Motor vehicle with a fuel cell

Abstract

Fuel cell system of a motor vehicle comprising a fuel cell (20) having an anode side and a cathode side, a compressor (34) rotaryally connected to a motor M (36) and connected to the cathode side of the fuel cell (20) via a supply line, a turbine (46) connected to the cathode side via an exhaust line and also rotaryally connected exclusively to a generator G (52) which is connected on its output side to an input of a first inverter (54), and low-voltage accessories of the fuel cell, preferably also low-voltage consumers (58) of the motor vehicle, are connected exclusively to the output of the first inverter (54) and optionally to a low-voltage battery (56) connected to the output of the first inverter (54).

Description

[0001] In a fuel cell, particularly one for use in a motor vehicle, air, used as an oxidizer, is supplied to the fuel cell at relatively high pressure and in large quantities over a period of time. After the pressure drops within the fuel cell, some of the air, still carrying considerable kinetic energy, escapes. This exhaust air stream is fed to an exhaust turbine, which is connected to a generator. The kinetic energy of this exhaust air stream is thus used to generate electrical energy.

[0002] To achieve a very high electrical output power from the fuel cell, which is equivalent to generating a high current, a high cathode pressure is required. In state-of-the-art devices, an increased mass flow of the air compressor is used; a throttle valve in the outlet line or an adjustable bypass valve that bypasses the fuel cell regulates the cathode pressure of the fuel cell to the desired value.

[0003] Even when operating at lower output power, a higher cathode pressure is required to maintain the stoichiometry in the individual fuel cell cells. If a bypass valve is used to bypass the fuel cell, the maximum cathode pressure can be achieved by closing the bypass valve, as this directs all air exclusively to the fuel cell, and / or by increasing the compressor speed. However, the resulting cathode pressure is limited by the design of the fuel cell's technical characteristics that cause pressure drop. The fuel cell's efficiency decreases when it is supplied with a higher airflow from the compressor.

[0004] From DE 10 2015 010 114 A1, a fuel cell system for a motor vehicle is known, comprising a fuel cell with an anode and a cathode. This system further includes a compressor, which is rotaryally connected to a motor and via a supply line to the cathode of the fuel cell, and a turbine, which is connected via an exhaust line to the cathode, which is exclusively rotaryally connected to a generator. The generator's output is connected to an inverter of a low-voltage battery. This allows a corresponding low-voltage DC / DC converter to be relieved of some load. Consequently, this converter can be selected or dimensioned to be smaller.

[0005] US patent 2016 / 0 315 334 A1 describes a fuel cell system for a motor vehicle with a compressor in the cathode supply and a turboexpander with variable geometry guide vanes, which is mechanically connected to the compressor, or a passive expander in the cathode exhaust. The fuel cell and high-voltage battery feed into the same power grid.

[0006] In JP 2005310429 A, a regulator 54 is provided for the generated power, which apparently represents an AC / DC converter. This is connected to a battery. The drawing shows an electrical connection from this battery to the compressor / air supply unit. The battery is only charged via the generator. Its capacity may not be sufficient to meet the high power demand of the compressor (if it is electrically driven). This architecture is only technically feasible if the battery is also recharged by a high-voltage circuit.

[0007] In JP 2006286559 A, a similar architecture is proposed, which essentially shows two electrical architecture variants: In variant 1, the electric compressor unit and the exhaust gas turbine are connected to the high-voltage circuit via controller 4. In variant 2, the compressor unit and the exhaust gas turbine are connected to a common energy storage device 7 (battery, capacitor bank, etc.) via a controller 8. Again, this architecture is only technically viable if the energy storage device 7 is recharged by the high-voltage circuit.

