Low-temperature cold start strategy of vehicle-mounted fuel cell system

Abstract

The invention discloses a low-temperature cold start strategy of a vehicle-mounted fuel cell system, and belongs to the technical field of fuel cells. The system is based on a whole vehicle system architecture and comprises a power battery pack, a power battery BMS, an all-in-one system, a whole vehicle VCU and a fuel cell system, and the fuel cell system comprises a fuel cell stack, a fuel cell controller FCU, a DC / DC converter, a hydrogen path, an air path and a water path. The low-temperature cold start strategy comprises four ordered steps of shutdown purging, auxiliary machine self-checking hot standby, fuel cell stack reverse heating and fuel cell stack self-starting. Liquid water residues are reduced to the maximum extent through shutdown purging, ice vaporization is achieved through fuel cell stack reverse heating, the hindering effect of ice on electrochemical reaction in the fuel cell stack self-starting step is effectively reduced, and meanwhile the waiting time at the beginning of fuel cell stack self-starting is shortened through auxiliary machine self-checking hot standby; therefore, the risk that vaporized ice is frozen again is reduced. Compared with a PTC heating method, time is saved, and the success rate of self-starting is increased.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and more specifically to a low-temperature cold start strategy for an on-vehicle fuel cell system. Background Art

[0002] A fuel cell is a power generation device that generates electrical energy through an electrochemical reaction between hydrogen and oxygen (from air). Due to the advantages of high conversion efficiency, zero pollution, and strong environmental adaptability of proton exchange membrane fuel cells, they are considered a new energy technology with broad prospects and have been demonstrated, promoted, and applied at the national and local levels in the automotive field.

[0003] Water is generated during the electrochemical reaction of the fuel cell. When the ambient temperature is below zero, the water will freeze and block the continuation of the reaction. Especially when the interval between shutdown and the next startup is relatively long, ice is more likely to form. The icing usually occurs on the membrane electrode and bipolar plate (the main drainage channel) of the fuel cell. Therefore, a major challenge faced by current on-vehicle fuel cell systems is the low cold start success rate and poor reliability in low-temperature environments (especially -30°C and below).

[0004] There are mainly two forms of existing low-temperature cold start methods: one is called the PTC heating method, which uses an electric heater (PTC) to heat the cooling circuit of the fuel cell system to a temperature above 0°C, and then executes the normal startup strategy. However, the disadvantage of this method is that the cold start time is relatively long; the other is called the direct start method, which uses a large amount of waste heat generated by the concentration polarization characteristics at a high current density of the stack to quickly heat the stack, thereby achieving the low-temperature startup of the stack. The disadvantage of this method is that as the ambient temperature decreases, the success rate of the direct start method becomes lower and lower, and frequent startup failures will cause irreversible damage to the stack. Therefore, we provide a low-temperature cold start strategy for an on-vehicle fuel cell system. Summary of the Invention

[0005] The present invention provides a low-temperature cold start strategy for an on-vehicle fuel cell system, aiming to solve the disadvantages of long low-temperature cold start time using the electric heater heating method, low success rate of the direct start method, and easy irreversible damage to the stack in existing on-vehicle fuel cells.

[0006] The present invention adopts the following technical solutions: A low-temperature cold start strategy for an on-vehicle fuel cell system, based on the vehicle system architecture, the vehicle system architecture includes a power battery pack, a power battery BMS, a multi-in-one system, a vehicle VCU and a fuel cell system, the fuel cell system includes a fuel cell stack, a fuel cell controller FCU, a DC / DC converter, a hydrogen path, an air path and a water path, the multi-in-one system includes a main positive relay K1 and a main negative relay K2, the DC / DC converter is provided with an auxiliary relay K3 and a main relay K4, the power battery, the main positive relay K1, the fuel cell stack, the main relay K4, and the main negative relay K2 form a reverse heating circuit, and the air compressor of the air path and the water pump of the water path are connected in parallel to the reverse heating circuit through the auxiliary relay K3. The low-temperature cold start strategy includes the following steps: (1) Shutdown purge: When the on-vehicle fuel cell system shuts down last time, execute the shutdown purge step to discharge most of the liquid water in the membrane electrode and bipolar plate inside the fuel cell; (2) Auxiliary machine self-check and thermal standby: Power on the vehicle to make the vehicle VCU enter the standby state, control the main positive relay K1, the main negative relay K2 of the multi-in-one system, and the auxiliary relay K3 of the DC / DC converter to close, and the water pump and air compressor to operate for self-check, and make the water pump and air compressor in the thermal standby state; (3) Reverse heating of the fuel cell stack: The power battery supplies power to the fuel cell stack in reverse, and the ice remaining inside the fuel cell stack is quickly melted by applying a reverse voltage; (4) Self-start of the fuel cell stack: Execute the fuel cell stack start-up program and judge whether the low-temperature cold start is completed.

