BIPOLAR PLATE WITH A FLOW CHANNEL WITH VARIABLE CROSS-SECTION

Abstract

A bipolar plate (10) and fuel cell system (100) for a fuel cell with a two-phase cooling system is specified, comprising: a coolant inlet (12), a coolant outlet (14), a large number of coolant channels (16), wherein the coolant inlet (12) is in fluid communication with the coolant outlet (14) via the plurality of coolant channels (16), wherein at least one of the plurality of coolant channels (16) has a variable cross-section such that a pressure gradient of a coolant along the plurality of coolant channels (16) and / or a backflow of the coolant is reduced.

Description

Field of the invention

[0001] The present description relates generally to the technical field of aviation. In particular, the description relates to a bipolar plate with a flow channel of variable cross-section and a fuel cell system with such a bipolar plate. 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 cell (PEMFC) stacks generate electricity through the electrochemical reaction of hydrogen and oxygen to produce water. This reaction generates heat that must be dissipated. In commercial fuel cell stacks, this is achieved through liquid cooling. A potential alternative with significant weight reduction potential at the system level is to replace the 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. Furthermore, the high heat transfer coefficient compared to single-phase cooling improves the performance of the thermal management system.Changing the flow regime in the fuel cell stack from liquid to two-phase cooling requires a special design of the flow fields. Summary of the invention

[0003] The invention can be considered to have an objective of improving the fluid dynamic design of the flow fields.

[0004] This object is achieved by the subject matter of the independent claim. Further embodiments emerge from the dependent claims and the following description.

[0005] According to one aspect, this is a bipolar plate for a fuel cell with an efficient two-phase cooling system. The bipolar plate has a coolant inlet, a coolant outlet, and a multitude of coolant channels. The coolant inlet is connected to the coolant outlet via these multiple coolant channels. At least one of the coolant channels has a variable cross-section, which serves to reduce the pressure gradient of the coolant along the channels and / or the backflow of the coolant.

[0006] The advantage of this bipolar plate with its efficient two-phase cooling system lies in its improved cooling performance. The variable cross-section of at least one, and in particular all, of the coolant channels reduces the pressure gradient of the coolant along the channels. This allows for a more uniform distribution of the cooling medium and prevents unwanted backflow of the coolant.

[0007] Restricting the flow of coolant in the channels further contributes to improved flow stability and a uniform flow distribution. In addition, restrictions can act as boiling initiators. Due to the cross-sectional narrowing, the fluid velocity must accelerate. The increased velocity reduces the local pressure, which can fall below the saturation pressure. The liquid then forms bubbles (flashing phenomenon), which can serve as boiling nuclei for the subsequent boiling process. This function can help to reduce the phenomenon of liquid superheating, also known as delayed boiling.

[0008] Reducing the pressure gradient and backflow results in more efficient cooling of the fuel cell. Uniform cooling is essential for maintaining optimal operating temperature and fuel cell performance.

[0009] Furthermore, the improved cooling performance helps to extend the lifespan of the fuel cell. The bipolar plate with the two-phase cooling system thus offers a more reliable and efficient solution for cooling fuel cells.

[0010] A fuel cell system with a bipolar plate according to the invention is a technological unit consisting of several components and used to convert chemical energy into electrical energy by means of electrochemical reactions. It typically comprises one or more fuel cells, which function as electrochemical cells and constitute the main component of the system. Additionally, the system includes further components such as a fuel supply mechanism, an oxidant supply mechanism, an electrolyte solution, electrodes, a catalyst, and an electrical connection.

[0011] The fuel cell system uses a chemical reaction between a fuel and an oxidant, typically hydrogen and oxygen, to generate electricity. Water is the only byproduct, making the fuel cell system an environmentally friendly energy source. The generated electricity can then be used to power electrical devices, vehicles, or for power generation in various applications.

[0012] A two-phase cooling system according to the invention is a cooling solution designed to efficiently dissipate large amounts of heat by utilizing the phase transition from liquid to vapor. It is frequently used in situations where conventional single-phase cooling systems reach their limits and cannot provide sufficient heat dissipation.

[0013] In a two-phase cooling system, a refrigerant is used that can exist in both liquid and gaseous forms at suitable temperatures and pressures. The refrigerant absorbs heat from the source being cooled and evaporates, thus changing from a liquid to a gaseous state. The resulting vapor absorbs large amounts of heat.

[0014] The phase change from liquid to vapor enables effective heat transfer and a higher heat transfer potential due to the utilization of latent heat. This leads to improved cooling performance and more efficient heat dissipation.

[0015] A two-phase refrigeration system typically consists of an evaporator, where the refrigerant absorbs heat and evaporates, and a condenser, where the vapor condenses back into water and releases the heat. The condensate is then returned to the evaporator to continue the refrigeration cycle.

