Fuel cell system

Abstract

This invention provides a fuel cell system that can estimate the nitrogen concentration of gases emitted from the fuel cell at low cost. [Solution] The fuel cell system 1 comprises a fuel cell 2 that generates electricity through an electrochemical reaction between hydrogen gas and oxygen gas and discharges hydrogen off gas containing unreacted hydrogen gas together with nitrogen gas and moisture; a hydrogen supply channel 40 that supplies hydrogen gas from a hydrogen tank 41 to the fuel cell 2; a recirculation channel 50 that recirculates the hydrogen off gas back into the hydrogen supply channel 40; a gas-liquid separator 51 that separates moisture from the hydrogen off gas to obtain dehumidified hydrogen gas and stores the separated moisture; an exhaust valve 54 that exhausts the dehumidified hydrogen gas from the recirculation channel 50; a pressure sensor 52 that detects the pressure in the recirculation channel 50; and a control device 6 that controls the exhaust valve 54. The control device 6 estimates the concentration of nitrogen gas contained in the dehumidified hydrogen gas based on the value detected by the pressure sensor 52, and controls the exhaust valve 54 to an open state when the concentration of nitrogen gas exceeds a predetermined value.

Description

Technical Field

[0001] The present invention relates to a fuel cell system that generates electricity by supplying hydrogen gas to a fuel cell.

Background Art

[0002] Conventionally, a fuel cell system has been developed that generates electricity through an electrochemical reaction between hydrogen gas and oxygen gas and supplies power to an electric motor that drives a vehicle or the like. A typical fuel cell system includes a fuel cell having an anode electrode (fuel electrode) to which hydrogen gas is supplied from a hydrogen gas tank through a hydrogen supply flow path and a cathode electrode (oxygen electrode) to which air containing oxygen gas is supplied, a gas-liquid separator that separates moisture from a hydrogen off-gas containing unreacted hydrogen gas and moisture discharged from the fuel cell, and a reflux flow path that refluxes the hydrogen off-gas from which moisture has been separated by the gas-liquid separator to the hydrogen supply flow path by a pump.

[0003] In a fuel cell, as the electrochemical reaction progresses, nitrogen in the air permeates through the electrolyte membrane from the cathode electrode to the anode electrode side. As a result, when the nitrogen partial pressure increases at the anode electrode and the hydrogen concentration decreases, the power generation ability of the fuel cell decreases. The fuel cell system described in Patent Document 1 calculates the amount of impurity gas present based on the propagation time of ultrasonic waves in the mixed gas in the hydrogen circulation path, and when the amount of impurity gas present is equal to or greater than a predetermined amount, it opens a purge valve (exhaust valve) to purge (exhaust) the impurity gas accumulated in the fuel cell and the hydrogen circulation path.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In the fuel cell system described in Patent Document 1, purging is performed when the amount of impurity gas exceeds a predetermined amount, so impurity gas can be discharged more efficiently compared to, for example, purging is performed periodically at predetermined time intervals. However, the fuel cell system described in Patent Document 1 has the problem of requiring an ultrasonic transmitter / receiver, which is costly.

[0006] Therefore, the present invention aims to provide a fuel cell system that can estimate the nitrogen concentration of gas emitted from a fuel cell at low cost. [Means for solving the problem]

[0007] To achieve the above objective, the present invention provides a fuel cell system comprising: a fuel cell that generates electricity by an electrochemical reaction between hydrogen gas and oxygen gas and discharges hydrogen off gas containing unreacted hydrogen gas together with nitrogen gas and moisture; a hydrogen supply channel that supplies the hydrogen gas from a hydrogen source to the fuel cell; a recirculation channel that recirculates the hydrogen off gas back into the hydrogen supply channel; a gas-liquid separator provided in the recirculation channel that separates the moisture from the hydrogen off gas to obtain dehumidified hydrogen gas and stores the separated moisture; an exhaust valve that exhausts the dehumidified hydrogen gas from the recirculation channel; a pressure sensor that detects the pressure in the recirculation channel; and a control device that controls the exhaust valve, wherein there is no pump in the recirculation channel to pump the dehumidified hydrogen gas, and the control device estimates the concentration of nitrogen gas contained in the dehumidified hydrogen gas based on the value detected by the pressure sensor, and controls the exhaust valve to an open state when the concentration of nitrogen gas exceeds a predetermined value. [Effects of the Invention]

