US20040001982A1 - Initiating operation of an electric vehicle or other load powered by a fuel cell at sub-freezing temperature - Google Patents
Initiating operation of an electric vehicle or other load powered by a fuel cell at sub-freezing temperature Download PDFInfo
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- US20040001982A1 US20040001982A1 US10/390,439 US39043903A US2004001982A1 US 20040001982 A1 US20040001982 A1 US 20040001982A1 US 39043903 A US39043903 A US 39043903A US 2004001982 A1 US2004001982 A1 US 2004001982A1
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- water
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- reactant gas
- fuel
- oxidant
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
- H01M8/04223—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
- H01M8/04007—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids related to heat exchange
- H01M8/04014—Heat exchange using gaseous fluids; Heat exchange by combustion of reactants
- H01M8/04022—Heating by combustion
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/02—Details
- H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
- H01M8/0258—Collectors; Separators, e.g. bipolar separators; Interconnectors characterised by the configuration of channels, e.g. by the flow field of the reactant or coolant
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/02—Details
- H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
- H01M8/0267—Collectors; Separators, e.g. bipolar separators; Interconnectors having heating or cooling means, e.g. heaters or coolant flow channels
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
- H01M8/04082—Arrangements for control of reactant parameters, e.g. pressure or concentration
- H01M8/04089—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants
- H01M8/04119—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying
- H01M8/04156—Arrangements for control of reactant parameters, e.g. pressure or concentration of gaseous reactants with simultaneous supply or evacuation of electrolyte; Humidifying or dehumidifying with product water removal
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
- H01M8/04223—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells
- H01M8/04225—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids during start-up or shut-down; Depolarisation or activation, e.g. purging; Means for short-circuiting defective fuel cells during start-up
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/04—Auxiliary arrangements, e.g. for control of pressure or for circulation of fluids
- H01M8/04298—Processes for controlling fuel cells or fuel cell systems
- H01M8/043—Processes for controlling fuel cells or fuel cell systems applied during specific periods
- H01M8/04302—Processes for controlling fuel cells or fuel cell systems applied during specific periods applied during start-up
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/24—Grouping of fuel cells, e.g. stacking of fuel cells
- H01M8/241—Grouping of fuel cells, e.g. stacking of fuel cells with solid or matrix-supported electrolytes
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/24—Grouping of fuel cells, e.g. stacking of fuel cells
- H01M8/2457—Grouping of fuel cells, e.g. stacking of fuel cells with both reactants being gaseous or vaporised
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
Definitions
- This invention relates to rapidly initiating operation of a fuel cell powered electric vehicle, at sub-freezing temperature, by means of one or more of: providing excess reactant gas or cold reactant gas to the proton exchange membrane (PEM) fuel cell stack which powers the load, such as a vehicle propulsion system, connecting the load to the stack within 20 seconds of reactant gas flow or when open circuit voltage is detected, previously draining hydrophilic support plates, connecting coolant only after several minutes or when sufficient water has melted.
- PEM proton exchange membrane
- the fuel cell is warmed up simply by connecting a load across it while stochiometric fuel and oxidant are supplied to the stack.
- stochiometric fuel and oxidant are supplied to the stack.
- application of hydrogen and air at room temperature caused a temperature in the core of a ten cell stack to advance from ⁇ 11° C. to 0° C. in about one minute.
- an electric propulsion system For use in vehicles, such as automobiles, an electric propulsion system must be operating in less than one minute, preferably less than one-half minute, after initiating startup. None of the foregoing are capable of providing fuel cells operable in subfreezing temperatures, particularly as low as ⁇ 40° C. ( ⁇ 40° F.).
- Objects of the invention include: operating, at subfreezing temperature, an electric vehicle powered by a fuel cell within seconds of initiation; improved initiation of fuel cell powered, electric vehicle operation at subfreezing temperature; initiating fuel cell powered electric vehicle operation at subfreezing temperature with a minimal of waste power used for raising the temperature of apparatus and/or fluids; avoiding the need for heat exchangers and other apparatus to heat reactants or coolants above freezing; and avoiding use of battery power to start a fuel cell for powering a vehicle.
- This invention is predicated on the discovery that the propulsion system of an electric vehicle powered by a PEM fuel cell can be powered from the fuel cell while the fuel cell stack is frozen.
- This invention is further predicated on the discovery that contrary to belief of the prior art, excess reactants, rather than reactant starvation, will permit extended operation of the fuel cell stack pending the ability to flow water through the stack.
- the invention is further predicated on the discovery that high flow of cold reactant gases through the reactant flow fields is not sufficient to cause freezing of product water, the heat generated in the membrane electrode assembly being sufficient, and sufficiently close to the reactant flow fields, to prevent freezing of product water or refreezing of melted water.
- the invention is also predicated on the discovery that fuel cell operation without loss of performance or damage to the cells can be extended during a frozen startup by providing at least one of the reactant gases at a pressure in excess of the pressure of any water in the stack, which before operation of a water circulation system is typically atmospheric.
- a PEM fuel cell stack at subfreezing temperature is connected to a vehicle propulsion system or other electric load within a few seconds or as soon as the stack provides open circuit voltage.
- the fuel cell stack is started with more than a stochiometric flow of fuel and at least stochiometric flow, but preferably two-five times stochiometric flow of oxidant, which may be at subfreezing temperatures, or not, whereby to prolong operation without localized heating, thereby permitting the vehicle (or other load) to be used during the time that the apparatus and fluids are being heated to suitable, operational temperatures.
- the invention not only permits, but prefers operation with reactants which are at the same subfreezing ambient temperature as the fuel cell stack itself, contrary to usage of the prior art, since this prolongs the onset of localized overheating.
