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Bloom Energy Will Take Homes Off the Power Grid

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Weeks before announcing a $300-million, three-year advertising campaign to raise awareness about global warming, Al Gore was conducting a slide show for a group of investors in Monterey, Calif., touting Bloom Energy. These bio-fuel and green technology firms could be poised to take off, Gore told his audience. Common consensus is that Bloom Energy is developing a 100 kW kW ceramic core regenerative solid oxide fuel cell for distributed generation in homes and business. A key innovation in their technology is its ability to process nearly any kind of hydrocarbon-based fuel, ranging from natural gas to ethanol to methane to biodiesel. Imagine your home completely free from the electrical power grid.


From an environmental perspective, fuel cells are one of the most attractive technologies for generating electricity. Solid oxide fuel cells operate by separating and transferring oxygen across a solid electrolyte membrane, where it reacts with a fuel - such as synthesis gas derived from coal, biofuels or natural gas - to produce steam and carbon dioxide (CO2). Condensing the steam results in a pure stream of CO2 gas; this can be readily captured for storage or other use in a central location. This feature, coupled with the well-known fact that fuel cell efficiency does not depend on high temperatures, results in near-zero emissions (e.g., NOx < 0.5ppm) at equivalent or reduced cost-of-electricity compared to today's power generation.


To realize the intrinsic advantages of solid oxide fuel cells requires achievement of SECA's cost reduction goals. The SECA project portfolio, including the Rolls-Royce and UTC Power projects, will research, develop and demonstrate fuel cell technologies that can support power generation systems as large as several hundred megawatts capacity. Key system requirements to be achieved include:


Cost of $100 per kilowatt (2002) for the minimum 40,000 hour fuel cell stack.

Cost of $400 per kilowatt (2002) for the integrated fuel cell power block.

Maintaining high power density in the large cells necessary for economic manufacturing.

SECA was established by DOE's Office of Fossil Energy in 2000 to research and develop low-cost, modular, fuel-flexible solid oxide fuel cell systems by 2010. In early 2005, the SECA program was accelerated to deliver megawatt-class fuel cell systems in response to the emerging national need for low-cost carbon capture technologies along with the more efficient and cost- effective use of fuels abundantly available in the United States and the need to address reduced water usage in power plants.


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 and Germany for more than 20,000 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 is developing high power density, flattened tube SOFC under the SECA program called the HPD Cell-Delta 9.


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.



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