JP2007115488A - Cathode gas control method of fuel cell and fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize improvement of fuel efficiency by restraining power consumption at a cathode compressor while taking a water budget balance of a fuel cell into consideration. <P>SOLUTION: The cathode compressor for supplying cathode gas is operated at an operation point where a cathode gas pressure P and a cathode gas flow volume q within a capacity of the cathode compressor become minimum, respectively. In this case, it is preferable to set the operation point on a graph so that a quantity of moisture exhausted from the fuel cell coincide with that to be exhausted therefrom, by utilizing the fact that a gradient of quantity of moisture to be exhausted is proportional to the gradient of a graph showing a relation between the cathode gas pressure P and the cathode gas flow volume q. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a fuel cell cathode gas control method and a fuel cell system. More specifically, the present invention relates to an improvement in control technology such as pressure when supplying a cathode gas.

A cathode compressor such as an air compressor is used to supply a cathode gas to the cathode (oxygen electrode) of the fuel cell. In such a cathode compressor, for example, gas flow rate control or gas pressure control may be performed so that the amount of water (membrane water content) contained in the electrolyte membrane in the fuel cell can be kept moderate. Further, from the viewpoint of improving the fuel consumption of the fuel cell, there is a case where control is performed to reduce the power consumption of the air compressor (cathode compressor) by reducing the gas pressure (see, for example, Patent Document 1).
JP 2004-335444 A

  However, it is difficult to say that the control technology that only lowers the gas pressure from the viewpoint of improving fuel efficiency makes it possible to generate power efficiently when viewed as a whole fuel cell system.

  In addition, when starting the fuel cell under a low temperature condition such as below freezing point, it is necessary to control the membrane water content while taking into consideration the situation where water is difficult to evaporate. Nevertheless, as described above, there is a problem that the water balance (the flow of water in and out of the fuel cell, or the balance of the amount) is lost just by reducing the gas pressure in order to improve the fuel efficiency.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a fuel cell cathode gas control method and a fuel cell system capable of improving fuel efficiency while taking into account the balance of water balance in the fuel cell.

  In order to solve this problem, the present inventor has made various studies. In the first place, in order to balance the water balance within the fuel cell, the amount of water to be discharged from the fuel cell (that is, the amount of water to be removed) must be taken into account, and the actual amount of water discharged (the amount of water to be removed) It is desirable that the control be adapted to the above. In this respect, in the fuel cell, the amount of generated water is determined according to the current or the like, and therefore the amount of water to be discharged (the amount of water necessary to be removed) is determined based on this. Here, the present inventor paid attention to the fact that the required water removal amount is proportional to the cathode gas flow rate and inversely proportional to the cathode gas pressure, and as a result of repeated studies, the fuel efficiency was improved while considering the balance of water balance. It came to know the technology that can realize.

  The present invention is based on such knowledge, and the fuel cell cathode gas control method according to claim 1 is configured such that the amount of water discharged from the fuel cell matches the amount of water to be discharged from the fuel cell. The cathode compressor is operated by setting operating points at which the cathode gas pressure and the cathode gas flow rate that can be taken by the cathode compressor for supplying the cathode gas are minimum values.

  The cathode gas pressure and the cathode gas flow rate are not all free values, and the cathode gas flow rate requires a certain minimum flow rate that is related to the minimum rotation speed of the cathode compressor (air compressor). The cathode gas pressure has a certain upper limit depending on the structure and performance of the cathode compressor. In addition, regarding the relationship between the cathode gas flow rate and the cathode gas pressure, a cathode gas pressure higher than the pressure loss increase due to the cathode gas flow rate is required (in other words, if a certain value of gas flow rate is to be secured, at least a value corresponding to it). It is necessary to set the above gas pressure). Therefore, since the range of the cathode gas pressure and the cathode gas flow rate that can be taken by the cathode compressor is determined from such a relationship, if the operating point at which the respective values are minimum within this range is set, the fuel cell system It is possible to improve fuel efficiency by suppressing power consumption while maintaining efficient power generation as a whole.

