Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2002-02-13
2004-07-20
Le, Hoa Van (Department: 1752)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000, C429S006000
Reexamination Certificate
active
06764781
ABSTRACT:
TECHNICAL FIELD
This invention relates to liquid cooled fuel cell systems, and more particularly to improving the efficiency thereof by reducing the parasitic drain imposed thereon by the coolant pump when the fuel cell is operating under low power conditions.
BACKGROUND OF THE INVENTION
A fuel cell is an electrochemical device for continuously converting a fuel and an oxidant into electricity. It consists of two electrodes (i.e. an anode and cathode) separated by an ion-conducting electrolyte with provision for the continuous movement of fuel (e.g. H
2
), oxidant (e.g. O
2
), reaction product (e.g. H
2
O), and coolant into and out of the cell. Fuel is continuously supplied to, and oxidized at, the anode (negative electrode), and provides electrons to an external circuit. Oxidant is continuously supplied to, and reduced at, the cathode (positive electrode) where it receives the electrons from the external circuit. An ionic current in the electrolyte completes the circuit. A plurality of individual fuel cells bundled together is often referred to as a fuel cell “stack”.
PEM (proton exchange membrane) fuel cells [a.k.a. SPE (solid polymer electrolyte) fuel cells] are well known H
2
/O
2
-based fuel cells that are ideal for mobile applications (e.g., electric vehicles), and may have the hydrogen stored in a compressed or liquefied state, or generated from reformed methanol, gasoline, diesel fuel or the like. PEM fuel cells include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton-transmissive, solid-polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements that serve as current collectors for the anode and cathode. The current collectors also serve to distribute the fuel cell's gaseous reactants over the surfaces of the anode and cathode. The solid polymer membranes are typically made from ion exchange resins such as perfluoronated sulfonic acid (e.g. NAFION from DuPont) that are hydrated to promote ionic conductivity therethrough. The anode and cathode typically comprise finely divided carbon and catalyst particles and proton conductive resin intermingled therewith.
In addition to the fuel cell itself, fuel cell systems require a variety of auxiliary equipment (e.g. pumps, heat exchangers, fuel processors, combusters, water separators, etc.) to support the operation of the fuel cell. Some of the auxiliary equipment is energized by the fuel cell in that they draw power from the fuel cell for their operation, and accordingly are essentially a parasitic power drain on the system (i.e. the power they consume is not available for useful work outside the fuel cell system). One such parasitic power-draining piece of auxiliary equipment is the pump used to circulate the liquid coolant through the fuel cell.
PEM fuel cells are temperature sensitive in that if the operating temperature is too high for too long, the membrane-electrolyte can begin to dehydrate causing a drop in the fuel cell's performance/voltage. Accordingly, PEM fuel cells (as well as other fuel cells) are cooled by flowing liquid coolant continuously therethrough. One cooling technique uses a constant speed pump that flows the coolant through the fuel cell at a constant rate set high enough to control the cell's temperature under full-power conditions. Another technique uses a variable speed pump that flows the coolant continuously through the stack at a variable rate determined by the load on the stack at any given point in time. However, due to design constraints of the fuel cell stack (i.e. the coolant flow channels within the stack), the coolant must flow at a certain minimum rate in order to achieve uniform flow distribution throughout the stack for uniform cooling of the stack. Such minimum flow rate is typically higher than is needed to remove the heat generated by the fuel cell stack when the stack is operating at low power levels (i.e. less than 25% full stack power), and hence generating relatively little heat. Hence even with a variable speed pump, more power is consumed by the coolant pump under low fuel cell load conditions than is necessary for cooling alone.
Highly efficient fuel cell systems are desirable. The efficiency of a fuel cell system is determined by dividing the net power of the system (i.e. power available for use outside of the system) by the potential power of the system, where the net power equals the amount of power produced by the fuel cell minus the parasitic power drain on the system, and the potential power equals the heating value of the fuel (kilojoules per gram) times the mass flow rate (grams per second) of the fuel supplied to the system. Hence, systems having high parasitic loads are less efficient then those having low parasitic loads. The present invention improves the efficiency of a fuel cell system by reducing the parasitic load imposed on the fuel cell system by the coolant pump when the fuel cell stack is operating at less than 25% full power.
SUMMARY OF THE INVENTION
The invention contemplates a process for improving the efficiency of a fuel cell system by only intermittently pumping coolant through the fuel cell when it is operating at low power levels. The invention is applicable to a fuel cell system that includes (1) a fuel cell that is designed for full power operation at a certain current density and has an anode and a cathode, (2) a liquid coolant flowing through the fuel cell, (3) a heat exchanger for extracting heat from the coolant, and (4) a pump energized by the fuel cell for pumping the coolant between the fuel cell and the heat exchanger. The process comprises the steps of: (i) supplying fuel to the fuel cell's anode; (ii) supplying oxidant to the fuel cell's cathode; (iii) drawing current from the fuel cell at a current density less then 25% of its full-power current density; (iv) while so drawing the current from the fuel cell, discontinuing the pumping of the coolant for an interval of time; (v) during the time interval that pumping is discontinued, allowing the temperature of the fuel cell to rise to a temperature that does not degrade the performance of the fuel cell; and (vi) restarting the pumping at the end of the time interval. The efficiency of the fuel cell system is increased by eliminating the parasitic load of the pump on said system during the interval that the pump is off. While the invention is applicable to any liquid-cooled fuel cell, it is particularly applicable to improving the efficiency of a fuel cell system having a temperature-sensitive PEM fuel cell operating at a current density less then 0.2 A/cm
2
.
According to one embodiment of the invention, during the “pump-off” interval, the temperature of the fuel cell is allowed to rise to a predetermined/preset temperature. When that temperature is reached, the cooling flow is resumed by energizing the cooling pump again. For this embodiment, the temperature of the fuel cell will preferably be determined by monitoring the temperature of either the fuel exiting the anode (a.k.a. anode tailgas), or most preferably the oxidant exiting the cathode (a.k.a. cathode tailgas). Once started, the pump will preferably remain on until the oxidant tailgas temperature of the fuel cell drops to within 2 degrees of the coolant temperature exiting the fuel cell. Then the pump is shut off again.
According to another embodiment of the invention, the “pump-off” time interval will have one or more preset/timed duration(s) that has/have been determined empirically under controlled test conditions (e.g. in a laboratory). In this embodiment, the electrical output of the fuel cell is monitored, and the duration of the interval adjusted in response thereto based on the results of the empirical tests.
The invention will be better understood when considered in the light of the following detailed description of a preferred embodiment thereof, which is given hereafter in conjunction with the drawing.
REFERENCES:
patent: 5503944 (1996-04-01), Meye
Brooks Cary W.
General Motors Corporation
Le Hoa Van
LandOfFree
Intermittent cooling of fuel cell does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Intermittent cooling of fuel cell, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Intermittent cooling of fuel cell will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3209942