[0007] The above DC / DC converter is equipped with a bidirectional power supply circuit, including a positive charging circuit that converts the electrical energy generated by the fuel cell stack into electrical energy stored in the power battery pack and the reverse heating circuit that supplies power to the fuel cell stack in reverse by the power battery pack.

[0008] The membrane electrode of the above fuel cell stack adopts a carbon-rich carrier membrane electrode or a microsphere graphene carrier membrane electrode.

[0009] The specific process of the above step (1) shutdown purge includes: 1.1 First, keep the hydrogen flow rate unchanged under the idle condition and increase the air flow rate for purging; 1.2 Then, reduce the speed of the air compressor, open the bypass valve of the air circuit, close the intake throttle valve and the back pressure valve, and keep the hydrogen flow rate unchanged and continue purging; 1.3 Detect the high-frequency impedance value R1 of the fuel cell stack through electrochemical impedance EIS. When R1 > the set value M1, close the hydrogen supply and the air compressor to complete the shutdown process.

[0010] The specific process of the auxiliary machine's self-check and hot standby in step (2) above includes: 2.1 Sending a high-voltage request command from the fuel cell controller FCU to the vehicle VCU. The vehicle VCU controls the main positive relay K1 and the main negative relay K2 of the multi-in-one system to close for self-check. If the self-check fails, repeat this step; 2.2 The fuel cell controller FCU controls the auxiliary relay K3 of the DC / DC converter to close for self-check. If the self-check fails, repeat this step; 2.3 If possible, the FCU controls to apply high voltage to the water pump and checks whether the water pump can operate. If it can, keep it running at the lowest speed; 2.4 Open the air bypass valve, supply high voltage to the air compressor to check whether the air compressor can operate. If it can, make the air compressor operate at a fixed speed r0, and keep the water pump and the air compressor in a hot standby state.

[0011] The specific process of the reverse heating of the fuel cell stack in step (3) above includes: 3.1 The fuel cell controller FCU sends a request command to the vehicle VCU to make the power battery supply power to the fuel cell stack in the reverse direction, and controls the single-cell voltage of the stack not to exceed V1, while the total voltage is V0 = V1 * N, where N represents the total number of cells in the fuel cell stack, and V1 is required to be less than the standard potential of electrolyzing water , from the formula = 1.5241 - 1.2261×10 −3 T + 1.1858×10 −5 T ln( T ) + 5.6692×10 −7 T 2 calculate the standard potential at different temperatures, where T represents the temperature in the unit of K; 3.2 Detect the high-frequency impedance value R2 of the fuel cell stack through EIS. If R2 ≤ the set value M2, it is considered that the ice has completely melted. If the duration t1 exceeds t0 and R2 still does not fall below the set value M2, adjust the magnitude of V1 to meet the condition; 3.3 Continue to increase the single-cell voltage to V2, requiring V2 > V1 and V2 ≤ + 0.5V, detect the high-frequency impedance value R3 of the stack. If R3 > the set value M3, it is considered that all the liquid water has vaporized. If the duration t2 exceeds t0 and R3 still does not rise above the set value M3, adjust the magnitude of V2 to meet the condition; 3.4 Quickly cut off the power supply from the power battery to the fuel cell stack, open the main hydrogen valve of the hydrogen path, adjust the hydrogen ratio valve to make the hydrogen inlet pressure of the stack P1, at the same time close the air bypass valve, open the inlet throttle valve of the air path, and adjust the back pressure valve to make the inlet pressure of the air path P2.

