WO2009095157A1

WO2009095157A1 - Compressor system for a fuel cell arrangement, fuel cell arrangement and control method

Info

Publication number: WO2009095157A1
Application number: PCT/EP2009/000202
Authority: WO
Inventors: Michael Bader; Manfred Stute; Andreas Knoop
Original assignee: Daimler Ag; Ford Global Technologies, Llc
Priority date: 2008-01-30
Filing date: 2009-01-15
Publication date: 2009-08-06
Also published as: DE102008006739A1

Abstract

The object of the invention is to propose a compressor system, a fuel cell arrangement having the compressor system and a method of controlling a or the compressor system, the innovation residing in the possibility of reducing the number of components required and/or improving partial efficiency. To this end, a compressor system (12) is proposed for a fuel cell arrangement for supplying fuel cells (10) of the fuel cell arrangement with an oxidant, with a compression device (13) for compressing the oxidant, wherein the characteristics of the compression device (13) may be described by means of a compressor performance map (1) and a surge line (5), which subdivides the compressor performance map (1) into a stable I and an unstable Il region, with a control device (15), which is designed to control the compression device (13) along an operating line 6, the operating line 6 being offset at least in portions from the surge line (5) by a stall margin, the stall margin comprising a speed-dependent component which is dependent on the speed of the compression device (13).

Description

COMPRESSOR SYSTEM FOR A FUEL CELL ARRANGEMENT, FUEL CELL ARRANGEMENT AND CONTROL METHOD

The invention relates to a compressor system for a fuel cell arrangement for supplying fuel cells of the fuel cell arrangement with an oxidant, with a compression device for compressing the oxidant, wherein the characteristics of the compression device may be described by means of a compressor performance map and a surge line, which subdivides the compressor performance map into a stable and an unstable region, with a control device, which is designed to control the compression device along an operating line, the operating line being offset at least in portions from the surge line by a stall margin. The invention further relates to a fuel cell arrangement having the compressor system and a method of controlling a or the compressor system.

Fuel cell arrangements are mobile or stationary power generators, which allow a consumer of power to be supplied with current by converting chemical energy into electrical energy. Said conversion proceeds by way of an electrochemical process, a fuel, generally hydrogen, being catalytically combusted with an oxidant, oxygen, in particular ambient air. Using fuel cell technology it is possible, for example, to replace conventional combustion engines in vehicles with fuel-cell-supplied electric engines or at least hybrid designs, so as to provide low-exhaust or exhaust-less vehicles for everyday use.

An economically viable mode of operating fuel cell arrangements, which is therefore also fit for everyday use, appears to be possible only when all the individual components of the fuel cell arrangements are optimally designed for their intended application. As with a combustion engine, particular attention must be paid to the supply of the fuel cells with the working gases or fuels. Document DE 10120947 A1 discloses for example a fuel cell air supply which comprises a two-stage compressor for compressing ambient air as an oxidant for the fuel cells. The compressor comprises two continuous flow machines connected in series, which take the form for example of radial compressors. The operating range of the compressor is limited to lower mass flow rates by the "surge limit", which separates a stable region in a mass flow rate compression diagram from an unstable region. In the document it is proposed that, at low mass flow rates, part of the mass flow is diverted by connection of a branch line downstream of the compressor, in order to increase the mass flow rate artificially using the compressor and thus prevent the compressor from entering into the unstable region.

The document bearing application number DE 10 2004 035 575.4, which constitutes the closest prior art, describes a method and a device for controlling an internal combustion engine with a compressor, in particular with an exhaust turbocharger for compressing the aspirated air, a safety margin being adapted to the surge limit dynamically as a function of the change in volumetric flow rate at the compressor and/or the pressure change at the compressor outlet. This document starts from the position that, to ensure reliable operation, a safety margin needs to be maintained at all times between the working point of the compressor and the surge limit, but the safety margin may be automatically adapted as a function of vehicle dynamics.

