DE102021207396A1 - Method for operating a turbomachine and control unit - Google Patents

Abstract

The invention relates to a method for operating a turbomachine (4), in particular a gas-bearing thermal turbomachine (4), in which the turbomachine (4) is operated in an operating range (A) which is defined on the one hand by a surge limit (P) and on the other hand by a The choke limit (S) is limited, and in which, to expand the operating range (A) by an operating range (B) beyond the surge limit (P), the following steps are carried out to determine the surge limit (P) or at least one limit point lying on the surge limit (P). are: a) throttling of the mass flow (m) at a constant speed (n) of the turbomachine (4), so that the pressure ratio (pi) increases until a maximum pressure ratio (pimax) is reached and exceeded, b) recording and storing the maximum Pressure ratio (pimax) and the associated mass flow (rh) and the associated speed (n) as a limit point of the surge limit (P).The invention also relates to a tax Device for carrying out the process or individual process steps.

Description

The invention relates to a method for operating a turbomachine, in particular a gas-bearing thermal turbomachine. In addition, the invention relates to a control unit that is set up to carry out steps of the method according to the invention.

The turbomachine can be used in particular for air compression in a fuel cell system.

State of the art

Hydrogen-based fuel cells are considered to be the mobility concept of the future because they only emit water as exhaust gas and enable fast refueling times.

In addition to hydrogen, the electrochemical reaction in the fuel cells requires oxygen as a reaction gas. Ambient air is usually used as the oxygen supplier. In order to provide a certain air mass flow and a certain pressure level, the air required is supplied to the fuel cells of a fuel cell system by means of an air compression system. In particular, air compression systems with high-speed turbomachines are used. Since the supplied air must be oil-free to protect the fuel cells, gas-bearing turbomachines are usually used. In addition to being oil-free, gas bearings have the advantage that they enable almost frictionless and therefore wear-free operation above a certain speed (lift-off speed). However, if this is undercut, for example when starting or stopping, the wear is high. A start/stop operation therefore represents a significant additional load compared to continuous operation of the gas-bearing turbomachine. Frequent driving in traffic jams and/or driving in city traffic thus contributes to a reduction in the service life of the turbomachine.

To protect the gas bearings, a minimum speed must therefore be maintained during idling. A maximum speed is also specified. Furthermore, the operating range of a turbomachine is limited by a surge limit on the one hand and a choke limit on the other.

A typical map of a thermal turbomachine is shown in FIG 1 shown. It shows the connection between the speed (n), the pressure ratio (p _o /p _i ) and the delivered mass flow (m). The choke limit (S) indicates the maximum mass flow that can be enforced, the surge limit (P) the maximum pressure ratio. If the surge limit is exceeded, local stalls initially occur at the inlet and/or outlet of the turbomachine. As a result, the pressure build-up can collapse and the turbomachine can go into an unstable state, so that the required air mass flow can no longer be provided and it is no longer possible to operate the fuel cell system at the desired operating point. This has a direct impact on the efficiency, emissions and wear and tear of the system.

It is therefore important to avoid exceeding the surge limit. For this purpose, an applicable limit operating characteristic is usually implemented in the system control. Since the surge limit is not exactly known for all operating conditions in real operation and aging effects can lead to a shift in the surge limit, the implemented limit operating characteristic maintains a safety margin from the surge limit (see shaded area in the figure). 1 ). However, this safety distance further restricts the usable operating range of the turbomachine. This proves to be a disadvantage, especially in idling mode and in the other lower part-load ranges, since it has a negative effect on energy and fuel consumption.

An extension of the operating range is therefore desirable in idling and lower part-load operation. Ideally, not only is the safety distance to the surge limit that is usually to be maintained exceeded, but also the surge limit itself. This is largely uncritical, at least in the low speed ranges. Because before the pressure build-up completely collapses and there is a backflow through the turbomachine, the so-called "pumping" (English: "deep surge"), only local flow separations occur, which are limited to individual blade areas. The resulting local stall areas are not stationary, but run around the blade areas in the circumferential direction and are therefore also called “rotating stall”. The turbomachine is then in a quasi-stable state or quasi-stable range ("with surge"). In contrast to "pumping", the direction of flow does not reverse, but remains the same.

