Patent Application
20050084732
The present invention relates to fuel cells that are arranged in fuel cell stacks that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fuel cell stack having a current collector that has a low sensible heat compared to an end cell of the stack and an insulator wherein a rate of heat transfer across the insulator is no greater than a rate of heat production by the end cell during start up from subfreezing conditions.
Fuel cells are well-known and are commonly used to produce electrical energy from reducing and oxidizing reactant fluids to power electrical apparatus, such as apparatus on-board space vehicles, transportation vehicles, or as on-site generators for buildings. A plurality of planar fuel cells are typically arranged into a cell stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids as part of a fuel cell power plant. Each individual fuel cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A fuel cell may also include a water transport plate, or a separator plate, as is well known.
The fuel cell stack produces electricity from reducing fluid and process oxidant streams. A reaction portion of the fuel cell stack is formed from a plurality of fuel cells stacked adjacent each other. The plurality of fuel cells includes an end cell at an end of the stack of fuel cells. A pressure plate overlies the current collector and is secured to an opposed pressure plate at an opposed end of the cell stack to apply a compressive load to the stack. Most known pressure plates are made of large, conductive metal materials.
During operation of the fuel cell stack, current flows through and out of the reaction portion of the stack and into a current collector adjacent the end cell. A power take-off secured to the current collector or pressure plate directs the current out of the cell stack to a load, such as a motor.
During a “bootstrap” start up from subfreezing conditions, preferably no auxiliary heated fluids are applied to the fuel cell stack, while a reducing fluid, such as hydrogen, is supplied to the anode electrode, and an oxidant, such as oxygen or air, is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (“PEM”) as the electrolyte, the hydrogen electrochemically reacts at a catalyst surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy. Electricity produced by the fuel cell flows into and through the current collector and a conductive pressure plate.
During such a “bootstrap” start up, the fuel cells that are in a central region of the stack quickly rise in temperature compared to the end cells that are adjacent opposed ends of the stack. The end cells heat up more slowly because heat generated by the end cells is rapidly conducted into and through the current collector and into the large, conductive metallic pressure plate. If a temperature of the end cells is not quickly raised to greater than 0 degrees Celsius (“° C.”), water in the water transport plates will remain frozen thereby preventing removal of product water, which results in the end cells being flooded with fuel cell product water. The flooding of the end cells retards reactant fluids from reaching the catalysts and may result in a negative voltage in the end cells. The negative voltage in the end cells may result in hydrogen gas evolution at the cathode electrode and/or corrosion of carbon support layers of electrodes of the cell. Such occurrences would degrade the performance and long-term stability of the fuel cell stack.
Accordingly, there is a need for a fuel cell stack having an end cell wherein the temperature can be raised to greater than 0° C. as quickly as possible during start up from subfreezing conditions.
The invention is a fuel cell stack having an improved current collector and insulator. The fuel cell stack can be used in a fuel cell power plant (not shown), such as a plant that includes the stack and such other components as for example, a reactant management system, a thermal management system, and a controller, to produce a power plant that can interface with and supply electrical energy to an external load. Such plants and their various components are well known to one skilled in the art. The external load that receives power from the fuel cell may be a transportation or a stationery device, such as a vehicle or a building for example. The fuel cell stack produces electricity from reducing fluid and process oxidant streams, and comprises a plurality of fuel cells stacked adjacent each other to form a reaction portion of the fuel cell stack. The plurality of fuel cells includes an end cell at an end of the stack.
A current collector is secured in electrical communication with the end cell, wherein the current collector has a sensible heat no greater than a sensible heat of the end cell and an electrical resistivity no greater than 100 micro-ohm centimeters. The fuel cell stack also includes an insulator secured adjacent the current collector, wherein a thermal conductivity of the insulator is no greater than 0.500 Watts per meter per degree Kelvin. The stack also includes a pressure plate secured adjacent and overlying the insulator and overlying the end cell. Because of the low sensible heat of the current collector and because of the low thermal conductivity of the insulator, heat does not readily leave the end cell, resulting in a rapid warm up of the end cell during start up in subfreezing conditions.
In one embodiment, the current collector is made from a metal foil. In an alternative embodiment, the current collector may consist of a metal coating. A preferred current collector may be a gold plated layer of tin with a thickness of 0.25-0.50 millimeter (“mm”) and with a sensible heat of approximately 0.13-0.26 times the sensible heat of an end cell.
Preferred insulators may include a closed cell plastic with a thermal conductivity of no greater than 0.010 Watts per meter per degree Kelvin, a silica aerogel with a thermal conductivity of no greater than 0.010 Watts per meter per degree Kelvin, or a silica aerogel within a vacuum insulation panel with a thermal conductivity of no greater than 0.005 Watts per meter per degree Kelvin. Preferred insulators may also have a compressive strength in excess of 350 kilo Pascals.
The invention may utilize a pressure plate made of a metallic, conductive material or made of a non-metallic, non-conductive, reinforced plastic composite.
Accordingly, it is a general purpose of the present invention to provide a fuel cell stack having an improved current collector and insulator that overcomes deficiencies of the prior art.
