Method for operating a multi-stack system, and control device

Abstract

The invention relates to a method for operating a multi-stack system (9) comprising a plurality of stacks (4.1, 4.2) and a cooling system (1) for dissipating the waste heat of the stacks (4.1, 4.2), wherein, in the event of a start, in particular in the event of a cold start and / or a frozen start, an electrical power (P) that is required at a defined time (ttarget) and that exceeds the maximum electrical power (Pmax) of an individual stack (4.1, 4.2) is provided by a plurality of stacks (4.1, 4.2) by virtue of the stacks (4.1, 4.2) being thermally coupled via the cooling system (1) and the waste heat of an already heated first stack (4.1) being used to heat at least one further stack (4.2), and wherein the following steps are carried out in order to ensure an electrical power (PStack2) of the at least one further stack (4.2) at the time (ttarget): a) determining a minimum temperature (Tmin) required by the at least one further stack (4.2) in order to provide an electrical power (PStack2), b) determining a heating rate (dPStack2 / dt) required by the at least one further stack (4.2) in order to reach the minimum temperature (Tmin) at the time (ttarget), and c) adapting at least one operating parameter of the cooling system (1) in order to ensure the required heating rate. The invention also relates to a control device for a multi-stack system (9) for carrying out steps of a method according to the invention.

Description

[0001] Description

[0002] title

[0003] Method for operating a multi-stack

[0004] The invention relates to a method for operating a multi-stack system having the features of the preamble of claim 1. Furthermore, the invention relates to a control device for a multi-stack system for executing steps of the method.

[0005] The preferred area of ​​application is fuel cell vehicles, preferably commercial vehicles with a high electrical power requirement.

[0006] State of the art

[0007] Hydrogen-based fuel cells convert hydrogen and oxygen into electrical energy, heat, and water. The hydrogen is fed to an anode, and the oxygen to a cathode of the fuel cells.

[0008] In practice, several, perhaps several hundred, fuel cells are arranged one above the other and connected to form a fuel cell stack. The stack is supplied with the required reaction gases via a hydrogen system on the anode side and an air system on the cathode side. The heat generated in the stack during the electrochemical reaction is dissipated via a cooling system. The systems or subsystems required to supply the media to a stack, together with the stack, form a fuel cell system.

[0009] To increase electrical output, several stacks can be interconnected or coupled to form a so-called "multi-stack system." The media supply can be provided via shared or separate subsystems. If a cooling system is used to dissipate the waste heat of multiple stacks, these are usually thermally coupled via the cooling system. Thermal coupling enables one stack to be warmed up using the waste heat of another stack that is already operational, for example, during a cold or freeze-start of the system. Using the waste heat from a first stack to warm up another stack saves electrical energy and consequently hydrogen, since the energy required for warming up must first be generated using the fuel cell system.

[0010] A cooling system for a multi-stack system that enables thermal coupling of at least two stacks is shown as an example in Figure 1. The cooling system 1 shown has a cooling circuit 2, through which a coolant is circulated with the help of two coolant pumps 3.1, 3.2. The coolant pumps 3.1, 3.2 are assigned to two stacks 4.1, 4.2 that are integrated into the cooling circuit 2 via a directional control valve 5. Downstream of the stacks 4.1, 4.2, a radiator 6 is integrated into the cooling circuit 2, through which the heat absorbed by the coolant in a stack 4.1, 4.2 is dissipated to the environment. If the heat is to be retained entirely or partially in the system, for example in order to bring the stacks 4.1, 4.2 to operating temperature during a cold or freezing start, a bypass path 8 that bypasses the radiator 6 can be opened via a further directional control valve 7. Depending on the switching position of the directional control valves 5, 7, the waste heat of one stack 4.1, 4.2 can be used to warm up the other stack 4.2, 4.1.

[0011] The performance of a stack is temperature-dependent. This relationship is illustrated in the diagram in Figure 2, where the temperature T is plotted on the x-axis and the power P on the y-axis. To achieve a minimum power P min To achieve this, a minimum temperature T min , because at temperatures below this temperature, the material wall decomposition in the fuel cells occurs too slowly. The stack performance is therefore limited by the kinetics of the stack. In contrast to other performance-limiting effects, such as a gas shortage with a time constant of approximately one second, this kinetic limitation cannot be lifted quickly. To overcome the kinetic limitation, the stack must be heated to the minimum temperature T min warmed up, which usually takes several seconds to a few minutes.

