Method and system for starting fuel cell at low temperature

Abstract

To provide a method and a system for starting a fuel cell at low temperatures.SOLUTION: There are presented a method (100) and a system (10) for starting a fuel cell at temperatures below 0°C with a two-phase cooling system. In that case, the two-phase cooling system includes a pump (20) to convey a coolant present in the two-phase cooling system, the coolant in the two-phase cooling system being present at least partially in a gas phase. The method (100) includes a step (102) of starting the fuel cell (12), and a step (104) of activating the pump (20) after a defined period of time, the coolant being present basically in a gas phase within the fuel cell (12) during the defined period of time.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates generally to the field of aviation technology, and more particularly to a method for starting a fuel cell at low temperatures, as well as an associated system and an aircraft equipped with such a system. [Background technology]

[0002] Fuel cells are a possible solution for emission-free drive or emission-free on-board power supply (APU = Auxiliary Power Unit), for example in aircraft. Polymer electrolyte fuel cells (PEMFC) generate current and power by electrochemical reaction producing water from hydrogen and oxygen. During this reaction, heat is generated that needs to be dissipated. In commercially available fuel cell stacks, the heat dissipation is usually achieved by liquid cooling. One alternative that allows significant weight savings at the drive system level or on-board power system level is to replace the liquid cooling by a two-phase cooling circuit. In this case, the latent heat of vaporization is used to dissipate the large amount of heat from the fuel cell. Furthermore, the high heat transfer coefficient improves the performance of the cooling system compared to single-phase cooling. The two-phase cooling circuit is then based on a phase transition in the fuel cell (evaporator) and in a heat exchanger to the surroundings (condenser).

[0003] A typical fuel cell system, especially in mobile applications, requires that the system can be started even at sub-freezing temperatures. During this start-up scenario, the heat generated by the fuel cell contributes significantly to the heating of the system. This presents a challenge as the heat generated must be balanced against the ice formation from the water produced. Summary of the Invention [Problem to be solved by the invention]

[0004] The object of the present invention can be seen as accelerating the start-up process of a fuel cell at sub-zero temperatures. [Means for solving the problem]

[0005] This problem is solved by the subject matter of the independent claims. Further embodiments are given in the dependent claims and in the following description.

[0006] According to one aspect, a method is presented for starting up a fuel cell using a two-phase cooling system at a temperature below 0° C. The two-phase cooling system has a pump for pumping a coolant present in the two-phase cooling system, the coolant in the two-phase cooling system being at least partially in a gas phase.

[0007] The method comprises: starting up the fuel cell; activating the pump after a prescribed time period, during which the coolant is essentially in a gas phase within the fuel cell.

[0008] At sub-zero temperatures, ice forms in the gas pipes. For this reason, the time to reach temperatures above 0°C is considered a critical factor, and reducing the thermal mass to be heated is considered advantageous. To speed up this process compared to liquid cooling, a technical solution is proposed that exploits the two-phase state of the coolant in the fuel cell and precisely controls the coolant pump.

[0009] Thus, the above method for starting up a fuel cell using a two-phase cooling system at temperatures below 0° C. provides an effective method for starting up and operating a fuel cell in a low temperature environment. A two-phase cooling system uses a special pump to pump a coolant, where the coolant is at least partially in the gas phase.

[0010] The start-up process begins with starting up the fuel cell according to normal procedures: After a prescribed time required for fuel cell start-up, the pump is started, during which the coolant is essentially in the gas phase within the fuel cell.

[0011] This approach offers several advantages. First, the use of a two-phase cooling system allows for efficient heat dissipation, ensuring optimal cooling of the fuel cell even at low temperatures. Second, the start-up of the pump after a defined time allows for a shorter start-up time for fuel cells with a smaller thermal mass before the coolant becomes subcooled to reduce the start-up time or warm-up time. When the coolant is in gas phase, it has a lower density than in liquid phase. This lower density translates into a lower thermal mass, which allows for faster heating for the same heat input.

[0012] Use of this method therefore provides a reliable method for successfully starting and operating a fuel cell even at low temperatures, allowing for efficient cooling and contributing to the long term stability and performance of the fuel cell at low temperatures, and ensuring optimal operation of the fuel cell in low temperature environments improves the ease of use and reliability of the system.

[0013] According to one embodiment, during the starting step, the coolant is 100% in the gas phase within the fuel cell.

[0014] According to one embodiment, the defined time is 60 seconds, preferably 30 seconds, particularly preferably 20 seconds.

[0015] According to one embodiment, the coolant comprises methanol and / or ethanol. This choice of coolant provides several advantages to the system.

[0016] Methanol and ethanol have high thermal conductivity and can efficiently dissipate heat from the fuel cell, allowing for effective cooling of the components and keeping the fuel cell operating temperature at an optimal level.

