System and method for management of gas and water in fuel cell system

#The present invention relates generally to an apparatus and method for management of process gases for a fuel cell system. More particularly, the present invention relates to an apparatus and method for controlling the humidity, temperature and flow of fuel cell process gases.

#Fuel cell systems are seen as a promising alternative to traditional power generation technologies due to their low emissions, high efficiency and ease of operation. Fuel cells operate to convert chemical energy into electrical energy. Proton exchange membrane fuel cells comprise an anode, a cathode, and a selective electrolytic membrane disposed between the two electrodes. In a catalyzed reaction, a fuel such as hydrogen, is oxidized at the anode to form cations (protons) and electrons. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. The electrons cannot pass through the membrane and are forced to flow through an external circuit thus providing an electrical current. At the cathode, oxygen reacts at the catalyst layer, with electrons returned from the electrical circuit, to form anions. The anions formed at the cathode react with the protons that have crossed the membrane to form liquid water as the reaction product.

#Proton exchange membranes require a wet surface to facilitate the conduction of protons from the anode to the cathode, and otherwise to maintain the membranes electrically conductive. It has been suggested that each proton that moves through the membrane drags at least two or three water molecules with it (U.S. Pat. No. 5,996,976). U.S. Pat. No. 5,786,104 describes in more qualitative terms a mechanism termed “water pumping”, which results in the transport of cations (protons) with water molecules through the membrane. As the current density increases, the number of water molecules moved through the membrane also increases. Eventually the flux of water being pulled through the membrane by the proton flux exceeds the rate at which water is replenished by diffusion. At this point the membrane begins to dry out, at least on the anode side, and its internal resistance increases. It will be appreciated that this mechanism drives water to the cathode side, and additionally the water created by reaction is formed at the cathode side. Nonetheless, it is possible for the flow of gas across the cathode side to be sufficient to remove this water, resulting in drying out on the cathode side as well. Accordingly, the surface of the membrane must remain moist at all times. Therefore, to ensure adequate efficiency, the process gases must have, on entering the fuel cell, appropriate humidity and temperature which are based on the system requirements.

#For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, which show a preferred embodiment of the present invention and in which:

#FIG. 1 illustrates a schematic flow diagram of a first embodiment of a fuel cell gas and water management system according to the present invention;

#FIG. 2 illustrates a variant of the first embodiment of the fuel cell gas and water management system according to the present invention, in which only one cooling loop is shown; and

#Referring first to FIG. 1, this shows a schematic flow diagram of a first embodiment of a fuel cell gas management system 10 according to the present invention. The fuel cell gas management system 10 comprises a fuel supply line 20, an oxidant supply line 30, a cathode exhaust recirculation line 40 and an anode exhaust recirculation line 60, all connected to a fuel cell 12. It is to be understood that the fuel cell 12 may comprise a plurality of fuel cells or just a single fuel cell. For simplicity, the fuel cell 12 described herein operates on hydrogen as fuel and air as oxidant and can be a Proton Exchange Membrane (PEM) fuel cell. However, the present invention is not limited to this type of fuel cells and applicable to other types of fuel cells.

#The fuel supply line 20 is connected to a fuel source 21 for supplying hydrogen to the anode of the fuel cell 12. A hydrogen humidifier 90 is disposed in the fuel supply line 20 upstream from the fuel cell 12 and an anode water separator 95 is disposed between the hydrogen humidifier 90 and the fuel cell 12. The oxidant supply line 30 is connected to an oxidant source 31, e.g. ambient air, for supplying air to the cathode of the fuel cell 12. An enthalpy wheel 80 is disposed in the oxidant supply line 30 upstream of the fuel cell 12 and also in the cathode recirculation line 40. A cathode water separator 85 is disposed between the enthalpy wheel 80 and the fuel cell 12. The enthalpy wheel 80 comprises porous materials with a desiccant. In known manner, a motor 81 drives either the porous materials or a gas diverting element to rotate around the axis of the enthalpy wheel so that gases from the oxidant supply line 30 and the oxidant recirculation line 40 alternately pass through the porous materials of the enthalpy wheel. Dry ambient air enters the oxidant supply line 30 and first passes through an air filter 32 that filters out the impurity particles. A blower 35 is disposed upstream of the enthalpy wheel 80, to draw air from the air filter 32 and to pass the air through a first region of the enthalpy wheel 80. The enthalpy is wheel 80 may be any commercially available enthalpy wheel suitable for fuel cell system, such as the one described in the applicant's co-pending U.S. patent application Ser. No. 09/941,934.

#A fuel cell cathode exhaust stream contains excess air, product water and water transported from the anode side, the air being nitrogen rich due to consumption of at least part of the oxygen in the fuel cell 12.