Cold starting and purging method for proton exchange membrane fuel cell system

Abstract

The invention relates to the technical field of fuel cells, and provides a cold starting and purging method for a proton exchange membrane fuel cell system. According to the method, the water content in the membrane is calculated by acquiring the stack temperature and the high-frequency impedance value in real time, and then different purging and starting control strategies are adopted. And during shutdown purging, low-temperature purging is carried out, lambda is calculated according to the impedance difference value and the temperature, the cathode purging excess coefficient is adjusted according to the lambda, and after the purging is finished, a residual hydrogen discharge mode is entered, and the related water amount is recorded. During cold start, related water quantity is calculated according to in-reactor temperature, shutdown water quantity and proton exchange membrane parameters, and current is controlled according to various conditions. In a normal operation stage, continuously monitoring and calculating the water content in the membrane and adjusting the operation according to the water content. The method can quantitatively characterize the purge sufficiency, improve the cold start success rate, reduce the stack damage and the cold start and purge duration, and improve the hydrogen utilization efficiency.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and particularly to a cold start and purge method for a proton exchange membrane fuel cell system. Background Art

[0002] A hydrogen fuel cell is an electrochemical power generation device that uses hydrogen and oxygen as raw materials to carry out an electrochemical reaction to generate water and convert chemical energy into electrical energy. At low temperatures, the water generated by the electrochemical reaction will freeze, which may cause the performance of the fuel cell system to rapidly decay or be damaged. At low temperatures, there is also a tendency for more water vapor to remain inside the fuel cell, resulting in abnormal humidity inside the stack and affecting the normal operation of the fuel cell.

[0003] The current solutions mainly use the corresponding gases at the anode / cathode to purge the anode / cathode when the fuel cell is turned on and off, so as to reduce the water content in the membrane electrode of the fuel cell, thereby reducing the formation of solid ice, and at the same time minimizing the air in the anode. In addition, after starting up and running, the stack and its internal plates and membrane electrodes can be preheated by externally connecting a power supply to electrically heat the cooling water and other means to achieve warm-up of the stack.

[0004] Existing proton exchange membrane hydrogen fuel cell systems have problems of over-purging or insufficient purging during the low-temperature purge and cold start processes, resulting in cold start failure or low performance after cold start. Therefore, how to balance the residual water in the purge system during the fuel cell purge process and ensure that the performance of the stack after purging does not affect the next start-up, the start-up control method of the fuel cell purge and cold start has become an important factor affecting the cold start performance of the hydrogen fuel cell system. Summary of the Invention

[0005] The problem to be solved by the present invention is to accurately identify the water content in the membrane of the stack in a low-temperature environment and take effective protective measures to improve the cold start success rate and performance.

[0006] In view of the deficiencies of the prior art, the technical solution adopted by the present invention to solve its technical problems is: a cold start and purge method for a proton exchange membrane fuel cell system, including the following steps:

[0007] Obtain the temperature and high-frequency impedance value of the stack in real time;

[0008] Calculate the water content in the stack membrane in real time based on the temperature and high-frequency impedance value of the stack;

[0009] Adopt different purge and start-up control strategies based on different water contents in the stack membrane, and the purge and start-up control strategies include: shutdown purge, cold start, and normal operation.

[0010] Preferably, the shutdown purge steps are as follows:

[0011] When the ambient temperature is lower than 0°C or a calibrated target value, the arbitration enters the low-temperature purge;

[0012] During the purge, the EIS impedance measurement is used to measure the current high-frequency impedance value in real time. The difference between the high-frequency impedance value at the start of the purge and the current high-frequency impedance value is taken to obtain the impedance value increase of the stack during the purge. According to the number of stack plates, the impedance value increase R (mΩ) of a single cell is calculated. The inlet and outlet water temperatures of the stack are measured in real time using a temperature sensor, and λ is calculated through the calculation formula;

[0013]

[0014] Among them, h is the thickness of the proton exchange membrane (unit: \um), R is the high-frequency impedance value of a single cell (unit: mΩ), S is the activation area (unit: cm2), and T is the current temperature (unit: K); the threshold value λmin of λ is set (which is the actual calibrated control value). When λ reaches the set λmin, the low-temperature shutdown purge is completed and the residual hydrogen discharge stage is entered;

[0015] Record λ at the completion of shutdown when entering the residual hydrogen discharge mode 0 , calculate the water content n in the proton exchange membrane inside the stack at the end of the shutdown purge 0 , record the current integral value I during the residual hydrogen discharge 0 , calculate the amount of water n generated by the residual hydrogen discharge 1 , and use n 0 + n 1 as the total amount of water n in the stack after the fuel cell system shuts down 2 Record it in the controller memory.

