BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic block diagram of a hybrid fuel cell system;
FIG. 2 is a block diagram for a control scheme for controlling the power requirements of a fuel cell stack during low power transients to reduce RH cycling of the stack, according to an embodiment of the present invention;
FIG. 3 is a graph with battery state of charge on the horizontal axis and a multiplier on the vertical axis showing a state of charge weighting factor for controlling the battery charging at low power transients, according to an embodiment of the present invention; and
FIG. 4 is a flow chart diagram showing a process for calculating fuel cell stack power during a low power transient, according to another embodiment of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The following discussion of the embodiments of the invention directed to a control scheme for reducing RH cycling in a fuel cell stack associated with a hybrid fuel cell hybrid system is merely exemplary in nature, and is no way intended to limit the invention or its applications or uses.
FIG. 2 is a schematic block diagram 50 showing a control scheme for reducing relative humidity cycling in the fuel cell stack 12. A driver power request is provided at input node 52, and is applied to a variable first order damping filter 54. The first order filter 54 operates as a damping function that reduces the driver power request so that it is more slowly reduced during low power transients, such as from 50 kW to 10 kW. For example, if the vehicle operator is driving along a highway, and suddenly wishes to break to an idle power demand, the driver request signal at the input node 52 may be reduced quickly. As discussed above, the flow of the hydrogen can be quickly reduced, but the cathode airflow from the compressor 30 has a slow response time, which results in drying of the membranes in the stack 12. By damping the power request from the driver using the filter 54, the fuel cell stack 12 will continue producing power at a slower reduced rate, which produces product water, so that the response time of the compressor 30 does not cause air to dry out the membranes in the stack 12. A set-point signal at input node 56 adjusts the damping factor of the filter 54 so that different damping requirements are provided depending on the degree of the lower power transient.
Because the fuel cell stack 12 would generate more power than is requested by the driver during the damping operation provided by the filter 54, it is desirable to use that power for useful purposes, if possible. According to the invention, the extra power generated by the fuel cell stack 12 during the damping procedure is used to charge the battery 14. A battery state of charge (SOC) signal at input node 58 is provided to a state of charge weighting factor look-up table 60 that provides a weighting factor depending on the state of charge of the battery 14. For a higher battery SOC, the weighting factor would be smaller and for a lower battery SOC, the weighting factor would be larger. The weighting factor from the look-up table 60 is multiplied by the damped power demand signal from the filter 54 by a multiplier 62 to provide the actual power demand to the fuel cell stack 12.
FIG. 3 is a graph with battery SOC percentage on the horizontal axis and a multiplier factor (weighting factor) on the vertical axis showing a representative example for the weighting factor look-up table 60. An optimal battery SOC is selected for a particular system based on experimentation and other factors. In this example, the optimal battery SOC is about 60%. When the battery SOC is 60%, then the multiplier factor is 1 and the damped power request from the filter 54 is passed through the multiplier 62 unchanged. The extra power provided by the stack 12 above the requested power provided by the damping filter 54 will be used to charge the battery 14 so that the battery SOC will go slightly above the optimal battery SOC. If the battery SOC is less than 60%, then the multiplier factor is greater than 1 so that the power demand signal is greater than the damped power request to provide a higher power request for increased battery charging to more quickly charge the battery 14 to the optimal battery SOC. If the battery SOC is greater than 60%, then the multiplier factor is less than 1, which reduces the damped power demand signal from the filter 54 in the multiplier 62. In this condition, the hydrogen flow will be shut off quicker than the compressor flow, and may provide some membrane drying.
FIG. 4 is a flow chart diagram 70 showing a process for reducing RH cycling of the membranes in the fuel cell stack 12 during low power transients, according to another embodiment of the present invention. The algorithm monitors a driver power request signal P_req at box 72. The algorithm estimates the stack current density request j_req for the power request based on a stack polarization curve at box 74. The algorithm then commands a cathode air flow signal dm_air_req that would satisfy the current density request j_req at box 76. For low power transients, the algorithm monitors the cathode air flow through the flow meter 34 as the compressor speed is reduced to achieve the air flow dm_air_req at box 78. As discussed above, it will take some time for the compressor 30 to get to the air flow required for the low power during a low power transient.
As the speed of the compressor 30 decreases, the algorithm calculates a cathode feedback stoichiometry stoich_fb based on the current density request j_req at box 80 and the flow of air to the stack 12 using, for example, the equation:
Where molar_flow_air is the molar flow rate of air through the flow meter 34 converted from mass flow, cell_area is the area of the membrane in each fuel cell and F is Faradays constant.
The algorithm then calculates an intermediate current density j_star that will maintain the cathode stoichiometry at the set-point for the current density request j_req at box 82 using, for example, the equation:
The stack 12 produces output power for the intermediate current density j_star for the current compressor speed. Hydrogen will continue flowing to the stack 12 to satisfy the changing current density j_star as the compressor speed is reduced. The algorithm performs these calculations continuously during the low power transient until the compressor speed reaches the air flow demand dm_air_req. The excess power that is generated by the stack 12 during the low power transient can be used to charge the battery 14 at box 84, as discussed above.
The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.
Patent Application
20080050626
