By Christian A. Forbes, Raymond A. George, Stephen E. Veyo, and Allan C. Casanova
Fuel cells are electrochemical devices that oxidize fuel without combustion to convert the fuel s chemical energy directly into electricity. The solid oxide fuel cell (SOFC) technology is distinguished from other fuel cell types by its all solid state structure and its high operating temperature (1000 degrees C). The Siemens Westinghouse tubular SOFC stack is process air-cooled and has integrated thermally and hydraulically within its structure a natural gas reformer that requires no fuel combustion, and, except during start up, no external heat source and no externally supplied water. In addition, since the SOFC stack delivers high temperature exhaust gas and can be operated at elevated pressure, it can supplant the combustor in a gas turbine-generator set yielding a dry (no steam) combined cycle power system of unprecedented electrical generation efficiency (>70 per cent ac/LHV). Most remarkably, analysis indicates that an electrical efficiency of 58 per cent can be achieved at power plant capacities as low as 250 kW, 60 per cent as low as 1 MW, and 70 per cent should be achievable at the 2 to 3 MW capacity level.
TUBULAR SOFC DESCRIPTION
The Siemens Westinghouse SOFC is an air electrode (cathode) supported tubular design configured as a single cell per tube with an axial interconnection. This is referred to as the cell. The cell is nominally 2.2 cm (0.867 inches) in diameter by 150 cm (59 inches) in active length with one closed end. The cell active area is 834 sq. cm, equivalent to a flat plate 11.4 inches square. To generate electricity, the cell must be maintained at operating temperature optimally, about 1000 degrees C; air must be supplied to the cell interior and fuel to the cell exterior. At open circuit, a potential of about 1 volt will be generated. When an external circuit is connected, a current will flow in the external circuit that is in direct proportion to the flow of oxygen ions through the electrolyte. The fuel is oxidized electrochemically in complete isolation from atmospheric nitrogen with no potential for NOx production. At atmospheric pressure, a uniform temperature of 1000 degrees C, 85 per cent fuel utilization, and 25 per cent air utilization, a single tubular SOFC will generate a maximum power of about 210 watts dc. We can gain additional power output when we pressurize these cells. Thus, at an elevated pressure of 10 atmosphere we can increase maximum power output by 10 per cent.
TUBULAR SOFC STACK DESCRIPTION
Thus, to generate commercially meaningful quantities of electricity, many cells must be aggregated into a generator module or stack. In Siemens Westinghouse's stack design, the tubular cells have a vertical orientation with the closed end down. The Siemens Westinghouse stack has progressed most recently from successful tests of 25 kW modules that used cells 50 cm in active length to the present prototype module using the 150 cm. cells, which are the cell size that will be carried forward in our commercial systems. A standard module of 1152 cells can produce up to 200 kW dc but is given a nominal rating of 100 kW ac. The prototype cells are arranged into 3 x 8 cell bundles, and the bundles into bundle rows. Between each bundle row is placed an in-stack reformer. The reformer is radiantly heated by the adjacent rows of cells. The outer container as shown applies to an atmospheric pressure unit.
When configured for pressurized operation, this container is replaced by a cylindrical pressure vessel. The orientation of this vessel can be vertical for a single 200 kW stack of 1,152 cells or a twin stack, as shown in Figure 3, containing 2,304 cells. If multiple single or twin stacks are to be contained in a single pressure vessel, then the vessel would most likely be horizontally oriented. Because of this modularity, SOFC generators using the basic 1,152 cell stack layout can be configured easily to fit applications into the low megawatts range.
ATMOSPHERIC SOFC SYSTEMS
The process schematic for the atmospheric pressure tubular SOFC power generation system is shown in Figure 4. In the atmospheric design, ambient air is drawn through an air filter and compressed to the appropriate process pressure by a compressor or blower. The process air is then routed through a recuperator heated by the exhaust gas to increase the air temperature to approximately 600¡C before introduction to the SOFC generator module. Pipeline natural gas at a pressure between 1 and 3 atmospheres above process pressure (typically 30-45 psi gauge) is desulphurized before being introduced to the SOFC generator module. Within the SOFC generator module, the fuel is electrochemically oxidized producing dc electricity. Nominally 85 per cent of the fuel is electrochemically oxidized with the balance burned in the stack s combustion zone. The SOFC exhaust exits the generator module at a temperature of between 800 degrees C and 850 degrees C and in atmospheric pressure systems is passed through the exhaust gas heat recovery train. This heat can be adapted to generate process heat or hot water for a combined heat and power application (CHP).