- heating of the water stored as ice in the pores of the water transport plates by heating up the mass of the stack as well as the water, the heat of fusion as the ice melts, and evaporative cooling of some of that water, further prolongs the period of time at which the vehicle can be operated with power from the fuel cell stack, without circulating coolant, before there is impermissible local heating within the fuel cell.
- At least one of the reactant gases is provided to the fuel cell stack at a pressure of at least about 4 kPa (0.6 psi) above the pressure of any water in the water channels, which typically will be about atmospheric pressure. This prevents liquid water from pooling in the reactant channels, and flooding the electrode substrates, which is particularly important in the oxidant gas reactant channel where product water can accumulate.
- Principal aspects of the present invention include starting an electric load, such as a vehicle or other load, the fuel cell stack of which is at subfreezing temperatures, before awaiting for the fuel cell stack to reach normal operating temperature, by supplying the fuel cell with at least twice stochiometric quantities of oxidant, and using substantially empty hydrophilic support plates for temporary product water storage thereby to allow the fuel cell to operate without circulating coolant until such time as all of the water systems are functional.
- FIG. 1 is a simplified, sectioned side elevation of slightly more than one fuel cell which may be part of a stack with which the present invention may be practiced.
- FIG. 2 is a simplified, sectioned side elevation of an alternative to that shown in FIG. 1.
- FIG. 3 is a simplified, sectioned side elevation of another alternative to that shown in FIG. 1.
- FIG. 4 is a schematic illustration of a vehicle engine propulsion system, including a fuel cell stack by which it is powered, which may practice the present invention.
- FIG. 5 is a plot of fuel cell stack voltage and temperature as a function of time, during a startup of a vehicle from ⁇ 20° C.
- FIG. 6 is a plot of fuel cell stack voltage as a function of time during a startup from ⁇ 20° C. with the pressure of the reactant gases equal to the pressure of the water in the water channels.
- FIG. 7 is a plot of fuel cell stack voltage as a function of time during a startup from ⁇ 20° C. with the pressure of the reactant gases pressurized relative to the pressure of the water in the water channels.
- the invention may be used with a wide variety of fuel cell stacks, having fuel cells of various configurations.
- FIG. 1 there is shown a cross sectional view of a typical fuel cell 12 , which includes a membrane electrode assembly (MEA) 16 , an anode support plate 17 and a cathode support plate 19 .
- the MEA 16 comprises a polymer electrolyte membrane (“PEM”) 70 , an anode catalyst 72 and a cathode catalyst 74 .
- PEM polymer electrolyte membrane
- the anode catalyst 72 and the cathode catalyst 74 are secured on opposite sides of the PEM 70 .
- the fuel cell 12 also includes a hydrophobic cathode diffusion layer 78 and a hydrophilic cathode substrate layer 82 , which allow the oxidant reactant gas passing through a passageway 92 in a water transport plate 86 to reach the cathode catalyst 74 .
- the cathode diffusion layer 78 is adjacent to a side of the cathode catalyst 74
- the cathode substrate layer 82 is adjacent to the cathode diffusion layer 78 opposite the cathode catalyst 74 .
- the hydrophobic cathode diffusion layer 78 and the hydrophilic cathode substrate layer 82 also allow the product water, which forms in the cathode catalyst 74 , to migrate toward the water transport plate 86 .
- the diffusion layers 76 , 78 are applied to both the anode and cathode substrate layers 80 , 82 , within the anode support plate 17 and cathode support plate 19 , by procedures well known in the art.
- the diffusion layers 76 , 78 are typically constructed of porous conductive layers that are rendered hydrophobic or partially hydrophobic by means of a hydrophobic polymer.
- the anode water transport plate 84 is adjacent to the anode support plate 17
- the cathode water transparent plate 86 is adjacent to the cathode support plate 19 .
- the anode and cathode water transport plates 84 , 86 may be structured and/or oriented to cooperate with adjacent water transport plates 88 , 89 such that the passageways 96 and 98 simultaneously serve as the coolant stream for both the anode of one cell and cathode of the next cell.
- the water transport plates 84 , 86 , 88 , 89 are typically porous graphite having a mean pore size of approximately two (2) to three (3) microns and a porosity of about 35% to 40%. It is preferable to make the water transport plates 84 , 86 , 88 , 89 hydrophilic by treating them with tin oxide (SnO 2 ) such as described in U.S. Pat. No. 5,840,414, which is owned by the assignee of the present invention and hereby incorporated by reference.
- tin oxide SnO 2
- the hydrophilic porous nature of the cathode water transport plate 86 in conjunction with a negative pressure differential between the coolant and oxidant reactant gas streams, ensures proper removal of the product water formed at the cathode. Specifically, the water flows from the cathode support plate 19 , through the water transport plate 86 and into the coolant passageways 98 . Also, the anode water transport plate 84 furnishes the anode support plate 17 with a continuous supply of water, which eventually reaches the PEM and prevents it from becoming dry.
- FIG. 3 there is shown another alternative embodiment 12 b of a fuel cell which may be used with the present invention and has interdigitated reactant passageways 110 , 112 within the substrate layers 100 ′, 102 ′ rather then in the water transport plates 138 , 140 , thereby allowing the reactant gas streams to pass directly into and through the substrate layer in lieu of first entering channels in the water transport plates.
- the substrate layers 100 ′, 102 ′ are oriented such that the passageways 110 , 112 are adjacent to flat porous water transport plates 138 , 140 , respectively.
- the water transport plates 138 , 140 are flat on the side adjacent the anode and cathode support layers 17 ′, 19 ′.
- the opposite side of the water transport plates 138 , 140 have coolant passageways 134 .
- the water transport plates 138 , 140 are still porous and allow water to pass therethrough.