[0008] To optimize the efficiency of a fuel cell, especially a hydrogen fuel cell, careful attention must be paid to the electrical energy input into the system; this should be kept as low as possible. According to the current state of the art, an electrically driven turbocharger and a radial compressor with an impeller are used for the air supply on the inlet side. On the outlet side, a radial turbine is used, driven by the outgoing air. It is mechanically connected to the impeller shaft. The disadvantage of this arrangement lies in the limited freedom to operate the turbine in its optimal range independently of that of the compressor. A consequence of this is that the amount of recovered kinetic energy is relatively low, with low power demanded from the fuel cell by the vehicle's drive motor and a reduced airflow.Furthermore, it is not possible to control the air pressure differently by using an additional component, such as a control valve at the fuel cell's air outlet, a system bypass, or variable turbine geometry, as described in some prior art applications. Each of these solutions has a limited degree of flexibility in controlling the cathode pressure or achieving higher efficiency.

[0009] Starting from this, the object of the invention is to further develop a fuel cell suitable for powering a motor vehicle in such a way that the efficiency is improved, both at high and low loads, and more optimal control of the operation of the fuel cell is possible, and the voltage supply of the consumers operating at low voltage is simplified.

[0010] This problem is solved by a fuel cell having the features of claim 1 and by a method having the features of claim 6.

[0011] A radial turbine with an impeller is used, designed in accordance with the state of the art and connected only to a generator. This allows the cathode pressure of the air to be controlled independently of the compressor. This is achieved in particular by regulating the compressor's rotational speed and / or the turbine's rotational speed using individual settings and by using the cathode pressure. The value measured by the cathode pressure sensor is compared with a setpoint or a table value as the variable controlled by the generator's torque.

[0012] An additional device for pressure control, such as a controllable bypass valve bypassing the fuel cell, is not required. An advantage of this is that energy recovery by the turbine coupled to the generator increases with increasing back pressure at the cathode, resulting in higher fuel cell efficiency across all low and high output power levels.

[0013] According to the invention, the anode pressure at H2 is controlled in the same way as according to the prior art.

[0014] The anode pressure is regulated to a higher differential pressure than that of the cathode. This is achieved by measuring the anode pressure using a sensor. The value measured by this sensor is used to control the anode pressure, which is regulated via a control valve for the pressure of the supplied gas and purge valves for cleaning the anode.

[0015] In a control unit of the compressor, the offset of the control of the mass flow of air and the setpoint for the cathode pressure are used to control the operating point for the speed of rotation of the compressor.

[0016] In a control unit for the turbine, the offset for controlling the cathode pressure and the setpoint for the airflow are used to control the rotational speed of the turbine or to set the torque value of the generator.

[0017] The combination of an electric generator on the turbine and an inverter (voltage converter) allows operation in the low-voltage range. Low voltage refers to voltages that can be safely touched, especially voltages up to and including 48 V. At higher voltages, adequate protection against electric shock is necessary. The terms inverter, converter, and voltage converter are used synonymously.

[0018] The average power recovered via the turbine from a typical 100 kW vehicle fuel cell is sufficient to cover the energy requirements of the vehicle's fuel cell accessories that operate at low voltage. Since the low-voltage battery can deliver higher power for short periods, a further advantage of the arrangement according to the invention is that no converter is required to transform high voltage to low voltage in the DC / DC range. This reduces the overall costs and the required installation space. Therefore, no additional high-voltage / low-voltage DC / DC converter is needed to supply power to the vehicle's low-voltage consumers. Such a converter is thus eliminated. The vehicle's low-voltage consumers are powered exclusively by the energy recovered from the turbine.Accordingly, the generator output is connected exclusively to a low-voltage battery and the low-voltage loads. This circuit receives no further input, in particular not from the high-voltage side and not via a high-voltage / low-voltage DC / DC converter.

[0019] For this purpose, it is advantageous to employ a special operating mode that ensures, on the one hand, that the low-voltage battery for supplying at least the low-voltage accessories of the fuel cell and preferably also the low-voltage vehicle electrical system is not discharged, but rather maintains its state of charge as constant as possible on average during operation. Reference is made to the teachings of claims 6 and 7 in this regard. This type of operating mode simultaneously controls the cathode pressure and the state of charge of the low-voltage battery.