[0012] The present invention addresses the task of accelerating the warm-up of stacks in a multi-stack system in order to achieve the required, possibly full, system performance in the shortest possible time. At the same time, energy and hydrogen consumption should be kept to a minimum.

[0013] To achieve this objective, the method having the features of claim 1 is proposed. Advantageous embodiments are set forth in the subclaims. Furthermore, a control unit for a multi-stack system for executing steps of the method is specified.

[0014] Disclosure of the invention

[0015] A method is proposed for operating a multi-stack system comprising multiple stacks and a cooling system for dissipating the waste heat from the stacks. During a start-up, particularly during a cold and / or freezing start, an electrical power required at a defined time ttarget and exceeding the maximum electrical power of a single stack is provided by multiple stacks by thermally coupling the stacks via the cooling system and using the waste heat of an already warmed-up first stack to warm up at least one additional stack.To ensure an electrical output of the at least one further stack at time ttarget, the following steps are carried out: a) determining a minimum temperature required by the at least one further stack to provide an electrical output, b) determining a warm-up rate required by the at least one further stack to reach the minimum temperature at time ttarget, and c) adjusting at least one operating parameter of the cooling system to ensure the required warm-up rate.

[0016] The proposed method enables higher system dynamics and higher system efficiency during start-up.

[0017] The increased system dynamics result from the fact that the process takes into account the kinetic limitations of the stacks in the multi-stack system. This is because the process ensures that at the defined time ttarget, at least one additional stack is warmed up enough to generate electrical power in addition to the electrical power of the first stack. The total electrical power generated by the stacks enables the increased power demand to be met in a comparatively short time and without derating.

[0018] The higher system efficiency is achieved by using the waste heat of an already warmed-up first stack to warm up at least one additional stack.

[0019] According to a preferred embodiment of the invention, in step a), the minimum temperature of the at least one additional stack is determined using a model and / or a characteristic curve. The model or characteristic curve can be stored in a control unit of the multi-stack system, so that the minimum temperature can be determined very easily.

[0020] Furthermore, it is proposed that in step b), the warm-up rate for warming up the at least one additional stack is determined using a model and / or a characteristic curve. Here, too, the model or characteristic curve can be stored in a control unit of the multi-stack system and thus made available.

[0021] The warm-up rate indicates the ratio of available heat to thermal mass in a given time. The warm-up rate required to meet a specific power demand thus depends in particular on the available heat and the time remaining until the time ttarget is reached. If the remaining time cannot be influenced, the required warm-up rate can be achieved by increasing the available heat. This is achieved in the proposed method by adjusting at least one operating parameter of the cooling system in step c).

[0022] In step c), for example, the speed of at least one coolant pump in the cooling system can be adjusted to the required warm-up rate. This is because the speed of the at least one coolant pump can influence the heat supplied to the at least one other stack for warm-up and thus the warm-up rate.

[0023] Alternatively or additionally, it is proposed that in step c), the switching position of at least one directional control valve integrated into a cooling circuit of the cooling system for distributing a coolant circulating in the cooling circuit to the stacks and / or for bypassing a radiator downstream of the stacks is adjusted to the required warm-up rate. The switching position of the at least one directional control valve can also influence the available heat and thus the warm-up rate.

[0024] Within certain limits, the time remaining for warming up the at least one additional stack can also be influenced. This is because this depends not only on the time ttarget, but also on the time required to warm up the first stack. To influence this, it is proposed that the first stack be warmed up using its own heat and / or external heat. Warming up using its own heat is particularly energy-efficient, but generally requires more time. To shorten the warm-up time, the first stack can be warmed up using external heat, for example, with the help of an electric heater. The electric heater requires electrical energy, which is usually provided by a battery. Since the electrical energy was previously generated using the multi-stack system, the operation of the electric heater has a negative impact on system efficiency.

[0025] To achieve the required warm-up rate, further measures can be taken, which are described below.