- the water transport plates include additional grooves on their opposite side. When this opposite side abuts another water transport plate, or any other plate, these grooves serve as passageways for the coolant stream to pass therethrough. Additionally, when the anode and cathode water transport plates abut each other and these grooves align, these grooves jointly create a single coolant stream passageway that serves as water transport plates for both the anode of one cell and the cathode of an adjacent cell.
- FIG. 2 there is shown an alternative embodiment of a fuel cell 12 ′′.
- the fuel cell 12 ′′ in FIG. 2 differs from the fuel cell 12 in FIG. 1, in that the anode support plate 17 ′′ of FIG. 2 includes a hydrophilic substrate layer 108 but does not include a diffusion layer. Not using a diffusion layer on the anode support plate further increases the performance capability of the fuel cell by removing all hydrophobic or partially hydrophobic barriers to the transport of liquid water from the anode water transport plate 84 to the anode catalyst 72 .
- the fuel cell stack may have solid separator plates between fuel cells, which would appear between the anode water transport plates 84 , 89 and the cathode water transport plates 86 , 88 .
- each cell may be separated from an adjacent cell by solid separator plates having coolant channels therein, as shown in U.S. patent application Ser. No. 10/036,181, filed Dec. 28, 2001.
- the coolant channels would similarly be placed, at every fourth or so fuel cell, between an anode water transport plate, such as one of the plates 84 , 89 , and a cathode water transport plate, such as one of the transport plates 86 , 88 , of an adjacent fuel cell.
- the invention may also be used with passive water management fuel cells, such as that disclosed in the aforementioned application Ser. No. 10/036,181, in which the ends of water channels adjacent to corresponding reactant gas inlet manifolds are dead ended, and the other ends of the water channels drain excess water into a related reactant gas exhaust manifold.
- a vehicle 150 includes a fuel cell stack 151 comprising a plurality of contiguous fuel cells, only one fuel cell 12 being shown in FIG. 4.
- the electrical output at the positive and negative terminals of the fuel cell stack 151 is connected by a pair of lines 155 , 156 through a switch 158 to a vehicle propulsion system 159 .
- the output is also connected through a switch 160 to an auxiliary heater 161 in a reservoir 164 of a water circulation system, the reservoir having a vent 165 .
- a controller 185 responds to load current determined by a current detector 186 as well as to the voltage across the lines 155 , 156 ; it may also have temperature of the stack provided on a line 187 .
- the controller in turn, can control the valve 180 over a line 190 as well as controlling the other valves, the switches 158 , 160 and the pumps 174 , 170 , as shown in FIG. 4.
- the controller 185 responds to start and speed control signals from the vehicle propulsion system 159 on lines 193 and 194 , which will indicate when the fuel cell should commence operation, and the amount of power being demanded by the vehicle propulsion system.
- the vehicle propulsion system will be started up in a condition in which at least a portion of the fuel cell stack 151 is below the freezing temperature of water.
- the entire vehicle may be in an ambient environment which is below the freezing temperature of water.
- the fuel cell stack has had substantially all of the water in the porous support plates and the reactant gas flow fields removed, which may be achieved in accordance with a procedure disclosed in U.S. patent application Ser. No. 09/826,739, filed Apr. 5, 2001.
- the water transport plates themselves can hold ice within the pores without doing damage to the water transport plates, as in the case for the PEM.
- the substrates 80 , 82 (FIG. 1) are sufficiently close to the PEM itself (as seen in FIG. 1) that the temperature of the water that is transferred into the substrates will very nearly follow the temperature of the PEM, rather than the temperature of the reactant gases themselves.
- product water will not freeze in the substrates due to the transient heat transfer characteristic of the cell; this is an important aspect of the present invention.
- valves 180 , 181 and the pump 183 will be operated appropriately so as to provide fuel reactant gas to the flow fields of the anode 17 , and the valve 175 and pump 174 will be operated appropriately to provide ambient air to the flow fields of the cathode 19 .
- a stochiometric amount of hydrogen based on current density during startup will be provided to the anode 17 .
- at least twice the stochiometric amount of air is provided initially; preferably an amount up to about five times stochiometric requirement of air is initially provided. The more air that is provided, the more uniform will be the current distribution among the various cells of the stack 151 .
- the controller 185 closes the switch 158 so as to connect the fuel cell stack 151 to the vehicle propulsion system 159 .
- the controller 185 may also close the switch 160 at the same time so as to connect the fuel cell stack to an auxiliary load, which may comprise the heater 161 in the reservoir 164 , which will start to melt some of the ice in the reservoir 164 .
- the heat generated by the stack is absorbed as the heat of fusion of the ice within the stack, substantially all of which is in the water transport plates.
- the water transport plates may have as much as 10 or 20 times more ice than the PEM, causing the temperature of the stack to remain at 0° C. out to nearly three minutes after startup, depending upon the power level.
- heat generated by the operation of the stack is absorbed as sensible heat by the materials of the stack and the water inside the stack. Then, beginning at a point indicated as 203 in FIG. 5, evaporative cooling begins to occur.
- the air and hydrogen that are being brought into the stack may be very cold, and even below the freezing temperature of water, as the gases pass through the flow fields, they rapidly warm up and after about three and one-half minutes (at point 203 ), will begin evaporating product water and water in the water transport plates into the gas streams, thereby providing cooling to the cell stack.
- the stack is relying on air cooling and primarily evaporative cooling to prevent excessive local heating at any point within the stack.
- the vehicle was placed in condition for operation within about 15 seconds of starting the flow of reactant gases into the stack; the stack was maintained in a sufficiently cool state by the heat of fusion of the melting ice in the water transport plates, by the heating up of the mass of the stack and water, and by the evaporation of water from the water transport plates into the reactant gases, particularly the oxidant reactant gas (air).
