Fuel Cell Ejector Recirculation via Pulse Width Modulation
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Solution Overview
Problem
Fuel cell systems with ejector vacuum pumps have a narrow operating range, making them inadequate for handling abrupt changes in fuel and oxidizer consumption, especially in automotive applications, and existing solutions either degrade battery performance or reduce system efficiency.
Innovation Solution
The fuel cell system employs frequency and pulse width modulation of the fuel gas mass flow rate to control the mass flow through the ejector, allowing for a wider operating range without increasing system complexity, by maintaining a sufficient Venturi effect for reactant gas flow and pressure management.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If an ejector vacuum pump is used for recirculating off-gas, then the system structure is simplified and no lubrication is required, but the operating range becomes narrow
Solution Approach 1:
The patent applies dynamics by making the fuel gas mass flow rate variable through frequency and pulse width modulation of the supply valve. This allows the ejector to adapt to different operating conditions (from idle to full load) by dynamically adjusting the fuel gas flow, thereby widening the operating range while maintaining the simple ejector structure without moving parts.
Solution Approach 2:
The patent changes the parameter of fuel gas mass flow rate by modulating the supply valve's frequency and pulse width. This parameter adjustment enables the ejector to maintain effective operation across a wide range of power demands, resolving the contradiction between structural simplicity and adaptability to varying operating conditions.
2Loss of energy
If the fuel gas mass flow rate is reduced to match low power demand, then fuel efficiency improves, but the ejector can no longer maintain sufficient Venturi effect for proper off-gas recirculation
Solution Approach 1:
The system dynamically adjusts the fuel gas mass flow rate using frequency and pulse width modulation of the supply valve. This dynamic control allows the fuel flow to be optimized for fuel efficiency at low loads while maintaining sufficient flow rates during brief periods when the valve is fully open, ensuring the ejector consistently maintains the Venturi effect needed for reliable off-gas recirculation.
Solution Approach 2:
The patent employs periodic action through frequency modulation of the fuel supply valve. By opening and closing the valve at controlled frequencies with specific pulse widths, the system creates periodic fuel gas flow that maintains the ejector's Venturi effect during active phases while reducing average fuel consumption, thus resolving the contradiction between fuel efficiency and ejector performance reliability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enables the fuel cell system to maintain efficient operation across varying loads without degrading battery performance or reducing efficiency, ensuring consistent reactant gas flow and pressure management, thus extending the operating range of the ejector vacuum pump.
Implementation Method 1
it is necessary to pressurize the reactant gases entering the stack 10 in order that the reactants flow from the inlets to the outlets through all the fuel cells... the fuel gas and the oxidizer gas are driven along the flow channels of respective flow fields 50 by a pressure differential
Implementation Method 2
the off-gas from the fuel cell stack is usually recirculated... the off-gas discharged from the anode side of the fuel cells is first guided through a gas/water separator and then pumped and mixed with fresh fuel gas to be supplied to the fuel cells. It is known to pump the off-gas by means of an ejector vacuum pump
Data Source
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AI summary
The fuel cell system includes: - at least one fuel cell adapted to generate electrical energy from a fuel gas and an oxidizer gas; - a fuel feed duct (11) provided for supplying the fuel cell with fuel gas, the fuel feed duct including an upstream part (11A) and a downstream part (11B); - a Venturi effect ejector (113) including a high pressure inlet (233), a low pressure inlet (238) and an outlet (240), the upstream part of the fuel feed duct being connected to the high pressure inlet of the ejector and the downstream part extending between the ejector outlet and the fuel cell; - an off-gas recirculation duct (11R) extending between the fuel cell and the low pressure ejector inlet (238) so that, in the presence of a stream of fuel gas coming from the upstream part of the fuel feed duct (11A) and passing through the ejector (113), the ejector draws up off-gas from the recirculation duct and ejects it into the downstream part (11B) mixed with the stream of fuel gas coming from the upstream part; - a control circuit (15) and a valve (110) arranged in the upstream part (11A) of the fuel feed duct and arranged to be controlled by the control circuit, the valve being adapted to be placed in an open state, in which it lets the stream of fuel gas from the upstream part pass through the ejector (113), or in a closed state, in which no stream of gas from the upstream part can flow through the ejector. - characterized in that said control circuit (15) is arranged to place the valve alternately in the open state and then the closed state, so that the stream of fuel gas passing through the ejector (113) is intermittent, being frequency and/or pulse width modulated by the control circuit.