[0016] Preferably, the cold start steps are as follows:

[0017] Based on the temperature T inside the fuel cell stack, when the temperature inside the fuel cell stack is low, the arbitration determines that the fuel cell system enters the cold start mode and calls the amount of water n at the previous shutdown 2 ; Calculate the maximum water capacity n that the stack can accommodate according to the proton exchange membrane parameters max , calculate the amount of water n required for the stack to reach the saturated state under the current conditions 3 , n 3 = n max - n 2 ;

[0018] During the cold start process, record the current integral I of the fuel cell 1 , calculate the generated n during the cold start process 4 , where N is the number of single cell plates; Different control strategies are adopted according to the relationship between n 3 and n 4 and different temperature T conditions.

[0019] Preferably, according to n 3 and n 4 and different situations of temperature T, different control strategies are adopted as follows: If n 3 ≤n 4 at the initial stage of cold start, the current is allowed to be loaded only when the temperature T in the stack rises to 0°C; when T < 0 and n 3 > n 4 , the current is increased on the premise of ensuring the single-cell voltage, and the voltage is controlled to increase the heat generation of the stack; when T < 0 and n 3 ≤n 4 , the loaded current is adjusted according to the single-cell state of the stack; when T > 0 and n 3 ≤n 4 , the drainage valve frequency is increased. If an abnormality occurs, the anode drainage valve is opened and the loaded current is reduced; when T > 0 and n 3 > n 4 , the current loading rate is limited according to the single-cell voltage; when T reaches the target temperature and meets the normal operation conditions of the fuel cell system, the cold start is completed.

[0020] Preferably, the normal operation includes the following steps:

[0021] After the cold start is completed, enter the normal operation stage, clear n 1 ~n 4 and record it again. Monitor the average temperature T of the stack in real time, record the EIS high-frequency impedance in real time, calculate the water content λ in the membrane during the operation, set the lower limit of λ in the operation stage as λ min , and perform corresponding operations by comparing λ with λ min .

[0022] Preferably, the calculation formula of the said n 0 is: The calculation formula of the said n 1 is: The calculation formula of the said n max is: The calculation formula of the single-cell mass m is: where N is the number of single-cell plates, m is the single-cell mass (g), EW is the ion exchange equivalent (g / mol), h is the thickness of the proton exchange membrane (unit: \um), A is the activation area (unit: cm 2 ), ρ is the density of the proton exchange membrane (unit: kg / m 3 ), t is the remaining hydrogen discharge duration. The above parameters such as the ion exchange equivalent EW of the proton exchange membrane, the number of single-cell plates N, and the single-cell mass m are calibration values and are calibrated according to the actual parameters of the stack.

[0023] Preferably, during the operation, compare λ with λmin The corresponding operations based on the relationship are as follows: When the current is stable, if λ < λmin, a warning for low water content in the membrane is triggered, and the inlet and outlet water temperatures are interpolated and corrected according to the pre-calibrated operating condition curve. If λmin is still not reached within the check time after correction, the cathode operating conditions are corrected. If it still cannot reach, a fault is triggered and power is limited. If it reaches, the operating conditions are maintained until the power changes and the normal operating conditions are responded to, and this action is repeated after the current is stable.

[0024] Preferably, the standard for stable current is that the current fluctuation range is within ±5A within a certain period of time.

[0025] The beneficial effects of the present invention are as follows:

[0026] Precise control and quantitative evaluation: By obtaining the stack temperature and high-frequency impedance value in real time to calculate the water content in the membrane, it can quantitatively characterize whether the purge is sufficient at low temperatures of the fuel cell, realizing precise control and quantitative evaluation of the purge process, and changing the situation that it is difficult to accurately measure the purge effect in the past;

[0027] Improve cold start performance: During the cold start process, the current is precisely controlled in stages based on multiple factors such as the temperature and water content in the stack. Different current control strategies are adopted in different stages, avoiding stack performance problems caused by improper current control, greatly improving the success rate of cold start of hydrogen fuel cells, reducing damage to the stack, and enhancing the applicability and reliability of fuel cells in low-temperature environments;

