U.S. patent application number 10/132789 was filed with the patent office on 2003-10-30 for maximizing pem fuel cell power plant system efficiency at optimum system pressure.
Invention is credited to Clark, Thomas M., Meyer, Alfred P., Seshadri, Parthasarathy, Van Dine, Leslie L..
Application Number | 20030203264 10/132789 |
Document ID | / |
Family ID | 29248840 |
Filed Date | 2003-10-30 |
United States Patent
Application |
20030203264 |
Kind Code |
A1 |
Seshadri, Parthasarathy ; et
al. |
October 30, 2003 |
Maximizing PEM fuel cell power plant system efficiency at optimum
system pressure
Abstract
A PEM fuel cell power plant system (10) has a process air pump
(26), which may be a fan, a blower or a compressor, with an
adiabatic efficiency of between 40% and 70%. The process air at the
inlet 27 of the cathode reactant gas flow field 16 is between 1.07
atmospheres and 1.85 atmospheres, and may be at an optimal pressure
for maximum overall system efficiency
P={0.45+2.6E-1.8E.sup.2}atms.+-.0.2 atms where P is the air inlet
pressure and E is the adiabatic efficiency of the process air
pump.
Inventors: |
Seshadri, Parthasarathy;
(Manchester, CT) ; Meyer, Alfred P.; (Simsbury,
CT) ; Van Dine, Leslie L.; (Manchester, CT) ;
Clark, Thomas M.; (Suffield, CT) |
Correspondence
Address: |
M. P. Williams
210 Main Street
Manchester
CT
06040
US
|
Family ID: |
29248840 |
Appl. No.: |
10/132789 |
Filed: |
April 24, 2002 |
Current U.S.
Class: |
429/444 ;
429/457; 429/492; 429/505 |
Current CPC
Class: |
H01M 8/04291 20130101;
H01M 8/1007 20160201; H01M 8/04097 20130101; H01M 8/04089 20130101;
Y02E 60/50 20130101 |
Class at
Publication: |
429/32 ; 429/38;
429/34 |
International
Class: |
H01M 008/10; H01M
008/04 |
Claims
We claim:
1. A fuel cell power plant comprising: a plurality of fuel cells,
each including a proton exchange membrane having a cathode catalyst
on a first surface and an anode catalyst on a second surface
opposite to said first surface; a cathode flow field plate adjacent
to said cathode catalyst and having an oxidant reactant gas flow
field; an anode flow field plate adjacent to said anode catalyst
and having a fuel reactant gas flow field; a source for providing
hydrogen-containing gas to said fuel reactant gas flow field; and
an air pump for pumping air from the earth's atmosphere to said
oxidant reactant gas flow field; characterized by the improvement
comprising: said pump having an adiabatic efficiency, E, of between
0.3 and 0.70 and providing air to said oxidant reactant gas flow
field at a pressure, P, substantially equal to:
P={0.45+2.6E-1.8E.sup.2}a- tms.+-.0.2 atms
2. A fuel cell power plant comprising: a plurality of fuel cells,
each including a proton exchange membrane having a cathode catalyst
on a first surface and an anode catalyst on a second surface
opposite to said first surface; a cathode flow field plate adjacent
to said cathode catalyst and having an oxidant reactant gas flow
field; an anode flow field plate adjacent to said anode catalyst
and having a fuel reactant gas flow field; a source for providing
hydrogen-containing gas to said fuel reactant gas flow field; and
an air pump for pumping air from the earth's atmosphere to said
oxidant reactant gas flow field; characterized by the improvement
comprising: said pump having an adiabatic efficiency, E, of between
0.40 and 0.70 and providing air to said oxidant reactant gas flow
field at a pressure between 1.07 atmospheres and 1.85
atmospheres.
3. A fuel cell power plant according to claim 2 wherein: said pump
has an adiabatic efficiency between about 0.40 and about 0.50, and
air is provided to said oxidant reactant gas flow field at a
pressure between 1.10 atmospheres and 1.50 atmospheres.
