Fuel cell system for energy recovery

Abstract

The fuel cell system has a first fuel cell stack pair having a first fuel cell stack and a second fuel cell stack, a first compressor mechanism, and a first turbine mechanism, the first compressor mechanism linkable with a cathode inlet of a fuel cell stack of the first fuel cell stack pair, the first turbine mechanism linkable with a cathode outlet of a fuel cell stack of the first fuel cell stack pair, the first turbine mechanism including a shaft, a first turbine coupled to the shaft, a second turbine coupled to the shaft, and a generator coupled to the shaft, the first compressor mechanism including at least one electrically operable compressor, and the generator linkable to the at least one electrically operable compressor.

Description

[Technical field]

[0001] The present invention relates to a fuel cell system. [Background technology]

[0002] In particular in vehicles in which the drive energy is also provided by a fuel cell, an oxidant, usually oxygen from the ambient air, is used which reacts with hydrogen in the fuel cell to water and thus provides electrical power by electrochemical conversion. The ambient air is then usually fed to one or more fuel cell stacks by a compressor. For this purpose, an air mass flow rate that depends on the operating point and a corresponding pressure level are required. The compressor usually has a thermal flow machine. Optionally, energy recovery from the exiting air by a turbine for air compression can also be implemented.

[0003] In particular, the relatively high system pressures for fuel cell systems with high power output may require two-stage compression and energy recovery by means of a turbine. In the prior art, this is usually achieved by a two-shaft air system, in which an electrically operated compressor is combined with a turbine-driven compressor. Alternatively, electrically driven one-stage compressors with a turbine are also known. The disadvantage of these concepts is the always direct link between the intake air and the exhaust air, as well as the resulting conflicting goals regarding the design, cost and operation of fuel cells with such types of compressors. Summary of the Invention [Problem to be solved by the invention]

[0004] The object of the present invention is to propose a fuel cell system, in particular having multiple fuel cell stacks, which can be operated without a direct link between the intake and exhaust air and is nevertheless capable of allowing a sufficiently high pressure for the cathode path with simultaneous energy recovery. [Means for solving the problem]

[0005] This problem is solved by a fuel cell system having the features of independent claim 1. Preferred embodiments and developments can be seen from the dependent claims and the following description.

[0006] A fuel cell system is proposed having at least one fuel cell stack, each having an anode inlet, an anode outlet, a cathode inlet and a cathode outlet, a turbine mechanism and an oxidant supply device for supplying an oxidant to the corresponding cathode inlet, the turbine mechanism being arranged downstream of a corresponding cathode outlet of at least one of the at least one fuel cell stack and linkable with the corresponding cathode outlet, the turbine mechanism including a shaft, at least one turbine coupled to the shaft and a generator coupled to the shaft, and the oxidant supply device and the turbine mechanism being operable independently of each other.

[0007] The fuel cell stack can in particular have a fuel cell with a polymer electrolyte membrane. The fuel cell stack then comprises a number of fuel cells, which are in a stacked arrangement and in particular have an electrical series and / or parallel circuit in order to increase the voltage delivered and / or the current provided. The fuel cells are then preferably separated from one another by bipolar plates which distribute the extract over a large area, receive and remove the product water, provide active cooling and allow electrical current to flow. Each fuel cell stack has an anode channel and a cathode channel, the cathode inlet being supplied with air and the exhaust gas being discharged at the cathode outlet. The anode channel is supplied with hydrogen. For this purpose, corresponding piping, valves and the like are provided which regulate and maintain a pressure level that depends on the operating point of the respective fuel cell stack. Such an arrangement is also called "balance of plant". However, this is not important within the scope of the present invention, since the focus here is on the cathode channel.

[0008] A corresponding cathode outlet of at least one fuel cell stack is coupled to a turbine mechanism, in which at least one turbine is disposed, and which preferably comprises a turbine housing including a turbine inlet, a turbine outlet and a flow passage extending fluidically conveniently therebetween. The turbine comprises a turbine wheel coupled to a shaft. The turbine wheel is urged to rotate by exhaust gas flowing into the turbine inlet. Being arranged on a common shaft, the turbine or turbines deliver mechanical power to the shaft, thereby driving an electric generator located thereon.

