Water separator and method of determining the water content of a gas flowing out of a water separator

Abstract

A water separator (20) for separating water from an exhaust air stream (14) of a fuel cell (4) comprises: a gas inlet (22) for receiving the exhaust air stream (14) exiting the at least one fuel cell (4); a gas outlet (24) for discharging a substantially gaseous portion of the received exhaust air stream (14); a liquid outlet (26) for discharging liquid (23) that has been separated from the received exhaust air stream (14); an ultrasonic sensor (30) that is provided in or on the gas outlet (24) and that is designed to emit an ultrasonic signal (35) into the gas outlet stream (25) passing through the gas outlet (24) and to detect an ultrasonic signal (35) exiting the gas outlet stream (25);and an evaluation device (36) which is designed to determine the water content in the gas output stream (25) flowing out through the gas outlet (24) from the ultrasonic signal (35) detected by the ultrasonic sensor (30);

Description

The invention relates to a water separator for separating water from the exhaust gases of a fuel cell and to a method for determining the water fraction of a gas flowing out through a gas outlet of the water separator. The invention also relates to a fuel cell system having a water separator according to the invention and to a motor vehicle which is equipped with such a fuel cell system.Prior ArtIn order to supply electric motors in motor vehicles with electrical energy, fuel cell systems can be used.Polymer electrolyte membrane fuel cell (PEM) systems convert hydrogen, which is supplied as a gas to the fuel cell system, into water by oxygen. In this reaction, electric energy and waste heat are generated.A PEM fuel cell has an anode supplied with hydrogen, a cathode supplied with oxygen-containing air, and a polymer electrolyte membrane located between the anode and the cathode. In order to increase the electric voltage that can be provided by the fuel cell system, a plurality of such fuel cells can be stacked and electrically connected in series.The hydrogen supplied to the fuel cell does not completely react to water. Therefore, the anode off-gas flowing out of the fuel cell still contains a considerable amount of hydrogen.An approach has therefore become established in which the hydrogen-rich anode off-gas which exits from the fuel cell is mixed with fresh hydrogen with the aid of gas feed units and is fed back to the anode inlet. This is referred to as recirculation.On the anode side, a supply with anode off-gas is optimal, which contains no liquid water components. Therefore, the liquid water phase of the anode off-gas exiting from the fuel cell is separated from the gaseous anode off-gas with the aid of a water separator.The water separator has an efficiency (degree of separation) of less than 100%. Depending on the operating point of the fuel cell system and the efficiency of the water separator, the substantially gaseous anode off-gas exiting from the water separator therefore also contains liquid residual constituents.The efficiency of the water separator decreases with increasing fill level of water within the water separator.During operation of a fuel cell system with a water separator which is arranged in an anode exhaust gas recirculation path, there is therefore a need to determine the water fraction of the gas flowing out through a gas outlet of a water separator. This makes it possible to remove the liquid water phase separated from the anode off gas in the water separator from the water separator if necessary in order to improve the efficiency of the water separator.Disclosure of the InventionA water separator according to the invention for separating water from the exhaust air stream of a fuel cell has a gas inlet for receiving an exhaust air stream which is output by the anode of at least one fuel cell; a gas outlet for outputting a substantially gaseous fraction of the received exhaust air stream; and a liquid outlet for outputting liquid, in particular water, which has been separated from the received exhaust air stream. A water separator according to the invention furthermore has an ultrasonic sensor which is arranged in or at the gas outlet. The ultrasonic sensor is designed to emit an ultrasonic signal into the gas outlet stream flowing through the gas outlet and to detect an ultrasonic signal emerging from the gas outlet stream. A water separator according to the invention furthermore comprises an evaluation device which is designed to evaluate the ultrasonic signal detected by the ultrasonic sensor in order to determine from this the water content in the gas outlet stream flowing out through the gas outlet.A method according to the invention of determining the water content of a gas outlet stream flowing out through a gas outlet of a water separator comprises emitting an ultrasonic signal into the gas outlet stream flowing through the gas outlet, detecting an