- the coolant water is managed through porous water transport plates, and by recirculating the water through a restriction, the water is caused to be at between 7 and 21 kPa (1 to 3 psi) below the pressure of the reactants, which are typically at atmospheric pressure. This ensures that water will not pool in the reactant gas channels, that the water is forced into the water channels, and that the hydrophilic substrates will not be flooded and will have sufficient open porosity to permit reactant diffusion.
- pressurization of the oxidant can be achieved by the controller closing the valve 175 to restrict flow below that which occurs during normal operation
- pressurization of the fuel reactant gas can be achieved by the controller balancing the settings of the valves 180 , 181 suitably so that the pressure of the fuel reactant gas will be above the pressure in the water channels by at least 4 kPa (0.6 psi) and preferably 4 to 21 kPa (0.6-3 psi).
- the pressure differentials are measured between the reactant exhaust manifolds and the water inlet manifold of the stack.
- FIGS. 6 and 7 show voltage as a function of time following a startup at ⁇ 20° C. ( ⁇ 4° F.) with a current density of 300 mA/cm 2 .
- both reactant gases are at ambient pressure. And there is no pressure differential between the gases in the reactant channels and the water in the water channels.
- the voltage per cell is relatively constant, after about 120 seconds (2 minutes) then rises slightly beginning at 480 seconds (8 minutes) and begins to decline and have a negative slope versus time at about 540 seconds (9 minutes).
- the decline in voltage at 540 seconds (9 minutes) is indicative of the time when, because of flooding, the air cannot reach the cathode catalyst, and so the performance is reduced. This indicates that with no pressure differential between reactants and water, this particular cell of a fuel cell stack operated perfectly well for about 9 minutes.
- the voltage is substantially constant until about 800 seconds (13 minutes and 20 seconds) as a result of the reactant gases operating at a pressure of 4 kPa (0.6 psi) above the pressure of the water in the water channels.
- the time that the fuel cell can operate without reduced performance with no active water management and no circulating coolant water is extended from 9 minutes to 13 minutes and 20 seconds, which is a four minute and 20 seconds improvement, an improvement of about 32%.
- the invention may be utilized when a water transport plate is adjacent to only one of the support plates; the invention may be utilized with reactant to water pressure differentials higher than and slightly lower than 4 kPa (0.6 psi); and the invention may be used in fuel cells powering electric vehicles or other loads, which fuel cells have a variety of different configurations.
- the various aspects of the present invention may be used to advantage, where appropriate, singly or in combination with less than all of the aspects of the invention, and may be used to power loads which are selected from a propulsion system of an electric vehicle or other electric loads.
- the auxiliary load 161 may be selected to draw between about 20% and about 40% of rated power of the stack. “Rated power” is the maximum average power output of a device. If desired, auxiliary loads not within the reservoir 164 may be utilized, such as on or in conduits, within the stack itself, or otherwise. However, it is useful to utilize all of the generated power in a manner that enhances the ability to start the vehicle (or other ultimate load) within seconds of reactant gas flow, and drive it without endangering the fuel cell by overheating, until such time as coolant circulation may begin.
- both of the support plates are at least partially hydrophilic, and possibly totally hydrophilic.
- the invention can also be practiced where the support plates are partially or totally hydrophobic in order to enhance gas flow therethrough.
- One aspect of the invention is being able to start the vehicle almost immediately upon introduction of reactant gases while being able to utilize reactant gases which may be below the freezing temperature of water.
- the invention may also be practiced while utilizing reactant gases above the freezing temperature of water, even though at least some part of the stack may be below the freezing temperature of water.
- the invention has a principal value in serving electric vehicles powered by PEM fuel cell stack assemblies; however, it obviously can be used with loads other than vehicle propulsion systems.
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Abstract
Description
- This is a continuation-in-part of U.S. patent application Ser. No. 10/187,547 filed on Jul. 1, 2002.
- This invention relates to rapidly initiating operation of a fuel cell powered electric vehicle, at sub-freezing temperature, by means of one or more of: providing excess reactant gas or cold reactant gas to the proton exchange membrane (PEM) fuel cell stack which powers the load, such as a vehicle propulsion system, connecting the load to the stack within 20 seconds of reactant gas flow or when open circuit voltage is detected, previously draining hydrophilic support plates, connecting coolant only after several minutes or when sufficient water has melted.
- It is generally agreed that one difficulty with utilizing fuel cells to power the propulsion system of electric vehicles is the requirement that such vehicles be operable at temperatures below that at which water will freeze. Freezing provides potential mechanical damage as a consequence of the expansion of ice, and presents problems due to the inseparability of water and the fuel cell processes. Heretofore, various methods of initiating operation of a fuel cell, preparatory to the operation of an electric vehicle, have concentrated on providing heat, either by reaction or combustion of fuel, or by means of battery power, to various water and other coolant conduits and reservoirs. Other efforts are directed toward processes designed to accelerate the rate at which a fuel cell stack will heat up to above-freezing temperatures, as a consequence of its own operation. In U.S. Pat. No. 5,798,186, the fuel cell is warmed up simply by connecting a load across it while stochiometric fuel and oxidant are supplied to the stack. In one experiment, with the fuel, oxidant and coolant water passages all having been purged of water upon previous shutdown of the stack, application of hydrogen and air at room temperature caused a temperature in the core of a ten cell stack to advance from −11° C. to 0° C. in about one minute. A four cell stack, in which only the reactant channels (and not the coolant channel) were purged upon previous shut down, required five minutes, after circulation of hydrogen and oxygen began and a 50 amp load was connected, to increase from −19° C. to 0° C. Coolant was not circulated until about 23 minutes after startup. In a four cell stack in which none of the channels were purged at the prior shut down, flow of warm hydrogen did not begin to occur until after four minutes, and 12 minutes expired between startup at −23° C. and reaching 0° C. within the core of a four cell stack. In U.S. Pat. No. 6,329,089, individual fuel cells at −5° C. started with room temperature hydrogen and air reached 0.5 amps per cm2 in five minutes. With a short circuit load, a seven cell stack with a core temperature of −15° C. reached 0.5 amps per cm2 nine minutes after prolonged short circuiting of the stack output. Performance of other experiments were less satisfactory.
