Fuel cell system and method having an air-cooled compressor / turbine unit

Abstract

The present invention relates to a fuel cell system (1) comprising an air compressor (5) used to compress an air mass flow (4) supplied to at least one fuel cell (3) and a cooling air flow path (19) for branching off a cooling air mass flow (7) from the compressed air mass flow (6). The cooling of the air compressor (5) in the fuel cell system (1) is further improved by controlling (20) the compressed cooling air mass flow (6) according to demand.

Description

[Technical Field]

[0001] The present invention relates to a fuel cell system comprising an air compressor used to compress a mass air flow supplied to at least one fuel cell and a cooling air flow path for branching a mass cooling air flow from the compressed mass air flow, as well as a method for operating the air compressor in such a fuel cell system. [Background technology]

[0002] From Patent Document 1, a fuel cell system is known which has a fuel cell, an air supply line for supplying an oxidant to the fuel cell, and an exhaust gas line for removing the oxidant from the fuel cell, the fuel cell system comprising a turbomachine with a wheel formed as a compressor arranged in the air supply line, in which, in order to optimize the cooling of the turbomachine, cooled uncompressed air is sucked in from the opposite direction, instead of using air that has already been heated and compressed by the wheel. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] German Patent Application Publication No. 102017212815 [Patent Document 2] German Patent Application Publication No. 102012224052 Summary of the Invention [Problem to be solved by the invention]

[0004] The object of the present invention is to further improve the cooling of air compressors in fuel cell systems. [Means for solving the problem]

[0005] The above-mentioned problem is solved by a demand-dependent control of the compressed cooling air mass flow in a fuel cell system including an air compressor used to compress an air mass flow supplied to at least one fuel cell and a cooling air flow path that branches off a cooling air mass flow from the compressed air mass flow. The demand-dependent control is implemented as a control circuit having a setpoint, an actual value, a regulation variable, and a control variable. In conventional fuel cell systems, the mass flow occurring in the cooling air flow path depends crucially on the pressure prevailing after the air compressor. This results in a maximum mass flow occurring near the surge limit, especially when the pressure at the compressor outlet is high, and a minimum mass flow occurring near the choke limit, especially when the pressure at the compressor outlet is low. The cooling mass flow required to cool the air compressor during operation of the fuel cell system depends on various factors, such as the rotational speed, compression ratio, and power of the electric machine operated in the fuel cell system. The required control allows the cooling air mass flow to be oriented toward the cooling demand of the air compressor. This minimizes the cooling air mass flow. The cooling air mass flow is a major mass flow loss in the fuel cell system, since it is no longer available to the fuel cell stack. Since the cooling air mass flow is provided for internal cooling via the air compressor, energy is required to generate the cooling air mass flow. This energy has a negative impact on the overall efficiency of the electric machine. Therefore, controlling the cooling air demand has the effect of improving efficiency.

[0006] A preferred embodiment of the fuel cell system is characterized in that the control comprises a measuring point at the cooling air outlet, which measures the temperature of the cooling air at the cooling air outlet and uses it as a control variable, thereby enabling a simple and trouble-free control of the compressed cooling air mass flow in the cooling air channel.

[0007] Another preferred embodiment of the fuel cell system is characterized in that an experimentally determined limit value of at least one outlet temperature of the exhaust gas mass flow of the fuel cell is used as the target value for the control, which is the outlet temperature at the outlet. In the case of several outlets, there are several outlet temperatures.

[0008] Another preferred embodiment of the fuel cell system is characterized in that a fluid resistance is used as a control variable in the cooling air flow path, the flow resistance of the fluid resistance varying depending on the cooling air temperature at the cooling air outlet, thereby adjusting the compressed cooling air mass flow as required. The fluid resistance is, for example, a throttle. In the required control, for example, a conventional fixed throttle is replaced by a throttle with an adjustable throttle cross-section.

[0009] Another preferred embodiment of the fuel cell system is characterized in that the flow resistance is configured in such a way that a minimum cooling air mass flow always flows through the cooling air channel, thereby ensuring that the cooling air at the measuring point at the cooling air outlet always reaches a cooling air temperature representative of the internal temperature of the electric machine with the fuel cell system.

[0010] Another preferred embodiment of the fuel cell system is characterized in that a bimetallic matrix is ​​arranged in the cooling air flow path, the bimetallic matrix being used to indicate the adjustment amount of the control and comprising side-by-side bimetallic elements with different thermal expansion coefficients, thereby enabling the cooling air mass flow to be metered as needed. The bimetallic elements with different thermal expansion coefficients allow for precise control of the cooling air mass flow. This has the advantage that compressed or squeezed air is not unnecessarily diverted between the air compressor and the fuel cell.