[0020] In a special embodiment of the invention, a simpler generator, e.g. in the form of an alternator, with an output power of up to 3 kW and a standard charge controller with rectifiers can be used.

[0021] The term "battery" refers specifically to rechargeable batteries, commonly known as accumulators. More broadly, the term "battery" encompasses electrical charge storage devices for direct current (DC), including capacitor batteries.

[0022] The turbine is not mechanically connected to the compressor, i.e., the air supply unit at the inlet. An electric motor is used as a generator, and an inverter is employed to convert the dynamic pressure of the air at the outlet into electrical power. This achieves a higher energy yield with lower power output from the fuel cell or a lower airflow rate.

[0023] Since the turbine is decoupled from the compressor side and not mechanically connected to it, it can be set to an optimal speed independent of the input side, which is particularly advantageous for energy recovery. The turbine is connected to an electric motor, which acts as a generator and converts the kinetic energy into electrical power.

[0024] A second feature of this arrangement is the regulation of the fuel cell's back pressure. This can be achieved by controlling the generator's rotational speed or the electric motor's torque, and thus the turbine's rotational speed (per unit of time). A lower turbine rotational speed results in a higher turbine inlet pressure and therefore a higher back pressure at the fuel cell's outlet. The higher outlet pressure leads to an increase in the fuel cell's cathode pressure, which in turn increases the pressure at which the fuel cell can generate operating pressure, resulting in higher efficiency and a wider operating range. The inverter is designed for low voltage, thus avoiding the additional costs associated with high-voltage certification, safety, and other issues related to higher voltages.

[0025] The invention achieves - a decoupled control of the back pressure in the exhaust gas, which leads to an improvement in system efficiency, - improved recuperation of the exhaust gas mass or volume flow, - the elimination of a high-voltage / low-voltage DC / DC converter, and - the use of a low-voltage battery as an energy storage device (e.g. 12 V or 48 V) in conjunction with the appropriate turbine / generator system (possibly alternator with controllable charge controller).

[0026] In the proposed design, the inverter of the motor / compressor unit is connected to the high-voltage circuit, and the inverter of the turbine / generator unit is connected to the low-voltage battery. This differs, among other things, from variant 2 of the aforementioned JP 2006286559 A.

[0027] Additionally, the high-voltage / low-voltage step-down converter can be omitted, thereby enabling decoupling of the compressor and turbine unit. This allows the electrical recuperation power of the generator / turbine unit to be controlled largely independently.

[0028] While exhaust pressure control is the primary control objective in the control concept, in the case of high low-voltage vehicle electrical system loads and the associated drop in the low-voltage battery's state of charge, the recuperation power must be increased, which simultaneously requires an increase in boost pressure and compressor power. The overall efficiency then decreases somewhat at low electric fuel cell loads.

[0029] The control strategy in the control unit must coordinate the cathode pressure and the battery charge level accordingly.

[0030] Exemplary embodiments of the invention, which are not to be understood as limiting, are described in more detail below and explained with reference to the drawing. In this drawing: Fig. 1: A basic circuit diagram of an architecture for an arrangement with a fuel cell for a hybrid vehicle, Fig. 2: a basic circuit diagram like Fig. 1, but now in a 2nd variant, Fig. 3: a basic circuit diagram like Fig. 1, but now in a 3rd variant, Fig. 4: a basic circuit diagram like Fig. 1, but now in a variant for a plug-in hybrid vehicle, Fig. 5: a circuit diagram to explain the regulation of the cathode pressure on the air outlet side, and Fig. 6: a circuit diagram for a vehicle alternator with a controllable rectifier.

[0031] The following will Fig. 1 described in more detail, the Fig. 2, Fig. 3 to Fig. 4 contain many matches with Fig. 1. These figures are therefore only described insofar as they differ from Fig. 1. Differentiate. Then we will discuss the Fig. 5 and Fig. 6. Position taken.

