SOFC / Gas Turbine Hybrid
Hybrid cycle power system
If a SOFC is pressurized, an increased voltage results, thus leading to improved performance. For example, operation at 3 atmospheres increases the power output by ~10%. However, this improved performance alone may not justify the expense of pressurization, but what may is the ability to integrate the SOFC with a gas turbine, which needs a hot pressurized gas flow to operate. Since the SOFC stack operates at 1000°C it produces a high temperature exhaust gas. If operated at an elevated pressure, the exhaust becomes a hot pressurized gas flow that can be used to drive a turbine. If a SOFC is pressurized and integrated with a gas turbine, the pressurized air needed by the SOFC can be provided by the gas turbine's compressor, the SOFC can act as the system combustor, and the exhaust from the SOFC can drive the compressor and a separate generator. This yields a dry (no steam) hybrid-cycle power system that promotes unprecedented electrical generation efficiency.
SOFC/Gas Turbine Hybrid Cycle Diagram
During normal operation of the pressurized SOFC hybrid, air enters the compressor and is compressed to ~3 atmospheres. This compressed air passes through the recuperator where it is preheated and then enters the SOFC. Pressurized fuel from the fuel pump also enters the SOFC and the electrochemical reactions takes place along the cells. The hot pressurized exhaust leaves the SOFC and goes directly to the expander section of the gas turbine, which drives both the compressor and the generator. The gases from the expander pass into the recuperator and then are exhausted. At ~200°C the exhaust is hot enough to make hot water. Electric power is thus generated by the SOFC (dc) and the generator (ac) using the same fuel/air flow. Analysis indicates that with such SOFC/GT hybrids an electrical efficiency of 55% can be achieved at power plant capacities as low as 250 kW, and ~60% as low as 1 MW using small gas turbines. At the 2 to 3 MW capacity level with larger, more sophisticated gas turbines, analysis indicates that electrical efficiencies of up to 70% are possible.
SOFC/Gas Turbine Hybrid System
220-kW hybrid system with a Solid Oxide Fuel Cell (SOFC) generator integrated with a micro gas turbine demonstrated at the University of California, Irvine and sponsored by Southern California Edison
300-kW SOFC/gas turbine hybrid system demonstrated in Pittsburgh, Pennsylvania
While other types of fuel cell/gas turbine hybrid cycles are possible also, Siemens Power Generation has led the way in SOFC/GT hybrid development and is the only company to have demonstrated such a system. Shown below are photographs of the proof of concept 220-kW SOFC/GT hybrid system demonstrated in California, and a 300-kW hybrid proof-of-concept system demonstrated in Pittsburgh.
Future of SOFC / Gas Turbine Hybrids
Siemens Power Generation has studied numerous alternative SOFC/GT hybrid configurations under a Cooperative Agreement with the US Department of Energy, National Energy Technology Laboratory. These studies have included hybrids with pressurized and atmospheric pressure fuel cell modules, turbines heated only by SOFC exhaust or also with supplemental duct heating and multiple, staged (cascaded) fuel cell/turbine reheat cycles. These studies have shown that while many different cycles offer considerable promise efficiency is maximized in a pressurized SOFC hybrid without supplemental firing, where the SOFC acts as the combustor for the gas turbine. For large scale applications (~20 MWe) staged reheat cycles have indicated that electrical generating efficiencies could reach as high as ~70%. These studies encouraged us to conduct demonstrations of pressurized SOFC/GT hybrids in pursuit of these high electrical efficiencies. Through our testing of a 220 kW and a 300 kW pressurized hybrid we have verified the feasibility of the pressurized SOFC/GT hybrid concept. With these demonstrations we have also demonstrated that the close coupled pressurized hybrids are complex systems and quite costly, but because of their feasibility and potential further development is warranted.
Our studies of hybrid cycles indicated that atmospheric cycles, while offering somewhat lower efficiency horizons than pressurized cycles, would be less complex to develop and quicker to implement. Because they would require less integration of the SOFC and gas turbine they have the potential to also be less expensive and could accommodate a wider variety of gas turbines. Before making a decision to proceed further with SOFC hybrid technology we plan to examine further other SOFC/gas turbine hybrid cycles to assess their potential.
Further efforts are also needed on the part of government and the industry to ensure the availability of appropriately sized small gas turbines. For pressurized hybrids especially, suitable mass flow, pressure ratio and other characteristics are necessary.