Energie Baden-Württemberg AG (EnBW) and Siemens Power Generation (PG) are joining forces to build a highly efficient fuel cell hybrid power plant. Plans call for the construction of a megawatt-class demonstration plant. The goal of this research project is to convert up to 70% of the fuel energy into electricity.
The efficiency of this hybrid process is significantly greater than that of modern gas and steam turbine power plants, which can reach an efficiency approroaching 60 percent. This hybrid power plant combines a high-temperature fuel cell with a gas turbine in order to make more efficient use of the fuel and minimize emissions.
In solid oxide fuel cells (SOFC), an electrochemical reaction, directly and very efficiently, converts fuel energy into electricity and heat. In this hybrid power plant, the hot exhaust gases exiting the fuel cell are fed into the gas turbine, thereby reducing or totally eliminating the fuel consumption of the turbine. The gas turbine makes it possible to operate the fuel cell at increased gas pressure, which makes it more efficient.
The necessary groundwork is scheduled to be completed by 2008. This will provide the basis for construction of an initial, small pilot plant. Siemens will initially supply a high-temperature SOFC fuel cell with a capacity of five kilowatts. The test components themselves will be coupled together in the next phase of the project starting in 2009, and the configuration will be optimized beginning in 2012 , to be followed in 2012 by the start of construcion of a one megawatt fuel cell hybrid plant.
After successful completion of the project, this hybrid technology will become available roughly a decade sooner than expected by experts today.
The Siemens Power Generation SOFC is a tubular design configured as a single cell per tube. The cell is built up in layers on the air electrode (cathode) with an axial interconnection that makes the cathode accessible and allows cells to be connected together in series.
Currently manufactured as commercial prototypes at a pilot manufacturing facility in Pittsburgh, the cell is nominally 2.2 cm (0.867 inches) in diameter by 150 cm (59 inches) in active length with one closed end. To generate electricity efficiently, the cell must be maintained at an operating temperature of about 1000°C, air must be supplied to the cell interior using an air delivery tube and fuel is delivered to the cell exterior. At open circuit, a potential in the range of 900 mV to 1V will be generated per cell, thus cells are connected in series to build voltage. Power produced is proportional to the active surface area of the cells. At atmospheric pressure, a uniform temperature of 1000°C, 85% fuel utilization and 25% air utilization, a single tubular SOFC will generate power of up to 210 W dc.
While many aspects of the design and construction of all fuel cells are proprietary the basic materials used in SOFCs are well known. The commonly used electrolyte is yttria stabilized zirconia, or YSZ, chosen for its oxygen transport property. The anode, or fuel electrode, is nickel bonded to the electrolyte as a nickel/zirconia cermet. The cathode, fabricated as an extruded porous tube and thus the basic building block of the tubular cell, is made of lanthanum manganite, and the cathode interconnection consists of a thin strip of lanthanum chromite.
Siemans has conducted significient work on hybrid fuel cells, the folowing is taken from their website.
If a SOFC is pressurized, an increased voltage results, thus leading to improved performance. For example, operation at 3 atmospheres increases the power output by ~10%. However, this improved performance alone may not justify the expense of pressurization, but what may is the ability to integrate the SOFC with a gas turbine, which needs a hot pressurized gas flow to operate. Since the SOFC stack operates at 1000°C it produces a high temperature exhaust gas. If operated at an elevated pressure, the exhaust becomes a hot pressurized gas flow that can be used to drive a turbine. If a SOFC is pressurized and integrated with a gas turbine, the pressurized air needed by the SOFC can be provided by the gas turbine's compressor, the SOFC can act as the system combustor, and the exhaust from the SOFC can drive the compressor and a separate generator. This yields a dry (no steam) hybrid-cycle power system that promotes unprecedented electrical generation efficiency.
During normal operation of the pressurized SOFC hybrid, air enters the compressor and is compressed to ~3 atmospheres. This compressed air passes through the recuperator where it is preheated and then enters the SOFC. Pressurized fuel from the fuel pump also enters the SOFC and the electrochemical reactions takes place along the cells. The hot pressurized exhaust leaves the SOFC and goes directly to the expander section of the gas turbine, which drives both the compressor and the generator. The gases from the expander pass into the recuperator and then are exhausted. At ~200°C the exhaust is hot enough to make hot water. Electric power is thus generated by the SOFC (dc) and the generator (ac) using the same fuel/air flow. Analysis indicates that with such SOFC/GT hybrids an electrical efficiency of 55% can be achieved at power plant capacities as low as 250 kW, and ~60% as low as 1 MW using small gas turbines. At the 2 to 3 MW capacity level with larger, more sophisticated gas turbines, analysis indicates that electrical efficiencies of up to 70% are possible.
The “Stationary Fuel Cells” division of Siemens PG located in Pittsburgh, Pennsylvania in the U.S. is a world leader in the field of solid oxide fuel cells.
EnBW is the third largest energy company in Germany.
EnBW and Siemens plan first ever megawatt-class fuel-cell power plant with sights set on up to 70 percent efficiency, EnBW Energie Baden-Württemberg AG, press release, Sept. 12, 2006