ultrasonic signal flowing out from the gas outlet stream flowing through the gas outlet, and evaluating the detected ultrasonic signal in order to determine the water content of the gas outlet stream flowing through the gas outlet.The invention also comprises a fuel cell system having at least one fuel cell, which has an inlet air line and an outlet air line, and a water separator according to the invention. The gas inlet of the water separator is connected to the exhaust air line of the at least one fuel cell in order to receive an exhaust air stream which is output by at least one fuel cell; and the gas outlet of the water separator is connected to the supply air line of the at least one fuel cell in order to return the gas outlet stream which exits from the gas outlet of the water separator as a return air stream into the at least one fuel cell.The invention furthermore comprises a motor vehicle having at least one electric motor and having a fuel cell system according to the invention, wherein the at least one fuel cell of the fuel cell system is designed to supply the at least one electric motor with electrical energy.With a water separator according to the invention and with a method according to the invention, the proportion of liquid contained in the gas outlet stream flowing out through the gas outlet of the water separator can be reliably determined. On the basis of the liquid fraction in the gas outlet stream determined in this way, the liquid water phase separated in the water separator can be removed selectively from the water separator if necessary in order to improve the efficiency of the water separator.When using a water separator according to the invention, it is in particular not necessary to determine the water level in the water separator directly, for example with a float, in order to be able to remove the liquid water phase from the water separator in good time. Compared to a float, an ultrasonic sensor according to the invention has a high operating reliability, a low maintenance requirement and a long service life.In one embodiment, the invention comprises evaluating the amplitude of the detected ultrasonic signal in order to determine the water content in the gas outlet flow flowing through the gas outlet. By evaluating the amplitude of the detected ultrasonic signal, the water content in the gas outlet flow flowing through the gas outlet can be determined easily and reliably.In one embodiment, the water separator comprises an outlet valve arranged in the liquid outlet of the water separator, which outlet valve makes it possible to selectively release and block the liquid outlet in order to be able to control the outflow of liquid through the liquid outlet.In one embodiment, the outlet valve is designed as a simple switching valve, which can be switched over between an open state and a closed state. A simple switching valve is particularly cost-effective and reliable.In one embodiment, the outlet valve is designed as a controllable valve which can be opened and closed continuously or in a plurality of stages. With such a controllable valve, the outflow of liquid through the liquid outlet can be variably adjusted.In one embodiment, the evaluation device is designed to actuate the outlet valve as a function of the previously determined water fraction in the gas outlet stream flowing out through the gas outlet. In this way, the outlet valve can be opened in a targeted manner in order to discharge liquid from the water separator through the liquid outlet if the water content in the gas outlet flow flowing out through the gas outlet exceeds a predefined limit value.In one embodiment, the evaluation device is additionally designed to also determine the volume flow of the gas outlet flow flowing through the gas outlet with the aid of the ultrasonic sensor.The evaluation device can be designed in particular to evaluate the transit time and / or the frequency spectrum of the ultrasonic signal detected by the ultrasonic sensor in order to determine the volume flow of the gas outlet flow flowing through the gas outlet.In one embodiment, the evaluation device is designed to close the outlet valve when the volume flow of the gas outlet flow flowing through the gas outlet falls below a predefined limit value. This can prevent gaseous exhaust gases from the at least one fuel cell from flowing out of the water separator through the fluid outlet and thus not being available for recirculation into the at least one fuel cell.The invention also comprises a fuel cell system having a water separator according to the invention and having at least one fuel cell which has an inlet air line and an outlet air line. The gas inlet of the water separator is connected to the exhaust air line of the at least one fuel cell in order to receive an exhaust air stream which is output by at least one fuel cell. The gas outlet of the water separator is connected to the supply air line of the at least one fuel cell in order to return the gas outlet stream exiting from the gas outlet as a return air stream into the at least one fuel cell.The exhaust air recirculation valve may be a simple switching valve or a variable valve which can be opened and closed continuously or in several stages.In one embodiment, a gas conveying device is provided between the gas outlet and the supply air line, which makes it possible to convey the return air flow from the gas outlet into the supply air line.An embodiment of the invention will be described below with reference to the accompanying figures.Brief Description of the FiguresFIG. 