Fuel cell system shutdown purging method and optimal strategy determination method thereof

Abstract

The invention relates to a fuel cell system shutdown purging method and an optimal strategy determination method thereof.The shutdown purging method specifically comprises the following steps that when normal operation of a fuel cell system is finished, a purging process is prepared to be executed, the working current is reduced to I purging through a fuel cell DC / DC converter, and the working current is reduced to II purging; at the moment, the current single-chip voltage is monitored through a single-chip voltage inspection device CVM, and the current single-chip voltage is recorded as V0; calculating the single-chip voltage reduction amount delta V of the corresponding purging stage when purging is completed; according to the reduction amount delta V of the single-chip voltage in the purging stage, calculating the single-chip voltage V1 when purging is completed; and when the voltage inspection device CVM monitors that the current single-chip voltage is V1, blowing is stopped, and then the shutdown process is completed. According to the shutdown purging method for the fuel cell system, the internal resistance value of the electric pile does not need to be detected by using complex equipment, whether purging is completed or not can be accurately judged only by using the single-chip voltage inspection device to monitor the single-chip voltage, the shutdown purging method for the fuel cell system is simple and easy to implement, the use cost is effectively reduced, and meanwhile the purging completion degree is guaranteed.

Description

Technical Field

[0001] The present invention relates to the field of fuel cells, and in particular to a fuel cell system shutdown purge method and an optimal strategy determination method thereof. Background Art

[0002] When the fuel cell system is working, hydrogen and air react to produce water. Although most of the water will be discharged with the exhaust gas, the water in the membrane electrode is difficult to be blown out. When it is shut down, the temperature in the stack drops and the gaseous water condenses into liquid water. In the sub-zero ambient temperature, it is easy to freeze the membrane electrode, affecting the performance and life of the stack. Therefore, it is necessary to implement a purge strategy when shutting down to discharge the water in the membrane electrode as much as possible to ensure the performance at the next startup.

[0003] When the fuel cell system is purged, the internal resistance value in the stack will increase with the increase of purge time. This is caused by the membrane slowly drying up. In the existing technology, fixed conditions and purge time are generally used as the basis for purge judgment, and the change of internal resistance value in the stack cannot be monitored in real time. The purge time should be inconsistent under different operating conditions. With the development of the fuel cell industry, DC manufacturers are also gradually adding the function of measuring internal resistance. The equipment for detecting the internal resistance value of the stack is usually very complicated and is generally only used in the research and development stage, and has not been put into use.

[0004] In addition, different purging conditions can achieve the final purging effect, but the prior art lacks a criterion for finding the best one, resulting in poor purging efficiency.

[0005] Therefore, it is urgent to design a fuel cell system shutdown purge method and an optimal strategy determination method thereof to solve the problems existing in the above-mentioned prior art. Summary of the invention

[0006] In view of this, the present invention provides a fuel cell system shutdown purge method and an optimal strategy determination method thereof, the purpose of which is to determine the conditions for completing the purge without using complex equipment to detect the internal resistance of the fuel cell stack, and to seek the optimal purge strategy based on the balance between the air compressor power consumption and the purge time.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A fuel cell system shutdown purge method, as follows:

[0009] When the normal operation of the fuel cell system ends, the purge process is ready to be performed, and the operating current is reduced to I 吹扫 At this time, the current single-chip voltage is monitored by the single-chip voltage patroller CVM and recorded as V0;

[0010] When calculating the purging completion, calculate the reduction amount ΔV of the single-cell voltage corresponding to the purging stage;

[0011] According to the reduction amount ΔV of the single-cell voltage in the purging stage, calculate the single-cell voltage V1 at the end of purging;

[0012] When the voltage inspection device CVM monitors that the current single-cell voltage is V1, stop purging, and then complete the shutdown process.

[0013] Furthermore, when calculating the reduction amount ΔV of the single-cell voltage corresponding to the purging completion, it is specifically as follows:

[0014] For the voltage drop caused by the increase in proton resistance, there is the following expression:

[0015]

[0016] In the formula, ΔV is the reduction amount of the single-cell voltage in the purging stage, I 吹扫 is the purging current, and ΔR H is the increase amount of the proton resistance in the purging stage.

