Fuel cell unit

Abstract

A fuel cell assembly comprises at least one fuel cell (3), a compressor (7) for supplying compressed charge air to a charge air inlet (9) of the fuel cell (3), at least one heat exchanger (10) through which a charge air line (8) is routed from the compressor (7) to the charge air inlet (9) to cool the compressed charge air, and a turbine (16) connected to an exhaust air outlet (14) of the fuel cell (3) to generate drive energy for the compressor (7) by expanding the exhaust air. An exhaust air line (15) is routed from the exhaust air outlet (14) of the fuel cell (3) to the turbine (16) through the at least one heat exchanger (10) to heat the exhaust air using heat transferred from the charge air.

Description

[0001] The present invention relates to a fuel cell unit, in particular a fuel cell unit suitable for supplying a drive motor in a motor vehicle with electrical energy, and to a motor vehicle having such a fuel cell unit.

[0002] It is well known to expand exhaust air emitted by a fuel cell under high pressure in a turbine to use the resulting energy to power a charge air compressor. Unlike combustion engines, however, this exhaust air has a relatively low temperature and such a high water vapor content that condensation can occur in the exhaust air stream even before reaching the turbine. Premature condensation results in the loss of heat energy that could be recovered in the turbine. Furthermore, the turbine must be protected from the ingress of liquid water by an upstream separator.

[0003] From EP 3 449 523 B1, an energy generation system is known in which flue gas from a combustion process, which is to be fed to a cathode section of a fuel cell, first passes through a heat exchanger in order to be preheated there by exhaust gas from the fuel cell.

[0004] An object of the present invention is to provide a fuel cell unit which enables a more complete utilization of residual energy contained in the exhaust air stream of the fuel cell for compressing the charge air.

[0005] The object is achieved according to one aspect of the invention in that in a fuel cell unit which comprises at least one fuel cell, a compressor for supplying a charge air inlet of the fuel cell with compressed charge air, at least one heat exchanger through which a charge air line is guided from the compressor to the charge air inlet in order to cool the compressed charge air, and a turbine which is connected to an exhaust air outlet of the fuel cell in order to obtain drive energy for the compressor by expanding the exhaust air, an exhaust air line is guided from an exhaust air outlet of the fuel cell to the turbine through the at least one heat exchanger in order to heat the exhaust air by heat emitted by the charge air.

[0006] Heating the exhaust air before it enters the turbine not only increases its recoverable thermal energy content but also suppresses the tendency to condense. The heat generated during the adiabatic compression of the charge air in the compressor must be largely removed before the charge air can be fed into the fuel cell, and is thus available as a heat source for the exhaust air, so to speak, "free of charge." Since heating the exhaust air by 1°C is sufficient to reduce its relative humidity by several percentage points, even a moderate heating of 10 or 20°C can, under certain circumstances, be sufficient to suppress the tendency to condense to such an extent that a water separator upstream of the turbine is no longer necessary.

[0007] Since the heat absorption capacity of the exhaust air is generally not sufficient to cool the charge air to a temperature beneficial for the fuel cell, a cooling fluid line different from the exhaust air line can also be routed through the heat exchanger in order to increase the cooling capacity.

[0008] Such a heat exchanger is expediently divided into a first and a second assembly, through which the charge air line passes one after the other. If the charge air line from the compressor to the charge air inlet passes through the first assembly before the second, the first has the higher temperature; therefore, the exhaust air line is preferably routed through the first assembly and the cooling fluid line through the second assembly.

[0009] In order to dissipate large amounts of heat from the charge air with a small temperature difference, the cooling fluid line can be part of a cooling water circuit.

[0010] A humidifier for humidifying the charge air can be provided on the charge air line, preferably between the heat exchanger and the charge air inlet. The exhaust air can serve as a source of water for humidification; for this purpose, the humidifier can also be arranged on the exhaust air line between the exhaust air outlet and the heat exchanger.

[0011] Since the drive power required to operate the compressor is generally higher than the output power of the turbine, an additional electric motor should be provided to drive the compressor.

[0012] The invention also relates to a motor vehicle with a fuel cell unit as described above.

[0013] Further features and advantages of the invention will become apparent from the following description of embodiments with reference to the attached figure. Fig. 1 shows a block diagram of a fuel cell unit according to the invention.

[0014] Fig. Figure 1 schematically shows a battery 1 and an electric motor 2 for driving the chassis of a motor vehicle, as well as a fuel cell unit that supplies the battery 1 and the motor 2 with electrical energy. A fuel cell 3 of the fuel cell unit is shown in the figure; additional fuel cells can be connected to the one shown to form a stack.

[0015] An anode 4 of the fuel cell 3 is connected to a tank 5 for fuel supply. A cathode 6 of the fuel cell 3 receives compressed fresh air from a compressor 7 via a charge air line 8. On its way from the compressor to a charge air inlet 9 of the fuel cell 3, the charge air line 8 passes through a heat exchanger 10 with an upstream assembly 11 and a downstream assembly 12, as well as an air humidifier 13.

