A fuel-cell-based system's performance is mainly identified
in the overall efficiency, strongly depending on the amount of
power losses due to auxiliary devices to supply. In such a
situation, everything that causes either a decrease of the
available power output or an increment of auxiliary losses would
determine a sensible overall efficiency reduction. This situation
inevitably pops up in case of water flooding, a phenomenon that
causes a reduction of the number of chemical reactions between
reactants (O₂ and H₂) that can take place at the reaction sites, as
liquid water would obstruct the catalyst and gas diffusion layers
pores the gases have to flow through, while at the same time
determining a higher pressure drop of the flow passing along the
channels, as the liquid water will have to be blown away: the two
main effects are a drop in the production of electricity with
respect to its theoretical amount (corresponding to the
stoichiometric reaction) and an increment of the pump electric load
required to overcome the overall pressure drop across the cell. A
suitable solution has been identified in designing the fuel cell
bipolar plate's channels in such a manner to allow an optimal
water management at a desired fuel cell design point (depending on
the mission requirements).
In order to do that, the approach the research team has
experienced consists in simulating the PEM fuel cell through the
use of a proprietary model, based on the co-operation of a CFD
solver (Cd-Adapco StarCCM+) and a numeric computation software
(Mathworks Matlab), able to estimate the baseline cell's
performance. Starting from this point, in order to overcome the
impossibility of carrying out a suitable bi-phase simulation in
StarCCm+ (version 5.06), a dedicated analytical model evaluates the
amount of liquid water produced by the cell, this data being a
fundamental input to be used to address the performance reduction
due to the presence of the liquid water itself. Through the use of
a mesh morphing technique applied directly on the plate's
channels, from the baseline geometry a new channels shape will be
configured: the objective is to counteract the effect of an
increment of the pressure drop along the flow path, thus not
impacting on the reactants pump workload, i.e., preserving the
overall system's efficiency by containing the auxiliary
losses.
The baseline bipolar plate configuration and the modified one
have then been tested separately and the results have been
compared, showing a valuable impact of the morphing technique on
the overall cell performance: a reduction in the overall pressure
drop has been identified, this being the main result the
researchers have been working for, as well as a better membrane
electrical conductivity (due to a satisfactory membrane
humidification).
By the way, a CAE centric approach has been adopted to give to
the user a flexible and powerful means to design an optimal
geometry, while respecting the physical constraints.