Method for operating a fuel cell device associated with a motor vehicle and motor vehicle

Abstract

Method for operating a fuel cell device associated with a motor vehicle, comprising a fuel cell stack (1) integrated in a coolant circuit (2) with a main water cooler (3) and a coolant pump (4), comprising the steps of detecting a braking process (8) and using the recuperated energy gained during this process to lower the temperature of a coolant circulating in the coolant circuit (2), wherein, upon detection of a braking process (8), a reduction in the power output of the fuel cell stack (1) and continued operation of the coolant pump (4) by means of the recuperated energy gained during the braking process (8) and at least a partial diversion of the coolant into a bypass (5) bypassing the fuel cell stack (1), in which a heat pump (6) operated by the recuperated energy gained cools the coolant circulating in the coolant circuit (2).

Description

[0001] The invention comprises a method for operating a fuel cell device associated with a motor vehicle, which includes a fuel cell stack integrated in a coolant circuit with a main water cooler and a coolant pump, comprising the steps of detecting a braking process and using the recuperated energy gained therefrom to lower the temperature of a coolant circulating in the coolant circuit. The invention further relates to a motor vehicle.

[0002] There is a current effort to further strengthen electromobility and to provide suitable devices that can be operated as efficiently as possible. For motor vehicles equipped with an electric traction motor, the necessary electrical energy can be supplied by a battery, more precisely an accumulator, which should be as large as possible to ensure a reasonable vehicle range. Alternatively, the required electrical energy can be generated by a fuel cell system, as this allows for a smaller battery, thus saving installation space, reducing the vehicle's weight, and conserving the resources required for a larger battery.

[0003] Furthermore, it is known that the kinetic energy converted during a braking process can be recuperated and stored in the battery.

[0004] Vehicles equipped with fuel cell devices are at a disadvantage here, as the relatively small batteries used cannot utilize the power recuperated during braking, thus reducing the overall system efficiency.

[0005] DE 10 2016 117 153 A1 relates to a hybrid vehicle and a method for conditioning a vehicle battery, wherein the electric machine is configured to charge the battery with current generated from recuperation. An electrically driven cooling system for cooling the battery is also provided, wherein the control system is programmed such that, in response to a battery temperature exceeding a threshold value, a portion of the current generated from recuperation braking is directed to the cooling system.

[0006] In US 2016 / 0036072 A1 and DE 10 2014 225 381 A1, the problem of cold starts in a motor vehicle with a fuel cell system is addressed, and to mitigate the associated problems, a bypass is proposed for the air supply to the fuel cell stack. This bypass bypasses the humidifier and is routed through a coolant heat exchanger. This allows moisture to be removed from the fuel cell stack when the system is shut down in freezing conditions by raising the temperature of the air supplied to the fuel cell stack and preventing the introduction of additional moisture by bypassing the humidifier. An additional advantage is stated: even without coolant circulation, the coolant heat exchanger can be cooled by the airflow through the bypass during braking.

[0007] The invention is based on the objective of providing a method that reduces or eliminates the disadvantages of a motor vehicle equipped with a fuel cell device and a relatively small battery. A further objective is to provide an improved motor vehicle.

[0008] This problem is solved by a method having the features of claim 1 and by a motor vehicle having the features of claim 5.

[0009] Advantageous embodiments with appropriate further developments of the invention are specified in the dependent claims.

[0010] The invention is based on the understanding that the energy recovered from kinetic energy during recuperative braking does not necessarily have to be stored in the battery, but can also be used effectively to supply the auxiliary components present in a fuel cell device. Furthermore, it has been recognized that the provided energy is advantageously used to lower the temperature of a coolant circulating in the coolant circuit, as this allows for a subsequent greater temperature increase of the coolant, i.e., a longer and heavier load on the fuel cell device, before temperature-related derating, i.e., power throttling, becomes necessary.

[0011] When braking is detected, the power output of the fuel cell stack is reduced, and the coolant pump continues to operate using the energy recovered during braking. At least some of the coolant is diverted into a bypass around the fuel cell stack, where a heat pump, powered by the recovered energy, cools the coolant circulating in the cooling circuit. This process significantly increases the efficiency of the fuel cell device because, firstly, its power output can be reduced, and secondly, the additional energy input—covered by the recovered energy from braking—actively cools the coolant circulating in the cooling circuit. This process discharges the coolant from the heat load, creating a depleted storage capacity for future heat charging.The advantages are particularly noticeable during high-load motorway journeys with rapidly alternating full-load and braking phases, as more reproducible driving behavior is achieved because a volume flow continues to be circulated via the coolant pump during braking phases, and the coolant and the main water radiator are actively cooled down via the heat pump.

