Air-cooled fuel cell cathode water management method based on high-frequency impedance

Abstract

The invention discloses an air-cooled fuel cell cathode water management method based on high-frequency impedance, and belongs to the technical field of new energy power generation. According to the air-cooled fuel cell cathode water management method based on the high-frequency impedance, the high-frequency impedance in the process that the voltage of a galvanic pile is increased and then decreased along with the temperature is measured through the high-frequency impedance equipment, and the high-frequency impedance measured when the system is in the optimal operation state serves as the reference high-frequency impedance and serves as the control target of a cathode water management strategy. After the control target is determined, a traditional temperature control mode is not adopted, the high-frequency impedance serves as the control target, control over the water content in the film is achieved, and the problem that the control target is prone to being affected by the environment due to traditional temperature control is solved; and a cathode control method with higher adaptability to environmental conditions is established from the angle of the water content of the membrane.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy power generation, and particularly relates to a cathode water management method for air-cooled fuel cells based on high-frequency impedance. BACKGROUND

[0002] As a kind of clean energy, proton exchange membrane fuel cells have the characteristics of high efficiency, zero pollutant emission, long endurance, low working temperature, etc., and are one of the current research hotspots in the field of new energy. Air-cooled proton exchange membrane fuel cells save auxiliary equipment for realizing functions such as circulation of cooling liquid and humidification of reaction gas, and have the advantages of light weight, high efficiency, compact structure, etc. compared with traditional fuel cells, and are regarded as the ideal power supply for small power systems in the future.

[0003] The output performance of proton exchange membrane fuel cells is greatly related to the water content of the proton membrane during operation, and the fuel cell stack needs to maintain a suitable water content during operation. Too high or too low water content will cause the output performance of the stack to decrease. Therefore, in order to achieve good working performance, the water content in the membrane needs to be reasonably controlled and managed. The water content in the membrane inside the air-cooled fuel cell is mainly controlled by two ways: one is to control the stack temperature through the cooling fan on the cathode side to indirectly affect the water content in the membrane; the other is to open the exhaust valve on the anode side at some moments to exhaust the accumulated water and nitrogen on the anode side.

[0004] The so-called cathode water management method is actually to control the stack to a suitable temperature through the cooling fan, so as to ensure that the water content in the membrane is at a suitable level. It mainly includes two aspects: one is how to set the reference value of the temperature controller under different working conditions, so that the stack operates at the reference value with optimal performance; the other is how to control the actual temperature to the reference value nearby quickly and stably after the temperature reference value is determined. For the second point, many mature algorithms can meet the requirements, such as PID control, predictive control, active disturbance rejection control, etc. For the first problem, no matter what specific control method is used to control the cathode fan, the controller needs a reference temperature input to quickly control the temperature around the reference temperature value through the fan. The so-called reference temperature is the temperature at which the output voltage of the stack is maximum.

[0005] At present, the reference temperature point at which the voltage is maximum is usually measured by experiment under different currents, and a reference temperature-current curve is drawn as the reference input of the cathode temperature controller. However, the reference temperature under different environmental conditions is different. Once the controller controls the temperature too high, the water content of the stack will decrease sharply, which will cause the voltage to decay irreversibly and quickly.

[0006] The above problems are caused by the fact that the water content in the battery membrane cannot be measured, so it is necessary to develop an air-cooled fuel cell cathode water management method with environmental adaptability, so that the stack can operate in the best state under different environmental conditions. SUMMARY

[0007] In order to overcome the defects of the existing air-cooled fuel cell water management method, the present application introduces high-frequency impedance as an index, and provides an air-cooled fuel cell cathode water management method based on high-frequency impedance.

[0008] The background technology mentions that only relying on the reference temperature to control the water content of the proton membrane will produce errors disturbed by environmental conditions, mainly because the air-cooled fuel cell cathode is directly in contact with the environment, and the water content in the environment will directly affect the water content in the membrane of the stack, thereby causing the reference temperature to change. The present application introduces high-frequency impedance as an index, and high-frequency impedance can directly reflect the water content in the membrane, so taking high-frequency impedance as the control target can avoid the influence of environmental conditions on the control target. Based on this, the technical solutions adopted by the present application are as follows:

[0009] An air-cooled fuel cell reference high-frequency impedance measurement method, comprising the following steps:

[0010] Step 1. Open the anode hydrogen inlet valve, and adjust the hydrogen inlet flow to a fixed value;

[0011] Step 2. Start the cathode fan and send the required air to the cathode;

[0012] Step 3. Set the electronic load to constant current mode, and the electronic load obtains the set current from the battery, at this time the fuel cell outputs constant current;