  In addition, in the present invention, the operating point is set while matching the amount of water discharged from the fuel cell with the amount of water to be discharged from the fuel cell. It is possible to set an operating point at which the power consumption of the cathode compressor can be suppressed while matching the amount of water) to the amount of water to be discharged from the fuel cell (necessary amount of removed water). Accordingly, even when the fuel cell is started under a low temperature condition such as a freezing point, for example, the balance of water balance is prevented from being lost, and the fuel consumption can be improved while controlling the water content of the membrane.

  According to a second aspect of the present invention, the operating point is set on the graph using the fact that the amount of water to be discharged is proportional to the slope of the graph representing the relationship between the cathode gas pressure and the cathode gas flow rate. Is. That is, the operating point can be set using a function that can be represented on the graph.

  In the fuel cell cathode gas control method according to claim 3, the operating point is set within the range of the cathode gas pressure and the cathode gas flow rate that can be taken by the cathode compressor for supplying the cathode gas. In this case, the operating point is set in consideration of the balance of each element of moisture balance of fuel cell, fuel consumption, and stability during operation, and the cathode compressor is operated at the operating point. To do.

  The above-mentioned invention is suitable for improving fuel efficiency while taking into account the water balance, but this is not the only factor to be considered when operating the fuel cell. is there. In such a case, it is preferable to set an operating point in consideration of a comprehensive balance in consideration of the factor of stability in addition to the factor of water balance (moisture amount balance) and fuel consumption described above. As a result, it is possible to realize an operation in which each element (or various requirements during the operation) is balanced in a high dimension.

  Here, when priority is given to the stability during operation among the elements, as described in claim 4, the operating point is set while maintaining the minimum amount of the cathode gas flow rate capable of maintaining the stability. It is preferable to set. As a result, while maintaining the stability, it is possible to balance the water balance as much as possible and improve fuel efficiency.

  Further, in this case, as described in claim 5, the amount of water discharged from the fuel cell should be discharged from the fuel cell while maintaining the minimum amount of the cathode gas flow rate capable of maintaining the stability. Preferably, the operating point is set so as to approach the amount of water, and as described in claim 6, the amount of water discharged from the fuel cell is closest to the amount of water to be discharged from the fuel cell. More preferably, the operating point is set. By setting an operating point close to the amount of water to be discharged, the water balance becomes closer to a preferable state accordingly. Note that setting the operating point so as to approach the amount of water to be discharged in this way results in a case where the amount of discharged water (the amount of water taken away) coincides with the amount of water to be discharged (the amount of water required to be removed). including. If they match in this way, the water balance in the fuel cell becomes the most balanced, so that the membrane water content in the fuel cell can be kept in an optimal state.

  Furthermore, a fuel cell system according to a seventh aspect of the present invention includes a control device for performing the cathode gas control method according to any of the first to sixth aspects.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to implement | achieve the fuel consumption improvement by suppressing the power consumption in a cathode compressor, considering the balance of the water balance in a fuel cell. Further, when priority is given to other factors (for example, driving stability), it is possible to achieve both a balance of water balance and an improvement in fuel consumption within a possible range.

  Hereinafter, the configuration of the present invention will be described in detail based on an example of an embodiment shown in the drawings.

  1 to 5 show an embodiment of a fuel cell cathode gas control method and a fuel cell system according to the present invention. The fuel cell system 1 in this embodiment includes a cathode compressor (hereinafter referred to as an air compressor) 5 for supplying cathode gas to the fuel cell 2 (see FIG. 1). Among the operating points determined by the cathode gas pressure (hereinafter also referred to as air pressure) and the cathode gas flow rate (hereinafter also referred to as air flow rate), the operating point at which each of these air pressure and air flow rate is at a minimum value. The air compressor 5 is operated to supply cathode gas (air). The operating point in this specification represents the cathode gas pressure value and the cathode gas flow rate during the operation of the cathode compressor. For example, a certain point in the gas pressure-gas flow graph (see 4 etc.) Become. In the embodiment described below, the fuel cell system 1 will be described first and then the details of such a cathode gas control method will be described (see FIGS. 1 to 5).

