U.S. patent application number 12/086468 was filed with the patent office on 2009-02-26 for evaporative cooling of fuel cells employing antifreeze solution.
Invention is credited to Ryan J. Balliet, Jeremy P. Meyers.
Application Number | 20090053568 12/086468 |
Document ID | / |
Family ID | 38693902 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090053568 |
Kind Code |
A1 |
Meyers; Jeremy P. ; et
al. |
February 26, 2009 |
Evaporative Cooling of Fuel Cells Employing Antifreeze Solution
Abstract
A fuel cell power plant (19) has a stack of fuel cells (20)
cooled by a mixture of water with a non-volatile, miscible fluid
that sufficiently depresses the freezing point, such as
polyethylene glycol (PEG). The water and fluid are mixed in a
reservoir (21), a small pump (22, 60) flows the mixture through
coolant channels (28) in or adjacent water transport plates (29);
heat of the catalytic reaction warms the water transport plates
causing water to evaporate therefrom thereby cooling the stack. The
PEG is non-volatile at stack operating temperature and does not
evaporate; concentrated PEG is returned (33) to the reservoir (21).
Water in the process air flow channels (41), including evaporated
process water, is recovered in a condensation-rate-controlled (53,
54)) condenser (46) in communication (48) with the reservoir (21)
for remixture with the concentrated PEG solution. Hydrophobic gas
diffusion layers (72) shield the proton exchange membrane (70) from
the PEG.
Inventors: |
Meyers; Jeremy P.; (West
Hartford, CT) ; Balliet; Ryan J.; (West Hartford,
CT) |
Correspondence
Address: |
M.P. Williams
210 Main Street
Manchester
CT
06042
US
|
Family ID: |
38693902 |
Appl. No.: |
12/086468 |
Filed: |
December 1, 2005 |
PCT Filed: |
December 1, 2005 |
PCT NO: |
PCT/US2005/043942 |
371 Date: |
June 12, 2008 |
Current U.S.
Class: |
429/429 |
Current CPC
Class: |
C08K 5/548 20130101;
Y02T 10/86 20130101; C08K 3/36 20130101; C08L 7/00 20130101; B60C
1/0016 20130101; Y02T 10/862 20130101; C08C 19/06 20130101; C08K
3/04 20130101; C08L 15/00 20130101; C08L 7/00 20130101; C08L
2666/08 20130101; C08L 15/00 20130101; C08L 2666/08 20130101 |
Class at
Publication: |
429/26 |
International
Class: |
H01M 8/04 20060101
H01M008/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 17, 2006 |
JP |
2006-137862 |
Nov 30, 2006 |
JP |
2006-324195 |
Claims
1. A fuel cell power plant (19) comprising: a stack (20) of fuel
cells (63), each fuel cell including water transport plates (29)
with coolant channels (28) formed therein or adjacent thereto and
with fuel reactant gas flow field channels (65) and oxidant
reactant gas flow field channels (66) having inlets and outlets, at
least one of said plates being porous and hydrophilic; a source
(42) of oxidant reactant gas in fluid communication with inlets of
said oxidant reactant gas flow field channels; a source (55) of
fuel reactant gas in fluid communication with inlets of said fuel
reactant gas flow field channels; a coolant reservoir (21), each of
said fuel cell coolant channels being in fluid communication with
said coolant reservoir; a pump (22, 60) for circulating coolant
from said reservoir, through said fuel cell coolant channels and
back to said reservoir; characterized by: said coolant reservoir
containing a coolant mixture 23 of water with a miscible, freeze
depressing substance; and a condenser (46), connected to the outlet
of at least one of said oxidant reactant gas flow field channels of
said fuel cells, condensate of said condenser in fluid
communication (48) with said reservoir, said coolant mixture
migrating from said coolant channels into said at least one
hydrophilic, porous water transport plate of each fuel cell and at
least some water within said coolant mixture along with some
process water evaporating into at least said reactant gas flow
field channels of said at least one porous and hydrophilic plates
of each fuel cell to cool said fuel cells, at least some of the
water vapor in at least one of said reactant gas flow field
channels being condensed in said condenser and returned to said
reservoir where it mixes with coolant in said reservoir.
2. A power plant (19) according to claim 1 wherein: the reactant
gas flow field is a fuel channel.
3. A power plant (19) according to claim 1 further characterized
by: each fuel cell (63) including membrane electrode assembly (MEA)
(70) having a membrane with catalyst on both surfaces thereof, said
MEA configured to provide a wet-proofed barrier between at least
one surface of said MEA and said coolant channels.