The object of the invention is to propose a compressor system, a fuel cell arrangement having the compressor system and a method of controlling a or the compressor system, the innovation residing in the possibility of reducing the number of components required and/or improving partial efficiency.

This object is achieved with a compressor system having the features of claim 1 , a fuel cell arrangement having the features of claim 12, and a control method having the features of claim 13. Preferred or advantageous embodiments of the invention are disclosed by the subclaims, the following description and/or the attached figures.

In the context of the invention, a compressor system for a fuel cell arrangement is proposed which is designed to supply the fuel cells with an oxidant, i.e. for example ambient air. The fuel cells are preferably organized in fuel cell stacks with at least 100, preferably at least 150 fuel cells. In a preferred embodiment, each fuel cell comprises an anode and a cathode area, which are separated from one another by a membrane, in particular by a PEM (Proton Exchange Membrane). The compressor system is preferably designed to convert the oxidant from an initial state by energy transfer into a consumption state. Preferably, the oxidant exhibits a higher pressure and/or a higher velocity in the consumption state than in the initial state.

This energy transfer, also known as compression, proceeds by means of a compression device, of which the operating characteristics may be described in the manner of a model by means of a compressor performance map with a surge line.

The compressor performance map is preferably plotted as a characteristics map, the different characteristic lines differing by different speeds of the compression device and/or taking the form of iso-speed lines. The characteristic lines are - likewise preferably - plotted relative to the mass flow rate and the pressure ratio of the oxidant in the initial state and in the consumption state. The mass flow rate - also known as throughput - is stated for example in [kg/s] and the pressure ratio is stated as the quotient initial state pressure/consumption state pressure, i.e. dimensionlessly.

The surge line divides working points, i.e. mass flow rate-pressure ratio tuples, into a stable and an unstable region. In principle, in the case of compression devices configured as continuous flow machines, the mass flow delivered and the maximum pressure buildup are coupled together. Coupling is established by aerodynamic components, such as for example compressor wheel, compressor spiral, diffuser, fuel cells etc. However, not every desired working point is actually achievable, but rather, at an identical speed and diminishing mass flow rate, at some point measurable pressure and/or volumetric flow rate fluctuations, i.e. instabilities, will arise. The instabilities are substantially caused by excessive flow deceleration in the compression device. If the mass flow rate is reduced further, this leads to flow separation in the compressor geometry and to "surging", as it is known. Surging is characterized by in particular cyclical forward and backward flow of the compressed medium, conventionally accompanied by elevated levels of vibration etc.

A surge limit or surge line is positioned, according to a first possible definition, as a boundary line to the region in which the above-described instabilities are present and divides this already unstable region from the stable region. According to a second possible definition, the surge limit or surge line is positioned as a boundary line relative to the region in which "surging" is already taking place, while still, however, assigning the region of the instabilities to the stable region. Alternatively or in addition, the surge limit or surge line describes the maximum pressure build-up in the event of a specific mass flow rate and a specific or adapted speed of the compression device.

It is also possible to indicate the characteristics map and/or the surge line in an alternative and/or equivalent mathematical representation.

The compressor system further comprises a control device, which effects open- and/or closed-loop control of the compression device, specifically along at least one operating line. Although it is preferable, in particular in order to limit control effort, for the control device to use precisely one operating line, in modified embodiments the control device may nonetheless also use two or more operating lines, which may in particular even combine together into an operating map. However, the one or more operating lines are distanced at least in portions by a stall margin from the surge line.

According to the invention, the stall margin is selected such that it comprises a speed- dependent component, i.e. a component which is selected as a function of the speed and/or an effective speed of the compression device. For example, as the speed drops, the speed-dependent component and/or the stall margin becomes steadily and/or linearly smaller.