[0008] According to the fuel cell system of the present invention, it is possible to estimate the nitrogen concentration of the gas emitted from the fuel cell at low cost. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram illustrating an example of the configuration of a fuel cell system according to an embodiment of the present invention. [Figure 2] This graph shows an example of the change in nitrogen gas concentration (estimated value) in dehumidified hydrogen gas and the opening / closing state of the exhaust valve. [Figure 3] This is a schematic diagram showing an example configuration of a fuel cell system related to a comparative example. [Modes for carrying out the invention]

[0010] [Embodiment] Figure 1 is a schematic diagram illustrating an example of the configuration of a fuel cell system 1 according to an embodiment of the present invention. This fuel cell system 1 is mounted on, for example, an electric small mobility vehicle that has an electric motor as its drive source, and generates electricity to supply to the electric motor. Here, an electric small mobility vehicle refers to a vehicle that has a total weight lighter than a regular automobile or light automobile under the Road Transport Vehicle Act, and has an electric motor as its drive source. Specifically, examples of electric small mobility vehicles include golf carts, electric assist bicycles, electric kick scooters, and senior mobility scooters.

[0011] The fuel cell system 1 comprises a fuel cell 2, an air supply system 3 that supplies air containing oxygen gas to the fuel cell 2, a hydrogen supply system 4 that supplies hydrogen gas as fuel to the fuel cell 2, a hydrogen reflux system 5 that recovers unreacted hydrogen gas emitted from the fuel cell 2 and returns it to the air supply system 3, a control device 6, and a diluent 7. The electricity generated by the fuel cell 2 is converted by a PCU (Power Control Unit) 81 including a capacitor, a DC-DC converter, and an inverter, and supplied to an electric motor 82, etc., which is the drive source for a small mobility device.

[0012] The fuel cell 2 has a stack structure in which multiple unit cells 22 are stacked inside a case 21. Figure 1 shows the inside of the case 21 with a portion cut out. Each unit cell 22 comprises a flat electrolyte membrane 221, an anode electrode (fuel electrode) 222 provided on one side of the electrolyte membrane 221 in the stacking direction, a cathode electrode (oxygen electrode) 223 provided on the other side of the electrolyte membrane 221 in the stacking direction, and a pair of separators 224 positioned opposite each other with the anode electrode 222 and cathode electrode 223 in between.

[0013] In each unit cell 22 constituting the fuel cell 2, when hydrogen gas is supplied as fuel gas to the anode electrode 222 and oxygen gas is supplied to the cathode electrode 223, electricity is generated by an electrochemical reaction between the hydrogen gas and the oxygen gas. In addition, as the electrochemical reaction progresses in the fuel cell 2, some nitrogen from the air seeps out from the cathode electrode 223 through the electrolyte membrane 221 to the anode electrode 222 side. The fuel cell 2 discharges hydrogen off-gas, which includes unreacted hydrogen gas that did not react electrochemically with the oxygen gas, water generated during power generation in the fuel cell 2, and nitrogen gas that seeped out to the anode electrode 222 side, from the outlet 20.

[0014] The air supply system 3 comprises an air supply channel 30 through which air supplied to the fuel cell 2 flows, and a compressor 31 provided in the air supply channel 30. The oxygen off-gas discharged from the fuel cell 2 is discharged through the air exhaust channel 10. The air supplied to the fuel cell 2 is the ambient air surrounding the fuel cell system 1, and contains approximately 21% oxygen and approximately 78% nitrogen.

[0015] The hydrogen supply system 4 includes a hydrogen supply channel 40 that supplies hydrogen gas from a hydrogen tank 41, which is a hydrogen supply source, to the fuel cell 2; a main shut-off valve 42, which is an electromagnetic shut-off valve that shuts off or allows the supply of hydrogen gas from the hydrogen tank 41 to the fuel cell 2; and an ejector (also called an ejector) 43 that adjusts the amount of hydrogen gas supplied to the fuel cell 2. The main shut-off valve 42 and the ejector 43 are provided in the hydrogen supply channel 40. A pressure reducing valve may be provided between the main shut-off valve 42 and the ejector 43.

