With a spate of orders around the world for industrial installations and the development of technology for the residential market, fuel cells seem to be finally reaching commercial status. Meanwhile, manufacturers of microturbines, another technology aimed at the small-scale power market, continue to look for applications which will prove the technology and speed up its route to commercialization.

Now manufacturers of the equipment may help open the door for each other by developing a system which integrates both technologies. A project is underway to build and test a “hybrid” generating plant which will be the first that integrates a solid oxide fuel cell with a microturbine – the result being a power plant with lower capital costs than a standalone fuel cell, and twice the efficiency of a standalone microturbine.

The multi-million dollar programme is being led by Southern California Edison (SCE), in cooperation with the US Department of Energy, the California Energy Commission, Siemens Westinghouse and Northern Research and Engineering Corporation (NREC), a wholly-owned subsidiary of Ingersoll-Rand Company.

The two main components of the hybrid power plant are a pressurized 200 kW Siemens Westinghouse solid oxide fuel cell (SOFC) and the NREC PowerWorks microturbine rated at 50 kW.

SOFC experience


Figure 1. The Siemens Westinghouse SOFC is an air electrode (cathode) supported tubular design
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The Siemens Westinghouse SOFC is an air electrode (cathode) supported tubular design configured as a single cell per tube with an axial interconnection (see Figure 1).

To generate electricity the cell has to operate at an optimum temperature of about 1000°C; and air must be supplied to the cell interior and fuel to the cell exterior.

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At open circuit, a potential of about one volt will be generated. When an external circuit is connected, a current flows in the external circuit which is directly proportional to the flow of oxygen ions through the electrolyte.

The fuel is oxidized electrochemically in complete isolation from atmospheric nitrogen thus eliminating NOx production. At atmospheric pressure, a uniform temperature of 1000°C, 85 per cent fuel utilization, and 25 per cent air utilization; a single tubular SOFC will generate a maximum power of about 210 W dc.

To generate useful quantities of electricity, cells are linked together into a generator module. 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 bundles, and the bundles into rows. An instack reformer is placed between each bundle row. The reformer is radiantly heated by the adjacent rows of cells.

For atmospheric pressure SOFC systems, the electrical generating efficiency is close to 50 per cent (ac/LHV). Exhaust gas heat recovery in the form of steam and hot water will yield fuel effectiveness values of about 80 per cent. These systems are expected to find applications in the cogeneration market for various heating and cooling requirements.


Figure 2. Process schematic for a simple pressurized SOFC gas turbine combined cycle
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In December 1997 EDB/Elsam, a consortium of Dutch and Danish utilities, began operating a 100 kW SOFC power system at a test site in Westervoort near Arnhem in the Netherlands. EDB is an acronym that refers to the Dutch energy distribution companies NUON, Essent, and ENECO, and the Federation EnergieNed. Elsam is the partnership owned by the regional Danish utilities, Fynsværket, Nordjyllandsvaerket Midtkraft, Skærbækværket, Sønderjyllands Højspændingsværk and Vestkraft. Funding for the project was also provided by the Dutch government agency Novem, the US Department of Energy, and Siemens Westinghouse. This system is the largest SOFC system to operate in the world.

During its initial operating period of 3700 hours (more than five months), the system typically operated at 106 kWe at an electrical efficiency of 43 per cent. It also provided 45 kWth in the form of hot water for the district heating system in the town of Westervoort. After an inspection period at Siemens Westinghouse, which followed the initial operation period of about 4300 hours, the system was restarted at the test site in Westervoort in March 1999. Since restarting, it typically supplies 110 kWe into the grid and operates at an electrical efficiency of 46 per cent. It now supplies 64 kWth for district heating. On December 30, 1999 the total operating time of the 100 kW SOFC system reached 8760 hours, or one year. The emissions from the system are impressive with NOx, SOx, CO, and VHCs all measured below 1 ppm.

The unit normally operates unattended with visits one day a week to the site by technicians employed by the local utility, NUON, which operates the system for the EDB/Elsam consortium. The system is contracted to run for two years although EDB/Elsam can continue to operate the unit beyond the test and demonstration period if required.

Pressurized design

With this first atmospheric test now one year into its operation, Siemens Westinghouse is turning its attention to the development of its pressurized system. Tests and studies show that additional power is produced when the fuel cell is operated under pressure. At an elevated pressure of 10 atm., the maximum power can be increased by 10 per cent. The fuel cell in the hybrid system under test will operate at an elevated pressure of 3 atm.


Figure 3. Testing of the microturbine began in NREC’s Portsmouth, New Hampshire facilities in June 1999
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In the pressurized system design, turbine work is extracted from the exhaust gas of the SOFC by an expander before the the exhaust passes through the recuperator. Typically, such systems can be configured in various ways, depending on the turbine and the capacity required. In the simplest recuperated SOFC/GT configuration, electrical efficiencies of up to 60 per cent can be achieved. If a reheat cycle is used with a split shaft turbine, efficiencies of up to 70 per cent can be achieved.

