Fuel cell system comprising an ejector for recirculating off-gas from a stack

A fuel cell system including:

• at least one fuel cell adapted to generate electrical energy from hydrogen gas and oxygen gas;
• a pressurized hydrogen storage vessel connected to a fuel feed duct provided for supplying the fuel cell with hydrogen, the fuel feed duct including an upstream part and a downstream part;
• a pressurized oxygen storage vessel connected to an oxidizer feed duct provided for supplying the fuel cell with oxygen, the oxidizer feed duct including an upstream part and a downstream part;
• a first and a second Venturi effect ejector, the first and the second Venturi effect ejectors each including a high pressure inlet, a low pressure inlet and an outlet, the upstream part of the fuel feed duct connecting the hydrogen storage vessel to the high pressure inlet of the first ejector and the downstream part of the fuel feed duct extending between the outlet of the first ejector and an anode side of the fuel cell, the upstream part of the oxidizer feed duct connecting the oxygen storage vessel to the high pressure inlet of the second ejector and the downstream part of the oxidizer feed duct extending between the second ejector outlet and a cathode side of the fuel cell;
• a first and a second off-gas recirculation duct, the first off-gas recirculation duct extending between the anode side of the fuel cell and the low pressure inlet of the first ejector, and the second off-gas recirculation duct extending between a cathode side of the fuel cell and the low pressure inlet of the second ejector, so that, in the presence of a stream of hydrogen coming from the upstream part of the fuel feed duct and passing through the first ejector, the first ejector draws up off-gas from the first recirculation duct and ejects it into the downstream part of the fuel feed duct mixed with the stream of hydrogen, and so that, in the presence of a stream of oxygen coming from the upstream part of the oxidizer feed duct and passing through the second ejector, the second ejector draws up off-gas from the second recirculation duct and ejects it into the downstream part of the oxidizer feed duct mixed with the stream of oxygen;
• a control circuit associated with a first valve and a second valve arranged in the upstream parts of the fuel and oxidizer feed ducts, each of said first and the second valves being arranged to be controlled by the control circuit, and being adapted to be placed in an open state, in which it lets the stream of hydrogen or the stream of oxygen from the corresponding upstream part pass through one of said first and second ejectors, or in a closed state, in which no gas flux from the upstream part of the fuel feed duct or of the oxidizer feed duct can pass through said one of said first and second ejectors;
• wherein the control circuit is arranged to place the first valve alternately in the open state and in the closed state, so that the stream of hydrogen passing through the first ejector is intermittent, in such a way as to allow the control circuit to frequency and/or pulse width modulate the mass flow rate of the hydrogen without affecting the intensity of the stream of hydrogen passing through the first ejector at the times when the first valve is in the open state, and wherein the control circuit is arranged to place the second valve alternately in the open state and in the closed state, so that the stream of oxygen passing through the second ejector is intermittent, in such a way as to allow the control circuit to frequency and/or pulse width modulate the mass flow rate of the oxygen without affecting the intensity of the stream of oxygen passing through the second ejector at the times when the second valve is in the open state.