METHOD FOR REDUCING TEMPERATURE PEAKS IN A FUEL CELL WITH A TWO-PHASE COOLING SYSTEM

Description

Field of invention

[0001] The present description relates generally to the technical field of aviation. In particular, the description relates to a method for reducing temperature peaks in a fuel cell, as well as an associated system and an aircraft comprising such a system. Background of the invention

[0002] Fuel cells represent a potential solution for zero-emission propulsion or zero-emission onboard power supply (APU = Auxiliary Power Unit) in aircraft, for example. Polymer electrolyte fuel cells (PEMFCs) generate electricity through the electrochemical reaction of hydrogen and oxygen to form water. This reaction produces heat that must be dissipated. In commercial fuel cell stacks, this is typically achieved through liquid cooling. A possible alternative, offering significant weight savings at the propulsion or onboard power supply system level, is to replace liquid cooling with a two-phase cooling circuit. This utilizes the latent heat of vaporization to remove large amounts of heat from the fuel cells. Additionally, the high heat transfer coefficient compared to single-phase cooling improves the cooling system's performance.The two-phase cooling circuit is based on the phase transitions in the fuel cells (evaporator) and in the heat exchanger to the environment (condenser).

[0003] During two-phase cooling, particularly during the necessary core formation, temperature spikes occur, known as "liquid superheating" or bumping. This effect leads to higher temperatures at the interface because the boiling process has not yet fully begun. Once a certain superheat above the saturation point is reached, boiling starts and the temperature drops to the saturation temperature. This is a temporary effect that initiates the boiling process. This effect of bumping can lead to a higher degradation rate of the

[0004] Fuel cell due to the reached temperature peak.

[0005] US 2022 / 238896 A1, DE 102020212939 A1 and EP 2631567 A1 describe methods for starting a fuel cell system with a two-phase cooling system. Summary of the invention

[0006] The object of the invention can be considered to be a method for optimizing the operation and increasing the durability of fuel cells. This object is achieved by the subject matter of the independent claims. Further embodiments are described in the dependent claims and in the following description.

[0007] According to one aspect, a method for reducing temperature peaks during the start-up of a fuel cell with a two-phase cooling system has the following steps: Adjusting the saturation pressure of a coolant, via a pressure control module, to a first saturation pressure such that a maximum temperature induced by boiling delay of the coolant is below a maximum operating temperature of the fuel cell; starting the fuel cell; in response to the fact that the maximum temperature induced by boiling delay of the coolant has been exceeded, adjusting the saturation pressure of the coolant, via the pressure control module, to a second saturation pressure, the second saturation pressure being higher than the first saturation pressure.

[0008] In two-phase flows, the saturation temperature depends on the pressure. The proposed operating strategy is therefore to set the maximum boiling delay temperature during the start-up phase below a temperature limit of the cell components by reducing the saturation pressure in the cooling system. Once the peak value has been exceeded and the boiling process has begun, the setpoint of the pressure regulator can be increased to the nominal operating pressure / temperature.

[0009] One advantage of this method for reducing temperature peaks during the start-up of a fuel cell with a two-phase cooling system is that it allows effective control and regulation of the saturation pressure and thus the temperature of the coolant.

[0010] By setting the coolant saturation pressure to an initial saturation pressure below the fuel cell's maximum operating temperature, the undesirable effect of delayed boiling at excessively high temperatures, and thus overheating above the set boiling point, is avoided. This reduces the risk of temperature spikes that can occur during the start-up process.

[0011] After the fuel cell is started, the system continuously monitors the coolant temperature. If the maximum temperature induced by boiling delay is exceeded, the coolant saturation pressure is adjusted using the pressure control module. The saturation pressure is set to a second, higher saturation pressure. This allows for increased heat dissipation by the coolant, resulting in improved cooling performance and effective temperature control of the fuel cell.

[0012] The advantage of this approach lies in its ability to prevent or reduce temperature spikes during the start-up process. By adjusting the saturation pressure in a timely manner, optimal cooling performance can be ensured without overheating due to delayed boiling above the target operating temperature. This contributes to the stability and longevity of the fuel cell and enables smooth system operation.

[0013] In summary, the described method enables precise control of the coolant saturation pressure to minimize temperature peaks during the fuel cell start-up process. This leads to improved reliability, performance, and lifespan of the fuel cell, as well as an overall optimized operating efficiency of the fuel cell system.

