Fuel cell system and refrigerant flow rate estimation method for the same

A fuel cell system including: a fuel cell group; a refrigerant distribution passage; a pre-distribution refrigerant flow rate acquiring unit configured to acquire a first outlet temperature flow rate; a first outlet temperature detecting unit that is configured to detect a first outlet temperature; a voltage acquiring unit configured to acquire at least a first voltage that is a voltage of the first fuel cell; a current acquiring unit configured to acquire at least a first current; and a controller that calculates a first individual supply flow rate of the first fuel cell on the basis of the first voltage, the first current, and the first outlet temperature, and calculates a second individual supply flow rate of at least one second fuel cell other than the first fuel cell on the basis of the first individual supply flow rate and the pre-distribution refrigerant flow rate.

TECHNICAL FIELD

The present invention relates to a fuel cell system that cools a fuel cell by supplying a refrigerant, such as air, to the fuel cell and estimates a refrigerant flow rate at a refrigerant outlet of the fuel cell, and to estimation method for estimating a flow rate of a refrigerant supplied in the fuel cell system.

BACKGROUND ART

There is known a fuel cell system that supplies a refrigerant, such as cooling water or air, to a fuel cell to control the temperature of the fuel cell to a predetermined temperature. JP2007-188667 A has disclosed an example of such a fuel cell system, particularly, a fuel cell system provided with a plurality of fuel cell groups.

The fuel cell system disclosed in JP2007-188667 A includes a refrigerant supply system in which a plurality of fuel cell groups is arranged in parallel, and the refrigerant supply system includes a refrigerant supply passage for supplying a refrigerant to the fuel cell groups, an inlet temperature sensor provided on the refrigerant supply passage, refrigerant distribution passages that branch from the refrigerant supply passage to individually distribute the refrigerant from the refrigerant supply passage to the fuel cell groups, a voltage sensor that measures respective voltages of the cell groups, a refrigerant discharge passage where refrigerants discharged from the fuel cell groups are merged, and an outlet temperature sensor that detects a temperature of the merged discharged refrigerant.

Then, the fuel cell system estimates respective calorific values of cells from a measured voltage value, etc. of each cell group, and estimates a temperature difference (a difference between a pre-supply refrigerant temperature and a discharged refrigerant temperature) of each cell group on the basis of a detected inlet temperature value from the inlet temperature sensor, a detected outlet temperature value from the outlet temperature sensor, and the estimates of the calorific values.

SUMMARY OF INVENTION

However, in the fuel cell system, a uniform flow rate of refrigerant is not necessarily distributed from the refrigerant supply passage to each refrigerant distribution passage. The flow rate of refrigerant distributed and supplied to each refrigerant distribution passage varies, for example, according to differences in various conditions, such as a pressure and a temperature in a refrigerant passage in each fuel cell group.

Therefore, if there is such variation in the flow rate of supplied refrigerant, the degree of cooling differs among the fuel cell groups, and there is variation in real calorific value.

Meanwhile, in the fuel cell system, the detected inlet temperature value and the detected outlet temperature value that are used for estimation of a calorific value of each fuel cell group only include temperature information before the distribution of the supplied refrigerant and temperature information after the discharged refrigerants are merged, respectively. As a result, this estimation of the calorific value does not take into consideration the variation in real calorific value among the fuel cell groups, and therefore the estimation accuracy of an estimate of the calorific value and an estimate of a temperature difference of each cell group based on this may be insufficient.

Therefore, an object of the present invention is to allow a fuel cell system that cools a plurality of fuel cells by distributing a refrigerant to the fuel cells to estimate a flow rate of refrigerant supplied to each fuel cell with higher accuracy.

An aspect of the present invention provides a fuel cell system comprising a fuel cell group including a plurality of fuel cells, a refrigerant distribution passage through which a refrigerant is individually distributed to the fuel cells composing the fuel cell group, a pre-distribution refrigerant flow rate acquiring unit configured to acquire a pre-distribution refrigerant flow rate that is a flow rate of the refrigerant before distribution, a first outlet temperature detecting unit that is provided at a refrigerant outlet of at least one first fuel cell in the fuel cell group in the refrigerant distribution passage, and is configured to detect a first outlet temperature that is a refrigerant outlet temperature of the first fuel cell, a voltage acquiring unit configured to acquire at least a first voltage that is a voltage of the first fuel cell, a current acquiring unit configured to acquire at least a first current that is a current of the first fuel cell, and a controller, wherein the controller calculates a first individual supply flow rate that is a flow rate of the refrigerant individually supplied to the first fuel cell on a basis of the first voltage, the first current, and the first outlet temperature, and a second individual supply flow rate that is a flow rate of the refrigerant individually supplied to at least one second fuel cell other than the first fuel cell on a basis of the first individual supply flow rate and the pre-distribution refrigerant flow rate.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described below with reference to accompanying drawings.

First Embodiment

FIG.1is a diagram illustrating a configuration of a fuel cell system according to a first embodiment.

As shown in the drawing, a fuel cell system10includes a fuel cell group12including two solid oxide fuel cell (SOFC) stacks12-1and12-2that are a plurality of fuel cells, an air supply passage14that is a refrigerant supply passage through which air as a refrigerant is supplied to the fuel cell group12, and an air distribution passage16as a refrigerant distribution passage through which the air is individually distributed to the SOFC stacks12-1and12-2composing the fuel cell group12. It is to be noted that in the present embodiment, the “SOFC stack12-1” corresponds to a “first fuel cell”, and the “SOFC stack12-2” corresponds to a “second fuel cell”. Therefore, hereinafter, the “SOFC stack12-1” and the “SOFC stack12-2” are also referred to as the “first SOFC stack12-1” and the “second SOFC stack12-2”, respectively.

Then, the first SOFC stack12-1and the second SOFC stack12-2are arranged in parallel with an electric load100including a traction motor, various auxiliary machines, a predetermined battery, etc. that are not shown.

The first SOFC stack12-1and the second SOFC stack12-2are both a fuel cell stack in which a plurality of SOFC cells each obtained by holding an electrolyte layer made of solid oxide, such as ceramic, between an anode (a fuel electrode) and a cathode (an air electrode) is stacked. Then, the first SOFC stack12-1and the second SOFC stack12-2cause fuel gas (hydrogen) supplied to the fuel electrode from a fuel supply system (not shown) to react with oxidizing gas (air) supplied to the air electrode from an air supply system (not shown) or through the air distribution passage16, thereby generating electric power.

Furthermore, the fuel cell group10includes a pre-distribution air flow rate sensor50and a first outlet temperature sensor54. The pre-distribution air flow rate sensor50detects a pre-distribution air flow rate qairthat is a flow rate of air in the air supply passage14, i.e., a flow rate of air before it is distributed to all the SOFC stacks12-1and12-2. The first outlet temperature sensor54is provided at an air outlet of the SOFC stack12-1in the air distribution passage16, and detects a first outlet temperature To1[1]that is an outlet temperature of the SOFC stack12-1.

The air supply passage14is a passage for supplying air fed from an air blower or the like (not shown) to the fuel cell group12.

The air distribution passage16includes a first distribution path16-1through which air from the air supply passage14is distributed to the first SOFC stack12-1and a second distribution path16-2through which the air from the air supply passage14is distributed to the second SOFC stack12-2. In this configuration, the air supplied to the first SOFC stack12-1and the second SOFC stack12-2through the first distribution path16-1and the second distribution path16-2cools the first SOFC stack12-1and the second SOFC stack12-2, and then is discharged into, for example, a cooling device (not shown) or a discharged gas system (not shown).

Moreover, the fuel cell system10includes a voltage sensor56and a current sensor58. The voltage sensor56detects a first voltage V1[1]that is a voltage of the first SOFC stack12-1. The current sensor58detects a current I as a first current that is a current of the first SOFC stack12-1.

It is to be noted that in the present embodiment, the SOFC stacks12-1and12-2are arranged in parallel with the electric load100; therefore, the “first current” of the first SOFC stack12-1and a “second current” of the second SOFC stack12-2are both detected as a “current I”. The same applies to first to third modification examples and second to fifth embodiments to be described later.

Furthermore, the fuel cell system10includes a controller60. The controller60calculates a first calorific value Qgen1[1]that is a calorific value of the first SOFC stack12-1on the basis of the first voltage V1[1]received from the voltage sensor56and the current I received from the current sensor58.

