Method and device for cooling a fuel cell

Abstract

In order to quickly reach and maintain a stable supply temperature for cooling a fuel cell, a required initial fan speed is predicted based on operating parameters, and the supply temperature is subsequently controlled via the fan speed as a manipulated variable.

Description

[0001] The invention relates to a method for cooling a fuel cell and a device suitable for the method.

[0002] Fuel cells are used to supply energy in many technical fields, for example, for vehicle propulsion in cars or rail vehicles. They require cooling during operation. A coolant circuit is typically used for this purpose, and it is important that the coolant flow temperature—that is, the temperature of the coolant upstream of the fuel cell—is kept sufficiently constant. Both excessively high and excessively low flow temperatures impair fuel cell operation.

[0003] In order to achieve the required flow temperature and maintain it at a nearly constant level, it is known to cool the coolant in a cooling system that is connected to the fuel cell's coolant circuit (internal coolant circuit) via a 3-way valve, and to control and regulate the flow temperature via the 3-way valve. The coolant, or a portion of the coolant, is branched off from the internal coolant circuit and passes through the cooling system, where the ambient air cools the coolant with the help of a fan. The cooled coolant is fed back into the internal coolant circuit via the 3-way valve and mixed with the uncooled coolant there, so that the required flow temperature is achieved. The valve is controlled using a temperature sensor for the coolant upstream of the fuel cell, and it is regulated to the required flow temperature using the 3-way valve as the manipulated variable.

[0004] When the fuel cell is operated at partial load and therefore less waste heat is generated, the 3-way valve directs a large part of the coolant through the inner circuit, and only a small part of the coolant flows through the cooling system.

[0005] This causes the cooling system to operate inefficiently. Because the cooling system is not supplied with sufficient coolant mass flow, the average driving temperature difference between the coolant and the intake ambient air decreases, thereby reducing cooling performance.

[0006] This method has the advantage that the air flow rate of the cooling system does not need to be adjusted to set and maintain the flow temperature to the fuel cell stack. The fan can operate at a constant speed because the cooling capacity is controlled via the 3-way valve. However, this method also has the disadvantage that in these frequently occurring operating situations, the fan must operate at a higher speed than is necessary to generate the required cooling capacity. This leads to increased energy consumption, a reduced service life of the fan, and high noise emissions. The increased energy consumption has a negative impact on the usable energy of the fuel cell, for example, on the range of a rail vehicle.

[0007] The object of the invention is to provide an improved method for cooling a fuel cell, in particular in a vehicle, as well as a corresponding device.

[0008] This object is achieved by a method having the features of patent claim 1 and by an arrangement according to patent claim 8. Advantageous embodiments of the invention are specified in the subclaims.

[0009] The invention provides for controlling the flow temperature by controlling the air volume flow through the cooling system using the fan speed as a manipulated variable. The invention is also based on the finding that when the fan speed and thus the air volume flow through the cooler changes, the flow temperature only changes with a time delay, since the cooler and the coolant, with their masses and corresponding heat capacities, only adapt their temperature with a time delay when the fan speed changes. Therefore, the invention provides for predicting the fan speed (referred to as the first fan speed) required for the required flow temperature (target flow temperature, which is determined in advance) and setting the fan to this speed. The predictive calculation is based on at least one operating parameter of the entire coolant circuit (i.e. including the fuel cell).A subsequent correction is performed using the superimposed control loop with the fan speed as the manipulated variable and the coolant flow temperature to the fuel cell as the controlled variable. The measured flow temperature (actual flow temperature) is compared with the target flow temperature, and if a deviation is detected, the calculated initial fan speed is adjusted. By combining the precalculation and subsequent adjustment of the fan speed as needed, the required flow temperature is reached sufficiently quickly.

[0010] This is important for applications in vehicles, as the operating parameters that influence the required cooling capacity (e.g., the fuel cell's utilization rate) often change very rapidly. Control alone, without any advance calculation, would not keep the flow temperature sufficiently constant.

[0011] A device according to the invention comprises a coolant circuit with a fuel cell, wherein the coolant can be cooled in a cooling system by means of a fan, and a temperature sensor for detecting the flow temperature in the coolant circuit. The device further comprises at least one detection unit for detecting at least one operating parameter and a calculation unit for determining a first fan speed using the at least one operating parameter. The device further comprises a control unit for setting the fan speed to the first fan speed and a regulating unit for regulating the flow temperature using the fan speed as a manipulated variable.