[0014] According to one embodiment, the pressure control module is an accumulator.

[0015] Using a battery as a pressure control module offers several advantages. It enables rapid and precise adjustment of the saturation pressure to respond to changes in operating conditions and requirements. This ensures efficient cooling and optimal fuel cell performance.

[0016] Additionally, the accumulator contributes to pressure stabilization in the two-phase cooling system. By buffering pressure fluctuations and maintaining a constant pressure level, a consistent coolant supply is ensured, resulting in improved heat transfer and cooling.

[0017] The accumulator also offers a degree of redundancy and safety. It can serve as a reservoir for additional coolant and compensate for unforeseen fluctuations in system pressure. This minimizes potential damage to the fuel cell due to pressure spikes or drops.

[0018] According to one embodiment, the accumulator is connected to an outlet of the fuel cell and is configured to separate the vapor and liquid phases of the coolant. Separating the vapor and liquid phases is essential in two-phase cooling.

[0019] According to one embodiment, the accumulator is configured to set a temperature of the coolant.

[0020] Using a battery as a pressure control module with adjustable temperature offers several advantages. By adjusting the battery's temperature, the coolant's saturation pressure can be precisely controlled. This enables effective cooling of the fuel cell and prevents temperature spikes during operation.

[0021] According to one embodiment, the accumulator is configured to set a pressure of the coolant.

[0022] Similarly, using an accumulator as a pressure control module with adjustable pressure offers several advantages. By adjusting the pressure within the accumulator, the saturation pressure, and thus the temperature of the coolant, can be precisely controlled to meet the requirements of the fuel cell. This enables efficient cooling. Another advantage of a pressure-controlled accumulator is the ability to instantly set an evaporation pressure.

[0023] The accumulator's adjustable pressure or temperature function also offers flexibility in adapting the cooling system to different operating conditions and requirements. Depending on the specific needs, the pressure can be adjusted accordingly to ensure optimal cooling and fuel cell performance.

[0024] Overall, using an accumulator as a pressure control module with adjustable temperature or pressure enables precise control of the saturation pressure in the two-phase cooling system. This contributes to improved performance, efficiency, and reliability of the fuel cell system and supports stable and safe operation.

[0025] According to one embodiment, the pressure control module is a combination of a pump and a throttle valve.

[0026] Using a combination of a pump and a throttle valve as a pressure control module offers several advantages. The pump enables active control of the coolant pressure by feeding the coolant into the system at a specific speed and pressure. This allows for precise control of the coolant saturation pressure to maintain the optimal operating temperature of the fuel cell.

[0027] At the same time, a combination of a pump and a throttle valve offers the possibility of savings in weight and installation space requirements of the cooling system by reducing the required accumulator size or eliminating the accumulator altogether.

[0028] The throttle valve acts as a control valve to limit the coolant flow and regulate the pressure. By adjusting the throttle valve, the pressure in the system can be controlled and adjusted. This allows for precise control of the saturation pressure in the cooling system to meet the requirements of the fuel cell and prevent temperature spikes.

[0029] Furthermore, the use of a pump and throttle valve combination enables continuous monitoring and control of the coolant pressure. The system can react to changes in operating conditions and adjust the pressure accordingly to ensure stable cooling and safe operation of the fuel cell.

[0030] According to one embodiment, the first saturation pressure is in a range of 0 bar to 10 bar, preferably 0 bar to 5 bar, particularly preferably 1.25 bar.

[0031] According to one embodiment, the second saturation pressure is in a range of 0 bar to 10 bar, preferably 0 bar to 5 bar, particularly preferably 3 bar.

[0032] According to one embodiment, the maximum temperature of the coolant induced by superheating is 50% to 100% of the operating temperature, preferably 60% to 90%, particularly preferably 70% to 80%.

[0033] In one embodiment of the fuel cell system, methanol and / or ethanol is used as the coolant. This choice of coolant offers several advantages for the system.

[0034] Methanol and ethanol have high thermal conductivity, which means they can efficiently dissipate heat from the fuel cells and the bipolar plate.

[0035] This enables effective cooling of the components and keeps the operating temperature of the fuel cell at an optimal level.

[0036] Furthermore, methanol and ethanol have low boiling points. This is advantageous because they can evaporate quickly when flowing through the coolant channels. This evaporation absorbs heat from the components, resulting in effective cooling. The combination of boiling point and saturation pressure, which is particularly suitable for fuel cell operation, proves to be beneficial.

