INTERNAL STEAM GENERATION FOR FUEL CELL
BACKGROUND
[0001] This disclosure relates to a fuel cell system. More particularly, the
disclosure relates to a method and apparatus for generating steam within a fuel cell stack of a
fuel cell system.
[0002] One typical fuel cell system includes a fuel cell stack having an anode
plate and a cathode plate arranged on either side of a proton exchange membrane. The fuel
stack also typically includes coolant channels, which circulates coolant in a coolant loop
within the fuel cell system. One typical coolant is water.
[0003] Some fuel cell stacks produce coolant at temperatures below boiling point
with the coolant ambient pressure with the fuel cell stack. Thus, no steam is produced inside
such a fuel cell stack. To produce steam under such conditions, one example fuel cell system
incorporates a valve and a flash evaporator arranged externally of the fuel cell stack to
convert the low temperature coolant to steam. The steam is then used in a fuel reformation
system.
SUMMARY
[0004] A fuel cell system includes a fuel cell stack having an anode plate and a
cathode plate arranged on opposing sides of a proton exchange membrane. Cooling channels
are in thermal contact with at least one of the anode plate and the cathode plate and include
an internal coolant passage. A pressure-drop device is provided in the coolant channels and
is configured to provide a sub-atmospheric pressure within the coolant passage. A
compression device fluidly interconnects to and is downstream from the internal coolant
passage by a coolant system loop and configured to convey a sub-atmospheric pressure
coolant steam. The compression device is configured to increase the pressure and a
temperature of the sub-atmospheric coolant steam to a super-atmospheric pressure and
maintain the coolant steam within a steam region of a pressure-enthalpy curve.
[0005] A method of producing steam within the fuel cell system includes a step of
creating a pressure drop within a fuel cell stack to lower the boiling point of coolant within
the fuel cell stack. The coolant is boiled within the fuel cell stack to produce steam. The
steam is supplied to a component outside of the fuel cell stack via a coolant steam loop.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The disclosure can be further understood by reference to the following
detailed description when considered in connection with the accompanying drawings
wherein:
[0007] Figure 1 is a highly schematic view of an example fuel cell system with
steam generation internal to the fuel cell stack.
[0008] Figure 2A is a schematic view of one example fuel cell stack.
[0009] Figure 2B is a schematic view of another example fuel cell stack.
[0010] Figure 2C is a schematic view of yet another example fuel cell stack.
[0011] Figure 2D is a schematic view of still another example fuel cell stack.
[0012] Figure 3 is a schematic view of another example fuel cell system having
steam generation internal to the fuel cell stack.
DETAILED DESCRIPTION
[0013] A fuel cell system 10 is schematically illustrated in Figure 1. The system
10 includes a fuel cell stack 12 having multiple cells 19 stacked relative to one another to
produce a desired amount of electricity. Each cell 19 includes an anode plate 14 and a
cathode plate 16 arranged on opposing sides of a proton exchange membrane 18, which is
part of a unitized electrode assembly, for example.
[0014] Coolant channels 20 are arranged throughout the fuel cell stack 12,
typically between the cells 19. A coolant loop 22 is in fluid communication with the coolant
channels 20 and circulates a coolant, water in one example, throughout the system 10 to
regulate the temperature of the fuel cell stack 12. The coolant may also be used for other
purposes within the system 10, as needed.
[0015] Some low temperature fuel cell applications operate at a temperature that
heats the coolant to less than 100°C. With water as the coolant, steam will not be generated
under these conditions. However, steam can be useful within the system 10. To this end, the
system 10 includes a pressure drop device 24 arranged internally to the fuel cell stack 12. As
schematically illustrated in Figure 1, the coolant channels 20 provide an internal coolant
passage with the pressure drop device 24 to lower the pressure of the coolant to the point at
which it will boil and produce steam inside the fuel cell stack 12.
[0016] In the example, the coolant loop 22 includes a first coolant steam line 28
that conveys sub-atmospheric pressure steam to a compression device 26. The compression
device 26 compresses the sub-atmospheric pressure steam, thus, also raising its temperature,
to produce super-atmospheric pressure steam (for example, to 1.1 atmospheres and 150°C)
that is conveyed through a second coolant steam line 30 to a junction 34.
[0017] A fuel source 36 supplies fuel to the junction 34, which intermixes the fuel
and the super-atmospheric pressure coolant steam to provide a mixture. The mixture from
the junction 34 is supplied to a fuel processing system 38 that produces reformate, which is
provided to the anode plate 14 via a reformate line 40. The fuel source 36 may also provide
fuel to a burner 42, which drives, in part, the fuel processing system 38. Unused coolant may
be returned to the coolant channels 20 through a coolant return line 32.
[0018] The compression device 26 maintains the coolant steam within a steam
region of a pressure-enthalpy curve. By generating the steam at sub-atmospheric pressures
within the fuel cell stack, the sub-atmospheric pressure coolant steam can be quasiisentropically
compressed by the compression device.
[0019] The compression device 26, which may be a scroll compressor, for
example, can be driven by an electric motor. The additional efficiency enabled by generating
the steam internally within the fuel cell stack, rather than externally, is sufficient to provide
an overall fuel cell efficiency increase despite the losses associated with the compression
device.
