Method for cooling a fuel cell stack, control unit and fuel cell system

Abstract

A method for cooling a fuel cell stack (2) of a fuel cell system (1), preferably of a motor vehicle, is proposed. The fuel cell stack (2) is cooled by means of a cooling circuit (3) carrying a coolant, into which a pump (4), a radiator (5) with a fan (6), and a directional control valve (7) for opening and closing a bypass (8) for bypassing the radiator (5) are integrated. The temperature of the coolant is adjusted to a predefined standard value or standard range via the mixing ratio of the coolant flows conducted via the radiator (5) and / or the bypass (8) and via the air velocity at the radiator (5).The method is characterized in that the temperature of the coolant varies depending on a cooling capacity of the cooling circuit (3) to be provided over a predetermined period (T) and / or on an energy consumption of the cooling circuit (3) to be provided over a predetermined period (T) and is lowered or raised compared to the standard value or standard range.

Description

[0001] The present invention relates to a method for cooling a fuel cell stack of a fuel cell system, preferably of a motor vehicle, according to the preamble of claim 1. State of the art

[0002] The cooling of a fuel cell stack in a fuel cell system can be achieved with a cooling system that includes a pump and a radiator with a fan. In mobile applications, i.e., in a motor vehicle, the radiator is usually the vehicle's radiator.

[0003] DE 10 2016 213 533 A1 provides an example of a cooling system for controlling the temperature of a fuel cell system. The cooling system has a closed cooling line for a coolant. A pump for adjusting the pressure is integrated into the cooling line. The pump supplies the coolant to the fuel cell system to be cooled. There, the coolant absorbs the process heat of the fuel cell system. After passing through the fuel cell system, the coolant is fed to a cooler, for example, a vehicle radiator, where it is cooled again with the help of a fan.

[0004] Cooling systems with cooling circuits that feature a bypass to bypass the cooler or radiator are also known. A directional control valve can then be used to adjust the mixing ratio of the coolant flows through the radiator and the coolant flows through the bypass. Typically, a fixed target coolant temperature is set by controlling the directional control valve in combination with the fan control.

[0005] The coolant temperature influences the aging and thus the efficiency of the fuel cell stack over its lifetime. Since a low temperature has a positive effect on the efficiency of the fuel cell stack, it can be advantageous to set a low coolant temperature as a fixed target temperature. However, this results in a small temperature difference between the radiator and the ambient temperature. This can result in the cooling system's cooling capacity being exceeded or the cooling system operating at its limits, where energy consumption increases, since the required cooling capacity can only be achieved by increasing the coolant or air flow at the radiator.

[0006] The present invention is concerned with the task of optimizing the cooling of a fuel cell stack in such a way that the efficiency of the fuel cell stack is increased at low energy costs and with low system aging for the cooling.

[0007] To achieve this object, the method having the features of claim 1 is proposed. Advantageous developments of the invention are set forth in the subclaims. Furthermore, a control device for executing steps of the method is specified. Disclosure of the invention

[0008] A first aspect of the present invention relates to a method for cooling a fuel cell stack of a fuel cell system, preferably of a motor vehicle, by means of a cooling circuit carrying a coolant, in which a pump, a radiator with a fan, and a directional control valve for opening and closing a bypass for bypassing the radiator are integrated. The temperature of the coolant is set to a predefined standard value or standard range via the mixing ratio of the coolant flows conducted via the radiator and / or the bypass, as well as via the air velocity at the radiator. According to the invention, the temperature of the coolant is varied as a function of a cooling capacity of the cooling circuit to be achieved over a predetermined period of time and / or of an energy consumption of the cooling circuit to be achieved over a predetermined period of time, and is lowered or raised compared to the standard value or standard range.

[0009] The proposed method therefore does not specify a fixed target value for the coolant temperature, but rather varies the coolant temperature. Starting from a predefined standard value or standard range, the coolant temperature is lowered or raised. The decisive factors for lowering or raising the temperature are the cooling capacity of the cooling circuit to be achieved over a predetermined period of time and / or the energy consumption of the cooling circuit to be achieved over a predetermined period of time. Keeping these two parameters in mind, the coolant temperature can be selected in such a way that the efficiency and, at the same time, the aging of the fuel cell stack can be optimized.