By using the "mitd surge" range, the mass flow and thus the power consumption of the turbomachine can be reduced in idle operation or in the lower partial load ranges with very low pressure conditions. This is particularly advantageous in stationary operation, for example in start/stop operation of a vehicle having the fuel cell system with the compressor unit switched off, in stand-by operation and when the turbomachine is started or stopped. Furthermore, the operating points of the fuel cell stack can be met in a more targeted and energy-optimal manner at part load, since less air has to be flown away uselessly via a bypass path to bypass the fuel cell stack when the air mass flow is reduced.

Since idling operation is less energy-intensive according to the proposed method, idling phases can be extended. In this way, the number of start-stop processes can be reduced. The gas bearings of the turbomachine are therefore subjected to less wear, so that the service life of the turbomachine increases.

However, the use of the "mitd surge" range requires as precise knowledge as possible of the surge limit and the map in this range. Furthermore, knowledge of the maximum speed/limit speed between "mitd surge" and "deep surge" is required. Because at higher or high speeds of the turbomachine, exceeding the surge limit can suddenly turn into "pumping" and the turbomachine can be damaged. In the higher or high engine speed ranges, it is therefore not only sensible to maintain the surge limit, but also to maintain a safety distance from the surge limit.

Various methods are known from the prior art for monitoring the surge limit. For example, vibrations that occur when “pumping” on pressure sensors and/or air mass meters can be recorded and evaluated. Depending on the result of the evaluation and any additional information (e.g. pre-programming and/or observer structures in the software), the system control can then be used to switch off or reduce the load point. In the operating range addressed here, however, the detection of vibrations is not functional, or only functional to a limited extent, since it is too imprecise. This is because vibrations hardly occur or do not occur at all in idling or lower part-load operation.

The present invention tries to remedy this. In particular, a method for determining the surge limit in the low to medium speed range of a turbomachine as precisely as possible is to be specified.

To solve the problem, the method with the features of claim 1 is proposed. Advantageous developments of the invention can be found in the dependent claims. Furthermore, a control device for executing the method or individual method steps is specified.

Disclosure of Invention

1. a) Drosseln des Massenstroms bei konstanter Drehzahl der Strömungsmaschine, so dass sich das Druckverhältnis erhöht und ein maximales Druckverhältnis erreicht und überschritten wird,
2. b) Erfassen und Speichern des maximalen Druckverhältnisses sowie des zugehörigen Massenstroms und der zugehörigen Drehzahl als ein Grenzpunkt der Pumpgrenze.

In the proposed method for operating a turbomachine, in particular a gas-bearing thermal turbomachine, the turbomachine is operated in an operating range A, which is limited on the one hand by a surge limit and on the other hand by a choke limit. In order to extend the operating range A by an operating range B beyond the surge limit, the following steps are carried out to determine the surge limit or at least a limit point lying on the surge limit:

1. a) throttling of the mass flow at a constant speed of the turbomachine, so that the pressure ratio increases and a maximum pressure ratio is reached and exceeded,
2. b) detecting and storing the maximum pressure ratio and the associated mass flow and the associated speed as a limit point of the surge limit.

The method makes it possible to determine at least one limit point lying on the surge limit in real operation of the turbomachine, so that the result is very precise. If the limit point determined is not on a surge limit stored as a characteristic curve, this deviation can be recorded and taken into account. Reasons for the deviation can be component variations and/or changed component behavior.

In order to find out whether the maximum pressure ratio has been reached, this point must be exceeded. Because when entering the "mitd surge" range or range B, the pressure ratio drops again. Advantageously, the entire characteristic curve is run through and the curve is then evaluated.

Steps a) and b) of the method are preferably repeated for different speeds. In this way, a large number of limit points can be determined, which in their entirety reflect the surge limit. Furthermore, the operating characteristics for individual speeds, preferably the entire operating map, can be determined, preferably for speeds greater than or equal to the idle speed of the unit and less than the maximum permissible limit speed for the "mitd surge" range or range B.

