Patent Application
20110097639
This invention relates to reducing gas bubble blockage in small, low flow coolant passageways of fuel cells, by means of gas segregating passageway configurations, and/or layers of liquiphilic and liquiphobic material adjacent to reactant gas flow field plates.
In order to reduce the exposure of proton exchange membrane (PEM) fuel cells to problems resulting from freezing temperatures, and to reduce reliance on water pumps and other plumbing as accessories to the fuel cells, the total volume of coolant in the fuel cells may be reduced.
However, in systems which do not use either a coolant pump or a gas venting micropump, and have minimal external water plumbing, the flow rate of coolant in the cells is quite low. If only gas is vented from the coolant passageways, water flow is limited to that which evaporates into the reactant gases. With small passageways and low water flow, gas bubbles may join together (aggregate) and block water flow from large areas of the water channels, sometimes referred to as “gas breakthrough” of the reactant gas flow field plates, thereby causing localized heating and dehydration of the membrane at various spots, resulting in degradation of components and reduced performance.
Systems employing between-cell coolant plates, which have glycol or other coolants, may have extremely small, in-cell water passageways to provide low flows of water to the anode side of the fuel cells for hydration, with insignificant cooling resulting from the water flow.
Aspects of the invention include: assuring coolant flow in small fuel cell coolant passageways; mitigating bubble blockage of coolant flowing at a low flow rate in fuel cell coolant passageways; assisting gas bubbles to flow through coolant passageways from PEM fuel cells; improved venting of gas in PEM fuel cell coolant passageways; reducing or eliminating ingestion of gas into fuel cell coolant passageways; increasing gas flow toward a vent in PEM fuel cells; improved hydration in PEM fuel cells; and improved evaporative cooling in PEM fuel cells.
Although the ultimate purpose herein is to assure that coolant flow will not be blocked in small coolant passageways of a fuel cell, the removal of coolant-blocking gas bubbles is achieved by addressing the gas flow characteristics of the coolant passageways. This invention is predicated on the realization that gas, typically gas bubbles, may be present in small fuel cell coolant passageways having low coolant flow rates, and that properly designed passageways will assure sufficient flow of coolant to propel gas bubbles through the coolant passageways to the vents, thereby assuring an adequate flow of coolant for operational purposes, rather than blockage of coolant by gas bubbles.
As coolant flow passageways become smaller, they become more prone to being blocked by bubbles of any gas therein. For a given cross sectional area of a prior art coolant passageway, there is a threshold coolant flow rate at or below which bubbles will block the coolant passageways if gas is present in the coolant; at coolant flow rates above the threshold, bubbles will be swept along by the flow of coolant, and will not block the coolant passageway.
As used herein, the term “fuel cell” may mean the apparatus for a complete, operative fuel cell capable of producing electricity from oxidant and fuel reactant gases, or where the context so requires, that term may mean the portion of a fuel cell exclusive of layers that provide coolant to the fuel cells, which may be disposed between adjacent ones of said fuel cell portions.
As used herein, the term “liquid coolant” means a coolant which is in the liquid state when the fuel cell is operating, even though such liquid coolant may freeze and become a solid when the fuel cell is not operating and the ambient temperature is below the freezing point of the coolant. The coolant may be water or other liquid; herein, “liquiphobic” is defined as having an aversion to the coolant, or lacking an affinity for the coolant, and “liquiphilic” is defined as having an affinity for, or lacking an aversion to, the coolant. The coolant flow assisted by the invention may, in some cases, be used primarily for hydration, with minimal cooling effect. Coolant flow paths and passageways herein are distinct from fuel and oxidant reactant gas flow field channels.
As used herein, the term “coolant passageways” refers to those portions of the coolant flow path in a fuel cell stack which are substantially coextensive with the planform (the in-plane outline) of the fuel cells.
According to the present invention, gas tolerant coolant passageways of fuel cells promote flow of coolant within such passageways with or without the presence of gas. In accordance further with the present invention, the gas tolerant configurations are provided along substantially the entire length of coolant passageways, thereby to provide a contiguous gas flow capability from inlets of said passageways to outlets thereof.