[0013] Step 4. Adjust the fan PWM (Pulse Width Modulation) signal value P A to P A =100%, at this time the stack temperature is controlled at a low level, and the high-frequency impedance value R A of the fuel cell is also at a low level;

[0014] Step 5. After the impedance value remains stable at the current fan speed, reduce the fan PWM signal value to P A -ΔP, measure the output voltage and impedance, and after the impedance value remains stable again, reduce the fan PWM signal value to P A -2ΔP, measure the output voltage and impedance, in this way, the fan PWM signal value is reduced in a fixed step ΔP ladder, and the output voltage and impedance value are measured, the high-frequency impedance rises in a ladder, and when the voltage continuously decays is found, the reduction of the fan PWM signal value is stopped, and the high-frequency impedance R optReference high frequency impedance.

[0015] Further, in step 5, the impedance value remains stable means that the fluctuation range of the impedance value is within 3% within 5 minutes.

[0016] Further, in step 5, the step size ΔP = 5%.

[0017] Further, in step 5, when it is found that the voltage continues to decay when the exhaust valve is closed, stop reducing the fan PWM signal value.

[0018] Reference high frequency impedance R opt Can reflect the specific membrane water content of the stack, and the system has the same R under different environmental conditions opt Therefore, the reference high frequency impedance R opt As the control target of the fuel cell cathode fan controller, by controlling the temperature and measuring the high frequency impedance of the battery in real time, it is controlled near R opt , Realize reasonable cathode water management. The invention provides a reference high frequency impedance based air-cooled fuel cell cathode water management method, which comprises the following steps:

[0019] Step 1. Open the hydrogen inlet valve and the cathode fan, and adjust the hydrogen inlet flow to a fixed value; the anode exhaust valve opening period and the opening duration each time are the same as when measuring the reference high frequency impedance, the electronic load is in constant current mode, and the fuel cell starts to work;

[0020] Step 2. The measured reference high frequency impedance R opt As the control target of the controller, set the initial value of the fan PWM signal to X%, the step size to ΔP, and the control dead zone to ΔR;

[0021] Step 3. Real-time acquisition of high frequency impedance data R of the stack under stable operation by using impedance measurement equipment;

[0022] Step 4. According to the difference between the real-time acquired high frequency impedance data and the reference high frequency impedance R opt , regulate the fan PWM signal value until the difference between the real-time acquired high frequency impedance data and the reference high frequency impedance R opt is within the control dead zone range, and the specific regulation method is: when the real-time acquired high frequency impedance value is greater than R opt + ΔR, the battery presents a dry membrane state, at which time the fan PWM signal value needs to be increased by ΔP step size to reduce the stack temperature; when the real-time acquired high frequency impedance value is less than R opt - ΔR, the stack does not reach the optimal working point, at which time the fan PWM signal value needs to be reduced by ΔP step size to increase the stack temperature;

[0023] This is because the difference between the real-time high-frequency impedance data and the reference high-frequency impedance R opt is less than ΔR, indicating that the reference high-frequency impedance has been reached, at which point the water content of the stack membrane is optimal, and the system remains stable, while the difference between the real-time high-frequency impedance data and the reference high-frequency impedance R opt is greater than ΔR, indicating that the stack has not yet reached the optimal operating state, and the stack temperature needs to be adjusted.

[0024] Further, in step 1, the hydrogen inlet flow rate is the same as when measuring the reference high-frequency impedance.

[0025] Further, in step 1, the electronic load obtains the same current from the battery as when measuring the reference high-frequency impedance.

[0026] Further, in step 2, the initial value X% of the PWM signal is more than 20% higher than the PWM signal value corresponding to the reference high-frequency impedance.

[0027] Further, in step 2, the step size ΔP = 3-5%.

[0028] Further, in step 3, stable operation means that the environmental conditions remain unchanged, and the sampling interval for obtaining high-frequency impedance data is 0.1-3s.

[0029] The present application provides a high-frequency impedance-based air-cooled fuel cell cathode water management method, which measures the high-frequency impedance of the stack voltage during the temperature rise and fall process through a high-frequency impedance device, and uses the high-frequency impedance measured under the optimal operating state of the system as the reference high-frequency impedance, as the control target of the cathode water management strategy. After determining the control target, the present application does not use the traditional temperature control method, but uses the high-frequency impedance as the control target to control the membrane water content. Compared with the prior art, the present application has the following advantages:

[0030] (1) An effective method for measuring the reference high-frequency impedance point is provided, which provides an initial control value for the controller.

[0031] (2) The problem of the control target being easily affected by the environment caused by traditional temperature control is improved, and a cathode control method with stronger adaptability to environmental conditions is established from the perspective of membrane water content.

