FUEL-CELL SYSTEM WITH EXHAUST-AIR MASS FLOW DETERMINATION

Abstract

A fuel-cell system has at least one fuel cell, an oxidant line, a compressor, an exhaust-air line, a turbine, which is arranged in the exhaust-air line and is coupled to the compressor, an anode-purging line, which is connected to the exhaust-air line and has an anode-purging valve, and a control unit. The fuel-cell system is characterized in that a temperature-detecting unit is arranged at a turbine input, or upstream of the turbine input, for detecting the temperature of exhaust air flowing into the turbine, in that a pressure-detecting unit is coupled at least to the turbine input or a component lying upstream and is designed to detect a pressure of the exhaust air flowing into the turbine, in that the control unit is designed to ascertain a momentary mass flow of the exhaust air from the measured temperature of the exhaust air, the pressure upstream of the turbine and a specified turbine characteristic map, and in that the control unit is designed to activate the compressor and/or the turbine so as to achieve a minimum mass flow of the exhaust air.

Claims

1. A fuel-cell system (2) having at least one fuel cell (4), an oxidant line (16), a compressor (24), an exhaust-air line (32), a turbine (30) which is arranged in the exhaust-air line (32) and is coupled to the compressor (24), an anode-purging line (47) which is connected to the exhaust-air line (32) and has an anode-purging valve (46), and a control unit (54), wherein a pressure-detecting unit (56, 58) is coupled at least to the turbine input (31) or a component lying upstream and is configured to detect a pressure of the exhaust air flowing into the turbine (30), in that the control unit (54) is configured to ascertain a reduced mass flow of the exhaust air from the pressure upstream of the turbine (30) and a specified turbine characteristic map, and in that the control unit (54) is configured to activate the compressor (24) and/or the turbine (30) so as to achieve a maximum water flow concentration.

2. The fuel-cell system (2) according to claim 1, wherein a temperature-detecting unit (60) is arranged at a turbine input (31) or upstream of the turbine input (31) for detecting the temperature of exhaust air flowing into the turbine (30) and in that the control unit (54) is configured to determine an absolute mass flow from the reduced mass flow knowing the temperature.

3. The fuel-cell system (2) according to claim 1, wherein the control unit (54) is configured to activate the anode-purging valve (46) and to regulate the mass flow when purging an anode of the at least one fuel cell (4).

4. The fuel-cell system (2) according to claim 1, wherein the compressor (24) is additionally connected to an electric motor (26), wherein the electric motor (26) is configured to provide a speed signal, and in that the control unit (54) is configured to support the determination of the momentary mass flow with the speed signal.

5. The fuel-cell system (2) according to claim 1, wherein the pressure-detecting unit (56, 58) comprises a differential pressure sensor or two pressure sensors (56, 58) and is configured for detecting the pressure drop between the turbine input (31) and a turbine output (33).

6. The fuel-cell system (2) according to claim 1, wherein the control unit (54) is configured to determine an expansion ratio through the turbine (30) from the pressure at the turbine input (31) and an estimated value of the pressure at the turbine output (33).

7. The fuel-cell system (2) according to claim 6, wherein the control unit (54) is configured to replace the estimated value with an ambient pressure measured by means of an ambient pressure sensor and the known pressure drop characteristic of the exhaust-air system.

8. The fuel-cell system (2) according to claim 1, wherein the control unit (54) is configured to determine the shortfall of a boundary line (64) in the turbine characteristic map in order to validate that the minimum mass flow has been achieved.

9. The fuel-cell system (2) according to claim 1, wherein the control unit (54) is configured to carry out a model-based simulation of the turbine for determining the mass flow, which is tracked at least by means of the measured pressure and the measured temperature of the actual turbine (30).

