Patent Application
20160141651
This application claims priority to Japanese Patent Application No. 2014-232249 filed on Nov. 15, 2014, the entire contents of which are incorporated by reference herein.
The present invention relates to a fuel cell system and to fuel cell system control method.
Conventionally, there has been known a fuel cell system in which hydrogen stored in a high-pressure hydrogen tank is supplied to a fuel cell via an injector and moreover in which an anode exhaust gas containing hydrogen that, although once supplied to a fuel cell, has not been used for power generation is circulated by a circulation pump so as to be supplied once again to the fuel cell (JP 2008-235020 A).
It has been desired heretofore that fuel cell systems be improved in power generation efficiency. As one of causes that lead to degradation of the power generation efficiency of fuel cells, there is a case in which hydrogen supplied to the anode permeates through an electrolyte membrane without being used for power generation reaction so as to leak to the cathode side (hereinafter, also referred to simply as ‘cross-leak’), with the result that hydrogen is not utilized for power generation but consumed. Particularly in intermittent operations, most of hydrogen supplied from the injector is consumed for the cross-leak, resulting in degraded power generation efficiency. The term, intermittent operation, in this case include to a state of a vehicle with a fuel cell system mounted thereon in which state the power generation of fuel cells is temporarily halted during a low-load operation while electric power is supplied from a secondary battery to a load (vehicle motor or the like), and to a state in which small power generation is executed at an extremely low load during a process of high-potential avoidance control or the like. The low-load operation is exemplified by such a state as an idling state, a low-speed running state or a regenerative braking state. The term, high-potential avoidance, refers to such control that an output voltage of the fuel cell is made to be a high-potential avoidance voltage smaller than an open circuit voltage (OCV) to order to suppress progression of deformation of a catalyst metal contained in the anode or the cathode in above-mentioned intermittent operations.
The present invention, having been accomplished to solve the above-described problems, can be implemented in the following aspects.
(1) In one aspect of the invention, there is provided a fuel cell system. The fuel cell system comprises: a fuel cell containing a unit cell with an anode and a cathode included therein; a hydrogen supply unit that supplies hydrogen gas to the anode; a circulation pump that supplies an anode exhaust gas containing hydrogen that has not been used for power generation by the fuel cell and is discharged from the anode, once again to the anode to circulate the anode exhaust gas; and a controller that controls supply quantity of the hydrogen gas by the hydrogen supply unit as well as rotating speed of the circulation pump, wherein the controller drives the circulation pump so that the rotating speed of the circulation pump approaches an optimum rotating speed of the circulation pump at which a total hydrogen loss quantity becomes a minimum under a specified current value, the total hydrogen loss quantity being a sum of a hydrogen quantity corresponding to an electric power necessary for driving the circulation pump and a hydrogen quantity passing from anode side to cathode side of the fuel cell. With this constitution, hydrogen quantity consumed by drive of the circulation pump for reduction of the cross-leak can be suppressed while the cross-leak level is reduced. That is, by raising the rotating speed of the circulation pump to increase the circulation quantity of the anode exhaust gas, the hydrogen partial pressure on the anode side of the fuel cell is lowered to reduce the cross-leak level while a hydrogen stoichiometric ratio necessary for generation of a target current value is ensured. On the other hand, by preventing excessive rise in the rotating speed of the circulation pump, the hydrogen quantity consumed by the drive of the circulation pump for reduction of the cross-leak can be suppressed. Thus, the power generation efficiency of the fuel cell can be improved.
(2) In the fuel cell system of the above described aspect, the controller may switch over between normal operation and intermittent operation of the fuel cell, drive the circulation pump at a rotating speed smaller than an optimum rotating speed at which the total hydrogen loss quantity becomes a minimum in the normal operation of the fuel cell, and drive the circulation pump so that the rotating speed of the circulation pump approaches the optimum rotating speed at which the total hydrogen loss quantity becomes a minimum under a specified current value in the intermittent operation of the fuel cell. With this constitution, in the intermittent operation of the fuel cell, the hydrogen quantity consumed by the drive of the circulation pump for reduction of the cross-leak can be suppressed while the cross-leak level is reduced.