[0012] The specific process of the fuel cell stack self-start in step (4) above includes: 4.1 The fuel cell controller FCU controls the start of the fuel cell stack and delivers electrical energy to the power battery pack, sets the single-cell loading voltage of the stack as V3, where V3 < 0.5V, and detects the coolant inlet temperature T1 of the stack at this time; 4.2 Controls the magnitude of the single-cell voltage V3 by changing the air and hydrogen flow rates, and maintains the current state. If V3 < 0.2V, the startup fails. If 0.2V < V3 < 0.5V and the time t3 required for T1 ≥ T0°C exceeds 10 minutes, the startup fails; 4.3 Otherwise, when the time t3 required for T1 ≥ T0°C < 10 minutes, switches the control mode of the DC / DC converter to the current mode, provides the hydrogen and air flow rates and pressures according to the idle condition of the stack, and controls the target current of the stack as the idle current I0; 4.4 If there is no fault during the stable operation time t4 at the idle current point exceeding 1 minute, it is considered that the cold start is completed.

[0013] As can be seen from the above description of the present invention, compared with the prior art, the present invention has the following advantages: 1. The present invention mainly includes four ordered steps: shutdown purge, auxiliary machine self-check and thermal standby, fuel cell stack reverse heating, and fuel cell stack self-start. Through the first step of shutdown purge, the residual liquid water can be minimized. Through the third step of fuel cell stack reverse heating, the ice can be vaporized, effectively reducing the hindrance of ice to the electrochemical reaction in the fuel cell stack self-start step. At the same time, the second step of auxiliary machine self-check and thermal standby can reduce the waiting time at the start of the fourth step of fuel cell stack self-start, thereby reducing the risk of the already vaporized ice refreezing. It saves time compared with the PTC heating method and improves the success rate of self-start at the same time.

[0014] 2. The cold start strategy of the whole vehicle of the present invention adopts the standard potential of electrolyzed water at different temperatures as the logical judgment basis to achieve precise regulation of the reverse heating voltage of the stack, thereby realizing the controllable melting and vaporization of ice on the membrane electrode; in addition, based on the high-frequency resistance value of the electrochemical impedance (EIS), the quantitative judgment from the shutdown purge time to the end of the stack reverse heating is realized. Description of the Drawings

[0015] Figure 1 It is a schematic diagram of the whole vehicle system architecture of the present invention.

[0016] Figure 2 It is a schematic diagram of the fuel cell system of the present invention.

[0017] Figure 3 It is a flowchart of the low-temperature cold start strategy of the on-vehicle fuel cell system of the present invention.

[0018] Figure 4 It is a flowchart of the shutdown purge step of the present invention.

[0019] Figure 5 This is a flow chart of the auxiliary machine self-check hot standby steps of the present invention.

[0020] Figure 6 It is a flow chart of the reverse heating and self-starting steps of the fuel cell stack of the present invention. DETAILED DESCRIPTION

[0021] Refer to the following Figure 1 The specific implementation of the present invention is described. In order to fully understand the present invention, many details are described below, but for those skilled in the art, the present invention can be implemented without these details. For well-known components, methods and processes, they are not described in detail below.

[0022] A low-temperature cold start strategy for a vehicle fuel cell system based on the vehicle system architecture. Figure 1 The vehicle system architecture includes a power battery pack, a power battery BMS, an all-in-one system, a vehicle VCU and a fuel cell system. The fuel cell system includes a fuel cell stack, a fuel cell controller FCU, a DC / DC converter, a hydrogen circuit, an air circuit and a water circuit. The all-in-one system includes a main positive relay K1 and a main negative relay K2, and the DC / DC converter is provided with an auxiliary relay K3 and a main relay K4. The above-mentioned DC / DC converter is equipped with a bidirectional power supply circuit, including a positive charging circuit that converts the electric energy generated by the fuel cell stack into electric energy stored in the power battery pack and a reverse heating circuit that reversely supplies power to the fuel cell stack from the power battery pack. The power battery, the main positive relay K1, the fuel cell stack, the main relay K4 and the main negative relay K2 form a reverse heating circuit, and the air compressor of the air circuit and the water pump of the water circuit are connected in parallel through the auxiliary relay K3 on the reverse heating circuit.

