Method for controlling pressure of air entering stack of fuel cell system

Abstract

The invention relates to the field of fuel cells, in particular to a fuel cell system air stack entering pressure control method. The method has the following three control modes: mode 1: when the system is just started and the air flow is small, the VNT adopts open-loop control, the opening degree is kept at the opening degree with the highest recovery efficiency, the air inlet reactor pressure is controlled only through a back pressure valve in a closed-loop manner, and when the opening degree of the back pressure valve exceeds 50%, the mode 2 is switched; in the second mode, the VNT and the back pressure valve are both in closed-loop control and jointly control the air inlet pressure, when the opening degree of the back pressure valve is smaller than 40% and the opening degree of the VNT is smaller than 25%, the mode 1 is switched, and when the opening degree of the back pressure valve is larger than 75% and the opening degree of the VNT is larger than 40%, the mode 3 is switched; in the mode 3, the back pressure valve is fully opened, the air inlet reactor pressure is controlled only through the VNT closed loop, and when the opening degree of the VNT is smaller than 30%, the mode 2 is switched. According to the invention, the high energy recovery efficiency is ensured on the premise of ensuring stable control of the air in-pile pressure.

Description

Technical Field

[0001] The present invention relates to the field of fuel cells, and in particular to a method for controlling air inlet pressure of a fuel cell system. Background Art

[0002] In terms of fuel cell development, air compressors with turbo expanders are an important development direction for the development of the next generation of more efficient fuel cell systems. Among them, variable-section turbo expanders have become a common means of improving the efficiency of high-power fuel cell systems because they can adapt to the wide operating range of the system. The expander air compressor uses the principle of turbine expansion to recycle part of the energy in the exhaust of the fuel cell stack, thereby reducing the air compressor's demand for power.

[0003] The VNT (variable nozzle turbine) in the air compressor with turbo expander will work together with the original structural back pressure valve of the fuel cell system to control the air inlet pressure of the battery system. Figure 1 The prior art solution shown includes the following steps: S1, obtaining the required stack pressure of the fuel cell and detecting the first gas pressure of the fuel cell; S2, detecting the opening of VNT and judging whether the opening of VNT is less than 7%; if so, outputting the first opening adjustment value of the exhaust throttle valve through PID control to adjust the first gas pressure to the required stack pressure; if not, proceeding to step S3; S3, judging whether the opening of VNT is not greater than 98%; if so, the exhaust throttle valve is fully opened, and the desired turbine inlet pressure of the turbine end inlet of the air compressor is calculated through PID control according to the required stack pressure; detecting the second gas pressure of the turbine end inlet of the air compressor, and outputting the second opening adjustment value of VNT through PID control to adjust the second gas pressure to the desired turbine inlet pressure; if not, adjusting the opening of the turbine bypass valve to adjust the first gas pressure to the required stack pressure.

[0004] In the above existing solutions, the back pressure valve will suddenly open completely, which leads to the problem that the pressure control stability cannot be guaranteed. Summary of the invention

[0005] The purpose of the present invention is to address the problems existing in the background technology and to propose a method for controlling the air inlet pressure of a fuel cell system, so as to ensure high energy recovery efficiency under the premise of ensuring stable control of the air inlet pressure.

[0006] The technical solution of the present invention is a method for controlling the air inlet pressure of a fuel cell system, which has the following three control modes:

[0007] Mode 1: When the system is just started and the air flow is small, VNT adopts open-loop control, and the opening is maintained at the opening with the highest recovery efficiency. The air inlet pressure is only controlled by the back pressure valve in a closed loop. When the opening of the back pressure valve exceeds 50%, it switches to mode 2.

[0008] Mode 2: Both VNT and back pressure valve are closed-loop controlled to jointly control the air inlet pressure. When the back pressure valve opening is less than 40% and the VNT opening is less than 25%, it switches to mode 1. When the back pressure valve opening is greater than 75% and the VNT opening is greater than 40%, it switches to mode 3.

[0009] Mode 3: The back pressure valve is fully open, and the air inlet pressure is controlled only through the VNT closed loop. When the VNT opening is less than 30%, it switches to Mode 2.

[0010] Preferably, the opening degree at which the VNT recovery efficiency is highest is 20%.

[0011] Preferably, temperature sensors and air pressure sensors are provided at both the air inlet and air outlet locations.

[0012] Preferably, air enters the battery stack via an air compressor and a shut-off valve or a bypass valve, and the air inlet pressure of the battery system is jointly controlled by the VNT in the expander and the back pressure valve.

[0013] Preferably, when the flow rate entering the fuel cell stack is determined, when the opening of the VNT or the opening of the back pressure valve decreases, the air entering the stack pressure increases; when the opening of the VNT or the opening of the back pressure valve increases, the air entering the stack pressure decreases.

