Fuel cell system and control method for turbine

A fuel cell system includes: a turbine including a changing mechanism that adjusts a pressure difference between an upstream pressure and a downstream pressure of the turbine, the turbine recovering at least a part of energy of the cathode off-gas using the pressure difference and assisting driving of the motor with the recovered energy; and a control unit configured to drive the changing mechanism to increase or decrease the recovered energy. The control unit acquires a correlation temperature correlated with a temperature of the cathode off-gas discharged from the turbine and performs freezing avoidance control of not setting the degree of opening to be equal to or less than a predetermined degree of opening when the correlation temperature is lower than a predetermined threshold temperature at which the turbine is able to become frozen.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2017-212389 filed on Nov. 2, 2017 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND

1. Technical Field

The disclosure relates to a fuel cell system including a turbine that assists driving of a compressor, and a control method for the turbine.

2. Description of Related Art

In the related art, a fuel cell system including a compressor that supplies a cathode gas to a fuel cell is known. In a fuel cell system described in Japanese Unexamined Patent Application Publication No. 2012-221657 (JP 2012-221657 A), a turbine is rotated by the energy of a cathode off-gas of which the temperature has increased due to emission of heat from a fuel cell and driving of a compressor is assisted with the power generated by such rotation.

SUMMARY

However, in the fuel cell system described in JP 2012-221657 A, there is a problem in that an amount of energy recovered from the cathode gas may not be sufficient. Therefore, the inventor of the disclosure attempted to increase the amount of energy recovered by narrowing a flow passage for the cathode off-gas to increase a pressure difference between an upstream pressure and a downstream pressure of the turbine. When the flow passage for the cathode off-gas is narrowed, the temperature decreases due to expansion of the cathode off-gas and thus there is a problem in that there is a likelihood that there will be dew condensation on and freezing of the turbine due to a large amount of water vapor included in the cathode off-gas.

According to a first aspect of the disclosure, there is provided a fuel cell system including: a fuel cell; a compressor that supplies a cathode gas to the fuel cell; a motor that drives the compressor; a cathode gas discharge passage into which a cathode off-gas discharged from the fuel cell flows; a turbine that is disposed in the cathode gas discharge passage and is driven by the cathode off-gas, the turbine including a changing mechanism that changes a degree of opening of a flow passage for the cathode off-gas passing through the turbine to adjust a pressure difference between an upstream pressure and a downstream pressure of the turbine, the turbine recovering at least a part of energy of the cathode off-gas using the pressure difference and assisting driving of the motor with the recovered energy; and a control unit configured to drive the changing mechanism to increase or decrease the recovered energy. The control unit is configured to acquire a correlation temperature correlated with a first temperature of the cathode off-gas discharged from the turbine and to perform freezing avoidance control of not setting the degree of opening to be equal to or less than a predetermined degree of opening when the correlation temperature is lower than a predetermined threshold temperature at which the turbine is able to become frozen. In this fuel cell system, since the correlation temperature correlated with the first temperature is acquired and the degree of opening is not set to be equal to or less than the predetermined degree of opening when the correlation temperature is lower than the predetermined threshold temperature at which the turbine is able to become frozen, it is possible to curb a decrease in an amount of energy recovered from the cathode off-gas, to curb a decrease in the first temperature of the cathode off-gas discharged from the turbine, and to curb freezing of the turbine.

The control unit may be configured to increase the degree of opening as the freezing avoidance control. According to this configuration, when the correlation temperature is lower than the predetermined threshold temperature at which the turbine is able to become frozen, the degree of opening is increased. Accordingly, it is possible to curb expansion of the cathode off-gas discharged from the turbine, to further curb a decrease in the first temperature of the cathode off-gas, and to further curb freezing of the turbine.

The control unit may be configured to acquire a second temperature of the cathode gas which has not yet been introduced into the compressor as the correlation temperature. According to this configuration, since the second temperature of the cathode gas which has not yet been introduced into the compressor is acquired as the correlation temperature, it is possible to accurately detect a low-temperature environment in which the turbine is able to become frozen.

The control unit may be configured to perform the freezing avoidance control when the correlation temperature is lower than the threshold temperature and a predetermined time has not elapsed after the fuel cell system has been started. According to this configuration, the freezing avoidance control is performed when the correlation temperature is lower than the threshold temperature and a predetermined time has not elapsed after the fuel cell system has been started. Accordingly, it is possible to prohibit a decrease in the degree of opening when a predetermined time has elapsed after being started and there is a low likelihood that the turbine will freeze and to curb a decrease in an amount of energy recovered from the cathode off-gas.

The fuel cell system may further include a temperature sensor that detects a third temperature of the cathode off-gas which has been discharged from the fuel cell and has not yet been introduced into the turbine. The control unit may be configured to estimate the first temperature based on the detected third temperature of the cathode off-gas and to acquire the estimated first temperature as the correlation temperature. According to this configuration, since the first temperature is estimated based on the detected third temperature of the cathode off-gas and the estimated first temperature is acquired as the correlation temperature, it is possible to control the degree of opening using a temperature having a high correlation with the actual first temperature.

The control unit may be configured to calculate the degree of opening at which the correlation temperature is equal to the threshold temperature and to control the degree of opening such that the degree of opening becomes the calculated degree of opening as the freezing avoidance control. According to this configuration, since the degree of opening is calculated such that the estimated first temperature is equal to the threshold temperature and the degree of opening is controlled such that the degree of opening becomes the calculated degree of opening, it is possible to curb an excessive increase in the degree of opening and a decrease in an amount of energy recovered from the cathode off-gas.