[0016] A coolant inlet is an opening or connection in a cooling system through which the coolant is introduced. The coolant inlet allows the controlled entry of the coolant into the relevant area or components that require cooling.

[0017] A coolant outlet is an opening or connection in a cooling system through which the coolant is discharged from the system. The coolant outlet allows the controlled release of the heated coolant from the relevant area or components to cool them and ensure heat exchange.

[0018] A plurality of coolant channels, as defined in the invention, are multiple channels or passages within a component or structure through which the coolant flows to dissipate heat. These coolant channels serve to efficiently distribute the coolant over the areas or components to be cooled and to facilitate heat exchange. The plurality of coolant channels creates a larger surface area for the heat transfer process, resulting in effective cooling.

[0019] The fluid connection between the coolant inlet and the coolant outlet via the multitude of coolant channels, at least one of which has a variable cross-section, results in a reduced pressure gradient along the channels and / or a reduced backflow of the coolant.

[0020] According to one embodiment, the bipolar plate has a stepped cross-sectional transition in the area of ​​the coolant inlet. This stepped transition enables improved flow guidance and distribution of the coolant due to a pressure drop in the area of ​​the stepped transition. Channels with a wedge-shaped cross-section are also conceivable to reduce backflow and induce a uniform pressure drop dp / dx along the respective channels.

[0021] According to one embodiment, at least one of the coolant channels has an increased cross-sectional area along a section from the coolant inlet to the coolant outlet. This advantage allows for a reduction of the pressure gradient along the channel and reduces coolant backflow.

[0022] When boiling in channels, the bubbles expand both upstream and downstream, causing instabilities in the form of fluctuations in pressure and / or local flow velocity. To prevent these flow fluctuations, either flow restrictions or expanding channels can be used. The increasing diameter of the vapor bubbles reduces the surface tension, resulting in a net force that pushes the vapor bubbles downstream.

[0023] According to one embodiment, the multitude of coolant channels are at least partially in fluid contact with each other via at least one groove. This enables improved coolant exchange between the channels and promotes uniform cooling of the bipolar plate.

[0024] The use of straight microchannels with cross-sectional grooves results in stable operating conditions over a wide range. This is because the vapor bubbles mix in the grooves, ensuring a uniform flow distribution into the next channel section.

[0025] According to one embodiment, at least one of the coolant channels has periodic cross-sectional changes. Firstly, these periodic cross-sectional changes create turbulence in the coolant flow, resulting in improved heat transfer. Secondly, periodic constrictions reduce backflow, as each constriction generates a pressure drop, causing the vapor bubbles to preferentially flow downstream.

[0026] According to one embodiment, at least one flow resistance element is integrated into at least one, preferably all, coolant channels, thereby changing the cross-section of the coolant channel(s). These flow resistance elements increase the flow resistance, resulting in a more uniform distribution of the coolant along the channels.

[0027] These channel designs are stable over a wide range of operating conditions. Additionally, their pressure drop at low Reynolds numbers (Re < 300) is lower compared to straight channels, and their heat transfer is better (increased Nusselt number). The cross-sectional expansions and constrictions have a significant influence on the temperature field, as they can improve fluid mixing between the wall and the core flow regions.

[0028] According to one embodiment, a plurality of flow resistance elements form a periodic sequence along the at least one coolant channel. This periodic arrangement of the flow resistance elements results in turbulent coolant flow, which improves heat transfer. This achieves efficient cooling of the bipolar plate and optimal coolant utilization.

[0029] A combination of flow resistance elements could be used to form diamond shapes and reduce flow resistance. Another combination can be used in conjunction with the aforementioned channel widenings.

[0030] According to one embodiment, the variable cross-section along the multitude of coolant channels is designed to allow a constant pressure drop per unit length, preferably from the smaller to the larger cross-section. This ensures a uniform flow of the coolant along the channels and reduces the pressure drop. This results in efficient cooling of the bipolar plate and optimal heat dissipation.

[0031] According to one embodiment, the coolant can comprise methanol and / or ethanol. This type of coolant offers the advantage of high thermal conductivity and efficient cooling.

[0032] 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.

[0033] 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.

[0034] Another aspect of this system is its use as a fuel cell system. The integration of at least one fuel cell, or fuel cell stack, a two-phase cooling system, and a specially designed bipolar plate ensures reliable cooling and efficient system operation. The reduced pressure gradient along the coolant channels and the minimized coolant backflow ensure optimal heat dissipation and uniform cooling of the fuel cell. This improves the performance of the fuel cell system and guarantees stable operation.