[0020] An example fuel cell stack 12 is illustrated in Figure 2A. In the example,
the anode and cathode plates provide first and second porous layers 44, 46. An internal
coolant passage 48 is provided between the first and second porous layers 44, 46. A coolant
manifold 50 provides coolant to the first and second porous layers 44, 46 for desired
humidification during fuel cell operation. Passage of processed water through the porous
layers 44, 46 during fuel cell operation provides the pressure drop device 24, which enables
the coolant that is at a temperature less than 100°C to boil in the sub-atmospheric pressure.
[0021] Another example fuel cell stack 112 is illustrated in Figure 2B. In this
example, a spray nozzle 52 is used to provide droplets of water to the internal coolant passage
48, which will become steam in the sub-atmospheric pressures within the coolant passage 48
created by the porous layers.
[0022] Another fuel cell stack 212 is illustrated in Figure 2C. The fuel cell stack
212 includes cell 119 having the first porous layer 44 and a second solid plate 56. That is, a
porous plate provides one of the anode and cathode plates, and a solid plate provides the
other plate. The coolant supplied by the coolant manifold 150 humidifies the first porous
layer 44, which provides the pressure drop device 124. Steam is generated in the subatmospheric
pressures.
[0023] Referring to Figure 2D, a fuel cell stack 312 includes cells 219 that utilize
first and second solid plates 54, 56. The internal coolant passage 248 is configured to provide
a sub-atmospheric pressure, for example, by introducing restrictions in the coolant channels.
Water droplets are introduced by the spray nozzle 52. The water is converted to steam in the
sub-atmospheric pressure within the internal coolant passage 248.
[0024] Another fuel cell system 110 is illustrated in Figure 3. The coolant loop
122 generates steam in the same manner as described relative to Figure 1 above. The system
110 cooperates with a fluid loop 60 of a building 58, for example, to transfer heat from the
coolant loop 122 to the fluid loop 60 via a heat exchanger 64. Heat is transferred between the
coolant loop 122 and the fluid loop 60 to achieve a desired temperature of fluid within the
fluid loop 60 for a building sub-system 62, for example, such as a building hot water system.
[0025] Although an example embodiment has been disclosed, a worker of
ordinary skill in this art would recognize that certain modifications would come within the
scope of the claims. For that reason, the following claims should be studied to determine
their true scope and content.

CLAIMS
What is claimed is:
1. A fuel cell system comprising:
a fuel cell stack including an anode plate and a cathode plate arranged on opposing
sides of a proton exchange membrane, and coolant channels including an internal coolant
passage in thermal contact with at least one of the cathode and anode plates;
a pressure drop device provided in the coolant channels and configured to provide a
sub-atmospheric pressure within the coolant passage; and
a compression device fluidly interconnect to and downstream from the internal
coolant passage by a coolant steam loop configured to convey a sub-atmospheric pressure
coolant steam, the compression device configured to increase the pressure and a temperature
of the sub-atmospheric coolant steam to a super-atmospheric pressure and maintain the
coolant steam within a steam region of a pressure-enthalpy curve.
2. The fuel cell system according to claim 1, wherein the coolant channels are
provided by a porous layer of at least one of the anode plate and the cathode plate.
3. The fuel cell system according to claim 2, wherein the porous layer provides
the pressure drop device.
4. The fuel cell system according to claim 3, comprising a spray nozzle arranged
in the coolant passage configured to provide spray water droplets into the coolant passage for
conversion to the coolant steam.
5. The fuel cell system according to claim 1, wherein the coolant channels are
provided by a solid non-porous plate provided by at least one of the anode plate and the
cathode plate.
6. The fuel cell system according to claim 5, comprising a spray nozzle arranged
in the coolant passage configured to provide spray water droplets into the coolant passage for
conversion to the coolant steam.
7. The fuel cell system according to claim 1, wherein the compression device
includes a scroll compressor.
8. The fuel cell system according to claim 1, comprising a fuel source in fluid
communication with the coolant steam loop at a junction via a fuel supply line, the junction
downstream from the compression device and configured to intermix a fuel and the superatmospheric
pressure coolant steam to provide a mixture.
9. The fuel cell system according to claim 8, comprising a fuel processing system
in fluid communication with the junction and configured to receive the mixture, the fuel
processing system fluidly interconnected to the anode plate via a reformate line and
configured to provide a reformate thereto through the reformate line.
10. The fuel cell system according to claim 1, wherein the fuel cell stack is
configured to operate at an equilibrium operating condition providing an internal cell stack
coolant temperature of less than 100°C.
11. The fuel cell system according to claim 1, comprising a building fluid loop,
and a heat exchanger including the building fluid loop and the coolant steam loop configured
to transfer heat there between.
12. The fuel cell system according to claim 1, wherein the coolant steam is
configured to undergo quasi-isentropic compression in the compression device in comparison
to an entropy of the coolant steam within the fuel cell stack.
13. A method of producing steam within a fuel cell system comprising:
creating a pressure drop within a fuel cell stack to lower the boiling point of coolant
within the fuel cell stack;
boiling the coolant within the fuel cell stack to produce steam; and
supplying the steam to a component outside of the fuel cell stack via a coolant steam
loop.
14. The method according to claim 13, wherein the creating step includes
providing a coolant temperature within the stack of less than 100°C and a pressure of less
than atmospheric pressure.
15. The method according to claim 13, wherein the supplying step includes quasiisentropically
compressing the steam, in comparison to an entropy of the steam within the
fuel cell stack, to a pressure greater than atmospheric pressure and maintaining the steam
within a steam region of a pressure-enthalpy curve.