DE102022200319A1 - Method for operating a fuel cell system, control unit - Google Patents

Abstract

The invention relates to a method for operating a fuel cell system (1), comprising a fuel cell stack (2) which is connected to a cooling circuit (3) with an integrated coolant pump (4). According to the invention, when the fuel cell system (1) is started, the coolant pump (4) is operated at an initial starting speed (nini), which is selected as a function of a currently measured and/or modeled temperature (Tini), preferably the coolant temperature a control unit for a fuel cell system (1).

Description

The invention relates to a method for operating a fuel cell system, comprising a fuel cell stack which is connected to a cooling circuit with an integrated coolant pump. The method is particularly suitable for operating fuel cell systems in mobile applications.

In addition, the invention relates to a control device that is set up to carry out steps of the method.

State of the art

A fuel cell converts a fuel, such as hydrogen, and oxygen into electrical energy, heat, and water. To increase the output, a large number of fuel cells are usually connected to form a fuel cell stack, the so-called "stack", and supplied with the reaction gases via supply channels running through the fuel cell stack. The heat generated during the electrochemical process in the fuel cells is dissipated to the environment with the help of a cooling circuit and a cooler integrated into it - in mobile applications usually the vehicle cooler. A further component of the cooling circuit is a coolant pump which, during operation of the fuel cell system, pumps the coolant through the coolant supply channels running through the fuel cell stack. A directional control valve can be integrated into the cooling circuit to bypass the cooler. Bypassing the radiator can be advantageous, for example, when starting. This is because when starting, in particular at ambient temperatures below 0° C., the fuel cell stack should be heated up as quickly as possible in order to avoid accumulations of water and/or ice, which could delay or even prevent the start.

However, the danger of icing is only averted if the coolant has been safely warmed above 0°C before it enters the fuel cell stack.

When starting the fuel cell system, the coolant mass flow through the fuel cell stack must be high enough to avoid local temperature peaks, so-called "hot spots". On the other hand, the coolant mass flow must be low enough to prevent the temperature from dropping too much and possibly icing up when entering the fuel cell stack. The coolant mass flow is controlled via the speed of the coolant pump.

In the case of a freeze start in particular, the initial starting speed of the coolant pump is not sufficiently defined. The invention tries to remedy this. In particular, the invention is intended to make it possible to determine the optimum possible starting speed of the coolant pump, specifically before the system switches to regulated operation.

To solve the problem, the method with the features of claim 1 is specified. Advantageous developments of the method can be found in the dependent claims. Furthermore, the control device with the features of claim 10 is proposed.

Disclosure of Invention

A method is proposed for operating a fuel cell system that includes a fuel cell stack that is connected to a cooling circuit with an integrated coolant pump. According to the invention, when the fuel cell system is started, the coolant pump is operated at an initial starting speed, which is selected as a function of a currently measured and/or modeled temperature, preferably the coolant temperature.

In the proposed method, the initial starting speed of the coolant pump is therefore varied depending on the temperature. Since the viscosity of the coolant depends on the temperature, this is taken into account when setting the starting speed. This is important because the viscosity of the coolant affects the coolant mass flow through the fuel cell stack as well as the pressure differential across the fuel cell stack. With the help of the proposed method, the starting speed of the coolant pump can therefore be set in such a way that a previously defined initial pressure difference is reached.

At a given temperature, the pressure difference correlates with the coolant mass flow. The required coolant mass flow can also depend on the temperature, but will only exactly compensate for the temperature dependence of the initial pressure difference to be achieved in special cases. An exact setting of the required coolant mass flow when starting the fuel cell system is not possible using the proposed method, but it helps to avoid major deviations.

When carrying out the method, the initial starting speed is preferably selected from at least two different initial starting speeds, each of which is associated with a specific temperature. The assignment enables the initial starting speed of the coolant pump to be determined first. If this is inaccurate, so the advance defined initial pressure difference is not reached, the starting speed can be adapted.

A higher initial starting speed is preferably selected, the lower the currently measured and/or modeled temperature is. Because the lower the temperature, the higher the viscosity of the coolant. This means that the required pressure difference and/or the required coolant mass flow can only be achieved with a higher starting speed.