Furthermore, the controller60calculates a first individual supply flow rate qair1_d[1]that is a flow rate of air individually supplied to the first SOFC stack12-1through the first distribution path16-1on the basis of the pre-distribution air flow rate qairreceived from the pre-distribution air flow rate sensor50, the first outlet temperature To1[1]received from the first outlet temperature sensor54, and the calculated first calorific value Qgen1[1]. Moreover, the controller60calculates a second individual supply flow rate qair2_d[2]that is a flow rate of air individually supplied to the second SOFC stack12-2on the basis of the calculated first individual supply flow rate qair1_d[1].

It is to be noted that the controller60is configured of a computer, particularly, a microcomputer that includes a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM), and an input/output (I/O) interface. Then, the controller60is programmed, at least, to be able to execute processing required to perform respective processes associated with the present embodiment or any of later-described modification examples 1 to 3, or any of the second to fifth embodiments.

It is to be noted that the controller60may be configured as one device, or may be configured of multiple devices so that the multiple devices share and process controls of the present embodiment.

Below are described details of calculation of the first individual supply flow rate qair1_d[1]and the second individual supply flow rate qair2_d[1].

FIG.2is a flowchart illustrating the flow of the calculation of the first individual supply flow rate qair1_d[1]and the second individual supply flow rate qair2_d[2].

At Step S110, using the following Expression (1), the controller60calculates a first calorific value Qgen1[1]due to power generation of the first SOFC stack12-1on the basis of a first voltage V1[1]and a current I.
[Math. 1]
Qgen1[1]=I(E0−V1[1])  (1)

It is to be noted that “E0” in the expression denotes a theoretical electromotive force of the first SOFC stack12-1.

At Step S120, the controller60sets a theoretical formula of an assumed first outlet temperature value TO1exp[1]shown in the following Expression (2).

It is to be noted that “Tin” in Expression (2) denotes a temperature of air in the air supply passage14(hereinafter, also referred to as a “pre-supply air temperature Tin”); “qair/2” denotes a value obtained by dividing a pre-distribution air flow rate qairdetected by the pre-distribution air flow rate sensor50by 2 that is the number of SOFCs disposed in the fuel cell group12in the present embodiment; and “cair” denotes a specific heat capacity of air (hereinafter, referred to simply as an “air specific heat cair”).

It is to be noted that a predetermined fixed value determined in an experiment or the like is used as the air specific heat cair. Furthermore, hereinafter, “qair/2”, a value obtained by dividing the pre-distribution air flow rate qairby the number of the disposed SOFC stacks12, is also referred to as an “ideal distribution air flow rate qair/2”.

Furthermore, the assumed first outlet temperature value TO1exp[1]is a value of an outlet air temperature expected to be detected by the first SOFC stack12-1if a uniform flow rate of air is distributed from the air supply passage14to the first distribution path16-1and the second distribution path16-2(i.e., if there is no variation in the flow rate of air at the time of distribution).

Moreover, a denominator of a second term on the right side in Expression (2), i.e., a value obtained by multiplying the ideal distribution air flow rate qair/2 and the air specific heat caircorresponds to a heat capacity per unit time of air supplied to the first SOFC stack12-1(hereinafter, also referred to simply as a “supply air heat capacity”). Therefore, the second term on the right side, where the first calorific value Qgen1[1]of the first SOFC stack12-1is divided by the supply air heat capacity, corresponds to a real temperature rise of the air due to heat generation of the first SOFC stack12-1.

Therefore, the second term on the right side is a value that the real temperature rise of the air due to heat generation of the first SOFC stack12-1based on the ideal distribution air flow rate qair/2 is added to the pre-supply air temperature Tin. That is, as shown in Expression (2), the second term on the right side equals to the assumed first outlet temperature value TO1exp[1]on the left side.

Next, at Step S130, the controller60sets a theoretical formula of a first outlet temperature To1[1]shown in the following Expression (3) on the basis of the first calorific value Qgen1[1]of the first SOFC stack12-1obtained at Step S110.

Then, at Step S140, the controller60calculates a first individual supply flow rate qair1_d[1] on the basis of Expressions (2) and (3). Specifically, the first individual supply flow rate qair1_d[1]is calculated as shown in the following Expression (4).

It is to be noted that in the above Expression (4), the term of the pre-supply air temperature Tinincluded in Expressions (2) and (3) is eliminated. Therefore, in the present embodiment, the controller60can calculate the first individual supply flow rate qair1_d[1]on the basis of the pre-distribution air flow rate qairdetected by the pre-distribution air flow rate sensor50, the first outlet temperature To1[1]detected by the first outlet temperature sensor54, and the first calorific value Qgen1[1]calculated at Step S110.

Then, at Step S150, the controller60calculates a second individual supply flow rate qair2_d[2], which is a flow rate of air supplied to the second SOFC stack12-2, from the obtained first individual supply flow rate qair1_d[1]. Specifically, as shown in the following Expression (5), the second individual supply flow rate qair2_d[2]is calculated by subtracting the first individual supply flow rate qair1_d[1]from the pre-distribution air flow rate qair(=2q).
[Math. 5]
qair2_d[2]=qair−qair1_d[1](5)

Therefore, according to the present embodiment, it is possible to find the first individual supply flow rate qair1_d[1]of the first SOFC stack12-1and the second individual supply flow rate qair2_d[2]of the second SOFC stack12-2that take into consideration the variation in the flow rate of air distributed from the air supply passage14to the first distribution path16-1and the second distribution path16-2.

The fuel cell system10according to the present embodiment described above achieves the following functions and effects.

The fuel cell system10according to the present embodiment includes: the fuel cell group12including the first SOFC stack12-1and the second SOFC stack12-2that are a plurality of fuel cells; the air distribution passage16as a refrigerant distribution passage through which air is individually distributed to the first SOFC stack12-1and the second SOFC stack12-2that compose the fuel cell group12; the pre-distribution air flow rate sensor50as a pre-distribution refrigerant flow rate acquiring unit that acquires, as a pre-distribution refrigerant flow rate, a pre-distribution air flow rate qairthat is a flow rate of air before distribution; the first outlet temperature sensor54as a first outlet temperature detecting unit that is provided at a refrigerant outlet of the first SOFC stack12-1as a first fuel cell, which is one of the fuel cell group12, in the first distribution path16-1of the air distribution passage16and detects a first outlet temperature (a first outlet temperature To1[1]) that is an air outlet temperature of the first SOFC stack12-1; the voltage sensor56as a voltage acquiring unit that acquires a first voltage V1[1]that is a voltage of the first SOFC stack12-1; the current sensor58as a current acquiring unit that acquires a current I (a first current) that is a current of the first SOFC stack12-1; and the controller60.

Then, the controller60calculates a first individual supply flow rate qair1_d[1]that is a flow rate of refrigerant individually supplied to the first SOFC stack12-1on the basis of the first voltage V1[1], the current I, and the first outlet temperature To1[1](Steps S110to S140inFIG.2), and calculates a second individual supply flow rate qair2_d[2]that is a flow rate of refrigerant individually supplied to the one second SOFC stack12-2other than the first SOFC stack12-1on the basis of the first individual supply flow rate qair1_d[1]and the pre-distribution air flow rate qair(Step S150inFIG.2).

That is, the present embodiment provides a refrigerant flow rate estimation method for estimating a flow rate of air to be supplied in the fuel cell system10that individually distributes and supplies air as a refrigerant to the first SOFC stack12-1and the second SOFC stack12-2that are a plurality of fuel cells composing the fuel cell group12. Then, in this refrigerant flow rate estimation method, a first individual supply flow rate qair1_d[ 1]that is a flow rate of refrigerant individually supplied to the first SOFC stack12-1is calculated on the basis of a pre-distribution air flow rate qairthat is a flow rate of refrigerant before distribution, a first outlet temperature To1[1]detected at the air outlet of the first SOFC stack12-1as at least one first fuel cell of the fuel cell group12, a first voltage V1[1]that is a voltage of the first SOFC stack12-1, and a current I as a first current that is a current of the first SOFC stack12-1(Steps S110to S140inFIG.2). Then, a second individual supply flow rate qair2_d[2]that is a flow rate of refrigerant individually supplied to the second SOFC stack12-2other than the first SOFC stack12-1is calculated on the basis of the first individual supply flow rate qair1_d[1]and the pre-distribution air flow rate qair(Step S150inFIG.2).