[0009] The turbine mechanism is only in contact with the exhaust gas and is not connected to any device through which air passes, for example belonging to a compressor or other oxidant supply device. The cathode supply path and the cathode exhaust path of the at least one fuel cell stack are thus completely separated from each other mechanically and possibly spatially. The mechanical power resulting from the rotation of the turbine is converted with a certain efficiency into an electrical power. This can be supplied to the on-board electrical system and / or an electrical energy store, possibly after having been converted beforehand to a desired voltage level, or can be consumed in the corresponding vehicle assembly or even in the fuel cell system itself. This can also be at least partially, depending on the circumstances, by driving an air compressor in the cathode supply path. A direct connection between the generator and the electrically operable compressor is not necessary here.

[0010] The structure according to the invention allows for a more flexible, modular and optimized design of the cathode supply and cathode exhaust paths. The cathode supply path can be, for example, an air supply path and can be supplied with pressurized air. A supply by an oxygen source is also conceivable. The oxidant supply device can therefore in particular include a compressor mechanism with at least one compressor or other device. Since no synchronization of the cathode supply and cathode exhaust paths is required, a more flexible operating strategy is obtained. The various target conflicts with respect to fuel consumption, degradation and required dynamics can be resolved significantly better than in the case of conventional fuel cell systems. No special freeze-start processes are required for maintenance. Possible firmly frozen / blocked turbines can be melted with hot exhaust gas without interfering with the operation of the fuel cell system. Various integration of the compressor and turbine into the on-board electrical system of the fuel cell vehicle is possible, which system has an additional degree of freedom that is particularly useful in the event of a fault. The structure of the turbine mechanism is possible in a modular manner, just like in the case of a turbine-driven compressor, allowing for a concept based on the modular principle. As regards the bearings of the turbine mechanism, in particular due to the separation of the air supply path, it is not necessary to use air bearings, but it is also possible to use oil-lubricated bearings. For example, two radial bearings and one axial bearing can be used with the resulting scaling effect. At least one turbine may or may not be equipped with a position adjustment device, for example a variable turbine geometry.

[0011] The at least one fuel cell stack can refer to any number of fuel cell stacks. In one simple embodiment, the fuel cell system includes only one fuel cell stack. However, two, three, four or more fuel cell stacks can be used. It is contemplated that a single common compressor is used to provide air to the cathode inlets of the multiple fuel cells. However, it is also contemplated that multiple compressors are utilized for each group of fuel cell stacks, or that a unique compressor is provided for each fuel cell stack.

[0012] Furthermore, the fuel cell system can have at least one turbine bypass that can be selectively opened to control the turbine power of at least one turbine, thereby influencing the exhaust mass flow into the respective turbine. The bypass is preferably infinitely adjustable, so that the mass flow can be infinitely divided between the exhaust flow through the turbine and the exhaust flow directly into the environment. The turbine bypass can also be used for pressure control in the fuel cell stack.

[0013] At least one pressure control valve can be arranged upstream or downstream of the at least one turbine, which can be influenced or optimized with respect to its operating point by corresponding control of the corresponding pressure control valve. It can be expedient to control the pressure control valve as a function of the respective pressure and / or exhaust mass flow rate, so that the turbine power output is continuously optimized for the current operating point.

[0014] At least one fuel cell stack may have at least one cathode bypass configured for at least partially directly supplying the oxidant flow provided by the oxidant supply device, i.e. in particular the compressed air from the compressor mechanism, to a corresponding turbine of the turbine mechanism, so that energy from the compressor mechanism can be converted into electrical power by the turbine mechanism, for example in the event of a fault.