[0012] According to one embodiment, one or more of the following operating parameters are recorded to calculate the first fan speed: - Flow temperature of the coolant to the fuel cell - Return temperature of the coolant to the radiator - Volume flow or mass flow of the coolant through the radiator - Temperature of the ambient air sucked in by the cooler

[0013] Knowing the operating characteristics of the cooling system, i.e., the cooling capacity as a function of the fluid mass flows and fluid temperatures, these parameters can be used to approximately calculate the required air flow rate and thus the required fan speed (first fan speed) to achieve a specific flow temperature. The operating parameters are recorded either directly using suitable sensors or derived from other parameters; in particular, the coolant flow rate can be determined by measuring the speed of the coolant pump 14.

[0014] Alternatively or additionally, the initial fan speed required to achieve the target flow temperature can be determined for different fuel cell utilization levels. The utilization level also represents an operating parameter and can be recorded by a suitable recording unit. In addition, the ambient air temperature and / or the return flow temperature can be considered as further operating parameters.

[0015] The required air volume or the initial fan speed can be calculated in advance during the cooling process based on the currently measured values ​​of the operating parameter(s), but it can also be calculated in advance and saved. In the latter variant of the process, a parameter array is determined and saved in advance, which is then used during the actual cooling operation. Thus, the required air volume or the initial fan speed is calculated in advance for different values ​​of at least one operating parameter. In particular, an initial fan speed can be determined and saved in advance for different fuel cell utilization levels.Preferably, further operating parameters, in particular the ambient temperature, are also taken into account, so that a multi-dimensional parameter field is available in advance, which can be used during operation and thus the first fan speed for the current parameter values ​​can be determined - if necessary with the help of interpolation.

[0016] To adjust the actual flow temperature to the desired flow temperature, the determined first fan speed can be corrected by a factor: If the setpoint is higher than the actual value, the calculated first fan speed is multiplied by a correction factor less than one, thus decreasing the value. If the setpoint is lower than the actual value, the calculated first fan speed is multiplied by a correction factor greater than one, thus increasing the value. The correction factor is generated using a controller, such as a PID controller.

[0017] Preferably, the method runs cyclically with a predetermined cycle duration. The method is run through within a predetermined period of time (cycle duration), i.e., the operating parameter(s) are recorded, the first fan speed is determined and corrected, and the fan is controlled accordingly. This process is then run through again. The cycle duration can be a few milliseconds to 10 seconds, preferably 100 ms to 1 second, particularly preferably 200 ms.

[0018] In another embodiment, the operating parameter(s) are recorded continuously, but the subsequent process steps—i.e., determining the initial fan speed, controlling the fan, and regulating the flow temperature—are only performed when an operating parameter changes by a specified deviation. The term "continuous" also includes recording at specific time intervals.

[0019] According to one embodiment, the coolant circuit comprises an inner coolant circuit, in which the fuel cell is arranged, and a connected cooler circuit, in which the cooling system is arranged. Additionally, the two circuits can be connected via a 3-way valve. In this case, the 3-way valve can be used during the fuel cell's warm-up phase, or if excessive cooling performance is still achieved even with the fan switched off. During the warm-up phase, the desired flow temperature of, for example, 60°C should be reached quickly, even at low ambient temperatures. Therefore, the cooler circuit is isolated, and the coolant circulates only in the inner cooling circuit. Likewise, the cooler circuit is isolated by the 3-way valve if the cooling performance is still too high with the fan switched off.

[0020] In another embodiment, the cooling system is not arranged parallel to the internal cooling circuit, but is integrated in series with it.

[0021] The invention is explained in more detail below using exemplary embodiments. They show a schematic representation Fig. 1 a device for cooling a fuel cell according to an embodiment Fig. 2 a flowchart of the method according to an embodiment

[0022] Fig. 1 shows a coolant circuit with an internal coolant circuit 1 and a cooler circuit 2, which is connected to the internal coolant circuit via a branch and a 3-way valve 15. A fuel cell 12, a coolant pump 14, the 3-way valve 15, and a pressure equalization tank 16 are arranged in the internal coolant circuit. Furthermore, near the fuel cell 12, there is a first temperature sensor 11, which measures the flow temperature of the coolant upstream of the fuel cell, and a second temperature sensor 13, which measures the return temperature of the coolant downstream of the fuel cell. The cooler circuit 2 comprises a cooling system 21, in which the coolant is cooled via heat exchange with the ambient air. One or more fans 22 are provided for this purpose.