Accordingly, in a case where air as a refrigerant is distributed to a plurality of disposed SOFCs, it is possible to find a first individual supply flow rate qair1_d[1]of the first SOFC stack12-1whose outlet temperature (first outlet temperature To1[1]) is detected by the first outlet temperature sensor54and also estimate a flow rate of air supplied to the second SOFC stack12-2on the basis of this first individual supply flow rate qair1_d[1]and a pre-distribution air flow rate qair.

Therefore, even if there is variation in the flow rate of supplied air between the first SOFC stack12-1and the second SOFC stack12-2, the accurate first individual supply flow rate qair1_d[1]is found in the first SOFC stack12-1whose outlet temperature is detected, and thus the flow rate of air supplied to the second SOFC stack12-2in which the variation in the flow rate of supplied air is reflected can be estimated by referring to this first individual supply flow rate qair1_d[1]and the pre-distribution air flow rate qair. That is, it is possible to estimate individual flow rates of air supplied to the first SOFC stack12-1and the second SOFC stack12-2with higher accuracy.

Consequently, temperature control, power generation control, etc. using the flow rates of air supplied to the first SOFC stack12-1and the second SOFC stack12-2can be also performed with higher accuracy.

In the present embodiment, particularly, the controller60calculates the second individual supply flow rate qair2_d[2]of the second SOFC stack12-2from the first individual supply flow rate qair1_d[1]of the first SOFC stack12-1provided with the first outlet temperature sensor54-1at its air outlet. Therefore, it is possible to find the second individual supply flow rate qair2_d[2]without installing a temperature sensor or a flow rate detection sensor at an air outlet of the second SOFC stack12-2. Accordingly, as compared with an existing fuel cell system in which a temperature sensor is installed at an air outlet of each fuel cell composing the fuel cell group12, it is possible to reduce the number of temperature sensors installed, and is possible to reduce the cost of manufacturing the system and the maintenance cost of the temperature sensors.

First Modification Example

Subsequently, the first modification example of the first embodiment is described. It is to be noted that a component similar to that of the first embodiment is assigned the same reference numeral, and its description is omitted. Furthermore, in the present modification example, particularly, there is described estimation of a first individual supply flow rate and a second individual supply flow rate in the fuel cell system10in which the fuel cell group12is composed of n (n is any positive integer) SOFC stacks. It is to be noted that the above-described first embodiment corresponds to a case of n=2 in the present modification example.

FIG.3is a diagram illustrating a configuration of the fuel cell system10according to the first modification example.

As shown in the drawing, in the first modification example, the fuel cell group12is composed of n SOFC stacks12-1,12-2, . . . ,12-(n−1), and12-n. Then, the air distribution passage16through which air from the air supply passage14is distributed to these SOFCs is composed of n passages that are first to nth distribution paths16-1to16-ncorresponding to the n SOFCs.

Then, the SOFC stacks12-1,12-2, . . . , and12-(n−1) are provided with voltage sensors56-1,56-2, . . . , and56-(n−1) corresponding to the voltage sensor56in the first embodiment, respectively. Furthermore, first outlet temperature sensors54-1,54-2, . . . , and54-(n−1) corresponding to the first outlet temperature sensor54in the first embodiment are provided at air outlets of the SOFC stacks12-1,12-2, . . . , and12-(n−1), i.e., the SOFC stacks other than the SOFC stack12-n, respectively.

Therefore, in the present modification example, the SOFC stacks12-1,12-2, . . . , and12-(n−1) correspond to the “first SOFC stack12-1” in the first embodiment. Furthermore, the SOFC stack12-ncorresponds to the “second SOFC stack12-2” in the first embodiment.

It is to be noted that in the present modification example, for the sake of simplicity of description, description about the n−1 first SOFC stacks12-1,12-2, . . . , and12-(n−1) and their peripheral structure is condensed by using an integer k in a range of 1≤k≤n−1 as needed. However, this description holds true for any integer k that meets 1≤k≤n−1.

That is, in the present modification example, the “first SOFC stack12-1” or the like in the first embodiment is changed into the “first SOFC stack12-k”, and the “second SOFC stack12-2” or the like in the first embodiment is changed into the “second SOFC stack12-n”. In accordance with this, respective reference numerals of the parameters described in the first embodiment are replaced as follows.“first outlet temperature To1[1]”⇒“first outlet temperature To1[k]”“first voltage V1[1]”⇒“first voltage V1[k]”“first calorific value Qgen1[1]”⇒“first calorific value Qgen1[k]”“assumed first outlet temperature value TO1exp[1]”⇒“assumed first outlet temperature value TO1exp[k]”“first individual supply flow rate qair1_d[1]”⇒“first individual supply flow rate qair1_d[k]”“second individual supply flow rate qair2_d[2]”⇒“second individual supply flow rate qair2_d[n]”

Below are described details of calculation of a first individual supply flow rate qair1_d[k]and a second individual supply flow rate qair2_d[n]in the present modification example.

FIG.4is a flowchart illustrating the flow of the calculation of the first individual supply flow rate qair1_d[k]and the second individual supply flow rate qair2_d[n]in the present modification example.

At Step S210, using the following Expression (6), the controller60calculates a first calorific value Qgen1[k]due to power generation of the first SOFC stack12-kon the basis of a first voltage V1[k]and a current I.
[Math. 6]
Qgen1[k]=I(E0−V1[k])  (6)

At Step S220, the controller60sets a theoretical formula of an assumed first outlet temperature value TO1exp[k]shown in the following Expression (7).

It is to be noted that as described in the above embodiment, “qair/n” in Expression (7) corresponds to the ideal distribution air flow rate described in the first embodiment.

At Step S230, the controller60sets a theoretical formula of a first outlet temperature To1[k]shown in the following Expression (8) on the basis of the first calorific value Qgen1[k]obtained at Step S210.

Then, at Step S240, the controller60calculates a first individual supply flow rate qair1_d[k]on the basis of Expressions (7) and (8). Specifically, the first individual supply flow rate qair1_d[k]is calculated as shown in the following Expression (9).

Therefore, also in the present modification example, the term of the pre-supply air temperature Tinincluded in Expressions (2) and (3) is eliminated, and the controller60can calculate the first individual supply flow rate qair1_d[k]from the pre-distribution air flow rate qair, the first outlet temperature To1[k], the first voltage V1[k], and the current I that are detected values.

In the present modification example, particularly, the above-described Steps S210to S240are performed on all integers k that meet 1≤k≤n−1, and thereby all of first individual supply flow rates qair1_d[1], qair1_d[2], . . . , and qair1_d[n-1] are found.

Then, at Step S250, the controller60calculates a second individual supply flow rate qair2_d[n]on the basis of the obtained first individual supply flow rates qair1_d[1], qair1_d[2], . . . , and qair1_d[n−1]. Specifically, as shown in the following Expression (10), the second individual supply flow rate qair2_d[n]is calculated by subtracting the total sum of the first individual supply flow rates qair1_d[1], qair1_d[2], . . . , and qair1_d[n−1]from the pre-distribution air flow rate qair.
[Math. 10]
qair2_d[n]=qair−(qair1_d[1]+qair1_d[2]+ . . . +qair1_d[n−1])  (10)

Therefore, according to the present modification example, even the fuel cell system10provided with the n SOFC stacks12can calculate the first individual supply flow rate qair1_d[k]of the first SOFC stack12-kwhose air outlet temperature (first outlet temperature To1[k]) is detected and also accurately find the second individual supply flow rate qair2_d[n]on the basis of the total sum of the first individual supply flow rates qair1_d[k]and the pre-distribution air flow rate qair.

That is, according to the present modification example, also in the fuel cell system10that distributes air as a refrigerant to the n SOFC stacks12, particularly, three, four, or any more number of SOFC stacks, it is possible to find the respective first individual supply flow rates qair1_d[k]of the first SOFC stacks12-kand the second individual supply flow rate qair2_d[n]of the second SOFC stack12-nin which the variation in the flow rate of air due to distribution is reflected. Therefore, it is possible to accurately estimate a flow rate of supplied air to each SOFC stack12even if the temperature sensor at the air outlet of the second SOFC stack12-nis omitted as with the first embodiment.

Second Modification Example

Subsequently, the second modification example of the first embodiment is described.

FIG.5is a diagram illustrating a configuration of the fuel cell system10according to the second modification example. It is to be noted that a component similar to that of the first embodiment or the first modification example is assigned the same reference numeral, and its description is omitted.

As shown in the drawing, in the present modification example, as with the first modification example, the fuel cell group12is composed of the n SOFC stacks12-1,12-2, . . . ,12-(n−1), and12-n.

