Aircraft generator unit with high-temperature fuel cell

Abstract

Generator unit for an aircraft, comprising an electric machine (1) and a fuel cell (3), as well as comprising a rotatable compressor (5) with compressor blades, a first combustion chamber (7), and a rotatable turbine (9), wherein the compressor (5), the first combustion chamber (7) and the turbine (9) are arranged in series such that a gas stream from ambient air is compressed by the compressor (5) during operation of the generator unit, is further used in its flow direction for the combustion of a fuel (T) in the first combustion chamber (7), in order to drive the turbine (9) upon subsequent flow through the turbine (9), wherein the turbine (9) can be coupled to the electric machine (1) via a shaft in a torque-transmitting manner.and comprising a return line (11) for returning exhaust air from the fuel cell (3) to the gas stream upstream of the fuel cell (3), and comprising a supply line (13) for supplying at least a portion of the exhaust air from the first combustion chamber (7) to the fuel cell (3), and comprising a first bypass line (12) that bypasses the fuel cell (3).

Description

The invention relates to a generator unit for electric power generation for an aircraft.A variety of conventional aircraft engines utilize fuel that is oxidized in a combustion chamber. This oxidation with an oxidizer such as atmospheric oxygen is an exothermic reaction; the released and usable energy is used to generate thrust for the aircraft. There are some degrees of freedom in the configuration of a combustion chamber. Various concepts are known in the art, which are also based in part on different fuels. While hydrocarbon-based liquid fuels are typically converted to primarily carbon dioxide and water in a high temperature combustion chamber, the use of hydrogen as a power carrier provides the possibility of being used in a fuel cell, which may also be used as an electrical power source using an oxidizer, typically atmospheric oxygen, to drive an electric machine. Besides, such a reductant used for a fuel cell, such as said hydrogen, may be used for combustion in a conventional engine configuration with a thermal combustion chamber. The fuel cell and the thermal combustion chamber can also be used together in order to use hydrogen both in the engine with the combustion chamber and in the fuel cell as a reducing substance for the power generation.U.S. Pat. No. 2022 / 0 297 844 A1 relates in this context to a hybrid engine having functions which are intended to meet the requirements for aircraft thrust, passenger air flow and fuel cells. An engine has a combustion chamber that burns the same fuel as the fuel cell. The motor has electric motors for this purpose in order to use the power of the fuel cell. The engine shafts are provided with clamping members to allow the motors to drive the compressors and drive the turbines. The engine has a variable flow path geometry to bypass the combustion chamber.DE 10 2006 002 882 A1 additionally relates to a fuel cell system comprising a combination of a first fuel cell of a first type and a second fuel cell of a second type connected downstream of the first fuel cell, wherein an anode side of the first fuel cell is connected to a cathode side of the second fuel cell.U.S. Pat. No. 10,644,331 B2 further relates to a quick-starting assembly and a method for operating a quick-starting assembly. A fuel cell converts combustible fuel into electrical energy during a normal operating phase after first generating little to no electrical energy. A combustor receives the non-burned fuel emitted from the fuel cell and burns the non-burned fuel to generate a first heated gas stream. Another combustor receives combustible fuel and burns the combustible fuel to generate a second heated gas stream during the first start-up phase. A turbine receives and is driven by the first and second heated gas streams to drive a drive shaft. A generator coupled to the drive shaft generates electrical energy during the first start-up phase and additional power during the normal operating time. In an alternative embodiment, a two-stage combustor is used instead of two separate combustors arranged in series.US 2023 / 0 039 759 A1 additionally relates to an integrated fuel cell and combustion chamber assembly having a combustion chamber which is fluidically coupled to at least one upstream compressor which generates compressed air. A fuel cell stack having a cathode and an anode is fluidly coupled to the combustor. The fuel cell stack is configured to receive intake fuel and a portion of the compressed air as intake air, to generate fuel cell performance using the intake fuel and the intake air, and to direct a fuel and air outlet from the fuel cell stack into the combustion chamber. An autonomous air supply system is fluidly coupled to the at least one upstream compressor and the fuel cell stack and is configured to supply the intake air to the fuel cell stack. A fault tolerant controller is configured to detect a transient event within the combustor and control the autonomous air supply system during the transient event.Fuel cells as used in