Microtubular designs

Other cell designs, such as radial configurations and more recently microtubular designs, have also been developed and demonstrated [21]. The microtubular designs can achieve lower operating temperature, since it has long been recognized that thinner electrolyte and electrode layers promote operation at lower temperature [22]. 

Another approach is to adopt a design similar to the lead acid battery and to use wires brought out from the electrodes and connected externally. This is the approach adopted by Adelan in their microtubular design. Clearly, the design of the cells and how they fit into the overall stack is vitally important in deciding such issues. 

The second major benefit of the microtubular design is a high thermal shock resistance [41]. Whereas the large-diameter tubular SOFCs are prone to cracking if they are rapidly heated, the microtubular SOFCs do not crack even when heated in a blow torch to their operating temperature of about 850°C in as little as 5 s. This is a marked advantage in applications where start-up time is critical. 

Each design may have variants, but the seal-less tubular designs, microtubular designs, and bipolar flat-plate (planar) designs have received wide attention. Hence, the discussion in the present section is limited to tubular (seal-less), microtubular, and planar designs only. The design of SOFC usually depends on three criteria as follows. 

Sammes N.M., Du Y., Bove R. (2005) Design and fabrication of a 100 W anode supported microtubular SOFC stack. Journal of Power Sources 145, 428 434. 

The cytoplasm of the cell is designated hyaloplasm in electron microscopy and cytosol in biochemistry. It constitutes about 51% of the cell volume and shows a microtubular lattice, (s. fig. 2.17) Basophilia, caused by ribonucleic acid, is the histochemical index for increased functional activity of the hepatocytes. The smooth endo-... 

A typical design of a microtubular SOFC is shown in Figure 8.25. A YSZ electrolyte tube (typically 2 mm in diameter and about 150 pm wall thickness), is used as a support for the electrodes, as a gas inlet tube, and also as a combustor tube at its outlet. The overall length of the tube is between 100 and 200 mm, whereas the cell region only occupies a length of about 30 mm towards the outlet end of the tube. The Ni + YSZ anode, 30 mm long, is coated on the inner wall of... 

Co-extruding a strip of lanthanum chromite based interconnect along the length of a YSZ microtube has also been demonstrated [45], although a number of difficulties remain. Firstly, the tubes are much weakened by the interconnect strip, and secondly the mixing of lanthanum chromite and YSZ at the boundary of the co-extruded materials leads to a dead-zone of material, about 350 pm in extent. Thus any microtubular cell design with co-extruded interconnect will require much further development to be successful. 

In 2000, Acumentrics Corp built a 1000-cell stack to illustrate the possibility of providing reliable power for computer systems back up. Since then, Acumentrics has designed and built several 2-5 kW systems using microtubular cells for use as back up power sources for broadband and computer systems. 

Microtubular cells were first made in 1990 by Michaela Kendall which were 1-5 mm in diameter, possessed 100-200 pm wall thickness and were made from extruded yttria-stabilised zirconia. After this achievement, Kendall and his co-workers demonstrated 20 cell, 200 cell, and 1000 cell reactors. Only a hot-air-fed reactor could heat up to the desired operating temperature and the cell could withstand an increase of temperature up to 200°C/min. Professor Kendall set up the first company in 1996 for microtubular (MT) cells known as Adelan (UK) Ltd. It was demonstrated by Bujalski that the operational temperature of these MT cells could be ramped at a rate of 4000°C/min, which indicates the rapid start-up time of 12 s for SOFC without any operational or structural damage to these cells. Second, these cells could be cooled extremely fast without any damage. The typical design of MT-SOFC and a 50 W portable MT is shown in Fig. 4.19a, b. The YSZ tube of thickness 150 pm and... 

Table 4.1 Microtubular cells stack designs by various firms...

For a more economical fabrication of microtubular SOFC with more reliability and flexibility in quality control, an advanced dry-jet wet extrusion technique, that is, a phase inversion-based co-extrusion process, followed by co-sintering and reduction processes was employed to fabricate a novel electroly te/anode dual-layer hollow fiber. Using the co-extrusion technique, one of the layers has to be thick in order to provide mechanical strength to the fiber, and in this design, the anode is chosen to be the thick layer due to the much lower ohmic losses (as shown in Figure 11.16). Use of co-extrusion has many advantages over conventional dry-jet wet extrusion methods such as simplified fabrication and better control over the manbrane structure. Furthermore, the risk of defects formation can be reduced and at the same time greater adhesion between the layers can be achieved. 

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