The Solid Oxide Fuel Cell (SOFC) uses a ceramic, solid-phase electrolyte which reduces corrosion considerations and eliminates the electrolyte management problems associated with liquid electrolyte fuel cells. To achieve adequate ionic conductivity in such a ceramic, however, the system must operate at temperatures between 650 and 1000°C (1200 and 1830°F). High temperature operation removes the need for precious-metal catalyst, thereby reducing cost. At these temperatures, internal reforming of carbonaceous fuels is possible, and the waste heat from such a device can be utilized by conventional thermal electricity generating plants to yield excellent fuel efficiency. Solid Oxide Fuel Cells (SOFCs) are currently being demonstrated in sizes from 1kW up to 250-kW plants, with plans to reach the multi-MW range. Commercial SOFCs are expected to have around 50-60 percent efficiency. In co-generationapplications overall efficiencies should be in the 80-85 percent range.
In operation, hydrogen or carbon monoxide (CO) in the fuel stream reacts with oxide ions (O=) from the electrolyte to produce water or CO2 and to deposit electrons into the anode. The electrons pass outside the fuel cell, through the load, and back to the cathode where oxygen from air receives the electrons and is converted into oxide ions which are injected into the electrolyte. It is significant that the SOFC can use CO as well as hydrogen as its direct fuel. This allows SOFCs to use gases made from coal.
High-temperature operation has disadvantages. It results in a slow startup and requires significant thermal shielding to retain heat and protect personnel, which may be acceptable for utility applications but not for transportation and small portable applications. The high operating temperatures also place stringent durability requirements on materials. The development of low-cost materials with high durability at cell operating temperatures is the key technical challenge facing this technology.
The most common electrolyte material, dense yttria-stabilized zirconia, is an excellent conductor of negatively charged oxygen (oxide) ions at high temperatures. Common electrodes are porous nickel/zirconia cermet while typical cathodes are magnesium-doped lanthanum manganate. The SOFC is a solid state device and shares certain properties and fabrication techniques with semi-conductor devices.
As in the MCFC, CO does not act as a poison and can be used directly as a fuel. The SOFC is also the most tolerant of any fuel cell type to sulfur. It can tolerant several orders of magnitude more sulfur than other fuel cells.
To supply the air and fuel an air blower and fuel compressor are usually needed. Other equipment, such as a heat exchanger, start-up and shutdown systems, instrumentation and controls, as well as safety equipment are also needed for a safe and reliable power system.
Because the electrolyte is solid, SOFC's can be made in other forms than a planar stack. Siemens-Westinghouse and Acumentric use a tubular form. This design eliminates the need for seals required by other types of fuel cells, and also allows for thermal expansion. In a tubular SOFC design, air flows through the interior of the cell, and fuel flows on the outside of the cell. Rolly-Royce uses a flat tubular form and Westinghous-Siemens is considering a flat tubular form for its next generation of fuel cells.
Rolls-Royce and ZTEK have designed an electrical power system that integrates a solid oxide fuel cell with a micro turbine. This power system promises to be significantly more efficient than any conventional gas turbine or reciprocating engine, with far less impact on the environment. The ZTEK unit is 200 kW and the prototype was scheduled for operation by the end of 2005. Rolls-Royce plans to have a system providing around 1 megawatt of electricity, for market delivery some time before the end of 2008.
ESL Electro-Science has developed, an improved electrolyte material, scandia stabilized zirconia (ScSZ) that has been shown to more than double the power density achieved with yttria stabilized zirconia (YSZ). ScSZ is now commercially available in both tape form and as a fired substrate. The new ScSZ ceramic is made by adding scandia (Sc2O3) to zirconium oxide (ZrO2) to optimize the crystal structure.
Several Researchers have been devloping SFCs that operate at lower temperatures than conventional SOFCs. In addition to the weight reduction of the stack the weight and size is reduced due to lesser amounts of heat insulation being required. SOFCs are claimed to be more rugged than PEMFC's and the military has expressed an interest in these small, lightweight and more efficient cells to replace batteries in backpacks and the larger fuel cells now used on the shuttle. Lower temperatures obviously reduce the amount of thermal power that could be generated from the stack, one of the primary advantages of SOFC's.
- Toto LTD, Japan has developed a SOFC stack that operates at 500°C. To increase the integration density and the start-up time, the fuel electrode is molded from lanthanum gallate ceramics to be a tubular member of 5mm in diameter.
- Stanford researchers have also reduced the operating temperature of SOFC's, to as low as 400°C, by making the electrolyte layer as thin as 50 nanometers.
- The University of Houston has developed SOFCs that are one micron thick and operate at 450°C to 500°C.
No one has SOFCs in commercial production. The following companies are leading the development, with a very brief characterization of their technology:
Acumentric, Westwood, MA--tubular, 750°C, efficiency-mid 30's, 19 five kW beta units
Ceramic Fuel Cells Ltd. Victoria, Austrailia--circular stacks, 850°C, 40-50% efficiency, overall 80%, 1 kWe, 1kWt, Field trials
Rolls Royce Fuel Cells Ltd., Derby, UK--flat tube ceramic, 1 MW system by 2007-2008
Siemens Westinghouse Fuel Cells, Erlangen, Germany--tubular, <1000°C, 125 kWe, 100kWt pre-commercial product, 45-47% efficient, system efficiency >80%
ZTEK Corporation, Woburn, MA, USA--circular stacks, 200 kW SOFC/gas turbine unit, estimated 60% efficiency, $1,600 per kW, under construction.
Solid Oxide Fuel Cells, DOE Energy Efficiency and Renewable Energy, Hydrogen, Fuel Cells & Infrastructure Technologies Program, Fuel Cells, Types of Fuel Cells, Updated July, 1, 2004
Solid Oxide Fuel Cells, DOD Fuel Cell ERDC/CERL Projects, Fuel Cell Descriptions