[0010] The proposed method allows for determining optimal system efficiency and aging of the fuel cell stack over a predetermined period, preferably over the lifetime of the fuel cell stack. For this purpose, an optimal coolant temperature is determined. Optimally setting the coolant temperature can ensure optimal consumption and / or optimal aging of the fuel cell stack and, in particular, the fuel cell system.

[0011] With the proposed method, a standard range for the coolant temperature can first be defined. This could, for example, be 62-68°C. If the temperature falls below a lower limit of the standard range, the cooling circuit can operate within a reduced temperature range. If the temperature exceeds an upper limit of the standard range, the cooling circuit can operate within a raised temperature range. Specifying a standard range instead of a single standard value allows for greater flexibility in design. It should be noted that a temperature increase can be considered as soon as the temperature is raised by at least 2%, 5%, or 10% above the standard value or the standard range. Analogously, a temperature decrease can be considered as soon as the temperature is lowered by at least 2%, 5%, or 10% below the standard value or the standard range.

[0012] The coolant temperature can correlate very strongly with the operating temperature of the fuel cell stack or fuel cell system. An increase in the coolant temperature inevitably leads to an increase in the operating temperature. The opposite is also true.

[0013] It is advantageous if an aging factor of the fuel cell stack is determined based on the cooling capacity to be provided by the cooling circuit over a predetermined period of time and / or the energy consumption to be provided by the cooling circuit over a predetermined period of time. In other words, aging of the fuel cell stack can be quantified for a given cooling capacity over the predetermined period of time and / or for a given energy consumption over the predetermined period of time. In principle, an aging factor can be derived from a temporal progression of energy consumption over the predetermined period of time. This can be determined, for example, by means of an integral calculation of the said temporal progression. The area of ​​such an integral calculation can correlate with the aging factor. The smaller the area, the smaller the aging factor can be.The temperature of the cooling circuit can be optimized with regard to the area of ​​the integral calculation.

[0014] Alternatively, or in principle, an aging factor can be derived from a temporal progression over the predetermined period of the cooling capacity to be provided. Analogous to energy consumption, the aging factor can be determined by means of an integral calculation of the aforementioned temporal progression. The smaller the area of ​​the integral calculation, the smaller the aging factor can be.

[0015] It is advantageous if the coolant temperature is further varied depending on a determined system efficiency of the fuel cell stack, in particular by means of an integral calculation of a determined system efficiency, and is lowered or raised compared to the standard value or standard range. In other words, the efficiency of the fuel cell stack can be quantified. The system efficiency can be defined as the relationship between (at least) two system parameters. The system efficiency can be, for example, the relationship between the aging factor and the energy consumption of the cooling circuit or between the aging factor and the cooling capacity.

[0016] In principle, the method can be carried out in such a way that the proposed variation of the coolant temperature can enable an increase in efficiency without an increased aging factor.

[0017] It is advantageous if the energy consumption is determined, preferably estimated, based on the current and / or predicted ambient temperature and / or the current and / or predicted air velocity at the radiator. In other words, the cooling capacity to be provided can be determined for a predetermined period of time, wherein the period can cover the past, the present, and / or the future. A prediction of operating parameters such as the ambient temperature and / or the air velocity at the radiator can be made based on a preset curve. Alternatively or additionally, such operating parameters can be predicted using empirical data and / or (stored) planned operations of the fuel cell system.

[0018] It is advantageous if the temperature of the coolant is raised above the standard value or standard range if it is predicted, or in particular if there is a risk of exceeding the cooling capacity of the cooling circuit, and the operating temperature of the fuel cell stack can be kept below a maximum permissible limit.

[0019] Raising the coolant temperature is therefore also subject to conditions. Only if these conditions are met can the temperature be raised above the standard value or the standard range. Compliance with these conditions ensures that neither the cooling capacity of the cooling circuit nor the maximum permissible operating temperature of the fuel cell stack is exceeded.