According to a second aspect of the disclosure, there is provided a control method for a turbine that is driven by a cathode off-gas discharged from a fuel cell to recover at least a part of energy of the cathode off-gas and to assist driving of a driving motor of a compressor supplying the cathode gas to the fuel cell using the recovered energy. The control method includes: acquiring a correlation temperature correlated with a first temperature of the exhaust gas discharged from the turbine; and not setting a degree of opening of a flow passage for the exhaust gas passing through the turbine to be equal to or less than a predetermined degree of opening when the correlation temperature is lower than a predetermined threshold temperature at which the turbine is able to become frozen.

The disclosure may be embodied in various forms other than the fuel cell system and the control method for a turbine. For example, the disclosure may be embodied in forms such as a control method for a fuel cell system and a vehicle including a fuel cell system.

DETAILED DESCRIPTION OF EMBODIMENTS

A. First Embodiment

A-1. Configuration of Fuel Cell System

FIG. 1is a diagram schematically illustrating a configuration of a fuel cell system according to an embodiment of the disclosure. A fuel cell system10is a system that supplies a driving power source and is mounted in a fuel-cell vehicle which is not illustrated.

The fuel cell system10includes a fuel cell15, a cooling system20, an anode gas supply/discharge system30, a turbine-attached compressor unit100(hereinafter simply referred to as a “unit100”), a cathode gas supply system40, a cathode gas discharge system70, and a control unit90.

The fuel cell15is a so-called solid polymer type fuel cell and generates electric power with reactant gases (an anode gas and a cathode gas) being supplied. The fuel cell15has a stacked structure in which a plurality of unit cells is stacked.

The cooling system20cools the fuel cell15. The cooling system20includes a coolant supply passage21, a coolant discharge passage22, a radiator23, and a coolant pump24.

The coolant supply passage21supplies cooling water as the coolant to the fuel cell15. An antifreeze fluid such as ethylene glycol, air, or the like may be used as the cooling water. The coolant discharge passage22sends the coolant discharged from the fuel cell15to the radiator23. The radiator23dissipates heat from the coolant. The coolant pump24is disposed in the coolant supply passage21and circulates the coolant.

The anode gas supply/discharge system30supplies hydrogen as an anode gas to the fuel cell15and discharges hydrogen therefrom. The anode gas supply/discharge system30includes an anode gas tank31, an anode gas supply passage32, a main stop valve33, a pressure regulating valve34, an anode gas circulation passage35, a gas-liquid separator36, a circulation pump37, an anode off-gas/liquid valve38, and an anode off-gas/liquid passage39.

The anode gas tank31stores high-pressure hydrogen. The anode gas supply passage32connects the anode gas tank31and the fuel cell15. The main stop valve33and the pressure regulating valve34are provided in the anode gas supply passage32. The main stop valve33turns on or off supply of an anode gas from the anode gas tank31in accordance with a command from the control unit90. The pressure regulating valve34is disposed downstream from the main stop valve33and adjusts a pressure of the anode gas supplied to the fuel cell15in accordance with a command from the control unit90.

The anode gas circulation passage35is connected to the fuel cell15and the anode gas supply passage32and circulates an anode off-gas discharged from the fuel cell15in the anode gas supply passage32. The gas-liquid separator36and the circulation pump37are provided in the anode gas circulation passage35. The gas-liquid separator36separates liquid water from the anode off-gas containing liquid water which is discharged from the fuel cell15. The gas-liquid separator36also separates impurity gases contained in the anode off-gas, for example, nitrogen gas. The anode off-gas containing non-used hydrogen gas is circulated in the anode gas supply passage32by the circulation pump37. The anode off-gas/liquid valve38is opened at a predetermined time in accordance with a command from the control unit90. Accordingly, the separated liquid water and the separated nitrogen gas are discharged from the system via the anode off-gas/liquid passage39.

The unit100includes a compressor60and a turbine80. The compressor60is assembled into the cathode gas supply system40, and the turbine80is assembled into the cathode gas discharge system70. The compressor60and the turbine80are connected to each other via a motor62. The unit100recovers energy from a cathode off-gas and feeds the cathode gas to the fuel cell15using the recovered energy as auxiliary power.

FIG. 2is a diagram schematically illustrating a configuration of the unit100. InFIG. 2, a part of the unit100is illustrated in a sectional view including an axis CX of the motor62.

The compressor60includes a housing61, a motor62, a shaft63, an impeller64, an inlet duct65, and a compressor scroll66. The compressor60suctions in and compresses air as a cathode gas and feeds the compressed air to the fuel cell15illustrated inFIG. 1.

The housing61has the elements of the compressor60accommodated therein. The motor62operates in accordance with a command from the control unit90and drives the compressor60. The shaft63is configured as a rotation shaft of the motor62and transmits a rotational torque of the motor62to the impeller64. The impeller64is constituted by a vane wheel, and compresses and feeds the cathode gas sucked from the inlet duct65to the compressor scroll66using a centrifugal force when it rotates. The inlet duct65is connected to an upstream supply passage41of the cathode gas supply system40illustrated inFIG. 1. The compressor scroll66illustrated inFIG. 2has a spiral appearance and guides the compressed cathode gas to a downstream supply passage51of the cathode gas supply system40illustrated inFIG. 1.

