FUEL CELL ASSEMBLY AND METHOD OF CONTROL
BACKGROUND
[0001] Typical fuel cell arrangements include multiple fuel cells placed together
in a cell stack assembly (CSA). A cathode reactant gas, such as air, and an anode reactant
gas, such as hydrogen, are used in an electro-chemical reaction to produce electrical energy.
Humidified membranes may separate the anode reactant from the cathode reactant, and
conduct ionic current between anode and cathode. A controller monitors operation
parameters of the CSA and controls the flow of the anode and cathode reactant gases and the
electrical current or voltage to produce a desired CSA power output level.
[0002] There are times when the desired power output from the CSA varies. This
can be in response to a change in load or power demand. It may also be a result of a change in
fuel cell operation such as a transition from startup to normal operation.
[0003] CSA durability can be limited by decay mechanisms associated with cyclic
operation. For example, voltage cycling may cause performance decay over time. Local
membrane humidity cycling may cause the membrane to wear out. Both of these types of
cycling may occur in response to changes in load or power demand. While such cycling may
result in only modest decay or wearout rates at lower temperatures, the negative effects
associated with such cycling is exacerbated by high temperature operation. Therefore, it is
desirable to limit the time spent at higher temperatures and the amount of cycling during
high temperature excursions. One approach to limiting negative effects from voltage cycling
is to use voltage clipping. Voltage cycling may be considered benign below a certain
voltage, to which the CSA is clipped. For example, at nominal operating temperatures, it
may be acceptable to clip the voltage to a specified value but at higher operating temperatures
the voltage clip may not be acceptable. Therefore, voltage clipping is not a complete
solution.
SUMMARY
[0004] An exemplary method includes operating a fuel cell at a first power output
level that includes a plurality of operation parameters. Each operation parameter has a value
to satisfy a first power demand. A change between the first power demand and a second
power demand is determined. At least a first one of the operation parameters is maintained at
a value corresponding to the first power output level while at least a second one of the
operation parameters is changed to a value corresponding to a second power output level to
satisfy the second power demand. The first operation parameter is delayed from changing to a
value corresponding to the second power output level until a predetermined criterion is met.
[0005] An exemplary fuel cell assembly includes a cell stack assembly and a
controller configured to operate the cell stack assembly at a first power output level that
includes a plurality of operation parameters. Each operation parameter has a value to satisfy a
first power demand. The controller determines a change between the first power demand and
a second power demand. The controller maintains at least a first one of the operation
parameters at a value corresponding to the first power output level and changes at least a
second one of the operation parameters to a value corresponding to a second power output
level to satisfy the second power demand. The controller delays changing first operation
parameter to a value corresponding to the second power output level until a predetermined
criterion is met.
[0006] These and other features of the disclosed examples can be understood from
the following description and the accompanying drawings, which can be briefly described as
follows.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] Figure 1 schematically illustrates an example fuel cell assembly.
[0008] Figure 2 schematically illustrates another example fuel cell assembly.
[0009] Figure 3 is a flowchart that illustrates an example method of operating a
fuel cell.
DETAILED DESCRIPTION
[0010] Figure 1 schematically illustrates an example fuel cell assembly 10
including a cell stack assembly (CSA) 12 having a plurality of fuel cells. In this example
embodiment, a fuel source 14 provides a reactant, such as hydrogen, which is directed to a
fuel inlet 16 of the CSA 12 by a supply device 18 such as a fuel pump. The hydrogen (or
another reactant) passes through the CSA 12 in a known manner to facilitate power
generation. Exhaust fuel including hydrogen exits the CSA 12 through a fuel outlet 20.
[0011] An air source 22 provides air to an air inlet 24 driven by a supply device
26, such as a blower or compressor. The air (or another gas) passes through the CSA 12 in a
known manner to facilitate power generation. Exhaust air exits the CSA 12 through an air
outlet 28 and is directed to an evaporative cooling system 30. The evaporative cooling system
30 operates in a known manner and condenses water from the exhaust air, which is directed
to the CSA 12 through a water inlet 32.
