Design of Hydrogen Fuel Cell Systems for Road Vehicles - Hydrogen Fuel Cells for Road Vehicles - page 121

parasitic energy consumption. The basic material used in membrane technology of
PEMFC (Nafion) has been also proposed for membrane fabrication in humidifier
devices [ 35 ] (see Sect. 3.2 ).
In the last decade some companies have patented new solutions that combine
humidification and cooling [ 36 ]. Plug Power Inc. has designed an innovative cooler-
humidifier [ 37 ], constituted by a plate with a flow of liquid water to remove heat on
one side, and wicks that moisten the reactant gases on the other side. Evaporation of
water at the humidification side provides additional cooling.
There have also been attempts to control the water in the cell by using porous
graphite plates or by external wicking connected to the membrane to either drain
or supply water by capillary action. More reliable forms of H 2 O management are
based on continuous flow field design and appropriate operating conditions [ 38 ].
A temperature rise can be used between the inlet and outlet of the flow field to
increase the water vapor carrying capacity of the gas streams. Recent studies have
been addressed towards the development of advanced materials for improving
membrane performance. A new method involves multilayer composite polymer
electrolytes [ 39 ] where the humidifying layer contains platinum catalyst particles.
Another widely used method involves evaporation of liquid water injected into
the gas stream [ 36 ]. An atomizing nozzle could be used to inject water into the gas
flow streams of hydrogen and air. The latent heat required for H 2 O evaporation
causes gas temperature decrease, thus a heater around the injection chamber is
necessary. In addition, a long pipe equipped with heaters is useful to fully water
evaporation, avoiding condensation. In this case humidity control could be difficult
as well as the transient response would result very low, limiting the practical
application of this solution.
The water balance inside FCS requires at least a water condenser located at the
outlet of the stack. This device has the dual role to condense a significant part of
the water content of the warm and wet cathodic outlet stream (a mixture of
nitrogen, oxygen, and water vapor with the probable presence of small water liquid
droplets) for re-cycling in humidification devices, and to transfer/recover a part of
significant thermal energy content of the cathode stream. The condenser could also
collect, but in minor quantities, the excess water content of the anode compart-
ment. The condenser is then aimed at maintaining the vehicle endurance and to
cooperate to efficiency maximization.
4.6 Integrated Fuel Cell System: Efficiency,
Dynamics, Costs
Life cycle assessment studies demonstrate the potentialities of vehicles powered by
fuel cells in terms of both total emission and energy consumption reduction [ 40 - 42 ].
In particular, while the global environmental benefits of hydrogen are essen-
tially depending on the selected production technology (see Sect. 2.1 ), and the