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Page | 006 1.4 Hybrid Gas Turbine Fuel Cell Systems
requires heat to proceed. The high temperature heat also allows significant co-generation and/or integration with a heat engine cycle.
The disadvantages of high temperature operation include the need to insulate the technology to protect from injury and the requirement
of more costly materials of construction.
SOFCs have higher overall fuel-to-electricity efficiency than lower temperature fuel cells (e.g., PEMFC) operated on available
hydrocarbon fuels (e.g., natural gas). When integrated with a heat engine cycle, efficiency can be increased even further. The hybrid
SOFC cycle, which integrates a SOFC into a gas turbine cycle, offers the potential of fuel-to-electricity efficiencies in the 75-80% range.
This remarkably high efficiency is unmatched by any other technology. Although in the early stages of development, hybrid designs and
systems are now emerging with the first demonstration being accomplished by Southern California Edison and Siemens Westinghouse
Power Corporation at the National Fuel Cell Research Center.
SOFC systems are being advanced by a number of companies and organizations with three major fuel cell stack designs emerging.
The major design types are tubular, planar, and monolithic. Tubular SOFC designs are closer to commercialization and are being
produced by Siemens Power Corporation, Mitsubishi Heavy Industries, Acumentrics, among others. The planar and the monolithic
designs are at an earlier stage of development typified by sub-scale, single cell and short stack development (kW scale). More than
100 companies are advancing and commercializing SOFC technology around the world and especially in the U.S., Europe and Japan.
Primary U.S. SOFC companies include GE, Acumentrics, FuelCell Energy, Versa Power, Ceramatec, Inc., Technology Management,
Inc., SOFCo, Cummins, and Ztek, Inc., among others.
SOFC systems have been operated all over the world, proving SOFC performance and features. Examples include the tubular
SOFC design of Siemens Westinghouse Power Corporation that has demonstrated over 85,000 hours of operation with low cell
degradation, and the SOFCo planar SOFC design exhibiting power densities up to 1000W/l.
Because of the high potential of SOFC technology to produce robust (long lasting), high power density, fuel flexible, and low
cost fuel cell systems, significant industry and agency investment is currently focused on SOFC technology, especially in Europe, Japan
and the United States. Notably, the Solid State Energy Conversion Alliance of the U.S. Department of Energy includes six industry-
led teams (General Electric, Siemens, Cummins, FuelCell Energy, Acumentrics, and Delphi) and a core technology research program
including national laboratory and university researchers that is focused on developing low cost, high power density and robust SOFC
technology.
Gas Turbine Technology for Hybrid Applications
A typical hybrid system recovers the thermal energy in the fuel cell exhaust and converts it into additional electrical energy
through a heat engine. Several heat engines have been considered for this type of system including gas turbines, steam turbines and
reciprocating engines. The only conversion device that has been tested in this role to-date is a micro-gas turbine (or micro-turbine
generator, MTG). An MTG is a type of gas turbine engine that is particularly amenable to integration with a high temperature fuel cell
in a hybrid system. This is due to several features of an MTG that are well matched to the requirements of the high temperature fuel cell
in the hybrid system such as:
• MTGs require relatively low turbine inlet temperature, which can be supplied by the exhaust of a high temperature fuel
cell,
• MTGs operate at relatively low pressure ratios that are amenable to either direct use in the high temperature fuel cell or in
other components of the hybrid system,
• MTGs often use recuperation to improve efficiency, which introduces components and features (e.g., heat exchangers,
large gas volume between compressor and turbine) that make the gas turbine engine design more amenable to a hybrid
cycle,
• The fuel cell can be operated under pressurized conditions improving it’s output and efficiency,
• Sufficient thermal energy is contained in the fuel cell exhaust to power an MTG compressor (for fuel cell pressurization)
and an electric generator (to produce additional electricity,
• The current size of most fuel cell systems is relatively small (between 250kW and 1.5 MW), which matches well with the
smaller output MTG,
• The power density of the system can be increased, and
• Overall system cost is potentially lower on a $/kW basis (primarily due to the increased output from the fuel cell).
Note that the gas turbine engine characteristics noted above as desirable for hybrid applications are not necessarily those that are
desired for stand-alone gas turbine engines. Usually one desires higher turbine inlet temperatures and higher pressure ratios to improve
the performance of a gas turbine engine. In hybrid cycles with a high temperature fuel cell, the gas turbine engine is not required to
operate at either high pressure ratios or with high turbine inlet temperature making the performance characteristics of the gas turbine
relatively simpler to achieve. That is, less sophisticated gas turbine technology may be all that is required for a hybrid system, although
improvements in compressor and turbine efficiency etc. are still desirable.
Although the MTG is currently well-suited for integration into hybrid gas turbine fuel cell systems, future high temperature fuel
cell technologies may become large and able to withstand significantly higher pressures. Analyses have shown that synergistic effects
of the combined gas turbine fuel cell system lead to electrical conversion efficiencies of 72-74 percent (LHV) for systems under 10 MW,
whereas efficiencies greater than 75% could be achieved with larger systems. As fuel cells advance and scale-up and pressurization
of MCFC and/or SOFC technology becomes viable, larger and more sophisticated gas turbine engines (e.g., axial compressors and
turbines, higher pressure ratios, high turbine inlet temperature) will be required.
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