FUEL CELL SYSTEM AND MOTOR VEHICLE EQUIPPED WITH IT

Abstract

A fuel cell system comprises at least one fuel cell (3), a cooling circuit with a first heat exchanger (15) in thermal contact with the at least one fuel cell (3), a second heat exchanger (7) in thermal contact with the environment, and a pump (6) for circulating heat transfer fluid between the heat exchangers (15, 7), temperature sensors (13, 14) for detecting a temperature difference (dT). ist ) between the flow and return (11, 5) of the first heat exchanger (15) and a control loop (16) for controlling the flow rate (q) of the pump (6) based on the temperature difference (dT ist The control loop (16) is set up at low power (P). min ) the fuel cell a lower setpoint (dT) soll ) the temperature difference is determined as at high power (P max ).

Description

[0001] The present invention relates to a fuel cell system and a motor vehicle in which the fuel cell system supplies a drive motor directly or indirectly via a battery.

[0002] To be suitable for use in a motor vehicle, a fuel cell system must be able to meet highly variable power requirements. The waste heat generated in the fuel cell(s) of such a system during operation varies proportionally to the power output. Cooling must first and foremost ensure that a maximum operating temperature, above which the cell is damaged, is not exceeded, even at high power output. Attempting to achieve this by increasing the coolant flow rate proportionally to the cell's power output quickly leads to the problem that flow resistance increases significantly with the flow rate, and the proportion of the cell's power required to circulate the coolant becomes greater as the cell's power output increases.

[0003] Another possibility is to lower the coolant supply temperature as the power output increases. While this can prevent the cell from overheating even at moderate coolant flow rates, the consequence is that parts of the cell near a coolant inlet are cooled more intensely the higher the cell's power output. If condensation accumulates on the catalytically active surfaces in these areas, it hinders the reaction and promotes further cooling, resulting in more condensation. Over time, the portion of the cell where the reaction proceeds unimpeded decreases, and the cell's power output declines.

[0004] To prevent this, it has been proposed to monitor the temperature difference between the coolant supply and return lines of the fuel cell and to regulate the refrigerant flow rate so that the temperature difference closely matches a setpoint. This can prevent localized undercooling of the cell under most operating conditions, but a problem arises at low power levels: the effects of changing cell operating conditions on the return temperature are not immediately apparent, but only become visible when the refrigerant affected by these conditions leaves the cell. The time required for this to occur increases with lower flow rates. This makes precise temperature control of the cell at low power levels difficult.

[0005] The object of the present invention is to create a fuel cell system that enables precise temperature control even at low power, without having to accept the disadvantages of the prior art described above.

[0006] According to one aspect of the invention, the problem is solved by a fuel cell system with at least one fuel cell and a cooling circuit comprising a first heat exchanger in thermal contact with the at least one fuel cell, a second heat exchanger in thermal contact with the environment, a pump for circulating heat transfer fluid between the heat exchangers, temperature sensors for detecting a temperature difference between the supply and return of the first heat exchanger, and a control loop for controlling the flow rate of the pump based on the temperature difference, wherein the control loop is configured to set a lower setpoint for the temperature difference at low power of the fuel cell than at high power.Reducing the setpoint causes the control loop to specify a higher coolant flow rate, thus shortening the time interval between a change in the cell's operating conditions and its manifestation as a temperature change in the coolant return.

[0007] Preferably, the control loop is configured to set a consistently high target temperature difference for every power output of the fuel cell above a certain power threshold, and to set a lower target value as a function increasing with power output for power outputs below the threshold. This makes it possible, on the one hand, to set a very small target temperature difference at extremely low power outputs, thereby limiting the coolant residence time in the cell to an appropriate level, and on the other hand, to avoid an unnecessarily high coolant flow rate at power outputs just below the threshold, which would result from selecting too small a target temperature difference.

[0008] In general, the low target value of the temperature difference can be set in such a way that a predetermined minimum pump flow rate is required to maintain it.

[0009] The low setpoint should be greater than the measurement inaccuracy of the temperature sensors. This prevents a falsely high temperature difference measurement from causing the control loop to continuously increase the pump flow rate without maintaining the setpoint temperature difference, or a falsely low temperature difference measurement from leading to a pump flow rate so low that increasing the cell's power output could cause it to overheat before the power increase is observable as an increase in the temperature difference.

[0010] To enable rapid adaptation of the fuel cell system to changing operating conditions, a functional unit of the control loop can be implemented. This unit defines a control parameter, which it outputs to the pump to control its flow rate, as a function of the fuel cell's power output and a setpoint deviation of the temperature difference. Such a function can be empirically optimized based on the behavior of a prototype of the system.

[0011] To provide the setpoint deviation, the control loop can include an error observer that uses the measured temperature difference and the power-dependent setpoint of the temperature difference as inputs. An observer is a system known in control engineering that reconstructs quantities not directly measurable, such as temperature differences inside the fuel cell, from known inputs (here, the setpoint) and outputs of an observed reference system (such as temperatures measured at the inlet and outlet of a fuel cell heat exchanger).