[0012] A controller 34 controls the fuel cell assembly 10 through communicating
with the supply devices 18 and 26, the evaporative cooling system 30, the cell stack assembly
12, and a temperature sensor 36. Although the temperature sensor 36 is shown adjacent the
CSA 12 in the illustrated example, the temperature sensor 36 could also be located remotely
from the CSA 12.
[0013] Power generated by the CSA 12 is selectively directed by the controller 34
to a load 38 or a power sink 40, such as a battery, a capacitor, or a resistor.
[0014] Figure 2 schematically illustrates another example fuel cell assembly 10.
In this example embodiment, water exits the CSA 12 through a water outlet 29 and is directed
to a sensible cooling system 130 that operates in a known manner. The water returns to the
CSA 12 though the water inlet 32 to cool the CSA 12 in a known manner.
[0015] Figure 3 is a flowchart diagram 200 that illustrates an example method of
operating a fuel cell. The example method includes operating the fuel cell at a first power
output level (step 202) that includes a plurality of operation parameters. Each operation
parameter has a value that corresponds to the first power output level, which satisfies a
current, first power demand. In this example, the operation parameters include at least one of
CSA voltage, reactant flow rate, reactant pressure, and reactant humidity.
[0016] The controller 34 determines when a second power demand occurs such
that there is a change between the first power demand and the second power demand (step
204).
[0017] At least a first one of the operation parameters is maintained at a value
corresponding to the first power output level while at least a second one of the operation
parameters is changed to a value corresponding to the second power output level (step 206).
Rather than instantly changing all of the operation parameters to satisfy the second power
demand, this example includes selectively delaying changing at least one of the operation
parameters. This approach prevents cycling of CSA voltage, reactant flowrate, reactant
pressure, and reactant humidity during high temperature excursions, which can reduce the
lifespan of the fuel cell. This is particularly useful in situations where the power demand
fluctuates repeatedly. If all operation parameters change immediately in response to a change
in power demand, the cycling associated with those changes can have negative effects.
However, if the CSA temperature is allowed to relax before detrimental cycling of the
voltage or local membrane humidity occurs, the expected durability of the CSA can be
extended.
[0018] One exemplary implementation occurs when the first power demand is
greater than the second power demand. In certain instances of sustained operation at high
power, the CSA makes an excursion to temperatures higher than normal due to the increased
rate of heat production associated with high power operation. Additionally, during sustained
high power operation, the temperature may be further increased by a required increase in
reactant pressure, since the pressure required for sustained operation at high power may be
higher than normal. In this case, as soon as power demand decreases, the reactant pressure
should be decreased accordingly. This allows the CSA to begin cooling, or to cool more
rapidly than if a higher pressure were maintained. However, although the pressure is
immediately cut to a level corresponding to the second power demand, other parameters such
as CSA voltage and reactant flow rate are either maintained at values corresponding to the
first power demand, or changed to values corresponding to an intermediate between the first
and second power demands.
[0019] In particular, while the CSA temperature is still higher than nominal, the
voltage is clipped lower than nominal. In one example, the clip voltage is specified as a
function of temperature. For example, if a cell voltage of 0.6V corresponds to the first power
demand, and a cell voltage of 0.88V corresponds to the second power demand, the voltage
may be clipped to a value lower than 0.88V, depending on the CSA temperature, until that
temperature reaches a nominal temperature and the cell voltage is allowed to float to 0.88V.
The clip voltage serves to avoid voltage cycling above a certain voltage, and clipping to a
lower voltage when operating at higher temperature serves to mitigate even more cycling.
[0020] In one example, the clip voltage is the maximum voltage at which the CSA
is allowed to operate. The clip voltage limits the minimum power draw and diverts excess
power to a power sink whenever the power demand is below the minimum power draw. The
clip voltage in one example decreases by 3mV/cell for every 1°C increase in temperature.
[0021] Additionally, while the CSA temperature is higher than nominal, the
reactant flow rate may be set to an intermediate level between those corresponding to the first
and second power demands, in an effort to maintain a steady balance between local
evaporation and local water production, to avoid local membrane humidity cycling until the
CSA temperature reaches its nominal range where such cycling is acceptable.