[0032] (3) The control method provided can accurately control the membrane water content inside the stack, thereby ensuring that the stack is in the optimal operating state.

[0033] (4) The control method provided can be fully automatically controlled through programming, and the implementation process is simple and efficient.

[0034] (5) The control method provided is conducive to combination with specific engineering applications, and facilitates actual solution of problems of the air-cooled fuel cell in engineering applications. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 Structure diagram of a cathode temperature control system of an air-cooled fuel cell based on the present application;

[0036] Figure 2 Output voltage variation curve of the air-cooled fuel cell obtained by monotonously increasing the stack temperature;

[0037] Figure 3 Stack high-frequency impedance variation curve obtained by measuring the reference high-frequency impedance at 11A;

[0038] Figure 4 Reference high-frequency impedance points measured under different experimental conditions;

[0039] Figure 5 Flowchart for measuring the reference high-frequency impedance;

[0040] Figure 6 Control block diagram of the control method provided by the present application;

[0041] Figure 7 Control flowchart of the control method provided by the present application. DETAILED DESCRIPTION

[0042] The specific embodiments of the present application are described below in conjunction with the accompanying drawings, so that those skilled in the art can better understand the present application. It should be particularly noted that, in the following description, when detailed description of known functions and designs may obscure the main content of the present application, these descriptions will be omitted here.

[0043] EMBODIMENT

[0044] In the present embodiment, the air-cooled fuel cell cathode water management method based on high-frequency impedance provided by the present application mainly provides two sub-methods, including an air-cooled fuel cell reference high-frequency impedance measurement method and an air-cooled fuel cell cathode fan control method taking the reference high-frequency impedance as a control target.

[0045] An air-cooled fuel cell reference high-frequency impedance measurement method, comprising the following steps:

[0046] Step 1. The experiment is based on Figure 1 The air-cooled fuel cell cathode temperature control system shown in the figure, open the anode hydrogen gas inlet valve, adjust the hydrogen gas inlet pressure to 20 KPa, set the anode exhaust valve opening period to 30 seconds, and each opening duration to 2 seconds;

[0047] Step 2. Set the cathode fan speed to a fixed value so that the fan can supply oxygen to the fuel cell stack while maintaining a constant temperature;

[0048] Step 3. Set the electronic load to constant current mode. The electronic load draws a fixed current of 11A from the battery. At this time, the fuel cell outputs a constant current, and the air-cooled fuel cell stack starts to work.

[0049] Step 4. Set the fan PWM (Pulse Width Modulation) signal value P A Adjust to P A =100%, at which point the stack temperature is controlled at a low level, and the high-frequency impedance value R of the fuel cell is measured. A It is also at a relatively low level;

[0050] Step 5. After the impedance value stabilizes at the current fan speed, reduce the fan PWM signal value to 95% and measure the output voltage and impedance. Once the impedance value stabilizes again, reduce the fan PWM signal value to 90% and measure the output voltage and impedance again. Continue this process of reducing the fan PWM signal value in 5% increments while measuring the output voltage and impedance. As the high-frequency impedance increases stepwise, observe the fuel cell output voltage curve. Figure 2 As shown, when a continuous voltage decay is detected, the decrease in the fan PWM signal value is stopped, and the high-frequency impedance corresponding to the voltage peak is recorded. This impedance is the reference high-frequency impedance. Figure 3 As shown;

[0051] Step 6. Repeat the experiment under different current densities and environmental conditions to obtain the reference high-frequency impedance at 11A under different humidity and temperature conditions, such as... Figure 4 As shown. The final reference high-frequency impedance of the fuel cell is 27mΩ.

[0052] A method for controlling the cathode fan of an air-cooled fuel cell, using a reference high-frequency impedance as the control target, includes the following steps:

[0053] Step 1. Open the hydrogen inlet valve and the cathode fan. The hydrogen inlet pressure is fixed at 20 kPa. Set the anode exhaust valve opening cycle to 30 seconds and the duration of each opening to 2 seconds.

[0054] Step 2. Set the electronic load to constant current mode. The electronic load draws a fixed current of 11A from the battery, and the air-cooled fuel cell stack starts to work.

[0055] Step 3. Use the measured reference high-frequency impedance as the control target of the controller, set the initial value of the fan PWM signal to 60%, the step size ΔP = 3%, and the control dead zone to ΔR = 0.5mΩ;

[0056] Step 4. Use an impedance measurement device to acquire the high-frequency impedance data R of the fuel cell stack under stable operation in real time. R may be higher or lower than the reference high-frequency impedance. In both cases, the voltage cannot reach the peak value. Therefore, it is necessary to control the fan PWM signal, mainly in two ways: one is to increase the PWM duty cycle to reduce the fuel cell stack temperature, thereby controlling R to decrease; the other is to decrease the PWM duty cycle to increase the fuel cell stack temperature, thereby controlling R to increase.

