Patent Application
20060263663
The present invention relates generally to fuel cell stacks and more particularly to a fuel cell stack having a unipolar plate with a modified coolant rate relative to at least a majority of the bipolar plates.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, gas impermeable, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. By way of example, some typical arrangements for multiple cells in a stack are shown and described in U.S. Pat. No. 5,763,113.
The electrically conductive end plates sandwiching the MEAs typically contain an array of grooves in the faces thereof that define a reactant flow field for distributing the fuel cell's gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels. These end plates are commonly referred to as unipolar plates.
In a fuel cell stack, a plurality of cells are stacked together in electrical series while being separated by a gas impermeable, electrically conductive bipolar plate. In some instances, the bipolar plate is an assembly formed by securing a pair of thin metal sheets having reactant flow fields formed on their external, face surfaces. Typically, an internal coolant flow field is provided between the metal plates of the bipolar plate assembly. Various examples of a bipolar plate assembly of the type used in PEM fuel cells are shown and described in commonly-owned U.S. Pat. No. 5,766,624.
Fuel cell stacks produce electrical energy efficiently and reliably. However, as they produce electrical energy, losses in the electrochemical reactions and electrical resistance in the components that make up the stack produce waste thermal energy (heat) that must be removed for the stack to maintain a constant optimal temperature. Typically, the cooling system associated with a fuel cell stack includes a circulation pump for circulating a single-phase liquid coolant through the fuel cell stack to a heat exchanger where the waste thermal energy (i.e., heat) is transferred to the environment. The two most common coolants used are de-ionized water and a mixture of ethylene glycol and de-ionized water.
The capability of end cells to reliably produce voltage as high as normal repeating cells has long been an issue affecting the reliability of the fuel cell stack. When the end cell voltage falls below a critical threshold the entire fuel cell stack may become inoperative. It would be desirable to provide an end cell that operates more like a normal repeating cell to thereby increase the life of a fuel cell stack.
In at least one embodiment, the present invention comprises a fuel cell assembly having opposite first and second ends, a wet end fuel cell adjacent the first end, a dry end fuel cell adjacent the second end, and a plurality of repeating fuel cells disposed between the wet end and dry end fuel cells. In accordance with this embodiment, the wet end fuel cell comprises a unipolar plate and the repeating fuel cells each comprise one-half of each adjacent bipolar plate. Further in accordance with this embodiment, the unipolar plate has a first coolant rate and at least one of the bipolar plates has a second coolant rate, with the first coolant rate being less than the second coolant rate.
The present invention also comprises a method of cooling a wet end cell in a fuel cell stack. In at least one embodiment, the method comprises operating a fuel cell system comprising the fuel cell stack. In accordance with this embodiment, the fuel cell stack comprises a wet end fuel cell comprising a unipolar plate, a dry end fuel cell, and a plurality of repeating fuel cells disposed between the wet end and dry end fuel cells, with each of the repeating fuel cells comprising one-half of each adjacent bipolar plate. Further in accordance with this embodiment, coolant is provided to the wet end unipolar plate having a first coolant rate, and coolant is provided to at least one of the bipolar plates having a second coolant rate with the second coolant rate being greater than the first coolant rate.
In still yet at least another embodiment, the present invention comprises a fuel cell assembly having opposite first and second ends, a wet end fuel cell adjacent the first end, a dry end fuel cell adjacent the second end, and a plurality of repeating fuel cells disposed between the wet end and dry end fuel cells. In accordance with this embodiment, the wet end fuel cell comprises a unipolar plate having a first coolant flow rate and the repeating fuel cells each comprise one-half of each adjacent bipolar plate with at least one of the bipolar plates having a second coolant flow rate with the first coolant flow rate being less than the second coolant flow rate. Further in accordance with this embodiment, the unipolar plate has a first number of coolant inlet openings and a second number of coolant outlet openings, with the second number being smaller than the first number such that the first coolant flow rate is less than 75% of the second coolant flow rate.