4. A fuel cell power plant according to claim 2 wherein: said pump
has an adiabatic efficiency between about 0.50 and about 0.70, and
air is provided to said oxidant reactant gas flow field at a
pressure between 1.20 atmospheres and 1.70 atmospheres.
Description
TECHNICAL FIELD
[0001] This invention relates to operation of a proton exchange
membrane (PEM) fuel cell power plant at a pressure which is
selected to correspond with the adiabatic efficiency of the
reactant air pump.
BACKGROUND ART
[0002] Heretofore, one type of PEM fuel cell power plant utilizes
compressors to pump the reactant air to the cathode reactant gas
flow fields, and has typically been operated with air inlet
pressures of two or three atmospheres. This takes advantage of the
ability of a fuel cell to operate at higher average cell
temperatures, exhaust dew point temperatures and coolant
temperatures as a consequence of higher operating pressures.
Another type of known fuel cell power plant may utilize a fan or
blower to provide the reactant air, and consequently works near
ambient pressure. While this type of fuel cell power plant cannot
take advantage of the efficiencies that result from higher pressure
and therefore higher temperature, the overall system efficiency is
higher than that of the fuel cell power plants operating at two or
three atmospheres with a compressor, due to the extremely low
parasitic power required of the blower or fan compared to the
parasitic power required by compressors.
[0003] It is known that water removal from a fuel cell power plant
is controlled by the temperature, pressure and volume of the
process exhaust streams leaving the fuel cell power plant. The
volume of the process exhaust streams are related to the reactant
utilizations and the composition of the fuel used to operate the
fuel cell power plant. Process exhaust is defined to be the sum of
any exhaust streams from the fuel or oxidant sides of the fuel cell
power plant. The fuel stream may be burned prior to exhausting the
power plant as is known. The process exhaust from a hydrogen-fueled
fuel cell is primarily unreacted air since the fuel utilization
approaches 100% to maximize power plant efficiency. The process
exhaust from a gasoline-fueled fuel cell includes significant
quantities of carbon dioxide and nitrogen, which are by-products of
the fuel reforming process. For a partial oxidation reformer, the
exhaust volume from the fuel side of the system is approximately
equal to exhaust volume from the air side of the system. The
greater volume of the process exhaust stream in a gasoline-fueled
fuel cell power plant vs. a hydrogen-fueled fuel cell power plant
requires a lower system exhaust dew point for the gasoline-fueled
fuel cell power plant to maintain water balance.
[0004] It is known that the thermodynamic efficiency of PEM fuel
cell power plants increases as the fuel and oxidant inlet pressures
increase. One of the benefits of higher operating pressure is that
it increases the exhaust dew point at which water balance can be
maintained, as is illustrated in FIG. 1. In FIG. 1, the leftmost
five traces 13 illustrate the exhaust dew point temperature as a
function of process exhaust pressure of a gasoline-fueled fuel cell
power plant for pressures between one atmosphere and two
atmospheres. The rightmost five traces 14 illustrate the increase
in system exhaust dew point with increases in process exhaust
pressure of a hydrogen-fueled fuel cell power plant for pressures
between one atmosphere and two atmospheres. At water balance, water
removed in the process exhaust stream is just sufficient to balance
the water created at the cathode by the fuel cell process, and
leaves adequate water to maintain sufficient humidification of the
membrane. Providing process exhaust with the proper dew point, so
as to achieve water balance, eliminates the need for additional
components such as condensers and enthalpy recovery devices which
add weight, volume and complexity to the overall system. When a
system is operated essentially at the water balance exhaust dew
point, water is nonetheless cooled and returned to the water inlet
channels to ensure adequate presence of water thereby permitting
water removal to achieve balance. Provision of a higher process
exhaust temperature permits operation at a higher cell temperature
and coolant temperature, and utilization of a smaller radiator for
removal of heat from the coolant to ensure that the water returning
to the water flow fields is at a lower temperature than the process
exhaust (or operation within higher ambient temperatures),
typically reducing overall system cost and weight.