1 shows a schematic view of a motor vehicle having a fuel cell system which comprises a water separator according to the invention. FIG. 2 shows a schematic view of a water separator according to the invention.DESCRIPTION OF THE FIGURESFIG. 1 shows a schematic view of a motor vehicle 1 having a fuel cell system 2 which is equipped with a water separator 20 according to an exemplary embodiment of the invention.The motor vehicle 1 has four wheels 3 and at least one electric motor 5, which is provided for driving at least two wheels 3 of the motor vehicle 1. The at least one electric motor 5 can also be provided for driving all four wheels 3 of the motor vehicle 1. In an alternative exemplary embodiment, which is not explicitly shown in the figures, an electric motor 5 can be provided on at least one of the wheels 3, in particular on each of the wheels 3, of the motor vehicle 1.The electric motor 5 is supplied with electrical energy via a motor controller 7, which is made available by the fuel cell system 2.The fuel cell system 2 comprises at least one fuel cell 4 and a supply air line 6, which is designed to supply hydrogen 37 from a hydrogen storage 38 to the at least one fuel cell 4, in particular to an anode of the at least one fuel cell 4.The fuel cell system 2 also comprises an exhaust air line 8, which is designed to discharge an exhaust air stream 14 from the at least one fuel cell 4 and to discharge it into the environment.The fuel cell system 2 can also have at least one fuel cell stack which comprises a plurality of fuel cells 4.The fuel cell system 2 has an exhaust air recirculation path 10 configured to recirculate a recirculation air flow 19 branched from the exhaust air flow 14 from the exhaust air line 8 into the supply air line 6.In the exhaust air line 8, a branching device 12 is provided, which is designed to branch off a partial stream as return air stream 19 from the exhaust air stream 14 and to lead it into the exhaust air return path 10.A fuel cell system 2 including a plurality of fuel cell stacks may also include a plurality of exhaust air recirculation paths 10. The exhaust air return paths 10 can be configured so as to be coupled across the stack.The transfer of the return air flow 19 from the exhaust air line 8 into the supply air line 6 can take place passively, i.e. solely on the basis of a pressure difference between the exhaust air line 8 and the supply air line 6.Optionally, a gas conveying device 13, e.g. a fan or compressor, can be provided in the exhaust air recirculation path 10, which is designed to convey the return air flow 19 from the exhaust air line 8 into the supply air line 6.In the exhaust air recirculation path 10, a water separator 20 is provided, which is designed to separate liquid 23, in particular water, from the recirculation air stream 19 before the recirculation air stream 19 reaches the supply air line 6.FIG. 2 shows a schematic view of a water separator 20 according to the invention.The water separator 20 has a gas inlet 22 for receiving the exhaust air stream 14, which is output from the at least one fuel cell of the fuel cell system 2.The water separator 20 also has a gas outlet 24 for the discharge of a gas outlet stream 25 which essentially contains the gaseous constituents of the exhaust air stream 14. The water separator 20 also has a liquid outlet 26 which allows liquid 23, which has been separated from the exhaust gases and collects at the bottom of the water separator 20, to be discharged from the water separator 20.An outlet valve 28 is provided in or at the liquid outlet 26, which makes it possible to selectively enable and block the outflow of liquid 23 from the water separator 20 through the liquid outlet 26.The outlet valve 28 can be designed as a simple switching valve 28, which can be switched between an open state and a closed state.Alternatively, the outlet valve 28 can also be designed as a controllable valve 28, which can be opened and closed continuously or in a plurality of stages in order to be able to regulate the outflow of liquid 23 through the liquid outlet 26.The water separator 20 is also equipped with an ultrasonic sensor 30 which is arranged in or at the gas outlet 24. The ultrasonic sensor 30 is designed to emit at least one ultrasonic signal 35 into the gas outlet stream 25 flowing through the gas outlet 24 and to receive and detect at least one