The turbine80includes a turbine housing81, a turbine wheel82, a turbine scroll83, an outlet duct84, and a changing mechanism85. The turbine80recovers at least a part of the energy of the cathode off-gas discharged from the fuel cell15illustrated inFIG. 1, and assists driving of the motor62using the recovered energy.

The turbine housing81is formed integrally with the housing61, and has the elements of the turbine80accommodated therein. The turbine wheel82is configured as a vane wheel, and rotates with the energy of the cathode off-gas. The turbine wheel82is connected to the impeller64via the shaft63. Power generated by the rotation of the turbine wheel82is transmitted to the motor62and is used as auxiliary power for the motor62. The turbine scroll83has a spiral appearance, is connected to a cathode gas discharge passage71illustrated inFIG. 1, and takes the cathode off-gas discharged from the fuel cell15into the turbine housing81. The outlet duct84illustrated inFIG. 2guides the cathode off-gas passing through the turbine wheel82to the cathode gas discharge passage71illustrated inFIG. 1, more specifically, to a muffler78. The temperature of the cathode off-gas passing through the turbine80decreases according to an expansion stroke of the turbine80.

The changing mechanism85has a so-called variable nozzle type structure, changes a degree of opening of a flow passage (hereinafter simply referred to as a “degree of opening”) of the cathode off-gas passing through the turbine80, and adjusts a pressure difference between an upstream pressure and a downstream pressure of the turbine80. The degree of opening of the flow passage for the cathode off-gas passing through the turbine80is also referred to as a degree of opening of the changing mechanism85. The changing mechanism85includes a changing mechanism driving motor86, a plurality of variable vanes87, and a plurality of shaft portions88. The changing mechanism driving motor86rotates the variable vanes87in accordance with a command from the control unit90.

FIGS. 3 and 4are sectional views of the turbine80taken along line3-3inFIG. 2.FIG. 3schematically illustrates an example in which the degree of opening is relatively large (a large degree of opening) andFIG. 4schematically illustrates an example in which the degree of opening is relatively small (a small degree of opening).

A plurality of variable vanes87is arranged in a circumferential direction outside the turbine wheel82in a radial direction. Each variable vane87is configured to be rotatable by a predetermined angle about the corresponding shaft portion88. Each shaft portion88is connected to the changing mechanism driving motor86via a unison ring and a link mechanism which are not illustrated. When the variable vanes87pivot, the magnitude of a gap between neighboring variable vanes87changes and the degree of opening changes.

A passage sectional area in which the cathode off-gas flows is large when the degree of opening is large, and is small when the degree of opening is small. The degree of opening may be expressed, for example, by a proportion of an actual passage sectional area when the passage sectional area in a fully open state is set to 100%. When the degree of opening decreases, the pressure difference of the cathode off-gas between the upstream pressure and the downstream pressure of the turbine80increases and an expansion ratio of the cathode off-gas passing through the turbine wheel82increases.

Here, the expansion ratio of the cathode off-gas refers to a ratio of a pressure P4of the cathode off-gas at an inlet of the turbine80(hereinafter simply referred to as an “inlet pressure P4”) to a pressure P6of the cathode off-gas at an outlet of the turbine80(hereinafter simply referred to as an “outlet pressure P6”). In general, as the expansion ratio (P4/P6) becomes larger, a temperature T6of the cathode off-gas at the outlet of the turbine80(hereinafter also referred to as a “discharge temperature T6”) becomes lower.

The cathode gas supply system40illustrated inFIG. 1supplies a cathode gas to the fuel cell15. The cathode gas supply system40includes an upstream supply passage41, an air cleaner42, an atmospheric pressure sensor43, an air cleaner temperature sensor44, an air flowmeter45, a downstream supply passage51, an intercooler52, a supply gas temperature sensor53, a supply gas pressure sensor54, and an inlet valve55, in addition to the compressor60of the unit100.

The upstream supply passage41constitutes a flow passage upstream from the compressor60in the cathode gas supply system40. The air cleaner42removes dust when the cathode gas is taken in. The atmospheric pressure sensor43detects an atmospheric pressure. The air cleaner temperature sensor44detects an outside air temperature. The air flowmeter45detects an amount of cathode gas taken in the air cleaner42. The detection results of the atmospheric pressure sensor43, the air cleaner temperature sensor44, and the air flowmeter45are transmitted to the control unit90.

The downstream supply passage51constitutes a flow passage downstream from the compressor60in the cathode gas supply system40. The intercooler52cools the cathode gas which has been compressed by the compressor60and has increased in temperature. The supply gas temperature sensor53measures the temperature of the cathode gas supplied to the fuel cell15. The supply gas pressure sensor54measures a pressure of the cathode gas supplied to the fuel cell15. The inlet valve55is disposed closer to the fuel cell15than a junction to a bypass flow passage73in the downstream supply passage51. The inlet valve55adjusts a flow rate of the cathode gas in accordance with a command from the control unit90.

The cathode gas discharge system70discharges a cathode gas from the fuel cell15. The cathode gas discharge system70includes a cathode gas discharge passage71, a pressure regulating valve72, a bypass flow passage73, a bypass valve74, and a muffler78in addition to the turbine80of the above-mentioned unit100.