[0028] Shorten the time: The optimized purge and cold start strategies reduce the cold start and purge duration, improve the response speed of the fuel cell system, enable the equipment to be put into use faster, improve work efficiency, and meet the requirements for quick start in practical applications;

[0029] Improve energy utilization efficiency: Reasonable purge and current control strategies reduce unnecessary hydrogen consumption, improve the hydrogen utilization efficiency during the cold start and purge processes of fuel cells, reduce operating costs, and contribute to the wide application and sustainable development of fuel cell technology. Description of the Drawings

[0030] Figure 1 is a schematic diagram of the purge strategy of the present invention under low-temperature environments;

[0031] Figure 2 is a schematic diagram of the cold start strategy of the present invention;

[0032] Figure 3 is a schematic diagram of the normal operation process strategy of the present invention;

[0033] Figure 4It is a schematic diagram of the relationship between λ and the excess coefficient during the shutdown purge process involved in the present invention;

[0034] Figure 5 It is a schematic diagram of the lift coefficient correction of the current based on the single-cell voltage involved in the present invention. Specific embodiments

[0035] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments. The embodiments of the present invention are given for the purpose of illustration and description, and are not exhaustive or limit the present invention to the disclosed form. Many modifications and variations are obvious to those of ordinary skill in the art. The embodiments are selected and described to better illustrate the principles and practical applications of the present invention, and to enable those of ordinary skill in the art to understand the present invention and thus design various embodiments with various modifications suitable for specific purposes.

[0036] The present invention provides a cold start and purge method for a proton exchange membrane fuel cell system, which is used to solve the defects of over-purging or insufficient purging during the purging and cold start processes of a proton exchange membrane hydrogen fuel cell system in the prior art, resulting in cold start failure or low performance after cold start, and to realize the identification of the water content in the membrane of the stack in a low-temperature environment and take protective measures to improve the cold start success rate and cold start performance.

[0037] The present invention provides a cold start and purge control method for a proton exchange membrane fuel cell system, and the specific process includes:

[0038] Using a DCDC converter to realize real-time acquisition of the temperature and high-frequency impedance value of the stack;

[0039] Calculating the water content in the stack membrane in real time based on the temperature and high-frequency impedance value of the stack;

[0040] 1. Based on different purge stages and the water content in the stack membrane, adopt different purge and start control strategies as follows: shutdown purge, when the ambient temperature is lower than 0°C, perform low-temperature purge; during the purge, use EIS impedance measurement to measure the current high-frequency impedance value in real time, take the difference from the high-frequency impedance value when entering the purge to obtain the impedance value increased by the stack during the purge process, and calculate the impedance value R (mΩ) increased by a single cell based on the number of stack plates. Use a temperature sensor to measure the water inlet temperature and water outlet temperature of the stack in real time and take the average value of the water inlet and water outlet temperatures as the current temperature of the stack, and obtain the water content λ in the membrane using the following calculation formula;

[0041] Let the thickness of the proton exchange membrane be h (unit: \um), the high-frequency impedance value of a single cell be R (unit: mΩ), the active area be S (unit: cm 2 ), and the current temperature be T (unit: K),

[0042]

[0043] Parameters such as activation area S, proton exchange membrane thickness h, and proton exchange membrane density are derived from the inherent parameters of the membrane electrode and provided by the manufacturer; the single cell high-frequency impedance value R is provided by the high-frequency impedance acquisition device;

[0044] Set the threshold λmin of λ (the actual calibration control value). When λ decreases continuously with the purge, the cathode purge excess coefficient follows the following relationship. The following is the cathode excess coefficient setting value based on the actual data performance of the cold shutdown purge of the stack (a calibrable array, calibrated according to the actual cold shutdown purge performance of the stack). It ensures that at the beginning of the purge, the moisture in the membrane can be quickly taken away to reduce the purge time, and at the end of the purge, the air volume can be reduced to avoid excessive air flow purge leading to rapid reduction of stack performance, such as low stack performance faults, single cell single low, reverse polarity and other faults; when λ reaches the set λmin, the cold shutdown purge is completed, the residual hydrogen discharge mode is entered, and the shutdown is completed;