[0012] The energy recovered during braking is not necessarily stored exclusively in a battery, but can be considered a cooling equivalent. Based on the reduced temperature in the coolant circuit, a strong acceleration following braking can be sustained for a longer period.

[0013] It is also advantageous if the power output of the fuel cell stack is reduced completely and the auxiliary components within the fuel cell system are powered by the energy recovered during braking. This further protects the fuel cell system, reduces the consumption of reactants required for its operation, and thus increases the overall efficiency of the fuel cell system.

[0014] It is also planned that when the braking process ends, the bypass will be closed again and the reduction in power output by the fuel cell stack will be ended, which is kept ready for an extended high-load phase starting from a lower coolant temperature.

[0015] It is also possible that the bypass is used as a coolant reservoir, i.e., opening the bypass provides an increased amount of coolant, whereby this amount of coolant, which was not previously circulating in the coolant circuit, is not thermally charged to the same extent, so that a reduction in the temperature of the total circulating coolant is also achieved.

[0016] The motor vehicle according to the invention has a fuel cell device comprising a fuel cell stack integrated into a cooling circuit with a main water cooler and a coolant pump. A bypass is connected to the coolant circuit downstream of the coolant pump and upstream of the fuel cell stack. This bypass bypasses the fuel cell stack and rejoins the cooling circuit downstream of it. A heat pump is arranged in the bypass, and supply lines are provided to supply the coolant pump and the heat pump with the energy recovered during braking. These supply lines can, in particular, be the supply lines provided for standard operation, where it is only necessary to ensure that the energy recovered during the recuperation process can be fed, at least indirectly, into the corresponding network.

[0017] A rapid change between operating states can be achieved by assigning at least one valve to the bypass, and a valve can also be arranged at each branch of the bypass.

[0018] Furthermore, the cross-sectional area of ​​the bypass pipe may be determined by its suitability as a coolant storage medium; that is, it is not necessary for the cross-sectional area of ​​the bypass pipe to be identical to the cross-sectional area of ​​the coolant circuit, but it may also be larger if this allows for a larger coolant storage medium without increasing the space requirements.

[0019] Further advantages, features, and details of the invention will become apparent from the claims, the following description of preferred embodiments, and the drawings. These show: Fig. 1 a schematic representation of the part of a fuel cell device required to explain the invention, Fig. 2 a schematic representation of the addition of the cooling circuit by the bypass, Fig. 3 a time-dependent representation of the speed of a motor vehicle with rapidly changing full-load and recuperation phases, and Fig. 4 a time-dependent representation of the temperature of the coolant at the inlet 207 of the main water cooler and at the outlet 227 of the main water cooler, shown for the cases of use of recuperated energy 21 and without use 23 (bottom figure) and the power available through the total system shown for the cases with 24 and without 25 use of the heat pump supplied by the recuperated energy.

[0020] In the Fig. Figure 1 shows only the part of a fuel cell assembly necessary to explain the invention, namely the fuel cell stack 1 with a plurality of fuel cells arranged in series, for whose temperature control a coolant circuit 2 is provided with a main water cooler 3 and a coolant pump 4. This system serves to cool the coolant, which is loaded with heat in the fuel cell stack 1, down to a limit temperature, so that in the event of a subsequent increased power demand on the fuel cell stack 1, its operation within the preferred temperature range is ensured and no derating, i.e., no power reduction, is necessary to ensure compliance with the permissible limits of the temperature range.The limit temperature is chosen to ensure trouble-free operation; 50°C is a suitable value that also prevents the coolant from freezing during a subsequent downhill drive. If the coolant remains in the bypass, the limit temperature can be reached by operating the heat pump at 20°C.