[0026] A first measure provides for the operating temperature of the first stack to be temporarily lowered while the at least one additional stack is warming up. The operation of multiple stacks in a multi-stack system is most efficient when the stacks are operated at similarly high temperatures. By temporarily lowering the operating temperature of the first stack, the difference to the temperature of the at least one additional stack can be reduced and efficiency increased. The additional heat thus introduced into the coolant of the cooling system can, in turn, be used to quickly warm up the at least one additional stack.

[0027] A second measure involves temporarily reducing the efficiency of the first stack in favor of increased waste heat production during the warm-up of at least one additional stack. Since this measure has a negative impact on efficiency and hydrogen consumption, it is preferably only applied if the previously described measures do not lead to the required warm-up rate.

[0028] A third measure involves derating, which means that the power demand is reduced if it can be predicted that the electrical power available at the target time is lower than the originally required electrical power. Since derating should generally be avoided, this third measure should also be considered secondary.

[0029] Furthermore, a control unit for a multi-stack system is proposed. The control unit is configured to execute steps of a method according to the invention. In particular, at least one model and / or a characteristic curve for executing steps a) and / or b) of the method according to the invention can be stored in the control unit. Furthermore, step c) can be executed with the aid of the control unit by appropriately controlling the at least one coolant pump of the cooling system and / or actuating the at least one directional control valve integrated into the cooling circuit of the cooling system.

[0030] The invention and its advantages are explained in more detail below with reference to the accompanying drawings. These show:

[0031] Figure 1 is a schematic representation of a cooling system for a multi-stack system, through which two stacks are thermally coupled,

[0032] Figure 2 is a diagram showing the dependence of the power P of a stack on the temperature T,

[0033] Figure 3 a) to c) each show a diagram illustrating the power P over time t (a), the temperature T over time t (b) and the heat balance (c) during a start of a multi-stack system with two stacks and

[0034] Figure 4 is a flow chart illustrating a preferred sequence of a method according to the invention.

[0035] Detailed Description of the Drawings Since Figures 1 and 2 have already been referred to in the introduction to the description, reference is made to the introduction to avoid repetition. Therefore, the following description begins with Figure 3.

[0036] Figure 3 contains three diagrams, all of which relate to the start of a multi-stack system 9 with two stacks 4.1, 4.2 and a cooling system 1 analogous to Figure 1. This means that the two stacks 4.1, 4.2 are thermally coupled or can be thermally coupled via the cooling system 1.

[0037] Starting the multi-stack system 9 requires warming up the stacks 4.1, 4.2, whereby the first stack 4.1 is warmed up or conditioned first. From time h, the first stack 4.1 supplies an electrical power P (see Figure 3a)), so that the waste heat of the first stack 4.1 can now be used to warm up the second stack 4.2 (see Figures 3b) and 3c)). At a time ttarget, a power P aii stacks is required that is greater than the maximum power Pmax.stacki of the first stack 4.1 (see Figure 3a). This means that in order to cover the power requirement at time ttarget, the second stack 4.2 must be warmed up sufficiently so that it also produces electrical power (see Figures 3a and 3b). To achieve this, the method according to the invention can be used. A preferred embodiment of the method according to the invention is described below with reference to Figure 4.

[0038] The method according to the invention is carried out to start a multi-stack system 9. First, a first stack 4.1 is warmed up so that, with the aid of the waste heat from the first stack 4.1, a further stack 4.2 can be warmed up in order to provide a required electrical power at a time ttarget that exceeds the maximum electrical power of the first stack 4.1. This is where the steps of a method according to the invention illustrated in Figure 4 begin.

[0039] In a step 10, the electrical power required at time ttarget is first determined. Then, in a step 11, the minimum temperature of the additional stack 4.2 required to produce electrical power is determined using a model and / or a characteristic curve. In a further step 12, the stacks 4.1, 4.2 are thermally coupled via a cooling system 1, so that in a step 13 the waste heat of the first stack 4.1 can be used to heat up the additional stack 4.2. In a step 14, the available heat and the time remaining until time ttarget then determine the warm-up rate required to bring the additional stack 4.2 to the minimum temperature within the remaining time. Only then will the additional stack 4.2 also produce electrical power. Then, in a step 15, a check is carried out to determine whether the waste heat of the first stack 4.1 is sufficient to heat the additional stack 4.2 to the minimum temperature within the remaining time. If the test result is positive ("+"), the electrical power required at time ttarget is delivered in a further step 16. If the test result is negative. At least one further measure must be taken to achieve the set goal.