The cathode off-gas discharged from the fuel cell15flows in the cathode gas discharge passage71. The pressure regulating valve72is disposed closer to the fuel cell15than a junction with the bypass flow passage73in the cathode gas discharge passage71. The pressure regulating valve72adjusts a pressure of the cathode gas in the fuel cell15in accordance with a command from the control unit90. The bypass flow passage73connects the downstream supply passage51to the cathode gas discharge passage71. The bypass valve74is disposed in the bypass flow passage73. The bypass valve74adjusts a flow rate of the cathode gas flowing in the bypass flow passage73in accordance with a command from the control unit90.

A downstream end of the anode off-gas/liquid passage39of the anode gas supply/discharge system30is connected to a part downstream from the turbine80in the cathode gas discharge passage71. The muffler78is disposed downstream from a junction with the anode off-gas/liquid passage39in the cathode gas discharge passage71. The muffler78reduces exhaust sound of the cathode off-gas.

The control unit90is a microcomputer including a central processing unit (CPU) and a main storage device and is constituted as an electronic control unit. The control unit90controls the operation of the fuel cell system10. The control unit90acquires output signals from various sensors such as the atmospheric pressure sensor43, the air cleaner temperature sensor44, the air flowmeter45, the supply gas temperature sensor53, and the supply gas pressure sensor54. The control unit90outputs drive signals to various valves such as the main stop valve33, the pressure regulating valve34, the anode off-gas/liquid valve38, the inlet valve55, the pressure regulating valve72, and the bypass valve74or the units associated with generation of power in the fuel cell15such as the motor62, the changing mechanism driving motor86, the coolant pump24, and the circulation pump37. The control unit90increases and decreases an amount of energy recovered from the cathode off-gas by driving the changing mechanism85. The control unit90performs freezing avoidance control which will be described later.

The control unit90increases an amount of energy recovered from the cathode off-gas by determining and controlling the degree of opening depending on the flow rate of the cathode off-gas in normal control. In this embodiment, first, the control unit90determines a target value of an expansion ratio of the cathode off-gas (hereinafter referred to as a “target expansion ratio”) and determines the degree of opening with reference to a turbine characteristics map indicting a relationship between the target expansion ratio, the flow rate of the cathode off-gas, and the degree of opening.

In this embodiment, an upper limit value of an expansion ratio P4/P6which is determined depending on a pressure P3at an outlet of the fuel cell15(hereinafter simply referred to as a “fuel cell outlet pressure P3”) is used as the target expansion ratio. In this embodiment, the fuel cell outlet pressure P3can be calculated in consideration of a pressure loss in the fuel cell15or the like based on the pressure measured by the supply gas pressure sensor54and the degrees of opening of the inlet valve55, the pressure regulating valve72, and the bypass valve74. The fuel cell outlet pressure P3may be measured by providing a pressure sensor in the pressure regulating valve72. In this embodiment, the outlet pressure P6is calculated as an approximate value of the atmospheric pressure detected by the atmospheric pressure sensor43, but may be calculated by applying a correction coefficient to the atmospheric pressure in consideration of the pressure loss in the muffler78or the like. It is preferable that the inlet pressure P4not be greater than the fuel cell outlet pressure P3in order to maintain the pressure in the fuel cell15at an appropriate value. Accordingly, the upper limit value of the inlet pressure P4depends on the fuel cell outlet pressure P3. Accordingly, the upper limit value of the expansion ratio P4/P6is determined by the outlet pressure P6and the fuel cell outlet pressure P3, and the target expansion ratio is determined.

FIG. 5is a diagram illustrating an example of a turbine characteristics map which is used for normal control. InFIG. 5, the vertical axis represents the expansion ratio P4/P6, and the horizontal axis represents a flow rate G4of the cathode off-gas flowing into the turbine80. InFIG. 5, for the purpose of convenience of explanation, two curves in a case in which the degree of opening is relatively large (a large degree of opening) and a case in which the degree of opening is relatively small (a small degree of opening) are illustrated and other curves with different degrees of opening are not illustrated. The turbine characteristics map is stored in advance in the main storage device of the control unit90.

In this embodiment, a flow rate of a cathode gas detected by the air flowmeter45is used as a flow rate G4of the cathode off-gas flowing into the turbine80. Instead, the flow rate G4may be calculated by applying a correction coefficient to the flow rate of the cathode gas detected by the air flowmeter45or may be measured by providing a flow rate sensor in the cathode gas discharge passage71.

The control unit90determines a degree of opening from an intersection between the flow rate G4of the cathode off-gas and the determined target expansion ratio with reference to the turbine characteristics map illustrated inFIG. 5. InFIG. 5, an example in which the flow rate G4of the cathode off-gas is x and the target expansion ratio is y is illustrated. Since the curve located at the intersection R1between x and y is a curve of a “large degree of opening,” the control unit90determines the degree of opening to be the “large degree of opening.” The control unit90outputs a drive signal to the changing mechanism driving motor86such that the degree of opening of the changing mechanism85becomes the determined degree of opening.

A large amount of water vapor or droplets which are products generated by a chemical reaction of the fuel cell15is included in the cathode off-gas. Accordingly, when the temperature of the cathode off-gas decreases at the time of passing through the turbine80, the turbine wheel82, the turbine housing81, or the like is dew-condensed and frozen due to a large amount of water vapor or droplets included in the cathode off-gas. The freezing of the turbine80is likely to occur in a cold region in which an outside air temperature is low. Therefore, in the fuel cell system10according to this embodiment, it is possible to curb a decrease of the discharge temperature T6of the cathode off-gas discharged from the turbine80, to curb a decrease in temperature of the turbine80, and to prevent freezing of the turbine80by performing freezing avoidance control which will be described later.

