Auxiliary Power SOFC System

Auxiliary Power Units (APUs) are another type of atmospheric system with power capacity of 1-10 kW. A typical small system APU fuelled by gasoline is shown in Fig. 9.14. 

Gasoline fuelled APUs can provide electrical efficiency at a maximum 30%, whereas residential systems working on natural gas can have efficiency up to 40%. 

14 Small auxiliary/residential unit fuelled by gasoline 

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Given these requirements, hybrid and nonhybrid PEMFC systems are the leading contenders for automotive fuel cell power, with additional attention focusing on the direct-methanol fuel cell (DMFC) version of the technology and the possibility of using solid oxide fuel cell (SOFC) systems as auxiliary power units for cars and trucks. 

In recent years, SOFC technology has been developed over a broad spectrum of power generation applications, with systems ranging from portable devices (e.g., 500 W battery charger) to small power systems (e.g., 5kW residential power or automobile auxiliary power units), and to distributed generation power plants (e.g., 100-500 kw systems). SOFCs can also be integrated with a gas turbine to form large (several hundred kW to multi-MW) pressurized hybrid systems [87]. 

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. 

The DOE has continued its support of McDermott s program by selecting the Cummins Power Generation/McDermott team for a 74 million award under its Solid-State Energy Conversion Alliance (SECA). The team will develop a 10 kW SOFC system for auxiliary power in RVs and commercial work vehicles and for emergency power at remote telecommunications sites. MTI has offices in Alliance, Ohio and Lynchburg, Virginia." ... 

Small SOFC systems in the 10-500 W power range operating on hydrocarbon fuels have been considered and developed for portable applications. Examples of portable apphcations are battery charging, remote power, and low-level auxiliary power [50, 51]. Portable SOFC systems are particularly suitable for tactical military applications (e.g., soldier power and unmanned vehicle power) due to its potential for operation on logistics fuels such as JP-8 [52]. 

From the mid-1990s, there arose anew wave in fuel cell applications, that is, the automotive application and closely related applications of small stationary. It began with PEFC. Even so, SOFC has also received the impacts of new applications. This leads to two important efforts on SOFC developments (1) severer requirements for mechanical stability in automotive applications [22] and (2) smaller stationary application. The former has come from the new application proposed by BMW, Delphi, and Renault to utilize SOFC systems as auxiliary power unit in combustion engine cars. Since BMW and Delphi have cooperated with National Laboratories in their own countries, materials development has been made to improve mechanical instability. Development of simulation technique also helps to improve the stack technologies. The latter case of the small stationary systems gave rise to a big impact in Japan, Australia, Switzerland, and UK where the SOFC cogeneration systems have attracted strong attention. 

SOFC technology, on the other hand, offers interesting options to improve the effectiveness of CHP units. The reasons are many the prospects for high efficiency, low emissions, low noise, scalability, reliabihty, and, finally, potentially low cost. However, the main attractiveness in CHP is the high system efficiency even for small units and at part load operation [14]. Among the most interesting applications are stationary distributed power generation (incl. CHP), small residential combined heat and power units (micro CHP), various transport apphcations (auxiliary power units, APU), and medium-sized portable systems up to 5 kWei-... 

Because both electricity and heat are desirable and useful products of SOFC operation, the best applications are those which use both, for example residential combined heat and power, auxiliary power supplies on vehicles, and stationary power generation from coal which needs heat for gasification. A residential SOFC system can use this heat to produce hot water, as currently achieved with simple heat exchangers. In a vehicle the heat can be used to keep the driver warm. A stationary power system can use the hot gas output from the SOFC to gasify coal, or to drive a heat engine such as a Stirling engine or a gas turbine motor. 

A typical schematic for a small SOFC system is given in Figure 1.9. The electrical power output for a mobile power application could be 100 We for communications up to 5 kWe to power a house or to supply air conditioning and auxiliary power in a vehicle. The heat output is less important for such devices because electrical efficiency is not the main performance criterion. 

A major application of SOFC systems is perceived at the 1-10 kW level to supply power to residential buildings and as auxiliary power units in vehicles and trucks. A number of projects are underway at present to show the feasibility... 

Thus, SOFC/turbine hybrid systems are better than conventional combustion systems in terms of electrical efficiency, part-load efficiency, and emissions. Reliability and maintenance cost of mature SOFC systems will probably be comparable with gas turbine systems and certainly better than gas engines. Attributes of SOFC systems in the field of control speed, control range, investment cost and lifetime have yet to be fully established. The longer start-up time of SOFC systems is a clear disadvantage for certain market applications, for instance, the uninterruptible power supply market and the auxiliary power units for the transportation market. 

In the SOFC systems, fuel and air enter the SOFC stack, and electricity, exhaust gas and possibly hot water or steam exit the system. Such systems Include atmospheric SOFC CHP systems pressurised SOFC/turbine hybrid systems atmospheric SOFC residential and auxiliary power systems and oxygen separating systems. The difference between an SOFC stack and an SOFC system is generally referred to as the balance-of-plant (BOP). BOP equipment may differ for each application, depending on the size of the system, the operating pressure, and the fuel used. 

Residential, Auxiliary Power and Other Atmospheric SOFC Systems... 

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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