[0023] The device for cooling the fuel cell further comprises a detection unit 31, with which at least one operating parameter is determined during operation of the device. In the exemplary embodiment, the ambient air temperature is measured by a sensor not shown in the figure, the flow temperature is measured by the first temperature sensor 11, and the return temperature is measured by a second temperature sensor 13. As a further operating parameter, the volume flow of the coolant in the internal coolant circuit 1 is determined by detecting the speed of the coolant pump 14.

[0024] The device for cooling the fuel cell further comprises a calculation unit 32. This calculates the air flow required in the cooling system 21 for the desired flow temperature and a first fan speed required for the fan 22 from the determined operating parameters.

[0025] The device further comprises a control unit 33 for controlling the fan speed, which is connected to the calculation unit. The control unit 33 is further connected to a control unit 34. An input of the control unit 34 is connected to the first temperature sensor 11, and an output of the control unit 34 is connected to the fan 22 such that the fan speed is used as a control variable for controlling the flow temperature.

[0026] Fig. Figure 2 shows an example of the method sequence. During ongoing operation of the fuel cell, the values ​​51b - 51e for the flow temperature, the return temperature, the ambient air, and the volume flow of the coolant are determined as operating parameters in step 51, partly by direct measurement using sensors 11, 13, among others, and partly calculated from other operating parameters such as pump speed and others. Furthermore, the target flow temperature 51a is specified.

[0027] From these operating parameters, the operating characteristics of the cooling system, and the required target flow temperature, the required air volume flow that must be achieved in the cooling system can be calculated in step 52. From thermodynamic considerations regarding heat balance and heat transfer (see below), the relative volume flow f FC_s of the air (ie the volume flow related to the maximum possible volume flow, therefore a dimensionless quantity): fFC_s=(1V˙max)1cpa+1kAaTr_FC+Tf_FC_s−2⋅Tai⋅cc⋅m˙c⋅(Tr_FC−Tf_FC_s)paRa(273.15 K+Tai)

[0028] Here and in the following formulas c pa - Heat capacity of air (J / KgK) ṁ a_FC - Mass flow of air (kg / s) T ai - Temperature of the air at the cooler inlet (= approx. ambient temperature) (°C) T ao - Air temperature at the radiator outlet (°C) c c- Heat capacity of the cooling medium (J / KgK) ṁ c - Mass flow of the cooling medium (kg / s) T r_FC -Temperature of the cooling medium at the cooler inlet (= approx. ambient temperature) (°C) T f FCs - Temperature of the cooling medium at the cooler outlet (°C) (target flow temperature) k - heat transfer coefficient of the cooler A - Heat exchange surface of the cooler (T c -T a ) - average driving temperature difference between cooling medium and air (K) V FC_s - Air volume flow (m 3 / s) (target) p a = ambient pressure R a = 289 J / kgK = Specific gas constant of air (with a water vapor content of 11 g per kg of dry air). V max - Maximum air flow rate (m 3 / s) (plant-specific) f FC_s- Relative volume flow of air related to the maximum possible volume flow (dimensionless).

[0029] The calculation is based on the energy balance around cooler 21: cpa⋅m˙a_FC⋅(Tao−Tai)=cc⋅m˙c⋅(Tr_FC−Tf_FC)→Tao=cw⋅m˙ccpa⋅m˙a_FC⋅(Tr_FC−Tf_FC)+Tai

[0030] On the other hand, the calculation is based on the energy balance around the part of the cooler through which air flows, with the formulation of the heat transfer within the cooler - i.e. from the part through which coolant flows to the part through which air flows: cpa⋅m˙aFC⋅(Tao−Tai)=k⋅A⋅(Tc−Ta)

[0031] Furthermore, the heat flow from the coolant to the air is assumed to be approximate k⋅A⋅(Tc−Ta)≅2⋅a⋅m˙a_FC⋅k⋅A⋅(Tr_FC+Tf_FC2−Tai+Tao2) and for air the laws for ideal gases are adopted, in particular: V˙FC_s=m˙a_FCpaRa(273.15 K+Tai) From this, the above-mentioned formula for the relative volume flow f FC_s which allows the air to escape.