[0016] The ejector 43 is located in the hydrogen supply channel 40 between the main shut-off valve 42 and the fuel cell 2, and discharges hydrogen gas from the hydrogen tank 41 to the fuel cell 2. The amount of hydrogen gas supplied to the fuel cell 2 via the ejector 43 is controlled by the control device 6. The ejector 43 has a valve that opens and closes in response to a pulse signal output from the control device 6. When the period of the pulse signal output from the control device 6 to the ejector 43 becomes shorter, the valve closes before the hydrogen gas flow rate in the hydrogen supply channel 40 increases after it opens, thus reducing the amount of hydrogen gas supplied to the fuel cell 2. Conversely, when the period of the pulse signal becomes longer, the hydrogen gas flow rate in the hydrogen supply channel 40 increases, and the amount of hydrogen gas supplied to the fuel cell 2 increases. The control device 6 controls the ejector 43 according to the period of the pulse signal so that an amount of hydrogen gas corresponding to the amount of power the fuel cell 2 needs is supplied to the fuel cell 2.

[0017] The hydrogen reflux system 5 includes a reflux channel 50 that recirculates the hydrogen off-gas discharged from the fuel cell 2 to the hydrogen supply channel 40, a gas-liquid separator 51 installed in the middle of the reflux channel 50, a pressure sensor 52 that detects the pressure in the reflux channel 50, a drain valve 53 connected to the gas-liquid separator 51, and an exhaust valve 54 connected to the reflux channel 50.

[0018] The gas-liquid separator 51 separates moisture from the hydrogen off-gas to obtain dehumidified hydrogen gas. The reflux channel 50 refluxes the dehumidified hydrogen gas, from which moisture has been separated in the gas-liquid separator 51, to the hydrogen supply channel 40. The moisture separated in the gas-liquid separator 51 is temporarily stored in the gas-liquid separator 51 in liquid form and is discharged to the outside when the drain valve 53 is opened. The exhaust valve 54 is connected downstream of the gas-liquid separator 51 in the reflux channel 50 and exhausts the dehumidified hydrogen gas in the reflux channel 50 to the diluent 7. The diluent 7 dilutes the dehumidified hydrogen gas to a hydrogen concentration that is safe to release into the atmosphere and releases the diluted exhaust gas into the atmosphere.

[0019] Hereinafter, the portion of the reflux passage 50 upstream of the gas-liquid separator 51 is referred to as the first reflux passage 501, and the portion of the reflux passage 50 downstream of the gas-liquid separator 51 is referred to as the second reflux passage 502. The pressure sensor 52 is connected to the second reflux passage 502, detects the pressure of the dehumidified hydrogen gas in the second reflux passage 502, and outputs the detection result to the control device 6.

[0020] The reflux passage 50 is connected to the ejector 43. A pump for pumping the dehumidified hydrogen gas is not arranged in the reflux passage 50. The ejector 43 adds the dehumidified hydrogen gas from the second reflux passage 502 to the hydrogen gas supplied from the hydrogen tank 41 via the hydrogen supply passage 40 and discharges it to the fuel cell 2 side. More specifically, when the ejector 43 discharges the hydrogen gas from the hydrogen tank 41 to the fuel cell 2 side, the ejector 43 sucks the dehumidified hydrogen gas from the reflux passage 50 due to the negative pressure generated, and discharges the sucked dehumidified hydrogen gas together with the hydrogen gas from the hydrogen tank 41 to the fuel cell 2.

[0021] The opening and closing states of the drain valve 53 and the exhaust valve 54 are controlled by the control device 6. The control device 6 estimates the concentration of nitrogen gas contained in the dehumidified hydrogen gas based on the detection value of the pressure sensor 52, and when the estimated concentration of nitrogen gas becomes a predetermined value or more, controls the drain valve 53 and the exhaust valve 54 to be in an open state for a predetermined time. Thereby, the nitrogen concentration at the anode electrode 222 of the fuel cell 2 decreases and the power generation ability of the fuel cell 2 is maintained. That is, when the concentration of nitrogen gas increases and the concentration of hydrogen gas decreases at the anode electrode 222 of the fuel cell 2, the power generation ability of the fuel cell 2 decreases. However, in the present embodiment, when the concentration of nitrogen gas estimated based on the detection value of the pressure sensor 52 becomes a predetermined value or more, the exhaust valve 54 is controlled to be in an open state and the nitrogen partial pressure at the anode electrode 222 decreases, so the power generation efficiency of the fuel cell 2 is increased.