  FIG. 1 shows a schematic configuration of a fuel cell system 1 according to the present embodiment. The fuel cell system 1 shown in the present embodiment can be used as, for example, an in-vehicle power generation system of a fuel cell vehicle (FCHV; Fuel Cell Hybrid Vehicle), but is not limited to this, and various mobile objects (for example, ships) Naturally, it can also be used as a power generation system mounted on a self-propelled device such as a robot or a robot. A fuel cell stack (not shown) has a stack structure in which a plurality of single cells are stacked in series, and is composed of, for example, a solid polymer electrolyte fuel cell.

  The oxidizing gas supply system to the fuel cell 2 includes an air compressor 5, an intercooler 3, and an intercooler cooling water pump 4 (see FIG. 1). The air compressor 5 compresses air taken from outside air through an air filter (not shown). The intercooler 3 cools the air that has been compressed to a high temperature. The intercooler cooling water pump 4 circulates cooling water for cooling the intercooler 3. The air compressed by the air compressor 5 is cooled by the intercooler 3 in this way, passes through the humidifier 17, and is supplied to the cathode (oxygen electrode) of the fuel cell 2. The oxygen off-gas after being subjected to the cell reaction of the fuel cell 2 flows through the cathode off-gas channel 16 and is exhausted out of the system. This oxygen off gas is in a highly moist state because it contains moisture generated by the cell reaction in the fuel cell 2. Therefore, the humidifier 17 exchanges moisture between the oxidant gas before supply in a low wet state and the oxygen off gas in high wet state flowing through the cathode offgas flow path 16, and the oxidant gas supplied to the fuel cell 2 Humidify moderately.

  A cooling water pipe 11 for circulating the cooling water is provided at the inlet / outlet of the cooling water (LLC) of the fuel cell 2. The cooling water pipe 11 is provided with a water pump 10 that sends out the cooling water and a flow path switching valve 12 for adjusting the cooling water supply amount.

  Part of the DC power generated by the fuel cell 2 is stepped down by the DC / DC converter 14 and charged to the secondary battery (battery) 15. The motor inverter 7 converts the DC power supplied from the fuel cell 2 into AC power and supplies the AC power to the traction motor 8. Further, the water pump inverter 9 converts the DC power supplied from the fuel cell 2 into AC power and supplies the AC power to the water pump 10. Further, the air compressor driving inverter 6 converts the DC power supplied from the fuel cell 2 into AC power and supplies the AC power to the air compressor 5.

  For example, if the control device 13 is mounted on a fuel cell vehicle, the control device 13 obtains the system required power (the sum of the vehicle travel power and the auxiliary power) based on the accelerator opening, the vehicle speed, etc. It is a device that controls the system to match the power. More specifically, the control device 13 controls the air compressor driving inverter 6 to adjust the rotational speed and the oxidizing gas supply amount of a motor (not shown) that drives the air compressor 5. Further, the temperature of the compressed air is adjusted by controlling the water pump 4 for cooling the intercooler. Further, the motor inverter 7 is controlled to adjust the rotational speed of the traction motor 8, and the water pump inverter 9 is controlled to adjust the water pump 10. Further, the operation point (output voltage, output current) of the fuel cell 2 is adjusted by controlling the DC / DC converter 14 so that the output power of the fuel cell 2 matches the target power.

  Further, the fuel cell system of the present embodiment is configured such that surplus regenerative power during braking of the fuel cell vehicle can be consumed by the air compressor 5 (see FIG. 1). That is, in the fuel cell vehicle having the secondary battery 15, the torque equivalent to the engine brake is generated by regenerating the drive motor on a downhill road, for example, and the regenerative energy generated at this time is the above-mentioned regenerative energy. The secondary battery 15 is stored. However, if the charging capacity is exceeded, it is necessary to consume energy by another method. For example, in this embodiment, the regenerative energy is consumed by rotating the air compressor 5 to stabilize the energy. Brake torque is obtained.