[0011] Another preferred embodiment of the fuel cell system is characterized in that the air compressor is configured as an electric turbocompressor, which is electrically driven, for example, by an electric motor. The turbocompressor comprises at least one compressor wheel used to compress the air in the air supply of the fuel cell. The turbocompressor preferably further comprises at least one turbine wheel of an exhaust gas turbine, which is assigned to the air outlet or the exhaust gas outlet of the fuel cell. Such a turbocompressor is also called a turbomachine.

[0012] The invention further relates to a method for operating an air compressor in a fuel cell system as described above, in which the required method allows the provision of cooling air in the cooling air flow path to be adapted to the actual cooling air requirement.

[0013] The invention further relates to a computer program product comprising a computer program, the computer program product having software means for carrying out the above method when the computer program is run on a computer. The invention optionally also relates to a control device for a fuel cell system comprising such a computer program product.

[0014] The invention further relates to a fluid resistance, in particular a bimetallic matrix, for the fuel cell system described above.The fluid resistance, in particular the bimetallic matrix, can be addressed separately.

[0015] Other advantages, features and details of the invention will become apparent from the following description in which various embodiments are set forth in detail with reference to the drawings. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram of a fuel cell system. DETAILED DESCRIPTION OF THE INVENTION

[0017] The only attached figure shows a schematic diagram of a fuel cell system having an air compressor cooled by a cooling air mass flow branched off from the compressed air mass flow.

[0018] 1 shows a schematic representation of a fuel cell system 1. Fuel cell systems are known per se, for example from US Pat. No. 5,649,499. The fuel cell system 1 comprises a fuel cell 3, which is simply represented by a dashed rectangle. The fuel cell 3 comprises at least one stack 2, which is instead represented by a valve symbol.

[0019] The air mass flow supplied to the fuel cell 3 via the air compressor 5 is indicated by arrow 4. The compressed air mass flow 6 is indicated by arrow 6, from which a cooling air mass flow 7 branches off. The cooling air mass flow 7 is also indicated only by an arrow and is the part of the cooling air flow path 19 which supplies the air compressor 5 with cooling air.

[0020] The cooling air supplied via the cooling air flow path 19 is used, for example, to cool air bearings that rotatably support the shaft of the air compressor 5. The diverted cooling air mass flow 7 is a loss of the compressed air mass flow 6, since it is no longer available to the stack 2 of the fuel cell 3.

[0021] Since the cooling air mass flow 7 is provided for interior cooling via the air compressor 5, energy, in particular electrical energy, is required to generate the cooling air mass flow, which has a negative impact on the overall efficiency of the electric drive machine of the vehicle powered via the fuel cell system 1.

[0022] The remaining air mass flow 6 is supplied to the fuel cell 3 via an air supply line 8. The fuel cell 3 is a galvanic cell that converts the energy of the chemical reaction of fuel and oxidant supplied via a fuel supply line (not shown) into electrical energy.

[0023] The oxidant is air, which is supplied to the fuel cell 3 via an air supply line 8. The fuel can be, in particular, hydrogen, methane, or methanol. Water vapor and carbon dioxide are correspondingly produced as exhaust gases. As indicated by arrow 10, the exhaust gases are removed via an exhaust gas line 9 in the form of an exhaust gas mass flow 10.

[0024] The exhaust gas mass flow 10 is led via an exhaust gas turbine 11 to an exhaust gas outflow 12 indicated by an arrow. The air compressor 5 is arranged in the air supply line 8. The exhaust gas turbine 11 is arranged in the exhaust gas line 9. The air compressor 5 and the exhaust gas turbine 11 are mechanically connected via a shaft.

[0025] The shaft can be electrically driven by an electric motor 14. The exhaust gas turbine 11 is used to assist the electric motor 14 in driving the air compressor 5. The air compressor 5, the exhaust gas turbine 11, the shaft, and the electric motor 14 together form a turbocompressor 15, also called a turbomachine.

[0026] The fuel cell system 1 further comprises a bypass line 13 in which a bypass valve 16 is arranged. Via the bypass line 13 with the bypass valve 16, a bypass air mass flow 17 can be led around the stack 2 of the fuel cell 3 into the exhaust gas line 9 in order to reduce the pressure in the air supply line 8. This is advantageous, for example, for reducing the pressure of the air mass flow fed via the air supply line 8 of the fuel cell 3.

