Fuel cell system with improved fuel recirculation

1. A method for operating a fuel cell system comprising a fuel cell stack comprising a fuel inlet and fuel outlet and an oxidant inlet and oxidant outlet and at least one fuel cell comprising a membrane, an anode and a cathode, the anode fluidly connected to the fuel inlet and fuel outlet, the cathode fluidly connected to the oxidant inlet and oxidant outlet and, a fuel supply line for supplying a fuel to the fuel inlet, a jet pump connected to the fuel supply line, a recirculation line connecting the fuel outlet with the jet pump, a solenoid valve arranged in the fuel supply line between the fuel supply and upstream of the jet pump for opening or closing fuel supply to the stack and capable of being controlled to produce a pulsed fuel supply to the stack, a bypass line connected between the fuel supply and the jet pump, and a control unit for operating the valve in response to an operational parameter indicative of the anode-cathode pressure differential, the method comprising the steps of:

determining the load on the fuel cell at which there is the minimum anode pressure differential between the anode inlet and anode outlet to ensure adequate removal of water droplets accumulated in the anode flow field channels, and assigning this load a value L;

determining the load currently applied to the fuel cell system;

closing the solenoid valve in the fuel supply line to direct fuel to the fuel cell stack via the bypass line, a pressure regulator, the jet pump, and the fuel inlet when it is determined that the load applied to the fuel cell system is above the value L; and

controlling the solenoid valve to produce a pulsed fuel supply to the stack when the load applied to the fuel cell system is below the value L by

(a) initially opening the solenoid valve in the fuel supply line and supplying fuel to the fuel cell via the fuel supply line,

(b) completely closing the solenoid valve in the fuel supply line and keeping closed the valve in the bypass line to stop the fuel supply to the stack when the operational parameter indicative of the anode-cathode pressure differential is above a first predetermined threshold value A, and

(c) opening the solenoid valve to restart the fuel supply to the stack when the operational parameter indicative of the anode-cathode pressure differential drops below a second predetermined threshold value B, the second predetermined threshold value B being lower than the first predetermined threshold value A.

2. The method of claim 1 , wherein the control unit operates the valve in response to an operational parameter indicative of the anode-cathode pressure differential, and controls the solenoid valve to produce a pulsed fuel supply to the stack when the load applied to the fuel cell system is below a value L.

3. A method for operating a fuel cell system comprising a fuel cell stack comprising a fuel inlet and fuel outlet and an oxidant inlet and oxidant outlet and at least one fuel cell comprising a membrane, an anode and a cathode, the anode fluidly connected to the fuel inlet and fuel outlet, the cathode fluidly connected to the oxidant inlet and oxidant outlet and, a first fuel supply line for supplying fuel to the fuel inlet, a jet pump connected to the first fuel supply line, a second fuel supply line for supplying fuel to the fuel inlet, a pulsing jet pump connected to the second line, a recirculation line connecting the fuel outlet with the jet pump and/or pulsing jet pump, a solenoid valve arranged in the first fuel supply line between the fuel supply and upstream of the jet pump for opening or closing fuel supply to the stack, a valve arranged in the second fuel supply line between the fuel supply and upstream of the pulsing jet pump for opening or closing fuel supply to the stack via the second fuel supply line, and a control unit for operating the valves in response to an operational parameter indicative of the anode-cathode pressure differential, the method comprising the steps of:

determining the load on the fuel cell at which there is the minimum anode pressure differential between the anode inlet and anode outlet to ensure adequate removal of water droplets accumulated in the anode flow field channels, and assigning this load a value L;

determining the load currently applied to the fuel cell system;

closing the valve in the second fuel supply line and opening the valve in the first fuel supply line to direct fuel to the fuel cell stack via the first fuel line when it is determined that the load applied to the fuel cell system is above the value L; and

controlling the valves to direct fuel to pass through the pulsing jet pump to produce a pulsed fuel supply to the stack when the load applied to the fuel cell system is below the value L by

(a) initially opening the solenoid valve in the first fuel supply line and supplying fuel to the fuel cell via the first fuel supply line,

(b) completely closing the solenoid valve in the first fuel supply line, and keeping closed the valve in the second line, to stop the fuel supply to the stack when the operational parameter indicative of the anode-cathode pressure differential is above a first predetermined threshold value A, and

(c) opening the solenoid valve in the first fuel supply line to restart the fuel supply to the stack when the operational parameter indicative of the anode-cathode pressure differential drops below a second predetermined threshold value B, the second predetermined threshold value B being lower than the first predetermined threshold value A.