FIG. 6is a flowchart illustrating a flow of freezing avoidance control. The freezing avoidance control is repeatedly performed after a starter switch (not illustrated) of a vehicle mounted with the fuel cell has been pushed and the fuel cell system10has been started. The freezing avoidance control may be performed at the same time as starting of the fuel cell system10or may be performed at an arbitrary other time.

The control unit90performs a freezing determining process (Step S300). The freezing determining process refers to a process of determining whether there is a likelihood that the turbine80will freeze.

FIG. 7is a flowchart illustrating a flow of the freezing determining process. The control unit90acquires an outside air temperature T0which is measured by the air cleaner temperature sensor44(Step S310). The control unit90determines whether the acquired outside air temperature T0is lower than a predetermined threshold temperature Tmin (Step S320). The threshold temperature Tmin refers to a temperature correlated with the discharge temperature T6at which there is a likelihood of freezing of the turbine80, and is stored in advance in the main storage device of the control unit90. In this embodiment, the threshold temperature Tmin is set to 5° C., but may be set to an arbitrary other temperature which is correlated with the discharge temperature T6at which there is a likelihood of freezing of the turbine80.

When it is determined in Step S320that the outside air temperature T0is not lower than the threshold temperature Tmin (NO in Step S320), the control unit90determines that there is no likelihood of freezing (Step S350), ends the freezing determining process, and returns to the freezing avoidance control illustrated inFIG. 6.

On the other hand, when it is determined in Step S320illustrated inFIG. 7that the outside air temperature T0is lower than the threshold temperature Tmin (YES in Step S320), the control unit90determines whether the fuel cell system10has been just started (Step S330). Specifically, the control unit90determines whether X seconds has not elapsed after the fuel cell system10has been started. X seconds is determined in advance and stored in the main storage device of the control unit90. In this embodiment, X second is set to 60 seconds, but may be set to an arbitrary other value indicating that the fuel cell system10has been just started.

When the fuel cell system10has been just started, the temperature of the fuel cell15does not become so higher and thus the temperature of the cathode off-gas discharged from the fuel cell15is low. Accordingly, when the fuel cell system10has been just started, there is a relatively high likelihood of freezing of the turbine80.

When it is determined in Step S330that the fuel cell system10has not been just started (NO in Step S330), the control unit90determines that there is no likelihood of freeze (Step S350), ends the freeze determining process, and returns to the freeze avoidance control illustrated inFIG. 6.

On the other hand, when it is determined in Step S330illustrated inFIG. 7that the fuel cell system10has been just started (YES in Step S330), the control unit90determines that there is a likelihood of freezing (Step S340), ends the freezing determining process, and returns to the freezing avoidance control illustrated inFIG. 6.

In the freezing avoidance control illustrated inFIG. 6, it is detected whether it has been determined that there is a likelihood of freezing as the result of the freezing determining process of Step S300(Step S210). When it is detected in Step S210that there is no likelihood of freezing (NO in Step S210), the control unit90returns to Step S300.

On the other hand, when it is detected in Step S210that there is a likelihood of freezing (YES in Step S210), the control unit90increases the degree of opening of the changing mechanism85, that is, the degree of opening of a flow passage for the cathode off-gas passing through the turbine80(Step S220). More specifically, the control unit90rotates the variable vanes87to increase the degree of opening by outputting a command to the changing mechanism driving motor86. Increasing the degree of opening means that the degree of opening is increased in comparison with that before Step S220has been performed. In this embodiment, the degree of opening is increased by 30%, but may be increased by an arbitrary other degree of opening such as 10% or 20%. A relatively large degree of opening at which there is a low likelihood of freezing of the turbine80in which the expansion ratio of the cathode off-gas passing through the turbine80is relatively low and the decrease in temperature of the cathode off-gas is relatively small may be set in advance and the degree of opening of the changing mechanism85may be increased to be greater than such a degree of opening.

When the degree of opening is increased, a passage sectional area increases and the pressure loss of the cathode off-gas decreases, and thus the expansion ratio (P4/P6) decreases. Accordingly, expansion of the cathode off-gas discharged from the turbine80is curbed, the decrease of the discharge temperature T6is curbed, the decrease in temperature of the turbine80is curbed, and freezing of the turbine80is prevented. After Step S220has been performed, the control unit90returns to Step S300.

In this embodiment, the outside air temperature T0can be considered to be a subordinate concept of the correlation temperature in the SUMMARY and a subordinate concept of the temperature of the cathode gas which has not yet been introduced into the compressor.

In the fuel cell system10according to this embodiment, the degree of opening is increased when the outside air temperature T0is lower than the threshold temperature Tmin and the fuel cell system10has been just started. Accordingly, since the expansion ratio P4/P6of the cathode off-gas can be decreased, it is possible to curb expansion of the cathode off-gas discharged from the turbine80and to curb a decrease of the discharge temperature T6. Accordingly, it is possible to curb a decrease in temperature of the turbine80and to prevent freezing of the turbine80.