[0023] Reference Figure 2 The air circuit includes air filter, air compressor, intercooler, humidifier, bypass valve, intake throttle, back pressure valve, etc.; the water circuit includes water pump, electronic thermostat, PTC heater, cooling fan, deionizer, etc. The connection of each component in the air circuit and water circuit refers to Figure 2 This is a conventional technology in the art and will not be described in detail in this embodiment.

[0024] The low temperature cold start strategy of the above vehicle fuel cell system refers to Figures 3 to 6 , including the following steps: 1. Shutdown purge: The shutdown purge step is performed when the vehicle fuel cell system is shut down last time to discharge most of the liquid water in the membrane electrode and bipolar plate inside the fuel cell. The details are as follows: 1.1 First, keep the hydrogen flow rate unchanged under the idle condition and increase the air flow rate for purging; 1.2 Then reduce the speed of the air compressor, open the bypass valve of the air circuit, close the intake throttle and the back pressure valve, keep the hydrogen flow rate unchanged and continue purging; 1.3 Detect the high-frequency impedance value R1 of the stack through electrochemical impedance spectroscopy (EIS). When R1> set value M1 (the set value M1 of different stacks is different, and it can be 1500mΩ·cm in this embodiment) 2 ), shut down the hydrogen supply and air compressor to complete the shutdown process.

[0025] 2. Auxiliary machine self-check hot standby: When the vehicle is powered on, the VCU of the vehicle enters the standby state, and the main positive relay K1, the main negative relay K2 of the all-in-one system, the auxiliary machine relay K3 of the DC / DC converter, and the operation of the water pump and air compressor are self-checked, and the water pump and air compressor are in the hot standby state. The details are as follows: 2.1 The whole vehicle is powered on, the whole vehicle VCU is in standby mode, and the whole vehicle VCU controls the power distribution box to power on the fuel cell controller FCU and the power battery BMS. The fuel cell controller FCU sends a high-voltage request command to the whole vehicle VCU, and the whole vehicle VCU controls the main positive relay K1 and the main negative relay K2 of the all-in-one system to pull in for self-test. If the self-test fails, repeat this step; 2.2 The fuel cell controller FCU controls the auxiliary relay K3 of the DC / DC converter to pull in for self-test. If the self-test fails, repeat this step; 2.3 If possible, the FCU controls the high pressure on the water pump to check whether the water pump can operate. If it can, it will maintain the lowest speed; 2.4 Open the air bypass valve, supply high voltage to the air compressor to check whether the air compressor can operate. If it can, the air compressor will operate at a fixed speed r0, so that the water pump and air compressor are in hot standby state.

[0026] 3. Reverse heating of the fuel cell stack: The power battery supplies power to the fuel cell stack in reverse, and the ice remaining inside the fuel cell stack is quickly melted by applying reverse voltage. The details are as follows: 3.1 After the auxiliary machine self-test and hot standby are completed, the fuel cell controller FCU sends a request command to the vehicle VCU to make the power battery reversely supply power to the fuel cell stack, and control the voltage of the single cell of the stack not to exceed V1, and the total voltage is V0=V1*N, N represents the total number of cells in the fuel cell stack, and V1 is required to be less than the standard potential of electrolyzed water , according to the formula =1.5241−1.2261×10 −3 T +1.1858×10 −5 T ln( T )+5.6692×10 −7 T 2Calculate the standard potential at different temperatures, where T represents the temperature in K; 3.2 Detect the high-frequency impedance value R2 of the fuel cell stack through EIS. If R2 ≤ the set value M2, it is considered that the ice has completely melted. If the duration t1 exceeds t0 (t0 is specifically 2 min in this embodiment) and R2 still does not fall below the set value M2 (the set value M2 is different for different stacks, and in this embodiment, it can be specifically 150 mΩ·cm 2 ), then adjust the magnitude of V1 to meet the condition; 3.3 Continue to increase the single-cell voltage to V2, requiring that V2 > V1 and V2 ≤ +0.5 V, and detect the high-frequency impedance value R3 of the stack. If R3 > the set value M3 (the set value M3 is different for different stacks, and in this embodiment, it can be specifically 1000 mΩ·cm 2 ), it is considered that all the liquid water has vaporized. If the duration t2 exceeds t0 and R3 still does not exceed the set value M3, then adjust the magnitude of V2 to meet the condition; 3.4 Quickly turn off the power battery from discharging to the fuel cell stack, open the main hydrogen valve of the hydrogen path, adjust the hydrogen ratio valve so that the hydrogen inlet pressure of the stack is P1. In this embodiment, P1 is specifically 50 kPa. At the same time, close the air bypass valve, open the intake throttle valve of the air path, and adjust the backpressure valve so that the intake pressure of the air path is P2. In this embodiment, P2 is specifically 30 kPa.