[0045] 2. At present, some industries will avoid using residual hydrogen discharge after shutdown when purging at low temperatures, so as to try to prevent water from being generated due to residual hydrogen discharge after purging and causing freezing at low temperatures. However, such use will bring safety hazards to the use of fuel cell systems. Therefore, the water content in the membrane is used to quantitatively characterize the amount of water generated during the residual hydrogen discharge stage, and to guide the next cold start control. When the residual hydrogen is discharged, the λ at the completion of the shutdown is recorded. 0 , and based on this λ, the water content n in the proton exchange membrane inside the stack at the end of the shutdown purge is calculated 0 (mol), can be calculated by the following formula: At the same time, because the proton exchange membrane has been purged to a relatively dry state during the residual hydrogen discharge, and there is no gas flow in the cathode, it is approximately assumed that all the generated water exists inside the fuel cell stack. The current integral value I during the residual hydrogen discharge process is recorded. 0 ; and calculate the amount of water n produced by residual hydrogen discharge 1 (mol); And n 2 =n 0 +n 1 After the fuel cell system is shut down, the total amount of water in the stack is recorded in the controller memory;

[0046] 3. Based on the fuel cell stack temperature T, when the fuel cell stack internal temperature is lower than 0°C, the arbitration fuel cell system enters the cold start mode and calls the water volume n at the last shutdown. 2 ; and according to the proton exchange membrane parameters, the maximum amount of water that the stack can hold is calculated as n max(mol) The above parameters such as the ion exchange equivalent of the proton exchange membrane, the number of single cells in the stack, and the mass of a single cell are calibration values, which are calibrated according to the actual parameters of the stack;

[0047] Calculate the amount of water n required for the stack to reach the saturated state under the current condition 3 (mol), n 3 = n max - n 2 ;

[0048] Based on the above amount of water and temperature, when the fuel cell system is in the cold start mode, there are the following situations. Denote the current integration of the fuel cell during cold start as I 1 , then during the cold start process, the amount of water n 4 (mol) = I 1 / 96485 / 2 * the number of single cells in the stack:

[0049] If at the initial stage of cold start, n 3 ≤ n 4 , it is considered that the previous shutdown purge is insufficient and cold start with current loading cannot be directly carried out. It is only allowed to load current when the stack temperature T rises to 0 °C; denoted as the 110 stage

[0050] When T < 0, n 3 > n 4 , it is recorded as the 120 stage. At this time, since the water content inside the current stack has not reached the saturated state, the overall performance of the fuel cell stack may be too low. As the stack continues to load current, the voltage continues to rise. At this time, the current should be increased as much as possible while ensuring the single cell voltage, and the voltage should be controlled within a reasonable range to increase the heat generation of the stack as much as possible to accelerate the cold start process. At this time, the current is increased based on the voltage; when n 3 ≤ n 4 , it is recorded as the 121 stage, and it is considered that the stack is close to saturation at this time. If the stack continues to draw current, since the internal temperature of the stack is still below 0 °C, the water generated by subsequent reactions will freeze inside the stack, and the stack performance may decline after reaching the peak. At this time, the loading current should be adjusted in a timely manner according to the single cell state of the stack to avoid large reverse polarization causing damage to the stack. When the reverse polarization is serious, the current loading should be stopped, and external auxiliary heating should be used to heat it above 0 °C before starting again;

[0051] When T > 0, n 3 ≤ n 4When it is in this state, it is recorded as the 210 stage. Since the external air is cold and the temperature of the fresh hydrogen is low, while the temperature inside the stack has risen above 0°C, the residual ice cubes in the anode and cathode circuits melt. Attention should be paid to the water blockage phenomenon that may occur at low temperatures, and the drainage valve frequency in this process needs to be increased. If a single low or reverse polarization phenomenon occurs at this time, the anode drainage valve should be opened in time, and the load current should be reduced to prevent damage to the stack.

[0052] When T > 0, n 3 > n 4 When it is in this state, it is recorded as the 220 stage. Although the temperature inside the stack has reached 0°C, since the membrane inside the stack is still not saturated, at this time, the current loading rate should be limited according to the minimum single-cell voltage and the average single-cell voltage to prevent the voltage from decreasing too quickly due to too fast current loading, resulting in shutdown failure or damage of the fuel cell system. The limiting rate of the current is corrected by a coefficient based on the single-cell voltage:

[0053] When T reaches the target temperature to meet the conditions for the normal operation of the fuel cell system, the cold start is completed.