(3) In another aspect of the invention, there is provided a fuel cell system control method. The control method comprises the steps of: increasing supply quantity of hydrogen gas with increasing current obtained from a fuel cell in normal operation of the fuel cell; and increasing a flow rate of an anode exhaust gas which is discharged from an anode of the fuel cell so as to be supplied again to the anode as an electric current obtained from the fuel cell increases in intermittent operation of the fuel cell so that a hydrogen partial pressure of the anode is made lower than a hydrogen partial pressure necessary for generating the current in normal operation of the fuel cell. With this constitution, in the intermittent operation, the hydrogen partial pressure on the anode side of the fuel cell can be lowered while a hydrogen stoichiometric ratio necessary for generation of a target current value is ensured. Thus, the power generation efficiency of the fuel cell can be improved.
In addition, the present invention can be implemented in various aspects. For example, the invention can be implemented in such forms as vehicles with the fuel cell system mounted thereon, methods for supplying anode gas to the fuel cell, computer programs for implementing the fuel cell system control method, and recording mediums with those computer programs recorded thereon.
The fuel cell 10 is a solid polymer electrolyte fuel cell which is supplied with hydrogen as an anode gas and oxygen as a cathode gas to generate electric power. The fuel cell 10 has a plurality of power generators which are stacked and arrayed in series and which are called unit cells as well. Each unit cell includes a membrane electrode assembly having an electrolyte membrane 1, a cathode 2 placed on one surface of the electrolyte membrane 1, and an anode 3 placed on the other surface of the electrolyte membrane 1. In
The controller 20 is implemented by a microcomputer including a CPU (Central Processing Unit) and a main storage unit. The controller 20 accepts a power request from the external, acquires measured values based on output signals derived from various types of sensors within the system, and issues control commands for causing the fuel cell 10 to perform power generation in response to the power request, to various component sections within the system. The controller 20 controls the individual sections of the fuel cell system 100 to switch over the operating state of the fuel cell 10. More specifically, the controller 20 switches over between normal operation and intermittent operation of the fuel cell 10. The term “normal operation of the fuel cell 10” in this case refers to a state that the fuel cell 10 is enabled to supply generated power to external loads, the state excluding, for example, a halt state, the intermittent operation and the like of the fuel cell 10. The term “intermittent operation of the fuel cell 10” refers to a state that the requested power level for the fuel cell 10 is equal to or lower than a specified value (e.g., a level necessary to execute high-potential avoidance control or the like). In addition, a decision as to whether the intermittent operation is selected is made, without being limited to the requested power level, based on temperature of the fuel cell 10, voltage, hydrogen pressure, and the like.
Generally, fuel cells are subject to occurrence of the so-called cross-leak that hydrogen supplied to the anode is partly allowed to move to the cathode via the electrolyte membrane without being used for the power generation reaction. The controller 20 performs operational control for suppressing the loss quantity of hydrogen in terms of loss quantity (cross-leak level ML) of hydrogen due to the cross-leak and a power consumption WP required for drive of a later-described circulation pump 64 for reducing the cross-leak level ML during the intermittent operations of the fuel cell 10. Concrete contents of the control will be described later.
The cathode gas supply section 30 supplies high-pressure air containing oxygen to the cathode 2 of the fuel cell 10. The cathode gas supply section 30 includes a cathode gas pipe 31, an air compressor 32, an air flow meter 33, and a supply valve 34. The cathode gas pipe 31 has one end opened outward of the fuel cell system 100 via an air filter. The cathode gas pipe 31 has the other end connected to an inlet of the cathode gas flow path in the fuel cell 10 via the air flow meter 33, the air compressor 32, the pressure measuring unit 35, and the supply valve 34. The air compressor 32 supplies high-pressure air, which results from compression of outside air taken in via the cathode gas pipe 31 and the cathode gas flow path, to the cathode 2 of the fuel cell 10. The air flow meter 33 measures a quantity of outside air taken in by the air compressor 32 to transmit a measuring result to the controller 20. Based on this measured value, the controller 20 drives the air compressor 32 to control the quantity of air supplied to the cathode 2. The supply valve 34 is provided as a three-way valve to supply high-pressure air fed from the air compressor 32 to the cathode 2 or to downstream of a discharge valve 43 of a later-described cathode exhaust gas pipe 41. The supply valve 34 is opened and closed in response to a pressure of the cathode gas supplied to the cathode 2 to control inflow of the high-pressure air to the cathode 2. The pressure measuring unit 35 measures the pressure of a cathode gas located between the supply valve 34 and the air compressor 32 and transmits a measured value to the controller 20.