ultrasonic signal 35 emerging from the gas outlet stream 25.For this purpose, the ultrasonic sensor 30 can comprise, in particular, an ultrasonic transmitter 32 and an ultrasonic receiver 34, wherein the ultrasonic transmitter 32 is designed to emit at least one ultrasonic signal 35 into the gas output stream 25 flowing through the gas outlet 24, and wherein the ultrasonic receiver 34 is designed to receive and detect at least one ultrasonic signal 35 which exits from the gas output stream 25.The ultrasonic transmitter 32 and the ultrasonic receiver 34 can be arranged in particular on a common line L which extends substantially orthogonally to the flow direction of the gas outlet stream 25.The common line L can also deviate at an angle α from the previously described orthogonal orientation to the flow direction of the gas outlet stream 25. The angle α may have a value of up to 30°, for example.The water separator 20 also comprises an evaluation device 40, which is designed to evaluate the ultrasonic signal 35 detected by the ultrasonic receiver 34 in order to determine the proportion of liquid 23, in particular the proportion of water, in the gas outlet stream 25.The liquid component in the gas outlet stream 25 influences in particular the amplitude of the ultrasonic signal 35 received by the ultrasonic receiver 34.A high water content in the gas outlet stream 25 indicates a low degree of separation, i.e. a low effectiveness, of the water separator 20. A low degree of separation of the water separator 20 can be the result, in particular, of a high water level H of liquid 23, which has been separated from the return air stream 19 and has accumulated in the water separator 20.In order to reduce the water level H in the water separator 20 and thus to increase the degree of separation and the effectiveness of the water separator 20 again, the evaluation device 40 can be designed to actuate the outlet valve 28 such that it opens when the water content in the gas outlet stream 25 exceeds a predefined limit value.When the outlet valve 28 is open, liquid 23, in particular water which has been separated from the return air stream 19 and has accumulated in the water separator 20, can flow out of the water separator 20 through the liquid outlet 26 and in particular through the open outlet valve 28, so that the water level H in the water separator 20 falls.After the liquid 23 that has accumulated in the water separator 20 has completely flowed out of the water separator 20, a portion of the gaseous exhaust gases can also emerge from the water separator 20 through the liquid outlet 26, since the liquid outlet 26 is not blocked by liquid 23. This is not desirable.When gaseous exhaust gases exit the water separator 20 through the dry-laid liquid outlet 26, the volume flow of the gas outlet flow 25 flowing through gas outlet 24 of the water separator 20 is reduced. In other words, by detecting a drop in the volume flow of the gas outlet flow 25 which flows out of the water separator 20 through gas outlet 24, it is possible to detect the point in time at which the liquid 23 has completely exited the water separator 20 when the outlet valve 28 is open, such that liquid outlet 26 is dry and is no longer covered by liquid 23.The volume flow of the gas outlet stream 25 reduced as a result results in longer transit times of the ultrasonic signals 35 which are sent through the gas outlet stream 25. A reduced volume flow of the gas outlet flow 25 can therefore be detected by an increase in the transit times of the ultrasonic signals 35 which are sent through the gas outlet flow 25.Gas escaping from the water separator 20 through the liquid outlet 26 is replaced by pure hydrogen in the case of an empty water separator 20. This increases the hydrogen content in the gas outlet stream 25 which flows through the gas outlet 24. An increased proportion of hydrogen in the gas outlet stream 25 which flows through the gas outlet 24 results in a frequency shift or a change in the spectrum of the ultrasonic signals 35 detected by the ultrasonic receiver 34.A state in which the water separator 20 no longer contains liquid 23 so that the liquid outlet 26 is dry can therefore be detected both on the basis of an increase in the propagation times of the ultrasonic signals 35 and on the basis of a frequency shift / spectrum change of the ultrasonic signals 35 detected by the ultrasonic receiver 34.The evaluation device 40 can be designed to recognize that the water separator 20 no longer contains liquid 23 and then close the previously opened outlet valve 28 again by comparing the transit times of the ultrasonic signals 35 detected by the ultrasonic sensor 30 and / or by comparing a frequency shift of the detected ultrasonic signals 35 with at least one predefined limit value, in order to prevent a part of the return air flow 19 from escaping from the water separator 20 through the liquid outlet 26 and not being available for return to the fuel cell system 2.