  Here, for example, from the viewpoint of improving the fuel consumption of a fuel cell vehicle, it is desirable to lower the air pressure of the air compressor (cathode compressor) 5 to lower the power consumption as described above. Since it is necessary to consider the water balance in 2 (the balance of water flow in and out), it is not necessary to simply lower the air pressure. Therefore, in the present embodiment, the membrane water content in the fuel cell 2 is controlled as follows to achieve a well-balanced water balance.

First, in the fuel cell 2, the amount of water generated in the fuel cell 2 is determined (uniquely determined) according to the magnitude of the current generated at that time. Therefore, according to the magnitude of the current, the amount of water at the time point at which the fuel cell 1 should be drained (this is referred to as the necessary amount of removed water or simply the amount of removed water in this specification) is also determined. Here, the amount of water taken away is proportional to the flow rate of air flowing through the fuel cell 2 and inversely proportional to the pressure. That is, if the air flow rate is q and the air pressure is P, the amount of water taken away is q / P.
It has a relationship. Therefore, if the Pq plane is created with the air pressure P as the x-axis and the air flow rate q as the y-axis, the required water removal amount is proportional to the slope of the straight line passing through the origin (see FIG. 2). Therefore, it can be said that the slope of this straight line has a character as an index of the amount of water taken away.

  Given that the slope of the straight line (operating line) shown in FIG. 2 has the above-described character, if the air pressure P and the air flow rate q are determined by the point (operating point) located on this straight line (operating line). This means that the amount of water removed is constant. Therefore, if you want to improve fuel efficiency while maintaining the required amount of water to take away, set the operating point located in the direction that lowers the air pressure P while maintaining the operating line, that is, the operating line that is as close as possible to the origin. This should be done (see FIG. 4).

  On the other hand, the air pressure P and the air flow rate q do not take free values in all regions on the Pq plane shown in FIG. 2, but are in a certain range (shown by a right triangle) as shown in FIG. It is possible to take only the value in the range). This will be described. First, regarding the air flow rate q, it is necessary to be at least a certain minimum air flow rate in relation to the minimum rotation speed of the air compressor 5. This determines the base of the right triangle. Regarding the air pressure P, a certain upper limit is determined depending on the structure and performance of the air compressor 5. This determines the right side (vertical side) of the right triangle. Furthermore, regarding the relationship between the air flow rate q and the air pressure P, an air pressure P that is greater than the pressure loss increase due to the air flow rate q is required. That is, if an air flow rate q of a certain value is to be secured, it is necessary to set an air pressure P that is at least equal to the value, and if the air flow rate q is increased, the pressure loss increases correspondingly (necessary) As a matter of course, the required air pressure also increases. This relationship determines the hypotenuse of the right triangle (see FIG. 3). From the above, the operating point determined by the air pressure P and the air flow rate q must be within the range of this right triangle.

  As described above, in the present embodiment, the balance of the amount of water taken away is maintained while satisfying the required relationship between the air pressure P and the air flow rate q, and the fuel efficiency can be most improved under these conditions. The intersection of the hypotenuse and the operation line is set as an operation point (see FIG. 4). That is, if the operating point is set on the operating line, it is possible to maintain the necessary water removal amount. Further, if the air pressure and the air flow rate are minimized within the range of the right triangle described above, the air pressure of the air compressor 5 is also minimized, and accordingly, power consumption can be reduced. . By performing such control, it is possible to achieve both the balance of water balance (in / out of moisture) in the fuel cell 2 and the improvement of fuel consumption as a system.

  Subsequently, the contents of the above-described control (membrane water content control) will be described using a block diagram (see FIG. 5).

  First, the target water content in the fuel cell 2 is set, and this is input to the comparison unit as a target value. In the comparison unit, a deviation which is a difference between this target value (target water content) and a feedback element (membrane water content calculated from impedance) is obtained. The deviation obtained in this way is input to the addition point after PI control. On the other hand, the amount of generated water is calculated from the magnitude of the current (FC current) at a certain point in time of the fuel cell 2, and this is used as the basic carry-out water amount. This basic carry-out amount is also input to the addition point. At the addition point, the necessary amount of water to be removed is obtained by adding a deviation to this basic amount of removal.