[0005] It is known that the overall system efficiency must take
into account efficiency of electrical and mechanical components
exterior of the fuel cell power plant. In particular, utilizing
electricity generated by the fuel cell power plant to operate the
pump (fan, blower or compressor) which provides the process air to
the fuel cell power plant comprises a significant load of parasitic
power consumption. It has heretofore been known that a fuel cell
power plant operating at or near ambient pressure utilizing a
relatively low power pump has an overall system efficiency which is
greater than fuel cell power plants operating at two or three
atmospheres, which have higher thermodynamic efficiency, but lower
overall system efficiency, due to the increased parasitic power
requirement of the required air pump.
DISCLOSURE OF INVENTION
[0006] Objects of the invention include provision of a PEM fuel
cell power plant: which operates at or near maximum system
efficiency; which operates at a system pressure which provides
overall maximum system efficiency, taking into account
thermodynamic efficiency as well as the adiabatic efficiency of the
process air pump; which permits achieving the highest possible
water balanced system exhaust dew point consistent with overall
system efficiency; and provision of an improved PEM fuel cell power
plant.
[0007] This invention is predicated on the discovery that the
overall efficiency of a fuel cell power plant, including the
parasitic power requirements of a process air pump, does not
increase with increases in system pressure for any attainable
adiabatic efficiency of the pump, but rather that there is a
critical system pressure, related to the adiabatic efficiency of
the process air pump, at which the efficiency of the overall system
is maximal.
[0008] According to the present invention, a proton exchange
membrane fuel cell power plant utilizes a process air pump which
has an adiabatic efficiency, E, of between 40% and 70%, and the
system pressure, P, is chosen to be substantially a pressure at
which the overall system efficiency is maximal as a function of the
adiabatic efficiency of the process air pump. According further to
the invention, the system pressure is substantially equal to
P={0.45+2.6E-1.8E.sup.2}atms+0.2 atms.
[0009] As used herein, the term "pump" includes fans, blowers and
compressors of all types. By selecting a system pressure as a
function of adiabatic efficiency of the process air pump, a fuel
cell stack can be composed of a fewer number of cells, without
sacrificing any performance; alternatively, a fuel cell stack
having a usual number of cells can operate at usual performance
with higher efficiency and lower fuel consumption.
[0010] In accordance with the invention, a proton exchange membrane
fuel cell power plant utilizes a process air pump which has an
adiabatic efficiency, E, of between 40% and 70% and a system
pressure, P, between 1.07 atmospheres and 1.85 atmospheres, and
preferably between 1.10 and 1.50 atmospheres for pump efficiencies
between 0.40 and 0.50, and between 1.20 and 1.70 atmospheres for
pump efficiencies between 0.50 and 0.70.
[0011] 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
[0012] FIG. 1 is a plot of system air utilization for various
exhaust dew points that achieve system water balance, for PEM fuel
cell power plants fueled with gasoline and for PEM fuel cell power
plants fueled with hydrogen, illustrating how water balanced dew
point increases with system pressure for any air utilization.
[0013] FIG. 2 is a simplified, schematic illustration of an
exemplary PEM fuel cell power plant known to the prior art, with
which the present invention may be utilized.
[0014] FIG. 3 is a chart of system efficiency as a function of
pressure while holding current density constant while the fuel cell
stack is configured with a different number of fuel cells, for
compressors (or other pumps) having adiabatic efficiency between
0.4 and 0.7, respectively.
[0015] FIG. 4 is a plot of system efficiency as a function of
system pressure for a fuel cell stack with a fixed number of fuel
cells, while allowing the current density to vary, for compressors
(or other pumps) having adiabatic efficiencies of between 0.4 and
0.7, respectively.
[0016] FIG. 5 is a plot of number of cells in a fuel cell stack in
order to maintain a given current density as system pressure
varies, for compressors (or other pumps) having adiabatic
efficiency of between 0.4 and 0.7, respectively.
[0017] FIG. 6 is a plot of optimum system pressure as a function of
adiabatic compressor efficiency.