the above-mentioned prior art typically require a certain minimum temperature in order to function at all or with high efficiency. In a high-temperature fuel cell such as a solid oxide fuel cell (also referred to as a "solid oxide fuel cell", abbreviated "SOFC"), inlet temperatures of reductant material (typically molecular hydrogen) and oxidant material (typically air oxygen) of approximately 800° C. may be desired or required. As a result, preheating of the material streams supplied to the fuel cell is necessary. While the fuel cell itself generates waste heat during its operation when used, which can in principle be used for preheating, the waste heat is not available when a drive unit having such a fuel cell is started up from standstill.It is an object of the invention to improve an electric generator unit with a fuel cell for generating an electric energy current for an aircraft.The invention results from the features of the independent claims. Advantageous refinements and refinements are the subject matter of the dependent claims.A first aspect of the invention relates to a generator unit for an aircraft, having an electric machine and a fuel cell, and having a rotatable compressor with compressor blades, a first combustion chamber, and a turbine, wherein the compressor, the first combustion chamber and the turbine are arranged in series such that a gas stream from ambient air is compressed by the compressor during operation of the generator unit, is further used in its flow direction for combustion of a fuel in the first combustion chamber in order to drive the turbine when a subsequent flow through the turbine takes place, wherein the turbine can be coupled to the electric machine in a torque-transmitting manner via a shaft, in particular by a controllable clutch, or is coupled in a torque-transmitting manner permanently, and having a return line for returning exhaust gas air of the fuel cell into the gas stream upstream of the fuel cell, preferably upstream of the first combustion chamber, and having a feed line for feeding at least part of the exhaust gas air of the first combustion chamber to the fuel cell, and having a first bypass line which, starting from the feed line and / or starting from a bypass around the combustion chamber or starting from a line after mixing the feed line with a second bypass line around the combustion chamber, leads around the fuel cell.The fuel cell is preferably a high temperature fuel cell, for example, a solid oxide fuel cell (abbreviated to "SOFC"). The compressor is constructed to rotate analogously to the turbine and therefore, in contrast to piston-based compressors, falls within the component category of the turbomachines-therefore the term rotatable compressor.The fuel for the combustion chamber is taken from a fuel tank; alternatively or additionally, exhaust gas from the fuel cell, which is fed via the return line into the gas flow upstream of the combustion chamber, can still contain fuel fraction and this can be used as fuel for the combustion chamber. In particular when the generator unit is started up, preferably only fuel from the fuel tank is used for combustion in the combustion chamber, since in this case no exhaust gas from the fuel cell is available yet, and therefore no fuel fraction in the exhaust gas from the fuel cell can be used. In regular operation of the generator unit, on the other hand, fuel present in the exhaust gas of the fuel cell can be transferred via the return line for combustion in the combustion chamber.Instead of a combustion chamber, a plurality of combustion chambers can also be provided, which can be connected in series or in parallel.By using the first bypass line, it is possible to use an architecture with a return line and a supply line, both of which are used for thermal optimization of the gas flows in the generator unit: exhaust air of the fuel cell heated by the fuel cell can be guided through the return line into the gas flow upstream of the combustion chamber, wherein the gas flow heated by the combustion in the combustion chamber can in turn be guided at least partially to the fuel cell by means of the supply line. The supply of oxygen-containing fresh air both to the fuel cell and to the combustion chamber, which are integrated in this partially closed circuit, is achieved on the one hand by the first bypass line in order not to conduct the complete gas flow through the fuel cell, but rather a part around it, and preferably by a second bypass line which leads around the combustion chamber, such that it is not possible for the combustion chamber to draw an excessively high degree of oxygen from the gas flow and to cause an oxygen deficiency in the fuel cell.There is thus firstly a circuit through the combustion chamber to the fuel cell and back to the inlet of the combustion chamber, secondly a path of the ambient air, which is absorbed by the generator unit, through the second bypass line and subsequently through the first bypass line back through the turbine into the environment.However, both paths (in the circuit through the combustion chamber and through the fuel cell and past the combustion chamber and