A method for low-temperature shutdown, purging and dehumidification of PEM fuel cells

Abstract

The present invention discloses a low-temperature shutdown purging and dehumidification method for a PEM fuel cell, which relates to the technical field of fuel cells. The method comprises the following steps: firstly, increasing the air metering ratio of an air path, extending the drainage time of a hydrogen path drain valve, and increasing the drainage frequency of the drain valve, controlling the coolant temperature of a cooling path within a set temperature range, and performing a high-temperature purge on a fuel cell stack based on the above steps, thereby effectively removing free water and weakly bound water from the fuel cell; then, after the high-temperature purge is completed, controlling the coolant in the cooling path to cool down so that the temperature of the coolant is less than a set value, thereby performing a low-temperature purge on the fuel cell stack, and ending the low-temperature purge when the low-temperature purge lasts for a set time, thereby avoiding the generation of condensed water after the temperature drops after the high-temperature purge of the PEM fuel cell is completed, thereby improving the performance and life of the PEM fuel cell.

Description

Technical Field

[0001] The present invention relates to the technical field of fuel cells, and in particular to a low-temperature shutdown purge and dehumidification method for a PEM fuel cell. Background Art

[0002] A PEM fuel cell, or proton exchange membrane fuel cell, is an energy device that directly converts the chemical energy of a hydrogen-oxygen reaction into electrical energy. The reaction directly generates water, emits no pollutants, and is highly efficient. Currently, the application of PEM fuel cells in road vehicles is in the early stages of commercialization. The main obstacles are high cost, short lifespan, a lack of hydrogen refueling facilities, high hydrogen storage and transportation costs, and environmental adaptability. Environmental adaptability primarily includes adaptability to low temperatures, high altitudes, and high temperatures. Low temperatures refer to environments below 0°C, and low-temperature adaptability involves low-temperature startup, low-temperature shutdown, and low-temperature storage. Water is an essential medium for mass transfer in PEMs. During operation, the anode and cathode flow channels, diffusion layer, catalyst layer, and proton membrane all contain large amounts of water vapor or even liquid water. If water isn't purged from the flow channels and PEM during low-temperature shutdown, the remaining water will freeze. Ice expands by 9%, and the heat generated by the battery melts the ice, causing the volume to recover. Repeated freezing and thawing can cause mechanical damage, including reduced membrane electrode bonding, increased contact resistance, fiber breakage in the diffusion layer, and catalyst agglomeration. In severe cases, ice can directly cause severe membrane perforation, rapidly reducing the fuel cell's lifespan. The presence of ice can also complicate low-temperature startups and lead to failures.

[0003] Currently, specialized low-temperature purge strategies are widely used. These involve a prolonged purge during low-temperature shutdown, with voltage used as the criterion for determining the end of the purge. However, after the shutdown purge is complete, the gas in the stack cools down and condenses. This condensed water not only degrades fuel cell performance and lifespan but can also condense on sensors, forming ice and causing sensor malfunctions. Summary of the Invention

[0004] The purpose of the present invention is to provide a low-temperature shutdown purging and dehumidification method for a PEM fuel cell, which performs high-temperature purging on the fuel cell stack, thereby effectively removing free water and weakly bound water in the fuel cell; then, after the high-temperature purging is completed, the fuel cell stack is purged at a low temperature, thereby avoiding the generation of condensed water after the temperature drops after the high-temperature purging of the PEM fuel cell, thereby improving the performance and life of the PEM fuel cell.

[0005] To achieve the above object, the present invention provides the following solutions:

[0006] A low-temperature shutdown purge and dehumidification method for a PEM fuel cell, wherein the PEM fuel cell includes a stack, an air path control system, a hydrogen path control system, and a cooling system, and the method comprises:

[0007] Step 1: Increase the air metering ratio of the air path; the air metering represents the ratio of the air supply to the actual reaction consumption;

[0008] Step 2: Extend the drain time of the hydrogen line drain valve and increase the drain frequency of the drain valve;

[0009] Step 3: Control the coolant temperature in the cooling path within a set temperature range and perform high-temperature purge on the fuel cell stack;

[0010] Step 4: When the EIS condition and / or the highest cell voltage reaches the set condition, the high-temperature purge is terminated;

[0011] Step 5: Control the cooling liquid in the cooling path to cool down so that the temperature of the cooling liquid is lower than a set value, so as to perform low-temperature purge on the fuel cell stack;

[0012] Step 6: When the low-temperature purge lasts for a set time, the low-temperature purge is terminated.

