Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2000-07-28
2002-07-09
Maples, John S. (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06416892
ABSTRACT:
TECHNICAL FIELD
The present invention relates to fuel cell power plants that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to an interdigitated enthalpy exchange device for a fuel cell power plant that exchanges heat and water exiting the plant back into the plant to enhance water balance and energy efficiency of the plant.
BACKGROUND OF THE INVENTION
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles, or on-site generators for buildings. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. 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.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane (“PEM”) electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
In operation of PEM fuel cells, it is critical that a proper water balance be maintained between a rate at which water is produced at the cathode electrode including water resulting from proton drag through the PEM electrolyte and rates at which water is removed from the cathode and at which water is supplied to the anode electrode. An operational limit on performance of a fuel cell is defined by an ability of the cell to maintain the water balance as electrical current drawn from the cell into the external load circuit varies and as an operating environment of the cell varies. For PEM fuel cells, if insufficient water is returned to the anode electrode, adjacent portions of the PEM electrolyte dry out thereby decreasing the rate at which hydrogen ions may be transferred through the PEM and also resulting in cross-over of the reducing fluid leading to local over heating. Similarly, if insufficient water is removed from the cathode, the cathode electrode may become flooded effectively limiting oxidant supply to the cathode and hence decreasing current flow. Additionally, if too much water is removed from the cathode, the PEM may dry out limiting ability of hydrogen ions to pass through the PEM, thus decreasing cell performance.
As fuel cells have been integrated into power plants developed to power transportation vehicles such as automobiles, trucks, buses, etc., maintaining a water balance within the power plant has become a greater challenge because of a variety of factors. For example, with a stationary fuel cell power plant, water lost from the plant may be replaced by water supplied to the plant from off-plant sources. With a transportation vehicle, however, to minimize fuel cell power plant weight and space requirements, the plant must be self-sufficient in water to be viable. Self-sufficiency in water means that enough water must be retained within the plant to offset water losses from gaseous streams of reactant fluids passing through the plant. For example, any water exiting the plant through a cathode exhaust stream of gaseous oxidant or through an anode exhaust stream of gaseous reducing fluid must be balanced by water produced electrochemically at the cathode and retained within the plant.
An additional requirement for maintaining water self-sufficiency in fuel cell power plants is associated with components necessary to process hydrocarbon fuels, such as methane, natural gas, gasoline, methanol, diesel fuel, etc., into an appropriate reducing fluid that provides a hydrogen rich fluid to the anode electrode. Such fuel processing components of a fuel cell power plant typically include a boiler that generates steam; a steam line that directs the steam from the boiler; and a reformer that receives the steam and a hydrocarbon fuel mixture along with a small amount of a process oxidant such as air and transforms the mixture into a hydrogen-enriched reducing fluid appropriate for delivery to the anode electrode of the fuel cell. The fuel processing components include water requirements that are part of an overall water balance of the fuel cell power plant. For example, water made into steam in the boiler must be replaced by water recovered from the plant.
It is known to use a direct mass and heat transfer device to enhance water balance of a fuel cell power plant, such as disclosed in U.S. Pat. No. 6,007,931 that issued on Dec. 28, 1999 to Fuller et al., and is owned by the assignee of all rights in the interdigitated enthalpy exchange device invention disclosed herein, and which Patent is hereby incorporated herein by reference. The direct mass and heat transfer device passes the process oxidant stream upstream of the fuel cell in mass transfer relationship with the plant exhaust stream exiting the plant so that mass and heat such as water vapor and entrained liquid water in the exhaust stream pass directly through a mass transfer medium into the process oxidant stream to heat and humidify the oxidant stream without the complexities of traditional condensing heat exchangers. Therefore the Fuller et al. mass and heat recovery system substantially enhances water recovery and plant energy efficiency because the recovered water and heat need no parasite power from the fuel cell to pump or otherwise transfer the mass and heat to humidify and heat the process oxidant stream.
Another difficulty associated with fuel cell power plants utilized to power transportation vehicles involves a coolant system necessary to maintain the fuel cell within an appropriate heat range. Heat must be removed from the fuel cell, and it is common to cycle a cooling fluid through cooler plates adjacent reactant stream flow fields of the fuel cell. Such cooling fluids must also be tolerant of temperature extremes to which transportation vehicles are exposed, and therefore the cooling fluids consist of various antifreeze solutions as is well known. However, as a cooling fluid contacts cell components, especially where fuel processing components have supplied a reformate fuel to the fuel cell, it is also known that the cooling fluid will be contaminated with dissolved gases, such as ammonia (NH
3
), hydrogen (H
2
), as well as carbon dioxide (CO
2
). Additionally, it is known that dissolved metals and other ions must be removed in order to limit conductivity of the coolant fluid to avoid shunt current corrosion. Therefore it is known to use cooling fluid treatment
Chisholm, Jr. Malcolm J.
Maples John S.
UTC Fuel Cells LLC
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