[0040] As a further measure, the operating temperature of the first stack 4.1 can be temporarily lowered in step 17, for example, by increasing the coolant flow through the first stack 4.1. As a result, more heat is introduced into the cooling system 1, which can be used to warm up the additional stack 4.2. Since more heat is available, a new warm-up rate is determined in step 18. In a subsequent step 19, a check is then performed again to determine whether the available heat is sufficient to bring the additional stack 4.2 to the minimum temperature within the remaining time. If the result of the check is positive ("+"), step 16 or the initially defined target is reached. If the result of the check is negative at least one further measure must be taken.

[0041] The further measure may consist of temporarily reducing the efficiency of the first stack 4.1 by adjusting the cathode operating parameters in step 20. This results in the first stack 4.1 generating more waste heat. Since more heat is available, a new warm-up rate is determined in step 21. Subsequently, in step 22, a check is carried out to determine whether the available heat is sufficient to heat the additional stack 4.2 to the minimum temperature within the remaining time. If the result of the check is positive, the procedure ends with step 16. If the result of the test is negative The initially defined goal can only be achieved with the help of a

[0042] Step 23, which provides for derating.

Claims

Claims 1. A method for operating a multi-stack system (9) comprising a plurality of stacks (4.1, 4.2) and a cooling system (1) for dissipating the waste heat of the stacks (4.1, 4.2), wherein in the case of starting, in particular during a cold and / or freezing start, an electrical power (P) required at a defined time (target) and exceeding the maximum electrical power (Pmax) of an individual stack (4.1, 4.2) is provided by a plurality of stacks (4.1, 4.2) by thermally coupling the stacks (4.1, 4.2) via the cooling system (1) and using the waste heat of an already warmed-up first stack (4.1) to warm up at least one further stack (4.2), and wherein the following steps are carried out to ensure an electrical power (Pstaciö) of the at least one further stack (4.2) at the time (target): a) determining a Minimum temperature (T min) which the at least one further stack (4.2) requires to provide an electrical power (Pstack2), b) determining a warm-up rate (dPstack dt) which the at least one further stack (4.2) requires to reach the minimum temperature (T min ) at the time (target) required and c) adjusting at least one operating parameter of the cooling system (1) to ensure the required warm-up rate.

2. Method according to claim 1, characterized in that in step a) the minimum temperature (T min ) is determined using a model and / or a characteristic curve.

3. Method according to claim 1 or 2, characterized in that in step b) the warm-up rate (dPstack dt) is determined with the aid of a model and / or a characteristic curve.

4. Method according to one of the preceding claims, characterized in that in step c) the speed of at least one coolant pump (3.1, 3.2) of the cooling system (1) is adapted to the required warm-up rate.

5. Method according to one of the preceding claims, characterized in that in step c) the switching position of at least one directional control valve (5, 7) integrated in a cooling circuit (2) of the cooling system (1) for distributing a coolant circulating in the cooling circuit (2) to the stacks (4.1, 4.2) and / or for bypassing a radiator (6) connected downstream of the stacks (4.1, 4.2) is adapted to the required warm-up rate.

6. Method according to one of the preceding claims, characterized in that the first stack (4.1) is heated by means of its own heat and / or external heat.

7. Method according to one of the preceding claims, characterized in that the operating temperature of the first stack (4.1) is temporarily lowered during the warming up of the at least one further stack (4.2).

8. Method according to one of the preceding claims, characterized in that the efficiency of the first stack (4.1) is temporarily reduced in favor of increased waste heat production during the warm-up of the at least one further stack (4.2).

9. Method according to one of the preceding claims, characterized in that the power requirement is reduced if it can be recognized by prediction that the electrical power available at the time (ttarget) is below the originally required electrical power (P).

10. Control device for a multi-stack system (9) which is configured to carry out steps of a method according to one of claims 1 to 9.

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