It is a more specific purpose to provide a fuel cell stack having an improved current collector and insulator that provides a current collector having a low sensible heat and an insulator having a low thermal conductivity so that an end cell of the fuel cell stack heats up rapidly during start up in subfreezing conditions.
These and other purposes and advantages of the present fuel cell stack having an improved current collector and insulator will become more readily apparent when the following description is read in conjunction with the accompanying drawings.
Referring to the drawings in detail, a fuel cell stack having an improved current collector and insulator is shown in
A current collector 30 is secured in electrical communication with the end cell 12. The current collector 30 is dimensioned so that a planar area of the current collector 30 is at least as large as a planar area of the end cell 12 in order to enhance the conduction of electricity between the end cell 12 and the current collector 30. The current collector 30 is secured in electrical communication with a first bus bar 32 and a second bus bar 34. The bus bars 32, 34 may be formed from a conductive material, such as copper, so that a current flowing from the current collector 30 can be directed to the bus bars 32, 34. Additionally, a first power take-off 36 is secured to the first bus bar 32 and a second power take-off 38 is secured to the second bus bar 34. The first and second power take-offs 36, 38 may be formed from conductive material for conducting electricity from the stack 10 to a load (not shown) for performing work. The current collector 30 has a sensible heat that is less than a sensible heat of the end cell 12, and the current collector has an electrical resistivity no greater than 100 micro-ohm centimeters. In other words, the end cell 12 has first sensible heat, and the current collector 30 has a second sensible heat that is less than the first sensible heat.
The stack 10 also includes an insulator 40 secured adjacent the current collector 30 or adjacent at least a portion of the current collector 30. A pressure plate 42, such as a layer of an electrically non-conductive, non-metallic, fiber reinforced composite material is secured to an outer end 41 of the stack 10 adjacent and overlying the insulator 40 and overlying the end cell 12. For purposes herein, the phrase “the pressure plate 42 overlying the end cell 12”, will mean that the pressure plate 42 is dimensioned to have a planar area at least as large as a planar area of the end cell 12. A second current collector, insulator and pressure plate (not shown) would be secured to an opposed second end cell (not shown) of the stack 10. As is known, the pressure plates are secured to each other, such as by tie rods (not shown), to apply a compressive load to the stack 10.
The stack 10 may also include a carbon paper cushion 44 secured adjacent the current collector 30. As is known in the art, the carbon paper cushion 44 is compressible for enhanced conductivity between adjacent surfaces of the stack 10. Furthermore, the stack 10 may include a first gasket 46 secured between the current collector 30 and first reactant manifold 26 and a second gasket 48 secured between the current collector 30 and second reactant manifold 28. The first and second gaskets 46, 48 prohibit movement of fluids out of the manifolds 26, 28.
If the pressure plate 42 is an electrically non-conductive, non-metallic, fiber reinforced composite pressure plate 42, then the current collector 30 may include a first long-side extension 43 and an opposed second long-side extension 45 that extend from a planar surface 47 of the current collector 30 that is co-planar with a contact surface 49 of the end cell 12. The first and second long-side extensions 43, 45 contact the first and second bus bars 32, 34.
As is known, sensible heat of an item is the product of its mass multiplied by its specific heat multiplied by a temperature differential over which it is being heated. Therefore, for example, the sensible heat of one gram of water raised from 0 degrees Celsius (“° C.”) to 20° C. is different than the sensible heat of one gram of concrete raised from 0° C. to 20° C. Thus the lower the sensible heat of the current collector 30, the lower an amount of heat transferred from the end cell 12 to the current collector 30 to raise its temperature. Reducing an amount of heat transferred from the end cell 12 to the current collector 30 leaves more heat in the end cell 12, thereby facilitating a rapid warm up of the end cell 12 during start up in subfreezing conditions.
In
The alternative embodiment of the stack 60 includes a plurality of fuel cells 14′, 16′, 18′ that form a reaction portion 20′ of the stack 60. Also, the stack 60 includes an end cell 12′ secured adjacent a first end 24′ of the reaction portion 20′ of the stack 60.
A current collector 62 is secured in electrical communication with an insulator 40′ and a pressure plate 64. The current collector 62 may wrap around the insulator 40′. In such an embodiment, the current collector 62 would be a uniform piece folded so that a first folded layer 71 of the current collector 62 is secured adjacent a first contact surface 66 of the insulator 40′ and a second folded layer 73 of the current collector 62 is secured adjacent a second contact surface 68 of the insulator 40′.
A preferred total thickness across the current collector 30 of
In the
The stack 60 may also include a first carbon paper cushion 44′ secured between the current collector 62 and end cell 12′ and a second carbon paper cushion 72 secured between the current collector 62 and pressure plate 64. Because the pressure plate 64 is electrically conductive, no long-side extensions of the current collector 62 are necessary.
