DE102022213171A1 - Method for operating an air conveying and air compression system, control device - Google Patents

Abstract

The invention relates to a method for operating an air conveying and air compression system (1), comprising a compressor (3) driven by a turbine (2) with a rotor supported by gas bearings. According to the invention, a diagnosis of the current state of the compressor (3), in particular of the gas bearings of the rotor, is carried out during a start and/or stop process by determining and evaluating at least one evaluation variable for the start and/or stop behavior of the compressor (3). The invention further relates to a control device for carrying out steps of the method.

Description

The invention relates to a method for operating an air conveying and air compression system, comprising a compressor driven by a turbine. Furthermore, the invention relates to a control device for carrying out the method or for carrying out steps of the method.

The preferred field of application of the invention is air systems for fuel cell systems, in particular for mobile fuel cell systems or fuel cell vehicles.

State of the art

Fuel cell systems require a reducing agent, usually hydrogen, and an oxidizing agent, usually oxygen. Air, which is taken from the environment, is usually used as the oxygen supplier. Hydrogen and oxygen are converted into electrical energy, heat and water in the fuel cells. In practice, a large number of fuel cells are combined to form a fuel cell stack.

Since the electrochemical reaction in the fuel cells requires a certain air mass flow and a certain pressure level, the air supplied to the fuel cells is first compressed using an air conveying and air compression system. This can be single- or multi-stage and single- or multi-flow. In addition, a turbine can be provided for energy recovery, to which the used air or exhaust air is fed. In single-stage air conveying and air compression systems, the turbine is connected to an electric motor drive of a compressor via a common shaft. In multi-stage, particularly two-stage, systems, a first stage can be driven by an electric motor and a second stage by a turbine. The turbine-driven stage can be connected upstream or downstream of the electric motor-driven stage. Multi-stage compression has the advantage that higher system pressures are possible.

Thermal flow machines are often used as compressors in an air conveying and air compression system. These have a rotor that is usually mounted so that it can rotate using two radial bearings and one axial bearing. In fuel cell systems, these bearings are designed as gas or air bearings to avoid oil. If the thermal flow machine falls below a minimum speed, the so-called take-off speed, solid friction occurs instead of gas friction, resulting in high stress on the bearings. This is usually the case when the thermal flow machine stops or runs down. The same applies when the thermal flow machine starts or runs up until the take-off speed is reached. This can lead to material build-up and/or bearing seizure. Other irreversible effects due to aging and wear, as well as reversible effects due to ice formation, can lead to the thermal flow machine not starting or only starting with a delay and/or showing conspicuous run-down behavior.

The present invention is concerned with the task of detecting damage, in particular bearing damage, of a turbine-driven compressor of an air conveying and air compression system as early as possible in order to prevent a failure of the compressor.

To solve the problem, the method with the features of claim 1 is proposed. Advantageous further developments of the invention can be found in the subclaims. In addition, a control device for carrying out steps of the method is specified.

Disclosure of the invention

A method is proposed for operating an air conveying and air compression system which comprises a compressor driven by a turbine with a rotor supported by gas bearings. According to the invention, a diagnosis of the current state of the compressor, in particular of the gas bearings of the rotor, is carried out during a start and/or stop process by determining and evaluating at least one evaluation variable for the start and/or stop behavior of the compressor.

Using the proposed method, a SOH (“State of Health”) diagnosis can be carried out, which in particular allows the wear condition of the gas bearings of the turbine-driven compressor to be identified. This is because as the wear condition increases, both the start and stop behavior of the compressor change.

According to the proposed method, the start and/or stop behavior of the compressor is evaluated during a start and/or stop process using at least one evaluation variable. Preferably, several evaluation variables are used, since these can differ depending on whether they are intended to evaluate the start behavior or the stop behavior.

The proposed method has the advantage that, based on the at least one The evaluation parameter allows the condition of the compressor, in particular its gas bearings, to be monitored. Any damage can be detected early so that measures can be taken to limit and/or repair the damage. For example, the operating strategy can be adjusted to reduce the load on the compressor, in particular its gas bearings. Alternatively or additionally, a workshop can be visited and/or a spare part can be obtained. In this way, a failure of the compressor can be reliably prevented, so that reliability and availability are increased. This applies in particular when the invention is used in mobile fuel cell systems, such as commercial vehicles.