In accordance with one form of the invention, gas tolerant fuel cell coolant passageways are configured to provide portions for which gas has an affinity, that is, a propensity to migrate to such portions and to flow therein, in contrast with other portions of the passageways where coolant may flow without interference from gas, thereby to assure that there is room for coolant to flow without being blocked by gas bubbles.
According further to the invention, coolant passageways having gas tolerant cross sections may comprise first portions conducive to presence of gas preferentially in comparison with second portions which are less conducive to the presence of gas than the first portions.
Gas tolerant passageways of the invention may comprise first and second portions within the same passageways: the first portions may be adjacent to a wall of a passageway which is liquiphobic; the first portions may each comprise a corner which subtends more than 90° between two walls of a passageway; the first portions may comprise a portion of a triangle, a trapezoid or other polygon away from acute angular areas thereof; or they may comprise combinations of the foregoing. Gas tolerant passageways of the invention may comprise channels extending from surfaces of reactant gas flow field plates opposite to those surfaces from which reactant gas flow field channels extend. The gas tolerant coolant passageways may comprise intersecting polygons, or other closed plane figures bounded by lines, at least some of which may not be straight, of the same or a different shape in the same or adjacent flow field plates; for instance, the gas tolerant coolant passageways may comprise two channels, one larger than the other, or of two different cross sections, extending inwardly from one of the fuel reactant gas flow field plates, or one extending from a surface of a reactant gas flow field plate of one fuel cell, and the other extending from a surface of a reactant gas flow field plate of an adjoining fuel cell.
In further accord with the present invention, gas tolerant passageways of the invention may comprise liquiphilic and/or liquiphobic layers disposed between and extending across substantially the entire planform of the reactant gas flow field plates of adjacent fuel cells. In one form, a layer which has high coolant permeability is disposed between coolant passageways and a reactant gas flow field plate in a proton exchange membrane fuel cell. According to the invention, the highly permeable layer may be adjacent conventional, rectangular coolant channels, or may be adjacent gas tolerant coolant channels, such as T-shaped channels or triangular shaped channels, or otherwise. This form of the invention may be implemented with liquiphilic permeable sheets such as carbon fiber paper treated with tin, in a conventional fashion. Disposing the layer of material with high coolant permeability adjacent the fuel reactant gas flow fields prevents dryout and subsequent gas ingestion toward the membrane through the fuel reactant gas flow field plates, and also reduces the amount of gas ingested into the coolant channels (flowing away from the membrane).
In another form, the gas tolerant coolant passageways comprise either a sandwich of a liquiphobic layer between liquiphilic layers, or a liquiphilic layer adjacent to a layer which is both liquiphilic and liquiphobic, the layers being disposed between adjacent fuel cells. In one embodiment, the liquiphilic/liquiphobic layer may comprise a layer that is first formed to be entirely liquiphobic, thereafter treated to form non-liquiphobic regions; or the layer may simply have holes through it to conduct coolant.
Gas tolerant passageways may comprise a combination of first channels having a first cross sectional area or shape and other channels, interspersed with said first channels, having a cross sectional area or shape different than said first cross sectional area or shape. A particular embodiment has channels of a larger cross section interspersed with separate channels of a smaller cross section, in pairs.
The flow of coolant is the purpose, rather than the flow of gas; however, the control of gas flow is accomplished by affecting the characteristics of the coolant channels with respect to gas: that is, while the coolant flow characteristics at low flow rates are essentially the same in one portion of the gas tolerant coolant passageways as in other portions thereof, the invention provides certain portions of gas tolerant channels for which gas has an affinity. For instance, gas prefers the larger channel of an intersecting channel configuration, and gas prefers the areas adjacent obtuse angles rather than acute angles in a triangular configuration. Therefore, affecting the location of gas within the passageway accomplishes the flow of coolant within the passageway, whether or not gas is present.