4. A power plant (19) according to claim 3 wherein said wet-proofed
barrier comprises: at least one wet-proofed gas diffusion layer
(72) adjacent said MEA (70) in each of said fuel cells.
5. A power plant (19) according to claim 3 wherein said wet-proofed
barrier comprises: at least one bilayer (70) adjacent said MEA in
each of said fuel cells.
6. A power plant (19) according to claim 3 wherein said wet-proofed
barrier comprises: a solid water transport plate (29) on at least
one side of said MEA (70).
7. A power plant (19) according to claim 1 further characterized
by: said pump (22) being disposed at an inlet of said reservoir
(21) receiving (33) circulating coolant from said coolant channels
(28).
8. A power plant (19) according to claim 1 further characterized
by: said pump (60) being disposed in a conduit (33) interconnecting
said coolant channels (28) with an inlet (37) of said reservoir
(21).
9. A power plant (19) according to claim 1 further comprising: a
condenser controller (53, 54) for controlling the rate of
condensation of water vapor in said condenser (46).
10. A power plant (19) according to claim 9 wherein: said condenser
(46) is cooled by a stream of air (52) and said condenser
controller is a controller (53) that varies the speed of an air fan
(54).
11. A power plant (19) according to claim 9 wherein: said condenser
(46) is cooled by a controlled flow of freeze-proof coolant through
flow passages in said condenser.
Description
TECHNICAL FIELD
[0001] This invention relates to circulating an antifreeze solution
from a reservoir through water channels of porous, hydrophilic
water transport plates and back to the reservoir; the mixture
enters the fine pores of the water transport plates which are
warmed by the heat of the fuel cell process, thereby evaporating
water which may include product water (but not antifreeze) from the
plates into the process oxidant flow channels, cooling the fuel
cells. Water is condensed out of the process air oxidant exhaust
and returned to re-mix with the concentrated antifreeze.
BACKGROUND ART
[0002] It is known that water produced at the cathodes of fuel
cells has to be removed from the cathodes in order to prevent the
water from blocking the flow of oxidant gas, such as air, from
reaching the electrodes. It is also known that fuel cells, when
operating, must be cooled to keep the fuel cells at a proper
operating temperature. Some fuel cells are cooled only by
conduction of heat into cooler plates which are interspersed
between some or all of the fuel cells.
[0003] One known type of fuel cells employ reactant gas flow field
plates which are porous and hydrophilic, having fine pores to allow
water to pass from the cathode into the oxidant reactant gas flow
channels, and to allow water to pass from the fuel reactant gas
flow channels toward the membrane. These are typically called water
transport plates. Cooling is typically accomplished by sensible
heat transfer to water in the water flow channels formed in or
adjacent to the water transport plates.
[0004] It has been known to cool fuel cells by evaporation,
typically by providing atomized water to the reactant gas streams,
which water evaporates, thereby cooling the stack.
[0005] In fuel cells which have employed separate cooler plates,
the use of an antifreeze mixture as coolant in place of water is
known. The use of separate cooler plates requires a fuel cell stack
to occupy a larger volume than it would without cooler plates.
Similarly, atomizing water into reactant gas streams for
evaporative cooling requires additional equipment, which increases
cost and volume and presents difficulty, especially at shut down,
for fuel cell power plants operating in freezing environments.
[0006] In any of the cases referred to, even when antifreeze is
used in cooler plates, the requirement to eliminate all water from
the stack and auxiliary plumbing before freezing, or to otherwise
accommodate the likelihood of freezing temperatures during fuel
cell power plant shut down poses additional difficulties, requiring
apparatus that adds cost and volume, which are most undesirable
when a fuel cell is used as a power source for an electric
vehicle.
DISCLOSURE OF INVENTION
[0007] Objects of the invention include: reducing the volume of a
fuel cell power plant; eliminating or reducing freezable water in a
fuel cell power plant system; improving fuel cell power plant for
use where freezing temperatures may be encountered when the fuel
cell is not operating; avoiding having freezable liquid in contact
with moving parts in a fuel cell power plant; shortening fuel cell
power plant startup time by reducing cell stack thermal mass; and
improved fuel cell power plant.
[0008] According to the invention, fuel cells in a fuel cell power
plant are evaporatively cooled by evaporation of at least some of
the water in an antifreeze mixture with a freeze depressing
substance in the porous, hydrophilic reactant gas flow field
plates, which typically have reactant gas flow channels extending
from a surface of reactant flow field plates opposite from coolant
passageways. The antifreeze coolant mixture circulates through the
coolant passageways in or adjacent the reactant gas flow field
plates. A more concentrated mixture returns to a coolant reservoir.