Meanwhile, in the present modification example, the (n−2) or less SOFC stacks12-1,12-2, . . . , and12-m(2≤m≤n−2) in the fuel cell group12are provided with the first outlet temperature sensors54-1,54-2, . . . , and54-m, respectively. Furthermore, the other SOFC stacks12-(m+1),12-(m+2), . . . , and12-nare not provided with a temperature sensor at their air outlet.

Therefore, in the present modification example, the SOFC stacks12-1,12-2, . . . , and12-mcorrespond to the “first SOFC stack”. Furthermore, the SOFC stacks12-(m+1),12-(m+2), . . . , and12-ncorrespond to the “second SOFC stack”. That is, in the present modification example, there are the multiple “second SOFC stacks” provided with no temperature sensor at their air outlet.

It is to be noted that in the present modification example, for the sake of simplicity of description, description about the m first SOFC stacks12-1,12-2, . . . , and12-mand their peripheral structure is condensed by using an integer k in a range of 1≤k≤m as needed. Furthermore, description about the (n-m) second SOFC stacks12-(m+1),12-(m+2), . . . , and12-nand their peripheral structure is condensed by using an integer z in a range of m+1≤z≤n. However, this description holds true for any integers k and z that meet 1≤k≤m and m+1≤z≤n.

Moreover, in the present modification example, the “first SOFC stack12-1” or the like in the first embodiment is changed into the “first SOFC stack12-k”, and the “second SOFC stack12-2” or the like in the first embodiment is changed into the “second SOFC stack12-z”. In accordance with this, the respective reference numerals of the parameters described in the first embodiment are replaced as follows.“first outlet temperature To1[1]”⇒“first outlet temperature To1[k]”“first voltage V1[1]”⇒“first voltage V1[k]”“first calorific value Qgen1[1]”⇒“first calorific value Qgen1[k]”“assumed first outlet temperature value TO1exp[1]”⇒“assumed first outlet temperature value TO1exp[k]”“first individual supply flow rate qair1_d[1]”⇒“first individual supply flow rate qair1_d[k]”“second individual supply flow rate qair2_d[2]”⇒“second individual supply flow rate qair2_d[z]”

Below are described details of calculation of a first individual supply flow rate qair1_d[k]and a second individual supply flow rate qair2_d[z]in the present modification example.

FIG.6is a flowchart illustrating the flow of the calculation of the first individual supply flow rate qair1_d[k]and the second individual supply flow rate qair2_d[z]in the present modification example.

At Steps S310to S340shown inFIG.6, the controller60performs similar processes to the processes at Steps S210to S240inFIG.4in the first modification example, and calculates respective first individual supply flow rates qair1_d[k]with respect to all integers k that meet 1≤k≤m. That is, the controller60calculates first individual supply flow rates qair1_d[1], qair1_d[2], . . . , and qair1_d[m].

Then, at Step S350, the controller60calculates a second individual supply flow rate qair2_d[z]from the obtained first individual supply flow rates qair1_d[1], qair1_d[2], . . . , and qair1_d[m]on the basis of the following Expression (11).

Here, the right side of Expression (11) denotes a value obtained by subtracting the total sum of the first individual supply flow rates qair1_d[1], qair1_d[2], . . . , and qair1_d[m]from the pre-distribution air flow rate qairand then dividing the obtained value by (n−m) that is the number of the second SOFC stacks12-z.

Therefore, the second individual supply flow rate qair2_d[z]in the present modification example is calculated to be the same value among all the second SOFC stacks12-z(m+1≤z≤n).

As understood from the above description, in the present embodiment, even in a case where there exist the multiple second SOFC stacks12-zprovided with no temperature sensor at their air outlet, it is possible to calculate the first individual supply flow rate qair1_d[k]and the second individual supply flow rate qair2_d[z].

Here, in the present modification example, as for at least all of the first SOFC stacks12-k(1≤k≤m), it is possible to find an individual flow rate of supplied air that takes variation into consideration. Meanwhile, as described above, as for the second SOFC stacks12-z(m+1≤z≤n), the same value is obtained as their second individual supply flow rate qair2_d[z]. Therefore, it is not possible to rigorously evaluate variation in the flow rate of supplied air among the second SOFC stacks12-z.

However, depending on the design, etc. of the fuel cell system10, without having to rigorously evaluate variation in the flow rate of supplied air among some of the second SOFC stacks12-z, there may not be large errors in temperature control, power generation amount control, etc.

In such a case, by adopting the system configuration according to the present modification example, it becomes possible to reduce the number of temperature sensors installed while maintaining the accuracy of temperature control, power generation amount control, etc. As a result, it is possible to further reduce the manufacturing cost and the maintenance cost of the fuel cell system10.

It is to be noted that in the present modification example, a ratio of the number of first SOFCs12-kprovided with a temperature sensor at their air outlet and the number of second SOFCs12-zprovided with a temperature sensor at their air outlet can be fittingly adjusted depending on the design, etc. of the fuel cell system10. Accordingly, it is possible to adjust the balance between the high accuracy of temperature control and power generation amount control and the low cost fittingly. For example, if the number of the first SOFCs12-kand the number of the second SOFCs12-zare about the same, the number of temperature sensors installed can be about half of that is in a case where all the SOFCs composing the fuel cell group12are provided with a temperature sensor at their air outlet.

Third Modification Example

Subsequently, the third modification example of the present embodiment is described.

FIG.7is a diagram illustrating a configuration of the fuel cell system10according to the third modification example. It is to be noted that a component similar to that of any of the first embodiment and the above-described modification examples 1 and 2 is assigned the same reference numeral, and its description is omitted.

As shown in the drawing, in the third modification example, the number n of the SOFCs disposed in the fuel cell group12in the first and second modification examples is set to be n=N (“N” is an even number). That is, the fuel cell group12is composed of N SOFC stacks12-1,12-2, . . . , and12-N. Then, the air distribution passage16through which air from the air supply passage14is distributed to these SOFCs is composed of N passages that are first to Nth distribution paths16-1to16-N corresponding to the N SOFCs.

It is to be noted that in the present modification example, for the sake of simplicity of description, description about the SOFC stacks12-1,12-2, . . . , and12-(N−2) and their peripheral structure is condensed by using an integer k in a range of 1≤k≤N−2 as needed. However, this description holds true for any integer k that meets 1≤k≤N−2.

The fuel cell system10according to the present modification example includes a merging path17[k, k+1]. In an SOFC group12[k, k+1]composed of two SOFC stacks12-kand12-(k+1), distribution paths16-kand16-(k+1) for supplying air to the SOFC stacks12-kand12-(k+1) are merged into the merging path17[k, k+1]at outlets of the SOFC stacks12-kand12-(k+1).

Furthermore, the merging path17[k, k+1]is provided with a first outlet temperature sensor54[k, k+1]that detects an air temperature of the merging path17[k, k+1]. Furthermore, the SOFC stacks12-kand12-(k+1) are each provided with a voltage sensor56-k.

That is, in the present modification example, the air temperature in the merging path17[k, k+1]where the air from the SOFC group12[k, k+1]composed of the two SOFC stacks12-kand12-(k+1) is merged is detected as a “first outlet temperature”.

Therefore, in the present modification example, the SOFC group12[k, k+1]composed of the two SOFC stacks12-kand12-(k+1) in the condition of 1≤k≤N−2 corresponds to the “first SOFC stack”.

Meanwhile, no temperature sensor is provided in a merging path17[N-1, N]where air from an SOFC group12[N-1, N]composed of two SOFC stacks12-(N−1) and12-N is merged. Therefore, in the present modification example, the SOFC group12[N-1, N]corresponds to the “second SOFC stack”. Hereinafter, the “SOFC group12[k, k+1]” and the “SOFC group12[N-1, N]” are referred to as the “first SOFC group12[k, k+1]” and the “second SOFC group12[N-1, N]”, respectively.

Furthermore, in the following description, the respective reference numerals of the parameters described in the first embodiment or each of the above-described modification examples are replaced as follows.“first outlet temperature To1[1]”⇒“first outlet temperature To1[k, k+1]”“first voltage V1[1]”⇒“first voltage V1[k, k+1]”“first calorific value Qgen1[1]”⇒“first calorific value Qgen1[k, k+1]”“assumed first outlet temperature value TO1exp[1]”⇒“assumed first outlet temperature value TO1exp[k, k+1]”“first individual supply flow rate qair1_d[1]”⇒“first individual supply flow rate qair1_d[k, k+1]”“second individual supply flow rate qair2_d[2]”⇒“second individual supply flow rate qair2_d[N-1, N]”

It is to be noted that in the present modification example, a “first voltage V1[k, k+1]” of the first SOFC group12[k, k+1]corresponds to “V1[k]+V1[k+1]” that is the sum of a first voltage V1[k]of the SOFC stack12-kand a first voltage V1[k+1]of the SOFC stack12-(k+1) in the first modification example.