[0032] The air flow rate through a fan is, to a first approximation, proportional to the fan speed. fFC_s=Ns / Nmax and it applies to the first fan speed: Ns=Nmax⋅fFC_s=(NmaxV˙max)1cpa+1kAaTr_FC+Tf_FC_s−2⋅Tai⋅cc⋅m˙c⋅(Tr_FC−Tf_FC_s)paRa(273.15 K+Tai)

[0033] The quantities V̇ max and N max plant-specific, as is the size askg; (Constant of the correction factor 2 · a · m a · ṁ a_FC) and kA. In this way, in step 52, the first fan speed corresponding to the required air flow is calculated in the calculation unit 32. Data on the operating characteristics of the cooling system, i.e., system-specific data, are also incorporated. In step 52, the required air volume flow and thus the required first fan speed to achieve a specific flow temperature are approximately calculated based on the operating characteristics of the cooling system, i.e., the cooling capacity as a function of the fluid mass flows and fluid temperatures.

[0034] Since not all influencing parameters are included in the prediction (e.g., heat capacity of the system) and there may therefore still be slight deviations from the setpoint for the flow temperature at the flow temperature achieved with the first fan speed, the actual flow temperature to the fuel cell is measured in step 53 using the first temperature sensor 11 and compared with the setpoint. The actual value flows into a control loop in which the fan speed forms the manipulated variable for the flow temperature (controlled variable). If the setpoint is higher than the actual value, the first fan speed is multiplied by a correction factor less than one and thus decreased. If the setpoint is lower than the actual value, the first fan speed is multiplied by a correction factor greater than one and thus increased. The correction factor is generated using a controller, e.g., a PID controller. Such a controller can regulate sufficiently quickly.

[0035] In step 54, the fan speed is set to the corrected value.

[0036] The process steps 51 to 54 are carried out cyclically, with the cycle duration being approximately 200 ms.

[0037] The features and aspects of the invention described in the exemplary embodiments can, of course, be combined with one another in various ways. In particular, the features can be used not only in the described combinations, but also in other combinations or on their own.

Claims

[1] Method for cooling a fuel cell in which the fuel cell (12) is arranged in a coolant circuit (1), the coolant of which is at least partially cooled in a cooling system (21) by means of a fan (22), where a target flow temperature for the coolant is determined, with the steps: a) determine at least one operating parameter of the coolant circuit b) determining a first fan speed using at least one operating parameter and the target flow temperature c) Measuring an actual flow temperature of the coolant in the coolant circuit (1) d) Control of the flow temperature of the coolant to the target flow temperature using the fan speed as a control variable. [2] Method according to the preceding claim, characterized by that as operating parameters at least one of the following quantities: Coolant flow temperature, coolant volume flow through the cooling system, ambient air temperature drawn in by the cooling system, coolant return temperature and / or fuel cell utilization rate recorded and used to determine the first fan speed. [3] Method according to one of the preceding claims, characterized by that the first fan speed is calculated during the cooling process. [4] Method according to one of the preceding claims, characterized by that the first fan speed during the cooling process is taken from a stored parameter field. [5] Method according to one of the preceding claims, characterized by that the process steps a) to e) are carried out cyclically. [6] Method according to one of claims 1-4, characterized bythat process step a) is carried out continuously and process steps b) - d) are only repeated if an operating parameter determined in step a) has changed by a predetermined deviation. [7] Method according to one of the preceding claims, characterized by that the coolant circuit (1) and the cooling system (21) are connected via a 3-way valve and the cooling system is disconnected when cooling of the coolant is not required. [8] Device for cooling a fuel cell with a fuel cell (12) arranged in a coolant circuit (1), the coolant of which can be cooled in a cooling system (21) by means of a fan (22), with a temperature sensor (11) for detecting an actual flow temperature in the coolant circuit, with a detection unit (31) for detecting at least one operating parameter of the coolant circuit, with a computing unit (32) for determining a first fan speed using the at least one operating parameter, with a control unit (33) for setting the fan speed to the first fan speed, with a control unit (34) for controlling the flow temperature of the coolant to a target flow temperature via the fan speed as a control variable. [9] Device according to the preceding claim, characterized by that the coolant circuit comprises an internal coolant circuit (1) with the fuel cell (12), and the cooling system is connected to the coolant circuit (1) via a 3-way valve. [10] Vehicle, in particular rail vehicle with a device according to one of claims 8 to 9.

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