• - aus der Drehzahl,
• - aus dem Druck und/oder Massenstrom am Eintritt der Turbine,
• - aus dem Druck und/oder Massenstrom stromaufwärts oder stromabwärts des Luftförder- und Luftverdichtungssystems und/oder
• - aus dem Differenzdruck über die Turbine,

ermittelt.The at least one evaluation variable for the start and/or stop behavior of the compressor is preferably derived from at least one sensor-detected variable, for example

• - from the speed,
• - from the pressure and/or mass flow at the turbine inlet,
• - from the pressure and/or mass flow upstream or downstream of the air conveying and compression system and/or
• - from the differential pressure across the turbine,

determined.

If a speed sensor is present, the speed can be recorded and used to determine the evaluation variable. Since there is usually at least one sensor in the air system for recording a thermodynamic variable, such as pressure or mass flow, these variables can also be used to determine the evaluation variable. Since existing sensors can be used, an additional sensor is not required.

During a start-up process, the starting point of the rotor and the conditions prevailing at this point can be determined from the speed and/or at least one thermodynamic variable. An evaluation variable for the start-up behavior can then be derived from these variables.

During a stopping process, the end of pure gas friction can be determined from the speed and/or at least one thermodynamic variable, for example. Pure gas friction ends when the rotor touches the bearing shells or the housing, i.e. when solid friction begins.

Preferably, the at least one evaluation variable for the start and/or stop behavior of the compressor is determined on a model-based basis. Furthermore, different models are preferably used to determine the evaluation variable for the start behavior and to determine the evaluation variable for the stop behavior.

• - für das Startverhalten das Losbrechmoment des Rotors oder mindestens eine mit dem Losbrechmoment zusammenhängende Größe und/oder
• - für das Stoppverhalten die Auslaufzeit des Rotors

ist.It is further proposed that at least one assessment variable

• - for the starting behaviour, the breakaway torque of the rotor or at least one value related to the breakaway torque and/or
• - for the stopping behaviour the run-down time of the rotor

is.

The breakaway torque is a measure of the friction at start-up when the speed is zero. The breakaway torque can be determined, for example, from the pressure at the turbine inlet, provided this is detected by a sensor. Alternatively or in addition, the differential pressure across the turbine and/or another thermodynamic quantity can be used to determine the breakaway torque.

Since the run-down time of a rotor shortens with increasing bearing wear, it represents an ideal evaluation parameter for the stopping behavior of the compressor.

Since the temperature can have a significant influence on the run-down time of a rotor, it is proposed in a further development of the invention that the temperature is recorded and taken into account when determining the evaluation variable for the stopping behavior of the compressor. If the evaluation variable is determined based on a model, the temperature can be taken into account as a further condition so that a temperature-compensated or temperature-normalized run-down time is obtained.

It is further proposed that the evaluation of the at least one determined evaluation variable for the start and/or stop behavior of the compressor includes a comparison with at least one reference value. In particular, a value of the evaluation variable determined when the compressor was new can serve as a reference value. The currently determined evaluation variable can also be compared with evaluation variables that were determined at an earlier point in time. This requires that the determined evaluation variables are saved.

Preferably, the at least one determined evaluation value is compared with at least one limit value. The at least one limit value is defined in advance and stored in a memory. If the at least one limit value is exceeded or not reached, a warning can be given optically and/or acoustically. Preferably, the at least one limit value defines an upper and/or lower limit of a non-critical operating range. If the limit value is reached, this signals the need for action. For example, the operating strategy can be adjusted and/or a workshop can be visited.

Advantageously, the at least one evaluation variable for the start and/or stop behavior of the compressor is recorded and evaluated over the entire running time of the air conveying and air compression system, preferably at each start and/or stop process of the air conveying and air compression system. The continuous monitoring enables control of the changes in status that occur over the running time and a forecast as to when a compressor failure is imminent. As a result, measures can be taken in good time to prevent the failure.

For continuous monitoring of the condition of the compressor and/or for forecasting purposes, preferably all sensor-detected variables for determining the at least one evaluation variable and/or the evaluation variables determined over the lifetime are stored in a memory, preferably in a memory of a control unit.

In addition, a control device is proposed which is set up to carry out steps of a method according to the invention. With the help of the control device, in particular, variables detected by sensors can be read in and evaluated. Furthermore, these variables can be stored. The evaluation can in particular include the determination of at least one evaluation variable for the start and/or stop behavior of the compressor from the variables detected by sensors. For this purpose, a model can be stored in the control device so that the determination of the at least one evaluation variable can be model-based. The evaluation can also include a comparison of the at least one determined evaluation variable with at least one reference and/or limit value. In this case, the at least one reference and/or limit value is stored in the control device.