Furthermore, in the present modification example, a “first calorific value Qgen1[k, k+1]” of the first SOFC group12[k, k+1]corresponds to “Qgen1[k, k+1]+Qgen1[k+1]” that is the sum of a “first calorific value Qgen1[k]” of the SOFC stack12-kand a “first calorific value Qgen1[k+1]” of the SOFC stack12-(k+1) in the first modification example.

Below are described details of calculation of a first individual supply flow rate qair1_d[k, k+1]and a second individual supply flow rate qair2_d[N-1, N]in the present modification example.

FIG.8is a flowchart illustrating the flow of the calculation of the first individual supply flow rate qair1_d[k, k+1]and the second individual supply flow rate qair2_d[N-1, N]in the present modification example.

At Step S410, the controller60calculates a first calorific value Qgen1[k, k+1]due to power generation of the first SOFC group12[k, k+1]on the basis of a first voltage V1[k+1]and a current I. Specifically, the controller60calculates a first calorific value Qgen1[k, k+1], replacing “V1[k]” and “E0” on the right side of Expression (6) described in the first modification example with “V1[k]+V1[k+1]” and “2E0”, respectively.

Furthermore, the controller60performs processes at Steps S420to S450in a similar way, replacing the respective parameters of Expressions (7) to (10) used at Steps S220to S250in the first modification example with the ones defined in the present modification example fittingly.

In the present embodiment, particularly, with two SOFCs as one group, the first SOFC groups12[k, k+1]and the second SOFC groups12[N-1, N]are an object of detection; therefore, an above-described ideal distribution air flow rate is determined on the basis of “N/2” that is the total number of these groups. That is, in Expressions (7) to (10), “qair/n” denoting the ideal distribution air flow rate is replaced with “2qair/N”.

Through the above-described processes at Steps S410to S450, the first individual supply flow rate qair1_d[k, k+1]and the second individual supply flow rate qair2_d[N-1, N]can be suitably calculated.

Therefore, also in the present modification example, it is possible to calculate the first individual supply flow rate qair1_d[k, k+1]of the first SOFC group12[k, k+1]provided with the outlet temperature sensor and also accurately estimate the second individual supply flow rate qair2_d[N-1, N]of the second SOFC group12[N-1, N]on the basis of the total sum of the first individual supply flow rates qair1_d[k, k+1]and the pre-distribution air flow rate qair.

Here, in the present modification example, the first individual supply flow rate qair1_d[k, k+1]and the second individual supply flow rate qair2_d[N-1, N]are estimated in units of the first SOFC group12[k, k+1]and the second SOFC group12[N-1, N]with two SOFCs as one group. Therefore, it is not possible to rigorously evaluate variation in the flow rate of supplied air among single SOFCs composing the fuel cell group12.

However, depending on the design, etc. of the fuel cell system10, without having to rigorously evaluate variation in the flow rate of supplied air among single SOFCs, there may not be large errors in temperature control, power generation amount control, etc. In such a case, by adopting the system configuration according to the present modification example, it becomes possible to reduce the number of temperature sensors installed while maintaining the accuracy of temperature control and power generation amount control. In particular, as in the present modification example, the first outlet temperature sensor54[k, k+1]is provided for each first SOFC group12[k, k+1]with two SOFCs as one group, and thus the number of temperature sensors installed can be reduced by more than half. As a result, it is possible to further enhance the cost suppressing effect.

It is to be noted that in the present modification example, with two SOFCs as one group, the first SOFC groups12[k, k+1]and the second SOFC groups12[N-1, N]are provided. However, with three or more SOFCs as one group, first SOFC groups and second SOFC groups12may be provided. Furthermore, the number of SOFCs included in each SOFC group may be different among the SOFC groups.

Second Embodiment

A second embodiment is described below. It is to be noted that a component similar to that of any of the first embodiment and the first to third modification examples is assigned the same reference numeral, and its description is omitted.

FIG.9is a diagram illustrating a configuration of the fuel cell system10according to the second embodiment.

As shown in the drawing, the fuel cell system10according to the present embodiment is based on the configuration of the fuel cell system10according to the first modification example illustrated inFIG.3. In particular, the fuel cell system10according to the present embodiment includes a pre-supply refrigerant temperature sensor59that detects a pre-supply air temperature Tinthat is an air temperature in the air supply passage14and a voltage sensor56-nthat detects a second voltage V2[n]that is a voltage of the second SOFC stack12-n, in addition to the configuration of the fuel cell system10according to the first modification example.

Furthermore, the controller60according to the present embodiment acquires the pre-supply air temperature Tinfrom the pre-supply refrigerant temperature sensor59and the second voltage V2[n]from the voltage sensor56-n, besides respective detection signals from the sensors described in the first modification example.

Then, the controller60calculates (estimates) a second outlet temperature To2exp[n]that is an air outlet temperature of the second SOFC stack12-non the basis of the pre-supply air temperature Tin, the second voltage V2[n], and a first individual supply flow rate qair1_d[k](k=1 to n) and a second individual supply flow rate qair2_d[n]that are found by executing the steps inFIG.4described in the first modification example.

FIG.10is a flowchart illustrating the flow of the calculation of the second outlet temperature To2exp[n]in the present embodiment.

At Step S510, using the following Expression (12), the controller60calculates a second calorific value Qgen2[n]due to power generation of the second SOFC stack12-non the basis of a second voltage V2[n]and a current I.
[Math. 12]
Qgen2[k]=I(E0−V2[n])  (12)

It is to be noted that the calculation of this second calorific value Qgen2[n]may be performed in advance, for example, in a stage of Step S210inFIG.4.

At Step S520, the controller60sets a theoretical formula of a second outlet temperature TO2exp[n]shown in the following Expression (13).

Then, at Step S530, the controller60calculates the second outlet temperature TO2exp[n]using the already-calculated second individual supply flow rate qair2_d[n]in Expression (13).

That is, the second outlet temperature TO2exp[n]can be calculated by substituting the second individual supply flow rate qair2_d[n]calculated in Expressions (9) and (10) for Expression (13).

It is to be noted that for example, in a case where the fuel cell group12is composed of two SOFC stacks12-1and12-2, “n=2”; therefore, the second outlet temperature TO2exp[n]in this case is determined as shown in the following Expression (14).

The fuel cell system10according to the present embodiment described above achieves the following functions and effects.

The fuel cell system10according to the present embodiment further includes the pre-supply refrigerant temperature sensor59as a pre-supply refrigerant temperature detecting unit that detects a pre-supply air temperature Tinas a pre-supply refrigerant temperature that is a temperature of a refrigerant before it is supplied to the SOFC stacks12-1to12-n.

Furthermore, the voltage sensor56-nas a voltage acquiring unit detects a second voltage V2[n]that is a voltage of the second SOFC stack12-n. Moreover, the current sensor58as a current acquiring unit detects a current I as a second current of the second SOFC stack12-n.

Then, the controller60calculates a second outlet temperature TO2exp[n]that is an air outlet temperature of the second SOFC stack12-non the basis of the second voltage V2[n], the current I, the pre-supply air temperature Tin, and the second individual supply flow rate first individual supply flow rate (Steps S510to S520inFIG.10).

Accordingly, it is possible to estimate the second outlet temperature TO2exp[n]that is an air outlet temperature of the second SOFC stack12-nprovided with no outlet temperature sensor.

In particular, the second individual supply flow rate qair2_d[n]in which the above-described variation in the flow rate of distributed air is reflected is used in the calculation of the second outlet temperature To2exp[n]; therefore, it is possible to obtain the accurate second outlet temperature To2exp[n]that takes into consideration the variation in the flow rate of distributed air. That is, this can be accurately estimated without having to detect an outlet temperature of the second SOFC stack12-n. As a result, it is possible to acquire an accurate air outlet temperature of the second SOFC stack12-nprovided with no outlet temperature sensor while reducing the number of temperature sensors at the air outlets of the second SOFC stacks12-nand thereby reducing the cost.