It is a consideration of the invention that, with the conventional designs of compressor systems, in particular compression devices, the pressure build-up is too slight in the case of low mass flow rates and speeds to build up the pressure level necessary for the fuel cells. If, on the other hand, the necessary pressure level is reached by open- or closed- loop control, the mass flow rate is too high, which may for example lead to desiccation problems in the fuel cells. It has been noted that the negative consequences of falling below the surge line during operation of a compressor system or a compression device are not uniform over the entire available speed range of the compression devices, but rather are particularly severe at high speeds. The invention makes use of this finding by selecting the stall margin of the operating line as a function of the speed of the compression device. In a preferred further development of the invention, the stall margin is selected such that the operating line extends at least in portions in the unstable region. In particular, said portion of the operating line extending in the unstable region arises at low speeds and/or is arranged in a partial load region of the compression device or of the compressor system. It is optionally preferred for the operating line in the region of the full-load operating point to be arranged in a stable region, such that the operating line intersects the surge line at least or precisely once. This further development emphasizes the inventive concept of making the speed-dependency of the stall margin, precisely at low speeds and/or in a partial load region of the compression device, such that the operating line extends in the unstable region, since at the low operating points the pressure variations in the compression device are so slight that no mechanical damage to the compression device need be feared.

In this development, the advantage also arises that no additional valves, bypass lines or blow-off lines have to be used for discharge of the compressed oxidant downstream of the compression device in order to maintain the necessary minimum mass flow rate or throughput. In this way, on the one hand, considerable savings are made in components and thus costs, while, on the other hand, a considerably improved partial load efficiency may be expected. For instance, experiments have already shown that compression devices under partial load with an operating line in the unstable region could complete a running time of more than four hundred hours without damage.

In a preferred embodiment of the invention, the operating line is such that it intersects the surge line approximately in the middle over the plotted mass flow rate and/or over the plotted pressure increase. This embodiment emphasizes the benefits of the invention, since it may cover approximately 50% of the operating states of the compression device.

As already explained, it is preferable for the operating line to extend in a full load working point or in the region thereof in the stable region of the compressor performance map. The full load working point is taken to mean the working point at which the fuel cell arrangement supplied with the compression device operates under full load conditions.

In this respect, it is particularly advantageous for the full load working point to be positioned at a local efficiency maximum of the compressor system and/or the compression device. In this way it is ensured that the compressor system operates most effectively and/or most economically when the fuel cell arrangement is under full load.

In an advantageous configuration of the compressor system, the operating line and/or the surge line take the form of straight lines or at least resemble straight lines, it being particularly preferred for the two to have a common point of intersection, about which they are rotated relative to one another. In particular, said rotation is such that the partial load region of the operating line is arranged in the unstable region of the compressor performance map and the full load operating region portion is arranged in a stable region of the compressor performance map.

In a structural embodiment of the compressor system, provision is made for the compression device to comprise one or more continuous-flow machines, preferably two continuous-flow machines. In the case of a plurality of continuous-flow machines, it is preferable for these to be connected together in series. The speed-dependent component is formed as a function of precisely one continuous-flow machine, any desired one of the continuous-flow machines or a cumulative value or an effective speed of a plurality of continuous-flow machines. The effective speed may be arrived at in any desired way, for instance by averaging a plurality of speeds.

The compression device preferably takes the form of a turbocharger, in particular an exhaust gas turbocharger, in particular with a electrical drive and/or an exhaust gas drive.

The surge line is preferably established in that steady states of the compression device are determined and evaluated. As already explained above, the surge line thus constitutes a characteristic of the compression device.

In a particularly preferred embodiment of the invention, the control device is designed so as, in normal operation, to control the compression device on the operating line without using a branch line, a blow-off means or the like for diverting part of the compressed mass flow. Provision may be made, on the one hand, for such a branch line etc. to be provided but not to be used in normal operation, but rather only in special operating states, such as for example venting or the like of the compression system. As an alternative to this, the compressor system is constructed without such branch, blow-off or bypass lines. The present invention also provides a fuel cell arrangement for mobile operation, which is suitable and/or designed in particular for supplying a drive train for a motor vehicle and which is characterized according to claim 12 by a compressor system as claimed in one of the preceding claims.

