Turbocharger-based hybrid systems: Modeling and validation of a free spool subject to compressor surge

Compressor surge is one of the main problem that may affect fuel cell gas turbine hybrid systems, because of the energy stored in the volume containing the high temperature pressurized fuel cell stack. The problem becomes even more crucial because in such kind of system, the fuel cell is the most sensitive and costly component that has to be preserved by abrupt pressure changes. In order to determine the behavior of a dynamic compressor in its whole range of operating conditions, a calculation model was implemented in TRANSEO, a software tool for transient and dynamic analysis of microturbine and fuel cell based-cycles (based on Matlab-Simulink environment). The modeling procedure has been derived from the Greitzer's 1976 nonlinear dynamic approach; the resulting T-RIG1 model predicts the transient response of a compression system and is able to simulate both normal and instable transient conditions. Several investigations have been done in order to characterize the impact of different parameters and configurations on the system response. The validation, in the frequency domain, was performed comparing calculations with experimental data measured from a dedicated test rig, where a small size turbocharger has been operated in stable and unstable conditions. In particular, the present work demonstrates the capability of the T-RIG1 model to simulate a free shaft turbocharger performance and instability, with the future purpose to develop feasible strategies for surge detection and recovery, applicable to turbocharger-based hybrid systems.