10. A method for operating a fuel-cell system (2) having at least one fuel cell (4), an oxidant line (16), a compressor (24), an exhaust-air line (32), a turbine (30) which is arranged in the exhaust-air line (32) and is coupled to the compressor (24), an anode-purging line (47) which is connected to the exhaust-air line (32) and has an anode-purging valve (46), and a control unit (54), wherein a pressure-detecting unit (56, 58) is coupled at least to the turbine input (31) or a component lying upstream and detects a pressure of the exhaust air flowing into the turbine (30), the method comprising: determining, via the control unit (54), a reduced mass flow of the exhaust air from the pressure upstream of the turbine (30) and a specified turbine characteristic map, and activating, via the control unit (54), the compressor and/or the turbine (30) so as to achieve a minimum mass flow of the exhaust air.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Shown are:

[0022] FIG. 1 a schematic illustration of the fuel-cell system;

[0023] FIG. 2 a schematic illustration of a swallowing characteristic of the turbine;

[0024] FIG. 3 a schematic illustration of a boundary line in a characteristic map of the turbine without consideration of turbine speed.

DETAILED DESCRIPTION

[0025] FIG. 1 shows a fuel-cell system 2 in a schematic illustration. The fuel-cell system 2 has a fuel cell 4 comprising an air input 6, an exhaust output 8, a hydrogen input 10, and a hydrogen output 12. The air input 6 is connected to an oxidant line designed as an air line 16 via a first shut-off valve 14. The first shut-off valve 14 can allow for an air supply to the fuel cell 4 and can disrupt it as needed. An intercooler 18 cools compressed air before it enters the fuel cell 4. Air enters a compressor 24 from the environment 20, for example by way of a particulate filter 22. This is coupled for example to an electric motor 26, which is supplied with electrical voltage via an inverter 28, which voltage is provided, for example, by the fuel cell 4.

[0026] The compressor 24 is further coupled to a turbine 30 arranged in an exhaust-air line 32 and having a turbine input 31 and a turbine output 33. The exhaust-air line 32 is arranged downstream of the cathode output 8 via a second shut-off valve 34. A cathode by-pass 36 is further provided between the air line 16 and the exhaust-air line 32, which is selectively activatable via a first bypass valve 38. The exhaust-air system 23 is arranged behind the turbine

[0027] An anode-purging valve 46 is coupled to the anode output 12 and the exhaust-air line 35 in order to purge nitrogen and water from the anode output 12, as needed, into the exhaust-air line 32 via an anode-purging line 47. Further, hydrogen present at the anode output 12 is recirculated to the anode input 10 via a second compressor 48 and a jet pump 50. Fresh hydrogen from a pressure tank 51, not shown, is mixed in via a throttle valve 52.

[0028] A control unit 54 is preferably coupled to all active elements, i.e., the valves 14, 34, 38, 42, 52, and the inverter 28, and is designed to activate the operation of the fuel-cell system 2 by activating these components. Furthermore, the control unit is coupled for example to a first pressure sensor 56 upstream of the turbine 30, as well as to a second pressure sensor 58 downstream of the turbine 30. Further, upstream of the turbine, there is arranged a temperature sensor 60 that is also connected to the control unit 54.

[0029] The control unit 54 is designed to determine a momentary mass flow of the exhaust air from the measured temperature of the exhaust air in the exhaust-air line 32, the pressure upstream of the turbine 30, and a turbine characteristic map associated with the turbine 30. The control unit 54 is thus enabled to activate the valve 46 as a function of the momentary mass flow, so that, when purging the anode of the fuel cell 4, the hydrogen concentration in the exhaust air does not exceed a certain value, for example 4%.

[0030] The inverter 28 and/or the electric motor 26 can further be designed so as to transmit a speed signal to the control unit 54. This simplifies the control unit 54 in the selection of a matching characteristic from the turbine characteristic map.

[0031] FIG. 2 shows an exemplary swallowing characteristic of the turbine 30. A plurality of curves 62a to 62f are shown here. Each of these characteristic curves is generated for a specific speed of the turbine 30. The y-axis shows the expansion ratio through the turbine 30, while the x-axis is the reduced mass flow at a reference temperature. Based on the knowledge of a speed as well as the expansion ratio, the reduced mass flow can consequently be read. Through the conversion as set forth above, the actual mass flow can be calculated knowing the actual temperature in the exhaust-air line 32 measured by the temperature sensor 60 and the pressure upstream of the turbine 30 measured by the first pressure sensor 56.

[0032] FIG. 3 shows a possible boundary line 64, which, in order to reduce the hydrogen concentration, should not be exceeded to the left or above, for which purpose no or no sensible speed signal must be present.