[0054] After the cold start is completed, clear n1 - n4, and start recording n1 - n4 again when entering operation. Using the same calculation method, calculate the water content inside the membrane during the operation process. Try to maintain n3 ≤ n4 as much as possible. Otherwise, during the operation process, the stack is prone to performance degradation due to excessive dryness, affecting the efficiency of the fuel cell system.

[0055] Adopting this cold start and purging method can quantitatively characterize whether the purging at low temperature of the fuel cell is sufficient. During the cold start process, for the control of the current and diagnosis, effective distinctions are made at different stages of the cold start, increasing the success rate of the cold start of the hydrogen fuel cell, reducing the damage to the stack, reducing the cold start and purging duration, and improving the hydrogen utilization efficiency during the cold start and purging process of the fuel cell.

[0056] Step 401, for shutdown purging, when the ambient temperature is lower than 0°C or a calibrated target value, the arbitration enters low-temperature purging; use the air compressor to purge the cathode, and control the purging excess coefficient as Figure 4 shown. Look up the table based on the currently calculated λ in real time to obtain the current purging excess coefficient. According to the excess coefficient and the current, the air flow required for purging the current stack can be determined, achieving the purpose of purging based on the water content inside the membrane during the low-temperature purging process. When the water content inside the membrane is low, reduce the purging flow rate to prevent over-purging and at the same time reduce the power of the air compressor.

[0057] During purging, the impedance value R1 is measured in real time using EIS impedance measurement, and the difference is taken with the average impedance value R0 within 5 s when the fuel cell status bit enters purging to obtain the impedance value increased by the stack during purging. Then, the impedance value increased by a single cell is calculated as R = (R1 - R0) / number of single cell plates (mΩ). The inlet water temperature and outlet water temperature of the stack are measured in real time using a temperature sensor, and the average of the inlet and outlet water temperatures is taken as the average temperature T of the stack.

[0058] The water content λ in the membrane is calculated using the following calculation formula:

[0059] Let the thickness of the proton exchange membrane be h (unit: \um), the high-frequency impedance value of a single cell be R (unit: mΩ), and the active area be S (unit: cm 2 ). The current temperature is T (unit: K).

[0060]

[0061] As purging progresses, λ continuously decreases, and the purge excess coefficient of the cathode follows the following relationship. Figure 4 is the set value of the cathode excess coefficient based on the actual data performance of the stack cold shutdown purge (an array that can be calibrated according to the actual cold shutdown purge performance of the stack). It ensures that in the initial stage of purging, the moisture in the membrane can be quickly removed to reduce the purging time, and in the final stage of shutdown purge, the air volume can be reduced to avoid faults such as rapid reduction of stack performance caused by excessive air flow purging, such as low stack performance and single cell low performance. At the same time, an appropriate excess coefficient is maintained to ensure the purging effect of the system. When λ reaches the set λmin (calibrated value), the low-temperature shutdown purge is completed, and the residual hydrogen discharge stage is entered.

[0062] Step 402, after the shutdown purge stage ends, enter the residual hydrogen discharge mode, and release the high voltage of the fuel cell system to a safe voltage. Currently, some strategies will skip this stage at low temperatures to keep the inside of the stack dry, but this method will pose a safety hazard to the use of the fuel cell system. Therefore, the water content method is used to quantitatively represent the moisture generated inside the stack during the residual hydrogen discharge process and is used for precise control during the next startup. When entering the residual hydrogen discharge mode, record λ at the end of shutdown. 0 and calculate the water content n (mol) in the proton exchange membrane inside the stack at the end of the shutdown purge based on this λ. 0 (mol) can be obtained from the following calculation formula:

[0063] Let the thickness of the proton exchange membrane be h (unit: \um), the active area be A (unit: cm 2 ), the density of the proton exchange membrane be ρ (unit: kg / m 3 ), the mass of a single cell be m (unit: g), the ion exchange equivalent be EW, and N be the number of single cell plates.