Furthermore, a temperature range, for example -40° C. to +60° C., and temperatures within the temperature range, for example in increments of 5 Kelvin, are preferably defined, each of which is assigned a starting speed. In this way, the initial starting speed can be determined immediately.

In order to optimize the initial starting speed, it is proposed that the selected initial starting speed be adapted if an initial pressure difference of the coolant pressure across the fuel cell stack is measured, which deviates from an initial pressure difference associated with the temperature. Therefore, the temperatures of a previously determined temperature range are preferably assigned not only an initial starting speed, but also an initial pressure difference. This can then be used to check whether the starting speed leads to the desired result or whether an adaptation is required.

Alternatively or additionally, it is proposed that the selected initial starting speed is adapted if an initial pressure difference of the coolant pressure across the fuel cell stack is measured, which lies outside a permissible pressure difference range that is defined by a predetermined maximum value and a predetermined minimum value. In this case, the adaptation of the starting speed can be limited to gross deviations of the measured initial pressure difference from the initial pressure difference to be achieved. In the other cases, the selected initial start speed is retained.

When the predetermined maximum value is exceeded, the starting speed of the coolant pump is preferably reduced, with the reduction preferably being carried out in steps.

Furthermore, when the value falls below the predefined minimum value, the starting speed of the coolant pump is preferably increased, with the increase preferably being carried out in steps.

The initial starting speed of the coolant pump is advantageously selected with the aid of a control unit in which at least two starting speeds are stored, each of which is associated with a specific temperature. The process can be automated in this way. However, this assumes that the coolant pump can be controlled via the control unit. Furthermore, a currently measured or modeled temperature, in particular coolant temperature, is to be made available to the control unit.

With the help of the control unit, the initial starting speed can also be adapted. For this purpose, at least two initial pressure differences can be stored in the control unit, each of which is assigned to a specific temperature. As an alternative or in addition, a maximum value and a minimum value can be stored in the control unit to define a permissible pressure difference range. If the measured initial pressure difference is outside the permissible pressure difference range, the initial starting speed is adapted with the help of the control unit. In this case, it is not necessary to respond to every deviation by adapting the starting speed.

With the help of the proposed method, a starting speed of the coolant pump that is as optimal as possible can be defined initially, that is to say in controlled operation, even before the fuel cell system changes over to regulated operation. After a certain time, ie in controlled operation, the temperatures of the coolant in the area where the fuel cell stack enters and in the area where it exits the fuel cell stack can then be used to regulate the pump speed.

In addition, a control unit for a fuel cell system is proposed. The control unit is set up to carry out steps of a method according to the invention. In particular, at least one temperature value and a starting speed assigned to the temperature value can be stored in the control unit. The initial starting speed of the coolant pump can then be selected with the aid of the control unit as a function of a currently measured and/or modeled temperature.

Furthermore, at least one initial pressure difference, which is assigned to the temperature and which has to be reached, can be stored in the control unit. In the event of a deviation, the starting speed can then be adapted accordingly.

Furthermore, a permissible pressure difference range, which is defined by a maximum value and a minimum value, can be stored in the control unit. If the measured pressure difference is outside This range can be corrected by adapting the starting speed.

• 1 eine schematische Darstellung eines mobilen Brennstoffzellensystems, das zur Durchführung eines erfindungsgemäßen Verfahrens geeignet ist,
• 2 ein Diagramm zur graphischen Darstellung des Kühlmittelmassenstroms ṁ über der Druckdifferenz Δp bei unterschiedlichen Temperaturen T,
• 3 ein Diagramm zur graphischen Darstellung der Druckdifferenz Δp des Kühlmittels über der Drehzahl der Kühlmittelpumpe bei unterschiedlichen Temperaturen T,
• 4 ein Diagramm zur graphischen Darstellung der Viskosität η über der Temperatur T des Kühlmittels und
• 5 ein Blockschaltbild zur Darstellung des Ablaufs eines erfindungsgemäßen Verfahrens.