[0027] The fuel cell system 1 further comprises an intercooler 18, indicated by a dashed rectangle, which is used to cool the compressed air mass flow 6 before the cooling air mass flow 7 is branched off via a cooling air channel 19.

[0028] The fuel cell system 1 is equipped with a control 20, by means of which the diverted cooling air mass flow 7 is directed towards the cooling air requirement of the turbocompressor 15. By controlling the cooling air mass flow 7 according to the requirement of the turbocompressor 15, also called turbomachine or machine for short, the diverted cooling air mass flow 7 can be kept low, which also makes it possible to increase the efficiency of a motor vehicle whose drive machine is driven via the fuel cell system 1.

[0029] The use of the temperature of the cooling air at the cooling air outlet 24 as a control variable for the control 20 is indicated by the arrow 21. The cooling air outlet is indicated by the arrow 24. Similarly, the cooling air inlet is indicated by the arrow 23.

[0030] The cooling air inlet 23 is assigned a flow resistance 22, which is combined with a bimetallic matrix 25, which is simply shown as a hatched rectangle to represent the control 20. The bimetallic matrix 25 is used to represent the adjustment amount of the control 20.

[0031] To meter the cooling air mass flow 7 as required, the bimetal matrix 25 consists of a number of different bimetals arranged side by side, each with a different thermal expansion coefficient, which allows for precise control of the cooling air mass flow 7 and prevents unnecessary diverting of squeezed or compressed air from the compressed air mass flow 6 provided by the turbocompressor 15.

[0032] To ensure that the cooling air temperature, which represents the temperature inside the machine, always prevails at the measuring point at the cooling air outlet 4, there must always be a minimum cooling air mass flow at the cooling air inlet 23. For this purpose, a complete closure of the flow resistance 22 must be avoided, thereby ensuring that the temperature at the cooling air outlet 24 does not change accordingly and that the inside of the machine does not overheat. [Explanation of symbols]

[0033] 1. Fuel cell system 2 Stacks 3 fuel cell 4 Air Mass Flow 5. Air compressor 6 Compressed air mass flow 7 Cooling air mass flow 8 Air supply line 9 Exhaust gas pipe 10 Exhaust gas mass flow 11 Exhaust gas turbine 12 Exhaust gas outflow 13 Bypass pipeline 14 Electric motor 15 Turbo compressor 16 Bypass valve 17 Bypass air mass flow 18 Intercooler 19 Cooling air passage 20 Control 21 Cooling air temperature 22 Fluid resistance 24 Cooling air outflow 25 Bimetallic Matrix

Claims

1. A fuel cell system (1) comprising an air compressor (5) used to compress an air mass flow (4) supplied to at least one fuel cell (3), and a cooling air flow path (19) branched from a compressed air mass flow (6) compressed by the air compressor (5) and supplying a cooling air mass flow (7) for cooling the air compressor (5), the system further comprising a controller (20) for controlling the cooling air mass flow (7) in accordance with a required amount, The controller (20) is provided with a measuring point at a cooling air outlet (24) which is an outlet for the cooling air after passing through the air compressor (5), and the measuring point detects the temperature of the cooling air at the cooling air outlet (24), and the temperature is used by the controller (20) to calculate the required amount.

2. A fuel cell system as described in claim 1, characterized in that a fluid resistance (22) is used as an adjustment variable for the controller (20) in the cooling air flow path (19), and the flow resistance of the fluid resistance (22) changes depending on the temperature of the cooling air in the cooling air outflow (24), thereby adjusting the cooling air mass flow (7) according to the required amount.

3. A fuel cell system as described in claim 2, characterized in that the fluid resistance (22) is formed so that a minimum of the cooling air mass flow (7) always flows through the cooling air flow path (19).

4. A fuel cell system as described in claim 2 or 3, characterized in that a bimetal matrix (25) is arranged in the cooling air flow path (19) and is used to represent the adjustment amount of the controller (20) and has bimetals with different thermal expansion coefficients arranged side by side.

5. A fuel cell system as described in any one of claims 1 to 4, characterized in that the air compressor (5) is formed as an electric turbo compressor (15).

6. A method for operating the air compressor (5) in a fuel cell system (1) described in any one of claims 1 to 5.

7. A computer program product having a computer program, the computer program product having software means for carrying out the method according to claim 6 when the computer program is run on a computer.

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