[0064]

[0065] Meanwhile, when discharging the residual hydrogen, the proton exchange membrane has been purged to a relatively dry state, and there is no gas flow at the cathode. It is approximately considered that all the generated water exists inside the fuel cell stack. Record the current value I during the residual hydrogen discharge process 0 ; the duration t of the residual hydrogen discharge, and calculate the amount of water n generated by the residual hydrogen discharge 1 (mol);

[0066] n 1 = I 0 / 96485 / 2 * the number of single-cell plates; and take n 0 + n 1 as the total amount of water n in the stack after the fuel cell system shuts down and record it in the controller memory; 2 Record it in the controller memory;

[0067]

[0068] Step 101, based on the temperature T inside the fuel cell stack, when the temperature inside the fuel cell stack is relatively low, arbitrate that the fuel cell system enters the cold start mode and call the amount of water n at the previous shutdown 2 ; and calculate the maximum water capacity n that the stack can accommodate according to the proton exchange membrane parameters max (mol) (n max = the maximum water capacity of a single cell (mol) * the number of single-cell plates; the maximum water capacity of a single cell = 14 * the sulfonate content of a single cell (mol); the sulfonate content of a single cell (mol) = the mass of a single cell (g) / the ion exchange equivalent (g / mol)); the mass of a single cell m is the same as the calculation formula in the above (Equation 3); the above proton exchange membrane ion exchange equivalent EW, the number of single-cell plates N, the mass of a single cell m and other parameters are calibration values and are calibrated according to the actual parameters of the stack;

[0069]

[0070] Calculate the amount of water n (mol) required for the current stack to reach the saturation state under the current conditions, n 3 (mol), n 3 = n max - n 2 ;

[0071] Based on the above amount of water and temperature, when the fuel cell system is in the cold start mode, there are the following situations. Denote the current integral of the fuel cell during the cold start as I 1 , then during the cold start process, the amount of water n generated 4 (mol),

[0072]

[0073] Step 110, if in the initial stage of cold start, n 3 ≤n 4 , it is considered that the previous shutdown purge is insufficient and the cold start with load current cannot be directly carried out. The load current is only allowed when the in-pile temperature T rises to 0 °C; denoted as the 110 stage;

[0074] Step 120, when T < 0, n 3 >n 4 , it is recorded as the 120 stage. At this time, since the water content inside the current fuel cell stack has not reached the saturation state, the overall performance of the fuel cell stack may be too low. As the stack continues to load current, the voltage will continue to rise. At this time, the current should be increased as much as possible while ensuring the single-cell voltage, and the voltage should be controlled within a reasonable range to increase the heat generation of the stack as much as possible to accelerate the cold start process. At this time, the current is increased based on the voltage;

[0075] Step 121, when n 3 ≤n 4 , it is recorded as the 121 stage. It is considered that the stack is close to saturation at this time. If the stack continues to draw current, since the internal temperature of the stack is still below 0 °C, the water generated by subsequent reactions will freeze inside the stack. It is possible that after the stack performance reaches its peak, the stack performance will decline. At this time, according to the single-cell state of the stack, when a single low or reverse polarization phenomenon occurs, the load current should be limited in time to avoid a large reverse polarization causing damage to the stack. When the reverse polarization is serious, the current loading should be stopped immediately, and it should jump to the 110 stage. After heating to 0 °C or above a calibrated limit value using external auxiliary heating, the current loading is carried out again;

[0076] Step 210, when T > 0, n 3 ≤n 4 , due to the low ambient temperature, low cathode inlet temperature, low anode fresh hydrogen temperature, and the internal temperature of the stack has risen above 0 °C, the residual ice cubes in the cathode and anode circuits melt. Attention should be paid to the possible water blockage phenomenon at low temperatures, and the drainage valve frequency in this process should be increased. The opening and closing response of the drainage valve in this stage has a separate calibrated value. At this time, if a single low or reverse polarization phenomenon occurs, the anode drainage valve should be opened in time, and the load current should be reduced to prevent damage to the stack;

[0077] Step 220, when T > 0, n 3 >n 4 , although the in-pile temperature has reached 0 °C, since the inside of the stack membrane is still not saturated, at this time, the current loading rate should be limited according to the minimum single-cell voltage and the average single-cell voltage to prevent the voltage from decreasing too quickly due to too fast current loading, resulting in a shutdown failure or damage of the fuel cell system, such as Figure 5The figure shows the principle of coefficient correction for the current limiting rate based on the single-cell voltage: when the single-cell voltage is at a relatively high level, the stack performance is high, and the normal load-up rate can be adopted at this time; during the load-up process, due to the increase in current, problems such as following operating conditions will cause polarization losses, resulting in a decrease in the single-cell voltage during the load-up process of the stack. If the current load-up rate is not restricted, it is very likely that the stack will experience single-cell voltage drop or even reversal during the load-up process, causing damage to the stack, or triggering protection, resulting in repeated oscillations of the current during the load-up process, affecting the normal power output of the fuel cell; using the current load-up rate limit based on the stack voltage can ensure that the stack is loaded at a relatively high voltage level. After the stack voltage recovers, the correction coefficient will also be increased, so as to respond to the target power demand as quickly as possible while protecting the stack safety to the greatest extent, and shortening the cold start duration.