Other aspects, 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. 4—intersecting rectangles formed in adjoining plates;
FIG. 5—intersecting rectangles formed in the oxidant reactant gas flow field plate;
FIG. 6—triangle formed in the oxidant reactant gas flow field plate;
FIG. 7—intersecting rectangle and triangle formed in adjoining plates;
FIG. 8—intersecting triangle and rectangle formed in the fuel reactant gas flow field plates;
FIG. 9—trapezoid formed in the oxidant reactant gas flow field plate;
A fuel cell system employing evaporative cooling is disclosed in U.S. Ser. No. 11/230,066 filed Sep. 19, 2005, and described with respect to
In
To prevent flooding, pressure of water in the passageways is between one and ten Kilopascals lower than the pressure of the reactant gases, such as provided by an air pump and pressure-regulated fuel.
The passageways may be formed other than by matching grooves as shown, such as by channels 76, 83 or 77, 84 provided in only one of the reactant gas flow field plates 75, 81.
The reactant gas flow field plates 75, 81 appear to be the same as water transport plates, sometimes referred to as fine pore plates, in a fuel cell power plant which utilizes significant water flow through the water transport plates, with external water processing, as is disclosed in U.S. Pat. No. 5,700,595. However, because there is about a 40 times improvement in cooling effectiveness per volume of water when evaporative cooling is used, the water flow channels of the aforesaid '595 patent have cross sections which are several tens of times larger than the cross sections of the water passageways 78, 85 in
In another embodiment of the aforementioned patent application, there is a material which is conductive, hydrophilic and has a high permeability to water, extending over substantially the entire planform of the reactant gas flow field plates 75, 81 in place of grooves forming passageways 78, 85. Such material may be carbon fiber paper, with fibers aligned in the direction of water motion to help with in-plane water permeability, or it may be other material conventionally used as fuel cell diffusion media.
The water passageways are typically very small and capable of providing the water required for evaporative cooling. Water flow in addition to that needed for evaporative cooling can assist in removing gas bubbles which leak in from the reactant gas channels and may assist in deionizing (demineralizing) water which is circulating in the system. Such water flow can be adequate if provided by a pump and plumbing external to the fuel cells.
According to the invention, that portion of fuel cell coolant paths which are substantially coextensive with the general planform of the fuel cells comprise gas tolerant coolant passageways, such as the passageways 78, 85 in
In accordance with the invention, there is a tendency for any gas within the passageways 78, 85 to migrate and/or be retained in the larger rectangles 76, 84, rather than in the smaller rectangles 77, 83. As illustrated by the bubble 87, the first portion of the coolant passageways formed by the large rectangles 77, 83 is conducive to the presence of gas preferentially over the second portion of the passageways which comprises the small rectangles 76, 84. As a consequence, bubbles that aggregate do so in the larger rectangles 76, 84, thereby leaving the smaller rectangles 77, 83 free from any bubble blockage. This assures that the flow of coolant necessary for proper fuel cell operation will occur in the smaller rectangles 77, 83.
As a result of the flow of coolant in the smaller rectangles 77, 83, as well as in the larger rectangles 76, 84 wherever bubbles are not blocking the larger rectangles, the flow of water will sweep the bubbles toward the vent 68 (
As illustrated in
Another form of the invention illustrated in
The triangular cross sections of the gas tolerant coolant passageways 85 may be equilateral, or they may be either scalene or isosceles right triangles. Or, they may be scalene triangles other than right triangles. However, since the gas will tend to migrate toward and/or remain in large, obtuse corners, in preference over small, acute corners, an equilateral triangle is probably not as useful as one in which the angles 96, 97 near the surface 93 are acute and smaller than the apex 98 away from the surface 93; the apex in such case is greater than 180°/N. Capillary flow may occur in a corner of a liquid passageway if
where α=half angle of the corner and θ=static contact angle of the liquid. For an equilateral triangle, 2α=60° and α=30°; θ<90-30=60; so if the contact angle is less than 60°, capillary flow of liquid will occur in the corner. High capillary pressure is defined as that which is sufficient to assure a non-zero flow of coolant through a coolant passageway.
In the general case, this form of the invention will have one angular intersection of sides of the coolant channel which has an angle greater than 180°/N, where N is the total number of sides (including an open portion substantially in the plane of one of the surfaces 91, 93). Thus, one of the angles of a triangular passageway, typically the apex, will be greater than 60°.