[0035] According to another aspect, the described fuel cell system is used in an aircraft. However, the system can also be used in other systems, such as motor vehicles, watercraft, spacecraft, or other fuel cell-powered units.

[0036] The integration of an efficient two-phase cooling system and a specially designed bipolar plate into an aircraft enables a reliable and efficient power supply. Optimal cooling of the fuel cell ensures stable performance and contributes to weight and emission reduction of the aircraft. This provides a sustainable and environmentally friendly energy source for the aircraft, resulting in improved overall efficiency and performance.

[0037] According to a further embodiment, the bipolar plate has a preheating section that is connected from the coolant inlet to the coolant outlet via a plurality of coolant channels. Within the preheating section, there is at least one preheating channel that serves to redirect the flow direction of the coolant before it enters the plurality of coolant channels. This reduces the temperature gradient along the preheating channel. In a specific embodiment, the preheating channel within the preheating section has at least one region in which the coolant flows against the usual flow direction.

[0038] The advantage is that the preheating channel in the preheating area redirects the flow direction of the coolant, thereby reducing the temperature gradient along the channel.

[0039] According to a further embodiment, at least one inner surface of the coolant inlet, coolant outlet and at least one of the plurality of coolant channels has a surface treatment to influence the flow pattern of a coolant along at least one inner surface and / or a phase transition of the coolant.

[0040] Surface treatment of the inner surface of coolant inlets, outlets, channels, or combinations thereof refers to the targeted modification or processing of the surface in contact with the coolant. This treatment can encompass various processes and techniques aimed at influencing the coolant flow or improving its phase transition.

[0041] Embodiments with coolant channels including a preheating channel located in a preheating area and wherein at least one of the plurality of coolant channels has undergone surface treatment are advantageous for cooling in a two-phase cooling system, as they are, for example, advantageous for the formation of boiling initiators of the coolant. Short description of the characters

[0042] 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 coolant channel of a bipolar plate with a variable cross-section; Fig. 2 another embodiment of a bipolar plate with a step-like transition of the cross-section of the coolant channel; Fig. 3 another embodiment of a bipolar plate wherein the plurality of coolant channels are at least partially in fluid communication with each other via at least one groove; Fig. 4 another embodiment of a bipolar plate with a plurality of Tesla valves; Fig. 5 another embodiment of a bipolar plate in which coolant channels have periodic cross-sectional changes in at least one area; Fig. 6 another embodiment of a bipolar plate in which flux resistance elements are attached to an inner surface of a coolant channel; Fig. 7 a fuel cell system; and Fig. 8 an aircraft with a fuel cell system. Detailed description of implementation examples

[0043] Fig. Figure 1 shows a coolant channel 16 of a bipolar plate 10 (not shown), which is an essential component of a fuel cell system 100 with a two-phase cooling system. This bipolar plate 10 is characterized by several features that enable effective cooling: First, there is the coolant inlet 12 (not shown), which serves as the entry point for the coolant. From there, the coolant flows through a multitude of coolant channels 16 in the bipolar plate 10 (not shown). These channels are designed to establish a fluid connection between the coolant inlet 12 and the coolant outlet 14 (not shown).

[0044] What makes the coolant channels 16 particularly distinctive is that at least one of them has a variable cross-section. This variable cross-section makes it possible to control the pressure gradient along the coolant channels and reduce coolant backflow. This ensures a uniform distribution of the coolant and achieves efficient cooling of the fuel cell.

[0045] Fig. Figure 2 represents a further embodiment of a coolant channel 16. In the area of ​​the coolant inlet 12, at least one of the many coolant channels 16 has a step-like transition in the cross-section.

[0046] This stepped transition means that the cross-section of the coolant channel changes abruptly in the area of ​​the coolant inlet. This creates a kind of step or ledge in the shape of channel 16. This design has certain effects on the flow and circulation of the coolant.

[0047] The stepped transition of the cross-section can be used to direct and optimize the coolant flow. It allows for better control of the pressure gradient along the channel and promotes efficient cooling of the fuel cell. This targeted design enables better distribution of the coolant flow and minimizes potential backflow.

[0048] Fig. Figure 3 shows a further embodiment of a bipolar plate 10 wherein the plurality of coolant channels 16 are at least partially, and in particular all, in fluid communication with each other via at least one groove 18.

[0049] The groove 18 on the bipolar plate 10 enables a connection between the individual coolant channels 16. This creates a common flow path for the coolant, allowing it to flow from one channel to another and resulting in pressure equalization in the groove 18.

[0050] This connection via the groove 18 is important because it allows the exchange of coolant between the channels 16.

[0051] Fig. Figure 4 shows another embodiment of a bipolar plate 10 with a plurality of Tesla valves which are used in recurring patterns to connect different channels together and form a network.