[0078] When T reaches the target temperature and can meet the conditions for the normal operation of the fuel cell system, the cold start is completed.

[0079] Step 300, after the cold start is completed, enter the normal operation stage, clear n 1 ~n 4 to zero, and start recording n 1 ~n 4 again when entering the operation. Using the same calculation method as in step 401, calculate the water content λ in the membrane during the operation process, and try to maintain n 3 ≤n 4 , otherwise, during the operation process, the stack is prone to performance degradation due to over-drying, affecting the efficiency of the fuel cell system.

[0080] Use a temperature sensor to monitor the water inlet temperature and water outlet temperature of the fuel cell system in real time, and take the average value of the water inlet and outlet temperatures as the average temperature T of the stack.

[0081] Step 301, record the EIS high-frequency impedance R in real time, and take the 5s average value of the impedance after entering the operation stage, denoted as R0, and use this value as the self-learning basic impedance value of the fuel cell system for this operation.

[0082] Step 302, during the operation process, record the impedance value R1 in real time, subtract it from R0, and divide by the number of single cells in the stack to obtain the impedance rise value R of the stack during the purging process: R=(R1 - R0) / number of single cells.

[0083] And calculate λ using the following calculation formula:

[0084] Set the lower limit of λ during the operation stage as λmin.

[0085] Step 303, during operation, when the current is stable, the way to judge the stable current is that within a certain period of time, the current fluctuation range is within ±5A; within a certain period of time, if λ < λmin, trigger a warning of low water content in the membrane.

[0086] According to the pre-calibrated operating condition curve of the water inlet and outlet temperature corresponding to λmin, and based on the current λ value, interpolate and correct the water inlet and outlet temperature.

[0087] If λ still does not reach λmin within the corrected check time, according to the pre-calibrated cathode pressure corresponding to λmin, the operating condition curve of the cathode excess coefficient, and based on the current λ value, interpolate and correct the water inlet and outlet temperature; perform cathode operating condition correction.

[0088] If λ still does not reach λmin within the check time, trigger a fault and perform power limit processing; if λ reaches λmin, maintain the operating conditions until the power changes and then respond to the normal operating conditions; and repeat this action after the current is stable.

[0089] The cold start and purge method of the present invention calculates the water content in the membrane by real-time obtaining the stack temperature and high-frequency impedance value, realizing precise control and quantitative evaluation of the purge process; during the cold start process, according to multiple factors such as the temperature and water content in the stack, the current is precisely controlled in a segmented manner. Different current control strategies are adopted in different stages, avoiding stack performance problems caused by improper current control, greatly improving the success rate of cold start of the hydrogen fuel cell, reducing damage to the stack, enhancing the applicability and reliability of the fuel cell in a low-temperature environment; the optimized purge and cold start strategies reduce the cold start and purge duration, improve the response speed of the fuel cell system, enable the equipment to be put into use faster, improve the work efficiency, and meet the requirements of rapid start in practical applications.

Claims

1. A cold start and purge method for a proton exchange membrane fuel cell system, characterized in that: The steps include: Obtain the temperature and high-frequency impedance value of the battery stack in real time; The water content in the stack membrane is calculated in real time based on the temperature and high-frequency impedance value of the stack; Different purge and start-up control strategies are adopted based on the different water content in the stack membrane. The purge and start-up control strategies include: shutdown purge, cold start, and normal operation.