It is to be noted that in the present embodiment, there is described an example where a second outlet temperature To2exp[n]is calculated in the fuel cell system10based on the configuration of the first modification example. However, a second outlet temperature may be calculated in the fuel cell system10based on the configuration of the first embodiment, the configuration of the second modification example, or the configuration of the third modification example as well. Also in these cases, a second outlet temperature can be calculated by performing processes similar to those at Steps S510to S530in the present embodiment.

Third Embodiment

A third embodiment is described below. It is to be noted that a component similar to that of any of the above-described embodiments and the modification examples is assigned the same reference numeral, and its description is omitted.

FIG.11is a diagram illustrating a configuration of the fuel cell system10according to the third embodiment.

As shown in the drawing, the fuel cell system10according to the present embodiment includes a fuel supply system that supplies fuel (fuel gas) for power generation to the SOFC stacks12-1to12-n, in addition to the configuration of the second fuel cell system10illustrated inFIG.9.

Specifically, the fuel supply system in the present embodiment includes a fuel pump80, a fuel supply passage82that is a passage through which fuel is supplied from the fuel pump80to the SOFC stacks12-1to12-n, and a fuel flow rate sensor84that detects a flow rate of fuel fed from the fuel pump80into the fuel supply passage82. Hereinafter, the flow rate of fuel detected by the fuel flow rate sensor84is also referred to as a “total supply fuel flow rate qfuel”.

Furthermore, the fuel supply passage82includes fuel distribution pipes82athrough which fuel is individually distributed to the SOFC stacks12-1to12-n. It is to be noted that the fuel distribution pipes82aare each provided with a device (not shown) such as an injector or an opening degree control valve that controls fuel supply to the corresponding SOFC stack12. By controlling these devices individually or collectively, respective distribution flow rates of fuel supply to SOFC stacks12-1to12-ncan be adjusted.

Then, in the fuel cell system10according to the present embodiment, the controller60acquires a total supply fuel flow rate qfuelthat is a detected value of the fuel flow rate sensor84besides respective detection signals from the sensors described in the second embodiment.

Furthermore, the controller60according to the present embodiment calculates a heat capacity of supply fuel on the basis of the total supply fuel flow rate qfuel, and corrects a first calorific value Qgen1[k]and a second calorific value Qgen2[n]on the basis of the heat capacity of supply fuel. Its details are described below.

FIG.12is a flowchart illustrating a method of calculating a first individual supply flow rate qair1_d[k], a second individual supply flow rate qair2_d[n], and a second outlet temperature To2exp[n], including a process of correcting a first calorific value Qgen1[k]and a second calorific value Qgen2[n].

At Step S610, the controller60calculates a corrected first calorific value Qgen1_cor[k]from a first calorific value Qgen1[k]. Specifically, as with Step S210(FIG.4) described in the first modification example, on the basis of Expression (6), the controller60first calculates a first calorific value Qgen1[k]on the basis of a first voltage V1[k]and a first current I.

Furthermore, on the basis of the following Expression (15), the controller60calculates a corrected first calorific value Qgen1_cor[k]by correcting the calculated first calorific value Qgen1[k]using a total supply fuel flow rate qfueldetected by the fuel flow rate sensor84.

However, “Cfuel1[k]” in Expression (15) denotes a heat capacity of fuel supplied to the first SOFC stack12-k. Hereinafter, this heat capacity is also referred to as a “first SOFC supply fuel heat capacity Cfuel1[k]”.

Here, the controller60can calculate the first SOFC supply fuel heat capacity Cfuel[k]on the basis of the following Expression (16).

In Expression (16), “cfuel” denotes a specific heat capacity of fuel supplied to the first SOFC stack12-k. Hereinafter, this is also referred to as a “fuel specific heat cfuel”. A fixed value determined in an experiment or the like in advance is used as the “fuel specific heat cfuel” in the present embodiment. In the present embodiment, particularly, for the sake of simplicity of calculation, the “fuel specific heat cfuel” is set to be the same value among the SOFC stacks12-1to12-n.

Furthermore, in Expression (16), qfuel/n on the right side, which is obtained by dividing the total supply fuel flow rate qfuelby n that is the number of all stacks, corresponds to a flow rate of fuel supplied to one SOFC stack. Therefore, according to Expression (16), by multiplying this qfuel/n by the fuel specific heat cfuel, a first SOFC supply fuel heat capacity fuel, Cfuel1[k]that is a heat capacity of fuel supplied to the first SOFC stack12-kcan be calculated.

Meanwhile, to return to Expression (15), “Cair1[k]” denotes a heat capacity of air supplied to the first SOFC stack12-k. Hereinafter, this is also referred to as a “first SOFC supply air heat capacity Cair1[k]”.

Here, the controller60can calculate the first SOFC supply air heat capacity Cair1[k]on the basis of the following Expression (17).

In Expression (17), “qair/n” on the right side corresponds to the already-described ideal distribution air flow rate. According to Expression (17), by multiplying this ideal distribution air flow rate qair/n by the air specific heat cair, the first SOFC supply air heat capacity Cair1[k]that is a heat capacity of air supplied to the first SOFC stack12-kcan be calculated.

Therefore, the controller60can calculate a corrected first calorific value Qgen1_cor[k]by applying the first SOFC supply fuel heat capacity Cfuel1[k]obtained by Expression (16), the first SOFC supply air heat capacity Cair1[k]obtained by Expression (17), and the first calorific value Qgen1[k]to Expression (15).

The meaning of calculating such a corrected first calorific value Qgen1_cor[k] is described.

In a basic operating state of the fuel cell system10, a flow rate of fuel supplied to the anode of the first SOFC stack12-kis reduced by about ten orders as compared with a flow rate of air supplied to the cathode of the first SOFC stack12-k. That is, it may be said that basically, a heat capacity of fuel supplied to the first SOFC stack12-kis negligibly small as compared with a heat capacity of air supplied to the first SOFC stack12-k.

Therefore, even a model based on the assumption that heat generated by the first SOFC stack12-kis normally transmitted to only air virtually supplied to the first SOFC stack12-kand not transmitted to fuel does not often cause large errors in the first individual supply flow rate qair1_d[k], the second individual supply flow rate qair2_d[n], and the second outlet temperature To2exp[n]that are calculated as a result.

However, under a special operating state, for example, such as at the time of high load, for example, if the flow rate of fuel supplied to the first SOFC stack12-kis increased, the heat capacity of fuel becomes higher than usual, and therefore, it can also be in a state that heat transmission to the fuel becomes not negligible.

On the other hand, in the present embodiment, the corrected first calorific value Qgen1_cor[k]that takes heat transmission to fuel into consideration is found, and therefore, even in a case where heat transmission to the fuel is not negligible, it is possible to calculate the first individual supply flow rate qair1_d[k], the second individual supply flow rate qair2_d[n], and the second outlet temperature To2exp[n]with high accuracy.

It is to be noted that as can be seen from Expression (15), as for (1−Cfuel1[k]/Cair1[k]) on the right side, in a case where heat transmitted to fuel is negligible, it can be considered that the SOFC supply fuel heat capacity Cfuel≈0; therefore, the corrected first calorific value Qgen1_cor[k]substantially agrees with the first calorific value Qgen1[k].

On the other hand, in a case where heat transmitted to fuel is not negligible, (1−Cfuel1[k]/Cair1[k])<1, and thus the corrected first calorific value Qgen1_cor[k]<the first calorific value Qgen1[k]. That is, the transmission of heat of the first SOFC stack12-kto fuel is reflected in the corrected first calorific value Qgen1_cor[k]. Therefore, in the calculation of the subsequent first individual supply flow rate qair1_d[k], etc., the corrected first calorific value Qgen1_cor[k]that has been reduced by the transmission to the fuel can be used, and therefore, the accuracy of estimation of the first individual supply flow rate qair1_d[k], etc. is further improved.

Next, the controller60performs processes at Steps S620to S650using the corrected first calorific value Qgen1_cor[k]. Specifically, the controller60performs processes similar to Steps S220to S250in the above-described first modification example, replacing the “first calorific value Qgen1[k]” with the “corrected first calorific value Qgen1_cor[k]”. That is, the controller60performs respective calculations based on the above-described Expressions (7) to (10), and calculates a first individual supply flow rate qair1_d[k]and a second individual supply flow rate qair2_d[n].

Next, at Step S660, the controller60finds a corrected second calorific value Qgen2_cor[n]in a similar way to the above-described Step S610.

