Tubular SOFCs

A 100-kW power plant was built by S-W in Westervoort (Netherlands) from tubular cells. The fuel cell stacks used in this plant contained four bundles of said type, combined in series to form a row, 12 rows then being placed in parallel. Between the rows, units for conversion of natural gas were installed. The plant also included units for desulfurization and pre-reforming of the natural gas. 

At a temperature of 1010°C and gas pressures of about 1 bar, the cell had an OCV of about 1.15 V at a current density of 110 mA/cm, the cell voltage was about 0.55 V. The current-voltage relation was strictly linear. The authors attributed the voltage drop seen with increasing current density to the ohmic resistance of the rather thick electrolyte layer between the electrodes. 

Not long after building this first model of a tubular solid-oxide fuel cell of electrolyte-supported design that was associated with large ohmic losses, Westinghouse switched to a new cathode-supported design admitting a much thinner electrolyte layer and thus much lower ohmic losses. Also, the YSZ electrolyte was used for all subsequent work. 

In 1998, Westinghouse joined forces with the German company Siemens, which up to then had worked on planar SOFCs, which gave rise to a new enterprise called Siemens-Westinghouse (S-W). This enterprise specialized in the further development and commercialization of tubular SOFCs and soon became the world leader in this field. 

In S-W cells, ceramic tubes produced by extrusion of the cathode material, lanthanum manganite (with some added alkaline-earth metal oxides), are used. 

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George, R.A., Status of tubular SOFC field unit demonstrations, /. Power Sources, 86(1-2), 134, 2000. 

George RA and Bessette NF. Reducing the manufacturing cost of tubular SOFC technology. J. Power Sources 1998 71 131—137. 

SOFC electrodes are commonly produced in two layers an anode or cathode functional layer (AFL or CFL), and a current collector layer that can also serve as a mechanical or structural support layer or gas diffusion layer. The support layer is often an anode composite plate for planar SOFCs and a cathode composite tube for tubular SOFCs. Typically the functional layers are produced with a higher surface area and finer microstructure to maximize the electrochemical activity of the layer nearest the electrolyte where the reaction takes place. A coarser structure is generally used near the electrode surface in contact with the current collector or interconnect to allow more rapid diffusion of reactant gases to, and product gases from, the reaction sites. A typical microstructure of an SOFC cross-section showing both an anode support layer and an AFL is shown in Figure 6.4 [24],... 

A more recent development in high power density large-scale tubular SOFCs is that of flat tubes, which consist of a tube with two flat, parallel sides, and two rounded sides, with cross-connected current paths connecting the two flat faces of the tubes through the interior to minimize the length of the current path, as shown schematically in Figure 6.6 [48],... 

Tubular SOFC cathode supports with diameters or distance between flat faces on the order of 1 to 2 cm are commonly prepared by extrusion. Extrusion is a wet-ceramic process used to prepare tubes, and one which facilitates the formation... 

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,... 

FIGURE 6.7 Extrusion process for fabricating tubular SOFC support layers, (a) Open-ended die with cathode slurry in it and (b) Mandril insertion into the die, extruding the cathode slurry into a closed-ended hollow tube. 

The potential benefits of plasma spraying as an SOFC processing route have generated considerable interest in the process. In the manufacture of tubular SOFCs, APS is already widely used for the deposition of the interconnect layers on tubular cells, and has also been used for the deposition of individual electrode and electrolyte materials, with increasing interest in utilizing APS rather than EVD for electrolyte deposition due to the high cost of the EVD process [48, 51,104],... 

In planar SOFCs, individual cathode, anode, and electrolyte layers have been deposited by PS [109-111], as well as coatings on interconnect materials and full cells [108, 110, 112]. In addition to the interconnect layers themselves in tubular SOFCs, dense protective layers with good adhesion have also been deposited to protect planar SOFC interconnects from oxidation [110], and diffusion barriers to inhibit inter-diffusion between the interconnects and anodes have been produced by PS [113]. 

Siemens Westinghouse is planning a number of tests on power plants that are prototypes of future products. All systems employ the tubular SOFC concept and most are combined with gas turbines in a hybrid configuration. Capacities of these systems are 250 kilowatts atmospheric, 300 kilowatt class hybrid, and 1 megawatt class hybrid. They are to operate at various sites in the U.S., Canada, and Europe. Some of them are discussed below. 

United States Siemens-Westinghouse projects on SOFC include a 250 kWe tubular prototype at the Irvine University campus (California), that will be operated by Southern California Edison Company. It is pressurized to 3.5 bar and thus is expected to give 200 kWe a coupled microturbine gives an additional 50 kWe. The have operated a tubular SOFC at pressures up to 15 atm. 

FIG. 24-54 Configuration of the tubular SOFC. Courtesy ofWesUnghouse Electric Corporation.)... 

Fig. A3.4 Schematic representation of a tubular SOFC for calculating the equivalent resistance [21]. (Reproduction under permission of Elsevier BV).

Campanari and Iora [21], for example, use the schematic representation of Figure A3.4 for calculating the equivalent resistance of a tubular SOFC. 

Campanari S., 2001. Thermodynamic model and parametric analysis of a tubular SOFC module. Journal of Power Sources 92, 26-34. 

Bove R., Sammes N.M, 2005. The effect of current collectors configuration on the performance of a tubular SOFC. In Proceedings of the Ninth International Symposium on Solid Oxide Fuel Cells (SOFC IX), May 15-20, Quebec City, Canada, S.C. Singhal and J. Mizusaki (Eds.), Electrochemical Society, Vol. 1, pp. 780-781. 

The micro-tubular SOFCs considered are depicted in Figure 4.19. Specifically, Figure 4.19 shows the anode (supporting structure), the anode plus the electrolyte, and the final single cells. More details about the production process, the cell properties and characteristics can be found in [13-15],... 

Nakajo A., Stiller C., Harkegard G. and Bolland O., 2006. Modeling of thermal stresses and probability of survival of tubular SOFC. Journal of Power Sources 158(1), 287-294. 

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.

Jiang, W., Fang, R., Khan, J. and Dougal, R. (2006) Parameter setting and analysis of a dynamic tubular SOFC model, Journal of Power Sources 162(1), 316-326. 

Li, P. and Suzuki, K. (2004) Numerical modeling and performance study of a tubular SOFC, Journal of the Electrochemical Society 151, A548-A557. 

Tubular SOFCs vary in length which generally ranges from 30 to 150 cm. There are also micro-tubular cells with a length of approximately 10 cm (Sammes et al., 2005). 

Fig. 7.3 Current flow path and voltage distribution at 0.656 V across a tubular SOFC section (left) and a detail (right).

Tested HPD5 cells have an active area of 900 cm2 and 75 cm of active length. Even compared to the longest tubular SOFC (active area of 850 cm2), a single HPD5 cell presents a higher power density in the same conditions of temperature and cell voltage. 

As for the power density of SOFC systems, a comparison between the different kinds of cells has been made (Vora, 2006). The measured specific power (at 1000 K, fuel utilization ratio of 80% and 0.65 V) of a tubular SOFC bundle (24 cells) is around 0.13 in W/cm3, whereas that of an HPD5 bundle (six cells) is of 0.17 in W/cm3. The Delta9 configuration, with bundles of nine cells, reaches over 0.4 W/cm3. A comparison is shown in Figure 7.9. 

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