SYSTEM WATER BALANCING
TECHNICAL FIELD
[0001] This disclosure relates generally to water balancing and, more
particularly, to maintaining water balance within a fuel cell system.
DESCRIPTION OF THE RELATED ART
[0002] Fuel cell systems are well known. One example fuel cell system
includes multiple individual fuel cells arranged in a stack. Each individual fuel
cell has an anode and a cathode positioned on either side of a proton exchange
membrane. A fuel, such as hydrogen, is supplied to the anode side of the proton
exchange membrane. An oxidant, such as air, is supplied to the cathode side of
the proton exchange membrane. The individual fuel cells make water during
operation.
[0003] Some fuel cell systems move liquid water through the fuel cell
assembly to remove thermal energy and hydrate the fuel cells. The supply of
liquid water may be limited, particularly in portable fuel cell systems. The fuel
cells may overheat, or fail due to dryout, if they receive inadequate amounts of
liquid water or exhaust an excess of water vapor. Balancing water within the fuel
cell system avoids overheating and dryout, and helps the fuel cell system operate
efficiently. Systems other than fuel cell systems may require water balancing.
SUMMARY
[0004] An example system water balancing method includes exhausting
water vapor from a system and varying the exhausting in a response to an
amount of water available for use by the system.
[0005] An exemplary fuel cell water balancing method includes
detecting an amount of water available for use by a fuel cell assembly and
limiting water vapor exhausted from the fuel cell in response to the detecting.
[0006] An exemplary fuel cell assembly includes a fuel cell and a
controller. The fuel cell receives water from a supply. The controller selectively
varies an amount of water vapor communicated from the fuel cell in response to
an amount of water within the supply.
DESCRIPTION OF THE FIGURES
[0007] The various features and advantages of the disclosed examples
will become apparent to those skilled in the art from the detailed description.
The figures that accompany the detailed description can be briefly described as
follows:
[0008] Figure 1 shows a highly schematic view of an example fuel cell
system.
[0009] Figure 2 shows a more detailed view of another example fuel cell
system.
[0010] Figure 3 shows a general method of maintaining water balance
within a fuel cell of the Figure 2 system.
[0011] Figure 4 shows a more detailed method of maintaining water
balance within a fuel cell of the Figure 2 system.
DETAILED DESCRIPTION
[0012] Referring to Figure 1, an example fuel cell system 10 includes a
fuel cell 12 and a supply 14 of water. The fuel cell 12 receives water from the
supply 14. The water is communicated to the fuel cell 12 along a path 16. The
water moves through the fuel cell 12 to hydrate and remove thermal energy.
After moving through the fuel cell 12, at least some of the water is exhausted
from the fuel cell 12 as water vapor at an exhaust 18. The water vapor moving
through the exhaust 18 moves to ambient to exit the fuel cell system 10. The
remaining water moves back to the supply 14 along a path 20 as liquid water. In
some examples, water vapor that has not exited the fuel cell system 10 through
the exhaust 18 may be condensed and added to the supply 14.
[0013] In this example, a controller 22 varies the amount of water vapor
exhausted from the fuel cell system 44 through the exhaust 18. The example
controller 22 alters the water vapor exiting the fuel cell 12 based on the
availability of water within the supply 14. The controller 22 may adjust the
pressure of air entering the fuel cell 12, or the airflow rate, to alter the amount of
water vapor exiting the fuel cell 12 through the exhaust 18.
[0014] Adjusting the pressure of air entering the fuel cell 12, such as by
increasing the pressure, is an example of how the controller 22 may vary the
amount of water vapor exiting the fuel cell 12 though the exhaust 18. The air is
considered a reactant in this example because the air contains oxygen.
[0015] In another example, the controller 22 varies the exhausting of
water by adjusting the pressure of another reactant entering the fuel cell 12, such
as by increasing the pressure of a reformate entering the fuel cell 12. The
reformate contains hydrogen.