  Subsequently, the inclination of the operation line in the Pq plane (see FIG. 2) is calculated. Here, a table is prepared in advance for obtaining a slope (that is, obtaining a ratio between the air flow rate q and the air pressure P) from the necessary amount of water taken away and the temperature (FC temperature) of the fuel cell 2 (see FIG. 5). Here, the above-mentioned required removal amount and the detected internal temperature (FC temperature) of the fuel cell 2 are input, respectively. The ratio between the air flow rate q and the air pressure P is determined from these two factors, and the slope of the operation line is obtained.

  When the inclination of the operation line is obtained, the intersection (operation point) with the hypotenuse of the right triangle in the Pq plane is obtained (see FIGS. 4 and 5). Since the operating point is determined in this manner, the air pressure P and the air flow rate q that can achieve both the balance of water balance and the improvement of fuel consumption are respectively obtained. Control is performed as follows. Further, the membrane water content of the fuel cell 2 is sequentially calculated based on the impedance, and a difference (deviation) from the target water content is obtained using this amount as a feedback element (see FIG. 5).

  The above-described embodiment is an example of a preferred embodiment of the present invention, but is not limited thereto, and various modifications can be made without departing from the scope of the present invention. For example, in the above-described embodiment, the mode in which the operating point is set on the operating line has been described. However, this is only a preferable example, and it is preferable that the operating point is appropriately set at a position other than the operating line depending on circumstances. A specific example will be described below (see FIGS. 6 and 7).

  For example, when priority is given to stable operation of the fuel cell 2, there may be a minimum air flow rate for maintaining such a state. In such a case, it is preferable to appropriately set the operating point in relation to the inclination of the operating line. For example, when the minimum air flow rate q for stable operation is the amount indicated by (1) in FIG. 6, the air flow rate q is larger than the line indicated by (1) in order to continue the stable operation. There is a need. Under such circumstances, when the operating line has an inclination as shown in FIG. 6, even if an operating point at which the air flow rate is larger than the line (1) is set on this operating line, it is a right angle. This is not possible because it falls outside the range of the triangle. Therefore, in order to bring the amount of removed water as close as possible to the required amount (necessary amount of removed water) while giving priority to stable operation, within the range of the right triangle above the line (1) and within the operating line. In the case of this example, in the case of the closest position, it is preferable to set the operating point at the position indicated by symbol A in FIG. In such a case, it is possible to keep the water balance as much as possible by bringing the amount of removed water close to the required amount under the assumption that the stable operation of the fuel cell 2 is continued.

  In addition, when the air flow rate q for continuing stable operation is a smaller amount than the above-described case, for example, the amount indicated by (2), as indicated by reference numeral B in FIG. An operating point can be set at the intersection with the operating line. In such a case, it is possible to maintain the balance of water balance by making the amount of removed water equal to the required amount under the premise that the stable operation of the fuel cell 2 is continued.

  In addition, when the air flow rate q for continuing the stable operation is an amount smaller than the above-described case, for example, the amount indicated by (3), this line (3) as indicated by reference numeral C in FIG. In the upper region, it is possible to set an operating point at the intersection of the hypotenuse of the right triangle and the operating line. In such a case, the air pressure P and the air flow rate q are set to the minimum values while maintaining the water balance by matching the amount of water taken away with the required amount under the premise that the stable operation of the fuel cell 2 is continued. Thus, it is possible to improve fuel efficiency.

  Next, another example will be described (see FIG. 7). As in the case described above, even when the operation line has a slope as shown in FIG. 7 (a slope smaller than that in FIG. 6) under the situation where stable operation of the fuel cell 2 is prioritized, the same as in the case described above. It is preferable to set the operating point appropriately. For example, when the minimum air flow rate q for stable operation is the amount indicated by (4) in FIG. 7, in order to bring the amount of removed water as close as possible to the necessary amount (necessary amount of removed water) while giving priority to stable operation. Is set within the range above the line (4) of the right triangle and the position closest to the operation line, in the case of this example, the operation point is set at the position indicated by the symbol D in FIG. Is preferred. Further, when the air flow rate q for continuing stable operation is a smaller amount than the above case, for example, the amount indicated by (5), as shown by the symbol E in FIG. An operating point can be set at the intersection with the operating line. Further, in the case where the air flow rate q for continuing the stable operation is an amount smaller than that in this case, for example, an amount that coincides with the bottom of the right triangle as shown by (6), the symbol F in FIG. As shown in FIG. 8, the operating point can be set at the intersection of the line (6) and the operating line (in other words, the intersection of the base of the right triangle and the operating line).

