Abstract
A large volume between compressor and turbine is present in fuel cell gas turbine hybrid systems. The substantially larger compressor plenum volume modifies the dynamic behaviour of these systems, increasing the risk of compressor surge during transients and subsequent destruction of both turbomachinery and fuel cell components. Diverting part of the compressor inlet flow directly to the turbine inlet through a cold-air bypass valve, bypassing the fuel cell stack, has been proven to be an effective method to increase the surge margin during normal operation and also to recover the machine from fully developed surge. This study investigates the dynamic effect of different cold-air bypass valve opening/closing procedures, especially steps and ramps changing the valve fractional opening. This analysis was carried out with reference to the Hybrid Performance (Hyper) facility: a hybrid system emulated using hardware and a cyber-physical fuel cell system at the National Energy Technology Laboratory (NETL), U.S. Department of Energy (DOE). Simulations performed on a Matlab®-Simulink® dynamic model of the system based on Greitzer’s theory showed a different behaviour varying the valve fractional opening with steps or ramps. Many experimental tests were performed on the Hyper facility to confirm the trends obtained from the simulations results. From the outcomes of this study, it has been possible to determine how to maximize the surge recovery effect of the cold-air bypass valve opening and to minimize surge related risks during the valve closure.
Highlights
Fuel cells are electrochemical reactors that generate electrical power from fuel oxidization without combustion and they are considered one of the most promising energy systems thanks to their high electric efficiency and low environmental impact [1]
Even if the system does not experience surge during the experimental tests, it is still possible to draw some conclusions comparing the surge margin trends during the valve openings and closings with the simulations showed in Fig. 2c and Fig. 2c
Since this work main goal is to study the effect on the compressor surge due to the large volumes that characterize fuel cell gas turbine hybrid systems, it was not necessary to run the SOFC real-time model and to emulate its thermal outcome
Summary
Fuel cells are electrochemical reactors that generate electrical power from fuel oxidization without combustion and they are considered one of the most promising energy systems thanks to their high electric efficiency and low environmental impact [1]. Many studies carried out on the Hyper facility [7]–[10] showed that diverting part of the compressor outlet flow to the compressor inlet, bypassing the stack volume through the CAB valve, is an effective way to increase the surge margin, because of the discharge pressure reduction and mass flow increment. The main goal of this work is to analyse the different responses of the compressor in an SOFC gas turbine hybrid system varying the CAB valve FO with steps or ramps, focusing the attention on the surge margin. This analysis is performed running both numerical simulations of a dynamic model and experimental tests on the Hyper facility
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