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

• 1 a schematic representation of a mobile fuel cell system that is suitable for carrying out a method according to the invention,
• 2 a diagram for the graphical representation of the coolant mass flow ṁ over the pressure difference Δp at different temperatures T,
• 3 a diagram for the graphical representation of the pressure difference Δp of the coolant over the speed of the coolant pump at different temperatures T,
• 4 a diagram for the graphical representation of the viscosity η over the temperature T of the coolant and
• 5 a block diagram to show the sequence of a method according to the invention.

Detailed description of the drawings

That in the 1 Fuel cell system 1 shown is used to generate electrical drive energy. For this purpose, the fuel cell system 1 comprises a fuel cell stack 2 with an anode 2.1 and a cathode 2.2. During operation of the fuel cell system 1, hydrogen is supplied to the anode 2.1 via an anode circuit 14 and air is supplied to the cathode 2.2 via an air supply path 20 as an oxygen supplier.

The hydrogen required by the anode 2.1 is stored in a tank 9 which can be shut off via a shut-off valve 10. Since hydrogen heats up when it is removed from the tank 9, a heat exchanger 11 for temperature control is provided downstream of the shut-off valve 10. The heat exchanger 11 is followed by a pressure regulator 12, with the help of which the pressure in the anode circuit 14 can be regulated. Since anode gas exiting from the fuel cell stack 2 still contains hydrogen, it is recirculated. In the present case, the recirculation is effected passively with the aid of a jet pump 13 and actively with the aid of a blower 15 . Since recirculated anode gas can contain liquid water, a water separator 16 is provided, which separates liquid water from the anode gas and collects it in a container 17 . When the container 17 is full, a drain valve 18 is opened and the container 17 is emptied. Recirculated anode gas also accumulates over time with nitrogen, which diffuses from the cathode side to the anode side. A purge valve 19, which is opened from time to time for this purpose, is provided for discharging the anode gas containing nitrogen. The amount of gas discharged via the purge valve 19 is replaced by hydrogen from the tank 9 .

The air required by the cathode 2.2 is taken from the environment and fed to an air compressor 22 via an air supply path 20 with an integrated air filter 21. Since the air heats up during compression, a heat exchanger 23 is provided downstream of the air compressor 22, which cools the air. The exhaust air emerging from the fuel cell stack 2 is discharged back to the environment via an exhaust air path 25 with an integrated shut-off valve 24 . In addition, a bypass path 26 with a bypass valve 27 arranged therein for bypassing the fuel cell stack 2 is provided.

During operation of the fuel cell system 1, the fuel cell stack 2 also generates heat in addition to electrical energy. The fuel cell stack 2 is connected to a cooling circuit 3 with an integrated coolant pump 4 in order to dissipate the heat. The coolant pump 4 is driven by an electric motor 5 . The coolant of the cooling circuit 3 releases the heat absorbed when flowing through the fuel cell stack 3 to a cooler 6, which is a vehicle cooler in the present case. To bypass the cooler 6 , a cooler bypass 7 is provided, which is opened via a directional control valve 8 .

When starting the fuel cell system 1, an initial starting speed n _ini of the coolant pump 4 must be specified. This should be selected so that the coolant mass flow ṁ is sufficiently high to avoid so-called "hot spots". At the same time, it should not be too high in order to avoid icing of the fuel cell stack 2--particularly in the case of a freeze start. The behavior of the coolant is strongly dependent on the temperature.

The 2 and 3 show the qualitative relationships between the mass flow ṁ, the pressure difference Δp and the starting speed n _ini at different starting temperatures T ₁ , T ₂ , T ₃ . 4 also shows the relationship between the viscosity η of the coolant and the temperature T. The higher the temperature T, the lower the viscosity η of the coolant. The starting speed n _ini of the coolant pump 4 can therefore vary and must be initially defined when starting.

When determining the initial starting speed n _ini , a temperature T _ini , in particular the coolant temperature, is taken into account according to the invention. The temperature T _ini can be measured or modeled become. The temperature T _ini is made available to a control unit in which a number of starting speeds n _ini are stored, each of which is assigned to a specific temperature T _ini . The following table shows an example of a possible assignment: _Tin [°C] n _ini [RPM] -40 80% -35 70% ... ... +60 10%

If the initial starting speed n _ini does not result in a specific pressure difference Δp _ini of the coolant across the fuel cell stack 2, the starting speed n _ini can be adapted. For this purpose, the actual pressure difference Δp _ini,meas can be measured and compared with a pressure difference Δp _ini assigned to the temperature T _ini . The following table shows a possible assignment: _Tin [°C] _Δpini [bar] -40 1.0 -35 0.9 ... ... +60 0.2

If the pressure difference Δp _ini assigned to the temperature T _ini is not reached, the starting speed n _ini can be adjusted or adapted accordingly.

