Controlling oxygen concentrations in fuel cell cooling water loops

In order to achieve good stack water chemistry and minimize corrosion in a fuel cell stack water cooling loop, the oxygen concentration in the water must remain within a relatively narrow range. Stacks using steam separators produce water in the separators which is relatively devoid of oxygen. Makeup water is used to replace the steam lost from the separator, which makeup water is relatively rich in oxygen. The flow rates of the makeup water and steam separator water are controlled to produce a proper oxygen concentration in the recirculated coolant water. Some of the makeup water will be added directly into the steam separator so that it will be stripped of oxygen and the rest will be added subsequent to the steam separator.

TECHNICAL FIELD 
This invention relates to water cooled fuel cell power plants, and more 
specifically to the control of the oxygen concentration in the water used 
to cool such plants. 
BACKGROUND ART 
The use of water circulated through a cooling loop to cool fuel cell power 
plants is known in the prior art. When water is used to cool fuel cell 
power plants, such as those using acid electrolytes, the coolant entering 
the power section will be water which will be heated to a two phase 
water-steam mixture by the time it leaves the power section. The two phase 
mixture will then be taken to a steam separator where the steam component 
will be removed from the water component. When this separation occurs, the 
water will be stripped of entrained oxygen so that the water leaving the 
steam separator will generally contain from about 0 to about 50 ppb of 
oxygen. In most of these systems, makeup water is added to the loop in 
order to replace the water which is lost in the form of steam from the 
steam separator. U.S. Pat. No. 3,969,145 granted July 13, 1976 to P. E. 
Grevstad, et al discloses a cooling system which operates in the aforesaid 
manner. The foregoing system does make provisions for replacing water in 
the loop lost as steam, but it does not ensure that the amount of oxygen 
in the recirculating water will be within the desired range needed to 
provide good water chemistry and minimize corrosion in the system. 
In order to achieve the desired fuel cell power plant water chemistry and 
minimize corrosion, the oxygen concentration in the water entering the 
power section should be in the range of about 50 to 150 ppb. Since the 
water leaving the steam separator is substantially devoid of oxygen, the 
makeup water is the only source of oxygen for the coolant prior to 
entering the power section. The amount of oxygen in the makeup water will 
be approximately 7,000 ppb. The rate at which the makeup water can be 
added to the loop cannot be varied however, since it must be enough to 
replace the water lost as steam from the loop. With the prior art system 
shown in U.S. Pat. No. 3,969,145 it will be apparent that the amount of 
oxygen in the water coolant as it reenters the power section is not 
controllable since the amount of makeup water needed is dictated by the 
amount of steam lost, and is thus not variable. 
DISCLOSURE OF INVENTION 
Our invention relates to a system which is similar to that shown in the 
prior art, but wherein the amount of oxygen in the coolant water 
reentering the power section can be controlled and can be kept in the 
desired range of about 50 to about 150 ppb. In one embodiment of our 
invention, the coolant flow rate in the coolant loop is selected by design 
so as to ensure that the water entering the power section after receiving 
the makeup water, has an oxygen concentration in the range of about 50 to 
about 150 ppb. 
The coolant flow rate can be selected within a broad range while still 
maintaining sufficient thermal cooling of the power section due to the two 
phase boiling water characteristics. In this embodiment, the flow rate and 
oxygen content of the makeup water are periodically monitored, as well as 
the coolant loop flow rate, and its oxygen content after addition of 
makeup water, and appropriate adjustments are made in the coolant flow 
rate to keep an oxygen concentration in the desired range when variations 
are noted. 
In another embodiment of our invention, the makeup water entering the 
system is split into two branches. In the first branch, some of the makeup 
water is fed into the steam separator where it will be stripped of oxygen 
and settle into the deoxygenated water already in the steam separator. The 
rest of the makeup water is fed through the other branch into the return 
loop water line downstream of the steam separator. The amount of oxygen in 
the returning coolant water is thus controlled by the percentage of makeup 
water fed through each of the two branches. If it is desired to lower the 
amount of oxygen in the returning coolant, then more makeup water is fed 
into the steam separator, and if the opposite is true, less makeup water 
is fed into the steam separator. Thus the coolant loop will only receive a 
fraction of the total oxygen in the makeup water and that fraction can be 
readily varied. The latter embodiment is the preferred, of the two, 
embodiments of the invention. 
It is, therefore, an object of this invention to provide a water cooling 
system for use in a fuel cell power plant having provisions for ensuring 
improved water chemistry for the water coolant. 
It is an additional object of this invention to provide a water cooling 
system of the character described wherein corrosion in the water coolant 
is minimized. 
It is another object of this invention to provide a water cooling system of 
the character described which operates by controlling the concentration of 
oxygen in the coolant water entering the power section of the plant. 
It is a further object of this invention to provide a water cooling system 
of the character described wherein the oxygen concentration is controlled 
by the use of makeup water which replaces water lost from the coolant in 
the form of steam. 
