300-1500 jm Electrolyte 20 pm Cathode -50 pm Temperature 600°C-800°C Typical thicknesses Anode -50 pm Electrolyte 20 pm Cathode -300-1500 pm 

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SOFC are produced with either tubular or planar stack configurations investments for planar design are a rough estimate, as no prototypes exist. Specific investments for PAFC are in the range 4000- 4500/kW (IEA, 2007). For further fuel-cell R D needs see IEA (2005). 

In order to analyze mixing phenomena occurring very close to the first fluid contact, i.e. directly behind the mixing element, a planar design completely covered by a transparent plate was chosen (see Figure 1.67) [112]. This permits characterization without any dead times, hence permitting observation of the entire mixing process. 

Solid oxide fuel cell — Figure 2. Configuration for a planar design SOFC [ii]... 

FIGURE 11.1 Planar design of solid-oxide fuel cell (a) a stack repeat unit and (b) details of a possible design. 

We decided to work with a planar design in order to compare the efficiency of block stacks to planar stacks. 

Our planar design does not rely on convection/air or pure oxygen feed (see Build Your Own Fuel Cells for more about convection and oxy-gen/hydrogen fuel cells). The planar design simply relies on ambient air ports. The intent was to see if these ports would give greater air intake, as well as more efficient water dispersion. We surmised that this design would take care of these two problems, and reduce maintenance problems (and thus cost) over a period of time. 

One of the advantages of mixed potential sensors is that it is possible for both electrodes to be exposed to the same gas. The elimination of a need to separate the two electrodes simplifies the sensor design, which in turn reduces fabrication costs. Although this simpler planar design is often used, the electrodes are sometimes separated to provide a more stable reference potential. As with equilibrium potentiometric sensors, the minimum operating temperature is often limited by electrolyte conductivity. However, the maximum operation temperatures for nonequilibrium sensors are typically lower than those of equilibrium sensors, because the electrode reactions tend towards equilibrium as the temperature increases. This operating temperature window depends on the electrode materials, as will be discussed later in the chapter. 

To build up functional or protective layers, various methods from other technical industries have been adapted to the specific needs of ceramic sensor materials. Important criteria for the selection of the various technologies are the shape of the ceramic substrate (e.g., thimble or planar design), if it is co- or post-fired, if the layer material is expensive (e.g., noble metals) or inexpensive (e.g., alumina, magnesia spinel), and if the geometry and thickness need to be controlled precisely. 

The essential tasks of the sensor housing are to hold the sensing element in place and to protect it against mechanical impacts or aggressive chemicals. Furthermore, it provides the electrical connection and keeps the reference side clean and separated from the exhaust side. Thimble and planar designs naturally differ from each other because of the tubular or rectangular geometry of the ceramic elements and the fact that the planar element typically includes a heater while the thimble heater is a separate part (Fig. 17.15.11). 

DOE has evaluated two different architectural approaches for ITM-based oxygen separation, planar and tubular however, engineering scale-up of the planar approach is currently being pursued. The planar design allows desired gas phase mass transfer and is amenable to standard ceramic processing technologies. 

Minatom institutions activity, All-Russian Research Institute of Technical Physics (VNIITF, Snezhinsk) and Institute of Physical and Power Engineering (IPPE, Obninsk) of Minatom develop SOFCs of tubular and planar design, respectively. 

There are mainly two different eoneepts under development - the tubular and the planar design. As far as proof of long term stability and demonstration of plant teehnology are eoneemed the tubular concept is far more advaneed. In eomparison, the planar development offers higher power density. In this ehapter the various design variants are presented, followed by a deseription of the main eompanies involved and the status of cell and staek teehnology. 

In contrast, tube-and-shell type membrane modules do not have these engineering challenges with deflection and compromise of seal integrity. For this reason, tubular membrane modules will generally be lighter in construction relative to stacked planar membrane modules of comparable membrane area. However, a drawback to the tubular membrane module is that the packing density of the membrane is often substantially less than that which can be achieved with the stacked planar designs. 

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