[0037] Furthermore, methanol and ethanol have a high enthalpy of vaporization. This allows for very small mass flow rates in the system, thereby reducing pressure loss. As a result, pipes and pumps can be smaller than with liquid cooling, leading to a weight reduction in the system.

[0038] According to one aspect, a system for operating a fuel cell with a two-phase cooling system, wherein the system is configured to reduce temperature peaks during startup, comprises the following: at least one fuel cell, a pressure control module, a heat exchanger, and a coolant circuit. The coolant circuit is configured to cool the fuel cell through the phase transition of the coolant. The pressure control module is configured to set a saturation pressure of the coolant contained in the coolant circuit.

[0039] The system comprises at least one fuel cell or fuel cell stack responsible for generating energy from the supplied fuel. A two-phase cooling system is used to effectively cool the fuel cell. This cooling system enables efficient heat dissipation and contributes to stabilizing the operating temperature.

[0040] A key component of the system is the pressure control module, which is responsible for controlling the saturation pressure of the coolant fed into the cooling circuit. The saturation pressure is set so that the maximum temperature of the coolant during the start-up process remains below the maximum operating temperature of the fuel cell. This prevents undesirable temperature spikes and ensures optimal cooling performance.

[0041] The coolant circuit is specially designed to enable efficient distribution and circulation of the coolant. The coolant is pumped through the fuel cell and the two-phase cooling system to dissipate the generated heat and ensure uniform cooling.

[0042] By combining these components and precisely adjusting the coolant saturation pressure during the start-up process, the described system can effectively regulate the fuel cell temperature, thereby improving its lifespan and performance. Reducing temperature peaks thus helps to minimize thermal stress and create a reliable and stable operating environment for the fuel cell.

[0043] Overall, the described system enables efficient cooling of the fuel cell with a two-phase cooling system and contributes to optimizing the fuel cell's performance and lifespan. This is crucial for the reliable operation of the fuel cell system and its application in various fields such as transportation, energy storage, and many other applications.

[0044] According to one aspect, an aircraft comprises a specified system. This system can also be explicitly applied in other systems, such as motor vehicles, watercraft, spacecraft, or other fuel cell-powered units.

[0045] Integrating this system into the aircraft provides a sustainable and environmentally friendly energy source for propulsion or onboard power supply. The fuel cell enables high energy efficiency while simultaneously reducing emissions compared to conventional combustion engines. The two-phase cooling system ensures efficient cooling and contributes to the reliability and longevity of the fuel cell system.

[0046] Using this system in the aircraft enables longer flight times and improved performance. It also reduces environmental impact and offers a sustainable alternative to conventional fuel sources. The aircraft with the integrated fuel cell system thus contributes to cleaner and more environmentally friendly aviation.

[0047] According to one embodiment, the method can further be supplemented by the following process steps for starting a fuel cell at temperatures below 0° Celsius, since avoiding temperature spikes for the cold material is particularly relevant at these temperatures: Starting the fuel cell, activating the pump after a defined period, whereby during the defined period the coolant is essentially in the gas phase inside the fuel cell.

[0048] Temperatures below freezing lead to ice formation in the gas channels. Therefore, the time required to reach temperatures above 0 °C is considered a crucial factor, and reducing the thermal mass to be heated is seen as advantageous. To accelerate this process compared to liquid cooling, the proposed technical solution involves utilizing the two-phase state of the coolant in the fuel cell and precisely controlling the coolant pump.

[0049] The described additional process steps for starting a fuel cell at temperatures below 0°C using a two-phase cooling system thus offer an effective method for activating and operating the fuel cell in cold environments. The two-phase cooling system uses a special pump to circulate the coolant, which is at least partially in a gaseous phase.

[0050] The start-up process begins with the fuel cell being started according to standard procedures. After a defined period required for the necessary activation of the fuel cell, the pump is activated. During this defined period, the coolant is essentially in the gaseous phase within the fuel cell.

[0051] This approach offers several advantages. First, the use of a two-phase cooling system enables efficient heat dissipation and ensures optimal cooling of the fuel cell, even at low temperatures. Second, activating the pump after a defined period allows for a shorter start-up time for the fuel cell with its low thermal mass, before the coolant enters at a supercooled state, thus reducing the start-up or warm-up time. When the coolant is in the gas phase, it has a lower density compared to the liquid phase. This lower density results in a lower thermal mass, leading to faster heating with the same heat input.