MODE(S) FOR CARRYING OUT THE INVENTION
[0018] Referring to FIG. 2, a fuel cell power plant 10 includes a
cell stack assembly 12 which comprises a plurality of individual
fuel cells stacked together in contiguous relationship, although
only a single cell is illustrated in FIG. 1. A fuel cell includes
an anode electrode 14, a cathode electrode 16, and a polymer
electrolyte membrane 18 disposed between the electrodes. Each
electrode consists of a catalyst, a porous support plate and a
reactant flow field as is well known. A water transport plate 19
(or coolant plate), adjacent to the cathode 16, is connected to a
coolant control loop including a coolant pump 20, a coolant
pressure control valve 21, and a heat exchanger 22. There may be a
heat exchange bypass and other water management apparatus, not
shown, as disclosed in U.S. Pat. No. 5,503,944. The pump 20 and
valve 21 will regulate both the pressure and the volume of flow
through the water transport plate 19 and through (or around) the
heat exchanger 22. Air is supplied by a blower 26 to a cathode
oxidant flow field inlet manifold 27, the depleted air exiting the
flow field through an exit manifold 28 and an oxidant pressure
control valve 28a to exhaust 29. Fuel from a pressurized source 32
of hydrogen, or hydrogen rich gas, passes through a fuel pressure
regulator 34, a fuel inlet manifold 35, the fuel flow field of the
anode 14, and a fuel exit manifold 36 to a flow control valve 37.
The valve 37, when open, vents the fuel flow field to ambient 39
(or any exhaust processor which may be used). The fuel flow field
effluent may typically pass through a recycle loop including a
blower 41. The fuel recycle blower typically has a flow rate that
is greater than the fuel inlet flow rate in order to maintain a
relatively uniform hydrogen composition across the anode flow
field.
[0019] Current in the power lines 42 feed the load 43 of the fuel
cell power plant. A controller 46 may adjust the setting of the
hydrogen pressure control valve 34 and the oxidant pressure control
valve 28a, if desired, to ensure proper flow of reactants to the
flow fields. The controller 46 may also control the hydrogen
exhaust flow valve 37 so as to purge trace contaminants
periodically, such as at a start up or shut down, or otherwise, as
is conventional.
[0020] Calculations of overall system efficiency were made for a
fuel cell power plant system of the type generally illustrated in
FIG. 2, but which may include cathode air recycling and/or solid
coolant plates dispersed between every several cells in the stack,
with a net power of 75 kW, and operating with a coolant exit
temperature of 85.degree. C., utilizing hydrogen and atmospheric
air as the reactants. The calculations were made for such a fuel
cell power plant having an air pump (a compressor in this case)
with several different adiabatic efficiencies, between 0.3 and 0.7,
as the system air inlet pressure was varied from one atmosphere to
several atmospheres. For one set of calculations, as the system
pressure of the fuel cell power plant was varied, the number of
cells in the fuel cell stack was varied in order to cause the
current density to remain constant. Results from this set of
calculations are illustrated in FIG. 3. For another set of
calculations, the number of cells in the fuel cell stack was held
constant, and the peak current density was allowed to vary with the
system pressure. Results from this set of calculations are
illustrated in FIG. 4. From the first set of calculations, the
number of fuel cells required to maintain a constant current
density, as system pressure (the pressure of air at the inlet to
the cathode flow field) varied, is illustrated in FIG. 5. The
pressure at which the number of fuel cells is minimal, for the
adiabatic pump efficiencies plotted in FIG. 5, is the same as the
pressure of maximum system efficiency illustrated in FIG. 3. The
optimum system pressure, for maximal system efficiency, for the
case illustrated in FIGS. 3 and 5 (constant load current) is
illustrated in FIG. 6. It is seen that optimum system pressure is
just over 1.30 atmospheres for adiabatic compressor efficiency of
0.5, and about 1.38 atmospheres for adiabatic compressor
efficiencies of 0.7.
[0021] The aforementioned patent is incorporated herein by
reference.
[0022] Thus, although the invention has been shown and described
with respect to exemplary embodiments thereof, it should be
understood by those skilled in the art that the foregoing and
various other changes, omissions and additions may be made therein
and thereto, without departing from the spirit and scope of the
invention.
* * * * *