Refrigeration – Processes – Gas and liquid contact
Reexamination Certificate
2003-02-24
2004-08-24
Jiang, Chen Wen (Department: 3744)
Refrigeration
Processes
Gas and liquid contact
C062S314000, C429S010000, C429S006000
Reexamination Certificate
active
06779351
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of electrochemical fuel cells such as proton exchange membrane (PEM) fuel cells with thermal and moisture management. More particularly, the present invention relates to fuel cell systems with evaporative cooling and methods for humidifying and adjusting the temperature of the reactant streams.
2. Discussion of the Background
Electrochemical fuel cells generate electrical energy by converting chemical energy derived from a fuel directly into electrical energy by oxidation of the fuel in the cell. A typical fuel cell includes an anode, a cathode and an electrolyte. The reactant streams as fuel and oxidant are supplied to the anode and cathode, respectively. In electrochemical fuel cells employing hydrogen as the fuel and oxygen containing gas as the oxidant, the reaction product is water. At the anode, the fuel permeates the electrode material and reacts at the anode catalyst layer to form cations, which migrate through the electrolyte to the cathode. At the cathode, the oxygen containing gas supply reacts at the cathode catalyst layer to form anions. The anions react with cations to form a reaction product. The fuel cell generates a useful electric current and the reaction product is removed from the cell. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen cations, the membrane isolates the hydrogen fuel stream from the oxidant stream. The anions O
2
−
formed at the cathode react with hydrogen ions 2H
2
+
that have crossed the membrane to form liquid water as the reaction product.
Unfortunately, it is not only electricity and product water that are generated during this process but also heat. The heat is produced primarily at the cathode when the hydrogen and oxygen ions combine. Some of this heat (about one third or less) can be removed by conventional evaporation of this product water, but the remaining heat must be removed by other means.
There is also another problem for reliable operation of fuel cells. Hydrogen ion conductivity through ion exchange membranes generally requires the presence of water molecules. The fuel and oxidant gases (especially fuel) are humidified prior to introducing them to the cell to maintain the water saturation of the membranes within the membrane electrode assembly. Currently, the most popular, perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its Nafion trade designation, must be hydrated or saturated with water molecules for ion transport to occur. It is well known that such perfluorosulfonic membranes transport cations using a “water pumping” phenomenon. Water pumping involves the transport of cations in conjunction with water molecules, resulting in a net flow of water from anode side of the membrane to the cathode side. Thus, membranes can dry out on the anode side if water molecules are not resupplied.
Fuel cells employing such membranes require water to be removed from the cathode side. There must not be so much water that electrodes, which are bonded to the electrolyte, flood and thereby block the pores in the electrodes or gas diffusion layer. A balance is therefore needed.
There are other important aspects of the methods of operation of fuel cell systems and fuel cell design. The most important considerations (especially for PEM fuel cells) are the method of cooling and adjusting the temperature of the reactant streams for the fuel cell, the process of humidifying the reactant streams, and the water management for the fuel cell.
Cooling for the fuel cell has been provided by reactants, natural convection, radiation, and possible supplemental cooling channels and/or cooling plates. The herein system uses an evaporative cooling process as the mechanism to provide cooling, either to coolants or to the reactant gases.
In this regard, U.S. Pat. No. 3,761,316 discloses a fuel cell with evaporative cooling. A fuel cell assembly utilizing the waste heat of a fuel cell to provide evaporative cooling of the cell is provided by a hydrophobic separator disposed in heat conducting relationship with the fuel cell. A coolant liquid (water) is fed under pressure to a cavity on one side of the hydrophobic separator, and as vapor evolves from the coolant liquid, it passes through the hydrophobic separator to ambient.
In U.S. Pat. No. 4,795,683, a method of evaporative cooling a PEM fuel cell is disclosed where liquid water mist is introduced into the anode. A desiccant material directs the liquid water mist to the ion exchange membrane. Evaporation of a portion of both the product water and the supplied liquid water cools the cell and eliminates the need for separate cooling chamber.
U.S. Pat. No. 4,824,741 discloses a fuel cell utilizing a solid polymer electrolyte membrane cooled by evaporation of water in the hydrogen reactant chamber of the cells. A porous graphite plate or water permeated membrane is disposed in the hydrogen reactant chamber adjacent to the electrolyte membrane. If a graphite plate is used, it is preferably grooved on the surface facing the electrolyte. The resultant lands preferably contact the supported catalyst layer on the membrane to cool the latter. Water is forced into the pores of the plate or membrane from the edge thereof, and the water vapor is carried out of the cells in the hydrogen reactant exhaust stream.
U.S. Pat. No. 5,262,249 describes an internally cooled PEM fuel cell device. It includes a pair of substantially coextensive electrode components each of which includes a porous central region and a fluid impermeable peripheral region circumferentially completely surrounding the central region. It also includes a proton exchange membrane component interposed between at least the central regions of the electrode component. The fuel cell device further includes an arrangement for cooling the fuel cell, including at least one enclosed cooling channel situated at the peripheral region of one of the electrode components and supplied with fresh cooling medium, with the spent cooling medium being discharged from the cooling channel. There is further provided a heat transfer device that is operative to transfer heat from the central region to the peripheral region on the one electrode component.
All previously known and available evaporative cooling methods and designs for fuel cells have one common disadvantage in that the maximum cool temperature that may be reached is the wet bulb temperature of outside air, which cannot guarantee efficient rejection of heat from a fuel cell. This limited maximum of cooling that can occur has proven to be commercially disadvantageous to current fuel cell systems and apparatuses especially for PEM fuel cells. Because of the indirect and direct cooling, the herein invention yields a lower temperature that is below the wet bulb temperature and approaching the dew point temperature.
There are many methods and designs for humidifying fuel cells. For example, U.S. Pat. No. 5,382,478 discloses an electrochemical fuel cell stack, which has a humidifying section located upstream from the electrochemically active section. The inlet fuel and oxidant streams are introduced into the humidifying section without first being directed through the electrochemically active section. The upstream location of the humidification section in the stack enables the number of manifold opening in the active section to be reduced.
U.S. Pat. No. 5,432,020 describes a process and an apparatus humidifying the process gas for operating fuel cell systems. To ensure high efficiency, the process gas must be introduced at a predetermined temperature and humidity. A metered quantity of fine water droplets is injected into the gas supply line, by way of which the process air is humidified. If the fuel cell is operated under pressure, the process air generally has to be cooled after it has been compressed. The process air is automatically cooled as a result of a partial evaporation of the water droplets whil
Gillan Alan D.
Gillan Leland E.
Heaton Timothy L.
Maisotsenko Valeriy
Bales Jennifer L.
Idalex Technologies, Inc.
Jiang Chen Wen
Macheledt Bales & Heidmiller LLP
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