- For use in vehicles, such as automobiles, an electric propulsion system must be operating in less than one minute, preferably less than one-half minute, after initiating startup. None of the foregoing are capable of providing fuel cells operable in subfreezing temperatures, particularly as low as −40° C. (−40° F.).
- Objects of the invention include: operating, at subfreezing temperature, an electric vehicle powered by a fuel cell within seconds of initiation; improved initiation of fuel cell powered, electric vehicle operation at subfreezing temperature; initiating fuel cell powered electric vehicle operation at subfreezing temperature with a minimal of waste power used for raising the temperature of apparatus and/or fluids; avoiding the need for heat exchangers and other apparatus to heat reactants or coolants above freezing; and avoiding use of battery power to start a fuel cell for powering a vehicle.
- This invention is predicated on the discovery that the propulsion system of an electric vehicle powered by a PEM fuel cell can be powered from the fuel cell while the fuel cell stack is frozen.
- This invention is further predicated on the discovery that contrary to belief of the prior art, excess reactants, rather than reactant starvation, will permit extended operation of the fuel cell stack pending the ability to flow water through the stack. The invention is further predicated on the discovery that high flow of cold reactant gases through the reactant flow fields is not sufficient to cause freezing of product water, the heat generated in the membrane electrode assembly being sufficient, and sufficiently close to the reactant flow fields, to prevent freezing of product water or refreezing of melted water.
- The invention is also predicated on the discovery that fuel cell operation without loss of performance or damage to the cells can be extended during a frozen startup by providing at least one of the reactant gases at a pressure in excess of the pressure of any water in the stack, which before operation of a water circulation system is typically atmospheric.
- According to the present invention, a PEM fuel cell stack at subfreezing temperature is connected to a vehicle propulsion system or other electric load within a few seconds or as soon as the stack provides open circuit voltage. According to the invention, the fuel cell stack is started with more than a stochiometric flow of fuel and at least stochiometric flow, but preferably two-five times stochiometric flow of oxidant, which may be at subfreezing temperatures, or not, whereby to prolong operation without localized heating, thereby permitting the vehicle (or other load) to be used during the time that the apparatus and fluids are being heated to suitable, operational temperatures. The invention not only permits, but prefers operation with reactants which are at the same subfreezing ambient temperature as the fuel cell stack itself, contrary to usage of the prior art, since this prolongs the onset of localized overheating.
- In further accord with the invention, in systems in which porous water transport plates are used for water management, heating of the water stored as ice in the pores of the water transport plates, by heating up the mass of the stack as well as the water, the heat of fusion as the ice melts, and evaporative cooling of some of that water, further prolongs the period of time at which the vehicle can be operated with power from the fuel cell stack, without circulating coolant, before there is impermissible local heating within the fuel cell.
- In accordance further with the invention, at least one of the reactant gases is provided to the fuel cell stack at a pressure of at least about 4 kPa (0.6 psi) above the pressure of any water in the water channels, which typically will be about atmospheric pressure. This prevents liquid water from pooling in the reactant channels, and flooding the electrode substrates, which is particularly important in the oxidant gas reactant channel where product water can accumulate.
- Principal aspects of the present invention include starting an electric load, such as a vehicle or other load, the fuel cell stack of which is at subfreezing temperatures, before awaiting for the fuel cell stack to reach normal operating temperature, by supplying the fuel cell with at least twice stochiometric quantities of oxidant, and using substantially empty hydrophilic support plates for temporary product water storage thereby to allow the fuel cell to operate without circulating coolant until such time as all of the water systems are functional.
- 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.
- FIG. 1 is a simplified, sectioned side elevation of slightly more than one fuel cell which may be part of a stack with which the present invention may be practiced.
- FIG. 2 is a simplified, sectioned side elevation of an alternative to that shown in FIG. 1.
- FIG. 3 is a simplified, sectioned side elevation of another alternative to that shown in FIG. 1.
- FIG. 4 is a schematic illustration of a vehicle engine propulsion system, including a fuel cell stack by which it is powered, which may practice the present invention.
- FIG. 5 is a plot of fuel cell stack voltage and temperature as a function of time, during a startup of a vehicle from −20° C.
- FIG. 6 is a plot of fuel cell stack voltage as a function of time during a startup from −20° C. with the pressure of the reactant gases equal to the pressure of the water in the water channels.
- FIG. 7 is a plot of fuel cell stack voltage as a function of time during a startup from −20° C. with the pressure of the reactant gases pressurized relative to the pressure of the water in the water channels.
- The invention may be used with a wide variety of fuel cell stacks, having fuel cells of various configurations.