[0032] Fig. Figure 1 shows a hydrogen fuel cell 20, depicted schematically. The anode side 19, or hydrogen side, is located at the top of the figure, while the cathode side 21, also called the air side, is located below it. Both sides are supplied with hydrogen in opposite directions. The anode side 19 is supplied with hydrogen. For this purpose, a hydrogen tank 22 is provided, in which the gas is stored under high pressure. It can be closed off via a tank valve and is connected to a supply line. A pressure regulator 26 is integrated into this supply line. The supply line opens into the anode side 19, and a jet pump 28 is also integrated into the supply line. This pump can also be designed as a circulation pump or blower. It serves to draw in hydrogen gas flowing from the anode side, which is routed in an exhaust line, and feed it back into the supply line in the area of ​​the jet pump 28.A pressure sensor 30 is provided in the exhaust pipe; it detects the exhaust pressure at the outlet of the anode side 19. A differential pressure gauge can also be provided, which detects the differential pressure between the anode and the cathode.

[0033] The cathode side 21 is charged with air. For this purpose, air is drawn in through an air inlet 32 ​​by a compressor 34. The compressor 34 is driven by a motor M 36. Electrically, a second inverter 38 is connected upstream of it, which also additionally performs the function of motor control. It is connected on the primary side to a distribution box 40, which in turn is connected to a high-voltage battery 42. The output of a first converter 44, which is a DC / DC converter, is also connected to the distribution box 40. This converter is connected on the input side to the fuel cell 20. The electrical power generated by the fuel cell 20 is extracted via the first converter 44 and supplied to the high-voltage battery 42 for charging. From there, the electrical power required for propelling the vehicle can be drawn; this is state of the art and is not shown here.

[0034] On the outlet side, air flows from the cathode side 21 through an exhaust duct to a turbine 46. The turbine is set in rotation by the kinetic energy of the exhaust air, and the air flows out of an air outlet. A cathode pressure sensor 48 is inserted into the exhaust duct.

[0035] A purge valve 31 is arranged at the outlet of the anode side 19 and is directly connected to the fuel cell 20. One purge outlet of this purge valve 31 is connected to a collection point 50, to which the exhaust line is also connected. In this way, during purging, hydrogen gas flows together with the exhaust air and reaches the turbine 46 together with it. In this way, the kinetic energy of the purge gas is also utilized.

[0036] Turbine 46 is exclusively driven by generator G 52, which in turn drives generator G 52. Generator G 52 is driven solely by turbine 46 and is exclusively connected to it. A second inverter 54, which also serves as a control unit for generator G 52, is connected downstream of generator G 52. Its output is connected to a low-voltage battery 56. It is also connected to low-voltage loads 58, which are not shown in detail here. These are, in particular, devices directly related to fuel cell 20.

[0037] The described arrangement is controlled by a controller FCU 60, which monitors and controls the entire system. For this purpose, it is connected via dashed control lines to individual components, in particular the second inverter 54, the jet pump 28, the flushing valve 31, and the second inverter 38. It receives its input signals via lines marked with a dash or dot; shown here as examples are an input line for the pressure sensor (cathode) 48, an input line for the pressure sensor (anode) 30, and an input line for a voltage sensor 62 on the positive side of the low-voltage battery 56. The negative side of the battery is connected to ground.

[0038] In the variant according to Fig. In section 2, the first converter 44, which is the inverter of the fuel cell 20, is omitted; instead, the electrical output of the fuel cell 20 is directly connected to the distribution box 40. The output voltage of the fuel cell 20 is thus directly available at the second inverter 38. The high-voltage battery 42 is now connected to the distribution box 40 via a second converter 64, which is the converter of the high-voltage battery 42.

[0039] In the third variant according to Fig. 3 is different from Fig. 1 Additionally, a second converter 64 is provided between the distribution box 40 and the high-voltage battery 42.