fuel cell) cross one another, whereby a continuous mixing of the gas streams at the nodes occurs. On the one hand, it is thus achieved that sufficient oxygen is available for oxidation both in the combustion chamber and in the fuel cell, and on the other hand, at least during steady-state operation of the generator unit, a gas stream supplied to the combustion chamber and a gas stream supplied to the fuel cell are preheated.An advantage of the generator unit is that the number and / or size of conventional heat exchangers can be reduced, and therefore mass and installation space can likewise be reduced. Instead of a configuration with heat exchangers, preheating of the air supply by means of return air flow is provided in the air supply. In addition, the heating of the air supplied to the fuel cell is promoted by the combustion chamber with respect to the air flow direction in front of the fuel cell. In addition, the reductant, such as hydrogen, may be preconditioned, especially with heat exchangers that absorb heat in the gas stream.Furthermore, the solid oxide fuel cell can advantageously be structurally integrated, instead of being provided as an independent component.According to a further advantageous embodiment, the generator unit further comprises a control unit which is designed to operate the electric machine in a generator mode or optionally in a motor mode and to set the motor mode at least when the generator unit is started up, and comprising valves which can be controlled by the control unit in order to adapt fluid flows in the generator unit.The fluid streams include at least the gas stream, but fuel streams may also be included: while some valves specifically adjust the ratios of the gas stream fractions through the combustion chamber, the fuel cell and a respective bypass, other valves may be used for adjusting the mass stream of the reductant material, for example hydrogen. A control unit advantageously optimizes the operation of the generator unit by coordinated actuation of the valves and can thus ensure that the required usable power of the generator unit is available in each operating range (starting, steady-state continuous operation, starting of the aircraft, etc.), and this is used in the highest possible efficiency range. In the event of a fault, the valves can also be controlled in such a way that a drive without a fuel cell is possible.The fuel cell is advantageously dimensioned relative to the other components such that at least half of the total heat generation in the generator unit originates from the fuel cell at least during stationary operation of the generator unit, for example during cruise flight of the aircraft. It is furthermore advantageously provided that during this operation of the generator unit, at least half of the gas mass flow is conducted through the fuel cell itself, instead of around a bypass around it.According to a further advantageous embodiment, the generator unit further comprises a propeller which can be coupled to the shaft and / or to the electric machine for torque transmission and which serves to generate aerodynamic thrust for the aircraft.According to a further advantageous embodiment, the electric machine and the shaft are connected to one another via a transmission.By using a summation gear, for example a planetary gear, the generator unit can be started up without running propellers, if provided, and also in the process provide electricity. In addition to the summation transmission, a clutch or a plurality of clutches can also be provided, so that the turbine, the electric machine and the propeller can be shifted (preferably independently of one another), i.e. can be connected into the torque chain to the at least one shaft between turbine and compressor or can be decoupled therefrom. This construction permits, on the one hand, controllability of the system; on the other hand, a start-up without propeller movement is made possible. Optionally, a brake unit is provided, which is connected in particular to the propeller in order to shut it down, and to operate the electric machine without rotating propellers by means of a clutch which is disengaged.According to a further advantageous embodiment, the return line has a first branch and a second branch which lead into different pressure stages in and / or downstream of the compressor.If a plurality of compressor stages are provided, a first branch can lead between the compressor stages. If another branch is led downstream of the last compressor stage in the direction of flow, a separate return line compressor is to be provided for this branch of the return line.According to a further advantageous embodiment, the compressor has a low-pressure stage and a high-pressure stage, wherein a return line compressor is arranged in the second branch.According to a further advantageous embodiment, the generator unit further comprises a second bypass line around the combustion chamber, wherein the second bypass line opens into the supply line.According to a further advantageous embodiment, the return line leads with a branch into the gas flow of the second bypass line.According to a further advantageous