- Referring to FIG. 1, there is shown a cross sectional view of a
typical fuel cell 12, which includes a membrane electrode assembly (MEA) 16, ananode support plate 17 and acathode support plate 19. TheMEA 16 comprises a polymer electrolyte membrane (“PEM”) 70, ananode catalyst 72 and acathode catalyst 74. Theanode catalyst 72 and thecathode catalyst 74 are secured on opposite sides of the PEM 70. - The
anode support plate 17 andcathode support plate 19 may includehydrophobic diffusion layers hydrophilic substrate layers anode diffusion layer 76 is adjacent to a side of theanode catalyst 72, and theanode substrate layer 80 is adjacent to theanode diffusion layer 76 opposite theanode catalyst 72. Theanode diffusion layer 76 and the hydrophilicanode substrate layer 80 allow the fuel reactant gas, which passes through apassageway 94 in awater transport plate 84, and the water, which passes through apassageway 96, to reach theanode catalyst 72. In the general case, the water passageways may be adjacent to the plate that provides the reactant gas passages. Thefuel cell 12 also includes a hydrophobiccathode diffusion layer 78 and a hydrophiliccathode substrate layer 82, which allow the oxidant reactant gas passing through apassageway 92 in awater transport plate 86 to reach thecathode catalyst 74. Thecathode diffusion layer 78 is adjacent to a side of thecathode catalyst 74, and thecathode substrate layer 82 is adjacent to thecathode diffusion layer 78 opposite thecathode catalyst 74. The hydrophobiccathode diffusion layer 78 and the hydrophiliccathode substrate layer 82 also allow the product water, which forms in thecathode catalyst 74, to migrate toward thewater transport plate 86. - The
diffusion layers cathode substrate layers anode support plate 17 andcathode support plate 19, by procedures well known in the art. - One procedure is described in U.S. Pat. No. 4,233,181. The
diffusion layers - As shown in FIG. 1, the anode
water transport plate 84 is adjacent to theanode support plate 17, and the cathode watertransparent plate 86 is adjacent to thecathode support plate 19. The anode and cathodewater transport plates water transport plates passageways - The
water transport plates water transport plates - The hydrophilic porous nature of the cathode
water transport plate 86, in conjunction with a negative pressure differential between the coolant and oxidant reactant gas streams, ensures proper removal of the product water formed at the cathode. Specifically, the water flows from thecathode support plate 19, through thewater transport plate 86 and into thecoolant passageways 98. Also, the anodewater transport plate 84 furnishes theanode support plate 17 with a continuous supply of water, which eventually reaches the PEM and prevents it from becoming dry. - Referring to FIG. 3, there is shown another
alternative embodiment 12 b of a fuel cell which may be used with the present invention and has interdigitatedreactant passageways water transport plates passageways water transport plates interdigitated passageways water transport plates water transport plates water transport plates coolant passageways 134. Additionally, thewater transport plates passageways - Referring to FIG. 2, there is shown an alternative embodiment of a
fuel cell 12″. Thefuel cell 12″ in FIG. 2 differs from thefuel cell 12 in FIG. 1, in that theanode support plate 17″ of FIG. 2 includes ahydrophilic substrate layer 108 but does not include a diffusion layer. Not using a diffusion layer on the anode support plate further increases the performance capability of the fuel cell by removing all hydrophobic or partially hydrophobic barriers to the transport of liquid water from the anodewater transport plate 84 to theanode catalyst 72. - The fuel cell stack may have solid separator plates between fuel cells, which would appear between the anode
water transport plates water transport plates plates transport plates - There are additional configurations disclosed in U.S. patent application Ser. No. 09/733,133, filed Dec. 8, 2000, with which the invention may be used.
- The invention may also be used with passive water management fuel cells, such as that disclosed in the aforementioned application Ser. No. 10/036,181, in which the ends of water channels adjacent to corresponding reactant gas inlet manifolds are dead ended, and the other ends of the water channels drain excess water into a related reactant gas exhaust manifold.
- In addition, other types of fuel cells, not employing water transport plates of any sort, which rely on external humidification of the reactant gases prior to entry into the flow fields of the fuel cell stack, and which rely on carrying product water from the stack by means of the oxidant reactant gas flow, may also take advantage of aspects of the present invention. Examples of this type of fuel cell are U.S. Pat. No. 6,117,577 to Wilson, as well as U.S. Pat. Nos. 5,366,818 and 5,773,160, to Wilkinson et al.
- Referring now to FIG. 4, a
vehicle 150 includes afuel cell stack 151 comprising a plurality of contiguous fuel cells, only onefuel cell 12 being shown in FIG. 4. The electrical output at the positive and negative terminals of thefuel cell stack 151 is connected by a pair oflines switch 158 to avehicle propulsion system 159. The output is also connected through aswitch 160 to anauxiliary heater 161 in areservoir 164 of a water circulation system, the reservoir having avent 165. The water circulation system may include atrim valve 166, water passages, such as those withinwater transport plates fan water pump 170. Ambient air at aninlet 173 is provided by a pump, such as ablower 174, to the oxidant reactant gas flow fields of thecathode 19, and thence through apressure regulating valve 175 toexhaust 176. Hydrogen is supplied from asource 179 through aflow regulating valve 180 to the fuel reactant gas flow fields of theanode 17, and thence through apressure regulating valve 181 toexhaust 182. A fuel recycle loop includes apump 183. - A
controller 185 responds to load current determined by acurrent detector 186 as well as to the voltage across thelines line 187. The controller, in turn, can control thevalve 180 over aline 190 as well as controlling the other valves, theswitches pumps - The
controller 185 responds to start and speed control signals from thevehicle propulsion system 159 onlines - It is assumed that the vehicle propulsion system will be started up in a condition in which at least a portion of the
fuel cell stack 151 is below the freezing temperature of water. When that is the case, the entire vehicle may be in an ambient environment which is below the freezing temperature of water. It is further assumed, for the explanation that follows, that the fuel cell stack has had substantially all of the water in the porous support plates and the reactant gas flow fields removed, which may be achieved in accordance with a procedure disclosed in U.S. patent application Ser. No. 09/826,739, filed Apr. 5, 2001. Thus, there will likely be ice in the PEM, in the anode and cathode catalyst layers, as well as within the pores of the water transport plates. However, it should be borne in mind that the water transport plates themselves can hold ice within the pores without doing damage to the water transport plates, as in the case for the PEM. Furthermore, thesubstrates 80, 82 (FIG. 1) are sufficiently close to the PEM itself (as seen in FIG. 1) that the temperature of the water that is transferred into the substrates will very nearly follow the temperature of the PEM, rather than the temperature of the reactant gases themselves. Thus, even though the water transport plates have ice in the pores thereof, product water will not freeze in the substrates due to the transient heat transfer characteristic of the cell; this is an important aspect of the present invention. - According to the invention, whenever a start signal is sent from the