2. The cold start and purge method of a proton exchange membrane fuel cell system according to claim 1, characterized in that: The shutdown and purge steps are as follows: When the ambient temperature is lower than 0℃ or a calibrated target value, the arbitration enters low-temperature purge; During purging, the EIS impedance measurement is used to measure the current high-frequency impedance value in real time, and the difference between the current high-frequency impedance value and the high-frequency impedance value when entering the purge is taken to obtain the impedance value of the stack during the purge process. The impedance value R (mΩ) of a single cell is calculated based on the number of cells in the stack. The temperature sensor is used to measure the water inlet and outlet temperatures of the stack in real time, and λ is calculated using the calculation formula. Wherein, h is the thickness of the proton exchange membrane (unit: \um), R is the high-frequency impedance value of a single cell (unit: mΩ), S is the activation area (unit: cm2), and T is the current temperature (unit: K); the threshold λmin of λ is set (the actual calibration control value). When λ reaches the set λmin, the low-temperature shutdown purge is completed and the residual hydrogen discharge stage is entered; When entering the residual hydrogen discharge mode, record λ0 when the shutdown is completed, calculate the water content n0 in the proton exchange membrane inside the stack at the end of the shutdown purge, record the current integral value I0 during the residual hydrogen discharge process, calculate the amount of water n1 generated by the residual hydrogen discharge, and record n0+n1 as the total amount of water n2 in the stack after the fuel cell system is shut down in the controller memory.

3. The cold start and purge method of a proton exchange membrane fuel cell system according to claim 2, characterized in that: The cold start steps are as follows: Based on the judgment of the temperature T inside the fuel cell stack, when the temperature inside the fuel cell stack is low, the arbitration fuel cell system enters the cold start mode and calls the water volume n2 at the last shutdown; the maximum water volume n that the stack can accommodate is calculated based on the proton exchange membrane parameters. max , calculate the amount of water n3 required for the stack to reach saturation in the current state, n3 = n max -n2; During the cold start process, record the current integral I1 of the fuel cell and calculate n4 generated during the cold start process. Where N is the number of cells in a single cell; different control strategies are adopted according to the relationship between n3 and n4 and the different conditions of temperature T.

4. The cold start and purge method of a proton exchange membrane fuel cell system according to claim 3, characterized in that: Different control strategies are adopted according to the relationship between n3 and n4 and different conditions of temperature T as follows: if n3≤n4 at the beginning of the cold start, current loading is allowed only when the temperature T in the stack rises to 0℃; when T<0 and n3>n4, the current is increased while ensuring the voltage of the single cell, and the voltage is controlled to increase the heat generation of the stack; when T<0 and n3≤n4, the loading current is adjusted according to the state of the single cell of the stack; when T>0 and n3≤n4, the drain valve frequency is increased, and if an abnormality occurs, the anode drain valve is opened and the loading current is reduced; when T>0 and n3>n4, the current loading rate is limited according to the single cell voltage; when T reaches the target temperature and meets the normal operating conditions of the fuel cell system, the cold start is completed.

5. The cold start and purge method of a proton exchange membrane fuel cell system according to claim 1, characterized in that: Normal operation includes the following steps: After the cold start is completed, the normal operation phase is entered. n1~n4 are reset and re-recorded. The average temperature T of the stack is monitored in real time. The EIS high-frequency impedance is recorded in real time. The water content λ in the membrane during operation is calculated. The lower limit of λ in the operation phase is set to λ min , compare λ with λ min The relationship is operated accordingly.

6. The cold start and purge method of a proton exchange membrane fuel cell system according to claim 4, characterized in that: The n0 calculation formula is: The n1 calculation formula is: The max The calculation formula is: The calculation formula of the single cell mass m is: Where N is the number of cells, m is the mass of the cell (g), EW is the ion exchange equivalent (g / mol), h is the thickness of the proton exchange membrane (unit: \um), and A is the activation area (unit: cm 2 ), ρ is the proton exchange membrane density (unit: kg / m 3 ), t is the residual hydrogen discharge time, and the above parameters such as proton exchange membrane ion exchange equivalent EW, number of single cell sheets N, and single cell mass m are calibration quantities, which are calibrated according to the actual parameters of the fuel cell stack.

7. The cold start and purge method of a proton exchange membrane fuel cell system according to claim 5, characterized in that: Compare λ with λ at runtime min The corresponding operation is as follows: when the current is stable, if λ<λmin, the low water content warning in the membrane is triggered, and the water inlet and outlet temperatures are corrected by interpolation according to the pre-calibrated operating condition curve. If λmin is still not reached within the check time after correction, the cathode operating condition is corrected. If it is still not reached, a fault is triggered and power is limited. If it is reached, the operating conditions are maintained until the power changes and responds to normal operating conditions, and the action is repeated after the current stabilizes.

8. The cold start and purge method of a proton exchange membrane fuel cell system according to claim 7, characterized in that: The standard for current stability is that the current fluctuation range is within ±5A within a certain period of time.

Images