The cathode exhaust gas discharge section 40 discharges an exhaust gas (hereinafter, referred to also as ‘cathode exhaust gas’) containing an unreacted gas, which has not been used for power generation reaction in the cathode 2, as well as generated water components. The cathode exhaust gas discharge section 40 includes the cathode exhaust gas pipe 41 and the discharge valve 43. The cathode exhaust gas pipe 41 has one end connected to an outlet of the cathode gas flow path in the fuel cell 10. The cathode exhaust gas pipe 41 has the other end opened outward of the fuel cell system 100 via the discharge valve 43. The discharge valve 43 adjusts the pressure of the cathode exhaust gas in the cathode exhaust gas pipe 41 (i.e., a back pressure on the cathode 2 side of the fuel cell 10). The discharge valve 43 is adjusted in its degree of opening by the controller 20.
The anode gas supply section 50 includes an anode gas pipe 51, a hydrogen tank 52, an opening/closing valve 53, a regulator 54, a hydrogen supply unit 55, and a pressure measuring unit 56. The anode gas pipe 51 has one end connected to the hydrogen tank 52. The anode gas pipe 51 has the other end connected to an inlet of the anode gas flow path in the fuel cell 10 via the opening/closing valve 53, the regulator 54, the hydrogen supply unit 55, and the pressure measuring unit 56. The hydrogen tank 52 supplies high-pressure hydrogen stored therein to the anode 3 of the fuel cell 10 via the anode gas pipe 51 and the anode gas flow path. The opening/closing valve 53 is opened and closed by a command from the controller 20 to control inflow of hydrogen from the hydrogen tank 52 to the upstream side of the hydrogen supply unit 55. The regulator 54 is a pressure reducing valve for adjusting hydrogen pressure on the upstream side of the hydrogen supply unit 55 and the degree of opening thereof is controlled by the controller 20. The hydrogen supply unit 55 can he implemented by, for example, an injector which is an electromagnetically driven opening/closing valve. The pressure measuring unit 56 measures the pressure of hydrogen on the downstream side of the hydrogen supply unit 55 and transmits a measuring result to the controller 20.
The anode gas circulation section 60 circulates an anode exhaust gas containing an unreacted gas (hydrogen, nitrogen etc.), which has not been used for power generation reaction in the anode 3, to the anode 3 of the fuel cell 10. Also, the anode gas circulation section 60 discharges water and an inert gas contained in the anode exhaust gas from the cathode exhaust gas pipe 41 to outside at a preset timing. The anode gas circulation section 60 includes an anode exhaust gas pipe 61, a gas-liquid separator 62, an anode gas circulation pipe 63, a circulation pump 64, an anode drain pipe 65, and a drain valve 66.
The anode exhaust gas pipe 61 connects the outlet of the anode gas flow path in the fuel cell 10 and the gas-liquid separator 62 to each other to guide the anode exhaust gas discharged from the fuel cell 10 to the gas-liquid separator 62. The gas-liquid separator 62 separates gaseous components and water components, from each other, contained in the anode exhaust gas guided from the anode exhaust gas pipe 61, then guides the gaseous components to the anode gas circulation pipe 63 and guides the water components to the anode drain pipe 65. The anode gas circulation pipe 63 is connected to the anode gas pipe 51 at a point downstream of the hydrogen supply unit 55. The anode gas circulation pipe 63, having the circulation pump 64 provided thereon, delivers out, to the anode gas pipe 51, hydrogen contained in the gaseous components separated in the gas-liquid separator 62 by driving force of the circulation pump 64. The anode drain pipe 65 discharges the water components separated in the gas-liquid separator 62 as well as the inert gas contained in the anode exhaust gas to the cathode exhaust gas pipe 41 at a point downstream of the discharge valve 43. The anode drain pipe 65, having the drain valve 66 provided thereon, is opened and closed in response to a command from the controller 20.
In addition, although their depiction and detailed description are omitted, the fuel cell system 100 mounted on the fuel cell vehicle further includes a secondary battery, and a DC/DC converter for controlling the output voltage of the fuel cell 10 and charging/discharging of the secondary battery. The secondary battery accumulates electric power and regenerative power outputted by the fuel cell 10 to function as a power source together with the fuel cell 10. The above-described individual component sections of the fuel cell system 100 can be driven even after an operation halt of the fuel cell 10 by using the electric power of the secondary battery.