Furthermore, the several boundary points determined are preferably connected to form a boundary line, which is then stored as a characteristic line. The characteristic curve then defines the surge limit. Knowing the exact surge limit, the operating range of the turbomachine can be determined exactly the. Furthermore, the operating range can be expanded in the low to medium speed ranges in order to optimize the operation of the turbomachine in terms of energy. Exact knowledge of the surge line helps when switching between the operating ranges. Exact knowledge of the operating map in the "mitd surge" area or in area B helps to regulate the air system in operating mode B.

Furthermore, the at least one detected boundary point or a boundary line resulting therefrom is preferably stored in a non-volatile memory of a control device or computer, for example an on-board computer. When the turbomachine is used as an air compressor in a fuel cell system, the non-volatile memory can in particular be a control device for system control.

The method is advantageously repeated at regular or irregular time intervals. In this way, aging effects influencing the component behavior can be recorded and taken into account. This enables adaptive system control over the lifetime of the turbomachine.

The proposed method can, for example, be carried out periodically, ie after the time has elapsed, and/or when the opportunity arises. This is the case in particular when the fuel cell stack is in stand-by mode or when the fuel cell stack is switched off, since system operation is then not disturbed. The compressor mass flow is then preferably discharged via a bypass bypassing the fuel cell stack. Further, the process can be performed as needed. The implementation can then be made dependent in particular on the load profile, the boundary conditions and/or the operating behavior of the turbomachine.

If no suitable point in time can be found during operation of the turbomachine, the implementation of the method can be given a higher priority. When the turbomachine is used as an air compressor in a fuel cell system of a vehicle, the operation of the fuel cell system can be temporarily suspended and driving via a battery can be enabled, so that the method can be carried out, for example by power splitting, i.e. by splitting the power.

Ideally, the procedure is carried out as an auto-calibration. The adaptation of the system control can thus be automated.

Furthermore, the method is preferably carried out for the first time during end-of-line calibration or when the turbomachine is started up. This means that data is available right from the start, which enables the surge limit to be precisely determined and thus energy-optimized operation of the turbomachine. The entire operating characteristics map is preferably determined at the same time.

During operation of the turbomachine, the at least one recorded and stored boundary point is preferably used as a trigger for switching between the two operating ranges A and B. Since exceeding the surge limit can damage the turbomachine in high speed ranges, it is also proposed that the surge limit is exceeded only in low to medium speed ranges. This means that switching only takes place in the low to medium speed ranges.

It is also proposed that vibrations be detected and evaluated in order to detect pumping of the turbomachine. This means that conventional detection and diagnostic functions can be used in addition, which indicate the onset of pumping via vibration detection, ie vibrations in the mass flow and/or the pressures, so that countermeasures can be initiated immediately. These can include shutting down the turbomachine or shifting the operating point. In addition, a limit speed can be determined in this way for which operation in the "mitd surge" range or in range B is still possible. The limit speed then defines the transition from the "mid surge" range or range B to the "deep surge" range or range D, both of which are beyond the surge limit.

In addition, a control unit is proposed that is set up to carry out steps of the method according to the invention. For example, a valve or a throttle flap can be activated with the aid of the control device in order to throttle the mass flow in step a). The valve or the throttle flap can in particular be arranged downstream of the turbomachine. When the flow machine is used in a fuel cell system, the valve or the throttle flap can be arranged in a bypass path for bypassing a fuel cell stack and/or in an exhaust air path for discharging the air emerging from the fuel cell stack.

Alternatively or additionally, the maximum pressure ratio, including the associated mass flow and the associated speed, can be recorded and stored using the control unit (step b) of the method). In addition, it can Operating map for the "mitd surge" range can be determined and saved. Furthermore, the limit speed between range B ("mild surge") and range D ("deep surge") can be determined and saved.

Furthermore, with the aid of the control device, the method according to the invention can be automated and carried out as an auto-calibration.

• 1 eine grafische Darstellung eines typischen Kennfelds einer thermischen Strömungsmaschine,
• 2 eine schematische Darstellung eines Brennstoffzellensystems mit einer Strömungsmaschine zur Luftverdichtung,
• 3 eine graphische Darstellung eines Kennfelds einer thermischen Strömungsmaschine mit erweitertem Betriebsbereich und
• 4 eine graphische Darstellung eines Kennfelds einer thermischen Strömungsmaschine mit einem nach dem erfindungsgemäßen Verfahren ermittelten Grenzpunkt bei einer Drehzahl n1.

