Environmental Engineering Reference
In-Depth Information
In Fig.
4.6
a scheme of cooling circuit related to FCS for automotive appli-
cation is shown. The main components are the pump for liquid circulation, the
water reservoir and the heat exchanger with fan. For small size stacks (from 100 to
500 W) it is possible to use only air forced by fan to cool directly the stack, while
for higher powers, more suitable for automotive requirements (1-100 kW), it is
reasonable to use an internal coolant circuit fed by a liquid, such as de-ionised
water or ethylene glycol-water mixtures, to improve the heat removing capacity of
an order of magnitude with respect to the gas.
The stack cooling is not the unique aspect to be examined in the context of FCS
thermal management, but also the possibility of heat recovery has to be also
considered. At this regard, additional devices have to be used, such as air-to-air
heat exchangers, humidifiers, or condensers that partially exploit the enthalpy
content of cathode outlet and coolant streams [
23
]. In particular enthalpy wheels,
which are rotary air-to-air heat exchanger, can be used to preheat the inlet
exploiting the enthalpy content of the wet and warm cathode outlet stream, which
is a gaseous mixture at the approximately the same temperature of the stack in
saturated conditions. They transfer mainly heat but allow also moisture to be
exchanged. The device is quite compact and can achieve high energy transfer
effectiveness. It is constituted by a cylinder filled with a permeable material with
high surface area (polymer, aluminum, or synthetic fiber), necessary for the sen-
sible heat transfer. The driving force for realizing the exchange is the thermal
gradient between the opposing air streams.
The enthalpy exchange is accomplished through the use of typical adsorbent
materials such as silica gel, zeolites or other molecular sieves which transfer water
molecules by the difference in the partial pressure of vapor within the opposing air
streams. The operation of a FCS that includes this component requires specific
Temperature
sensor
Temperature
sensor
Fuel Cell
Pump
Water
Reservoir
Heat
exchanger
Fan
Fig. 4.6
Water cooling sub-system for H
2
FCS