[0052] The application of this method therefore offers a reliable way to successfully start and operate a fuel cell even at cold temperatures. It enables efficient cooling and contributes to the long-term stability and performance of the fuel cell at low temperatures. By ensuring optimal operation of the fuel cell in cold environments, the system's usability and reliability are improved.

[0053] Such a method for reducing temperature peaks during start-up and additionally for starting a fuel cell at temperatures below 0°C is particularly feasible with a described system for operating a fuel cell with a two-phase cooling system. Brief description of the characters

[0054] The following section describes exemplary embodiments of the invention with reference to the accompanying drawings. The illustrations are schematic and not to scale. Identical reference numerals refer to identical or similar elements. The drawings show: Fig. 1 a flowchart of a process for reducing temperature peaks during the start-up of a fuel cell with a two-phase cooling system; Fig. 2A a system for operating a fuel cell with a two-phase cooling system according to a first embodiment; Fig. 2B a system for operating a fuel cell with a two-phase cooling system according to a second embodiment; Fig. 3 an aircraft with a system for operating a fuel cell with a two-phase cooling system; and Fig. 4 two graphical representations of temperature trends over time. Detailed description of implementation examples

[0055] Fig. 1 Figure 100 shows a flowchart of a method 100 for reducing temperature peaks during the start-up of a fuel cell with a two-phase cooling system, comprising the following steps: First, the saturation pressure of a coolant is set to a first saturation pressure 102 via a pressure control module 14 (not shown). This first saturation pressure is selected such that the maximum temperature of the coolant induced by boiling delay is below the maximum operating temperature of the fuel cell.

[0056] Once the saturation pressure is set, the fuel cell is started 104. During the start-up process, boiling delay can occur, which can lead to an increased temperature of the coolant. After exceeding the temperature peak at a first saturation pressure 102, the saturation pressure of the coolant is set to a second saturation pressure 106 via the pressure control module 14. This second saturation pressure is higher than the first saturation pressure and enables effective cooling of the fuel cell close to its maximum operating temperature.

[0057] By precisely adjusting the saturation pressure of the coolant, the process can ensure accurate temperature control during the start-up process, thus improving the operational reliability and lifespan of the fuel cell.

[0058] Figur 2A und 2B Each figure shows a system 10 for operating a fuel cell 12 with a two-phase cooling system designed to ensure efficient cooling of the fuel cell and to reduce temperature peaks during start-up. The system comprises several components that operate in an integrated coolant circuit 16 with a pump 20.

[0059] The central component of the system is the fuel cell 12, which converts chemical energy into electrical energy. To ensure the optimal operating temperature of the fuel cell 12, a two-phase cooling system is used. This system utilizes a coolant that can exist in both liquid and gaseous phases to efficiently dissipate heat.

[0060] A pressure control module 14 is integrated to control the coolant circuit. This module enables precise adjustment of the coolant saturation pressure in the coolant circuit 16. By controlling the saturation pressure, cooling performance can be optimized and temperature spikes in the form of delayed boiling during the start-up process can be reduced.

[0061] System 10 operates in a continuous circuit in which the coolant flows through the fuel cell 12 and absorbs the heat generated. The coolant is then cooled via the heat exchanger 18 and returned to the coolant circuit 16. This ensures constant cooling of the fuel cell 12 and prevents overheating.

[0062] The essential difference between the designs of Figur 2A Regarding 2B, it can be seen that the 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 minimize the thermal mass of the mass flow circulating through the fuel cell. A condenser bypass is a device or arrangement that allows a portion of the mass flow or coolant to be routed around the condenser instead of through it. The condenser bypass thus diverts the coolant around the condenser. This reduces the thermal mass of the circulating coolant in the loop and prevents unnecessary heat dissipation to the environment during the start-up process.

[0063] System 10 for operating a fuel cell with a two-phase cooling system offers efficient cooling, improved performance, and a longer service life for the fuel cell 12. It enables reliable operation even at extreme temperatures and ensures stable and effective energy generation.

[0064] Figur 3 Figure 200 shows an aircraft equipped with a System 10 for operating a fuel cell with a two-phase cooling system. System 10 operates according to the principles described above for reducing temperature peaks during start-up and for efficient cooling of the fuel cell.