[0040] According to the scheme Fig. 4 is different from Fig. 3. A charger 66 is additionally connected to the distribution box 40; this charger is preferably located on board the vehicle. It can be connected to a power supply, for example a household or public network, via a plug 68. The high-voltage battery 42 can be charged via the charger 66.

[0041] Fig. Figure 5 shows a block diagram illustrating the cathode pressure control process. Based on the current power requirement of the fuel cell system or the current demand, a cathode pressure and a setpoint for the airflow are calculated. Additionally, a decision offset is determined for both the airflow rate and the cathode pressure, prioritizing the required power of the fuel cell or the battery voltage / state of charge. The decision offset values ​​are then added to the setpoint values.

[0042] Four values ​​are fed into the system as input variables on the left side, namely: 1. the currently required power of the fuel cell or the required current (requested fuel cell power or current), 2. Measured value of the actual air mass flow, 3. Measured value of the current cathode pressure and 4. the measured voltage of the low-voltage battery (LV battery voltage).

[0043] The required power value is fed to a first stage 70 in the upper part of the block diagram. In this stage, a required value for the cathode air mass flow (cathode air mass flow request) is determined. This value is then fed to a first logic gate 72. There, it is combined with an arbitrary value for the air mass flow (air mass flow arbitration). Each combination is positive, as shown in the figure. This value is determined in a second stage 74, where the air mass flow, the cathode pressure 48, and the state of charge of the low-voltage battery 56 are combined. The input to this second stage 74 is the voltage signal of the low-voltage battery (LV battery voltage) and the required power value. The output of the first logic gate 72 contains a setpoint for the cathode air mass flow.This value is fed directly to a controller turbine 76, and in a second linkage 78, with the specified sign, is combined with the measured value of the actual mass flow of air and then fed to a controller compressor 80. This compressor receives a further input signal, which will be discussed later. The output receives a setpoint for the compressor speed.

[0044] The value for the required power is also available at a third stage 82. In this stage, a request value for the cathode pressure 48 is determined and fed to a fourth circuit 84. There, it is combined, according to the specified signs, with the arbitrary value for the cathode pressure 48 (cathode pressure arbitration) determined by the second stage 74. On the output side, the fourth circuit 84 is connected on one side to a second input of the controller compressor 80 and on the other side to a fifth circuit 86. The signs should also be noted here. The measured value for the current cathode pressure 48 is also fed to the fifth circuit 86.The output signal of the fifth link 86 is fed to the controller-turbine 76 as an input value; the controller-turbine 76 determines a setpoint for the turbine speed (turbine speed setpoint) and / or a setpoint for the generator torque (generator torque setpoint) from its two input values.

[0045] Fig. Figure 6 finally shows a more concrete embodiment for the use of a 3 kW motor vehicle alternator in conjunction with a controllable rectifier. The anode side and the air inlet side of the cathode are as shown in Fig.As shown in Figure 1, unlike in this figure, the second inverter 38 is directly connected to the high-voltage battery 42; therefore, no distribution box 40 is provided. On the exhaust side, a turbine 46 is again provided, which is now directly and mechanically connected to the alternator G. Here, it represents an example of a special configuration of the generator 52. Downstream of the alternator G is a controllable rectifier, e.g., a charge controller, which here is an example of a special configuration of a first inverter 54. On its output side, it is connected to a low-voltage battery 56.

[0046] In the procedure for controlling the fuel cell system, a setpoint for the compressor speed 34 is determined by considering an input value for the required power of the fuel cell 20, a measured value of the actual mass flow of air, a measured value of the current cathode pressure 48, and an input value for the voltage of the low-voltage battery 56. Based on the currently required power of the fuel cell 20, a value for the cathode pressure 48 and a setpoint for the airflow are calculated. A decision value is determined for both the mass flow of air and the cathode pressure, taking priority from the required power of the fuel cell 20 and / or the battery voltage of the low-voltage battery 56.