• 1 eine schematische Darstellung eines Luftsystems mit einem ersten mehrstufigen Luftförder- und Luftverdichtungssystem, das nach einem erfindungsgemäßen Verfahren betrieben werden kann,
• 2 eine schematische Darstellung eines Luftsystems mit einem zweiten mehrstufigen Luftförder- und Luftverdichtungssystem, das nach einem erfindungsgemäßen Verfahren betrieben werden kann,
• Fug. 3 ein Diagramm zur Darstellung sensorisch erfasster Größen während eines Startvorgangs des Luftförder- und Luftverdichtungssystems,
• 4 ein Diagramm zur Darstellung sensorisch erfasster Größen während eines Stoppvorgangs des Luftförder- und Luftverdichtungssystems,
• 5 ein Diagramm zur Darstellung einer Bewertungsgröße für das Startverhalten über der Anzahl der Starts und
• 6 ein Diagramm zur Darstellung einer Bewertungsgröße für das Stoppverhalten über der Anzahl der Stopps.

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

• 1 a schematic representation of an air system with a first multi-stage air conveying and air compression system which can be operated according to a method according to the invention,
• 2 a schematic representation of an air system with a second multi-stage air conveying and air compression system, which can be operated according to a method according to the invention,
• Fig. 3 is a diagram showing sensor-detected variables during a start-up process of the air conveying and air compression system,
• 4 a diagram showing sensor-detected variables during a stop process of the air conveying and air compression system,
• 5 a diagram showing an evaluation value for the starting behaviour over the number of starts and
• 6 a diagram showing an evaluation of the stopping behavior over the number of stops.

Detailed description of the drawings

The 1 an air system can be seen which serves to supply a cathode 4 of a fuel cell stack 5 with air. Since the electrochemical reaction in the fuel cells requires a certain air mass flow, the air system comprises an air conveying and air compression system 1, with a first compression stage 1.1 and a second compression stage 1.2. The first compression stage 1.1 has a compressor 3 arranged in an air supply path 6, which is driven by an electric motor. The second compression stage 1.2 has a compressor 3 arranged downstream of the first compressor 3 in the air supply path 6, which is driven by a turbine 2. The turbine 2 is arranged in an exhaust air path 7, so that the air or exhaust air emerging from the fuel cell stack 5 is fed to it. In this way, part of the energy previously used for compression can be recovered.

The air is taken from the environment 8 and fed to the compressor 3 of the first compression stage 1.1 via an air filter 9. Since the air heats up considerably during compression, a heat exchanger 10 and/or a cooler 11 for intermediate cooling can be provided between the two compression stages 1.1, 1.2. Downstream of the second compression stage 1.2, a further heat exchanger 10 for passive cooling and/or a cooler 11 for active cooling can be arranged in the supply air path 6. In order to separate the cathode 4 from the air system, for example in the In the event of a shutdown, a shut-off valve 12 is provided in the supply air path 6 and in the exhaust air path 7. When the shut-off valves 12 are closed, a bypass valve 14 in a stack bypass 13 can be opened so that the supply air path 6 and the exhaust air path 7 are short-circuited. Compressed air is then supplied to the turbine 2 instead of exhaust air. A turbine bypass 15 with an integrated bypass valve 16 is provided to bypass the turbine 2. A pressure regulator 17 is arranged in the exhaust air path 7 downstream of the turbine 2.

The 2 Another air system for supplying a cathode 4 of a fuel cell stack 5 with air can be seen. This has the same components as the air system of the 1 but in a different arrangement. The same reference numerals indicate the same components. The difference to the system of 1 Essentially, the first and second compression stages 1.1, 1.2 are swapped.

The air conveying and air compression systems 1 of the 1 and 2 The air systems shown have in common that they each comprise a compression stage with a purely turbine-driven compressor 3, which has a rotor supported by gas bearings. Thus, both air conveying and air compression systems 1 can be operated according to a method according to the invention in order to monitor the condition of the compressor 3, in particular its gas bearings.

The diagram of the 3 shows exemplary measurements on a system test bench during a start-up process of a two-stage air conveying and air compression system 1. The turbine-driven compressor 3, hereinafter referred to as TAC, was started after the electric motor-driven compressor 3, hereinafter referred to as EAC. The respective speed curve is shown by the curves n, TAC and n, EAC.

The speed curve n, TAC shows the start time t1 of the TAC rotor, so that an evaluation variable for the start behavior of the air conveying and air compression system 1 can be determined from this. This applies analogously to the pressure p at the inlet of the turbine 2 and the differential pressure Δp,T across the turbine 2, so that - alternatively or additionally - an evaluation variable for the start behavior can also be derived from this. The pressure p at the inlet of the turbine 2 can, for example, be used to determine a breakaway torque. The breakaway torque, in turn, is a measure of the friction during start-up when the speed is zero. The evaluation variable shows the condition of the compressor 3, in particular its gas bearings, since the friction in the gas bearings increases during start-up with increasing wear.