These and other objects and advantages will become more readily apparent 
from the following detailed description of a preferred embodiment thereof 
when taken in conjunction with the accompanying drawing.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring to the drawing the power section of the plant is denoted 
generally by the numerals 2 and 4. The power section 2 and 4 includes top 
and bottom end plates 6 and 8 for tapping current from the plant, and a 
plurality of fuel cells 10 stacked one atop the other. The fuel cells 10 
are the type that utilize an acid electrolyte, such as phosphoric acid, 
and operate at temperatures of about 375.degree. F. typically. The cell 
stacks are provided with coolant tube assemblies, denoted generally by the 
numeral 12 which can comprise serpentine tubes 14, through which the water 
coolant flows, the tubes 14 being mounted in carbon plates. Each of the 
tubes 14 is connected to a coolant water inlet manifold 16 which receives 
water from a return conduit 18. Feeder pipes 20 extend from the inlet 
manifold 16 to each of the tubes 14, and are connected to the latter by 
dielectric sleeves 22. The water-steam mixture which leaves the coolant 
tubes -4 is collected in an outlet manifold 24 via pipes 26 which are 
connected to the tubes 14 by dielectric sleeves 28. From the outlet 
manifold 24, the water-steam mixture passes into a steam separator 30 
where the water and steam fractions are separated from each other. The 
steam leaves the separator 30 through a line 32 for subsequent use in a 
fuel reformer. The water fraction leaves the separator through a down line 
34 which leads to a pump 36 which pumps the water into the return conduit 
18. As previously noted, the water exiting the separator 30 in the down 
line 34 is substantially devoid of oxygen. In order to replace the water 
lost as steam through the line 32, a makeup water feed system is included. 
The makeup water, which is rich in oxygen, is fed into the system from a 
trunk line 38 having a variable orifice valve 40, which can be modulated 
to increase or decrease the total amount of makeup water being added to 
the system, based on changes in the water level in separator 30 from the 
amounts of steam leaving the system. The power plant microprocessor 
control will control operation of the valve 40. An upper branch line 42 
and a lower branch line 44 receive makeup water from the trunk line 38. 
The upper branch line 42 feeds makeup water directly into the steam 
separator 30, and the lower branch line 44 feeds makeup water into the 
down line 34, downstream from the separator 30. Valves 46 and 48 control 
the amount of water flowing through the branch lines 42 and 44 
respectively. The valves 46 and 48 are periodically adjusted to achieve 
the desired O.sub.2 level in conduit 18. 
The disclosed system can operate in one of two ways. As previously noted, 
the objective of the system is to provide a water coolant in the return 
conduit 18 which has an oxygen content of between 50 and 150 ppb. The 
oxygen content of the water in the conduit 18 is monitored periodically. 
When a low oxygen condition is noted, the oxygen level in the returning 
coolant water can be raised in a number of ways. The pump output flow can 
be varied by valve 49. Thus the pump 36 can increase or decrease the rate 
of flow of the coolant water through the loop. If a low oxygen condition 
is present, the pump 36 output flow can be made to slow the flow rate of 
coolant through the loop. If this is done and the flow rate of makeup 
water through the lower branch 44 remains constant, then a greater 
percentage of the water in the return conduit 18 will be derived from the 
makeup water. Since the makeup water has a rich oxygen concentration, the 
oxygen content of the water in the return conduit 18 will rise. Using this 
approach, the valve 46 would preferably be throttled back or closed 
completely. Alternatively, if the oxygen content in the conduit 18 is too 
high the pump 36 output flow can be increased, so that the deoxygenated 
water in the down line will contribute more deoxygenated water to the 
conduit 18. This will lower the oxygen content in the return conduit 18. 
Thus the oxygen content of the water returning to the power section 2 and 
4 can be controlled merely by increasing or decreasing the flow rate of 
water being recirculated while leaving the flow rate of the makeup water 
unchanged. 
If the coolant flow rate inside of the coolant loop is desired to remain 
constant, the disclosed system can employ an alternative mode of operation 
to regulate the oxygen content in the coolant water. Altering the 
proportions of the makeup water which flows through each of the branch 
lines 42 and 44 will vary the oxygen content of the recirculating coolant 
water This is because the makeup water which enters the separator 30 from 
the line 42 is stripped of its oxygen by the steam in the separator 30, so 
that the only oxygen added to the recirculating coolant water will derive 
from the water flowing through the branch 44 Thus, if one desires to 
increase the oxygen content in the conduit 18, while leaving the coolant 
flow rate constant, the valve 46 will be throttled back to lessen the 
amount of makeup water entering the separator 30, and the valve 48 will be 
opened to increase the percentage of makeup water flowing into the down 
line 34 through branch 44. The total amount of makeup water entering the 
loop will not change, but more of it will be of the oxygen-rich variety, 
thereby raising the oxygen concentration in the conduit 18. The reverse 
procedure will be followed if one desire to lower the oxygen content in 
the coolant water conduit 18 without altering the coolant flow rate in the 
system. 
It will be readily appreciated that the system of this invention can be 
used to retain the oxygen concentration level of the recirculating coolant 
water in a desired range of values so that the water chemistry of the 
system is optimized, and corrosion within the system is kept at a minimum. 
The system operates with a defined makeup water flow rate range, so that 
water lost to the cooling loop as steam is constantly replenished. The 
system can operate by varying the flow rate of the coolant water in the 
cooling loop, or by varying the apportionment of deoxygenated and 
oxygen-rich makeup water which is fed into the loop. Both approaches could 
also be used concurrently should conditions warrant. 
Since many changes and variations of the disclosed embodiment of the 
invention may be made without departing from the inventive concept, it is 
not intended to limit the invention otherwise than as