The invention is explained in more detail below with reference to the accompanying drawings. These show:

• 1 a graphical representation of a typical map of a thermal turbomachine,
• 2 a schematic representation of a fuel cell system with a turbomachine for air compression,
• 3 a graphical representation of a characteristic map of a thermal fluid machine with extended operating range and
• 4 a graphical representation of a characteristic diagram of a thermal turbomachine with a limit point determined by the method according to the invention at a speed n1.

Detailed description of the drawings

1 , to which reference was already made at the outset, shows a typical characteristic diagram of a thermal turbomachine. The pressure ratio between the pressure p _o at the outlet and the pressure p _i at the inlet of the turbomachine is plotted against the mass flow m. Lines are shown between them, along which the compressor speed n is constant.

The map shows the stable operating range of the turbomachine. This is limited on the one hand by a surge limit P and on the other hand by a choke limit S. If the surge limit P is exceeded, the turbomachine leaves the stable operating range and so-called "pumping" of the turbomachine occurs. In order to prevent this, the safety distance to the surge limit P shown as a hatched area is maintained. However, this narrows the operating range of the turbomachine.

A thermal flow machine with a corresponding map can be used, for example, in a fuel cell system for air compression. the 2 shows an example of a fuel cell system 1 with a corresponding turbomachine 4 for air compression.

With the help of in the 2 Turbomachine 4 shown, a fuel cell stack 2 of the fuel cell system 1 can be supplied with compressed air via an air supply path 3 . The air is taken from the surroundings 11 and supplied to the turbomachine 4 via an air filter 12 . In the present case, the turbomachine 4 is driven by an electric motor 14 . Since the air heats up when it is compressed, a cooling device 13 is integrated into the supply air path 3 . A humidification device 15 for humidifying the compressed air can also optionally be integrated into the supply air path 3 and is therefore only shown in dashed lines.

The one in the 2 The topology shown for air compression is only selected as an example. Instead of the flow machine 4 shown, an electrically driven air compressor with a turbine arranged in an exhaust air path 6 or a purely turbine-driven air compressor can optionally also be used. The air compressor can also be designed with two stages and/or two flows.

To shut off the fuel cell stack 2 from the air supply, shut-off valves 9, 10 are provided, which are closed in particular when the vehicle is switched off. Furthermore, a partial mass flow or the entire air mass flow can be routed past the fuel cell stack 2 via a bypass path 5 . For this purpose, a valve 7 arranged in the bypass path 5 is partially or completely closed. The air mass flow supplied to the fuel cell stack 2 can be throttled by at least partially closing the valve 7 and/or a valve 8 arranged in an exhaust air path 6 . However, when the compressed air flows out via the bypass path 5, energy is lost without being used.

3 , which shows another characteristic diagram of a thermal turbomachine 4, the stable operating area is identified as “Area A”. Beyond the surge line P, two further areas are shown, identified as "Area B" and "Area D". "Area B" designates the "mild surge" area and "Area D" designates the "deep surge" area. A safety zone A1, which maintains a safety distance from the surge limit P, precedes the "deep surge" area.

For energy-optimized operation of the turbomachine, the stable operating range "Area A" can be expanded to include the operating range "Area B" in the low to medium speed ranges. This means that there is no safety margin to the surge limit P and the surge limit P is deliberately exceeded. In this speed range, the operating range of the turbomachine is not only about the area "Area B", but also further expanded to include a safety zone A2 that has not been complied with. In addition to the surge limit P, a limit speed can also be defined, which separates area B (“mild surge”) from area D (“deep surge”).

In the extended operating range “Area B”, the turbomachine is in a quasi-stable state or in a quasi-stable range. This means that stalls can occur, but these are locally limited. Furthermore, the main direction of flow remains unchanged, so that there is no "pumping" of the turbomachine. This is only the case in the operating range "Area D", so that in the corresponding speed range of the surge limit P, the safety zone A1 is upstream in order to reliably avoid "surge" of the turbomachine.