[0022] The membrane humidity may not be spatially uniform, but may vary at
different points in the cell. The membrane humidity at a given location, or "local" membrane
humidity, may also change with time as it responds to changes in the local water evaporation
rate and water production rate. The local evaporation rate depends on the reactant flow rates,
reactant humidity, and reactant pressures, while the local water production rate depends on
the local ionic current density, which is related to the CSA current, voltage, and power.
While local membrane humidity cycling may be acceptable at nominal temperatures, it may
be mitigated during high temperature excursions by, for example, tuning the reactant flow
rate and CSA current to maintain a steady balance between local evaporation and local water
production.
[0023] In one example, the intermediate values for the reactant flow rates are
chosen to maintain a steady balance between local evaporation (which depends on reactant
flow rate, humidity, and pressure) and local water production (which depends on local current
density, and in turn, on CSA voltage). Maintaining such a balance avoids local membrane
humidity cycling while the CSA is still at a relatively high temperature. The controller 34
may implement a simple or complicated algorithm to determine the reactant flow rates
required to maintain this balance.
[0024] In this particular example, the operation parameters that are not
immediately changed or that are set to intermediate values include reactant flow and cathode
potential. The operation parameters that are changed immediately (e.g., the "second"
parameters), to correspond with the second power demand, include reactant pressure.
Referring to Figures 1 and 2, the supply device 26 and fuel valve 18 reduce the reactant
pressures to match the second power demand, while reactant flow rates, humidity, and CSA
voltage remain unchanged or are temporarily set to intermediate values. Since the fuel cell is
producing more power than is necessary to meet the second power demand, the excess power
is directed to a power sink until the power output level matches the second power demand.
The reduction in reactant pressure and, if applicable, the intermediate settings of the other
operating parameters reduce the fuel cell operating temperature to a range where changes in
reactant flow, humidity, and CSA voltage will be less detrimental to the durability of the fuel
cell. Delaying large changes in at least one of reactant flow, humidity and CSA voltage until
the temperature reaches a desired range avoids otherwise potentially detrimental effects. This
example applies to fuel cells that utilize evaporative or sensible cooling systems 30, 130.
[0025] When the first power demand is less than the second power demand the
operation parameters that remain at a value corresponding to the first power demand may
include reactant pressure. The operation parameters that can be changed without delay in
response to the increase in power demand include reactant flow rate, humidity and CSA
voltage. In one example, the reactant pressure is maintained at a value corresponding to the
first power demand, which prevents the CSA 12 from heating up as much as it would if the
reactant pressure were increased. The reactant flow rate, humidity, and CSA voltage are
changed without delay to a value corresponding to the second power output level to satisfy
the second power demand. This example applies to fuel cells that utilize evaporative cooling
systems 30.
[0026] The first one of the operation parameters is delayed from changing until a
predetermined criterion is met (step 208). For example, the predetermined criterion includes
a preset length of time and the controller delays changes to the first one of the operation
parameters until the preset length of time passes. Avoiding frequent changes in response to
frequent power demand changes decreases cycling which can be harmful to the fuel cell.
[0027] In another example, the predetermined criterion includes temperature of
the cell stack assembly and the controller delays changes to the first one of the operation
parameters until the cell stack assembly reaches a preset temperature as measured by the
temperature sensor 36.
[0028] From the above discussion, it should be clear that in many cases, the
change in power demand will be only temporary, and after a few seconds the power demand
may revert from the second power demand back to the first power demand. In this case, the
"first parameters" will not have changed to their "second" values, and will still be at their
"first" values or intermediate values when the power demand reverts. If this is true, then in
the example, where the second power demand is lower than the first, an entire detrimental
cycle in voltage and in local membrane humidity will have been avoided, even though the
power demand was cycled. In the example where the second power demand is higher than
the first, the pressure will still be at its initial setting when the power demand reverts, so that
the temporary temperature increase will have been lower than if the pressure had been
increased. Thus, a potentially detrimental high temperature excursion will have been avoided
or minimized.