Generally, the cross-leak level ML is affected by the pressure in the fuel cell. For example, as the hydrogen partial pressure PH2 of the anode 3 in the fuel cell 10 decreases, the cross-leak level Ml also decreases. The controller 20 of this embodiment increases the circulation quantity of the anode exhaust gas in intermittent operations of the fuel cell 10 so that the hydrogen partial pressure PH2 of the anode 3 in the fuel cell 10 is decreased while a hydrogen stoichiometric ratio necessary for generation of a target current value is ensured. The term, hydrogen partial pressure PH2 of the anode in the fuel cell 10, herein means a hydrogen partial pressure inside the anode gas flow path out of the anode 3. The relationship between the circulation quantity of the anode exhaust gas and the hydrogen partial pressure PH2 in the intermittent operations of the fuel cell 10 will be described below.
With respect to a theoretical hydrogen quantity MN (mol/s) necessary for generation of a target current value I during intermittent operations, a hydrogen quantity MN* (mol/s) that ensures a hydrogen stoichiometric ratio C (e.g., C≈1.25) satisfies the following Equation (1):
M
N
*=C×M
N
=M
N
+M
O (1)
where MC, which is an additional hydrogen quantity necessary for ensuring the hydrogen stoichiometric ratio C relative to the theoretical hydrogen quantity MN, can be expressed as MC=(C·1)×MN.
The theoretical hydrogen quantity MN is consumed by the electrochemical reaction during power generation. Also, a hydrogen gas flow rate flowing through the anode is equal to an anode-exhaust-gas flow rate VH2 (m3/s). Therefore, these additional hydrogen quantity MC anode-exhaust-gas flow rate VH2 and hydrogen partial pressure PH2 (Pa) satisfy an equation of state expressed by the following Equation (2):
M
C
=P
H2
V
H2
/RT (2)
where R is the gas constant and T is the gas temperature (K) of the anode. It can be understood from Equation (2) that given a constant RT, increasing the anode-exhaust-gas flow rate VH2 allows the hydrogen partial pressure PH2 necessary for ensuring the additional hydrogen quantity MC to be decreased. The anode-exhaust-gas flow rate VH2 is correlated with the rotating speed RP of the circulation pump 64 (VH2=αRP, where α is a unit conversion factor). It can be understood from this that increasing the circulation quantity of the anode exhaust gas allows the hydrogen partial pressure PH2 to be decreased while the necessary hydrogen stoichiometric ratio C is ensured.
MT
L
=M
L
+βW
P (3)
where β is the unit conversion factor for conversion of the power consumption WP to the hydrogen loss quantity.
The controller 20 starts the following operational control for an intermittent operation of the fuel cell 10. While controlling the hydrogen supply unit 55 so that the actual hydrogen partial pressure PH2 of the anode gas flow path comes to the target hydrogen partial pressure PTH2, the controller 20 controls the circulation pump 64 so that the rotating speed RP of the circulation pump 64 becomes R1 (step S110). The value of R1 is a set value previously determined as a rotating speed at which the total hydrogen loss quantity MTL becomes a minimum when the current value I is I1. The hydrogen partial pressure PH2 of the anode gas flow path can be determined, for example, from a pressure of near an outlet of the anode estimated by giving considerations of pressure loss or the like into a pressure of a pressure gauge located near an inlet of the anode. Alternatively, with the pressure gauge provided near the outlet of the anode, the hydrogen partial pressure PH2 of the anode gas flow path may be determined from a pressure of the anode exhaust gas located near an outlet of the fuel cell 10 measured by the pressure gauge provided near the outlet.
The controller 20 holds the rotating speed RP of the circulation pump 64 remaining at R1 until the current value I becomes larger than I2 (step S120). The value of I2, being the set value described above, is a current value at which the total hydrogen loss quantity MTL becomes a minimum when the rotating speed RP of the circulation pump 64 is R2 (R1<R2). The current value I, being an actual value of an electric current flowing through the fuel cell 10, may be a current value detected by a current detector or a target current value set according to an output request from outside or the like. The current value I may also be determined from measured values based on output signals derived from various sensors or the like within the system. Furthermore, the current value I may also be derived, for example, from a target voltage value preset for high-potential avoidance and I-V characteristics of the fuel cell 10. The I-V characteristics of the fuel cell 10 may be stored beforehand in the controller 20, or may be set one by one from conditions of the fuel cell 10 including its temperature. In the case where the current value I is the target current value, the I-V characteristics may be set based on internal output requests for supplying electric power to auxiliary machines of the fuel cell system 100 or on external output requests by a user of the fuel cell system 100 such as a driver of the fuel cell vehicle.