The evaporation of water from the antifreeze mixture and product
water into the reactant streams (primarily the cathode) cools the
fuel cell stack. At least some water vapor is condensed out of at
least the oxidant reactant gas stream exiting from the stack, the
condensed water being returned to the mixture in the accumulator.
To avoid diluting the antifreeze mixture, less than all of the
water vapor in the air exhaust may be condensed. The rate of
condensing may be controlled using a condensate controller to
ensure proper water balance, such as a variable flow cooling fan
for the condenser, or by cooling the air in the condenser with a
controlled circulation of antifreeze.
[0009] A pump is used to pump the antifreeze mixture in a
conventional fashion similar to the manner of circulating coolant
water in conventional fuel cells. Since only the antifreeze is
present in the pump, freezing during shutdown is not a problem.
[0010] Other objects, features and advantages of the present
invention will become more apparent in the light of the following
detailed description of exemplary embodiments thereof, as
illustrated in the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIG. 1 is a block illustration of the invention.
[0012] FIG. 2 is a partial perspective view of an embodiment of the
invention.
[0013] FIG. 3 is a fragmentary view of a variation of FIG. 1.
[0014] FIG. 4 is a fragmentary view of an alternative to the
embodiment of FIG. 2.
[0015] FIG. 5 is a sectioned, side elevation view, with sectioning
lines omitted for clarity, of portions of fuel cells useful with
the invention.
MODE(S) FOR CARRYING OUT THE INVENTION
[0016] Referring to FIG. 1, a fuel cell power plant 19 has a stack
20 of fuel cells. The concept of the present invention is
illustrated by the density of stippling to provide a rough
indication of the fraction of fluid in the fuel cell stack coolant
that is antifreeze 23, such as polyethylene glycol (PEG) or other
non-volatile, miscible fluid that sufficiently suppresses the
freezing point of a mixture with water. In FIG. 1, PEG and water
are mixed in a reservoir 21, assisted by a pump 22, so as to
achieve the desired freeze point of the mixture. Because the pump
is in an antifreeze solution, the pump will not be rendered
inoperable due to freezing conditions.
[0017] The desired mixture, such as at the top 25 of the reservoir
21, is fed through a conventional coolant inlet manifold 26 into
coolant channels 28 of the fuel cells (described with respect to
FIG. 4, hereinafter) in the stack 20, in which the coolant channels
are formed within or in fluid communication with, porous,
hydrophilic water transport plates 29, which have fine pores that
contain a water-antifreeze mixture.
[0018] The fuel cell coolant channels 28 are connected to a coolant
exit manifold 32 which is interconnected by means of a conduit 33
to the reservoir 21. At the inlet 37 where the coolant returns to
the reservoir, the coolant may be substantially antifreeze 23
(e.g., PEG); that is to say, a very concentrated solution of
antifreeze 23. However, this is remixed in the reservoir 21, such
as by means of the pump 22, if desired; if the pump 22 is not
necessary in any given embodiment of the invention, it may be
omitted. Furthermore, other ways of assuring an adequate mixing of
the returned antifreeze 23 with the rest of the fluid in the
reservoir 21 may be used within the purview of the present
invention.
[0019] The water transport plates 29 absorb heat generated in the
catalytic reaction of oxygen and hydrogen. Although the antifreeze
23 is non-volatile at the operating temperature of the fuel cell
stack, on the order of 60.degree. C.-70.degree. C. (140.degree.
F.-158.degree. F.), water evaporates into the oxidant reactant gas
stream flow channels 41 that receive oxidant, such as air from an
air inlet manifold 42, cooling the fuel cells by the heat of
vaporization. The saturated (or nearly saturated) air exits the
fuel cells through an air exit manifold 45 and enters a condenser
46 where at least some water vapor is condensed out of the process
air, the dried air flows to exhaust 47, and the condensate, which
is essentially pure water, flows to the reservoir 21 directly or
through a conduit 48. In the area 49 where the condensate enters
the reservoir 21, the mixture is dilute. However, it is remixed
with concentrated antifreeze 23 within the reservoir before
reentering the fuel cells through the coolant inlet manifold
26.
[0020] FIG. 2 illustrates a portion of the fuel cell power plant 19
including the fuel cell stack 20, which employs evaporative
cooling. Air is provided to the air inlet manifold 42 and proceeds
through the oxidant flow field channels 41 (FIG. 1) to the air exit
manifold 45 and thence into the condenser 46. The air outflow from
the condenser 46 is above the water line 51 of the water reservoir
21. The cool dried air is expelled to exhaust 47. The coolant for
the condenser 46 may comprise ambient air as illustrated by arrows
52, the volume of which is controlled by a condenser controller 53
that varies the speed of a flow fan 54 in order to adjust the
condensation rate as needed. The condenser 46 may serve as a
manifold, and the air exit manifold 45 may then be omitted.