[0017] Furthermore, the increase amount ΔR of the proton resistance in the purging stage H has the following relationship with the reduction amount Δλ of the water content of the proton exchange membrane:

[0018]

[0019] In the formula, k and b are corresponding constants, both k and b are related to the thickness and active area of the proton exchange membrane. In the purging stage, for a fixed type of proton exchange membrane, the reduction amount Δλ of the water content is a fixed value.

[0020] Furthermore, when calculating the single-cell voltage V1 at the end of purging according to the reduction amount ΔV of the single-cell voltage in the purging stage, it is specifically as follows:

[0021] V1 = V0 - ΔV;

[0022] In the formula, V0 is the current single-cell voltage monitored by the single-cell voltage inspection device CVM at the start of the shutdown purging of the fuel cell system

[0023] The present invention also provides an optimal strategy determination method based on the above fuel cell system shutdown purging method, and the optimal strategy determination method is as follows:

[0024] S1. Set the maximum limit t of the purging time for a certain fixed fuel cell system lim ;

[0025] S2. Set different purge speeds of the air compressor at certain intervals during the purge stage, and execute the fuel cell system shutdown purge method in ascending order of the air compressor purge speed. Calculate the power consumption of the corresponding air compressor and record the purge time.

[0026] S3. When the purge speed of the air compressor is a certain value and the corresponding purge time is less than or equal to the maximum limit t of the purge time lim , determine this air compressor purge speed and the corresponding purge time as the optimal strategy of the shutdown purge method.

[0027] Furthermore, in step S2, different purge speeds of the air compressor are set at certain intervals during the purge stage, and the fuel cell system shutdown purge method is executed in ascending order of the air compressor purge speed. Calculate the power consumption of the corresponding air compressor and record the purge time as follows:

[0028] S2-1. Set the initial purge speed of the air compressor as N0, execute the fuel cell system shutdown purge method, and simultaneously obtain the purge air mass flow rate m, the air pressure P at the inlet of the air compressor 前 , the air pressure P at the outlet of the air compressor 后 and the air temperature T at the inlet of the air compressor 前 , calculate the current power consumption of the air compressor Record the current purge time t0;

[0029] S2-2. Resume the normal operation of the fuel cell system for 30 min to ensure that the proton exchange membrane is fully wetted; set the purge speed of the air compressor as N n , execute the fuel cell system shutdown purge method, and simultaneously obtain the purge air mass flow rate m, the air pressure P at the inlet of the air compressor 前 , the air pressure P at the outlet of the air compressor 后 and the air temperature T at the inlet of the air compressor 前 , calculate the current power consumption of the air compressor Record the current purge time t n ;

[0030] S2-3. Compare the purge time t n with the maximum limit t of the purge time lim . If t n > t lim , then set the purge speed of the air compressor as N n+1 , and N n+1 > N n , repeat step S2-2; if t n ≤ t lim , enter S3.

[0031] Furthermore, the specific calculation method of the power consumption of the air compressor is as follows:

[0032]

[0033] In the formula, T 前 represents the air temperature at the inlet of the air compressor; m represents the mass flow rate of the purging air; η represents the efficiency, which is obtained according to the characteristic curve of the air compressor; p r represents the pressure ratio of the air compressor, which is the ratio of the air pressure P 后 at the outlet of the air compressor to the pressure P 前 at the inlet of the air compressor.