Claims

Water separator (20) for separating water from an exhaust air stream (14) of a fuel cell (4), wherein the water separator (20) has: a gas inlet (22) for receiving the exhaust air stream (14) from the at least one fuel cell (4); a gas outlet (24) for outputting a substantially gaseous fraction of the received exhaust air stream (14); a liquid outlet (26) for outputting liquid (23) which has been separated from the exhaust air stream (14); an ultrasonic sensor (30) which is provided in or at the gas outlet (24) and which is designed to emit at least one ultrasonic signal (35) into the gas outlet stream (25) flowing through the gas outlet (24) and to detect at least one ultrasonic signal (35) emerging from the gas outlet stream (25) flowing through the gas outlet (24); and an evaluation device (36) which is designed to determine the water content in the gas outlet stream (25) flowing out through the gas outlet (24) by evaluating the ultrasonic signal (35) detected by the ultrasonic sensor (30).Water separator (20) according to Claim 1, wherein the evaluation device (36) is designed to evaluate the amplitude of at least one ultrasonic signal detected by the ultrasonic sensor (30) in order to determine the water fraction in the gas outlet stream (25) flowing through the gas outlet (24).Water separator (20) according to Claim 1 or 2, having an outlet valve (28) arranged in the liquid outlet (26), which outlet valve makes it possible to selectively open and block the liquid outlet (26), wherein the evaluation device (36) is designed to actuate the outlet valve (28) as a function of the previously determined water fraction in the gas outlet stream (25) flowing out through the gas outlet (24).Water separator (20) according to one of the preceding claims, wherein the evaluation device (36) is additionally designed to also determine the volume flow of the gas outlet flow (25) flowing through the gas outlet (24) with the aid of the ultrasonic sensor (30).Water separator (20) according to Claim 4, wherein the evaluation device (36) is designed to evaluate the transit time and / or the frequency spectrum of at least one ultrasonic signal (35) detected by the ultrasonic sensor (30) in order to determine the volume flow of the gas outlet flow flowing through the gas outlet (24).Water separator (20) according to Claim 4 or 5, wherein the evaluation device (36) is designed to close the outlet valve (28) if the volume flow of the gas outlet flow (25) flowing through the gas outlet (24) falls below a predefined limit value.Fuel cell system (2) having at least one fuel cell (4) which has an inlet air line (6) and an outlet air line (8), and a water separator (20) according to one of the preceding patent claims, wherein the gas inlet (22) of the water separator (20) is connected to the outlet air line (8) of the at least one fuel cell (4) in order to receive an outlet air stream (14) which is output by the at least one fuel cell (4); and wherein the gas outlet (24) of the water separator (20) is connected fluidically to the inlet air line (6) of the at least one fuel cell (4) in order to lead the gas outlet stream (25) which flows out of the gas outlet (24) of the water separator (20) as a outlet air stream (19) into the at least one fuel cell (4).Fuel cell system according to Claim 7, wherein a gas conveying device (13) is provided between the gas outlet (24) and the supply air line (6) in order to convey the return air stream (19) from the gas outlet (24) into the supply air line (6).Motor vehicle (1) having at least one electric motor (5) and a fuel cell system (2) according to Claim 7 or 8, wherein the at least one fuel cell (4) of the fuel cell system (2) is designed to supply the at least one electric motor (5) with electrical energy.Method of determining the water content of a gas outlet stream (25) flowing out through a gas outlet (24) of a water separator (20), the method comprising: emitting an ultrasonic signal (35) into the gas outlet stream (25) flowing through the gas outlet (24) and detecting an ultrasonic signal (35) flowing out of the gas outlet stream (25) flowing out through the gas outlet (24); and evaluating the detected ultrasonic signal (35) to determine the water content of the gas outlet stream (25) flowing out through the gas outlet (24); wherein the method in particular comprises evaluating the amplitude of the detected ultrasonic signal (35) to determine the water content in the gas outlet stream (25) flowing through the gas outlet (24).

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