[0013] Optionally, before step 1, the following steps may be included:

[0014] After receiving a PEM fuel cell shutdown instruction, the current density of the PEM fuel cell is set to a preset current density for purge.

[0015] Optionally, step 1 specifically includes:

[0016] Control the air path throttle to fully open and adjust the air compressor speed to increase the air metering ratio of the air path.

[0017] Optionally, the upper limit of the set temperature range is the maximum allowable operating temperature of the fuel cell stack, and the lower limit is 60°C.

[0018] Optionally, step 3 specifically includes:

[0019] When the temperature of the coolant is lower than the lower limit of the set temperature range, the PTC is used to heat the coolant until the temperature of the coolant reaches within the set temperature range.

[0020] Optionally, in step 1, the air stoichiometric ratio of the air path is increased to 1.3 to 1.5 times the air stoichiometric ratio during normal operation of the PEM fuel cell.

[0021] Optionally, in step 2, the drainage time of the hydrogen line drain valve is extended to more than 3 seconds.

[0022] Optionally, in step 2, the drainage frequency of the hydrogen path drain valve is increased to 1.5 to 2 times that of the normal operation of the PEM fuel cell.

[0023] Optionally, the set value in step 5 is 40°C.

[0024] Optionally, the setting time in step 6 is 1 minute.

[0025] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects: the present invention first increases the air metering ratio of the air path, prolongs the drainage time of the hydrogen path drain valve, and increases the drainage frequency of the drain valve, thereby controlling the coolant temperature of the cooling path within a set temperature range, and performing high-temperature purge on the fuel cell stack based on the above steps, thereby effectively removing free water and weakly bound water in the fuel cell; then, after the high-temperature purge is completed, the coolant in the cooling path is controlled to cool down so that the temperature of the coolant is less than the set value, so as to perform low-temperature purge on the fuel cell stack, and when the low-temperature purge lasts for the set time, the low-temperature purge is terminated, thereby avoiding the generation of condensed water after the temperature drops after the high-temperature purge of the PEM fuel cell is completed, thereby improving the performance and life of the PEM fuel cell. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0027] Figure 1 A schematic flow chart of the low-temperature shutdown purge dehumidification method for a PEM fuel cell provided by the present invention;

[0028] Figure 2 A specific flow chart of the two-stage temperature-controlled shutdown and purge in a low-temperature environment provided by the present invention;

[0029] Figure 3 The air path principle diagram of the fuel cell system provided by the present invention;

[0030] Figure 4 A schematic diagram of the hydrogen circuit of the fuel cell system provided by the present invention;

[0031] Figure 5 This is a schematic diagram of the cooling circuit of the fuel cell system provided by the present invention. DETAILED DESCRIPTION