[0012] Furthermore, a correction element can be connected between a setpoint deviation output of the observer and an input of the functional unit to apply a predefined correction term to the setpoint deviation. Such a correction element can be used to account for the case where the specifications of the fuel cell installed in the specific fuel cell system differ from those of the prototype used to optimize the function of the functional unit.

[0013] The correction term can be a function of the fuel cell's power output.

[0014] The invention further relates to a motor vehicle with a fuel cell system as described above.

[0015] Further features and advantages of the invention will become apparent from the following description of exemplary embodiments with reference to the accompanying figures. These show: Fig. 1 a block diagram of a fuel cell unit and its cooling system; Fig. 2 a block diagram of a cooling control system for fuel cells of the unit; Fig. 3 a target difference between coolant temperatures at the supply and return lines of a fuel cell as a function of its power output; and Fig. 4. The control of a pump of the cooling system depending on the power of the fuel cell and a deviation between the target difference and an actually measured difference.

[0016] Fig. Figure 1 shows, as a block diagram, the cooling circuit of a fuel cell assembly for a hydrogen-powered vehicle. One or more fuel cell stacks 1, 2, which can be electrically connected to each other in series or parallel, each contain several fuel cells 3 electrically connected in series. The stacks 1, 2 each have an anode-side inlet IA for hydrogen, which is connected to a storage tank (not shown), a cathode-side inlet IC for oxygen, which is connected to a compressor (not shown), and anode- and cathode-side outlets OA, OC for reaction products and residues. The individual fuel cells are connected in parallel between the pairs of inlets and outlets IA, OA and IC, OC.

[0017] Each cell 3 of stacks 1 and 2 integrates a heat exchanger 15. The heat exchangers 15 of a stack are connected in parallel to each other between the coolant inlets and outlets. Manifold lines 4 leading from the coolant outlets of stacks 1 and 2 converge to form a common return line 5, which leads to a suction port of a circulation pump 6. A cooler 7, a bypass line 8, and optionally one or more heat exchangers 9 are connected in parallel to each other to a pressure port of the circulation pump 6.

[0018] The coolant circulating in the coolant circuit is deionized water, possibly mixed with antifreeze.

[0019] The coolant flows via the radiator 7 and the bypass line 8 meet again at a three-way valve 10, run from there via a supply line 11 and distribution lines 12 to inlets of the stacks 1, 2 and are distributed from there again to the heat exchangers 15.

[0020] Temperature sensors 13, 14 arranged on the flow and return lines 11, 5 are each connected to a control loop 16, the structure and operation of which are described with reference to Fig. 2 will be explained in more detail.

[0021] The inputs of a differential circuit 17 are connected to the temperature sensors 13, 14, so that an output of the differential circuit 17 provides a signal dT representative of the temperature difference of the coolant in the supply and return lines 11, 5. ist delivers.

[0022] A functional unit 18 provides a setpoint value for the temperature difference dT for a current actual or target value of the power of the fuel cell 3. soll The functional unit 18 can be used as a reference table, in which values ​​of a relationship between the power of the fuel cell 3 and the setpoint of the temperature difference dT are given. sollThe function is stored as a descriptive function, or it can be implemented as a routine executed by a processor not specifically shown in the figure to calculate the function. According to a simple embodiment, the function can be piecewise linear; in particular, it can be as in Fig. 3 shown between a non-zero minimum power P min , which cannot be undercut without completely switching off fuel cell 3, and a power threshold P thr with an initial slope increasing linearly and reaching the power threshold P thr continuously transition into a second straight line, the slope of which is between the power threshold P thr and a maximum permissible power P max the cell has a second, smaller slope. This second slope can be zero, i.e., at a power output of cell 3 between the power threshold P thr and the maximum permissible power P max is dTsoll independent of the power output. Since the temperature difference dT soll Since the temperature difference (dT) reaches its highest value at high power, the amount of heat that the cooling fluid can absorb is large, and a moderate flow rate q of coolant is sufficient to achieve the setpoint of the temperature difference dT. soll to be able to comply.

[0023] The smallest value dT min , which is the setpoint of the temperature difference dT soll The value that can be assumed at low power is greater than the sum of the measurement inaccuracies of the temperature sensors 13, 14. This prevents the setpoint from being erroneously judged as being met in a situation where the coolant flow rate is zero and the measured values ​​of both sensors equalize to the ambient temperature, and the coolant flow rate remaining at zero even though waste heat is being generated inside cell 3.

