DESIGN AND MODELLING OF A PEM FUEL CELL AND SUPERCAPACITORS POWERTRAIN USING MATLAB-SIMULINK

This paper presents the modelling, design, and optimisation of the powertrain of a hybrid fuel cell electric vehicle (HFCEV), aiming to minimize consumption and thus maximising energy efficiency. The vehicle is the prototype “IDRAKronos” competing at the Shell Eco Marathon race and it is built by the H2Polito team. The hybridisation of a fuel cell electric vehicle (FCEV), through the use of supercapacitors, has a number of advantages, particularly the supercapacitors serve as an energy buffer to satisfy the load peaks requested by the electric motor, allowing a smoother (and closer to a stationary application) working condition for the fuel cell. Thus, the fuel cell can achieve higher efficiency rates and the fuel consumption is minimized. To optimally design and optimise the vehicle powertrain, several models were developed using MATLAB and SIMULINK and then experimentally validated through laboratory and track tests. Specifically, an in-depth model of the fuel cell has been developed to evaluate the maximum reversible cell voltage, open circuit losses, activation losses, ohmic losses and concentration losses as function of the main operating parameters (temperature, pressure, relative humidity and current density). To take into account the influence of relative humidity and temperature, a model was developed to correlate relative humidity with the water content of the membrane (Nafion) of the cell and a thermal model (0D and 1D) of the fuel cell to account for temperature variation during the race. In addition, models are presented which allow the evaluation of two main fuel cell/supercapacitors architectures: 1) configuration using a DC/DC converter which imposes a current equal to the desired value: 2) configuration using a direct parallel connection (“passive charging”) between fuel cell and supercapacitors. Therefore, a methodology for sizing the capacitance of supercapacitors has been proposed for both configurations: for the DC/DC converter configuration it depends mainly on the maximum charge variation and the voltage range to which the supercapacitors can undergo, while for the “passive charging” configuration the sizing of supercapacitors is obtained by evaluating the hydrogen consumption for the different capacitances, taking into account the influence of weight and thermal behaviour as the capacitance varies. In order to compare the different powertrain configurations, it was developed a model of dynamics which allows the powertrain energy demand to be assessed as the race track changes. The results obtained with the direct parallel connection (with the appropriate sizing of the overall capacitance) have highlighted a significant efficiency advantage, while the DC/DC converter insertion enables an improved control of the fuel cell current and requires a smaller capacitance. Finally, the results obtained from the models were validated by comparing them with experimental data obtained in the laboratory and on the track.