[0022] Here, a method for estimating the concentration of nitrogen gas in the dehumidified hydrogen gas based on the detected value of the pressure sensor 52 will be described. The molecular weight of hydrogen gas (H2) is 2.0158 g / mol, while the molecular weight of nitrogen gas (N2) is 28.0134 g / mol. Therefore, nitrogen gas has a higher viscosity than hydrogen gas, and as the concentration of nitrogen gas in the dehumidified hydrogen gas increases, the viscosity of the dehumidified hydrogen gas increases, and the pressure detected by the pressure sensor 52 increases. Thus, the concentration of nitrogen gas in the dehumidified hydrogen gas can be estimated based on the detected value of the pressure sensor 52.

[0023] Further, the control device 6 may estimate the concentration of nitrogen gas in the dehumidified hydrogen gas taking into account the influencing factors that affect the pressure detected by the pressure sensor 52. Examples of such influencing factors include the control amount of the ejector 43 by the control device 6 and the power generation amount of the fuel cell 2. In the present embodiment, the period of the pulse signal output to the ejector 43 corresponds to the control amount of the ejector 43 by the control device 6. By estimating the concentration of nitrogen gas in the dehumidified hydrogen gas taking into account these influencing factors, the concentration of nitrogen gas can be estimated more accurately.

[0024] FIG. 2 is a graph showing an example of the change in the concentration (estimated value) of nitrogen gas in the dehumidified hydrogen gas and the opening / closing state of the exhaust valve 54. The horizontal axis of the graph is the time axis, and Sh on the vertical axis in the graph of the concentration of nitrogen gas indicates the above-mentioned predetermined value.

[0025] When the exhaust valve 54 is in the closed state, the concentration of nitrogen gas in the dehumidified hydrogen gas gradually increases due to the nitrogen that permeates from the cathode electrode 223 through the electrolyte membrane 221 to the anode electrode 222. When the exhaust valve 54 is in the open state, the dehumidified hydrogen gas with a high concentration of nitrogen gas is exhausted and new hydrogen gas is supplied from the hydrogen tank 41, so that the concentration of nitrogen gas on the anode electrode 222 side of the fuel cell 2 and in the reflux passage 50 decreases. Thereby, the power generation ability of the fuel cell 2 is maintained. The interval (T1) of the time when the exhaust valve 54 is in the open state depends on the power generation amount of the fuel cell 2 and is, for example, about 30 seconds, and the time (T2) when the exhaust valve 54 is open is, for example, 1 second or less.

[0026] (Comparative example) Figure 3 is a schematic diagram showing an example configuration of fuel cell system 1A according to a comparative example. In Figure 3, components common to fuel cell system 1 shown in Figure 1 are denoted by the same reference numerals, and redundant explanations are omitted.

[0027] The fuel cell system 1A replaces the pressure sensor 52 of the fuel cell system 1 according to the above embodiment with a pump 90, a first pressure sensor 91, and a second pressure sensor 92. The pump 90 is provided in the second reflux channel 502 and supplies dehumidified hydrogen gas to the hydrogen supply channel 40. The first pressure sensor 91 is provided on the gas-liquid separator 51 side of the pump 90, and the second pressure sensor 92 is provided on the hydrogen supply channel 40 side of the pump 90.

[0028] The first pressure sensor 91 and the second pressure sensor 92 detect the pressure of the dehumidified hydrogen gas before and after the pump 90 and output the detection results to the control device 6A. Similar to the control device 6 of the fuel cell system 1 according to the above embodiment, the control device 6A controls the drain valve 53 and the exhaust valve 54 to be open for a predetermined time when the estimated value of the nitrogen gas concentration in the dehumidified hydrogen gas exceeds a predetermined value. However, the method of estimating the nitrogen gas concentration differs from that of the control device 6 according to the above embodiment, as the nitrogen gas concentration is estimated based on the difference between the pressure detected by the first pressure sensor 91 and the pressure detected by the second pressure sensor 92.