[0017] Additionally, methanol and ethanol have low boiling points, which is advantageous because they can evaporate quickly as they flow through the coolant channels, absorbing heat from the components, which provides effective cooling.

[0018] Furthermore, methanol and ethanol have a high enthalpy of vaporization, which allows for very low mass flows in the system, which reduces pressure losses and results in smaller piping and pumps than liquid cooling, which translates into a lighter system.

[0019] According to one embodiment, the coolant pump operates intermittently to keep the thermal mass of the coolant mass flow in the fuel cell small.

[0020] In this context, "intermittently" means switching on and off at predetermined time intervals. This mode of operation is used to keep the thermal mass of the coolant in the fuel cell small.

[0021] Intermittent operation of the coolant pump reduces the mass flow rate of the coolant in the fuel cell, which means that only a limited amount of coolant circulates through the cell, and in particular none at all, which leads to a reduction in thermal mass.

[0022] The advantage of this approach is that the less mass of coolant in the fuel cell, the faster the reaction times and the more efficient the heat dissipation, and the reduced thermal mass contributes to an overall more efficient and stable operation of the fuel cell.

[0023] The energy consumption of the system can also be optimized by an intermittent operation method for the coolant pump: the pump does not have to be operated continuously but can be controlled depending on the thermal requirements of the fuel cell, which leads to a more efficient use of the available energy and can improve the overall efficiency of the system.

[0024] According to one aspect, a system for starting a fuel cell using a two-phase cooling system at temperatures below 0° C. is configured to perform a method for starting a fuel cell at temperatures below 0° C. To that end, the system comprises at least one fuel cell, a pump, a heat exchanger, and a coolant circuit. The coolant circuit is configured to cool the fuel cell by two-phase cooling. The pump is configured to be startable after a defined time.

[0025] According to one embodiment, a condenser bypass is used to keep the thermal mass of the mass flow circulating through the fuel cell small.

[0026] In this embodiment, the condenser bypass is used to keep the thermal mass of the mass flow circulating to and / or from the condenser small. A condenser bypass is a device or arrangement that allows a portion of the mass flow or coolant to bypass the condenser instead of passing through the condenser. Thus, the condenser bypass forces the coolant to bypass the condenser. This reduces the thermal mass of the coolant circulating in the circuit and eliminates unnecessary heat dissipation to the surroundings during the start-up process.

[0027] The purpose of the condenser bypass is to reduce the amount of coolant circulating through the cooling circuit: by bypassing the condenser of a portion of the mass flow, the thermal mass of the coolant circulating in the fuel cell is reduced.

[0028] The advantages of this arrangement are shorter reaction times and more efficient heat dissipation. The amount of coolant circulating inside the fuel cell is reduced, so the operating temperature is reached faster. At the same time, a more uniform temperature distribution is achieved inside the fuel cell, which leads to improved system performance and stability.

[0029] Additionally, the reduced thermal mass due to the condenser bypass contributes to the fuel cell operating more efficiently overall: less coolant means less energy is required to operate the coolant pumps, which translates into optimized energy utilization.

[0030] The condenser bypass can also be used to control the temperature of the coolant: by diverting a portion of the mass flow around the condenser, the temperature of the coolant can be effectively adjusted to maintain an optimum operating temperature of the fuel cell.

[0031] According to one aspect, an aircraft includes such a system, although the system obviously has other applications as well, such as automobiles, surface vessels, spaceflight, or other fuel cell powered units.

[0032] The system design, in summary, contemplates that the fuel cells are geodetically located higher than the rest of the coolant system. The coolant collects in the lower half of the circuit, while the vapor phase prevails in the rest of the coolant system (e.g., the fuel cells). Instead of switching on the pump to push the liquid towards the fuel cell stack, the pump is started later. Furthermore, an isolation valve can be used to isolate the fuel cell stack from the coolant circuit. The start-up process of the fuel cell stack can be accelerated due to the low heat capacity of the vapor coolant in the coolant flow path (the gas has a lower heat capacity, hence a lower thermal mass compared to conventional liquid cooling). The temperature of the fuel cell stack rises quickly, and as soon as a sufficient operating temperature is reached, the isolation valve to the fuel cell stack is opened and cold coolant is poured into the stack. This causes a rapid drop in temperature. To limit the temperature drop, the pump flow rate can be controlled (ramp mode, cyclic on / off switching, circulation mode) for a smooth start-up process. Furthermore, the fuel cell stack can be heated only by liquid cooling, due to the lower heat transfer coefficient than two-phase cooling. Once the operating temperature is reached, the saturation temperature for the two-phase cooling can be established.

[0033] If the total coolant circulation is limited to the fuel cell and bypass and the condenser is not turned on, the thermal mass inside the fuel cell remains unchanged, but the thermal mass of the coolant heated outside the fuel cell is limited / less.

