Untersuchung des Wassertransportes in Gasdiffusionsmedien für Polymer-Brennstoffzellen
Aachen / Publikationsserver der RWTH Aachen University (2015) [Dissertation / PhD Thesis]
Page(s): 103 S. : Ill., graph. Darst.
The aim of this work was to develop a GDL water transport model based on measurable material specific properties. Therefore the Moisture Diffusion model was chosen as modelling approach and the required input parameter were determined experimentally. Capillary pressure saturation curves of the analysed materials show a wide hysteresis. Positive capillary pressures are needed to push water into the pores of a GDL. Once the water is fixed inside of a pore negative capillary pressures are required to remove the water completely. The examinations of effective permeability show that pressure drop over GDL thickness is dominated by the micro porous layer if typical liquid water flow rates are applied. The pressure drop in the GDL substrate is very low resulting in a low liquid water saturation. Additional to the ex-situ determinations in-situ tests were conducted. The results show that under wet operating conditions higher liquid water saturations are achieved compared to the ex-situ permeability measurements with equivalent liquid water flow rates. The higher liquid water saturation observed in in-situ tests is caused by condensation of water inside the GDL. The experimentally found data were used to conduct different simulations. The simulation results show that the modelling approach typically used in literature leads to very low saturations, analogue to the permeability measurements. In consequence the model was expanded by the drainage part of the capillary pressure saturation hysteresis which is applied for condensed water instead of the filling curve. This is to reflect that filling of a pore due to condensation occurs from inside out and thus is not depending on capillary pressure. The simulation results based on a modified non-isothermal model agree very well with in-situ measured saturations.