Environmental Engineering Reference
In-Depth Information
gradually reduces with the load (see the area comprised between the two curves of
Fig.
4.11
).
The performance of the air compressor affects both stack and system efficiency
by means of the impact of flow rate level and working pressure on both cell
voltage and compressor consumption [
48
,
49
]. Low pressures and low stoichi-
ometric ratios minimize the compressor consumption but stack efficiency results
negatively affected (see also
Sect. 3.3
).
On the other hand, the choice of working at high pressure assures improvements
in individual cell performance, beside the disadvantage related to the not acceptable
energy consumption of the compressor, but it determines other advantages,
essentially related to the possibility of reducing the severe requirements for thermal
and water/humidification devices. In particular the water necessary for stream
humidification is reduced as well as the higher temperature differences minimize
the costs and the sizes of heat exchangers [
1
,
45
]. The size of power plant could
address the choice of working pressure (see
Sect. 4.3
).
4.6.2 FCS Dynamics
Another important automotive qualification for a fuel cell engine is its dynamic
response to driving requirements. Vehicle drivability should require very fast
accelerations and rapid decelerations. It is desirable that the power generator is
able to deliver maximum power (almost up to 90%) with very short time delay.
Conventional engines are able to support the dynamic drivability requirements, but
the fuel consumption on overall driving cycles results strongly affected by the
contribution of these phases characterized by several not optimized working
conditions. Hybrid solutions have been recently proposed to improve pollutant
emission levels and fuel economy, exploiting the best efficiency conditions of
internal combustion engines. Hybridization approach, that involves a significant
contribution of energy storage systems (traction batteries, supercapacitors), should
be used also in fuel cell vehicles with the aim of minimizing hydrogen con-
sumption and maximizing the performance of the overall propulsion system.
Electric energy storage systems can assist the FCS or the internal combustion
engines in supporting high power dynamic requirements of the vehicle and permit
energy recovery during braking phases. Then the hybridization approach would
permit the steady state operation of the power generator. This possibility results
really interesting for IC engines, because emissions and fuel consumption could be
strongly reduced, while these advantages need to be carefully verified in fuel cell
vehicles. Hybridization does not change significantly the concern of evaluating the
dynamic characteristics of the FCS [
45
], as fuel cells present high efficiency at part
load and system efficiency is high in a wide range of loads. Load variations could
then intercept many working points at high efficiency. Thus, the analysis of their
dynamic performance in a wide range of operative conditions appears desirable.