[0029] In other words, as described above, the viscosity of dehumidified hydrogen gas increases as the nitrogen gas concentration increases. Therefore, even if the rotational speed of pump 90 remains the same, a change in nitrogen gas concentration will cause a change in the pressure difference between the suction and discharge sides of pump 90. The control device 6A determines this pressure difference based on the detection results of the first pressure sensor 91 and the second pressure sensor 92, and estimates the nitrogen gas concentration in the dehumidified hydrogen gas.

[0030] In this fuel cell system 1A, similar to the fuel cell system 1 according to the above embodiment, the power generation capacity of the fuel cell 2 can be maintained by controlling the exhaust valve 54 to be open for a predetermined time when the estimated concentration of nitrogen gas in the dehumidified hydrogen gas exceeds a predetermined value. However, since a pump 90 and two pressure sensors (first pressure sensor 91 and second pressure sensor 92) are required, the weight, cost, and power consumption are increased compared to the fuel cell system 1 according to the above embodiment.

[0031] In other words, according to the fuel cell system 1 of the above embodiment, the nitrogen concentration of the gas emitted from the fuel cell 2 can be estimated at low cost, and weight and power consumption can also be reduced. Furthermore, since there is no pump in the recirculation channel 50, the nitrogen concentration can be accurately estimated based on the detection result of the pressure sensor 52 without being affected by pressure changes caused by the pump.

[0032] (Note) The present invention has been described above based on embodiments, but these embodiments do not limit the invention as defined in the claims. Furthermore, it should be noted that not all combinations of features described in the embodiments are necessarily essential for solving the problem of the invention.

[0033] Furthermore, the present invention can be implemented by modifying it as appropriate, without departing from its spirit, by omitting some components or adding or substituting components. For example, in the above embodiment, the case in which the pressure of the second reflux channel 502 is detected by the pressure sensor 52 was described, but the arrangement of the pressure sensor 52 may be changed so that the pressure of the first reflux channel 501 is detected by the pressure sensor 52. However, if the pressure of the second reflux channel 502 is detected by the pressure sensor 52, the pressure of the dehumidified hydrogen gas obtained by separating moisture from the hydrogen off-gas by the gas-liquid separator 51 can be detected, so it is possible to estimate the nitrogen concentration more accurately while suppressing the influence of moisture. [Explanation of symbols]

[0034] 1…Fuel cell system 2…Fuel cell 40…Hydrogen supply channel 43…Ejector 50...reflux channel 51...gas-liquid separator 52...Pressure sensor 54...Exhaust valve

Claims

1. A fuel cell that generates electricity through an electrochemical reaction between hydrogen gas and oxygen gas, and emits hydrogen off-gas containing unreacted hydrogen gas together with nitrogen gas and water, A hydrogen supply channel for supplying the hydrogen gas from a hydrogen source to the fuel cell, A recirculation channel for recirculating the hydrogen off-gas back into the hydrogen supply channel, A gas-liquid separator is provided in the reflux channel and separates the moisture from the hydrogen off-gas to obtain dehumidified hydrogen gas and stores the separated moisture. An exhaust valve for exhausting the dehumidified hydrogen gas from the reflux channel, A pressure sensor for detecting the pressure in the aforementioned recirculation channel, The system includes a control device for controlling the exhaust valve, The aforementioned reflux channel does not have a pump for pumping the dehumidified hydrogen gas. The control device estimates the concentration of nitrogen gas contained in the dehumidified hydrogen gas based on the value detected by the pressure sensor, and controls the exhaust valve to an open state when the concentration of nitrogen gas exceeds a predetermined value. Fuel cell system.

2. The pressure sensor is located downstream of the gas-liquid separator in the reflux channel and detects the pressure of the dehumidified hydrogen gas. The fuel cell system according to claim 1.

3. An ejector is provided in the hydrogen supply channel for discharging the hydrogen gas from the hydrogen supply source to the fuel cell side. The aforementioned recirculation channel is connected to the ejector, The ejector adds the dehumidified hydrogen gas from the reflux channel to the hydrogen gas supplied from the hydrogen source via the hydrogen supply channel and discharges it to the fuel cell side. The fuel cell system according to claim 1 or 2.

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