[0034] Furthermore, the mass flow rate m of the purging air is obtained by a mass flowmeter arranged at the inlet of the air compressor, the air pressure P 前 at the inlet of the air compressor is obtained by a pressure sensor arranged at the inlet of the air compressor, the air pressure P 后 at the outlet of the air compressor is obtained by a pressure sensor arranged at the outlet of the air compressor; the air temperature T 前 at the inlet of the air compressor is obtained by a temperature sensor arranged at the inlet of the air compressor.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] (1) Based on the increase in the proton resistance R H caused by the decrease in the water content in the proton exchange membrane during the purging process, which leads to a voltage drop, the present invention accurately calculates the voltage reduction amount during the purging process by using the increase amount of the proton resistance. Therefore, there is no need to use complex equipment to detect the internal resistance value of the stack. Only by using a single-chip voltage inspection device CVM to monitor the single-chip voltage, it is possible to accurately determine whether the purging is completed. By monitoring the voltage, it can be ensured that the water in the membrane electrode is reduced to the required degree. The fuel cell system shutdown purging method adopted by the present invention is simple and easy to implement, effectively reducing the use cost, and at the same time ensuring the completion degree of the purging.

[0037] (2) On the basis of judging whether the purging is completed based on the voltage drop, the present invention makes an optimization judgment according to the purging time and the power consumption of the air compressor. According to the relevant parameter values obtained during the shutdown purging process of the fuel cell system, the power consumption of the air compressor is accurately calculated. Under the condition of ensuring the purging time, it is possible to ensure that the power consumption of the air compressor is the lowest as much as possible. The balance point of the purging time and the power consumption of the air compressor is selected as the optimal strategy of the shutdown purging method. Under the condition of the same purging current, it not only ensures the purging time of the fuel cell system, but also avoids the energy loss caused by excessive power consumption of the air compressor, improving the purging efficiency.

[0038] Other features and advantages of the present invention will be described in the subsequent specification, and, in part, will be obvious from the specification, or will be understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the structures pointed out in the specification and the drawings. Brief Description of the Drawings

[0039] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0040] Figure 1 Shows a diagram of the shutdown purge method of the fuel cell system according to an embodiment of the present invention;

[0041] Figure 2 Shows a diagram of the change of the internal resistance value and voltage with time during the purge process of the fuel cell system according to an embodiment of the present invention;

[0042] Figure 3 Shows a diagram of the change of the proton resistance with the purge time when the rotational speed of the air compressor in the embodiment of the present invention changes. Detailed Embodiments

[0043] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0044] An embodiment of the present invention provides a shutdown purge method for a fuel cell system, as Figure 1 shown, the shutdown purge method is specifically as follows:

[0045] When the normal operation of the fuel cell system ends, prepare to execute the purge process, and reduce the working current to I through the fuel cell DC / DC converter 吹扫 , at this time, monitor the current single-cell voltage through the single-cell voltage inspection device CVM and record it as V0;

[0046] Calculate the reduction amount ΔV of the single-cell voltage in the corresponding purge stage when the purge is completed;

[0047] Calculate the single-cell voltage V1 when the purge is completed according to the reduction amount ΔV of the single-cell voltage in the purge stage;

[0048] When the single-cell voltage inspection device CVM monitors that the current single-cell voltage is V1, stop the purge, and then complete the shutdown process.

[0049] The calculation of the reduction amount ΔV of the single-cell voltage in the corresponding purge stage when the purge is completed is specifically as follows:

[0050] When performing the purge procedure of the fuel cell system, as the water content in the proton exchange membrane gradually decreases, the internal resistance value will gradually increase, and at the same time, the voltage of the stack will gradually decrease. Specifically, as Figure 2 shown, during the purge, the internal resistance value gradually increases, and the corresponding voltage gradually decreases, which are all caused by the change of the internal water content.

[0051] Among the voltage losses of the stack, it is composed of activation loss E act and ohmic loss E ohm and concentration difference loss E conc . Among them, the expression of the voltage loss Δv of the stack is:

[0052] Δv = E act + E ohm + E conc ;

[0053] The voltage loss Δv of the stack is the product of the voltage reduction amount ΔV of a single cell and the number of single cells in the stack.