[0027] 4. Self-start of the fuel cell stack: Execute the fuel cell stack start-up program and determine whether the low-temperature cold start is completed. Specifically as follows: 4.1 The fuel cell controller FCU controls the fuel cell stack to start and deliver electrical energy to the power battery pack. Set the single-cell loading voltage of the stack as V3, V3 < 0.5 V, and detect the coolant inlet temperature T1 of the stack at this time; 4.2 Control the magnitude of the single-cell voltage V3 by changing the air and hydrogen flow rates and maintain the current state. If V3 < 0.2 V, the start-up fails. If 0.2 V < V3 < 0.5 V and the time t3 required for T1 ≥ T0 °C exceeds 10 min, the start-up fails; 4.3 Otherwise, when the time t3 required for T1 ≥ T0 (T0 is specifically 5 °C in this embodiment) < 10 min, switch the control mode of the DC / DC converter to the current mode, provide the hydrogen and air flow rates and pressures according to the idle condition of the stack, and control the target current of the stack as the idle current I0; 4.4 If the stable operation time t4 at the idle current point exceeds 1 min without failure, it is considered that the cold start is completed.

[0028] The above is only the specific implementation manner of the present invention, but the design concept of the present invention is not limited thereto. Any non-substantive modification made to the present invention using this concept shall fall within the scope of infringement of the protection of the present invention.

Claims

1. A low-temperature cold start strategy for a vehicle-mounted fuel cell system, based on a vehicle system architecture, the vehicle system architecture includes a power battery pack, a power battery BMS, an all-in-one system, a vehicle VCU and a fuel cell system, the fuel cell system includes a fuel cell stack, a fuel cell controller FCU, a DC / DC converter, a hydrogen path, an air path and a water path, the all-in-one system includes a main positive relay K1 and a main negative relay K2, the DC / DC converter is provided with an auxiliary relay K3 and a main relay K4, characterized in that: The power battery, the main positive relay K1, the fuel cell stack, the main relay K4, and the main negative relay K2 form a reverse heating circuit. The reverse heating circuit is connected in parallel to the air compressor of the air circuit and the water pump of the water circuit through the auxiliary relay K3. The low-temperature cold start strategy includes the following steps: (1) Shutdown purge: The shutdown purge step is performed when the vehicle fuel cell system is shut down last time to discharge most of the liquid water on the membrane electrode and bipolar plate inside the fuel cell; (2) Auxiliary machine self-check hot standby: When the vehicle is powered on, the vehicle VCU enters the standby state, controls the main positive relay K1, the main negative relay K2, the auxiliary machine relay K3 of the DC / DC converter of the all-in-one system, and the operation of the water pump and air compressor for self-check, and puts the water pump and air compressor into the hot standby state; (3) Reverse heating of the fuel cell stack: The power battery supplies power to the fuel cell stack in reverse, and the ice remaining inside the fuel cell stack is quickly melted by applying reverse voltage; (4) Fuel cell stack self-start: Execute the fuel cell stack startup procedure and determine whether the low-temperature cold start is completed.

2. A low temperature cold start strategy for a vehicle-mounted fuel cell system as claimed in claim 1, characterized in that: The DC / DC converter is equipped with a bidirectional power supply circuit, including a positive charging circuit for converting the electric energy generated by the fuel cell stack into electric energy stored in the power battery pack and a reverse heating circuit for reverse discharge of the power battery pack to the fuel cell stack.

3. The low temperature cold start strategy of a vehicle-mounted fuel cell system according to claim 1, characterized in that: The membrane electrode of the fuel cell stack adopts a carbon-rich carrier membrane electrode or a microsphere graphene carrier membrane electrode.