vehicle propulsion system 159 over theline 193 to thecontroller 185, signals from the controller will cause thevalves pump 183 to be operated appropriately so as to provide fuel reactant gas to the flow fields of theanode 17, and thevalve 175 and pump 174 will be operated appropriately to provide ambient air to the flow fields of thecathode 19. Initially, more than a stochiometric amount of hydrogen based on current density during startup will be provided to theanode 17. Similarly, at least twice the stochiometric amount of air is provided initially; preferably an amount up to about five times stochiometric requirement of air is initially provided. The more air that is provided, the more uniform will be the current distribution among the various cells of thestack 151. - When fuel and air of sufficient quantity have been provided uniformly to the cells, open circuit voltage will be detected on the
lines controller 185. At that time, which is illustrated atpoint 197 in FIG. 5, the controller closes theswitch 158 so as to connect thefuel cell stack 151 to thevehicle propulsion system 159. Optionally, thecontroller 185 may also close theswitch 160 at the same time so as to connect the fuel cell stack to an auxiliary load, which may comprise theheater 161 in thereservoir 164, which will start to melt some of the ice in thereservoir 164. With the fuel cell stack providing power to the loads, heat produced by the reactions within the fuel cell stack causes the fuel cell stack materials and the ice to begin warm up, in the period denoted 198 in FIG. 5. - When the temperature of the stack reaches about 0° C. (32° F.) in the range of times indicated as199 in FIG. 5, the heat generated by the stack is absorbed as the heat of fusion of the ice within the stack, substantially all of which is in the water transport plates. In fact, the water transport plates may have as much as 10 or 20 times more ice than the PEM, causing the temperature of the stack to remain at 0° C. out to nearly three minutes after startup, depending upon the power level. After about two and one-half minutes, at a point indicated as 202 in FIG. 5, heat generated by the operation of the stack is absorbed as sensible heat by the materials of the stack and the water inside the stack. Then, beginning at a point indicated as 203 in FIG. 5, evaporative cooling begins to occur.
- Although the air and hydrogen that are being brought into the stack may be very cold, and even below the freezing temperature of water, as the gases pass through the flow fields, they rapidly warm up and after about three and one-half minutes (at point203), will begin evaporating product water and water in the water transport plates into the gas streams, thereby providing cooling to the cell stack. During the entire initial time that the cell stack is operating, up to about seven and one-half minutes in FIG. 5, at the point identified as 204, the stack is relying on air cooling and primarily evaporative cooling to prevent excessive local heating at any point within the stack. At
point 204, about seven and one-half minutes after initiating stack operation, there typically will be sufficient liquid water in thereservoir 164 to fill the water circulating system, from the tank through thevalve 166, through the water transport plates, through theradiator 168, thepump 170 and back to thereservoir 164, can all be filled with water. Although there may not be sufficient water to replace all of the ice in thereservoir 164 at this time, that is not, however, necessary. Therefore, circulation of water flowing within the water circulation system, including the water passages in the cell stack, can begin atpoint 204. Thereafter, liquid cooling of the cell stack, as in normal cell stack operation, will take place. - According to the invention, it is important to note that the vehicle was placed in condition for operation within about15 seconds of starting the flow of reactant gases into the stack; the stack was maintained in a sufficiently cool state by the heat of fusion of the melting ice in the water transport plates, by the heating up of the mass of the stack and water, and by the evaporation of water from the water transport plates into the reactant gases, particularly the oxidant reactant gas (air).
- In one known type of PEM fuel cells, the coolant water is managed through porous water transport plates, and by recirculating the water through a restriction, the water is caused to be at between 7 and 21 kPa (1 to 3 psi) below the pressure of the reactants, which are typically at atmospheric pressure. This ensures that water will not pool in the reactant gas channels, that the water is forced into the water channels, and that the hydrophilic substrates will not be flooded and will have sufficient open porosity to permit reactant diffusion. However, when freezing temperatures are encountered, the water in the reactant channels, coolant channels, water pump and other conduits of the water circulatory system is drained upon shutdown of the fuel cell system; upon startup, there is no circulating water so there is no way to maintain negative pressure in the water channels. Therefore, coolant can build up in the reactant channels. According to another aspect of the invention, water buildup in the reactant flow fields prior to the operation of the water circulation system is avoided by pressurizing the reactant flow fields in the initial phases of startup.
- In FIG. 4, pressurization of the oxidant can be achieved by the controller closing the
valve 175 to restrict flow below that which occurs during normal operation, and pressurization of the fuel reactant gas can be achieved by the controller balancing the settings of thevalves - FIGS. 6 and 7 show voltage as a function of time following a startup at −20° C. (−4° F.) with a current density of 300 mA/cm2. In FIG. 6, both reactant gases are at ambient pressure. And there is no pressure differential between the gases in the reactant channels and the water in the water channels. Referring to FIG. 6, the voltage per cell is relatively constant, after about 120 seconds (2 minutes) then rises slightly beginning at 480 seconds (8 minutes) and begins to decline and have a negative slope versus time at about 540 seconds (9 minutes). The decline in voltage at 540 seconds (9 minutes) is indicative of the time when, because of flooding, the air cannot reach the cathode catalyst, and so the performance is reduced. This indicates that with no pressure differential between reactants and water, this particular cell of a fuel cell stack operated perfectly well for about 9 minutes.