[0034] According to one embodiment, the method further comprises the following method steps for reducing temperature peaks during start-up of the fuel cell using a two-phase cooling system: adjusting the saturation pressure of the coolant via a pressure control module to a first saturation pressure such that a maximum temperature of the coolant caused by boiling delay is less than a maximum operating temperature of the fuel cell; starting up the fuel cell; In response to an excess of the maximum temperature of the coolant caused by the boiling delay, the saturation pressure of the coolant can be adjusted via the pressure control module to a second saturation pressure higher than the first saturation pressure, which is particularly advantageous for low temperature materials.

[0035] In the case of two-phase flow, the saturation temperature is pressure dependent. Therefore, a proposed operating strategy of a further method step is to establish the maximum temperature due to boiling delay below the temperature limit of the battery components during the start-up phase by lowering the saturation pressure of the cooling system. Once the peak value is exceeded and the boiling process is initiated, the target value of the pressure regulator can be increased to the nominal operating pressure / temperature.

[0036] One advantage of the above additional method step of reducing temperature peaks during start-up of a fuel cell using a two-phase cooling system is that the saturation pressure of the coolant can be effectively controlled and regulated.

[0037] By adjusting the saturation pressure of the coolant to a first saturation pressure below the maximum operating temperature of the fuel cell, the undesirable effects of boiling delay and therefore overheating above the adjusted boiling temperature are reliably avoided. This can reduce the risk of temperature peaks that may occur during start-up.

[0038] After start-up of the fuel cell, the method continuously monitors the temperature of the coolant. If the maximum temperature caused by boiling delay is exceeded, the pressure control module is used to adjust the saturation pressure of the coolant. The saturation pressure is adjusted to a second, higher saturation pressure. This increases heat dissipation by the coolant, improves cooling performance, and effectively controls the temperature of the fuel cell.

[0039] The advantage of this approach is that it prevents or reduces temperature peaks during the start-up process, especially at low temperatures. By adjusting the saturation pressure in time, it ensures optimal cooling performance without overheating above the target operating temperature due to boiling delay. This contributes to the stability and lifespan of the fuel cell and allows the system to operate smoothly.

[0040] In summary, the above method allows for precise control of the saturation pressure of the coolant to minimize temperature peaks during the start-up process of the fuel cell. This improves the reliability, performance and lifespan of the fuel cell and generally optimizes the operating efficiency of the fuel cell system. Such a method for starting up a fuel cell at temperatures below 0° C. and further reducing temperature peaks during start-up can be implemented in particular with the above system for operating a fuel cell with a two-phase cooling system.

[0041] In the following, embodiments of the invention will be explained in more detail with reference to the accompanying drawings, in which the illustrations are schematic and not to scale, and in which the same reference signs refer to the same or similar elements. [Brief description of the drawings]

[0042] [Figure 1] 2 is a flow chart of a method for starting a fuel cell at temperatures below 0° C. [Figure 2A] FIG. 1 illustrates a system for operating a fuel cell using a two-phase cooling system according to a first embodiment. [Figure 2B] FIG. 2 illustrates a system for operating a fuel cell using a two-phase cooling system according to a second embodiment. [Diagram 3]FIG. 1 shows an aircraft equipped with a system for starting a fuel cell at temperatures below 0° C. [Figure 4] 1 is a graph showing a temperature curve over time. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] 1 shows a method 100 for starting up a fuel cell using a two-phase cooling system at temperatures below 0° C., the method including several steps to ensure a smooth start-up process. In this method, a two-phase cooling system is used, which includes a pump 20 for pumping coolant through the system 10. The coolant is at least partially in a gas phase within the two-phase cooling system.

[0044] The first step of the method is to start up 102 the fuel cell 12. In sub-freezing temperatures, it is especially important that the start-up process is performed properly. After the fuel cell has started up, the method proceeds to the next step.

[0045] The next step is to start 104 the pump 20 after a prescribed time period. During this time the coolant is essentially in the gas phase within the fuel cell 12. This is a critical step to ensure effective cooling. Starting the pump moves the coolant through the two-phase cooling system, thereby dissipating heat and stabilizing the temperature of the fuel cell.

[0046] The method allows the fuel cell to start up normally even at very low temperatures. Due to the two-phase cooling system and the proper start-up of the pump, the cooling is optimized. This can increase the life span of the fuel cell and the operational reliability of the fuel cell.

[0047] 2A and 2B each show a system 10 for starting up a fuel cell using a two-phase cooling system at temperatures below 0° C., the system 10 being configured to perform the method 100 described above. The system 10 is comprised of several components that cooperate to enable a successful start-up process.

[0048] The central component of the system is a fuel cell 12, which converts chemical energy into electrical energy. To ensure effective cooling of the fuel cell, a two-phase cooling system is used, which utilizes a combination of liquid and gas phases of a coolant to dissipate heat and regulate the temperature of the fuel cell.

