Tubular Solid Oxide Fuel Cell Technology

Solid oxide fuel cells differ in many respects from other fuel cell technologies. First, they are composed of all-solid-state materials--the anode, cathode and electrolyte are all made from ceramic substances. Second, because of the all-ceramic make-up, the cells can operate at temperatures as high as 1,800 degrees F (1,000 degrees C), significantly hotter than any other major category of fuel cell. This produces exhaust gases at temperatures ideal for use in combined heat and power applications and combined-cycle electric power plants. Third, the cells can be configured either as rolled tubes (tubular) or as flat plates (planar) and manufactured using many of the techniques now employed today by the electronics industry.

Although a variety of oxide combinations have been used for solid oxide electrolytes, the most common has been doping zirconia with yttria, which serves to facilitate the transport of oxygen ions. Formed as a crystal lattice, the hard ceramic electrolyte tube is coated on both sides with specialized porous electrode materials.

At the high operating temperatures, oxygen ions are formed from air in the interior of the tubes at the "air electrode" (the cathode). When a fuel gas containing hydrogen is passed over the outside of the tube in contact with the "fuel electrode" (the anode), the oxygen ions migrate through the crystal lattice to oxidize the fuel. Electrons generated at the anode move out through an external circuit, creating electricity. Reforming natural gas or other hydrocarbon fuels to extract the necessary hydrogen can be accomplished within the fuel cell, eliminating the need for an external reformer. The tubular design also eliminates the need for seals and allows for thermal expansion. The tubular stacks are cooled using process air, and during normal operation consume no external water.

The fuel-to-electricity efficiencies of solid oxide fuel cells are expected to be around 50 percent. If the hot exhaust of the cells is used in a hybrid combination with gas turbines, the electrical generating efficiency might exceed 70 percent. In applications designed to capture and utilize the system's waste heat, overall fuel use efficiencies could top 80-85 percent.

The technical roots of solid oxide technology extend as far back as the late 1930s when Swiss scientist Emil Bauer and his colleague H. Preis experimented with zirconium, yttrium, cerium, lanthanum, and tungsten as electrolytes. By the late 1950s, Westinghouse began experimenting with zirconium compounds and small-scale research into solid oxide fuel cells was being carried out by researchers in the Netherlands, and the Consolidation Coal Company in Pennsylvania, and General Electric in New York. Much of the research, however, was short-lived as melting, short-circuiting, and high electrical resistance inside the cell materials created numerous technical hurdles.

One company, Westinghouse Electric Corporation continued to develop tubular solid oxide fuel cells, and in 1962 one of the first federal research contracts by the newly-formed Office of Coal Research in the Department of the Interior was granted to Westinghouse to study a fuel cell using zirconium oxide and calcium oxide. By 1976, the Energy Research and Development Administration--one of DOE's predecessor agencies--embarked upon an R&D program with Westinghouse to develop tubular solid oxide fuel cells.

Throughout the 1980s Westinghouse experimented with the design of tubular SOFCs, starting with very short cells built on a porous support tube (PST). Stacks and systems were also demonstrated, starting with a 400-Watt stack sponsored by the Tennessee Valley Authority (TVA). In the late 1990s, Siemens AG Power Generation purchased the power generation business unit of Westinghouse.

In the 1990s, long cell lifetimes and commercially viable cell performance were established, air electrode supported (AES) cells were developed that eliminated the PST, and a new cooperative agreement with the DOE was initiated to commercialize tubular SOFCs. The turn of the century culminated in the current successful commercial prototype 150 cm cells, and a 100 kW cogeneration system that operated in the Netherlands, Italy, and Germany for more than 36,000 hours. Also, a world record for individual fuel cell operation (~8 years) still stands, and the prototype 150 cm cells have demonstrated two critical successes: the ability to withstand >100 thermal cycles and voltage degradation of less than 0.1 percent per 1,000 hours.

Today, Siemens Power Generation has gone beyond the tubular technology to develop new high power density and high active area SOFCs under the SECA Program (the HPD Delta SOFC).

Fuel Cell/Turbine Hybrids

The high-temperature operation of a solid oxide fuel cell and its capability to operate at elevated pressures makes it an attractive candidate for linking with a gas turbine in a hybrid configuration. The hot, high pressure exhaust of the fuel cell can be used to spin a gas turbine, generating an additional source of electricity.

Siemens has tested the world's first solid oxide fuel cell/gas turbine hybrid system. The system had a total output of 220 kW, with 200 kW from the fuel cell and 20 kW from the microturbine generator. This proof-of-concept system demonstrated an electrical efficiency of 53 percent.