[0016] Exhaust air leaves the fuel cell 3 at an exhaust air outlet 14 at a temperature of approximately 85°C and a relative humidity of approximately 90%. An exhaust air line 15 initially guides the exhaust air through the humidifier 13. In this humidifier, the charge air and exhaust air streams are separated by semipermeable walls that allow some of the water vapor from the exhaust air to diffuse into the considerably drier charge air coming from the heat exchanger 10, so that the latter reaches a moisture content sufficient for fuel cell operation.

[0017] The exhaust air then reaches assembly 11 of heat exchanger 10 via line 15. Here, the charge air, heated adiabatically in the compressor, transfers heat to the exhaust air. This heating increases the power that can be extracted from the exhaust air stream by a turbine 16 at the end of line 15; moreover, the temperature increase leads to a reduction in the relative humidity of the exhaust air and thus to a reduction in the tendency to condensate. In the best case scenario, no condensate separator is therefore required in line 15 to protect turbine 16 from the ingress of liquid water. Since the capacity of air to absorb water vapor increases by several percent with every degree of temperature increase, it can be assumed that heating the exhaust air in assembly 11 by approximately 20°C is sufficient to limit condensation in exhaust air line 15 to a negligible level.

[0018] While it would be entirely possible to design assembly 11 to heat the exhaust air even more, since the media between which the heat exchange takes place are both gaseous, such an assembly would require a large amount of space, which is not available in a motor vehicle. Therefore, heat exchanger 10 also includes a second assembly 12, in which the charge air, after initial cooling in assembly 11, is cooled to the operating temperature of humidifier 13 of approximately 85°C.

[0019] Assembly 12 uses a liquid cooling medium, typically water, possibly with antifreeze and other additives. The high heat capacity of the liquid coolant allows for a significantly more compact design than with heat exchange between gases.

[0020] A cooling fluid line 17 running through the assembly 12 is part of a cooling circuit in which a pump 18 circulates the cooling medium between the assembly 12 as a heat source and a cooler-fan unit 19 as a heat sink.

[0021] As an additional heat source, the fuel cell 3 itself can also be connected to the cooling circuit. As in Fig.1, the cooling circuit can be divided into two parallel branches downstream of the cooler-fan unit 19, one of which, 22, runs through the fuel cell 3 and the other comprises the cooling fluid line 17. If a higher flow temperature of the cooling water is desired for the assembly 12 than for the fuel cell 3, a low-power auxiliary pump 23 can be arranged between the flow and return lines of the cooling fluid line 17, as indicated by dotted lines in the figure, which pump returns part of the warm return water from the assembly 12 to the flow line and mixes it there with cool water from the cooler-fan unit 19.

[0022] The turbine 16 drives the compressor 7 via a shaft 20. An electric motor 21 is attached to the shaft 20 to support the operation of the compressor 7 at times when the power of the turbine 16 is insufficient. Reference symbol 1 battery 2 electric motor 3 Fuel cell 4 Anode 5 tanks 6 Cathode 7 Compressor 8 Charge air line 9 Charge air intake 10 heat exchangers 11 Assembly 12 Assembly 13 humidifiers 14 Exhaust air outlet 15 Exhaust air duct 16 turbines 17 Cooling fluid line 18 Pump 19 Radiator fan unit 20 Wave 21 Electric motor 22 branch 23 Auxiliary pump QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] EP 3 449 523 B1

[0003]

Claims

[1] Fuel cell unit, comprising at least one fuel cell (3), a compressor (7) for supplying a charge air inlet (9) of the fuel cell (3) with compressed charge air, at least one heat exchanger (10) through which a charge air line (8) is guided from the compressor (7) to the charge air inlet (9) in order to cool the compressed charge air, and a turbine (16) which is connected to an exhaust air outlet (14) of the fuel cell (3) in order to obtain drive energy for the compressor (7) by expanding the exhaust air, characterized by that furthermore an exhaust air line (15) is guided from the exhaust air outlet (14) of the fuel cell (3) to the turbine (16) through the at least one heat exchanger (10) in order to heat the exhaust air by heat emitted by the charge air. [2] Fuel cell unit according to claim 1, further comprising a cooling fluid line (17) different from the exhaust air line (15) passing through the heat exchanger (10) to cool the charge air. [3] Fuel cell unit according to claim 2, wherein the heat exchanger (10) comprises a first (11) and a second assembly (12), wherein the charge air line (8) from the compressor (7) to the charge air inlet (9) passes through the first assembly (11) before the second (12), and the exhaust air line (15) passes through the first assembly (11) and the cooling fluid line (17) passes through the second assembly (12). [4] Fuel cell unit according to claim 2 or 3, wherein the cooling fluid line (17) is part of a cooling water circuit. [5] Fuel cell unit according to one of the preceding claims, in which an air humidifier (13) for humidifying the charge air is arranged on the charge air line (8) between the heat exchanger (10) and the charge air inlet (9) and optionally on the exhaust air line (16) for dehumidifying the exhaust air between the exhaust air outlet (14) and the heat exchanger (10). [6] Fuel cell unit according to one of the preceding claims, in which the heat exchanger (10) is dimensioned to heat the exhaust air by at least 10°C, preferably at least 15°C or 20°C, during normal operation. [7] Fuel cell unit according to one of the preceding claims, further comprising an electric motor (21) for driving the compressor (7) together with the turbine (16). [8] Motor vehicle with a fuel cell unit according to one of the preceding claims.

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