Start date :13/03/2020
End date :13/03/2020
Location : salle des thèses (C002) de l'ENSEEIHT - 2 rue Charles Camichel, 31000, Toulouse
Doctoral School: GEET
The first part of the thesis works is devoted to the study and the modeling of performances as a function of operating conditions. An experimental database has been created using the Design of Experiments method. Several PEM fuel cell stacks have been tested in a broad operating field, considering classic operating conditions but also operating conditions linked to an aeronautical environment. A particular attention has been paid to low pressure functioning, knowing that experimental investigations at subatmospheric pressures have revealed lower performances compared to a classic pressure functioning. The experimental results have been exploited via a modelling approach, with the objective to define a model for the polarization curve as a function of operating conditions. First, a curve by curve model, in which a set of parameters (exchange current density, parasitic reactions’ equivalent current density and diffusion resistance) has been determined for every polarization curve, has been proposed. In the next step, some assumptions concerning the parameter dependency to the operating conditions have been introduced within the model. Different variation laws expressing the parameters as a function of operating conditions have been proposed. In all these steps, the results show good agreement between the experimental polarization curve and the polarization curve estimated by the model in the complete operating field. The predictive behaviour have finally been investigated with polarization curves carried out at operating conditions taken inside (interpolation cases) or outside (extrapolation cases) the initial definition range of the Design of Experiments. The prediction for interpolation cases has given promising results whereas the prediction for extrapolation cases has appeared limited for “exotic” operating conditions like low pressure or low temperature functioning. Future works will be devoted to improve variation laws and thus predictions using the model.
The second part of the thesis works is dedicated to the study and the modeling of the ageing of a PEM fuel cell stack submitted to a given mission profile. The presented approach is based on the superposition principle which states that degradation caused by several solicitations is the sum of the degradations that would have been caused by each solicitation individually. A decomposition of the mission profile in different sub-missions is proposed, considering quasi-static sub-missions linked to the mission profile current levels and dynamic sub-missions linked to the current transitory phases of the mission profile and to the start and stop phases. In the thesis works, dynamic sub-missions are supposed to have a limited impact on degradation and are not considered. Three quasi-static sub-missions, it is to say three different current levels, have been tested on dedicated campaigns. Mission profile defined by the industrialist partner has also been tested. A rich experimental database has been obtained and has given interesting informations about the stack degradations as a function of different current solicitations. Different methodologies have been defined to apply the superposition principle, considering degradation evaluation on polarization curves or on “voltage versus time” curves. The obtained results have pointed out that the proposed approach seems relevant to estimate the cell voltage degradation: most of the time, estimation of voltage degradation due to the mission profile has shown a quite good agreement with experimental voltage degradation observed. However, the approach application to another mission profile has given more mixed results. Future works will be necessary to confirm the potential of this approach.