Freezing of the turbine80is likely to occur particularly in a cold region in which the outside air temperature T0is low. In the fuel cell system10according to this embodiment, since the freezing determining process is performed based on the outside air temperature T0, it is possible to accurately detect a low-temperature environment in which the turbine80is likely to freeze.

It is determined that there is a likelihood of freezing of the turbine80when the fuel cell system10has been just started, and it is determined that there is no likelihood of freezing of the turbine80when the fuel cell system10has not been just started. Accordingly, when a predetermined time has elapsed after the fuel cell system10has been started, the temperature of the fuel cell15increases, the temperature of the cathode off-gas increases, and there is a low likelihood of freezing of the turbine80, it is possible to curb an excessive increase of the degree of opening due to determination that there is a likelihood of freezing and to curb a decrease of the amount of energy recovered from the cathode off-gas.

B. Second Embodiment

FIG. 8is a diagram schematically illustrating a configuration of a fuel cell system10aaccording to a second embodiment. The fuel cell system10aaccording to the second embodiment is different from the fuel cell system10according to the first embodiment, in a turbine inlet temperature sensor75awhich is additionally provided and specific means of the freezing determining process. The other configuration is the same as in the fuel cell system10according to the first embodiment, and thus the same elements and the same steps will be referred to by the same reference signs and detailed description thereof will be omitted.

The turbine inlet temperature sensor75ais disposed between the pressure regulating valve72and the turbine80in the cathode gas discharge passage71of a cathode gas discharge system70a. The turbine inlet temperature sensor75adetects a temperature T4of a cathode off-gas introduced into the turbine80(hereinafter also referred to as an “inlet temperature T4”). The result of detection from the turbine inlet temperature sensor75ais transmitted to the control unit90.

FIG. 9illustrates a flowchart illustrating a flow of freezing avoidance control according to the second embodiment. First, a freezing determining process is performed (Step S500).

FIG. 10is a flowchart illustrating a flow of the freezing determining process according to the second embodiment. The control unit90acquires the inlet temperature T4detected by the turbine inlet temperature sensor75a(Step S510). The control unit90estimates the discharge temperature T6of the turbine exhaust gas discharged from the turbine80when the above-mentioned normal control is performed based on the acquired inlet temperature T4and a turbine characteristics map indicating turbine efficiency ηt (Step S520).

The turbine efficiency ηt is calculated as a ratio of power Lt at the outlet of the turbine80to power (Lt)ad in adiabatic change at the inlet of the turbine80as expressed by Equation (1).
ηt=Lt/(Lt(ad)  (1)
The power (Lt)ad in the adiabatic change at the inlet of the turbine80is calculated by Equation (2) and the power Lt at the outlet of the turbine80is calculated by Equation (3).

(Lt)⁢ad=Cpg×G⁢⁢4×T⁢⁢4⁢{1-1(P⁢⁢4/P⁢⁢6)K-1K}(2)Lt=Cpg×G⁢⁢4⁢(T⁢⁢4-T⁢⁢6).(3)
Here, Cpg denotes specific heat and k denotes a ratio of specific heat (Cp/Cv). Cp denotes specific heat at a constant pressure and Cv denotes specific heat at a constant volume. Equation (2) can be replaced with Equation (5) by applying Equation (4).

FIGS. 11 and 12are diagrams illustrating an example of a turbine characteristics map indicating turbine efficiency ηt.FIG. 11illustrates a case in which the degree of opening is relative large (a large degree of opening) andFIG. 12illustrates a case in which the degree of opening is relative small (a small degree of opening). InFIGS. 11 and 12, the vertical axis represents the turbine efficiency ηt and the horizontal axis represents the expansion ratio P4/P6. InFIGS. 11 and 12, for the purpose of convenience of explanation, four curves corresponding to rotation speeds of the turbine wheel82are representatively illustrated, and other curves with other rotation speeds are not illustrated. A rotation speed is expressed by a proportion of an actual rotation speed when a maximum rotation speed is set to 100%. The main storage device of the control unit90stores a plurality of turbine characteristics maps with different degrees of opening in addition to the turbine characteristics maps illustrated inFIGS. 11 and 12.

In the above-mentioned normal control, the expansion ratio P4/P6is set as a target expansion ratio, and the degree of opening is determined from the turbine characteristics map illustrated inFIG. 5. The control unit90can calculate the turbine efficiency ηt based on the rotation speed of the turbine wheel82and the expansion ratio P4/P6with reference to the turbine characteristics map indicating the turbine efficiency ηt according to the degree of opening when the normal control is performed.

In Step S520illustrated inFIG. 10, the control unit90estimates the discharge temperature T6by applying the expansion ratio P4/P6and the turbine efficiency ηt when the normal control is performed to Equation (6) representing the discharge temperature T6.

The control unit90acquires the estimated discharge temperature T6and determines whether the estimated discharge temperature T6is lower than a predetermined threshold temperature Tmin (Step S530). The threshold temperature Tmin is set to, for example, 5° C.

When it is determined in Step S530that the estimated discharge temperature T6is not lower than the threshold temperature Tmin (NO in Step S530), the control unit90determines that there is no likelihood of freezing (Step S550), ends the freezing determining process, and returns to the freezing avoidance control illustrated inFIG. 9.

On the other hand, when it is determined in Step S530illustrated inFIG. 10that the estimated discharge temperature T6is lower than the threshold temperature Tmin (YES in Step S530), the control unit90determines that there is a likelihood of freezing (Step S540), ends the freezing determining process, and returns to the freezing avoidance control illustrated inFIG. 9.