- In FIG. 7, after approximately two minutes, the voltage is substantially constant until about 800 seconds (13 minutes and 20 seconds) as a result of the reactant gases operating at a pressure of 4 kPa (0.6 psi) above the pressure of the water in the water channels.
- Thus, according to the invention, by operating the fuel cells with the reactant gas pressure sufficient to assist water in passing through the porous plates adjacent to the support plates, the time that the fuel cell can operate without reduced performance with no active water management and no circulating coolant water, is extended from 9 minutes to 13 minutes and 20 seconds, which is a four minute and 20 seconds improvement, an improvement of about 32%. The invention may be utilized when a water transport plate is adjacent to only one of the support plates; the invention may be utilized with reactant to water pressure differentials higher than and slightly lower than 4 kPa (0.6 psi); and the invention may be used in fuel cells powering electric vehicles or other loads, which fuel cells have a variety of different configurations.
- After a few minutes of initiating operation, when the
controller 185 senses a reduction in voltage across the fuelcell output lines pump 170 and adjust thevalve 166 to have a sufficient restriction to operate the coolant water at a pressure of 4-21 kPa (0.6-3 psi) below the reactants. - The various aspects of the present invention may be used to advantage, where appropriate, singly or in combination with less than all of the aspects of the invention, and may be used to power loads which are selected from a propulsion system of an electric vehicle or other electric loads.
- There is a difference between the voltage characteristic in FIG. 6, in which the voltage begins a negative slope at about 9 minutes, and the voltage depicted in FIG. 5 which begins to have a negative slope at about just over 7 minutes. This is due to the fact that the load on the cell when the data of FIG. 5 were obtained was higher than when the data of FIG. 6 were obtained.
- In a typical situation, the
auxiliary load 161 may be selected to draw between about 20% and about 40% of rated power of the stack. “Rated power” is the maximum average power output of a device. If desired, auxiliary loads not within thereservoir 164 may be utilized, such as on or in conduits, within the stack itself, or otherwise. However, it is useful to utilize all of the generated power in a manner that enhances the ability to start the vehicle (or other ultimate load) within seconds of reactant gas flow, and drive it without endangering the fuel cell by overheating, until such time as coolant circulation may begin. - Although the example herein is illustrated with fuel cells having water transport plates adjacent both the anode the cathode, the invention may be practiced with water transport plates adjacent only one of the electrodes. In the example herein, both of the support plates are at least partially hydrophilic, and possibly totally hydrophilic. On the other hand, the invention can also be practiced where the support plates are partially or totally hydrophobic in order to enhance gas flow therethrough. One aspect of the invention is being able to start the vehicle almost immediately upon introduction of reactant gases while being able to utilize reactant gases which may be below the freezing temperature of water. However, the invention may also be practiced while utilizing reactant gases above the freezing temperature of water, even though at least some part of the stack may be below the freezing temperature of water.
- The invention has a principal value in serving electric vehicles powered by PEM fuel cell stack assemblies; however, it obviously can be used with loads other than vehicle propulsion systems.
- All of the aforementioned patents and patent applications are incorporated herein by reference.
- 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.
Claims (23)
Priority Applications (5)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US10/390,439 US6673481B1 (en) | 2002-07-01 | 2003-03-17 | Initiating operation of an electric vehicle or other load powered by a fuel cell at sub-freezing temperature |
PCT/US2003/020368 WO2004004047A1 (en) | 2002-07-01 | 2003-06-26 | Initiating operation of a fuel cell powered load at sub/freezing temperature |
JP2004517997A JP2006513528A (en) | 2002-07-01 | 2003-06-26 | Start-up operation under freezing of fuel cell-powered electric vehicles or other loads |
DE10392884T DE10392884T5 (en) | 2002-07-01 | 2003-06-26 | Initiate the operation of a fuel cell powered electric vehicle or other load at sub-freezing temperatures |
AU2003251632A AU2003251632A1 (en) | 2002-07-01 | 2003-06-26 | Initiating operation of a fuel cell powered load at sub/freezing temperature |
Applications Claiming Priority (2)
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US18754702A | 2002-07-01 | 2002-07-01 | |
US10/390,439 US6673481B1 (en) | 2002-07-01 | 2003-03-17 | Initiating operation of an electric vehicle or other load powered by a fuel cell at sub-freezing temperature |
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US18754702A Continuation-In-Part | 2002-07-01 | 2002-07-01 |
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US (1) | US6673481B1 (en) |
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US20070243428A1 (en) * | 2005-11-29 | 2007-10-18 | Richards Christopher J | Method of commencing operation of an electrochemical fuel cell stack from freeze-start conditions |
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- 2003-03-17 US US10/390,439 patent/US6673481B1/en not_active Expired - Lifetime
- 2003-06-26 WO PCT/US2003/020368 patent/WO2004004047A1/en active Application Filing
- 2003-06-26 JP JP2004517997A patent/JP2006513528A/en active Pending
- 2003-06-26 DE DE10392884T patent/DE10392884T5/en not_active Withdrawn
- 2003-06-26 AU AU2003251632A patent/AU2003251632A1/en not_active Abandoned
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Also Published As
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AU2003251632A1 (en) | 2004-01-19 |
US6673481B1 (en) | 2004-01-06 |
DE10392884T5 (en) | 2005-07-14 |
JP2006513528A (en) | 2006-04-20 |
WO2004004047A1 (en) | 2004-01-08 |
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