Figure 2 shows a process schematic for a simple pressurized SOFC gas turbine combined cycle. Here, the expander drives the compressor. Because the SOFC requires a process air inlet temperature of about 600°C, a recuperator is required.

For analysis purposes, Siemens Westinghouse has configured concepts ranging in capacity from 250 kW to several hundred megawatts. The best configuration will typically have a ratio of SOFC output to GT output of between three and five.

Microturbine

NREC has developed, tested, and delivered a PowerWorks microturbine to be incorporated into the first hybrid plant. NREC began building the microturbine for the hybrid system in the spring of 1999 and began testing it in its Portsmouth, New Hampshire facilities in June 1999 (Figure 3). The latter were basic functionality tests of the microturbine by itself using a pressure vessel to simulate the fuel cell portion of the system. The completed system was then shipped to Siemens Westinghouse.


Figure 4. The system assembly without the power dissipators
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PowerWorks’ gas-fueled, turbocharger-based, rugged engine design is ideal for the rigorous environment of industrial and commercial applications. The dual-turbine PowerWorks configuration provides excellent mechanical drive load flexibility while reducing stress and prolonging engine life to 80 000 hours.

A key component within each machine is NREC’s recuperator technology which captures turbine exhaust heat and uses the energy to preheat the engine’s combustion air. The recuperated gas turbine cycle yields very high efficiencies at modest temperatures and pressure ratios. In comparison to other designs, NREC’s recuperator is claimed to have a much higher strain tolerance and superior thermal cycle endurance (see PEi October 1998).

The hybrid power system consists of an SOFC module housed in a pressure vessel, a thermal management system, electricals and power dissipators. The thermal management system consists of the microturbine generator (MTG), a startup duct burner for the MTG, a startup duct burner to preheat the SOFC inlet air, and an auxiliary air system for shutdown. Because of the experimental nature of this power system, the system is not connected to the grid to avoid grid initiated transients during initial testing. The power dissipators are provided to dissipate the SOFC dc power and the MTG ac power. The system assembly without the power dissipators is shown in Figure 4.


Figure 5. General cycle diagram with the PowerWorks turbine
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As shown in the general cycle diagram in Figure 5, incoming air is pressurized by the PowerWorks gas generator compressor to about three atmospheres. The air then passes through the recuperator, which raises the air temperature using some of the remaining heat available from the hybrid’s exhaust gases. It is then introduced into the SOFC fuel cell along with fuel. The fuel cell produces about 200 kW of electricity and a very hot exhaust gas (about 815°C). This hot gas then passes into the microturbine’s combustor where further heat is provided if needed, for example during startup.

The hot gas is then introduced into the microturbine’s two turbine stages. The first turbine stage drives the compressor mentioned earlier. The second stage acts as a free power turbine and drives a synchronous 50 kW electrical generator. The gas exhausted from the power turbine is then routed through the other side of the recuperator to give up heat to the incoming air (and thus raise cycle efficiency substantially).

Pressurizing the fuel cell increases its power output and efficiency. Using the hot fuel cell exhaust gas eliminates the need for burning fuel in the microturbine combustor during normal operation.

Table 1 shows the performance of the hybrid power system. The total rated system output and electrical efficiency, if it were connected to the grid, is predicted to be 220 kWe (176 kWe from the SOFC and 47 kWe from the MTG minus 3 kWe system loads) and 57 per cent (net ac/LHV), respectively. For the same number of fuel cells as the 100 kWe SOFC cogeneration system, the system power output is approximately doubled (from 110 kWe to 220 kWe net ac) and the electrical efficiency is increased from 46 per cent to 57 per cent.

Progress

The microturbine has now been integrated into the overall hybrid power plant developed by Siemens Westinghouse at its Science and Technology Center located in Pittsburgh, Pennsylvania. Preliminary testing of the integrated power plant system is now underway to test the operation of system components and to determine system dynamics.

Prior to factory acceptance test, the system was tested without the cell stack but with all other components in place including the pressure vessel. The system functionality was verified and the control loops tuned. Once this was established, the cell stack was inserted into the pressure vessel in preparation for the factory acceptance test (FAT). Following the FAT, the unit will be installed at the National Fuel Cell Research Center (NFCRC) located at the University of California, Irvine. Site testing is scheduled to begin in the March 2000 timeframe.

Future demonstrations

Following the SCE proof-of-concept demonstration, several prototype demonstrations are planned. Siemens Westinghouse plans 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 220 kWe hybrid power system as well as from the 100 kWe atmospheric SOFC demonstration system at Westervoort. Siemens Westinghouse expects the new demonstrations to be contracted this year and delivered at various times by the end of 2003.