[0015] The generator of the turbine arrangement may be constructed as a motor / generator unit, and in an emergency operating mode in the event of a failure of the oxidant supply device, a voltage may be supplied to the generator to drive at least one turbine of the turbine arrangement, at least one of the turbines being configured to supply air in a reverse flow direction to at least one of the fuel cell stacks. For example, in the event of a failure of the electrical system to which the compressor arrangement is connected, the reverse operation of the turbine arrangement allows air to be compressed from the surroundings via the turbine and supplied to the respective fuel cell stack. Thereby, an emergency operation of the respective fuel cell stack can be performed and thus a redundancy is provided for preserving the driving function, in particular in fuel cell vehicles.

[0016] The turbine arrangement can have a first turbine and a second turbine, which can be linked to the corresponding cathode outlet of at least one fuel cell stack. The at least one fuel cell stack can, for example, only include one fuel cell stack. The cathode outlet of this one fuel cell stack can be linked to both turbines. Thus, the exhaust stream from this one cathode outlet is split to the two turbines. However, two fuel cell stacks can also be provided, which can be linked to both turbines or can be assigned to only one of the two turbines, respectively. If more than two fuel cell stacks are provided, various variations are conceivable, which can include all possible combinations or permutations of the links of the individual fuel cell stacks to both turbines. In particular, when two or more fuel cell stacks are applied, for example multiple pairs of fuel cell stacks or other system concepts, the mechanical construction is simplified and leads to a constant realization of the output from the multiple exhaust streams based on a common shaft, regardless of the respective operating state of the corresponding fuel cell stack.

[0017] The at least one fuel cell stack can have two or more fuel cell stacks, and both turbines can be linked respectively with the cathode outlets of at least two of the fuel cell stacks. Thus, the mass flow of the at least two fuel cell stacks is split to the turbines. The splitting of the mass flow into two separate streams for operating the two turbines is particularly favorable with regard to the bearing and construction of the turbines. Due to the lower generated forces, simpler and more compact turbine bearings can be used.

[0018] The at least one fuel cell stack can have at least two fuel cell stacks as mentioned above, and both turbines can be linked to different cathode outlets of the at least two fuel cell stacks. Thus, the individual exhaust streams of the individual fuel cell stacks or the exhaust streams of a subset of the fuel cell stacks present can be assigned to the individual turbines. In operating strategies where, for example, not all fuel cell stacks are operating simultaneously at any one time, for example the exhaust stream of one of the fuel cell stacks can be fed to just one turbine, so that a sufficiently high pressure is achieved at the turbine and thus a high efficiency is achieved during regeneration.

[0019] The at least one fuel cell stack may include at least one fuel cell stack whose cathode outlet can be operated independently of the turbine mechanism. This (additional) fuel cell stack may be designed differently from the other fuel cell stacks, in particular for partial load operation or for medium loads, so that high system pressure is not required in the corresponding cathode path, and therefore no regeneration is required.

[0020] An air intake heat exchanger can be arranged upstream of the cathode inlet of at least one of the fuel cell stacks, which is thermally coupled to the cathode outlet in order to transfer the heat of the air flowing into the cathode inlet to the exhaust air at the cathode outlet. This makes it particularly easy to cool the intake air to a certain temperature level. At the same time, the power converted by the turbine can be slightly increased.

[0021] A plurality of first fuel cell stack pairs can be produced. A plurality of fuel cell stack pairs, which may be constructed according to the above-mentioned principle, can therefore be used side by side. These can then be called second, third, fourth fuel cell stack pairs, and so on. It is also conceivable to use a plurality of third fuel cell stacks. In principle, an even and an odd number of fuel cell stacks can be used. In this case, it is particularly expedient to use a turbine arrangement with two turbines on one shaft in each fuel cell stack pair. In addition, a compressor arrangement with more than one compressor can be used. All conceivable combinations of fuel cell stack pairs and individual fuel cell stacks are conceivable. This can be intended on the basis of the principle of redundancy as well as on the basis of normal power and operating points.