The operation of the turbomachine 4 in the extended operating range “Area B” requires precise knowledge of the surge limit P and preferably of the operating map of the area B or the limit speed between the areas B and D. This can or can be determined using the method according to the invention, which is based on the 4 is explained in more detail.

4 shows an example of the measurement result and the evaluation according to the method according to the invention for an example speed n1 of a turbomachine 4.

The starting point S1 of the procedure is selected in such a way that the operating point is definitely in the "Area A" area. The measurement can start completely de-throttled (S1 right) or it can start time-optimized with a certain throttling (S1 middle or S1 left). The flow cross-section is then reduced or the throttling increased, so that the mass flow decreases and the pressure ratio increases (see arrow). The flow cross-section can be reduced step by step with timed holding phases or by a continuous ramp. The process is continued up to E1 or up to the desired expansion of the map. The pressure ratio drops slightly at the same speed, which can be attributed to local backflows.

The maximum pressure ratio p _{imax reached} indicates a limit point of the surge limit for the speed n1 at an air mass flow mAir 1 and can be determined from the measured trajectory. The limit point and preferably also the operating characteristic in area B is or are recorded and stored. The process is then repeated accordingly for further speeds n2, n3, etc. The result points p _imax [n], mAir[n] for n = n1... N then result in the limiting characteristic or the surge limit for the transition from "Area A" to "Area B". The resulting characteristic curves pi=f(mAir, n) to the left of the limit characteristic curve then result in the extended characteristic diagram for operation of the turbomachine in the “mitd surge” range. The speed limit between ranges B and D can be determined using the usual diagnostic functions for anti-surge protection.

The method can be carried out during operation of the turbomachine 4, in particular as an auto-calibration that is triggered by the passage of time and/or the given boundary conditions. By repeating the process over the service life of the turbomachine, component variations or aging effects can be eliminated.

To carry out the method in a fuel cell system 1 according to 2 For example, with the shut-off valves 9, 10 open, the air mass flow downstream of the turbomachine 4 can be throttled, for example by partially closing the valve 7 arranged in the bypass path 5 and/or the valve 8 arranged in the exhaust air path 6.

Claims (9)

Method for operating a turbomachine (4), in particular a gas-bearing thermal turbomachine (4), in which the turbomachine (4) is operated in an operating range (A) which is defined on the one hand by a surge limit (P) and on the other hand by a choke limit (S) is limited, and in which, in order to expand the operating range (A) by an operating range (B) beyond the surge limit (P), the following steps are carried out to determine the surge limit (P) or at least one limit point lying on the surge limit (P): a) Throttling of the mass flow (m) at a constant speed (n) of the turbomachine (4) so that the pressure ratio (p _i ) increases until a maximum pressure ratio ( _pimax ) is reached and exceeded, b) detecting and storing the maximum pressure ratio ( p _imax ) and the associated mass flow (rh) and the associated speed (n) as a limit point of the surge limit (P). procedure after claim 1 , characterized in that steps a) and b) are repeated for different speeds (n). procedure after claim 1 or 2 , characterized in that several boundary points are connected to form a boundary line, which is stored as a characteristic. Method according to one of the preceding claims, characterized in that the at least one detected boundary point or a boundary line resulting therefrom is stored in a non-volatile memory of a control unit or computer, for example an on-board computer. Method according to one of the preceding claims, characterized in that the method is repeated at regular or irregular time intervals, in particular depending on the load profile, the boundary conditions and/or the operating behavior of the turbomachine (4). Method according to one of the preceding claims, characterized in that the method is carried out for the first time during end-of-line calibration or when the turbomachine (4) is started up. Method according to one of the preceding claims, characterized in that during operation of the turbomachine (4) the at least one detected and stored limit point is used as a trigger for switching between the two operating ranges (A, B), with preferably only in low to medium speed ranges Surge limit (P) is exceeded. Method according to one of the preceding claims, characterized in that vibrations are detected and evaluated in order to detect pumping of the turbomachine (4). Control unit that is set up to carry out steps of a method according to one of the preceding claims.

Images