[0029] Although preferred embodiments of this invention have been disclosed, a
worker of ordinary skill in this art would recognize that certain modifications would come
within the scope of this invention. For that reason, the following claims should be studied to
determine the scope of legal protection provided to this invention.

CLAIMS
We claim:
1. A method of operating a fuel cell, comprising the steps of:
a) operating a fuel cell at a first power output level that includes a plurality of
operation parameters, each operation parameter having a value to satisfy a first power
demand;
b) determining a change between the first power demand and a second power
demand;
c) maintaining at least a first one of the operation parameters at a value corresponding
to the first power output level while changing at least a second one of the operation
parameters to a value corresponding to a second power output level to satisfy the second
power demand; and
d) delaying changing the first operation parameter to a value corresponding to the
second power output level until a predetermined criterion is met.
2. The method of claim 1, wherein the first power demand is greater than the second
power demand.
3. The method of claim 2, wherein changing the second operation parameter comprises
reducing fuel cell pressure.
4. The method of claim 2, wherein the first operation parameter includes at least one of
reactant flow, humidity, or cell stack assembly voltage.
5. The method of claim 2, comprising changing the first operation parameter to a value
corresponding to an intermediate power output level between the first and second power
output levels.
6. The method of claim 1, wherein the first power demand is less than the second power
demand.
7. The method of claim 6, wherein the first operation parameter is pressure.
8. The method of claim 1, wherein the predetermined criterion includes at least one of
time and temperature of the fuel cell.
9. The method of claim 1, wherein step c) comprises directing excess power to a power
sink when the first demand is greater than the second demand.
10. The method of claim 1, wherein the second operation parameter includes at least one
of reactant flow, humidity, or cell stack assembly voltage.
11. The method of claim 1, comprising
clipping a cell stack assembly voltage ; and
selecting a voltage value for the clipping based on temperature.
12. The method of claim 11, comprising decreasing the voltage value for the clipping as
the temperature increases.
13. A fuel cell assembly comprising:
a cell stack assembly; and
a controller configured to:
operate the cell stack assembly at a first power output level that includes a
plurality of operation parameters, each operation parameter having a value
corresponding to a first power demand;
determine a change between the first power demand and a second power
demand;
maintain at least a first one of the operation parameters at a value
corresponding to the first power output level while changing at least a second one of
the operation parameters to a value corresponding to a second power output level; and
delay a change in the first operation parameter to a value corresponding to the
second power output level until a predetermined criterion is met.
14. The fuel cell assembly of claim 13, wherein the first power demand is greater than the
second power demand.
15. The fuel cell assembly of claim 14, wherein changing the second operation parameter
comprises reducing a pressure of the cell stack assembly.
16. The fuel cell assembly of claim 14, wherein the first operation parameter includes at
least one of reactant flow, humidity, or cell stack assembly voltage.
17. The fuel cell assembly of claim 14, wherein changing the first operation parameter to
a value corresponding to an intermediate power output level between the first and second
power output levels.
18. The fuel cell assembly of claim 13, wherein the first power demand is less than the
second power demand.
19. The fuel cell assembly of claim 18, wherein the first operation parameter comprises
pressure.
20. The fuel cell assembly of claim 13, wherein the second operation parameter includes
at least one of reactant flow, humidity, or cell stack assembly voltage.
21. The fuel cell assembly of claim 13, wherein the predetermined criterion includes at
least one of time and temperature of the cell stack assembly.
22. The fuel cell assembly of claim 13, wherein the controller is configured to direct
excess power to a power sink when the first power demand is greater than the second power
demand.
23. The fuel cell assembly of claim 22, wherein the power sink includes at least one of a
battery, a capacitor, or a resistor.
24. The fuel cell assembly of claim 13, wherein the controller is configured to
clip a cell stack assembly voltage ; and
select a clip voltage value based on temperature.
25. The fuel cell assembly of claim 24, wherein the controller is configured to decrease
the clip voltage value as the temperature increases.