[0049] To move the coolant in the cooling system, there is a pump 20. This pump is designed to start after a defined time, which is important in order to ensure the correct condition of the coolant in order to guarantee optimal cooling of the fuel cell.

[0050] Incorporated into the system 10 is a coolant circuit 16 and a heat exchanger 18, in addition to a two-phase cooling system and a pump 20. The circuit allows for a continuous flow of coolant through the fuel cell and the cooling system. The coolant circuit ensures efficient circulation of the coolant and optimizes heat dissipation.

[0051] The basic difference between the embodiment of Figure 2A and Figure 2B can be seen in the fact that an accumulator / collector is connected to the return line, allowing the gas phase to mix with the liquid phase. In this embodiment, a condenser bypass is used to keep the thermal mass of the mass flow circulating through the fuel cell small. A condenser bypass is a device or arrangement that allows a portion of the mass flow or coolant to bypass the condenser instead of passing through it. The condenser bypass thus forces the coolant to bypass the condenser. This reduces the thermal mass of the coolant circulating in the circuit and eliminates unnecessary heat dissipation to the surroundings during the start-up process.

[0052] The system 10 for operating a fuel cell with a two-phase cooling system provides efficient cooling, improved performance, and extended life for the fuel cell 12. It allows for reliable operation even in extreme temperatures, resulting in consistent and efficient energy production.

[0053] 3 shows an aircraft 200 equipped with a system 10 developed for the operation of a fuel cell, which operates in conjunction with a fuel cell to enable efficient energy production.

[0054] In addition to the cooling system and pump 20, the system 10 also includes a coolant circuit 16. The coolant circuit 16 allows for a continuous flow of coolant through the system to ensure constant cooling of the fuel cell. The coolant circuit plays a key role in maintaining the optimum operating temperature of the fuel cell, and using the methods described above, it is possible to start up even at temperatures below 0°C.

[0055] Figure 4 shows two graphs of temperature curves over time. Both graphs start below 0°C (T0) and rise to the operating temperature T B The graph on the left (solid line) approaches the operating temperature T earlier than the graph on the right (dashed line). B It can be clearly seen that the specified time t P The pump starts after t and the above effects occur. In contrast, the graph on the right shows the curve where the pump is started from the beginning, which is quite flat at the beginning and therefore rises above the operating temperature T B is reached relatively slowly. Thus, the method described above makes it possible to accelerate the start-up process of a fuel cell at sub-zero temperatures.

[0056] As a supplementary note, it should be noted that "comprises" or "comprises" does not exclude other elements or steps, and "a" does not exclude a plurality. Furthermore, it should be noted that features or steps described with reference to one of the above embodiments can also be used in combination with other features or steps of other embodiments described above. Reference signs in the claims are not to be considered as limiting. [Explanation of symbols]

[0057] 10. System 12 Fuel Cell 14 Accumulator 16 Coolant Circuit 18 Heat exchanger 20 Pump Method for starting a fuel cell at temperatures below 1000°C 102 Fuel cell start-up 104 Pump start after specified time 200 aircraft T0 freezing point T B operating temperature t P Specified time

Claims

1. A method (100) for starting up a fuel cell (12) using a two-phase cooling system at a temperature below 0° C., wherein the two-phase cooling system includes a pump (20) for pumping a coolant present in the two-phase cooling system, the coolant in the two-phase cooling system being at least partially in a gas phase; starting up (102) the fuel cell (12); and starting (104) the pump (20) after a predetermined time, during which the coolant is essentially in a gas phase within the fuel cell (12).

2. 2. The method (100) of claim 1, wherein during the starting step, the coolant is 100% in the gas phase within the fuel cell (12).

3. 2. The method (100) according to claim 1, wherein the defined time is at least 60 seconds, preferably at least 30 seconds, particularly preferably at least 20 seconds.

4. The method (100) of any one of claims 1 to 3, wherein the coolant comprises methanol and / or ethanol.

5. The method (100) of claim 1, wherein the pump (20) operates intermittently to keep a thermal mass of the coolant mass flow in the fuel cell (12) low.

6. A system (10) for starting up a fuel cell with a two-phase cooling system at temperatures below 0° C., said system (10) being configured to carry out the method according to any one of claims 1 to 5, At least one fuel cell (12); A pump (20); a coolant circuit (16), the coolant circuit (16) is configured to cool the fuel cell (12) by two-phase cooling; The system (10), wherein the pump (20) is configured to be capable of being activated after a predetermined time.

7. The system (10) of claim 6, wherein a condenser bypass is used to keep the thermal mass of the mass flow to and / or from the condenser small.

8. An aircraft (200) comprising a system (10) according to claim 6 or 7.

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