Further areas of applicability of the present invention will become apparent from the detailed description provided herein. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.
The present invention will become more fully understood from the detailed description and the accompanying drawings where like structure is indicated with like reference numerals and in which:
The following description of the preferred embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative bases for the claims and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.
Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: percent, “parts of”, and ratio values are by weight; the term “polymer” includes “oligomer”, “copolymer”, “terpolymer”, and the like; the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.
Before further describing the invention, it is useful to understand an exemplary fuel cell system within which the invention operates.
With continued reference to
End plates 14 and 16 press up against primary current collector 34 on cathode face 4a of MEA 4 and primary current collector 40 on anode face 6b of MEA 6 while bipolar plate assembly 8 presses up against primary current collector 36 on anode face 4b of MEA 4 and against primary current collector 38 on cathode face 6a of MEA 6. An oxidant gas, such as oxygen or air, is supplied to the cathode side of the fuel cell stack from a storage tank 46 via appropriate supply plumbing 42. Similarly, a fuel, such as hydrogen, is supplied to the anode side of the fuel cell from a storage tank 48 via appropriate supply plumbing 44. In a preferred embodiment, oxygen tank 46 may be eliminated, such that ambient air is supplied to the cathode side from the environment. Likewise, hydrogen tank 48 may be eliminated and hydrogen supplied to the anode side from a reformer which catalytically generates hydrogen from methanol or a liquid hydrocarbon (e.g., gasoline). While not shown, exhaust plumbing for both the H2 and O2/air sides of MEAs 4 and 6 is also provided for removing H2-depleted anode reactant flow field and O2-depleted cathode gas from the cathode reactant flow field.
Coolant supply plumbing 50, 52, and 54 is provided for supplying a liquid coolant from an inlet header (not shown) of the fuel cell stack to the coolant flow fields of bipolar plate assembly 8 and end plates 14 and 16. The coolant flow fields of the bipolar plate assembly 8 and end plates 14 and 16 include long narrow channels 56 defining coolant passages within the plates 8, 14, and 16. As shown in
Coolant 90, oxidant 92 and hydrogen 94 are supplied to the fuel cell stack 70 via appropriate plumbing. As shown in the schematic illustration of
A fuel cell is formed when an MEA is interposed between adjacent plates. For instance, unipolar plate 84, MEA 80a, and roughly half of bipolar plate 82a form a fuel cell 110, while the other half of bipolar plate 82a, MEA 80b, and roughly half of bipolar plate 82b form another fuel cell 112. The cell 110 closest to the wet end is commonly referred to as the “wet end cell” or “cell n.” The cell n 110 is shown to be the last cell in the cell stack 70. Fuel cell 112 is an exemplary non-end cell or normal repeating cell. The cell opposite cell n 110 and adjacent the dry end is commonly referred to as the “dry end cell.”
The present invention helps to ensure that the end cell temperature is relatively consistent with the repeating cells temperatures and that the end cell is not substantially overcooled relative to the repeating cells thereby simultaneously reducing the opportunity to flood the end cell and increasing the end cell's capability to reliably produce voltage as high as the normal repeating cells.
The present invention accomplishes this by modifying the cooling rate of at least one of the unipolar plates 84 and 86 relative to the other bipolar plates 82a-c. Since unipolar plates 84 and 86 operate substantially similarly, and since the teachings of the present invention can be applicable to one or both of the unipolar plates 84 and/or 86, for the sake of simplicity, only unipolar plate 84 will be discussed in detail below, however it should be understood that the discussion of unipolar plate 84, where appropriate, alternatively can apply to unipolar plate 86, or to both unipolar plates 84 and 86, as well. Likewise, the discussion of the wet end cell 110, where applicable, can alternatively apply to the dry end cell or both the wet and dry end cells.