[0047] The fuel cell system of a motor vehicle comprises a fuel cell 20, which has an anode side and a cathode side; a compressor 34, which is rotaryally connected to a motor M 36 and is connected to the cathode side of the fuel cell 20 via a supply line; and a turbine 46, which is connected to the cathode side via an exhaust line and is also rotaryally connected exclusively to a generator G 52, which is connected at its output to the first inverter 54 and optionally to a low-voltage battery 56. Low-voltage accessories of the fuel cell, and preferably also low-voltage consumers 58 of the motor vehicle, are connected exclusively to the output of the first inverter 54 and optionally to the low-voltage battery 56 connected to the output of the first inverter 54. The voltage supply of these accessories, or rather, the low-voltage consumers 58 of the motor vehicle, is provided by the first inverter 54.Consumer 58 is therefore supplied exclusively via the generator G 52, possibly buffered by the low-voltage battery 56.

[0048] Terms such as "essentially", "preferably", and the like, as well as any information that may be considered imprecise, are to be understood as allowing a deviation of plus or minus 5%, preferably plus or minus 2%, and in particular plus or minus 1% from the normal value. The applicant reserves the right to combine any features and sub-features from the claims and / or any features and sub-features from a set of the description in any way with other features, sub-features, or sub-features, even outside the features of independent claims.

[0049] In the different figures, parts that are equivalent in function are always provided with the same reference symbols, so that they are usually only described once. Reference symbol list 19 Anode side 20 Fuel cell 21 Cathode side 22 hydrogen tank 24 tank valve 26 pressure regulators 28 jet pump 30 Pressure sensor (anode) 31 Flushing valve 32 Air intake 34 Compressor 36 Engine M 38 2. Inverter 40 distribution boxes 42 HV battery 44 1. Converter, DC / DC converter 46 Turbine 48 Pressure sensor (cathode) 50 collection point 52 Generator G 54 1. Inverter 56 Low-voltage battery 58 consumers 60 Controller FCU 62 Voltage sensor 64 2. Converter 66 charger 68 plugs 70 Level 1 72 1. Link 74 Level 2 76 Controller turbine 78 2. Link 80 Controller Compressor 82 3rd level 84 4. Link 86 5. Linking

Claims

[1] Fuel cell system of a motor vehicle comprising a fuel cell (20) having an anode side and a cathode side, a compressor (34) rotaryally connected to a motor M (36) and connected to the cathode side of the fuel cell (20) via a supply line, a turbine (46) connected to the cathode side via an exhaust line and also rotaryally connected exclusively to a generator G (52) which is connected on its output side to an input of a first inverter (54) and low-voltage accessories of the fuel cell, preferably also low-voltage consumers (58) of the motor vehicle, are connected exclusively to the output of the first inverter (54) and optionally to a low-voltage battery (56) connected to the output of the first inverter (54). [2] Fuel cell system according to claim 1, characterized by, that the electrical output of the fuel cell (20) is connected to an HV battery (42) via a first converter (44), and that preferably the HV battery (42) can be connected to the motor M (36) via a second inverter (38). [3] Fuel cell system according to claim 1, characterized by , that the electrical output of the fuel cell (20) is connected to a second inverter (38) and via this to the motor M (36). [4] Fuel cell system according to any of the preceding claims, characterized by , that a second converter (64) is connected upstream of the HV battery (42), which is connected on the input side either directly or via the first converter (44) to the electrical output of the fuel cell (20). [5] Fuel cell system according to any of the preceding claims, characterized by, that a decision value is determined for both the quantity flow of air and the cathode pressure, taking priority into account the required power of the fuel cell (20) and / or the battery voltage of the low voltage battery (56). [6] Method for controlling a fuel cell system according to one of the preceding claims, wherein a setting value for the rotational speed of the compressor (34) is determined by taking into account an input value for the required power of the fuel cell (20), an input value of the current mass flow of air, an input value of the current cathode pressure (48) and an input value for the voltage of the low-voltage battery (56). [7] Method according to claim 6, characterized by , that based on the currently required power of the fuel cell (20) a value for the cathode pressure (48) and a control value for the airflow are calculated.

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