For atmospheric pressure SOFC systems, the horizon for electrical generation efficiency is close to 50 per cent (ac/LHV) level. Exhaust gas heat recovery in the form of steam and hot water will yield fuel effectiveness values of approximately 80 per cent. Such systems are expected to find application as cogeneration systems for various heating and cooling requirements. In December 1997 EDB/ELSAM, a consortium of Dutch and Danish utilities, began operation of a 100 kW SOFC power system supplied by Siemens Westinghouse, at a test site in Westervoort near Arnhem in the Netherlands.
After a factory acceptance test of 335 hours at the company's facility in Pittsburgh, the system was shipped to Westervoort and began operation in early 1998, where it operated for an initial period of 3,700 hours (>5 months). It operated typically at 106 kWe of power at an electrical efficiency of 43 per cent, and also provided 45 kWth (thermal) in the form of hot water to the district heating system in the town of Westervoort. However, after these initial hours of testing, the plant never reached its maximum power and efficiency potential, and when after 5 months of site operation other anomalies were detected, the system was shut down after a total of 4,035 hours of operation. After careful system analysis and review of all operating parameters, the module was returned to the factory for further evaluation where the problem was found to be failed baffle boards that allowed air to leak into the anode (fuel) gas stream, causing oxidation of the nickel components.
This problem held down system efficiency and if not corrected would have eventually caused the stack to deteriorate over time. After design improvements were made to the baffle boards and some cells were replaced, the system was returned to the site and restarted in March 1999. Almost immediately a significant improvement in performance was noted. Since restarting, it typically supplies 109 kWe into the grid and 63 kWth of hot water into the local district heating system, and operates at 46 per cent electrical efficiency. As of October 11, 1999, the system has operated for a total of 7,150 hours and has achieved a 46 per cent electrical efficiency at 109 kW with negligible NOx and SOx emissions.
As stated previously, we are focusing on two types of systems. Atmospheric Pressure and Pressurized Systems.
PRESSURIZED SOFC SYSTEMS
In the pressurized systems design, the elevated pressure system, turbine work is extracted from the exhaust gas stream of the SOFC by an expander before the exhaust passes through the recuperator. Such systems can be configured in a number of ways depending on the turbine under consideration and the capacity required. In the simplest recuperated SOFC/GT configuration electrical efficiencies of approximately 60 per cent can be achieved. If a reheat cycle is used with a split shaft turbine, efficiencies of greater than 70 per cent can be achieved. In a simple pressurized SOFC gas turbine (GT) combined cycle, the expander directly drives the compressor. Because the SOFC requires a process air inlet temperature of around 600¡C, a recuperator is required. Analysis has shown that for recuperated gas turbines with a turbine inlet temperature at about the SOFC exhaust temperature (850 degrees C), there is no benefit to exceeding a maximum process pressure of 6 to 10 atmospheres. Further, analysis shows that there is no efficiency advantage to burning fuel in a gas turbine combustor to increase the turbine inlet temperature.
Siemens Westinghouse has configured for analysis purposes SOFC/GT concepts ranging in capacity from 250 kW to several hundred megawatts. In general, the best configuration will have a ratio of SOFC output to GT output of between 3 and 5. Because of their extraordinary high efficiencies this system configuration is expected to be one of our first commercial SOFC products.
The first SOFC/GT hybrid demonstration has already benefited from the design completed for the 100 kW stack which is capable of being integrated with and driving a small gas turbine. The 220 kW SOFC/GT system will thus result from integrating the stack shown in Figure 2, but now operating at 3 atmospheres pressure, with a 50 kW micro-turbine generator. This system is expected to achieve an electrical generation efficiency of 58 per cent (net ac/LHV). The 220 kW SOFC/GT system package is capable of a maximum output of 250 kW and is expected to be about the same size as the atmospheric pressure 100 kW SOFC system demonstrated at EDB/ ELSAM.
FUTURE DEMONSTRATIONS
Following the SCE hybrid proof-of-concept demonstration, several prototype demonstrations are planned. Siemens Westinghouse intends to design, build and test in collaboration with development partners and customers one or more 250 kWe SOFC atmospheric cogeneration systems, one or two 320 kWe SOFC/GT power systems, and two 1 MWe SOFC/GT power systems. The design of these units will benefit from the lessons learned from the 100 kWe SOFC cogeneration system and 220 kWe hybrid power system demonstrations. We expect all of these demonstrations to be under contract in Calendar Year 2000 and delivered at various times by the end of 2003.
Christian A. Forbes, Raymond A. George, Stephen E. Veyo, and Allan C. Casanova are with Siemens Westinghouse Power Corporation. ET