[0021] Fuel provided to a fuel inlet manifold 55 flows to the left,
then through a fuel turn manifold 56, after which fuel flows to the
right and out through a fuel exit manifold 57; the exhausted fuel
may be recycled or consumed in a related process.
[0022] Coolant from the reservoir 21 flows through a coolant
conduit 60 to the coolant inlet manifold 26. The coolant passes
into the coolant channels (as described with respect to FIG. 1
hereinbefore) to the top of the fuel cell stack 20, and through the
coolant exit manifold 32. Coolant flowing out of the coolant exit
manifold 32 is recirculated over the conduit 33 to the reservoir
21. The water in the coolant mixture entering through coolant inlet
manifold 26 replaces that which is evaporated into the process air
channels 41, as described with respect to FIG. 1 hereinbefore.
[0023] To ensure that adequate water will be present in the fine
pores for evaporation, the pump 22 (FIG. 1) may be disposed at the
inlet 37 to draw the coolant into the reservoir 21 from the line 33
as shown in FIG. 3. Or, a pump 60 (FIG. 4) may be used in the line
33 or in any other suitable location to ensure adequate coolant
circulation. Usually, a pump will be required in order to assure
that the flow of antifreeze mixture is sufficient to provide enough
water so that evaporation will occur throughout all portions of all
of the fuel cells, and to prevent the antifreeze component from
partially or completely filling the pores of the water transport
plates.
[0024] Because the PEG, or other antifreeze, has a viscosity many
times higher than that of pure liquid water, the pressure drop
across the coolant channels will be high, or, the coolant channels
will have to be larger (deeper) to accommodate the coolant flow
rates required to cool the stack. Deeper channels decrease the
number of cells per unit of stack length compared to fuel cell
stacks employing water transport plates and using evaporative
cooling. The channel depth will nonetheless be shallower than in
systems employing coolant water or similar systems employing an
antifreeze mixture to cool the stack using the fluid sensible heat
exchange. Thus, the invention will provide power density which is
greater than traditional water or antifreeze cooling systems.
[0025] Detailed descriptions of fuel cells having water transport
plates may be found in patent publication US2004/0106034.
[0026] Referring to FIG. 5, fuel cells 63 which may be used to
implement the present invention include anode water transport
plates (WTPs) 29a having fuel reactant gas flow field channels 65
and cathode water transport plates 29b having oxidant reactant gas
(air) flow field channels 66. A membrane electrode assembly (MEA)
70 includes a proton exchange membrane with catalyst on both
surfaces. Gas diffusion layers (GDLs) 72 are provided adjacent each
surface of the MEAs 70. In the prior art (such as in the
aforementioned patent publication US 2004/0106034) the GDLs are
typically constructed from carbon fiber sheet material, and are
usually wettable. In some known GDLs, there is an additional
wet-proof layer deposited on or joined to the GDL to form a
bi-layer. The carbon fiber layer may or may not be wet-proofed
whether a bi-layer is used or not used.
[0027] To prevent loss of fuel cell stack performance, the MEAs 70
must be shielded from the non-water component of the antifreeze
mixture in the coolant channels 28. Therefore, the invention
preferably employs fuel cells 63 with GDLs 72 which are treated,
such as with polytetrafluoroethylene (PTFE) to be wet-proofed, or
include an additional wet-proof layer.
[0028] If desired in any utilization of the invention, either the
anode WTP 29a or the cathode WTP 29b may be solid. A solid WTP will
block coolant from reaching the MEA on the side it is located. If
the cathode WTP 29b is solid, water will reach the air (oxidant)
flow field channels 66 by migration through the membranes of the
MEAs 70 and GDLs 72. Alternatively, the surfaces of one of the WTPs
29, adjacent to the GDL 72, including the reactant gas flow field
channels 65, 66, may be wet-proofed by treating with a wet-proofing
material, such as PTFE, to shield the membrane from the PEG or
other antifreeze on that side.
[0029] The coolant channels 28 may be formed by having grooves 75
on the opposite surface of the anode water transport plates 29a
from the fuel reactant gas flow field channels 65 which match up
with grooves 76 on the opposite surface of cathode water transport
plates 29b from oxidant reactant gas flow field channels 66. Or,
the grooves may be in only one plate 29a, 29b, the matching surface
of the other plate 29b, 29a being flat.
* * * * *