[0052] Fig. Figure 5 shows a further embodiment of a coolant channel 16 of a bipolar plate 10, in which at least one of the many coolant channels 16 has periodic cross-sectional changes in at least one region. These periodic cross-sectional changes are selected from the group consisting of spherical shapes, truncated cones, double truncated cones, angular steps, or combinations thereof.

[0053] Fig. Figure 6 shows a further embodiment of a bipolar plate 10, in which at least one flow resistance element 20 is attached to an inner surface of at least one of the many coolant channels 16. The shape of the at least one flow resistance element 20 is triangular. The at least one flow resistance element 20 consists of a plurality of flow resistance elements, and this plurality of flow resistance elements has a periodic arrangement along the at least one coolant channel.

[0054] Fig. Figure 7 shows a fuel cell system 100 consisting of several components, including a fuel cell 110, which can also be configured as a fuel cell stack, and a two-phase cooling system 120. Furthermore, the fuel cell system 100 has a condenser 130 in which the vapor condenses and releases the heat. The condensate is then returned to the evaporator to continue the cooling cycle.

[0055] The fuel cell 110 includes the bipolar plate 10, which has a large number of coolant channels 16.

[0056] Fig. Figure 8 shows an aircraft 200 with a fuel cell system 100 and a bipolar plate 10. The fuel cell system 100 consists of several components such as a fuel cell 110 and a two-phase cooling system 120, which work together to generate electrical energy from hydrogen.

[0057] The exemplary embodiments thus enable a more uniform distribution of the coolant and a reduction of unwanted backflow by means of a variable cross-section of a coolant channel 16.

[0058] 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 10 Bipolar plate 12 Coolant inlet 14 Coolant outlet 16 Coolant channel 18 groove 20 Flow resistance element 100 fuel cell systems 110 Fuel cell 120 Two-phase cooling system 130 capacitor 200 aircraft

Claims

[1] Bipolar plate (10) for a fuel cell with a two-phase cooling system, comprising: a coolant inlet (12), a coolant outlet (14), a plurality of coolant channels (16), wherein the coolant inlet (12) is in fluid communication with the coolant outlet (14) via the plurality of coolant channels (16), wherein at least one of the plurality of coolant channels (16) has a variable cross-section such that a pressure gradient of a coolant along the plurality of coolant channels (16) and / or a backflow of the coolant is reduced. [2] Bipolar plate (10) according to claim 1, wherein the at least one of the plurality of coolant channels (16) has a step-like transition of the cross section in the region of the coolant inlet (12). [3] Bipolar plate (10) according to one of the preceding claims, wherein the at least one of the plurality of coolant channels (16) has an enlargement of the cross-section in at least one region along a distance from the coolant inlet (12) to the coolant outlet (14). [4] Bipolar plate (10) according to one of the preceding claims, the plurality of coolant channels (16) are at least partially in fluid communication with one another via at least one groove (18). [5] Bipolar plate (10) according to one of the preceding claims, wherein at least one of the plurality of coolant channels (16) has periodic cross-sectional changes in at least one region. [6] Bipolar plate (10) according to claim 5, wherein the periodic cross-sectional changes are selected from the group consisting of: spherical shape, truncated cone, double truncated cone, angular steps, or combinations thereof. [7] Bipolar plate (10) according to one of the preceding claims, wherein the at least one of the plurality of coolant channels (16) has at least one flow resistance element (20). [8] Bipolar plate (10) according to one of the preceding claims, wherein the at least one flow resistance element (20) is attached to an inner surface of the at least one of the plurality of coolant channels (16). [9] Bipolar plate (10) according to one of claims 7 or 8, wherein the shape of the at least one flow resistance element (20) is selected from the group: sphere, cone, cuboid, wedge, deflections, loops, returns, or combinations thereof. [10] Bipolar plate (10) according to one of the preceding claims, wherein the at least one flow resistance element (20) is a plurality of flow resistance elements, and the plurality of flow resistance elements have a periodic sequence along the at least one coolant channel. [11] Bipolar plate (10) according to one of the preceding claims, wherein the cross section along the plurality of coolant channels (16) is arranged to enable a constant pressure loss per unit length, preferably from the smaller cross section to the larger cross section. [12] Bipolar plate (10) according to one of the preceding claims, wherein the coolant comprises methanol and / or ethanol. [13] Fuel cell system (100), comprising: at least one fuel cell (110), a two-phase cooling system (120), a bipolar plate (10) according to one of claims 1 to 12, wherein the bipolar plate (10) is configured to reduce a pressure gradient of a coolant along the plurality of coolant channels (16) and / or a backflow of the coolant. [14] Aircraft (200) with a fuel cell system (100) according to claim 13.

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