The Tubular Design
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.
Design of cylindrical-cell SOFC technology
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.
Single cylindrical cell operation and current path
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.