  As described above, various embodiments have been described with reference to FIGS. 6 and 7. In short, the various factors such as a request for a water balance in the fuel cell 2, a request for improvement in fuel consumption, and a request for stable operation are taken into account. It is desirable to appropriately set a balanced operating point (setting a balanced air pressure P and air flow rate q) in consideration of which is prioritized. If another example is given, for example, in the case where the operating point indicated by the symbol A in FIG. 6 is set, even if the stability of the operation is somewhat inferior, under circumstances where improvement of the water balance is regarded as important, It is also possible to reset the operating point to a position slightly closer to the operating line as indicated by the reference symbol A ′. In such a case, the water balance can be improved accordingly.

  In the Pq graphs of FIGS. 2 to 7 (excluding FIG. 5), the relationship between the air pressure P and the air flow rate q (portion indicated by hatching) is represented by a straight line. Therefore, such an example is merely shown as a representative example, and in some cases, the relationship may be expressed as a curve (see, for example, FIG. 5).

1 is a diagram showing a schematic configuration of a fuel cell system according to an embodiment of the present invention. The graph shows the air pressure P on the horizontal axis and the air flow rate q on the vertical axis, and the amount of water taken away in the fuel cell is proportional to the slope of the straight line passing through the origin of this graph. It is a figure which shows the range of the value which the air pressure P and the air flow rate q can take. It is a figure which shows the concept in the case of setting an optimal operating point. It is a block diagram which shows an example of the cathode gas control method demonstrated by this embodiment. It is a figure for demonstrating another embodiment of this invention, and represents the example of a setting of the operating point in case the stable operation of a fuel cell is prioritized. It is a figure for demonstrating another example of embodiment shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Fuel cell system, 2 ... Fuel cell, 5 ... Air compressor (cathode compressor), 13 ... Control apparatus, P ... Air pressure (cathode gas pressure), q ... Air flow rate (cathode gas flow rate)

Claims (7)

  An operation in which each of the cathode gas pressure and the cathode gas flow rate that can be taken by the cathode compressor for supplying the cathode gas is minimized while making the amount of moisture discharged from the fuel cell coincide with the amount of water to be discharged from the fuel cell. A cathode gas control method for a fuel cell, wherein a point is set to operate the cathode compressor.   2. The operating point is set on the graph using the fact that the amount of water to be discharged is proportional to the slope of the graph representing the relationship between the cathode gas pressure and the cathode gas flow rate. For controlling the cathode gas of a fuel cell.   When setting the operating point within the range of cathode gas pressure and cathode gas flow rate that can be taken by the cathode compressor for supplying cathode gas, each of the fuel cell moisture balance, fuel consumption, and stability during operation A cathode gas control method for a fuel cell, wherein the operating point is set in consideration of a balance of elements, and the cathode compressor is operated at the operating point.   The operation point is set while maintaining a minimum amount of the cathode gas flow rate capable of maintaining the stability when priority is given to stability during operation among the elements. A fuel cell cathode gas control method.   The operating point is set such that the amount of water discharged from the fuel cell approaches the amount of water to be discharged from the fuel cell while maintaining the minimum amount of the cathode gas flow rate capable of maintaining the stability. The cathode gas control method for a fuel cell according to claim 4.   6. The fuel cell cathode gas control method according to claim 5, wherein the operating point is set so that the amount of water discharged from the fuel cell is closest to the amount of water to be discharged from the fuel cell. A fuel cell system comprising a control device for carrying out the cathode gas control method according to claim 1.

Images