[0020] It is advantageous if the operation of the cooling circuit at elevated temperature is limited in time, for example per event and / or over the entire lifetime of the fuel cell stack.

[0021] This means that each individual increase and / or all increases as a whole may be subject to a time limit. If the time limit is exceeded, increasing the coolant temperature may no longer be permitted. It may then be necessary to reduce the power of the fuel cell stack to reduce the cooling capacity of the cooling circuit to the maximum possible cooling capacity.

[0022] A second aspect of the present invention relates to a control unit configured to carry out the steps of a method according to one of the preceding claims. With the aid of the control unit, for example, a control or regulation of the directional control valve and / or fan integrated into the cooling circuit can be realized in order to lower or raise the temperature of the coolant.

[0023] A third aspect of the present invention relates to a fuel cell system. The fuel cell system comprises a fuel cell stack and a cooling circuit carrying a coolant and is particularly designed for a motor vehicle. The cooling circuit extends partially through the fuel cell stack. The fuel cell system can be cooled by means of the cooling circuit according to a method according to one of claims 1 to 6. Such a fuel cell system can advantageously be operated efficiently and with low aging.

[0024] As a further optional aspect, a motor vehicle with such a fuel cell system can be provided.

[0025] All disclosures and advantages which have been explained in connection with the method according to the first aspect of the invention apply equally to the control device according to the second aspect of the invention and to the fuel cell system according to the third aspect of the invention and vice versa.

[0026] In the following, embodiments of the invention are described with reference to the figures. Fig. 1 schematically shows a fuel cell system according to an embodiment, and Fig. 2 shows a time course of an operating parameter of a fuel cell system.

[0027] Similar, similarly acting, identical, or equivalent elements are provided with similar or identical reference numerals in the figures. The figures are merely schematic and not to scale.

[0028] The illustrated fuel cell stack 2 of a fuel cell system 1 has an anode 2.1 and a cathode 2.2. During operation of the fuel cell system 1, the anode 2.1 is supplied with hydrogen via an anode circuit (not shown). Ambient air is supplied to the cathode 2.2 as an oxygen source via an air supply path (not shown). In the fuel cells of the fuel cell stack 2, hydrogen and oxygen are converted into electrical energy, heat, and water.

[0029] The heat generated during this process is dissipated via a cooling circuit 3, which is routed sectionally through the fuel cell stack 2 for this purpose. A pump 4, a radiator 5 with a fan 6, and a directional control valve 7 are integrated into the cooling circuit 3 outside the fuel cell stack 2. The pump 4 circulates a coolant in the cooling circuit 3. When the coolant enters the fuel cell stack 2, it absorbs heat, which it then later releases to the environment with the help of the radiator 5 and the fan 6. The directional control valve 7 can be used to control the coolant flow through the radiator 5. Depending on the switching position of the directional control valve 7, at least a partial flow of the coolant is not fed to the radiator 5, but branched off into a bypass 8 that bypasses the radiator 5. This means that the coolant can release less heat to the environment.The coolant temperature can therefore be adjusted by adjusting the mixing ratio of the coolant flows through the radiator 5 and the bypass 8. The air velocity at the radiator 5 also influences the coolant temperature. This can be adjusted by operating the fan 6. In mobile applications, the air velocity also depends on the vehicle speed.

[0030] A fuel cell system 1 that operates at lower temperatures generally exhibits less system aging. Thus, considering the service life of the fuel cell system 1, lower consumption can be achieved by a less aged fuel cell system 1. On the other hand, a fuel cell system 1 that operates at lower temperatures requires higher energy consumption by the actuators of the thermal system, i.e., the cooling circuit 3 and, for example, the pump 4. Generally, the fan 6 and the pump 4 consume more energy at comparatively lower temperatures. Furthermore, the fuel cell system 1 exhibits different system efficiencies depending on the operating temperature.Depending on these general conditions, an optimal operating temperature for the lowest possible consumption of the fuel cell system 1 (and in particular of the actuators) can be determined according to a method according to the invention.