[0024] To ensure that the waste heat generated in such a situation is detected quickly enough by the return-side sensor 14 to prevent critical overheating of cell 3, the value dT min Furthermore, it is chosen such that the residence time of the coolant in the heat exchangers 15 does not exceed a suitably predetermined value dt, typically a few seconds. If V is the flow volume of the heat exchangers 15 and of the coolant line sections extending between the heat exchangers 15 and the temperature sensors 13 and 14, respectively, and p is the specific heat capacity of the coolant per unit volume, then Q = pV is the specific heat capacity of the coolant located between the temperature sensors 13 and 14. If η denotes the efficiency of the fuel cell, then its waste heat output is at least (1 - η)P. min So that the difference between the actual flow and return temperatures matches the setpoint dT minThe time interval dt required for the cooling fluid to traverse the fuel cells 3 must not exceed ρV*dT. min / (1-η)P min , or better yet, not larger than ρV*(dT min -e) / (1-η))P min , where e denotes the sum of the inaccuracies of sensors 13, 14. Therefore, if dt is given, dT is given by min the requirement dTmin>(1−η)Pmin dt / ρV or, taking into account the inaccuracies e: dTmin>e+(1−η)Pmin dt / ρV.

[0025] A further differential circuit 19 is provided to measure the deviation dT. err between the temperature difference dT determined by the differential circuit 17 ist and the setpoint dT soll to determine this deviation dT. errcan serve directly as an input variable for a second functional unit 20, which will be described in more detail later; in the example considered here, a controller 21 is inserted between the two, e.g., an observer programmed to use the temperature difference dT to ist and the deviation dT err to determine an optimized deviation that replaces dT err can be passed on to functional unit 20 to achieve a faster reduction of the deviation, or a PI or PID controller that is known in itself.

[0026] The functional unit 20 implements - also computationally or using a table - a function that determines the deviation dT depending on the deviation (possibly optimized by the observer 21). err and the target flow rate q to be output to the pump 6 is specified for the power of the fuel cell 3. Fig.Figure 4 shows a typical curve of the function in a 3D diagram. The deviation dT is shown on the horizontal axes of the diagram. err and the power P of fuel cell 3 is plotted (assuming here that P min (is close enough to zero to be indistinguishable from zero on the scale of the diagram). Support points of the function for equal values ​​of power P are interpolated by straight line segments; the resulting curves, similar to the well-known sigmoid function, each have a maximum slope close to dT. err =0 and approach each other for negative values ​​of dT err a throughput q of zero or for positive dT err the maximum flow rate (100%) of the circulation pump 6; furthermore, the values ​​of q increase with increasing power P, from one curve to the next, which reflects the overall increasing cooling requirement with power P. The exact shape of the function q(P, dT) err) is optimized using a prototype of the fuel cell system, taking into account specifications from a fuel cell manufacturer regarding the target value of the temperature difference dT soll as well as pump properties, pipe geometries and other system specifics, to enable the fastest possible correction of an observed setpoint deviation dT. err to enable.

[0027] During the series production of a motor vehicle model, it may happen that, due to a lack of availability of the type of fuel cell for which the function of functional unit 20 has been optimized, fuel cells with a different specification for the temperature difference dT are used. soll must be installed. To facilitate the use of fuel cells with differing specifications, a correction element 22 can be connected upstream of the input of the functional unit 20, which is used to adjust the setpoint deviation dT (possibly optimized by the controller 21). erra correction value dT ref added.

[0028] In the simplest case, such a correction value could be a constant selected to suit the type of fuel cell being installed. Preferably, however, it is – just like dT – soll - specified as a function of the fuel cell's power output. A third functional unit 23 is provided here to implement such a function; its output and that of functional unit 18 are connected to inputs of a differential circuit 24. As long as the type of fuel cell is used for which functional unit 20 is optimized, the same function is selected for functional unit 23 as is implemented by functional unit 18. In this case, the output of differential circuit 24 is a constant zero, and the control loop behaves exactly as if components 22, 23, 24 were not present and dT errdirectly at the input of functional unit 20. If the functions of functional units 18 and 23 differ, their difference, which varies depending on the power P, is always present at the input of the correction element 22.

[0029] The control loop 16, based on the return temperature 5 measured by sensor 14, also controls the position of the three-way valve 10 in a manner known per se. If the return temperature threatens to rise above a permissible maximum, the measured temperature difference dT ist but the target value dT sollIf the required flow rate is reached, the proportion of cooling fluid from cooler 7 relative to that from the bypass line must be increased at the three-way valve 10, thus decreasing the supply and return temperatures. The temperature difference between them remains unaffected. Conversely, the proportion of cooling fluid from cooler 7 is reduced if the return temperature is too low. The time constant for these processes is longer than for adjusting the pump flow rate q in order to limit interactions between the two control processes. Reference sign 1 fuel cell stack 2 fuel cell stacks 3 Fuel cell 4 Collective line 5 Return 6 Circulation pump 7 coolers 8. Side line 9 heat exchangers 10 Three-way valve 11 preliminary round 12 distribution line 13 Temperature sensor 14 Temperature sensor 15 heat exchangers 16 Control loop 17 Differential circuit 18 Functional unit 19 Differential circuit 20 functional units 21 regulators 22 Correction element 23 Functional unit 24 Differential circuit

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