Power Generation - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 188

of the fuel cell, the gas is heated and by the well-known chimney effect, the
gas flows upward through the fuel cell without the use of a pump or a com-
pressor and then falls with gravity as it cools and becomes denser.
Since no power is consumed for moving the fuel or the oxidizing
agent through the cells, the efficiency of the fuel cell is increased. Operation
without pressurization eliminates the moving parts making the fuel cell less
expensive and more reliable. Pressurization can also be added for increased
performance where the whole assembly is placed under pressure.
The fuel and oxidizing-agent circuits include upper- and lower-
reversal points. At the upper-reversal points, fuel and oxidizing agents
flowing counter to gravity forces begin to flow in the direction of gravity.
The opposite takes place at the lower-reversal point.
The heat- and gravity-driven circulation is due to the arrangement
of the fuel cells between the upper- and the lower-reversal points. This ar-
rangement allows the fuel and oxidizing agent to pass through the fuel
cell after passing through the lower-reversal point and before passing
through the upper-reversal point. The fuel or oxidizing agent are heated
while passing through the fuel cell so the gravity-driven flow of the fuel
or the oxidizing agent through the fuel cell is sustained.
The gravity-driven flow through the fuel cell is further supported by
the arrangement of cooling fins or a cooling coil for removing the heat be-
tween the lower- and upper-reversal points. In the arrangement, the fuel
or oxidizing agent passes the fins or coil, removing heat after passing the
upper-reversal point and before passing the lower-reversal point.
The coil or zigzag fin shape provides a relatively large heat exchange
surface area. This permits a larger amount of heat to be transferred to the
cooler environment as compared to a return duct running in a straight line
between the upper- and the lower-reversal points.
The fuel cells and heat sinks occupy more than 80% of the straight
line distance between the lower- and the upper-reversal points. Then the
gravity forces are particularly effective for the transport of the fuel or the
oxidizing agent.
In the area of the upper-reversal point, the heated fuel and oxidizing
agent are cooled. As a result, the fuel's and oxidizing agent's density and
weight are increased after passing the upper-reversal point so that they
flow downward. In this way, a gravity-driven flow of fuel and oxidizing
agent through the fuel cell is generated. The gravity-driven flow through
the fuel cells with circulating system is particularly suitable for fuels such
as liquid methanol.