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0033] Water content in fuel cells is a crucial factor in their ability to adapt to low-temperature environments. If the water content is excessive during low-temperature storage, the remaining water will freeze. This ice expands by 9% in volume, but the volume recovers as the heat generated by the battery melts the ice. Repeated freezing and thawing can lead to mechanical damage, such as reduced membrane-electrode bonding, increased contact resistance, fiber breakage in the diffusion layer, and catalyst agglomeration. In severe cases, ice can directly cause serious perforation of the membrane, rapidly shortening the fuel cell's lifespan. The presence of ice can also complicate low-temperature startups and even cause them to fail. Water entering the fuel cell includes water introduced by the reactants and generated by the reactions. Exhaust water includes water vapor and liquid water carried over from the anode and cathode exhausts, as well as water discharged from the anode drain valve. Within the fuel cell, water is distributed in gaseous or liquid form within the flow field, diffusion layer, and catalyst layer. Within the proton membrane, water can be categorized as free water, bound water, and weakly bound water, depending on the strength of its binding to hydrophilic groups such as sulfonate groups. Bound water is water that forms strong chemical bonds with sulfonate groups through hydration; weakly bound water is water that is slightly away from the cluster walls and has weak hydrogen bonds with bound water; and free water is water that is not bound to the cluster walls or bound water. To ensure that residual water does not freeze during low-temperature storage, affecting fuel cell performance and life, liquid water and water vapor within the flow channels, diffusion layer, and catalyst layer, as well as free water and weakly bound water within the proton membrane, must be removed. Liquid water and water vapor in the flow channels can be removed using a high-stoichiometric gas. However, water in the diffusion layer, catalyst layer, and proton membrane is difficult to remove by gas flow alone. The only way to remove this water is by vaporizing it at high temperatures, which then flows out with the airflow. The saturated water vapor content of air is exponentially proportional to temperature. For example, at standard atmospheric pressure, the moisture content of saturated air at 40°C is 48.9 g / kg, at 60°C it is 153.2 g / kg, and at 80°C it is 550.9 g / kg. At a pressure of 220 kPaa, the moisture content of saturated air at 40°C is 21.6 g / kg, at 60°C it is 62.1 g / kg, and at 80°C it is 171.0 g / kg. Therefore, at low temperatures, prolonged operation of the fuel cell will prevent the effective removal of water from the fuel cell, leading to flooding. Therefore, the operating temperature of the fuel cell is typically ≥60°C. To effectively remove moisture during shutdown and purge, high temperatures must be used to evaporate the water in the diffusion layer, catalyst layer, and proton membrane and release it with the gas. At the end of the purge, the hot gas contains a high level of moisture. This is especially true during rainy summer weather, when ambient humidity can reach over 90%. This air can carry a significant amount of moisture into the system. For example, at an ambient temperature of 40°C, the moisture content of saturated air can reach 49.1g / kg. After the purge is complete and the system is shut down, as the system temperature drops, the water vapor in the gas condenses. If this condensed water is not removed, it can reduce the performance and life of the fuel cell.

[0034] To address the above issues, the present invention provides a low-temperature shutdown purge and dehumidification method for PEM fuel cells. This method employs a two-stage purge strategy, consisting of a high-temperature purge followed by a short low-temperature purge. First, the stack undergoes a high-temperature purge, effectively removing free and weakly bound water from the fuel cell. Following the high-temperature purge, the stack undergoes a low-temperature purge, thereby preventing the formation of condensed water after the high-temperature purge ends, thereby improving the performance and lifespan of the PEM fuel cell.

[0035] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] like Figure 1 and Figure 2 As shown, the present invention provides a low-temperature shutdown purge dehumidification method for a PEM fuel cell, wherein the PEM fuel cell includes a fuel cell stack, an air path control system, a hydrogen path control system, and a cooling system. The method includes:

[0037] Step 1: Increase the air metering ratio of the air path; the air metering represents the ratio of the air supply to the actual reaction consumption.

[0038] Step 2: Extend the drainage time of the hydrogen line drain valve and increase the drainage frequency of the drain valve.

[0039] Step 3: Control the coolant temperature in the cooling path within a set temperature range and perform high-temperature purge on the fuel cell stack.

[0040] Step 4: When the EIS condition and / or the highest cell voltage reaches the set condition, the high-temperature purge is terminated.

[0041] Step 5: Control the cooling liquid in the cooling path to cool down so that the temperature of the cooling liquid is lower than a set value, so as to perform low-temperature purge on the fuel cell stack.

[0042] Step 6: When the low-temperature purge lasts for a set time, the low-temperature purge is terminated.

[0043] This embodiment is divided into two stages of high-temperature purge and low-temperature purge to purge the fuel cell stack. It should be noted that in this embodiment, steps 1-3 are not in any order. In this embodiment, the fuel cell stack is purged at high temperature through the air path, hydrogen path and cooling path, specifically as follows:

[0044] 1. The principle of air path Figure 3As shown, the throttle is an electronic butterfly valve, whose opening adjusts the flow and pressure of air entering the stack. In this embodiment, the air path throttle is fully opened, and the air compressor speed is adjusted to increase the air stoichiometric ratio in the air path. It should be noted that in step 1, increasing the air stoichiometric ratio in the air path to 1.3 to 1.5 times the stoichiometric ratio during normal PEM fuel cell operation can increase the amount of water vapor carried away by the hot air by 10% to 20%.