In the freezing avoidance control illustrated inFIG. 9, as the result of the freezing determining process of Step S500, it is detected whether it is determined that there is a likelihood of freezing (Step S410). When it is detected in Step S410that there is no likelihood of freezing (NO in Step S410), the control unit90returns to Step S500.

On the other hand, when it is detected in Step S410that there is a likelihood of freezing (YES in Step S410), the control unit90calculates the degree of opening at which the discharge temperature T6is equal to the threshold temperature Tmin (Step S420). More specifically, the control unit90applies the threshold temperature Tmin as the discharge temperature T6in Equation (6), applies the turbine efficiency ηt when the normal control is performed, and calculates the expansion ratio P4/P6. Then, the control unit90calculates the degree of opening based on the calculated expansion ratio P4/P6and the flow rate G4of the cathode off-gas with reference to the turbine characteristics map illustrated inFIG. 5.

The control unit90outputs a command to set the degree of opening of the changing mechanism85to the degree of opening calculated in Step S420to the changing mechanism driving motor86(Step S430). Accordingly, the degree of opening of the changing mechanism85is increased (Step S440) and the control unit returns to Step S500. Since the discharge temperature T6increases with an increase in the degree of opening and becomes equal to the threshold temperature Tmin, a decrease of the discharge temperature T6is curbed, the decrease in temperature of the turbine80is curbed, and freezing of the turbine80is prevented.

In this embodiment, the turbine inlet temperature sensor75acan be considered to be a subordinate concept of the temperature sensor that detects the temperature of cathode off-gas which has been discharged from the fuel cell and has not yet been introduced into the turbine in the SUMMARY, the inlet temperature T4can be considered to be a subordinate concept of the temperature of the cathode off-gas which has been discharged from the fuel cell and has not yet been introduced into the turbine in the SUMMARY, and the estimated discharge temperature T6can be considered to be a subordinate concept of the correlation temperature in the SUMMARY.

The above-mentioned freezing avoidance control according to the second embodiment achieves the same advantageous effects as the freezing avoidance control according to the first embodiment. Since the discharge temperature T6is estimated, it is possible to control the degree of opening using a temperature with a high correlation with the actual discharge temperature T6. Since the discharge temperature T6is estimated based on the inlet temperature T4of the turbine located at a position physically close to the outlet of the turbine, it is possible to curb a decrease in estimation accuracy of the discharge temperature T6and to curb a decrease in determination accuracy of the freezing determining process.

The control unit90calculates the degree of opening at which the discharge temperature T6is equal to the threshold temperature Tmin and increases the degree of opening of the changing mechanism85such that it becomes equal to the calculated degree of opening. Accordingly, since an excessive increase in the degree of opening can be curbed, it is possible to curb a decrease in the amount of energy recovered from the cathode off-gas due to an excessive decrease in the expansion ratio P4/P6.

C. Modified Example

C-1. First Modified Example

In the above-mentioned embodiments, the degree of opening is increased when it is determined that there is a likelihood of freezing (YES in Steps S210and S410), but the disclosure is not limited thereto. For example, when it is determined that there is a likelihood of freezing (YES in Steps S210and S410), control may be performed such that the degree of opening is not equal to or less than a predetermined degree of opening. The predetermined degree of opening may be set in advance as a relatively large degree of opening at which there is a low likelihood of freezing of the turbine80. In other words, a degree of opening at which the expansion ratio of the cathode off-gas passing through the turbine80is relatively small and the decrease in temperature of the cathode off-gas is relatively small may be set in advance as the predetermined degree of opening. With this configuration, the same advantageous effects as those in the fuel cell systems10and10aaccording to the above-mentioned embodiment, can be achieved. When a current degree of opening is larger than the predetermined degree of opening, the degree of opening may be decreased within a range in which the degree of opening is not equal to or less than the predetermined degree of opening and thus it is possible to prevent freezing of the turbine80and to curb a decrease in the amount of energy recovered from the cathode off-gas. For example, when it is determined that there is a likelihood of freezing (YES in Steps S210and S410), the degree of opening may be maintained. Maintaining of the degree of opening means that the degree of opening is not changed after the determination of Steps S210and S410. According to this configuration, it is possible to curb a decrease in the amount of energy recovered from the cathode off-gas due to an excessive increase of the degree of opening. That is, in general, freezing avoidance control of not setting the degree of opening to be equal to or less than the predetermined degree of opening may be performed when the correlation temperature is lower than the predetermined threshold temperature Tmin at which there is a likelihood of freezing of the turbine80. With this configuration, the same advantageous effects as those in the fuel cell systems10and10aaccording to the above-mentioned embodiments can also be achieved.

C-2. Second Modified Example

In the freezing avoidance control according to the second embodiment, the degree of opening at which the discharge temperature T6is equal to the threshold temperature Tmin may not be detected and the degree of opening may be uniformly increased. In other words, in the second embodiment, the same freezing avoidance control as in the first embodiment may be performed. In this configuration or the freezing avoidance control according to the first embodiment, the degree of opening may be stepwise increased depending on the temperature difference between the outside air temperature T0or the estimated discharge temperature T6and the threshold temperature Tmin. For example, the degree of opening may be increased by 20% when the difference between the outside air temperature T0or the estimated discharge temperature T6and the threshold temperature Tmin is less than 5° C., and the degree of opening may be increased by 30% when the difference is equal to or greater than 5° C. With this configuration, the same advantageous effects as those in the fuel cell systems10and10aaccording to the above-mentioned embodiments can also be achieved.

