On design of Hybrid Power Unit with Partitioned Fuel-cell and Flywheel Energy Sorage System for City Transit Buses

Paper proposes the design of a novel hydrogen-fueled hybrid power unit for zero-emission city transit bus applications. In particular, the powertrain consists of an electric drive fed by a hybrid power unit composed of a functional coupling of a high power density Flywheel Energy Storage System (FESS) and a high energy density hydrogen Fuel Cell (FC).The objective of the study is to design an HPU with an operation that can better follow the dynamics of the road power demand, optimize energy efficiency and reduce the power of the FESS.For this purpose, the FC power unit is partitioned in a certain number of stacks: each of those is on-off operated around its best-efficiency operating point; power transients are covered by the FESS bench. The control strategy of the hybrid power unit is based on the state of charge of the FESS. The performances of the vehicle on a reference mission were simulated through a modular and parametric approach: each part of the model was separately calibrated and validated by experimental data from literature.Simulation results highlight that the proposed hybrid power unit and control strategy make it possible to obtain an overall power output for the FC stacks which better follows road power demands (if compared to more traditional FC power units) increasing the system energy efficiency. In addition, the proposed control strategy allows a relevant downsizing of the FESS to be achieved.