[0045] Before increasing the air stoichiometric ratio in the air path in step 1, this embodiment further includes: upon receiving a PEM fuel cell shutdown instruction, setting the current density of the PEM fuel cell to a preset purge current density. Specifically, the current density of the PEM fuel cell is reduced to the preset purge current density according to a program set in the fuel cell.

[0046] 2. The principle of hydrogen circuit Figure 4 As shown, the hydrogen output contains a large amount of water vapor and liquid water droplets. The liquid water and some water vapor are separated by a water separator. The drain valve is opened intermittently to drain the water, and some gas is also discharged. The hydrogen output also contains a large amount of hydrogen gas. The hydrogen returns to the hydrogen inlet device through the hydrogen return line and enters the fuel cell stack together with pure hydrogen to participate in the fuel cell chemical reaction again. Extend the drainage time of the hydrogen line drain valve by ≥3s to drain all the residual water in the water separator. Then increase the drainage frequency of the hydrogen line drain valve to 1.5 to 2 times the normal operation of the PEM fuel cell. If the normal operation is 1 second every 5 seconds, change it to 1 second every 2.5 to 3.5 seconds.

[0047] 3. The principle of cooling circuit Figure 5 As shown, the stack-radiator-electronic thermostat-water pump-stack is the large loop, while the stack-electric heater-electronic thermostat-water pump-stack is the small loop. The electronic thermostat can adjust the large and small loops through a ball valve. The large loop dissipates heat through the radiator, allowing the coolant to cool down quickly; the small loop uses the electric heater to quickly heat the coolant. The upper limit of the temperature range set in step 3 is the maximum allowable operating temperature of the stack, and the lower limit is 60°C. When the coolant temperature is lower than the lower limit of the set temperature range, the PTC is used to heat the coolant until the coolant temperature reaches the set temperature range. If the stack coolant temperature is ≥60°C, the electronic thermostat dynamically adjusts the coolant temperature using a PID algorithm to keep the coolant temperature above 60°C and below the maximum allowable operating temperature of the stack.

[0048] Based on the above steps, the stack is purged at high temperature, and a low-temperature shutdown and startup cycle test is performed specifically for the stack to confirm. When the EIS condition and / or the highest cell voltage reaches the set conditions, the high-temperature purge is terminated. Specifically:

[0049] If the PEM fuel cell has an EIS function, the end of the high-temperature purge can be based on the EIS conditions; if it does not have an EIS function, it depends on whether the highest cell voltage reaches the target value. The reason for using the highest cell voltage is to ensure that the moisture in all cells is purged to an acceptable level by the end of the high-temperature purge.

[0050] After the high-temperature purge is completed, this embodiment also performs a low-temperature purge on the stack, and the specific process is as follows:

[0051] Turn on the large circulation and cooling fan through the thermostat, increase the speed of the water pump, and quickly reduce the coolant temperature to the set value. In this embodiment, the set value is 40°C, that is, the coolant temperature is ≤40°C, and the operating conditions of the air circuit and the hydrogen circuit remain unchanged (that is, the operation of the air circuit and the hydrogen circuit are as in steps 1 and 2, respectively). When the coolant temperature is ≤40°C for the set time, the set time in this embodiment is 1 minute. Shut down (shutdown here means closing the three valves of the cooling circuit, air circuit and hydrogen circuit) to allow the stack body and the gas to cool down sufficiently. After the gas temperature drops below 40°C, the amount of water vapor that can be contained in the gas will be reduced by more than 60%, thereby significantly reducing the amount of water vapor condensed and precipitated in the gas after shutdown and cooling.