[0016] Referring now to Figure 2 with continuing reference to Figure 1,
another example fuel cell system 40 includes a fuel cell 44 having an anode 48
and a cathode 52. A proton exchange membrane 56 separates the anode 48 from
the cathode 52. The fuel cell 44 is one of several fuel cells within a fuel cell stack.
[0017] A fuel source 60 supplies a fuel, such as hydrogen, to the anode
48 of the fuel cell 44. Some of the fuel is exhausted from the fuel cell 44 at a fuel
exhaust 64. Some water vapor may be exhausted with the fuel through the fuel
exhaust 64.
[0018] A portion of the exhausted fuel may be recycled back into the
anode 48. Recycling fuel helps by improving fuel efficiency. Recycling some of
the fuel also may help maintain water balance because less water vapor is lost
out the fuel exhaust than if the fuel were not recycled.
[0019] An oxidant supply 68 supplies an oxidant, such as air, to the
cathode 52 of the fuel cell 44. Some of the air is exhausted from the fuel cell 44 at
an air exhaust 72.
[0020] Hydrogen-air PEM fuel cell systems, such as the system 40 shown
in Figure 2, produce water as a byproduct. Some of the water is exhausted from
the fuel cell 44 as water vapor, which is carried by the air exhausted from the fuel
cell through the air exhaust 72. Chemical reactions within the fuel cell 44 produce
the water vapor carried by the exhausted air.
[0021] The chemical reactions within the fuel cell 44 produce liquid
water in addition to the water vapor. In this example, the liquid water is moved
to an accumulator reservoir 76 along a path 80. Liquid water moving along path
80 may pass through a liquid-liquid heat exchanger that transfers heat to the
vehicle radiator fluid.
[0022] Air also may move along the path 80. This air may pass through
a condenser & separator, which condenses water vapor from the air. The
condensed water is then added to the accumulator reservoir 76. The rest of the
air (which still includes some water vapor) is then exhausted to ambient. If there
is only a liquid-liquid heat exchanger (and no condenser), then all the water
vapor which leaves cathode 52 is exhausted.
[0023] The accumulator reservoir 76 provides an external source of
water for the fuel cell 44. The liquid water then communicates back to the fuel
cell 44 along the path 82 as required. A pump (not shown) is used to move the
liquid water along the paths 80 and 82.
[0024] Some of the liquid water within the accumulator reservoir 76 may
thus be water that was produced by the fuel cell 44. Water from the accumulator
reservoir 76 may be used to cool the fuel cell "sensibly," by absorbing heat and
increasing in temperature as it traverses the fuel cell. Alternatively, or in
addition, the coolant water may cool the fuel cell "evaporatively," by
evaporating into the air (or other reactant gas) stream. The liquid water moving
back to the accumulator reservoir 76 consists of that liquid water that was
provided in excess of the evaporative demands, and that which is condensed
from the air moving along path 80.
[0025] The amount of liquid water communicated from the fuel cell 44
along the path 80 is thus reused by the fuel cell 44 and does not exit the fuel cell
system 40. By contrast, most of the water vapor moved from the fuel cell 44
through the air exhaust 72 exits the fuel cell system 40. The example fuel cell
system 40 is a portable system (such as a system for a vehicle) and does not have
access to an unlimited supply of water.
[0026] If the total amount of water vapor exhausted from the system 40
equals the water produced by the fuel cell 44, the system 40 can be said to be
operating in water balance. If the total amount of water vapor exhausted from
the system 40 is higher than the water produced by the fuel cell 44, the system 40
can be said to be operating in negative water balance. If the total amount of water
vapor exhausted from the system 40 is less than the water produced by the fuel
cell 44, the system 40 can be said to be operating in water excess.
[0027] In the example system 40, a controller 84 is operably connected to
sensors 88a and 88b that are secured to the accumulator reservoir 76. The first
sensor 88a is used to determine whether the level of water within the
accumulator reservoir 76 is higher than a level Li. The second sensor 88b is used
to determine whether the level of water within the accumulator reservoir 76
exceeds a level L2.
