| Solid oxide fuel cell gas turbine(SOFC/GT) hybrid systems have the potential for ultra-high efficiency, ultra-low emissions, extension of fuel cells lifetime, which are regarded as the effective means to promote the industrialization of SOFCs. The hybrid systems have been regarded as one of the most promising research areas.The operation temperature of SOFC is high, around 800 ~ 1 000 ℃. Thus, the thermal management inside the cells has been shown to be a critical issue for the operation and the lifetime of SOFC. This issue becomes more serious when SOFC stack is coupled with an gas turbine. The transients, suche as the changes of the fuel cell load, may result in the disturbance of the electrochemistry reactions, which may be followed by the changes of the thermal effluent of the fuel cell system. The off-gas from SOFC stack strongly affects the gas turbine, which may lead to a further change in SOFC cathode inlet conditions. Previous work showed that effective management and control of the cathode inlet airflow could be a method for the thermal management in the fuel cell component of a SOFC/GT hybrid system. But most of previous research either focused on the stand alone SOFC, or were simulation works. More experimental results based on the SOFC/GT hybrid systems are required for the design and the operation of the hybrid systems.Based on above, the dynamic responses of SOFC/GT hybrid system to the transient in the inlet airflow of the fuel cell cathode were evaluated in this thesis, using a hardware-based simulation facility designed and built by the U.S. Department of Energy, National Energy Technology Laboratory(NETL). The dynamic responses of both the fuel cell component and the hardware component of the hybrid system were studied in this work. An effective method was proposed for the thermal management of the hybrid systems. The research contents include:(1)The impacts of current load transients in SOFC subsystem on SOFC/GT hybrid system were analyzed. The operation conditions of fuel cell were changed and a huge disturbance in gas turbine speed was caused. The interaction between the hardware subsystem and the software subsystem was discussed.(2)The disturbances of the cathode inlet airflow were accomplished through the manipulation of a hot air bypass valve in open loop experiments. The impacts on the whole system were analyzed, including the temperature profiles, current density profiles in the cathode channels, and the turbine speed responses;(3) The impacts of the compressor bleeding airflow on SOFC/GT hybrid system were studied. Perturbations were accomplished by diverting part of the compressor discharge directly to atmosphere through the manipulation of a bleed air bypass valve in open loop experiments.The main results are summarized as follows:1) The transients in SOFC current load resulted in the changes in the operation conditions of the fuel cell stack, including the temperature gradient. SOFC current load was shown to have a huge impact on the gas turbine speed. The turbine speed increased 2 000 rpm when the SOFC current load was decreased by 30 A. The hybrid system was shutdown when the increase of the current load of the fuel cell stack was greater than 15 A.2) The transients of the cathode inlet airflow had a strong impact on the air temperature and the solid temperature in the SOFC cathode channels, resulting in an instant change in the thermal effluent from SOFC. 30% change of the cathode airflow did not affect the electrochemical reaction obviously, but caused changes in gas and solid temperature in cathode channels, which therefore resulted in 55 k W change in the thermal energy dissipation within five seconds. The transient caused a huge change in gas turbine speed, which was changed by 800 rpm within just 5 seconds. A further disturbance in cathode inlet airflow was resulted from the turbine speed change. The effective control of the cathode inlet airflow was thought to be the method to mitigate the imapact of current load transients in SOFC subsystem.3) The existence of SOFC stack has a strong impact on gas turbine speed. The small perturbation in the SOFC operation conditions can be amplified by the fuel cell subsystem and then causes a significant impact on the hardware system. Therefore, the transients in SOFC current load, fuel flow or cathode airflow need to be considered carefully.4) The disturbance of the compressor bleed air showed a strong impact on gas turbine speed. 4% change of the compressor bleed air resulted in 1 500 rpm speed change of the gas turbine within 20 seconds. The compressor bleed air was shown to be a critical parameter and needed to be considered sufficiently. 7% change of the compressor bleed air was shown to have very limited effect on the fuel cell electrochemical reaction, but resulting in 30 k W change in thermal energy dissipation. The results provided the indication that compressor bleed was the best and most sufficient parameter to control turbine speed without affecting the fuel cell subsystem.5) Manipulating the compressor bleed airflow and cathode inlet airflow synchronously was thought to be an extremely useful and effective method in terms of thermal management in SOFC and gas turbine speed to mitigate the impact on the hybrid system. |
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