That is, the controller60calculates a corrected second calorific value Qgen2_cor[n]on the basis of a second calorific value Qgen2[n]of the second SOFC stack12-n, determining a second SOFC supply fuel heat capacity Cfuel2[n]as a heat capacity of fuel supplied to the second SOFC stack12-nand a second SOFC supply air heat capacity Cair2[n]as a heat capacity of air supplied to the second SOFC stack12-n.

Then, at Steps S670and S680, the controller60calculates a second outlet temperature To2exp[n]in a similar way to Steps S520and S530(seeFIG.10) described in the second embodiment.

Accordingly, it is possible to estimate the second outlet temperature To2exp[n]that is an air outlet temperature of the second SOFC stack12-nwith higher accuracy, taking into consideration the transmission to fuel supplied to the SOFC stack12.

The fuel cell system10according to the present embodiment described above achieves the following functions and effects.

The fuel cell system10according to the present embodiment further includes the fuel flow rate sensor84as a fuel flow rate acquiring unit that detects a total supply fuel flow rate qfuelas a flow rate of fuel supplied to the fuel cell group12.

Then, the controller60calculates a first SOFC supply fuel heat capacity Cfuel1[k]and a second SOFC supply fuel heat capacity Cfuel2[n]that are a heat capacity of fuel on the basis of the total supply fuel flow rate qfuel, and corrects the first individual supply flow rate qair1_d[k], the second individual supply flow rate qair2_d[n], and the second outlet temperature To2exp[n]on the basis of the first SOFC supply fuel heat capacity Cfuel1[k]and the second SOFC supply fuel heat capacity Cfuel2[n](Steps S610to S680inFIG.12).

Therefore, even in a case where thermal energy transmitted to fuel is not negligible with respect to thermal energy transmitted to air because of the operating states of the SOFC stacks12-1to12-n, it is possible to accurately calculate a first individual supply flow rate qair1_d[k], a second individual supply flow rate qair2_d[n], and a second outlet temperature To2exp[n]that take into consideration the thermal energy transmitted to the fuel.

It is to be noted that in the above-described embodiment, there is described a case where both a first calorific value Qgen1[k]and a second calorific value Qgen2[n]are corrected; however, only either one of them may be corrected. For example, in a case where the second outlet temperature To2exp[n]is not be used as a parameter because of an aspect of intended control, a corrected first calorific value Qgen1_cor[k]obtained by correcting the first calorific value Qgen1[k]may be calculated, and the processes up to Step S650shown inFIG.12may be performed.

Furthermore, in the fuel cell system10according to the present embodiment, the fuel flow rate sensor84is provided to detect a total supply fuel flow rate qfuel. However, the total supply fuel flow rate qfuelmay be estimated by another means without providing the fuel flow rate sensor84. For example, the total supply fuel flow rate qfuelmay be estimated on the basis of changes in the fuel level of a fuel tank (not shown) as a fuel supply source or a duty ratio of the fuel pump80.

Moreover, in the present embodiment, in calculation of a first SOFC supply fuel heat capacity Cfuel1[k]and a second SOFC supply fuel heat capacity Cfuel2[n]on the basis of the above-described Expression (16), a flow rate of fuel supplied to each SOFC stack12is assumed to be qfuel/n.

However, for example, an injector, an opening degree control valve, or the like may be provided in each of the fuel distribution pipes82a, and respective individual supply fuel flow rates of the SOFC stacks12-1to12-nmay be estimated from respective individual fuel flow rate control amounts for the SOFC stacks12-1to12-n, and then, with respect to each individual stack, a first SOFC supply fuel heat capacity Cfuel1[k]and a second SOFC supply fuel heat capacity Cfuel2[n]may be calculated on the basis of the individual supply fuel flow rates.

Furthermore, in calculation of a first SOFC supply fuel heat capacity Cfuel1[k]and a second SOFC supply fuel heat capacity Cfuel2[n]using Expression (17), the ideal distribution air flow rate qair/n on the right side may be replaced with a value of a flow rate of air that takes into consideration the variation in the flow rate of air among the SOFC stacks12-1to12-n.

For example, a preliminary first individual supply flow rate qair1_d[k]and a preliminary second individual supply flow rate qair2_d[n]may be calculated from the uncorrected first calorific value Qgen1[k]and the uncorrected second calorific value Qgen2[n]in the present embodiment by a similar method to the first modification example, and these preliminary flow rates of air may be used instead of the ideal distribution air flow rate qair/n on the right side.

Fourth Embodiment

A fourth embodiment is described below. It is to be noted that a component similar to that of any of the above-described embodiments and the modification examples is assigned the same reference numeral, and its description is omitted.

FIG.13is a diagram illustrating a configuration of the fuel cell system10according to the fourth embodiment.

As shown in the drawing, the fuel cell system10according to the present embodiment includes an air pump86as a refrigerant adjusting device that adjusts the flow rate of air in the air supply passage14in addition to the configuration of the fuel cell system10according to the second embodiment illustrated inFIG.9.

In the fuel cell system10according to the present embodiment having the above-described configuration, the controller60controls the output of the air pump86on the basis of a first outlet temperature To1[k]detected by the first outlet temperature sensor54-kand a second outlet temperature To2exp[n]calculated through the processes (at Steps S510to S530inFIG.10) described in the second embodiment, and adjusts the flow rate of air in the air supply passage14.

It is to be noted that in the present embodiment, a flow rate control outlet temperature To_f_contis calculated by applying all values from 0 to n−1 to k. Therefore, the flow rate control outlet temperature To_f_contis set to the highest value in all the first outlet temperatures To1[1], To1[2], . . . , and To1[n−1]and the second outlet temperature To2exp[n].

FIG.14is a flowchart illustrating the flow of air flow rate control in the present embodiment.

At Step S710, the controller60sets either the first outlet temperature To1[k]or the second outlet temperature To2exp[n], whichever is higher as a flow rate control outlet temperature To_f_contfor controlling the flow rate of air in the air supply passage14. That is, the flow rate control outlet temperature To_f_cont=Max{To1[k], To2exp[n]} is defined.

At Step S720, the controller60controls the output of the air pump86so as to bring the flow rate control outlet temperature To_f_contcloser to a predetermined target temperature. Specifically, as the flow rate control outlet temperature To_f_contmoves away from the target temperature in a direction of getting lower, the controller60makes the output of the air pump86lower. Furthermore, as the flow rate control outlet temperature To_f_contmoves away from the target temperature in a direction of getting higher, the controller60makes the output of the air pump86higher.

The fuel cell system10according to the present embodiment described above achieves the following functions and effects.

The fuel cell system10according to the present embodiment further includes the air pump86as a refrigerant adjusting device that adjusts the flow rate of air in the air supply passage14. Then, the controller60adjusts the flow rate of air in the air supply passage14by controlling the air pump86on the basis of the first outlet temperature To1[k]and the second outlet temperature To2exp[n](Steps S710and S720inFIG.14).

Accordingly, the flow rate of air in the air supply passage14is controlled on the basis of the detected first outlet temperature To1[k]of the first SOFC stack12-kand the second outlet temperature To2exp[n]of the second SOFC stack12-nestimated through the processes described in the above-described second embodiment, etc. That is, the flow rate of air in the air supply passage14is controlled on the basis of air outlet temperature information that takes into consideration the variation in the flow rate of supplied air among the first SOFC stacks12-kand the second SOFC stack12-n, and therefore the flow rate of air to be supplied to each one (a target air flow rate) can be set more appropriately. As a result, it is possible to perform the temperature control of the first SOFC stacks12-kand the second SOFC stack12-nmore suitably.

In the present embodiment, particularly, the controller60controls the air pump86on the basis of the flow rate control outlet temperature To_f_contthat is either the first outlet temperature To1[k]or the second outlet temperature To2exp[n], whichever is higher (Step S720inFIG.14).

Therefore, the output of the air pump86is controlled on the basis of the highest air outlet temperature among the first SOFC stacks12-k(k=1 to n−1) and the second SOFC stack12-n.

As a result, it is likely to be controlled to a direction of increasing the flow rate of air supplied to the first SOFC stacks12-kand the second SOFC stack12-n, and therefore it is possible to further improve the safety in terms of the heat resistance of the first SOFC stacks12-kand the second SOFC stack12-n.

Fifth Embodiment

A fifth embodiment is described below. It is to be noted that a component similar to that of any of the above-described embodiments and the modification examples is assigned the same reference numeral, and its description is omitted.

FIG.15is a diagram illustrating a configuration of the fuel cell system10according to the fifth embodiment.