[0022] Other ways of improving the present invention will be described in detail below in conjunction with the description of the preferred embodiment of the present invention with reference to the drawings. [Brief description of the drawings]

[0023] [Figure 1] 1 is a first embodiment of a fuel cell system. [Figure 2a] 1 illustrates various aspects of a compressor. [Figure 2b] 1 illustrates various aspects of a compressor. [Figure 2c] 1 illustrates various aspects of a compressor. [Diagram 3] FIG. 2 is a cross-sectional view showing a first turbine mechanism. [Figure 4]2 is a second embodiment of the fuel cell system. [Diagram 5] 3 is a third embodiment of the fuel cell system. [Figure 6] 11 is a fourth embodiment of the fuel cell system. [Figure 7] 1 is a partial embodiment of a fuel cell system. [Figure 8] FIG. 13 is a cross-sectional view showing the first turbine mechanism with two piping options. [Figure 9] FIG. 13 is a cross-sectional view showing the first turbine mechanism with two piping options. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] FIG. 1 shows a schematic block-based diagram of a fuel cell system 2. Here, a first fuel cell stack pair 4 is provided, which comprises a first fuel cell stack 6 and a second fuel cell stack 8, both of which have a cathode inlet 10 and a cathode outlet 12. A first turbine arrangement 14 is provided, which comprises a first turbine 16 and a second turbine 18, which are arranged on a common shaft 20, on which a generator 22 is further arranged. Connected to the generator 22 is an inverter 24, which is configured to deliver an electrical output at a voltage adapted to the on-board electrical system (not shown here) of the fuel cell vehicle or other higher-level device. The first turbine arrangement 14 is coupled to the cathode outlets 12 of the fuel cell stacks 6 and 8. Exhaust gas flowing out of the cathode outlet 12 is guided through the turbines 16 and 18, thus leading to the generation of an electrical output.

[0025] Both turbines each have a turbine bypass 26, through which the exhaust mass flow can be guided at least partially past the respective turbine 16 to 18. Downstream of the turbines 16 and 18, by way of example only, a pressure control valve 28 is arranged in each case, by means of which the output pressure of the turbines 16 to 18 can be controlled. The pressure control valve 28 can preferably be omitted, and the pressure control of the stack is realized by a valve of the turbine bypass 26. Downstream, a surround 30 is suggested as a separate block, into which the exhaust flows out.

[0026] The generator 22 may be a motor-generator unit in a preferred embodiment, such that the turbines 16 and 18 can be utilized in a reverse flow direction to provide compressed air, at least temporarily, under emergency operation in the event of compressor failure as further described below.

[0027] Supply air is illustratively provided via a first compressor mechanism 32 including a first compressor 34 and a second compressor 36, each of which provides pressurized air drawn from the ambient 30 and delivered to the cathode inlet 10. To provide fast and accurate pressure control at the cathode inlet under variable supply air flow, the compressed supply air can be partially guided past the fuel cell stacks 6 and 8 directly into the exhaust stream via a cathode bypass 38. Embodiments of compressors 34 and 36 are shown in Figures 2a to 2c below.

[0028] Furthermore, both fuel cell stacks 6 and 8 have an anode inlet 40 and an anode outlet 42. Hydrogen from a hydrogen tank 44 is conducted to the anode inlet 40. Residual hydrogen from the anode outlet 42 can be recycled and sent back to the anode inlet 40. A flush or purge outlet 46 serves to carry off flush gases generated during repeated or periodic flushing of the anodes of the fuel cell stacks 6 and 8.

[0029] Figure 2a shows a first embodiment of the compressors 34 to 36. The views of Figures 2a to 2c can also cover the first compressor mechanism 32 as a whole, in case only a single compressor is utilized for both fuel cell stacks 6 and 8. In Figure 2a the compressor impeller 48 is linked to a motor 50 which receives voltage supply via an inverter 52. This is therefore a simple one stage compressor.