[0028] The level Li is higher than the level L2 in this example. As can be
appreciated, the amount of water within the accumulator reservoir 76 is greater
when the level of water is at the level Li than when the level of water is at the
level L2.
[0029] The sensors 88a and 88b detect the presence of water at a
particular height of the accumulator reservoir 76 to determine the amount of
water available for use by the fuel cell 44. Other examples may include other
techniques for determining the availability of water for use by the fuel cell 44.
[0030] The example controller 84 makes adjustments to the air
communicated to the fuel cell 44 in response to information provided by the
sensors 88a and 88b. In this example, the controller 84 makes the adjustments to
the air from the oxidant supply 68 in order to increase or decrease the amount of
water vapor exiting the fuel cell system 40 through the air exhaust 72.
[0031] In this example, the controller 84 actuates a valve 90, or another
device, to adjust the pressure of air entering the fuel cell 44, which changes the
amount of water vapor exiting the fuel cell system 40 through the exhaust 72. In
another example, the controller 84 actuates the valve 90 to adjust the flow rate of
air entering the fuel cell 44, which changes the amount of water vapor exiting the
fuel cell system 40 through the exhaust 72. Other examples utilize other
techniques for altering the amount of water vapor exiting the fuel cell system 40,
such as adjusting a compressor that supplies air to the fuel cell 44.
[0032] Many computing devices could be used to implement various
functions of the controller 84. In one example, the controller includes a
microprocessor that executes a program stored in a memory portion of the
controller.
[0033] Referring to Figure 3 with continuing reference to Figure 2, an
example method 100 utilized by the controller 84 for balancing water within the
system 40 includes exhausting water vapor from the fuel cell system 40 at a step
110, and then varying the exhausting based on an amount of water available for
use by the fuel cell 44 at a step 120.
[0034] The step 110 utilizes the information from the sensors 88a and 88b
to determine the amount of water available for use by the fuel cell 44. Although
the amount of water available for use in this example is shown as being entirely
contained within the accumulator reservoir 76, a person having skill in the art
and the benefit of this disclosure would understand that the amount of water
may extend to other areas, and could be monitored by suitable sensory (or other)
devices.
[0035] Figure 4 shows a more detailed method 200 of control utilized by
the controller 84 within the system 40. At a step 210, the controller 84 determines
whether the amount of water available for use by the fuel cell 44 is greater than
an amount Xi. In this example, the amount Xi corresponds to the water within the
accumulator reservoir 76 exceeding the level Li. At the step 210, the controller 84
also determines whether the pressure of the air being supplied to the fuel cell 44
is greater than a minimum possible pressure. Providing unnecessary pressure is
inefficient as is known.
[0036] If the controller 84 determines that the water is greater than Xi
and the pressure is greater than minimum potential pressure P m the controller
84 decreases the pressure of the air being supplied to the fuel cell 44 at a step 220.
[0037] If the available water is not greater than Xi and/or the pressure of
the supplied air is not greater than a minimum potential pressure Pmin , the
controller 84 moves to the step 230. At this step, the controller 84 determines if
the available water is less than Xi and if the pressure of the air supplied to the
fuel cell 44 is less than the maximum potential pressure Pmax. If so, the controller
84 moves to a step 240 where the controller 84 increases the pressure of air
supplied to the fuel cell 44.
[0038] If the answer to step 230 is no, the controller 84 next determines at
a step 250 if the available water is greater than X2. In this example, X2
corresponds to the level L2shown in Figure 2. The level L2 is less than the level L i
and indicates that there is less water available for use by the fuel cell 44 than if
the water were at the level Li.
[0039] More specifically, in this example, the level L i represents the
accumulator reservoir 76 being filled to about 75% of its total potential capacity.