[0021] In a vehicle equipped with a fuel cell system, braking maneuvers 8 frequently occur, during which kinetic energy is converted and stored, for example, as electrical energy in the battery of the fuel cell system or the vehicle itself through recuperation. Since this battery is relatively small in vehicles equipped with fuel cells, the potential for utilizing the recuperated energy provided by braking 8 is limited. Therefore, a method has been proposed to use this energy to increase efficiency by lowering the temperature of the coolant circulating in the coolant circuit 2. To achieve this, upon detection of a braking maneuver 8, the power output of the fuel cell stack 1 is initially reduced, potentially even to the point of completely shutting down the fuel cell stack 1.Continued operation of the coolant pump 4 using the energy recovered during braking is ensured, with the coolant being pumped into a bypass 5 that bypasses the fuel cell stack 1 at points 9 and 10. A heat pump 6 is located in this bypass 5 and is operated with the recovered energy to cool the coolant circulating in the coolant circuit 2. The bypass 5 can be opened and closed, in particular, by a valve located at the branch or within the bypass 5 itself.

[0022] When the braking process 8 is completed, the bypass 5 is closed again and the reduction of power output by the fuel cell stack 1 is terminated.

[0023] Fig. Figure 3 illustrates the application scenario where, for example, during highway driving, the fuel cell stack 1 operates under full load 7, leading to a significant heating of the coolant circulating in the coolant circuit 2. When the full-load phase 7 is terminated by braking 8, the energy provided by recuperation is used to further reduce the coolant temperature. This allows for a longer period of cooling of the coolant up to its permissible limit temperature during the subsequent full-load phase 7, thus ensuring reproducible driving behavior with an extended boost phase for full load 7.

[0024] Fig.Figure 4 shows the temperature behavior of the coolant during a braking process 8, in which the recuperated energy provided is used to lower the coolant temperature, so that it can be provided for a longer period when there is a subsequent increased power demand. p REFERENCE MARK LIST: 1 fuel cell stack 2 Coolant circuit 3 main water coolers 4 Coolant pump 5 Bypass 6 Heat pump 7 Full load 8 Braking process 9 Branch into Bypass 10 Branch from Bypass 20 Coolant temperature at the main water cooler inlet 21 Coolant temperature when using recuperated energy 22 Coolant temperature at the outlet of the main water cooler 23 Coolant temperature without using recuperated energy 24 available power using recuperated energy 25 available power without using the recuperated energy

Claims

[1] Method for operating a fuel cell device associated with a motor vehicle, comprising a fuel cell stack (1) integrated in a coolant circuit (2) with a main water cooler (3) and a coolant pump (4), comprising the steps of detecting a braking process (8) and using the recuperated energy gained during this process to lower the temperature of a coolant circulating in the coolant circuit (2), wherein, upon detection of a braking process (8), a reduction in the power output of the fuel cell stack (1) and continued operation of the coolant pump (4) by means of the recuperated energy gained during the braking process (8) and at least a partial diversion of the coolant into a bypass (5) bypassing the fuel cell stack (1), in which a heat pump (6) operated by the recuperated energy gained causes the cooling of the coolant circulating in the coolant circuit (2). [2] Method according to claim 1, characterized by , that the reduction of the power output of the fuel cell stack (1) is complete and the supply of the auxiliary units present in the fuel cell device is carried out using the energy recovered during the braking process. [3] Method according to one of claim 1 or 2, characterized by , that when the braking process (8) is completed the bypass (5) is closed again and the reduction of power output by the fuel cell stack (1) is terminated, which is kept ready for an extended high load phase starting from a lower temperature of the coolant. [4] Method according to any one of claims 1 to 3, characterized by , that the bypass (5) is used as a coolant reservoir. [5] Motor vehicle with a fuel cell device comprising a fuel cell stack 1 integrated in a coolant circuit (2) with a main water cooler (3) and a coolant pump (4), characterized by , that downstream of the coolant pump (4) and upstream of the fuel cell stack (1) a bypass (5) is connected to the coolant circuit (2), which bypasses the fuel cell stack (1) and downstream of it again leads into the coolant circuit (2), that a heat pump (6) is arranged in the bypass (5), and that supply lines are provided to supply the recuperated energy gained during a braking process to the coolant pump (4) and the heat pump (6). [6] Motor vehicle according to claim 5, characterized by , that at least one valve is assigned to the bypass (5). [7] Motor vehicle according to claim 5 or 6, characterized by , that the cross-sectional area of ​​the bypass (5) is determined by its suitability as a coolant storage medium.

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