[0014] Compared with the prior art, the present invention has the following beneficial technical effects:

[0015] The present invention selects the appropriate time according to the actual opening of the VNT and the back pressure valve when the fuel cell system is running, and smoothly switches the control mode of the VNT and the back pressure valve, thereby ensuring high energy recovery efficiency as much as possible under the premise of ensuring stable control of the air inlet pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 It is a flow chart of the control of the air inlet pressure of the fuel cell system by VNT and back pressure valve in the prior art;

[0017] Figure 2 A schematic diagram of the structure of a fuel cell system matching an air compressor with a variable-section turbo expander;

[0018] Figure 3 A schematic diagram of switching between three control modes in the method for controlling the air inlet pressure of a fuel cell system of the present invention;

[0019] Figure 4It is a schematic diagram of the PID control strategy of the back pressure valve to the air pressure;

[0020] Figure 5 Schematic diagram of VNT's PID control strategy for air pressure. DETAILED DESCRIPTION

[0021] like Figure 2-Figure 5 As shown, air enters the stack through the air compressor and the shut-off valve or bypass valve, and the VNT and back pressure valve in the expander jointly control the air inlet pressure of the battery system. When the flow rate entering the stack is determined, when the opening of the VNT or the opening of the back pressure valve decreases, the air inlet pressure increases; when the opening of the VNT or the opening of the back pressure valve increases, the air inlet pressure decreases. Temperature sensors and air pressure sensors are set at the air inlet and air outlet to collect the air temperature and air pressure at the inlet and outlet. According to the preset target air inlet pressure and the actual air inlet pressure collected by the sensor, adjust the opening of the back pressure valve and VNT so that the actual air inlet pressure accurately follows the target pressure to ensure that the fuel cell system operates at an appropriate pressure.

[0022] Since the opening changes of the back pressure valve and VNT will cause the change of the air inlet pressure, in order to reduce the air pressure fluctuation caused by the mutual coupling between the two and achieve a higher energy recovery efficiency, such as Figure 3 As shown, the fuel cell system air inlet pressure control method of the present invention is divided into the following three control modes that can be switched:

[0023] Mode 1: When the system is just started and the air flow is small, VNT adopts open-loop control, and the opening is maintained at the opening with the highest recovery efficiency (20%). The air inlet pressure is only controlled by the back pressure valve in a closed loop. Under the premise of avoiding stagnation, when the same air flow passes, the smaller the VNT opening, the higher the recovery efficiency; under the same air flow, as the back pressure valve opening increases, the influence of the back pressure valve on the flow resistance becomes smaller and smaller. When the back pressure valve opening exceeds 50%, the influence of the back pressure valve opening on the flow resistance decreases, and it is switched to mode 2 at this time;

[0024] Mode 2: Both VNT and back pressure valve are closed-loop controlled to jointly control the air inlet pressure. When the back pressure valve opening is less than 40% and the VNT opening is less than 25%, it switches to mode 1. When the back pressure valve opening is greater than 75% and the VNT opening is greater than 40%, the increase in the back pressure valve opening has almost no effect on the flow resistance, and it switches to mode 3.

[0025] Mode 3: The back pressure valve is fully open, and the air inlet pressure is controlled only through the VNT closed loop; when the VNT opening is less than 30%, it switches to Mode 2.

[0026] This embodiment smoothly switches the control mode of the VNT and the back-pressure valve when the appropriate timing, i.e., the appropriate switching conditions, are reached according to the actual opening of the VNT and the back-pressure valve when the fuel cell system is running, thereby ensuring high energy recovery efficiency as much as possible while ensuring stable control of the air inlet pressure.

[0027] In the low flow range, the VNT maintains the smallest opening for the highest recovery efficiency, and the back pressure valve is also in a smaller opening range for closed-loop control, which can meet the pressure control requirements.

[0028] When the back pressure valve opening reaches a certain limit, the pressure regulation capability weakens. At this time, VNT intervenes, and VNT and back pressure valve jointly participate in pressure control. When the back pressure valve opening continues to increase, the pressure regulation capability further weakens. At this stage, the back pressure valve becomes fully open, and only VNT participates in control, which can effectively reduce the pressure fluctuations in the process.

[0029] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited thereto, and various changes can be made within the knowledge scope of technicians in the relevant technical field without departing from the purpose of the present invention.

Claims

1. A method for controlling air inlet pressure of a fuel cell system, characterized in that: There are three control modes: Mode 1: When the system is just started and the air flow is small, VNT adopts open-loop control, and the opening is maintained at the opening with the highest recovery efficiency. The air inlet pressure is only controlled by the back pressure valve in a closed loop. When the opening of the back pressure valve exceeds 50%, it switches to mode 2. Mode 2: Both VNT and back pressure valve are closed-loop controlled to jointly control the air inlet pressure. When the back pressure valve opening is less than 40% and the VNT opening is less than 25%, it switches to mode 1. When the back pressure valve opening is greater than 75% and the VNT opening is greater than 40%, it switches to mode 3. Mode 3: The back pressure valve is fully open, and the air inlet pressure is controlled only through the VNT closed loop. When the VNT opening is less than 30%, it switches to Mode 2.

2. A fuel cell system air inlet pressure control method according to claim 1, characterized in that: The highest VNT recovery efficiency is at an opening of 20%.

3. A fuel cell system air inlet pressure control method according to claim 1, characterized in that: Temperature sensors and air pressure sensors are installed at the air inlet and outlet of the stack.

4. A fuel cell system air inlet pressure control method according to claim 3, characterized in that: Air enters the fuel cell stack through the air compressor and the shut-off valve or bypass valve, and the air entering the battery system is controlled by the VNT in the expander and the back pressure valve.

5. A fuel cell system air inlet pressure control method according to claim 4, characterized in that: When the flow rate entering the fuel cell stack is determined, when the opening of the VNT or the opening of the back-pressure valve decreases, the air entering the stack pressure increases; when the opening of the VNT or the opening of the back-pressure valve increases, the air entering the stack pressure decreases.

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