[0065] The aircraft 200 integrates the system 10 into its overall structure. The fuel cell 12, the pressure control module 14, and the coolant circuit 16 are specifically adapted to the aircraft's requirements.

[0066] System 10 enables the aircraft 200 to generate energy in an environmentally friendly and efficient manner. The fuel cell extracts electrical energy from the supplied fuel and atmospheric oxygen, while the two-phase cooling system ensures an optimal operating temperature, with boiling superheat occurring below the operating temperature.

[0067] Figur 4 Figure 1 shows two graphs plotting the temperature of a fuel cell 12 over time. The upper graph shows that the maximum temperature Ts induced by boiling delay is higher than the operating temperature TB. This can lead to the described undesirable effects.

[0068] The lower graph illustrates the temperature profile, showing that, in response to exceeding the maximum temperature of the coolant induced by superheating, the saturation pressure of the coolant is adjusted to a second saturation pressure via the pressure control module. This second saturation pressure is higher than the first saturation pressure.

[0069] This results in a lower maximum temperature Ts induced by delayed boiling, which is below the operating temperature TB, thus improving the operating behavior of the fuel cell.

[0070] It should also be noted that "comprehensive" or "comprising" does not exclude other elements or steps, and "a" or "an" does not exclude a plurality. Furthermore, it should be noted that features or steps described with reference to one of the above embodiments may also be used in combination with other features or steps of other embodiments described above. Reference numerals in the claims are not to be considered as limitations. Reference symbol list

[0071] 10 System 12 Fuel cell 14 Pressure control module, accumulator 16 Coolant circuit 18 Heat exchanger 20 Pump 100 Method for reducing temperature peaks 102 Setting a first saturation pressure 104 Starting the fuel cell 106 Setting a second saturation pressure 200 Aircraft TB Maximum operating temperature of the fuel cell Ts Maximum temperature due to superheating

Claims

1. Method (100) for reducing temperature peaks during the start-up of a fuel cell (12) with a two-phase cooling system, comprising the following steps: Adjusting (102) a saturation pressure of a coolant, via a pressure control module (14), to a first saturation pressure such that a maximum temperature (Ts ) of a coolant induced by boiling delay is below a maximum operating temperature (TB ) of the fuel cell; starting (104) the fuel cell; in response to the maximum temperature (Ts ) of the coolant induced by boiling delay being exceeded, adjusting (106) the saturation pressure of the coolant via the pressure control module (14) to a second saturation pressure, wherein the second saturation pressure is higher than the first saturation pressure.

2. Method (100) according to claim 1, wherein the pressure control module (14) is an accumulator (14).

3. Method (100) according to claim 2, wherein the accumulator (14) is connected to an outlet of the fuel cell (12) and is arranged to separate a vapor phase and a liquid phase of the coolant from each other.

4. Method (100) according to claim 2 or 3, wherein the accumulator (14) is adapted to adjust a temperature of the coolant.

5. Method (100) according to claims 2 to 4, wherein the accumulator (14) is adapted to adjust a pressure of the coolant.

6. Method (100) according to claim 1, wherein the pressure control module (14) is a combination of a pump (20) and a throttle valve.

7. Method (100) according to one of the preceding claims, wherein the first saturation pressure is in a range from 0 bar to 10 bar, preferably 0 bar to 5 bar, and particularly preferably 1.25 bar.

8. Method (100) according to one of the preceding claims, wherein the second saturation pressure is in a range from 0 bar to 10 bar, preferably 0 bar to 5 bar, particularly preferably 3 bar.

9. Method (100) according to one of the preceding claims, wherein the maximum temperature of the coolant induced by boiling delay is 50% to 100% of the operating temperature, preferably 60% to 90%, particularly preferably 70% to 80%.

10. Method (100) according to one of the preceding claims, wherein the coolant comprises methanol and / or ethanol.

11. System (10) for operating a fuel cell with a two-phase cooling system, wherein the system (10) is designed to reduce temperature peaks during start-up by performing a method according to one of claims 1 to 10, comprising: at least one fuel cell (12), a pressure control module (14), a heat exchanger (18), a coolant circuit (16), wherein the coolant circuit (16) is arranged to cool the fuel cell (12) by two-phase cooling via the heat exchanger (18), wherein the pressure control module (14) is arranged to adjust a saturation pressure of a coolant contained in the coolant circuit (16).

12. Aircraft (200) with a system (10) according to claim 11.