The present invention also provides a method having the features of claim 13 for controlling a compressor system for a fuel cell arrangement, the compressor system and/or the fuel cell arrangement being constructed as claimed in one of the preceding claims or the compressor system being constructed according to the preamble of claim 1 in any desired combinations of the following features of the subclaims and/or the description. The method is distinguished in that the operating line extends in a low load working region at least in portions in the unstable region of the compressor performance map.

Further features, advantages and effects of the invention are revealed by the following description of a preferred exemplary embodiment of the invention. In the figures:

Figure 1 shows a compressor performance map illustrating the mode of operation of an exemplary embodiment of the invention;

Figure 2 shows a block diagram of a compressor system as an or the exemplary embodiment of the invention.

Figure 1 is a schematic representation of a compressor performance map 1 , which is plotted using a system of coordinates whose x-axis 2 shows a mass flow rate, for example in kilograms per second, as a throughput through a compression device and whose y-axis 3 shows the pressure ratio PE/PA, i.e. the pressure ratio between inlet pressure and outlet pressure of the oxidant in the compression device. In the compressor performance map 1 there are entered a plurality of characteristic lines 4 a - f, which in each case represent characteristic lines of identical compression device speed (n - constant). Thus, at a fixed speed n, the characteristic lines 4 a to 4 f reproduce the relationship between the throughput and the pressure ratio. However, the characteristic lines 4 a - 4 f cannot be plotted over the entire throughput required in operation, but rather each characteristic line 4 a - f exhibits a boundary in the direction of diminishing throughput, which is represented for all the characteristic lines 4 a to 4 f by a common surge line 5. The surge line 5 illustrates for each characteristic line 4 a - 4 f the working point in the compressor performance map 1 at which, at a fixed speed n, a reduction in throughput is no longer possible with only slight disturbance. At the points of intersection formed by the characteristic lines 4 a - f and the surge line 5, according to a first definition of the surge limit, instabilities arise, which indicate the start of flow separation in the compressor geometry of the compression device or, according to a second definition, "surging" arises, i.e. partial back-flow of the gas stream contrary to the delivery direction in the compressed gas stream. The surge line 5 thus subdivides the compressor performance map 1 into an unstable region I₁ which lies above the surge line 5 and a stable region II, which is arranged below the surge line 5.

The design and/or the mode of operation of the compression device is selected according to an operating line 6, which extends in the compressor performance map 1 from a full load working point 7. The full load working point 7 corresponds to the mass flow rate which is required in order to supply a downstream fuel cell arrangement under full load with the oxidant.

Starting from the full load working point 7, the operating line 6 extends straight and intersects the surge line 5 approximately in the middle, with regard to throughput, at a point of intersection 8. As the operating line 5 proceeds after the point of intersection 8, it extends into the unstable region I. In this portion, starting from the point of intersection 8, the distance d between the operating line 6 and the surge line 5 becomes steadily larger.

In the example shown, the stall margin d is accordingly selected relative to the surge line 5 such that, in the region of the full load working point 7, a maximum stall margin Dmax is present, which diminishes steadily and/or linearly up to the point of intersection 8 and becomes larger again in amount after the point of intersection 8, but in. the unstable region II, while becoming smaller due to its sign. The stall margin d thus changes its sign relative to the surge line 5. The area shown in the vicinity of the origin of the coordinates in the unstable region I indicates the available area in which the operating tine 6 may be placed without causing damage to the compression device. Figure 2 shows in a schematic block diagram an exemplary embodiment of the invention in the form of a fuel cell arrangement 9, which comprises at least one fuel cell stack with fuel cells 10, which are in each case subdivided by a membrane 11 into a cathode and an anode compartment. The fuel cell arrangement 9 takes the form, for example, of a mobile unit for a motor vehicle.

To supply the fuel cells 10 with an oxidant, in particular ambient air, a compressor system

12 is provided, which comprises a compression device 13. The compression device 13 brings the oxidant, in particular ambient air, from an initial state A into a consumption state V, by inputting energy to the oxidant and/or compressing it. The compression device

13 is of single-, two-, or multistage construction, wherein one, several or each stage is embodied as a continuous-flow machine, for example as a radial compressor.