In the embodiments shown in
Also, the insulator 40, 40′ has a thermal conductivity that is no greater than 0.500 Watts per meter per degree Kelvin, and is secured to the current collector 30, 62 so that a total rate of heat transfer across the insulator from the end cell 12 is no greater than heat generated by the end cell 12. The insulator 40, 40′ may consist of: a) a closed or open cell plastic with a thermal conductivity of no greater than 0.010 Watts per meter per degree Kelvin; b) a silica aerogel with a thermal conductivity of no greater than 0.010 Watts per meter per degree Kelvin; or c) a silica aerogel within a vacuum insulation panel with a thermal conductivity of no greater than 0.005 Watts per meter per degree Kelvin. A preferred thickness of the insulator 40, 40′ is less that 20 mm, and most preferably less than 10 mm.
During operation of the stack 10, a rate of heat transfer into or across the insulator 40, 40′ is less than one-hundred percent (“%”) of the rate of heat generated by the end cell 12 during the first minute of a “bootstrap” start; a preferred rate of heat transfer into the insulator 40, 40′ is less than 50% of the rate of heat generated by the end cell 12; and, a most preferred rate of heat transfer into the insulator 40, 40′ is less than 25% of the rate of heat generated by the end cell 12 during the first minute of such a start up. The rate of heat generated by a single cell during such a start up is about 0.2 watts per square centimeter. The insulator 40, 40′ also preferably has a compressive strength in excess of 350 kilo Pascals.
An exemplary open cell plastic insulation is a product marketed under the trade name “Pyropel MD-50”, made from rigid, lightweight polyimide fiberboards, available from Albany International company of Mansfield, Mass., U.S.A. An exemplary silica aerogel insulator is a product marketed under the trade name of “Aspen Aerogel”, available from Aspen Aerogels, Inc. of Marlborough, Mass., U.S.A. An exemplary silica aerogel within vacuum panels is a product marketed under the trade name of “Barrier Ultra-R”, available from Glacier Bay company of Oakland, Calif., U.S.A.
Exemplary materials for making the non-conductive pressure plate 42 include a glass or fiber reinforced polymer or resin that is compatible with the operating conditions of the fuel cell stack 10. Exemplary fiber reinforced composite materials include products available from the Quantum Composites, Company, of Bay City Mich., U.S.A., distributed under the following trade designations: a) “LYTEX 9063”, 63% glass fiber epoxy SMC; b) “LYTEX 4149”, 55% carbon fiber epoxy SMC; c) “QC8560” glass fiber reinforced vinyl ester resin SMC; and, d) “QC8880” glass fiber reinforced vinyl ester resin SMC.
It is known that during a “bootstrap” start up, the fuel cells 14, 16, 18 that are not in contact with the current collector 30 quickly rise in temperature compared to the end cell 12 of the stack 10. The end cell 12 heats up more slowly because heat generated by the end cell 12 would move rapidly into a prior art current collector and pressure plate (not shown). For example, a common prior art pressure plate is a stainless steel pressure plate with a sensible heat approximately 41 times the sensible heat of a fuel cell. Because of the high sensible heat of the pressure plate and the end cell not heating up rapidly as possible, the end cell may be flooded with product water and frozen product water in sub-freezing ambient conditions. The flooding of the end cell may result in a negative voltage in the end cells and may degrade the performance and long-term stability of the fuel cell stack.
In solving the problem of heat loss by the end cells 12, 12′ the inventors contrasted various materials for minimal thickness of the current collectors 30, 62 for an exemplary fuel cell (not shown) at specific operating conditions. Although various materials can be used as current collectors, a tin current collector coated with gold is the preferred material because gold maintains a low electrical resistivity between the current collector 30 and carbon paper cushion 44 and because tin forms a virtually insoluble tin oxide in a PEM cell and is easily fabricated.
To raise the temperature of the end cell to 0° C. as quickly as possible and in less than 60 seconds, it is apparent that the 0.50 mm tin current collector with the 8.0 mm insulator and the 30.0 mm stainless steel pressure plate of line 74 achieve a remarkably rapid warming from −20° C. to 0° C. in less than or equal to 40 seconds. In contrast, the 2.0 mm stainless steel current collector and composite pressure plate of line 76 and the 38.0 mm stainless steel current collector and stainless steel pressure plate of line 78 do not warm up from −20° C. to 0° C. in less than 2 minutes. Thus, it is apparent that a thin current collector 40, 62 having a sensible heat less than the sensible heat of the end plate 12, 12′, with an insulator secured between the current collector and the pressure plate 42, 64 is a preferred configuration that results in a rapid heating of the end cell 12, 12′ during a bootstrap start.
While the present invention has been described and illustrated with respect to a particular construction of a fuel cell stack 10 having an improved current collector and insulator it is to be understood that the invention is not to be limited to the described and illustrated embodiments. For example, while the fuel cells 14, 16, 18 including individual fuel cells are described as having anode and cathode electrodes on opposed sides of PEM electrolytes, the invention may be applied to fuel cells utilizing other known electrolytes. Additionally, the current collector 30, insulator 40 and pressure plate 42 of the described and illustrated embodiments are shown being secured adjacent only the illustrated end cell 12. However, it is to be understood that the fuel cell stack 10 in most circumstances would include a second current collector, insulator and pressure plate (not shown) like the described components adjacent a second end cell (not shown). Accordingly, reference should be made primarily to the following claims rather than the foregoing description to determine the scope of the invention.