[0030] In Fig. 2b, two compressor impellers 48 are shown arranged in parallel to each other, operated via a common motor 50. The mass flow to be delivered is divided between the two compressor impellers. Thus, the mass flow can be doubled compared to the compressor of Fig. 2a, or alternatively the impellers can be made smaller. Both compressor impellers 48 can be arranged on a common shaft 54 ​​and can be operated with the same mechanical power.

[0031] In Fig. 2c a two-stage arrangement is shown in which air is guided from one of the compressor wheels 48 to the other compressor wheel 48. Both compressor wheels 48 are arranged on the same shaft 54 ​​and are driven by the same motor 50. Due to the higher pressure level, in this embodiment an intercooler 56 is provided which is arranged between both compressor wheels 48 in order to cool the air. Here, the pressure can be significantly increased under approximately the same mass flow rate as in the one-stage compressor of Fig. 2a.

[0032] In Fig. 3, the first turbine arrangement 14 is shown in a cross-sectional view. Here, the first turbine 16 and the second turbine 18 are embodied in the form of turbine impellers 58, each arranged in a turbine housing section 60. Between the respective turbine impellers 58, a generator 22 is provided. The turbines 16 and 18 and the generator 22 have only a single continuous housing and form a continuous unit, which is further independent of the air intake side. Here, the exhaust air flows axially outward from the turbines 16 and 18, and the exhaust air flows radially from the fuel cell stacks 6 and 8 into the turbine housing section.

[0033] FIG. 4 shows a fuel cell system 62 based on the fuel cell system 2 of FIG. 1. However, in addition to the two fuel cell stacks 6 and 8 of the first fuel cell stack pair 4, a third fuel cell stack 64 is provided, which is independent of the first fuel cell stack pair 4. A second compressor mechanism 66, which is independent of the first compressor mechanism 32, supplies air to the third fuel cell stack 64. The third fuel cell stack 64 can be designed for partial or medium loads. For this purpose, a particularly high air mass flow rate or pressure is not required. For this reason, the second compressor mechanism 66 could, for example, be a compressor as shown in FIG. 2a or FIG. 2b. The exhaust air from the cathode outlet 12 of the third fuel cell stack 64 is fed directly to the surroundings 30. In addition to this, a pressure control valve 68 is provided, which is used to throttle the exhaust air pressure before entering the surroundings 30. Accordingly, for example, the pressure level in the third fuel cell stack 64 or the intake air mass flow rate can be controlled. Downstream of the second fuel cell stack 8 there is arranged a pressure control valve 28 which serves to throttle the corresponding exhaust gas flow.

[0034] Figure 5 shows a fuel cell system 70 based on the fuel cell system 62 of Figure 4, in which exhaust from the cathode outlet 12 of a third fuel cell stack 64 is fed to a first turbine mechanism 14.

[0035] Figure 6 shows a fuel cell system 72 based on the fuel cell system 2 of Figure 1. Here, an air supply heat exchanger 74 is additionally provided, which cools the air supply to both fuel cell stacks 6 and 8, by discharging the heat of the air supply into the exhaust gas flow of the fuel cell stacks 6 and 8. In addition to this, a cooler 76, which can be flowed through by a coolant, can be provided immediately before each cathode inlet 10. Thereby, the maximum permissible inlet temperature for each fuel cell stack 6 and 8 can be guaranteed.

[0036] Figure 7 shows yet another embodiment, where a fuel cell stack 6-8 or 64 is shown linked to a first turbine arrangement 14. Here the exhaust stream is split equally to both turbines 16 and 18 by flow splitting. Such a two-path arrangement may make sense if only a single exhaust stream is to be utilized for regeneration.