Alternatively or additionally, a pressure difference range can be specified, which is limited by a maximum value Δp _ini,max and a minimum value Δp _ini,min . If the actually measured pressure difference Δp _ini,mess is outside of this range, the starting speed n _ini can also be adapted.

A method for adapting the starting speed n _ini over a pre-defined pressure differential range is based on the 5 described in more detail.

In step S1, when the fuel cell system 1 is started, an initial starting speed n _ini of the coolant pump 4 is selected as a function of a temperature T _ini . In step S2, the actual pressure difference Δp _ini,mess of the coolant across the fuel cell stack 2 is determined. If the pressure difference Δp _ini,mess is outside a defined pressure difference range, which is specified by a maximum value Δp _ini,max and a minimum value Δp _ini,min , the starting speed n _ini is adapted. In step S3 it is checked whether Δp _ini,mess > Δp _ini,max . If the answer is yes ("+"), the starting speed n _ini is reduced in step S4. If the answer is no, in step S5 it is checked whether Δp _ini,mess <Δp _ini,min . If the answer is yes ("+"), in step 6 the starting speed n _ini is increased. However, if it is determined in step S2 that the pressure difference Δp _ini,mess is within the defined pressure difference range, so that Δp _ini,min <Δp _ini,mess <Δp _ini,max applies, the initial starting speed n _ini can be retained in step S7.

Claims (10)

Method for operating a fuel cell system (1), comprising a fuel cell stack (2) which is connected to a cooling circuit (3) with an integrated coolant pump (4), characterized in that when the fuel cell system (1) is started, the coolant pump (4) is equipped with a initial starting speed (n _ini ) is operated, which is selected as a function of a currently measured and/or modeled temperature (T _ini ), preferably the coolant temperature. procedure after claim 1 , characterized in that the initial starting speed (n _ini ) is selected from at least two different initial starting speeds (n _ini ), each of which has a specific temperature (T _ini ) associated with it. procedure after claim 1 or 2 , characterized in that a higher initial starting speed (n _ini ) is selected, the lower the currently measured and/or modeled temperature (T _ini ). Method according to one of the preceding claims, characterized in that the selected initial starting speed (n _ini ) is adapted if an initial pressure difference (Δp _ini,mess ) of the coolant pressure across the fuel cell stack (2) is measured, which depends on one of the temperatures (T _ini ) associated initial pressure difference (Δp _ini ) and/or is outside a permissible pressure difference range which is defined by a predetermined maximum value (Δp _ini,max ) and a predetermined minimum value (Δp _ini,min ). procedure after claim 4 , characterized in that when the maximum value (Δp _ini,max ) is exceeded, the starting speed of the coolant pump (4) is reduced, the reduction preferably being carried out in steps. procedure after claim 4 or 5 , characterized in that when the minimum value (Δp _ini,min ) is undershot, the starting speed of the coolant pump (4) is increased, the increase preferably being carried out in steps. Method according to one of the preceding claims, characterized in that the initial starting speed (n _ini ) is selected with the aid of a control unit in which at least two starting speeds (n _ini ) are stored, each of which is associated with a specific temperature (T _ini ). procedure after claim 7 , characterized in that the initial starting speed (n _ini ) is adapted with the aid of the control unit, in which at least two initial pressure differences (Δp _ini ) are stored, each of which is associated with a specific temperature (Tini). procedure after claim 7 , characterized in that the initial starting speed (n _ini ) is adapted with the aid of the control unit, in which a maximum value (Δp _ini,max ) and a minimum value (Δp _ini,min ) are stored for defining a permissible pressure difference range. Control unit for a fuel cell system (1), the control unit being set up to carry out steps of a method according to one of Claims 1 until 9 to execute.

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