[0054] Since the current is small and remains the same during the purge, it is considered that the influences of the activation loss E act and the concentration difference loss E conc are fixed, that is, the voltage loss during the purge stage is all caused by the ohmic loss E ohm . And the ohmic resistance R ohm is composed of the proton resistance R H , the electron resistance R e and the contact resistance R C . The expression of the ohmic resistance R ohm is:

[0055] R ohm = R H + R e + R C ;

[0056] When performing the purge stage, since the working current is very low and remains the same, and at the same current, the electron resistance R e and the contact resistance R C are fixed. Therefore, it is mainly due to the increase of the proton resistance R H caused by the decrease of the water content in the proton exchange membrane that leads to the voltage drop. Therefore, there is the following expression for the voltage drop caused by the increase of the proton resistance:

[0057]

[0058] In the formula, ΔV is the voltage reduction amount of a single cell during the purge stage, I 吹扫 is the purge current, and ΔR HIt is the increase in proton resistance during the purging stage.

[0059] The increase in proton resistance ΔR during the purging stage H has the following relationship with the decrease in water content Δλ of the proton exchange membrane:

[0060]

[0061] In the formula, k and b are corresponding constants, and both k and b are related to the thickness and active area of the proton exchange membrane.

[0062] According to experimental tests, the increase in proton resistance ΔR during different purging stages H and the corresponding decrease in water content Δλ of the proton exchange membrane, and then the above relationship is obtained through data fitting.

[0063] During the purging process, for a fixed type of proton exchange membrane, the decrease in water content λ is often a certain value. Then, the increase in proton resistance ΔR during the purging stage H is a certain value, and the decrease in single-cell voltage ΔV during the purging stage is also a fixed value.

[0064] For a certain fixed type of proton exchange membrane, during the purging stage, its water content λ changes from 14 to 2. Therefore, according to the decrease in water content of the proton exchange membrane Δλ = 14 - 2, the increase in proton resistance ΔR during the purging stage can be calculated H , and then combined with the purging current I 吹扫 , it is calculated that the decrease in single-cell voltage ΔV during the purging stage is also a fixed value.

[0065] Calculating the single-cell voltage V1 at the end of purging according to the decrease in single-cell voltage ΔV during the purging stage is as follows:

[0066] V1 = V0 - ΔV;

[0067] In the formula, V0 is the current single-cell voltage monitored by the single-cell voltage inspection device CVM at the start of purging when the fuel cell system shuts down.

[0068] Based on the increase in proton resistance R caused by the decrease in water content in the proton exchange membrane during the purging process H and the resulting voltage drop, the decrease in voltage during the purging process is accurately calculated using the increase in proton resistance. Thus, there is no need to use complex equipment to detect the internal resistance value of the stack. By only using the single-cell voltage inspection device CVM to monitor the single-cell voltage, it is possible to accurately determine whether the purging is completed. By monitoring the voltage, it can be ensured that the water in the membrane electrode is reduced to the required level. The purging method for the fuel cell system shutdown adopted by the present invention is simple and easy to implement, effectively reducing the use cost, and at the same time ensuring the completion degree of purging.

[0069] When performing the shutdown purge procedure of the fuel cell system, at the same purge current, there are still different purge conditions, such as different air flow rates, which results in inconsistent purge times. However, for on-vehicle applications, the purge time is an important indicator and should be reduced as much as possible. And for the air flow rate, it is related to the rotational speed of the air compressor, that is, related to the power consumption of the air compressor. During shutdown purge, the power consumption of the air compressor should be taken into account to avoid excessive energy consumption losses.

[0070] As Figure 3 shown, at the same purge current, when the rotational speed of the air compressor changes, the proton resistance based on the voltage drop strategy changes with the purge time. It can be seen from the figure that the rotational speeds of the air compressors corresponding to curves ①②③④ increase in sequence, the corresponding power consumptions of the air compressors also increase in sequence, and the corresponding purge completion times decrease in sequence.