[0031] The following values ​​can be compared and / or evaluated using the method according to the invention: • The aging of the fuel cell system 1 can be determined as a function of the operating temperature. This means that fuel consumption can be improved by reducing aging by lowering the operating temperature relative to the standard value and / or standard range. • The efficiency of the fuel cell system 1 can be determined depending on the operating temperature. • The energy consumption of cooling circuit 3 can be determined depending on the temperature and optionally the ambient temperature.

[0032] By adjusting the temperature of the coolant (and accordingly the operating temperature of the fuel cell system 1), an optimal consumption of the fuel cell system 1 and / or an optimal aging factor can be ensured.

[0033] Fig. Figure 2 shows a time course of an operating parameter P of a fuel cell system 1. The horizontal axis represents time t. In Fig. 2, in particular, two temporal profiles of an operating parameter P, such as the energy consumption of the cooling circuit 3 or the cooling capacity of the cooling circuit 3 over a predefined period of time T, are shown. Based on an integral calculation of the corresponding profile, a value V1, V2 can be derived, which correlates with an aging factor. In other words, an integral calculation can be used to derive which temporal profile for the operating parameter leads to optimal aging and efficiency of the fuel cell stack 2 or the fuel cell system. It follows that not only the current value of the operating parameter P is considered or optimized, but rather the value seen over the predefined period of time. In the example of the Fig. 2, V2 is smaller than V1, which is why this curve may be preferred. However, the current value of the resulting V2 curve is sometimes higher than the resulting V2 curve.

[0034] Additionally, it should be noted that the terms "comprising" and "having" do not exclude other elements, and the indefinite articles "a" or "an" do not exclude a plurality. Furthermore, it should be noted that features and steps described with reference to one of the above embodiments may also be used in combination with other features and steps of other embodiments described above. Reference signs in the claims are not to be considered limitations. QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2016 213 533 A1

[0003]

Claims

[1] Method for cooling a fuel cell stack (2) of a fuel cell system (1), preferably of a motor vehicle, by means of a cooling circuit (3) carrying a coolant, into which a pump (4), a radiator (5) with a fan (6) and a directional control valve (7) for opening and closing a bypass (8) for bypassing the radiator (5) are integrated, wherein the temperature of the coolant is set to a predefined standard value or standard range via the mixing ratio of the coolant flows guided via the radiator (5) and / or the bypass (8) and via the air speed at the radiator (5), characterized by that the temperature of the coolant varies depending on a cooling capacity of the cooling circuit (3) to be provided over a predetermined period (T) and / or on an energy consumption of the cooling circuit (3) to be provided over a predetermined period (T) and is lowered or raised compared to the standard value or standard range. [2] Method according to claim 1, characterized by that an aging factor (A) of the fuel cell stack (2) is determined based on the cooling capacity of the cooling circuit (3) to be provided over a predetermined period of time (T) and / or the energy consumption of the cooling circuit (3) to be provided over a predetermined period of time (T). [3] Method according to one of the preceding claims, characterized by that the temperature of the coolant varies further depending on a determined system efficiency, in particular by means of an integral calculation of a determined system efficiency, of the fuel cell stack (2) and is lowered or raised compared to the standard value or standard range. [4] Method according to one of the preceding claims, characterized bythat the energy consumption is determined, preferably estimated, based on the current and / or predicted ambient temperature and / or the current and / or predicted air speed at the radiator (5). [5] Method according to one of the preceding claims, characterized by that the temperature of the coolant is raised above the standard value or standard range when (i) an exceeding of the cooling capacity of the cooling circuit (3) is predicted, and (ii) the operating temperature of the fuel cell stack (2) can be kept below a maximum permissible limit. [6] Method according to one of the preceding claims, characterized by that the operation of the cooling circuit (3) at an elevated temperature is limited in time, for example per event and / or over the entire service life of the fuel cell stack (2). [7] Control device which is arranged to carry out the steps of a method according to one of the preceding claims. [8] Fuel cell system (1) comprising, a fuel cell stack (2) and a cooling circuit (3) carrying a coolant, wherein the cooling circuit (3) is guided in sections through the fuel cell stack (2), and wherein the fuel cell system can be cooled by means of the cooling circuit (3) according to a method according to one of claims 1 to 6.

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