Referring to
In
In the general case, gas tends to leak from the fuel reactant gas flow field plate into the coolant passageways of a fuel cell. For that reason, the portions of gas tolerant coolant passageways for which the gas has an affinity might preferably be disposed toward the fuel reactant gas flow field plate, whereas those portions of gas tolerant coolant passageways which are less conducive to gas bubbles will be positioned toward the oxidant reactant gas flow field plate. In the case of gas tolerant water passageways utilized in evaporatively cooled fuel cells, this assures that water will easily reach and be evaporated into the oxidant reactant gas flow field channels 82.
Each of
Referring to
Although a gas bubble may form in one of the channels 102 thereby tending to block water in that channel, the highly permeable layer 100 will conduct water, freely, around the bubble-blocked channel, from other channels 102 to the entire planform of the fuel reactant gas flow field plates 75. The water flows faster within the permeable layer 106 than in channels 102, and causes a pressure drop from the coolant inlet to the vent 68 (
Thus, the highly permeable layer 100 is disposed between the oxidant reactant gas flow field plate of each fuel cell and the fuel reactant gas flow field plate of the fuel cell adjacent thereto in said stack; of course, the highly permeable layer adjacent to the oxidant reactant gas flow field plate in a fuel cell which is at the cathode end of the stack will not be adjacent, in the usual case, to any fuel reactant gas flow field plate. As used herein, the term “each fuel cell” excludes the last fuel cell at the cathode end of the stack, which has a highly permeable layer next to its oxidant reactant gas flow field plate, but that layer is not typically adjacent to any fuel reactant gas flow field plate, there being none beyond the last fuel cell at the cathode end of the stack.
In addition, the highly permeable layer 100 allows water to move rapidly around any bubble that may be blocking an adjacent water channel. This prevents local dry-out of the fuel gas flow field plate, thereby avoiding “breakthrough, which allows gas to pass freely through the flow field plate. In this way, the layer 100 substantially “blocks” the flow of reactant gas from the fuel reactant gas flow field channels 74 and the oxidant reactant gas flow field channels 82 to the water passageways 102. Thus, the formation of gas bubbles within the coolant water passageways 102 is reduced, and the effect of such bubbles, in blocking water from various parts of a fuel cell planform, is also reduced.
In
However, if deemed necessary in any given utilization of the invention, such as in the case of desiring to have water passageways 102 of extremely small cross section, the cross section of the water passageways 102 may provide additional gas tolerance. For instance, a channel configuration illustrated in
The passageways may be triangular, as shown by the passageway 102b in
The water passageways can be of other configurations since employment of the highly permeable layer 100 avoids the problem of water passageways being blocked by air bubbles when there is a very low flow rate of water.
Another form of the invention, illustrated in
Another form of the invention employs layers which are conductive, highly permeable to water, and including hydrophilicity and hydrophobicity. In one embodiment illustrated in
The layers 114-116 may comprise porous carbon fiber layers of the type typically used for gas diffusion layers in fuel cells, with appropriate treatment to render them either hydrophilic or hydrophobic.
In another embodiment illustrated in
The hydrophobic/hydrophilic plate 120 may comprise porous conductive hydrophobic material 121 with a plurality of non-hydrophobic regions 122 therein. The non-hydrophobic regions 122 may be the same material as the rest of the plate 120, which however are treated to make them hydrophilic, such as with a coating of tin. On the other hand, the non-hydrophobic regions 122 may simply be passageways (e.g., punched holes through the material 121) to allow water to pass through the layer 120. Thus, the non-hydrophobic regions 122 may be hydrophilic, or may simply be passageways.
The present invention has been described with respect to fuel cells employing a water coolant, by which is meant a coolant which is substantially pure water. The embodiment of
In
Gas tolerant coolant passageways according to the present invention may be configured in groupings, such as pairs or larger series of alternatively sized or shaped channels. In one embodiment of this form of the invention, illustrated in
In another embodiment of this form of the invention illustrated in
In
In the embodiments of
The aforementioned patent and patent application 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.
This is a division of U.S. patent application Ser. No. 11/323,767 filed Dec. 30, 2005.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11323767 | Dec 2005 | US |
| Child | 12928854 | US |