[0071] Based on this, the present invention proposes to optimize the shutdown purge strategy using two boundary conditions of the power consumption of the air compressor and the purge time. Specifically, the embodiments of the present invention also provide a method for determining the optimal strategy based on the above fuel cell system shutdown purge method. The method for determining the optimal strategy is as follows:

[0072] S1. Set a maximum limit t for the purge time for a certain fixed fuel cell system lim ;

[0073] S2. Set different purge rotational speeds of the air compressor at a certain interval during the purge stage, and in the order from small to large of the purge rotational speeds of the air compressor, sequentially execute the fuel cell system shutdown purge method, calculate the corresponding power consumption of the air compressor, and record the purge time;

[0074] S3. When the purge rotational speed of the air compressor is a certain value and the corresponding purge time is less than or equal to the maximum limit t of the purge time lim , determine this purge rotational speed of the air compressor and the corresponding purge time as the optimal strategy of the shutdown purge method.

[0075] Step S2 is specifically as follows:

[0076] S2-1. Set the initial purge rotational speed of the air compressor as N0, execute the fuel cell system shutdown purge method, and at the same time obtain the purge air mass flow rate m, the air pressure P at the inlet of the air compressor 前 , the air pressure P at the outlet of the air compressor 后 and the air temperature T at the inlet of the air compressor 前 , calculate the current power consumption of the air compressor Record the current purge time t0;

[0077] S2-2. Resume the normal operation of the fuel cell system for 30 min to ensure that the proton exchange membrane is fully wetted; set the purge rotational speed of the air compressor as Nn Execute the fuel cell system shutdown purge method, and simultaneously obtain the purge air mass flow rate m, the air pressure P at the inlet of the air compressor 前 , the air pressure P at the outlet of the air compressor 后 and the air temperature T at the inlet of the air compressor 前 , and calculate the power consumption of the current air compressor Record the current purge time t n ;

[0078] S2-3. Compare the purge time t n with the maximum limit t of the purge time lim . If t n >t lim , then set the air compressor purge speed to N n+1 , and N n+1 >N n . Repeat step S2-2; if t n ≤t lim , enter S3.

[0079] The air compressor purge speed N n , where n takes values of positive integers, and N n+1 >N n >N0.

[0080] The specific calculation method of the power consumption of the air compressor is as follows:

[0081]

[0082] In the formula, T 前 represents the air temperature at the inlet of the air compressor; m represents the purge air mass flow rate; η represents the efficiency, which is obtained according to the air compressor characteristic curve; p r represents the pressure ratio of the air compressor, which is the ratio of the air pressure P 后 at the outlet of the air compressor to the pressure P 前 at the inlet of the air compressor.

[0083] The purge air mass flow rate m is obtained by a mass flow meter installed at the inlet of the air compressor, the air pressure P 前 at the inlet of the air compressor is obtained by a pressure sensor installed at the inlet of the air compressor, the air pressure P 后 at the outlet of the air compressor is obtained by a pressure sensor installed at the outlet of the air compressor; the air temperature T 前 at the inlet of the air compressor is obtained by a temperature sensor installed at the inlet of the air compressor.

[0084] Based on determining whether purging is completed by voltage drop, an optimization judgment is made according to the purging time and the power consumption of the air compressor. Based on the relevant parameter values obtained during the shutdown purging process of the fuel cell system, the power consumption of the air compressor is accurately calculated. Under the condition of ensuring the purging time, the power consumption of the air compressor is minimized as much as possible. The balance point between the purging time and the power consumption of the air compressor is selected as the optimal strategy for the shutdown purging method. Under the condition of the same purging current, it not only ensures the purging time of the fuel cell system, but also avoids the energy loss caused by excessive power consumption of the air compressor, improving the purging efficiency.

[0085] Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fuel cell system shutdown purge method, characterized in that: The shutdown purge method is specifically as follows: When the fuel cell system is finished operating normally, it is ready to perform the purge process, and the operating current is reduced to I 吹扫 At this time, the current single-chip voltage is monitored by the single-chip voltage patroller CVM and recorded as V0; Calculate the single chip voltage reduction ΔV during the corresponding purge phase when the purge is completed; According to the single chip voltage reduction ΔV during the purge phase, the single chip voltage V1 when the purge is completed is calculated; When the voltage monitor CVM detects that the current single chip voltage is V1, the purge is stopped and the shutdown process is completed.