The level L2 represents the accumulator reservoir 76 being filled to 25% of its
capacity.
[0040] If at the step 250 the available water is greater than X2, the method
200 and the controller 84 maintain the pressure of air supplied to the fuel cell 44
at a step 260. If the available water is not greater than X2 at the step 250, the
method 200 and the controller 84 may limit the power drawn from the fuel cell 44
at a step 270.
[0041] In one example, limiting the power drawn from the fuel cell at the
step 270 involves reducing an existing limit on the potential power drawn from
the fuel cell 44. For example, if the fuel cell 44 is used to power a vehicle, an
existing limit on the power drawn from the fuel cell 44 may be 80 kilowatts. If the
answer to the step 250 is that the available water is less than X2, the controller 84,
at the method step 270, reduces the existing limit to a lower level, say 60
kilowatts.
[0042] The controller 84 may define a lower limit as well, say 40
kilowatts, to ensure that there is enough water being produced by fuel cell 44 to
replenish the accumulator reservoir 76 via path 80.
[0043] The preceding description is exemplary rather than limiting in
nature. Variations and modifications to the disclosed examples may become
apparent to those skilled in the art that do not necessarily depart from the
essence of this disclosure. Thus, the scope of legal protection given to this
disclosure can only be determined by studying the following claims.

CLAIMS
We claim:
1. A water balancing method, comprising:
exhausting water vapor from a system; and
varying the exhausting in response to an amount of water available for
use by the system.
2. The system water balancing method of claim 1, wherein the varying comprises
increasing a pressure of a reactant entering the system.
3. The system water balancing method of claim 1, wherein the varying comprises
decreasing a flow rate of a reactant entering the system.
4. The system water balancing method of claim 1, wherein the amount of water
comprises a level of water within an accumulator reservoir.
5. The system water balancing method of claim 1, including limiting power
drawn from the system in response to an amount of water available for use by
the system.
6. The system water balancing method of claim 5, including limiting power
drawn by applying a maximum power draw and a minimum power draw limit.
7. The system water balancing method of claim \ , wherein the system is a fuel
cell system.
8. A fuel cell water balancing method, comprising:
detecting an amount of water available for use by a fuel cell; and
limiting water vapor exhausted from the fuel cell system in response to
the detecting.
9. The fuel cell water balancing method of claim 8, including limiting power
drawn from the fuel cell.
10. The fuel cell water balancing method of claim 9, including limiting by
applying a maximum power draw and a minimum power draw limit.
11. The fuel cell water balancing method of claim 8, including limiting water
vapor exhausted from the fuel cell if the amount of water is less than a first
reference amount of water, and limiting power drawn from the fuel cell if the
amount of water is less than a second reference amount of water that is less than
the first reference amount of water.
12. The fuel cell water balancing method of claim 11, wherein the first reference
amount of water is about 75% of an accumulator capacity, and the second
reference amount of water is about 25% of the accumulator capacity.
13. A fuel cell assembly, comprising:
a fuel cell that receives water from a supply; and
a controller that selectively varies an amount of water vapor
communicated from the fuel cell in response to an amount of water within the
supply.
14. The fuel cell assembly of claim 13, wherein the supply comprises an
accumulator reservoir.
15. The fuel cell assembly of claim 14, wherein the fuel cell produces liquid water
that is communicated to the supply.
16. The fuel cell assembly of claim 13, wherein the water vapor is exhausted from
the fuel cell to ambient.
17. The fuel cell assembly of claim 13, including a conduit that communicates
liquid water from the supply to the fuel cell.
18. The fuel cell assembly of claim 17, wherein the controller initiates actuation of
a device to change a pressure of a reactant entering the fuel cell.
19. The fuel cell assembly of claim 18, wherein the device is a valve, a
compressor, or both.
20. The fuel cell assembly of claim 17, wherein the controller initiates actuation of
a device to change a flow rate of a reactant entering the fuel cell.