Designs of SOFC System

A typical SOFC system takes fuel and air as the input and electricity hot water or steam is the main output. Major commercial organisations have shown interest and made large investments in this technology. They have come up with their own designs for different applications. Different SOFC systems include residential and auxiliary power systems, oxygen separating systems, pressurised SOFC or turbine hybrid systems, and atmospheric SOFC combined heat and power (CHP) systems. 

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Thus far, tubular design cells are the most reliable cells against thermal stresses and chemical degradation of the performance. Siemens Power Generation has demonstrated the performance and reliability of SOFC systems with tubular cells [6], The 100 kW CHP system has been operated for more than 20000 h, and it was reported that no degradation of the system was observed. To prove the feasibility of a GT/SOFC hybrid system, a 220-kW hybrid system with an SOFC generator integrated with a micro gas turbine has been demonstrated at the University of California this system has shown a conversion efficiency of around 49% (HHV). 

Cell design is also an important factor to improve the performance of SOFC stack/module [15-18]. Use of small diameter SOFC may also give opportunity to reduce operating temperature by increasing the volumetric power density [19]. Thus, they are expected to accelerate the progress of SOFC systems which can be applied to portable devices and auxiliary power units for automobile. 

System-level modeling of SOFCs aims to ensure that nearly uniform temperatures, currents, and fuel and air flows are seen by each individual SOFC cell in the system. This can be a daunting task in large power systems however, modeling can help in the design of the systems to ensure that there are a sufficient number of sensors, valves, and so on, to control the system and its operation correctly. 

SOFC system-level models provide valuable insight into the overall operation and design of SOFC power systems. They provide a means to investigate an SOFC... 

Peksen, M., Peters, R., Blum, L., and Stolten, D. (2011) Hierarchical 3D multiphysics modelling in the design and optimisation of SOFC system components. Int. J. Hydrogen Energy, 36, 4400-4408. 

The performance of a stack, reflected in its cell voltage and current density, is an important operating parameter in the design of SOFC power systems. System efficiency is obviously a strong function of cell voltage/current Figure 33.10 shows the effect of voltage and current density on the net system efficiency (defined as... 

In recent decades, the operation temperature of SOFCs was reduced to lower than 1,073 K. In such a temperature zone, metallic intercormectors can be applied instead of oxides [4,5]. The application of metallic materials is expected to reduce the materials cost effectively. In addition, they have high thermal and electrical conductivities in comparison with oxide ceramics, which has an advantage of designing compact SOFC systems. 

Another important contribution to lowering efficiency comes from the overpotential of electrodes. To analyze the efficiency of SOFC systems, it is essential to evaluate the amount and temperature of evolved heats and to design how to use these heats in the system. An attempt was made to derive differences of electrolytes to be used and of fuels to be oxidized in SOFC systems within the framework of exergetic analyses [6]. 

Solid oxide fuel cell technology has been reviewed from the point of view of lowering the operation temperatures. This process is closely related with the development of new materials, processing techniques, and stack designs. The most striking fact is that the recent achievement by Kyocera may indicate the start of a new era in the development and demonstration of SOFC systems for stationary applications. 

In designing practical SOFC systems with associated components such as fans, heat exchangers, etc., modelling of a SOFC as a power generating burner is very helpful (see Figure 3.7). The system is defined as a module consisting of SOFC cells connected in electrical parallel into stacks supplying a common burner with... 

This chapter first considers the trends in the energy markets that are driving towards utilisation of SOFC technology for electricity generation in various applications, Then SOFCs are compared with combustion engine generators and the potential application areas of SOFC systems are explored in detail. Designs and performance of SOFC systems are then analysed, followed by examples of SOFC systems and their demonstrations. 

Compressor Intercooling Whether a compressor should be intercooled or not depends on the trade-off between the increased efficiency of the intercooled compressor and its increased capital cost. In general, intercooling is required for large compressors with pressure ratios that exceed approximately 5 1 (44). The designer also should consider whether the heat is advantageous to the process. For example, when near the 5 1 pressure ratio, it may not be appropriate to intercool if the compressed stream will subsequently require preheating as it would with the process air stream of an MCFC or SOFC system. 

There have already been a number of excellent reviews describing many aspects of SOFC, and we will not try to duplicate that literature. For information on stack designs and cells operating on H2 or on synthesis gas (a mixture or H2 and CO) produced by reforming, we refer the reader to those other reviews. In this review, we will focus on recent work aimed at operating SOFC directly on hydrocarbon fuels and on anode materials that are compatible with direct hydrocarbon utilization. We have further restricted the scope to the performance and analysis of single-cell systems. While stacks that operate directly on hydrocarbons will likely need to be different from more traditional stacks, the development of direct-utilization SOFC is still in its infancy, with only one report of preliminary data on the stack level. 2 ... 

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. 

Fig. 2.17 The influence of the heat engine design on the system efficiency )Syst of SOFC-heat engine hybrid cycles.

An example of a system transient is shown in Figure 9.3. The figure shows a 20 amp load increase on a nominal 3 kW SOFC system using a stream-reformed methane fuel. Stack load current, stack voltage, and input fuel flow rate are shown. Here, the system is pre-warmed to the conditions shown (ca. 20 amps), following which the controller permits exporting more power to a load at the ramp rate shown. As the transition occurs, numerous other system variables also adjust, some in direct response to the increase in load, and others imposed by the control system in order to keep all system components within their design limits. 

Petruzzi L., Cocchi S., Fineschi F. (2003) A global thermo-electrochemical model for SOFC systems design and engineering. Journal of Power Sources 118, 96-107. 

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