As the current value I becomes larger than I2, the controller 20 switches over the rotating speed RP of the circulation pump 64 from R1 to R2 (R1<R2) (step S130). Because the target hydrogen partial pressure PTH2 is changed due to an increase in the current value I and a rise of the rotating speed RP, the controller 20 controls the hydrogen supply quantity by the hydrogen supply unit 55 so that the hydrogen partial pressure PH2 approaches a new target hydrogen partial pressure PTH2.
When the rotating speed RP of the circulation pump 64 is switched over from R1 to R2 through the above-described processes, the total hydrogen loss quantity MTL can be reduced, as compared to the case where the rotating speed RP is held remaining at R1. That is, with the current value I larger than I2, the optimum rotating speed corresponding to the current value I becomes larger than R2. In
Reverting to
The controller 20 repeats the processing procedure of above-described. steps S110 to S140 until the intermittent operation status of the fuel cell 10 is canceled. In the fuel cell system 100 of this embodiment, since the rotating speed RP of the circulation pump 64 is changed so as to approach an optimum rotating speed at which the total hydrogen loss quantity MTL becomes a minimum in the intermittent operations of the fuel cell 10 as described above, the total hydrogen loss quantity MTL can be reduced to more extent, as compared with cases where such change is not made.
In the intermittent operation of the fuel cell 10, as described above, the controller 20 controls the rotating speed RP of the circulation pump 64 so that the total hydrogen loss quantity MTL becomes smaller. That is, the controller 20 controls the circulation pump 64 so that the rotating speed RP of the circulation pump 64 approaches the optimum rotating speed. Meanwhile, in the normal operation of the fuel cell 10, the controller 20 controls the circulation pump 64 so that the rotating speed RP of the circulation pump 64 is raised with increasing current value I, as shown in
According to the fuel cell system 100 of this embodiment described hereinabove, the controller 20 controls the circulation pump 64 so that the rotating speed RP of the circulation pump 64 approaches the optimum rotating speed. As a result of this, hydrogen less quantity corresponding to the power consumption WP required for driving the circulation pump 64 to reduce the cross-leak level ML can be suppressed while the reduction of the cross-leak level ML is fulfilled. That is, by raising the rotating speed of the circulation pump 64 to increase the circulation quantity of the anode exhaust gas, the hydrogen partial pressure PH2 of the anode gas flow path of the fuel cell 10 is lowered to reduce the cross-leak level ML while a hydrogen stoichiometric ratio C necessary for generation of a target current value is ensured. On the other hand, by preventing excessive rise in the rotating speed RP of the circulation pump 64, the hydrogen loss quantity corresponding to the power consumption WP required for driving the circulation pump to reduce the cross-leak level ML can be suppressed. Thus, the power generation efficiency of the fuel cell can be improved.
Modifications:
In addition, the present invention, not being limited to the above-described embodiments and examples, can be implemented in various aspects without departing from the gist of the invention. For example, part or entirety of the functions and processes implemented by software in the foregoing embodiment may be implemented by hardware. Also, part or entirety of the functions and processes implemented by hardware may be implemented by software. As the hardware, for example, various types of circuitry such as integrated circuits, discrete circuits or circuit modules in combination of those circuits may be used. Further, such modifications as described below are implementable.
Modification 1:
In the first embodiment, the controller 20 switches over the rotating speed RP of the circulation pump 64 to either R1 or R2. However, the controller 20 may be so constituted that an optimum rotating speed is determined in response to the current value I and then the rotating speed RP of the circulation pump 64 is changed to the determined optimum rotating speed. For example, it is assumed that optimum rotating speeds and target hydrogen partial pressures PTH2 for individual current values I of the fuel cell 10 have been stored in the controller 20 beforehand. In this case, the controller 20 is enabled to determine an optimum rotating speed and a target hydrogen partial pressure PTH2 from the current value I. It is also assumed, for example, that RP-PTH2 maps for individual current values I of the fuel cell 10 as well as a map (RP-WP map) showing the relationship between rotating speed RP and power consumption WP shown in
Modification 2:
In the foregoing embodiment, it is described that the operational control of
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-232249 | Nov 2014 | JP | national |
