Case Study B: Fuel Cell Power Train for Cars - Hydrogen Fuel Cells for Road Vehicles - page 208

Fig. 7.10 Air compressor
power consumption versus air
flow rate for different pres-
sure drop conditions
always convenient, in particular when increasing the load. Moreover, no signifi-
cant effect is observed for R [ 2.5 for all loads.
In Fig. 7.10 , the behavior of the side channel air compressor is reported in terms
of power consumption as a function of air flow rate for three different settings of
pressure drop at the outlet of the device. The air flow rate can be varied by
regulating the compressor motor speed and controlling the motor reference volt-
age. It can be noted that a value of 135 kPa of pressure drop permits the air flow
rate necessary for the stack maximum load (about 45 Nm 3 h -1 ) to be reached with
a power consumption of about 1.4 kW. This consumption decreases at partial load,
which is a condition which frequently occurs for a FCS operating on a vehicle.
Another consumption reduction can be achieved by choosing the minimum
humidification temperature to maintain a correct stack membrane functionality.
A specific experiment is performed humidifying the stack at room temperature
(287 K) and allowing the stack temperature to reach 313 K with reactant pressures
derived from the compressor characteristics shown in Fig. 7.10 . The acquisition of
stack voltage and current versus time is reported in Fig. 7.11 a, while the stack and
humidification temperatures are presented in Fig. 7.11 b. After 3,500 s, the stack
temperature reaches 313 K with a voltage of 62 V at 60 A, only 5% less than the
value measured when humidifying at the same stack temperature (Fig. 7.8 ). In this
case, the correct membrane operation is verified by monitoring the individual cell
voltage at the end of the test as shown in Fig. 7.12 . A similar regular behavior is
observed also at higher loads, in particular for a stack power up to 12 kW.
The whole characterization of the FCS in the experimental conditions of
Fig. 7.11 allows the stack and FCS electric efficiency to be calculated (see
Sect. 6.4 ) and reported in Fig. 7.13 as a function of the stack power. The FCS
efficiency is about 47% in a wide range of load conditions, with a slight decrease
after 8 kW of stack power.
For a vehicle application, the humidification system at room temperature should
be designed taking into account criteria of constructive simplicity and reliability.