4. The low temperature cold start strategy of a vehicle fuel cell system according to claim 1, characterized in that: The specific process of shutdown purging in step (1) includes: 1.1 firstly maintaining the flow rate of hydrogen unchanged under the idle condition and increasing the air flow rate for purging; 1.2 then reducing the speed of the air compressor, opening the bypass valve of the air circuit, closing the intake throttle and the back pressure valve, maintaining the hydrogen flow rate unchanged and continuing the purging; 1.3 detecting the high-frequency impedance value R1 of the stack through electrochemical impedance spectroscopy (EIS); when R1>the set value M1, shutting down the hydrogen supply and the air compressor to complete the shutdown process.

5. A low temperature cold start strategy for a vehicle-mounted fuel cell system as claimed in claim 4, characterized in that: The specific process of the step (2) auxiliary machine self-test hot standby includes: 2.1 sending a high-voltage request instruction to the vehicle VCU through the fuel cell controller FCU, and the vehicle VCU controls the main positive relay K1 and the main negative relay K2 of the all-in-one system to be energized for self-test. If the self-test fails, repeat this step; 2.2 the fuel cell controller FCU controls the auxiliary machine relay K3 of the DC / DC converter to be energized for self-test. If the self-test fails, repeat this step; 2.3 if possible, the FCU controls the high pressure on the water pump to check whether the water pump can operate. If so, maintain the lowest speed; 2.4 open the air bypass valve, supply high voltage electricity to the air compressor to check whether the air compressor can operate. If so, make the air compressor operate at a fixed speed r0, so that the water pump and the air compressor are in hot standby state.

6. A low temperature cold start strategy for a vehicle-mounted fuel cell system as claimed in claim 5, characterized in that: The specific process of reverse heating of the fuel cell stack in step (3) includes: 3.1 The fuel cell controller FCU sends a request instruction to the vehicle VCU to make the power battery reversely supply power to the fuel cell stack, and control the voltage of a single cell of the stack not to exceed V1, and the total voltage is V0=V1*N, where N represents the total number of cells in the fuel cell stack, and V1 is required to be less than the standard potential of electrolyzed water. , according to the formula =1.5241−1.2261×10 −3 T +1.1858×10 −5 T ln( T )+5.6692×10 −7 T 2 Calculate the standard potential at different temperatures, T represents temperature, unit is K; 3.2 Detect the high-frequency impedance value R2 of the fuel cell stack through EIS. If R2≤set value M2, it is considered that the ice is completely melted. If the duration t1 exceeds t0 and R2 is still not lower than the set value M2, adjust the size of V1 to meet the conditions; 3.3 Continue to increase the single-chip voltage to V2, requiring V2>V1 and V2≤ +0.5V, detect the high-frequency impedance value R3 of the stack. If R3> the set value M3, it is considered that all the liquid water has vaporized. If the duration t2 exceeds t0 and R3 is still not higher than the set value M3, adjust the size of V2 to meet the conditions; 3.4 Quickly shut off the power battery to discharge the fuel cell stack, open the main hydrogen valve of the hydrogen circuit, adjust the hydrogen proportional valve so that the hydrogen inlet pressure of the stack is P1, and at the same time close the air bypass valve, open the air circuit inlet throttle, and adjust the back pressure valve so that the air circuit inlet pressure is P2.

7. A low temperature cold start strategy for a vehicle-mounted fuel cell system as claimed in claim 6, characterized in that: The specific process of the fuel cell stack self-start in step (4) is as follows: 4.1 The fuel cell controller FCU controls the start of the fuel cell stack and delivers electrical energy to the power battery pack, sets the single-cell loading voltage of the stack as V3, where V3 < 0.5V, and detects the coolant inlet temperature T1 of the stack at this time; 4.2 Controls the magnitude of the single-cell voltage V3 by changing the air and hydrogen flow rates, and maintains the current state. If V3 < 0.2V, the startup fails. If 0.2V < V3 < 0.5V and the time t3 required for T1 ≥ T0°C exceeds 10 minutes, the startup fails; 4.3 Otherwise, when the time t3 required for T1 ≥ T0°C is < 10 minutes, the control mode of the DC / DC converter is switched to the current mode, and the hydrogen and air flow rates and pressures are provided according to the idle condition of the stack, and the target current of the stack is controlled as the idle current I0; 4.4 If the stable operation time t4 at the idle current point exceeds 1 minute without faults, it is considered that the cold start is completed.

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