embodiment, the generator unit further comprises a second combustion chamber which is arranged between the fuel cell and the turbine in the flow direction of the gas flow.In certain operating modes of the generator unit, in a further embodiment, the first combustion chamber can be switched off, so that a complete gas flow is guided around the first combustion chamber by means of a bypass. Preferably, a central control unit, which serves for controlling valves for setting a mass flow ratio of bypass and combustion chamber or fuel cell and for controlling the flow of the fuel supply, is also designed for shutting down the first combustion chamber.According to a further advantageous embodiment, a nozzle is arranged downstream of the turbine in the flow direction in order to generate thrust for the aircraft by the gas stream emerging through the nozzle into the environment.The nozzle preferably has a cross-sectional profile with local constriction in order to bring the gas flow through the generator unit before its outlet into the environment to higher speeds. Thus, elevated pressure and / or elevated temperature of the gas stream is at least partially converted into kinetic energy in terms of energy. The impulse thereby produced on the aircraft generates thrust.According to a further advantageous embodiment, the generator unit further comprises at least one heat exchanger which is designed to remove heat from the gas stream and to transfer it to fuel in a fuel supply line.A further aspect of the invention relates to an aircraft having a generator unit as described above and below.Further advantages, features and details are evident from the following description, in which--possibly with reference to the drawing--at least one exemplary embodiment is described in detail. Identical, similar and / or functionally identical parts are provided with the same reference numerals.The following are shown: FIG. 1 : shows a generator unit according to an exemplary embodiment of the invention in section. FIG. 2 : shows a schematic functional principle of a generator unit according to a further exemplary embodiment of the invention. FIG. 3 : shows a schematic functional principle of a generator unit according to a further exemplary embodiment of the invention.The representations in the figures are schematic and not to scale.FIG. 1 shows a generator unit for an aircraft. The generator unit serves primarily as a power source for electric motors distributed along a wing of the aircraft and connected to respective small pusher propellers. For the purpose of generating power, the generator unit has, on the one hand, an electric machine 1 and an annular fuel cell 3. The electric machine 1 is an electric motor, which is preferably used as an electric generator during cruise flight. The fuel cell 3 uses hydrogen as the fuel T and the oxygen of the supplied ambient air. Since the fuel cell 3 requires a certain temperature in order to carry out the oxidation of the fuel T and also to carry out it with high efficiency, two mechanisms are used for preheating the mass flows of ambient air and fuel T supplied to the fuel cell 3: Firstly, a return line 11 is provided which leads exhaust gases of the fuel cell 3 even upstream of a combustion chamber 7. In this way, the gas stream from ambient air introduced into the generator unit from the environment is heated up. In addition, when ambient air is supplied through a supply line 13 (see FIG. 2 ), exhaust gas is conducted from the combustion chamber 7 to the fuel cell 3, which in turn is heated by combustion in the combustion chamber 7. On the one hand, this results in a possibility of electric power generation by means of the fuel cell 3, and on the other hand, there is a further possibility of power generation by means of the electric machine 1. The compressor 5 has a first stage 27 and a second stage 29. In this embodiment, the return line 11 leads between the two stages of the compressor 5, but there are design degrees of freedom in order to find an optimum configuration for the respective individual generator unit. The gas stream compressed by the compressor 5 is, heated by the exhaust gases of the fuel cell 3 conducted through the return line 11, conducted to the combustion chamber 7; there, fuel T is burned and the energetically charged gas mixture, after further heating, is conducted into the fuel cell 3, in which fuel T is consumed, and then expanded in a turbine 9. Possible accurate conduits of the gas stream and the fuel streams are shown in Figures 2 and 3. In the embodiment of FIG. 1, the turbine 9 also has a high-pressure stage and a low-pressure stage, wherein the low-pressure stage 35 of the turbine 9 (see FIG. 2 ) is connected to the low-pressure stage 27 of the compressor 5 by means of a first shaft, while the high-pressure stage 33 of the turbine 9 is connected to the high-pressure stage 29 of the compressor 5 by means of a second shaft. At least one of the two shafts is furthermore connected to a transmission 21, which in turn is coupled to the electric machine 1. In addition, a propeller 15 is provided, so that the generator unit, in addition to the electric power generation