To drive the compression device 13, on the one hand an electric drive may be provided, while on the other hand the compression device 13 may be embodied as an exhaust gas turbocharger, the drive energy being provided wholly or in part by an exhaust gas turbine 14, which is connected either mechanically or electrically to the compression device 13.

In order to actuate the compressor system 12 in the mode of operation illustrated by way of Figure 1 , the compressor system 12 exhibits a control device 15, which may take the form for example of a microcontroller or a central control device or the like, which controls the compression device 13 in accordance with power requirements along the operating line 6 in Figure 1 , the compressor performance map 1 in Figure 1 representing the characteristics of the compression device 13 or of the compressor system 12 as one possible example.

In particular, it should be noted that no blow-off or bypass line or the like is provided between the outlet side of the compression device 12 and the inlet side of the fuel cell 10 in order to increase the mass flow rate in the compression device 13 artificially.

Through operation of the compression device 13 in the region of Figure 1 indicated by the broken line, i.e. under partial load in the unstable region II, a very high level of efficiency is achieved, on the one hand, in region Il and, on the other hand, savings may be made with regard to components, such as for example for the blow-off or bypass line.

Claims

Patent Claims

1. A compressor system (12) for a fuel cell arrangement for supplying fuel cells (10) of the fuel cell arrangement with an oxidant,

with a compression device (13) for compressing the oxidant, wherein the characteristics of the compression device (13) may be described by means of a compressor performance map (1) and a surge line (5), which subdivides the compressor performance map (1) into a stable (I) and an unstable (II) region,

with a control device (15), which is designed to control the compression device (13) along an operating line (6), the operating line (6) being offset at least in portions from the surge line (5) by a stall margin,

characterized in that

the stall margin (d) comprises a speed-dependent component, which is dependent on the speed of the compression device (13).

2. The compressor system (12) as claimed in claim 1 , characterized in that the operating line (6) intersects the surge line (5).

3. The compressor system (12) as claimed in claim 1 , characterized in that the operating line (6) intersects the surge line (5) in the middle when plotted linearly.

4. The compressor system (12) as claimed in one of the preceding claims, characterized in that the operating line (6) extends at a full load working point (7) and/or in a region of the full load working region (7) in the stable area (I) of the compressor performance map (1).

5. The compressor system (12) as claimed in claim 4, characterized in that the full load working point (7) is positioned at a local efficiency maximum of the compressor system (12) and/or the compression device (13).

6. The compressor system (12) as claimed in one of the preceding claims, characterized in that the operating line (6) extends in a low load working region in the unstable region (II) of the compressor performance map (1).

7. The compressor system (12) as claimed in one of the preceding claims, characterized in that the operating line (6) and/or the surge line (5) take the form of straight lines or at least in portions resemble straight lines.

8. The compressor system (12) as claimed in claim 7, characterized in that the operating line (6) and the surge line (5) are rotated or swiveled about a common point of intersection (8).

9. The compressor system (12) as claimed in one of the preceding claims, characterized in that the compression device (13) comprises one or more continuous-flow machines, the speed-dependent component being formed as a function of the, any desired or a cumulative value of a plurality of continuous-flow machines.

10. The compressor system (12) as claimed in one of the preceding claims, characterized in that the surge line (5) is determined by evaluation of steady states of the compression device (13).

11. The compressor system (12) as claimed in one of the preceding claims, characterized in that the control device (15) is designed so as, in normal operation, to control the compression device (13) on the operating line (6) without using a branch line for diverting part of the ejected mass flow.

12. A fuel cell arrangement (9) for example for mobile operation, in particular for supplying a drive train for a motor vehicle, characterized by a compressor system (12) as claimed in one of the preceding claims.

13. A method of controlling a compressor system (12) for a fuel cell arrangement (9), preferably as claimed in one of the preceding claims and/or according to the preamble of claim 1 , characterized in that the operating line (6) extends in a low-load working region at least in portions in the unstable region (II) of the compressor performance map (1).