[0052] The present invention first increases the air stoichiometric ratio in the air path, prolongs the drain time of the hydrogen path drain valve, increases the drain frequency of the drain valve, and controls the coolant temperature in the cooling path within a set temperature range. Based on these steps, the fuel cell stack is subjected to a high-temperature purge, thereby effectively removing free water and weakly bound water from the fuel cell. After the high-temperature purge, the coolant in the cooling path is cooled to a temperature below a set value, thereby performing a low-temperature purge on the fuel cell stack. The low-temperature purge is terminated after the set time. This prevents the formation of condensed water after the temperature drops after the high-temperature purge, thereby improving the performance and lifespan of the PEM fuel cell. Furthermore, the purge time is significantly reduced.

[0053] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the method and core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A method for low-temperature shutdown and purging dehumidification of a PEM fuel cell, wherein the PEM fuel cell comprises a fuel cell stack, an air path control system, a hydrogen path control system, and a cooling system, characterized in that: The method comprises: Step 1: Increase the air metering ratio of the air path; the air metering ratio represents the ratio of the air supply amount to the actual reaction consumption; Step 2: Extend the drainage time of the hydrogen line drain valve and increase the drainage frequency of the drain valve; extend the drainage time of the hydrogen line drain valve to ≥3s to drain all the residual water in the water separator; Step 3: Control the coolant temperature in the cooling path within a set temperature range and perform high-temperature purge on the fuel cell stack; fuel cell stack-radiator-electronic thermostat-water pump-fuel cell stack is a large cycle, and fuel cell stack-electric heater-electronic thermostat-water pump-fuel cell stack is a small cycle. The electronic thermostat adjusts the large cycle and the small cycle through a ball valve. The large cycle dissipates heat through the radiator to quickly cool the coolant; the small cycle quickly heats the coolant through the electric heater; the upper limit of the set temperature range is the maximum allowable operating temperature of the fuel cell stack, and the lower limit is 60°C. When the coolant temperature is lower than the lower limit of the set temperature range, the PTC is used to heat the coolant until the coolant temperature reaches the set temperature range. If the temperature of the fuel cell stack coolant is ≥60°C, the electronic thermostat dynamically adjusts the coolant temperature through the PID algorithm to control the coolant temperature to be above 60°C and lower than the maximum allowable operating temperature of the fuel cell stack; Step 4: When the EIS condition and / or the highest cell voltage reaches the set condition, the high-temperature purge is terminated; Step 5: Control the cooling liquid in the cooling path to cool down so that the temperature of the cooling liquid is lower than a set value, so as to perform low-temperature purge on the fuel cell stack; Step 6: When the low-temperature purge lasts for a set time, the low-temperature purge is terminated; The low-temperature purge process includes: turning on the large circulation and cooling fan through the thermostat, increasing the water pump speed, and quickly reducing the coolant temperature to the set value. The operating conditions of the air and hydrogen circuits remain unchanged. When the coolant temperature is ≤40°C for the set time, shut down to allow the fuel cell stack and gas to fully cool down. Shutdown means closing the three valves of the cooling circuit, air circuit, and hydrogen circuit.

2. The method for low-temperature shutdown and purging dehumidification of a PEM fuel cell according to claim 1, characterized in that: Before step 1, also include: After receiving a PEM fuel cell shutdown instruction, the current density of the PEM fuel cell is set to a preset current density for purge.

3. The method for low-temperature shutdown and purging dehumidification of a PEM fuel cell according to claim 1, characterized in that: Step 1 specifically includes: Control the air path throttle to fully open and adjust the air compressor speed to increase the air metering ratio of the air path.

4. The method for low-temperature shutdown and purging dehumidification of a PEM fuel cell according to claim 1, characterized in that: In step 1, the air stoichiometric ratio of the air path is increased to 1.3 to 1.5 times the air stoichiometric ratio during normal operation of the PEM fuel cell.

5. The method for low-temperature shutdown and purging dehumidification of a PEM fuel cell according to claim 1, characterized in that: In step 2, the drainage frequency of the hydrogen path drain valve is increased to 1.5 to 2 times that of the normal operation of the PEM fuel cell.

6. The method for low-temperature shutdown and purging dehumidification of a PEM fuel cell according to claim 1, characterized in that: The set value in step 5 is 40°C.

7. The method for low-temperature shutdown, purging and dehumidification of a PEM fuel cell according to claim 1, characterized in that: The set time in step 6 is 1 minute.

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