itself, can directly generate thrust in two ways: on the one hand, through the outlet of the gas flow behind the turbine 9 through a nozzle, and on the other hand, through the rotation of the propeller 15 when it is coupled to at least one of the shafts in a torque-transmitting manner by a coupling to be provided. By way of example, but not necessarily in all embodiments, the propeller 15 is illustrated in the embodiment of FIG. 1, which propeller can be provided, but does not have to be provided. Particularly at the start of the aircraft, the propeller 15 can supply additional thrust, while furthermore the electric machine 1 can be operated as an electric motor in order to assist the rotation of the propeller 15. In this case, an additional battery can optionally be provided in order to drive the electric machine 1. When the generator unit is started up from the standstill, the electric machine 1 can likewise be used by supplied electric current in order to start up the system, in particular comprising the compressor 5 and the turbine 9, i.e. in order to start up the gas flow. Alternatively, the start-up can be carried out by means of compressed air. In both cases, the fuel cell 3 is slowly heated, while preferably a bypass 12 (see FIG. 2 ) around the fuel cell 3 remains closed. When the combustion chamber 7 is started, the bypass 12 around the fuel cell 3 is regulated in such a way that a further slow temperature rise takes place in the fuel cell 3 and the heat exchanger 25 which may be provided is put into operation effectively for preheating the fuel T (cf. FIG. 2 ). After successful preheating of the fuel cell 3, the latter is put into operation. After the aircraft has been started, on the other hand, the propeller 15 can optionally be switched off and the electric machine 1 can be used in the generator mode by receiving torque and delivering electric power by means of corresponding coupling to at least one of the two shafts. In cruise flight or after a steady state has been reached, the combustion chamber 7 can also be switched over to operation only with residual fuel from the fuel cell 3. Alternatively, the combustion chamber 7 can be switched off and the residual fuel from the fuel cell 3 is burned in a further combustion chamber 23 (see FIG. 3 ). Alternatively, excess mechanical energy from a shaft connection of compressor 5 and turbine 9 can be divided between electric machine 1 and propeller 15. In the event of excess electrical energy, it can be conducted to another pusher propeller of the aircraft or to a battery. In addition, in the event of a fault in the fuel cell 3, the bypass 12 around the fuel cell 3 can advantageously be opened completely, and the generator unit can be used as a gas turbine coupled to an electric machine 1. Depending on the specific architecture, the generator unit allows further optimizations in the actuation using the degrees of freedom of clutches, transmissions 21, valves 25, possibly connected batteries, etc.;FIG. 2 shows a topology of a possible embodiment of the generator unit. Fuel T is removed from a tank and preconditioned in a fuel preconditioning unit. In particular, when the generator unit is started up, this fuel preconditioning unit automatically ensures a correct temperature control of the fuel T. In addition, ambient air is taken up at the left-hand end of the diagram, compressed by a low-pressure stage 27 of the compressor 5, further compressed by a high-pressure stage 29 of the compressor 5 and divided into a second bypass 17 around a combustion chamber 7 and also into the supply to the combustion chamber 7. exhaust gases of the combustion chamber 7 and of the second bypass are subsequently joined together again in order to be divided into a first bypass line 12 around the fuel cell 3 and into a supply to the fuel cell 3. Exhaust gases of the fuel cell 3, after being mixed with the first bypass line 12, are partially recirculated into the gas stream through a recirculation line 11 between the compressor stages 27 and 29 and behind the compressor 5, wherein a recirculation line compressor 31 is provided in this branch directly behind the compressor 5 during the recirculation. Alternative arrangements and numbers of branches of the return line 11 are possible. In addition, a plurality of heat exchangers 25 are provided, which absorb heat along the gas stream, preferably from the fuel cell 3, and emit it to the fuel T. In this case, a guidance of heat flows can be provided, as in the paths shown in dashed lines. Depending on the state of the generator unit and on the exact design, these heat exchangers 25 can be connected in series, or alternatively only a subset of these heat exchangers 25 can be used. A control unit can adjust this by means of corresponding valve controls.FIG. 3 shows a variant of the generator unit of FIG. 2, in which a second combustion chamber 23 is provided behind the fuel cell 3 with respect to the flow direction of the gas stream. This does not continuously make the first combustion chamber 7 superfluous, but rather, depending on the operating state, both combustion chambers 7, 23 can be used, or the first combustion chamber 