The diagnosis can also be model-based, for example by using a TAC rotor-bearing model to determine an evaluation parameter for breakaway from the measured parameters.

In the 3 the further curve Δp,C shows the course of the differential pressure across the compressor 3 and the further curve ṁ shows the course of the mass flow.

The diagram of the 4 shows exemplary measurements on a system test bench during a stop process of a two-stage air conveying and air compression system 1, i.e. during free run-down of the rotor of the compressor 3. The speed curve of EAC and TAC are represented by the curves n, EAC and n, TAC.

The run-down time dt_Coast begins at time t1, when the EAC is switched off, and ends at time t2, when the TAC rotor is placed on the bearing shells or on the housing. Shortly before time t2, the pure gas friction ends and changes to solid friction. This point in time can be determined either from the speed n, TAC or from another sensor-detected variable, such as the pressure p at the turbine inlet, the mass flow ṁ, the differential pressure Δp,C across the compressor 3 of the TAC. If there is significant bearing wear, the run-down time dt_Coast is significantly shortened.

Alternatively, a model-based diagnosis can also be carried out here, i.e. an evaluation value for the run-out can be derived from the existing conditions using a TAC rotor-bearing model. For example, the existing temperatures could also be taken into account here, as these have a significant influence on the run-out behavior. In this way, temperature-compensated or temperature-normalized run-out times would be obtained.

The 5 and 6 each show by way of example how an evaluation value can be derived from a sensory value.

In the 5 the pressure p at the inlet of turbine 2 is used to determine an evaluation value, since - as already mentioned - this indicates the breakaway torque. The breakaway torque in turn is a measure of the friction during start-up when the speed is zero. The pressure p at the inlet of turbine 2 is plotted on the y-axis, the x-axis shows the number of start-ups that have been completed. In the diagram shown For example, a running-in behavior of the TAC can be seen, i.e. a rising curve or an increase in friction during the first starts. After that, an approximately constant behavior occurs. Since the friction parameters vary depending on the position of the rotor, there is also a scatter of the values. It is advantageous to average the measured values. Several limit values can be used for monitoring. A first limit value p,max1 can, for example, define an upper limit of a non-critical range, a further limit value p,max2 an upper limit of a permissible range.

6 shows the run-down time Δt_Coast over the individual stopping processes. The values shown are not temperature compensated. Limit values can be defined for the run-down time Δt_Coast, which in this case each define a lower limit. Δt_Coast, min1 can indicate the lower limit of a non-critical range and Δt_Coast, min2 can indicate the lower limit of a permissible range.

Claims (10)

Method for operating an air conveying and air compression system (1), comprising a compressor (3) driven by a turbine (2) with a rotor mounted via gas bearings, characterized in that during a start and/or stop process a diagnosis of the current state of the compressor (3), in particular the gas bearings of the rotor, is carried out by determining and evaluating at least one evaluation variable for the start and/or stop behavior of the compressor (3). Procedure according to Claim 1 , characterized in that the at least one evaluation variable is determined from at least one sensor-detected variable, for example - from the speed, - from the pressure and/or mass flow at the inlet of the turbine (2), - from the pressure and/or mass flow upstream or downstream of the air conveying and air compression system (1) and/or - from the differential pressure across the turbine (2). Procedure according to Claim 1 or 2 , characterized in that the at least one evaluation variable is determined on a model-based basis. Method according to one of the preceding claims, characterized in that the at least one evaluation variable - for the starting behavior is the breakaway torque of the rotor or at least one variable related to the breakaway torque and/or - for the stopping behavior is the run-down time of the rotor. Method according to one of the preceding claims, characterized in that the temperature is recorded and taken into account when determining the evaluation variable for the stopping behavior of the compressor (3). Method according to one of the preceding claims, characterized in that the evaluation of the at least one determined evaluation variable for the start and/or stop behavior of the compressor comprises a comparison with at least one reference value. Method according to one of the preceding claims, characterized in that the determined evaluation variable is compared with at least one limit value, which preferably defines an upper and/or lower limit of a non-critical operating range. Procedure according to Claim 7 , characterized in that if at least one limit value is exceeded, the operating strategy is adjusted and/or a workshop is visited. Method according to one of the preceding claims, characterized in that the at least one evaluation variable for the start and/or stop behavior of the compressor (3) is recorded and evaluated over the entire running time of the air conveying and air compression system (1). Control device configured to carry out steps of a method according to one of the preceding claims.

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