[0037] This is further shown in Figures 8 and 9. In Figure 8 the exhaust flows are also combined downstream of the turbines 16 and 18, whereas in Figure 9 these are fed separately to the environment. Alternatively (not shown in detail here), the turbine arrangement 14 for a single exhaust mass flow could be made with only one turbine impeller 58. [Explanation of symbols]

[0038] 2,62,70,72 Fuel Cell System 6,8,64 Fuel Cell Stack 10 Cathode inlet 12 Cathode outlet 14 Turbine mechanism 16,18 Turbine 20 Shaft 22 Generator 26 Turbine Bypass 28 Pressure Control Valve 38 Cathode Bypass 40 Anode inlet 42 Anode outlet 74 Supply air heat exchanger

Claims

1. In a fuel cell system (2, 62, 70, 72), at least one fuel cell stack (6, 8) each having an anode inlet (40), an anode outlet (42), a cathode inlet (10), and a cathode outlet (12); A turbine mechanism (14); and an oxidant supply device for supplying an oxidant to the corresponding cathode inlet (10); the turbine mechanism (14) is arranged downstream of a corresponding cathode outlet (12) of at least one of the at least one fuel cell stacks (6, 8, 64) and is linkable to the corresponding cathode outlet (12); The turbine mechanism (14) includes a shaft, at least one turbine (16, 18) coupled to the shaft, and a generator (22) coupled to the shaft (20); The oxidant supply device and the turbine mechanism (14) are operable independently of one another.

2. 2. The fuel cell system (2, 62, 70, 72) of claim 1, further comprising at least one turbine bypass (26) selectively openable to control turbine output of at least one of said turbines (16, 18).

3. 3. The fuel cell system (2, 62, 70, 72) of claim 1 or 2, wherein at least one pressure control valve (28) is arranged upstream or downstream of at least one of said turbines (16, 18).

4. 4. The fuel cell system (2, 62, 70, 72) of claim 1, wherein at least one of the fuel cell stacks (6, 8, 64) has at least one cathode bypass (38) configured to at least partially directly supply an oxidant flow provided from the oxidant supply device to a corresponding turbine (16, 18) of the turbine mechanism (14).

5. the generator (22) of the turbine mechanism (14) is fabricated as a motor / generator unit, and in an emergency operation mode when the oxidizer supply device fails, a voltage is supplied to the generator (22) to drive at least one of the turbines (16, 18) of the turbine mechanism (14); 5. A fuel cell system (2, 62, 70, 72) as described in any one of claims 1 to 4, wherein at least one of the turbines (16, 18) is configured to supply air in a reverse flow direction to at least one of the fuel cell stacks (6, 8, 64) when in a driven state.

6. The turbine mechanism (14) includes a first turbine (16) and a second turbine (18); 6. The fuel cell system (2, 62, 70, 72) of claim 1, wherein the first turbine (16) and the second turbine (18) are linkable with a corresponding cathode outlet (12) of at least one of the fuel cell stacks (6, 8, 64).

7. At least one of the fuel cell stacks (6, 8, 64) comprises two or more fuel cell stacks (6, 8, 64); 7. The fuel cell system (2, 62, 70, 72) of claim 6, wherein both turbines (16, 18) are respectively linkable with the cathode outlets (12) of at least two of the fuel cell stacks (6, 8, 64).

8. At least one of the fuel cell stacks (6, 8, 64) comprises at least two fuel cell stacks (6, 8, 64); 7. The fuel cell system (2, 62, 70, 72) of claim 6, wherein both turbines (16, 18) are linkable with different cathode outlets (12) of at least two of the fuel cell stacks (6, 8, 64).

9. 9. The fuel cell system (2, 62, 70, 72) of claim 1, wherein the at least one fuel cell stack (6, 8, 64) includes at least one fuel cell stack (64) capable of operating the cathode outlet (12) independently of the turbine mechanism (14).

10. 10. A fuel cell system (2, 62, 70, 72) as described in any one of claims 1 to 9, wherein an air supply heat exchanger (74) is arranged upstream of the cathode inlet (10) of at least one of the fuel cell stacks (6, 8, 64) and thermally coupled to the corresponding cathode outlet (12) to release heat of the air flowing into the cathode inlet (10) to exhaust from the cathode outlet (12).

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