2. The fuel cell system shutdown purge method according to claim 1, characterized in that: The calculation of the single chip voltage reduction ΔV when the purge is completed is as follows: The voltage drop caused by the increase in proton resistance can be expressed as follows: Where ΔV is the voltage reduction of the single chip during the purge phase, I 吹扫 is the purge current, ΔR H is the increase in proton resistance during the purge phase.

3. The fuel cell system shutdown purge method according to claim 2, characterized in that: The increase in proton resistance during the purge phase is ΔR H The decrease in the water content of the proton exchange membrane, Δλ, has the following relationship: Wherein, k and b are corresponding constants, both of which are related to the thickness and active area of ​​the proton exchange membrane. During the purge stage, the reduction Δλ of the water content of a proton exchange membrane of a fixed model is a constant.

4. The fuel cell system shutdown purge method according to claim 3, characterized in that: The single chip voltage V1 when the purge is completed is calculated according to the single chip voltage reduction ΔV during the purge phase, as follows: V1 = V0 - ΔV; Wherein, V0 is the current single chip voltage monitored by the single chip voltage monitor CVM when the fuel cell system is shut down and purged.

5. A method for determining an optimal strategy for a fuel cell system shutdown purge method according to any one of claims 1 to 4, characterized in that: The optimal strategy determination method is as follows: S1. Set the maximum limit of purge time t for a fixed fuel cell system lim ; S2. In the purge stage, different air compressor purge speeds are set at certain intervals, and the fuel cell system shutdown purge method is executed in order from small to large air compressor purge speeds, and the corresponding air compressor power consumption is calculated and the purge time is recorded; S3. When the air compressor purge speed is a certain value, the corresponding purge time is less than or equal to the maximum limit of the purge time t lim When the air compressor purge speed and the corresponding purge time are determined, it is the optimal strategy for the shutdown purge method.

6. The optimal strategy determination method according to claim 5, characterized in that: Step S2 is specifically as follows: S2-1. Set the initial purge speed of the air compressor to N0, execute the fuel cell system shutdown purge method, and obtain the purge air mass flow m and the air pressure P at the air compressor inlet. 前 , Air pressure P at the outlet of the air compressor 后 And the air temperature T at the compressor inlet 前 , calculate the current power consumption of the air compressor Record the current purge time t0; S2-2. Restore the normal operation of the fuel cell system for 30 minutes to ensure that the proton exchange membrane is fully wetted; set the air compressor purge speed to N n , execute the fuel cell system shutdown purge method, and obtain the purge air mass flow m, the air pressure P at the air compressor inlet 前 , Air pressure P at the outlet of the air compressor 后 And the air temperature T at the compressor inlet 前 , calculate the current power consumption of the air compressor Record the current purge time t n ; S2-3. Set the purge time t n and the maximum limit of purge time t lim Compare, if t n >t lim , then set the air compressor purge speed to N n+1 , and N n+1 >N n , repeat step S2-2; if t n ≤t lim , enter S3.

7. The optimal strategy determination method according to claim 6, characterized in that: The specific calculation method of the power consumption of the air compressor is as follows: Where, T 前 Represents the air temperature at the compressor inlet; m represents the purge air mass flow rate; η represents the efficiency, which is obtained according to the characteristic curve of the air compressor; p r Represents the pressure ratio of the air compressor, which is the air pressure P at the outlet of the air compressor 后 The pressure P at the air compressor inlet 前 ratio.

8. The optimal strategy determination method according to claim 7, characterized in that: The purge air mass flow rate m is obtained by the mass flow meter installed at the inlet of the air compressor, and the air pressure P at the inlet of the air compressor is 前 The air pressure P at the air compressor outlet is obtained by the pressure sensor installed at the air compressor inlet. 后 The air temperature T at the air compressor inlet is obtained by the pressure sensor installed at the air compressor outlet. 前 Obtained by the temperature sensor installed at the inlet of the air compressor.

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