During normal operation, each of the normal repeating cells, such as fuel cell 112, produce roughly the same amount of heat. As such, each of these normal repeating cells, which have roughly the same rate of coolant flow through passages 98b-d, cool at substantially similar rates. For cell n 110, heat q1 is generated from MEA 80a that is dispelled in generally opposing directions, as shown by arrows 114, away from MEA 80a into unipolar plate 84 and bipolar plate 82a and 92 respectively.
Each normal repeating cell can be expected to operate in substantially the same manner as fuel cell 112, described below. Normal repeating fuel cell 112 produces heat q2 at MEA 80b that is dispelled in generally opposing directions, as shown by arrows 116, into bipolar plates 82a and 82b. During normal operation, q1 and q2 are substantially equal amounts of heat. As can be seen in
In conventional systems, flow rate of coolant directed to the wet end unipolar plate, such as 84, is typically the same as the flow rate of coolant directed to bipolar plates, such as 82a-c. Since less heat is being directed to unipolar plate 84 than the bipolar plates 82a-c, and because the amount of coolant being directed to plates 84 and 82a-c are the same, applicants believe that unipolar plate 84 is “overcooled”, such that the temperature of the coolant exiting unipolar plate 84 through passage 98a is less than the temperatures of coolant exiting bipolar plates 82a-c through passages 98b-d. Applicants believe that this overcooling of the unipolar plate 84 can cause flooding of the end cell 110 thereby decreasing the voltage produced in end cell 110 and thus decreasing performance and lifetime of the fuel cell stack 70.
In accordance with the present invention, applicants have provided a coolant system in which a first amount (or flow rate) of coolant is provided to the coolant passage 98a of the unipolar plate 84 and a second amount of coolant is substantially provided to each of the coolant passages 98b-d of the bipolar plates 82a-c, respectively, in the normal repeating cells. The first amount is at least substantially less than the second amount. In at least one embodiment, the first amount is no more than 75% (based on mass flow rate) of the second amount, while in another embodiment the first amount is between 30%-70% of the second amount, while yet in another embodiment is between 45%-55% of the second amount. For instance, if the coolant flow rate in bipolar plate 82a is about 13.4 grams per second, the coolant flow rate in the unipolar plate 84 is, in at least one embodiment, no more than 10 grams per second, in at least another embodiment between 4-9.5 grams per second, and in yet another embodiment between 6-7.4 grams per second.
The manner in which the reduced (i.e., first) amount of coolant directed to the unipolar plate 84 is achieved is not necessarily important. The amount of coolant introduced into unipolar plate 84 can be reduced relative to the amount of coolant introduced into the bipolar plates 82a-c in a variety of manners. To help illustrate at least one of these manners, a schematic representation of the coolant side of a unipolar plate 122 is provided in
The unipolar plate 122 illustrated in
Coolant flow field, generally designated at 150, comprises channels extending between the inlet and outlet openings 144 and 146, respectively. One manner to reduce the amount of coolant introduced into the unipolar plate 122 is to provide plate 122 with at least some of the inlet and/or outlet openings 144 and/or 146 and/or the channels 150 that are smaller in size or area (i.e., diameter) than those in the bipolar plates 82a-c used in the same stack. For illustration purposes only, the outlet openings 146 in plate 122 are schematically illustrated as being smaller in size than inlet openings 144.
To help illustrate another manner in which the coolant introduced into unipolar plate 84 can be reduced relative to the amount of coolant introduced into the bipolar plates 82a-c, schematic representation of a unipolar plate 122′ is provided in
In the embodiments where a portion of the outlet openings 146′ are closed, the closing of the openings can be accomplished by designing the unipolar plate 122′ to have fewer outlet openings 146′. In another embodiment, the outlet openings 146′ could be blocked with a suitable material such as epoxy resin.
In the exemplary embodiment illustrated in
In yet another embodiment, as best illustrated in
While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. For instance, the modification to the wet end unipolar plate and/or the manner in which coolant is directed to the wet end unipolar plate can apply equally to the dry end unipolar plate, either in addition to or in the alternative to wet end unipolar plate. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.
