Tubular SOFC Design

Two general types of tubular cells are currently being pursued, cells with a large diameter ( 15 mm), and microtubular cells with a very small diameter ( 5 mm) the microtubular cells are discussed in Section 8.4. 

In addition to eliminating the porous support tube, the active length of the cells was continually increased to Increase the power output per cell a greater cell power output decreases the number of cells required in a given power size generator and thus improves power plant economics. The active length (the length of the interconnection) was Increased from 30 cm for pre-1986 thick-wall PST cells to 150 cm for today s commercial prototype air electrode-supported cells. Additionally, the diameter of the cells has been 

I ignre H.ll Representative inicroiirapli of the electrocbemicallfi vapour deposited VSZ electroli/te over a 

The Ni/YSZ anode, 100-150 pm thick, is deposited over the electrolyte by a two-step process. In the first step, nickel powder slurry is applied over the electrolyte. In the second step, YSZ is grown around the nickel particles by the same EVD process as used for depositing the electrolyte. Deposition of a 

Ni/YSZ slurry over the electrolyte followed by sintering has also yielded anodes that are equivalent in performance to those fabricated by the EVD process. Deposition of the anode by a thermal spraying method is also being investigated. Use of these non-EVD processes should result in a substantial reduction in the cost of manufacturing SOFCs. 

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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... 

Several other organisations, notably Mitsubishi Heavy Industries and TOTO in Japan and also Adelan Ltd in the United Kingdom have been developing tubular SOFC designs. 

Figure 7.5 Schematic of seaUess tubular SOFC design. (Image courtesy of Siemens Power Generation.)...

The system power density is lower than PEFC systems, especially for tubular SOFC designs. 

Figure 11.6 shows the steady-state modelling results for a tubular SOFC design, as reported by Ferguson et al. [29]. Clearly, the characteristic operating... 

The most advanced tubular SOFC design is the sealless one, realized by Siemens-Westinghouse (SW). 

Tubular SOFC design is also attractive for residential units because of its reliable and stable performance over long periods. Kyocera (Japan) has developed anode-supported flat tubular cells, and these cells are being utilized by several organizations in Japan (Osaka Gas, Tokyo Gas, etc.) to fabricate and test SOFC-based residential CHP systems. The first trial operation of such a 1 kW unit in a residential house was conducted in 2005-2006 for a period of 2000 h the average electric efficiency and hot water heat recovery efficiency were 44.1 % (LHV) and 34 % (LHV), respectively. Over 75 such units, illustrated in Fig. 8, are now installed and operating in homes throughout Japan, and are expected to be commercialized in the next two years. 

FIGURE 6.6 High power density (HPD) SOFC, consisting of a flattened tube with two flat faces. The vertical struts between the two flat faces provide shorter paths for the electronic current collection, eliminating the need for all of the electronic current to travel around the circumference of the cathode, as in the standard tubular cell design shown in Figure 6.5 [48], Reprinted from [48] with permission from Elsevier. 

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,... 

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. 

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 tubular SOFC was the first to be studied and is the more advanced in terms of scale-up and commercialization. In the longer term, however, the planar version appears to be more amenable to mass production and cost reduction, once its outstanding technical problems are fully resolved. The advantages of each design have to be weighed against the difficulties encountered in its fabrication. 

Fabrication and characterization of tubular SOFC stacks designed for low temperature operation were shown. Two types of stacks were proposed and demonstrated using the micro tubular SOFC bundles the type A was a vertically assembled stack, four-story cube-type stack with the volume of about 0.8 cm The performance of the type A stack was shown to be 3.6 V OCV and 2 W maximum output power under 500 °C operating temperature. Another type of the stack was also fabricated, which consists of three bundles and fuel manifolds, where the air was applied only by natural convection. Open circuit voltages of the type B stack were shown to be 2.85 and 2.73 V at 450 and 500 C operating temperatures, with the maximum power outputs of 0.91 and 1.54 W. Overall, both types of stacks showed reasonably sufficient performance for application use. 

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... 

Cali M., Santarelli M.G., Leone P., Design of experiments for fitting regression models on the tubular SOFC CHPlOO kWe screening test, response surface analysis and optimization, accepted for publication on International Journal of Hydrogen Energy, 2006. 

Cali M., Orsello G., Santarelli M., Leone P., Experimental activity on the tubular SOFC CHP 100 kWe Field Unit in Italy factor significance, effects and regression model analysis. Proceedings of ESDA2006, 8th Biennial ASME Conference on Engineering Systems Design and Analysis, Torino (Italy), July 2006. 

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