C-3. Third Modified Example

In the freezing determining process according to the first embodiment, Step S330may be skipped. That is, the freezing determining process may be performed regardless of whether the fuel cell system10has been just started. A configuration in which the outside air temperature T0may be acquired via the Internet may be employed instead of the configuration in which the outside air temperature is detected by the air cleaner temperature sensor44. The freezing determining process may be performed using a temperature of the cathode gas which has not yet been introduced into the compressor60and which is measured by another temperature sensor disposed in the upstream supply passage41instead of the outside air temperature T0. With this configuration, the same advantageous effects as those in the fuel cell system10according to the first embodiment can also be achieved.

C-4. Fourth Modified Example

In the first embodiment, the freezing determining process may be performed using the inlet temperature T4of the turbine detected by the turbine inlet temperature sensor75adisposed in the fuel cell system10aaccording to the second embodiment. In this example, since the inlet temperature T4of the turbine is assumed to be higher than the outside air temperature T0due to the cathode gas passing through the compressor60and the fuel cell15, the threshold temperature Tmin may be set to a higher value. For example, the threshold temperature Tmin may be set to 10° C. With this configuration, the same advantageous effects as those in the fuel cell system10according to the first embodiment can also be achieved.

C-5. Fifth Modified Example

In the freezing avoidance control according to the above-mentioned embodiments, the freezing determining process is performed in accordance with the flowcharts illustrated inFIGS. 7 and 10, but the freezing avoidance control may be performed using a map which is stored in advance in the main storage device of the control unit90. For example, instead of the freezing determining process, the degree of opening may be set to be greater than a lower limit value by storing at least the lower limit value of the degree of opening and the correlation temperature as a map indicating a correlation in which freezing of the turbine80can be avoided in advance, applying the acquired correlation temperature to the map, and referring to the map. In the above-mentioned embodiments, the turbine characteristics map illustrated inFIG. 5is used for determining the degree of opening in the normal control, but the degree of opening may be determined using a relational expression indicating a relationship between the flow rate G4of the cathode off-gas, the expansion ratio P4/P6, and the degree of opening instead of the turbine characteristics map. In calculating the turbine efficiency ηt in the second embodiment, a turbine characteristics map in which the inlet temperature T4is further reflected may be used or a correction coefficient based on the inlet temperature T4may be applied thereto. With this configuration, the same advantageous effects as those in the fuel cell systems10and10aaccording to the above-mentioned embodiments can also be achieved.

C-6. Sixth Modified Example

The system configuration of the fuel cell system10aaccording to the second embodiment is only an example and can be modified in various forms. For example, the temperature of the cathode off-gas which has been discharged from the fuel cell15and has not yet been introduced into the turbine80may be used instead of the inlet temperature T4measured by the turbine inlet temperature sensor75a. In this configuration, the temperature of the cathode off-gas may be measured, for example, by another temperature sensor disposed in the outlet of the fuel cell15or the like. For example, a pressure sensor that measures the inlet pressure P4of the turbine80or a pressure sensor that measures the outlet pressure P6of the turbine80may be additionally provided, and the expansion ratio P4/P6may be calculated based on a value measured by the pressure sensor. For example, by additionally providing the temperature sensor that measures the discharge temperature T6, Steps S510and S520can be skipped and the freezing determining process may be performed based on the discharge temperature T6measured by the temperature sensor. With this configuration, the same advantageous effects as those in the fuel cell system10aaccording to the second embodiment can also be achieved.

C-7. Seventh Modified Example

In the freezing determining process according to the first and second embodiments, the changing mechanism85has a so-called a variable nozzle type configuration, but the disclosure is not limited thereto. The changing mechanism85may have an arbitrary other configuration in which the degree of opening of the flow passage for the cathode off-gas passing through the turbine80can be changed to adjust the pressure difference between the upstream pressure and the downstream pressure of the turbine80, such as a movable flap type and a variable nozzle width type. With this configuration, the same advantageous effects as those in the fuel cell systems10and10aaccording to the above-mentioned embodiments can also be achieved.

C-8. Eighth Modified Example

In the above-mentioned embodiments, the turbine80is connected to the compressor60via the motor62and driving of the motor62is assisted by the energy recovered from the cathode off-gas, but the energy of the cathode off-gas may be recovered as power of the turbine80and driving of the motor62may be assisted by electric power generated by rotating another motor with the recovered power. That is, in general, the turbine80may recover at least a part of energy of the cathode off-gas using the pressure difference between the upstream pressure and the downstream pressure of the turbine80and may assist driving of the motor62using the recovered energy. In the above-mentioned embodiment, the fuel cell systems10and10aare mounted and used in a fuel-cell vehicle, but may be mounted in an arbitrary other moving object or may be used for a stationary fuel cell.

The disclosure is not limited to the above-mentioned embodiments and can be embodied in various configurations without departing from the scope thereof. For example, technical features in the embodiments corresponding to technical features in the aspects described in the SUMMARY can be appropriately subjected to replacement or combination in order to solve some or all of the above-mentioned problems or to achieve some or all of the above-mentioned advantageous effects. The technical features can be appropriately deleted unless they are described to be essential in this specification.