[0057] Step 5. Adjust the fan PWM signal value based on the difference between the real-time acquired high-frequency impedance data and the reference high-frequency impedance until the difference between the real-time acquired high-frequency impedance data and the reference high-frequency impedance is within the control dead zone. Specifically, when the real-time acquired high-frequency impedance value is greater than R... opt When the voltage is +ΔR, the battery exhibits a film-dry state. At this point, the fan PWM signal value needs to be increased in increments of ΔP to reduce the stack temperature. When the real-time high-frequency impedance value is less than R... opt When the temperature is -ΔR, the fuel cell stack has not reached its optimal operating point. At this time, the fan PWM signal value needs to be reduced by a step size of ΔP to increase the fuel cell stack temperature.

Claims

1. A method for measuring the reference high-frequency impedance of an air-cooled fuel cell, characterized in that, Includes the following steps: Step 1. Open the anode hydrogen inlet valve and adjust the hydrogen inlet flow rate to a fixed value; turn on the cathode fan and set the electronic load to constant current mode; Step 2. Adjust the fan PWM signal value to P A =100%, measure the high-frequency impedance value of the fuel cell at this time; Step 3. Once the impedance value stabilizes, decrease the fan PWM signal value by a fixed step size ΔP and measure the output voltage and impedance value. When the voltage in the output voltage curve continues to decay, stop decreasing the fan PWM signal value and set the high-frequency impedance R corresponding to the maximum voltage value in the output voltage curve to the specified value. opt As a reference high-frequency impedance.

2. The method for measuring the reference high-frequency impedance of an air-cooled fuel cell according to claim 1, characterized in that, In step 3, keeping the impedance value stable means that the fluctuation range of the impedance value is within 3% within 5 minutes.

3. The method for measuring the reference high-frequency impedance of an air-cooled fuel cell according to claim 1, characterized in that, In step 3, the step size ΔP = 5%.

4. The method for measuring the reference high-frequency impedance of an air-cooled fuel cell according to claim 1, characterized in that, In step 3, when it is found that the voltage continues to decay when the exhaust valve is closed, the reduction of the fan PWM signal value is stopped.

5. A method for managing cathode water in an air-cooled fuel cell based on reference high-frequency impedance, characterized in that, Includes the following steps: Step 1. Open the hydrogen inlet valve and cathode fan to adjust the hydrogen inlet flow rate to a fixed value; set the electronic load to constant current mode, and the fuel cell will start working; Step 2. Measure the reference high-frequency impedance R obtained by the method described in claim 1. opt As the control target of the controller, the initial value of the fan PWM signal is set to X%, the step size is ΔP, and the control dead zone is ΔR. Step 3. Acquire the high-frequency impedance data R of the fuel cell stack under stable operation in real time; Step 4. Based on the real-time acquired high-frequency impedance data and the reference high-frequency impedance R... opt The difference between the values ​​adjusts the fan PWM signal value until the real-time acquired high-frequency impedance data matches the reference high-frequency impedance R. opt The difference between them is within the control dead zone range. The adjustment method is as follows: when the real-time high-frequency impedance value is greater than R... opt When +ΔR, the fan PWM signal value is increased in steps of ΔP to reduce the fuel cell stack temperature; when the real-time high-frequency impedance value is less than R... opt When -ΔR is applied, the fan PWM signal value is reduced in increments of ΔP to increase the fuel cell temperature.

6. The method for managing cathode water in an air-cooled fuel cell based on reference high-frequency impedance according to claim 5, characterized in that, In step 1, the hydrogen inlet flow rate is the same as when measuring the reference high-frequency impedance, and the electronic load draws the same current from the battery as when measuring the reference high-frequency impedance.

7. The method for managing cathode water in an air-cooled fuel cell based on reference high-frequency impedance according to claim 5, characterized in that, In step 2, the initial value X% of the PWM signal is more than 20% higher than the PWM signal value corresponding to the reference high-frequency impedance.

8. The method for managing cathode water in an air-cooled fuel cell based on reference high-frequency impedance according to claim 5, characterized in that, In step 2, the step size ΔP = 3-5%.

9. The method for managing cathode water in an air-cooled fuel cell based on reference high-frequency impedance according to claim 5, characterized in that, In step 3, stable operation means that the environmental conditions remain unchanged and the sampling interval for acquiring high-frequency impedance data is 0.1s to 3s.

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