7 can be switched off, in particular during cruise flight of the aircraft, if after a certain operating time of the generator unit the fuel cell 3 itself already produces sufficient heat to be able to provide electrical energy effectively and efficiently. The operation of the electric machine 1 may then no longer be required or may be required only to a significantly lesser extent, and the first combustion chamber 7 may be shut down.Although the invention has been illustrated and explained in more detail by preferred exemplary embodiments, the invention is not restricted by the disclosed examples and other variations can be derived therefrom by the person skilled in the art without departing from the scope of protection of the invention. It is therefore clear that a large number of possible variations exist. It is also clear that embodiments mentioned by way of example represent only examples which are not to be understood in any way as limiting, for example, the scope of protection, the possible applications or the configuration of the invention. Rather, the preceding description and the description of the figures enable the person skilled in the art to implement the exemplary embodiments in concrete terms, wherein the person skilled in the art, knowing the disclosed inventive concept, can make various changes, for example with regard to the function or the arrangement of individual elements mentioned in an exemplary embodiment, without departing from the scope of protection defined by the claims and their legal equivalents, such as further explanations in the description.List of reference characters1 Electric machine 3 Fuel cell 5 Compressor 7 First combustion chamber 9 Turbine 11 Return line 12 First bypass line 13 Supply line 15 Propeller 17 Second bypass line 19 Valve 21 Transmission 23 Second combustion chamber 25 Heat exchanger 27 Low-pressure stage of the compressor 29 High-pressure stage of the compressor 31 Return line compressor 33 High-pressure stage of the turbine 35 Low-pressure stage of the turbine

Claims

Generator unit for an aircraft, having an electric machine (1) and a fuel cell (3), and having a rotatable compressor (5) with compressor blades, a first combustion chamber (7) and a rotatable turbine (9), wherein the compressor (5), the first combustion chamber (7) and the turbine (9) are arranged in series such that a gas stream from ambient air is compressed by the compressor (5) during operation of the generator unit, is further used in its flow direction for combustion of a fuel (T) in the first combustion chamber (7) in order to drive the turbine (9) when a subsequent flow through the turbine (9) occurs, wherein the turbine (9) can be coupled to the electric machine (1) in a torque-transmitting manner via a shaft, and having a return line (11) for returning exhaust gas air of the fuel cell (3) into the gas flow upstream of the fuel cell (3) and having a feed line (13) for feeding at least part of the exhaust gas air of the first combustion chamber (7) to the fuel cell (3), and having a first bypass line (12) which leads around the fuel cell (3).Generator unit according to Claim 1, having a control unit which is designed to operate the electric machine (1) in a generator mode or optionally in a motor mode, and to set the motor mode at least when starting up the generator unit, and having valves (19) which can be controlled by the control unit in order to adapt fluid flows in the generator unit.Generator unit according to one of the preceding claims, further comprising a propeller (15) or fan which can be coupled to the shaft and / or to the electric machine (1) for torque transmission and which serves to generate aerodynamic thrust for the aircraft.Generator unit according to one of the preceding claims, wherein the electric machine (1) and the shaft are connected to one another via a transmission (21).Generator unit according to one of the preceding claims, wherein the return line (11) has a first branch and a second branch which lead into different pressure stages in and / or behind the compressor (5).Generator unit according to one of the preceding claims, further comprising a second bypass line (17) around the first combustion chamber (7), wherein the second bypass line (17) opens into the supply line (13).Generator unit according to any one of the preceding claims, wherein the first branch and / or the second branch of the return line (11) comprises a compressor (31).Generator unit according to one of the preceding claims, further comprising a second combustion chamber (23) which is arranged between the fuel cell (3) and the turbine (9) in the direction of flow of the gas stream.Generator unit according to one of the preceding claims, wherein a nozzle is arranged downstream of the turbine (9) in the flow direction in order to generate thrust for the aircraft by the gas stream emerging through the nozzle into the environment.Generator unit according to one of the preceding claims, having at least one heat exchanger (25) which is designed to remove heat from the gas stream and to transfer it to fuel (T) in a fuel feed line.

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