As shown in the drawing, the fuel cell system10according to the present embodiment is based on the configuration of the fuel cell system10according to the fourth embodiment illustrated inFIG.13. Then, the fuel cell system10according to the present embodiment further includes a power adjusting device90as a power adjusting device that adjusts generating power (an extraction current) of each of the SOFC stacks12-1to12-nin addition to the configuration of the fourth embodiment. It is to be noted that this power adjusting device90includes a DC/DC converter, etc.

Furthermore, the power adjusting device90is configured to be able to individually adjust the generating power of the first SOFC stacks12-k(1≤k≤n−1) and the second SOFC stack12-n.

Therefore, in the present embodiment, the controller60can basically control the generating power of the first SOFC stack12-k(1≤k≤n−1) on the basis of the first outlet temperature To1[k], and control the generating power of the second SOFC stack12-non the basis of the second outlet temperature To2exp[n].

However, in the present embodiment, to more certainly prevent the temperatures of the first SOFC stacks12-kand the second SOFC stack12-nfrom increasing above a predetermined temperature set in terms of heat resistance due to heat generated from the first SOFC stacks12-kand the second SOFC stack12-n, the controller60performs generating power control based on a power control outlet temperature To_g_contthat takes a safety margin into consideration. Its details are described below.

FIG.16is a flowchart illustrating the generating power control in the present embodiment.

At Step S810, the controller60sets either the first outlet temperature To1[k]or the second outlet temperature To2exp[n], whichever is higher as a power control outlet temperature To_g_contfor controlling the generating power. That is, the power control outlet temperature To_g_cont=Max{To1[k], To2exp[n]} is defined.

It is to be noted that in the present embodiment, the power control outlet temperature To_g_contis calculated by applying all values from 0 to n−1 to k. Therefore, the power control outlet temperature To_g_contis set to the highest value in all the first outlet temperatures To1[1], To1[2], . . . , and To1[n−1]and the second outlet temperature To2exp[n].

At Step S820, the controller60controls the power adjusting device90so as to bring the power control outlet temperature To_g_contcloser to a predetermined target temperature. Specifically, as the power control outlet temperature To_g_contmoves away from the target temperature in a direction of getting lower, the controller60controls the power adjusting device90to make the electric power (the calorific value) extracted from each SOFC stack12lower. Furthermore, as the further the power control outlet temperature To_g_contmoves away from the target temperature in a direction of getting higher, the controller60controls the power adjusting device90to make the electric power (the calorific value) extracted from each SOFC stack12higher.

At Step S830, the controller60determines whether or not the power control outlet temperature To_g_contis higher than a predetermined threshold temperature Tthin a state where the power adjusting device90is controlled at Step S820. The threshold temperature Tthhere is a value determined in an experiment or the like in terms of preventing the temperatures of the first SOFC stacks12-kand the second SOFC stack12-nfrom getting too high in consideration of the heat resistance, etc. according to the specifications of the first SOFC stacks12-kand the second SOFC stack12-n.

When having determined that the power control outlet temperature To_g_contis not higher than the threshold temperature Tth, the controller60continues the generating power control at Step S820. On the other hand, when having determined that the power control outlet temperature To_g_contis higher than the threshold temperature Tth, the controller60performs a process at Step S840.

At Step S840, the controller60controls the power adjusting device90to cause the power generation of the first SOFC stacks12-kand the second SOFC stack12-nto be stopped. That is, if the power control outlet temperature To_g_contbecomes higher than a certain level, the controller60causes the power adjusting device90to stop the power generation of the first SOFC stacks12-kand the second SOFC stack12-nin terms of heat-resistance protection, etc.

The fuel cell system10according to the present embodiment described above achieves the following functions and effects.

The fuel cell system10according to the present embodiment further includes the power adjusting device90that adjusts the generating power of the SOFC stacks12. Then, the controller60controls the power adjusting device90on the basis of the first outlet temperature To1[k]and the second outlet temperature To2exp[n](Steps S810to S840inFIG.16).

Accordingly, the generating power of the SOFC stacks12is controlled on the basis of the detected first outlet temperature To1[k]of the first SOFC stack12-kand the second outlet temperature To2exp[n]of the second SOFC stack12-nestimated through the processes described in the above-described second embodiment, etc. That is, the generating power of the SOFC stacks12is controlled on the basis of the air outlet temperature information that takes into consideration the variation in the flow rate of supplied air among the first SOFC stacks12-kand the second SOFC stack12-n, and therefore it is possible to perform control of the respective calorific values of the first SOFC stacks12-kand the second SOFC stack12-nbased on the adjustment of the generating power with higher accuracy.

In the present embodiment, particularly, the controller60controls the power adjusting device90on the basis of the power control outlet temperature To_g_contthat is either the first outlet temperature To1[k]or the second outlet temperature To2exp[n], whichever is higher (Step S810inFIG.16).

That is, the power control outlet temperature To_g_contis set to be either the first outlet temperature To1[k]or the second outlet temperature To2exp[n], whichever is higher. Therefore, respective amounts of power generation by the first SOFC stacks12-kand the second SOFC stack12-nare controlled on the basis of an air output temperature of, of the first SOFC stacks12-k(k=1 to n−1) and the second SOFC stack12-n, the one estimated to have the highest calorific value.

As a result, the generating power of the first SOFC stacks12-kand the second SOFC stack12-nis likely to be controlled to a direction of further decreasing the calorific value, and therefore it is possible to further improve the safety in terms of the heat resistance of the first SOFC stacks12-kand the second SOFC stack12-n.

Furthermore, in the present embodiment, if the power control outlet temperature To_g_contexceeds the predetermined threshold temperature Tth, the controller60causes the power adjusting device90to stop the power generation of the first SOFC stacks12-kand the second SOFC stack12-n(Steps S830and S840inFIG.16).

That is, if the power control outlet temperature To_g_contset to the safety side in terms of heat-resistance protection exceeds the threshold temperature Tth, the power generation is stopped to stop the heat generation of the first SOFC stacks12-kand the second SOFC stack12-n.

Accordingly, it is possible to certainly prevent the first SOFC stacks12-kand the second SOFC stack12-nfrom having a temperature exceeding an upper temperature limit or the like determined in terms of heat-resistance protection, and therefore the safety is further improved in terms of the heat resistance of the first SOFC stacks12-kand the second SOFC stack12-n.

The embodiments of the present invention are described above; however, the above-described embodiments are merely some of application examples of the present invention, and are not meant to limit the technical scope of the present invention to the specific configurations of the above-described embodiments.

For example, in the fuel cell system10according to any of the above-described embodiments and the modification examples, a pre-distribution air flow rate qairthat is the sum of respective flow rates of air supplied to the SOFC stacks12-1and12-nis detected by the pre-distribution air flow rate sensor50. However, instead of providing the pre-distribution air flow rate sensor50, the pre-distribution air flow rate qairmay be estimated from, for example, a set output of the air pump86used to supply air into the air supply passage14.

Furthermore, in the fuel cell system10according to any of the above-described embodiments and the modification examples, the SOFC stacks12-1and12-nare arranged in parallel with the electric load, and this one current sensor58is provided and shared by them. However, in a case where the SOFC stacks have different currents, such as in the case where the SOFC stacks are not arranged in parallel with the electric load, all the SOFC stacks may be provided with a current sensor, or at least SOFC stacks having different currents from each other may be provided with a current sensor.

Moreover, in the fuel cell system10according to any of the above-described embodiments and the modification examples, for example, in calculation of a first calorific value Qgen1[k]or a second calorific value Qgen2[n], the same theoretical electromotive force E0is set in all the SOFC stacks12-1to12-n. However, for example, in a case where an output enable voltage per stack is different among the SOFC stacks12-1to12-n, such as in the case where the number of stacks of unit cells is different among the SOFC stacks12-1to12-n, a calorific value may be calculated with a different value of theoretical electromotive force fittingly set for each of the SOFC stacks12-1to12-n.

Furthermore, in the fuel cell system10according to any of the above-described embodiments and the modification examples, there is described an example where the fuel cell group12is composed of the SOFC stacks12-1to12-n. However, the calculation method, the supply air flow rate control, and the generating power control according to any of the above-described embodiments and the modification examples can also be applied to a system in which at least part of the fuel cell group12is composed of unit fuel cells.

Moreover, the “air specific heat cair” used in calculation in the above-described embodiments and the modification examples and the “fuel specific heat cfuel” used in calculation in the third embodiment both take a fixed value; however, for example, a corrected value may be used fittingly in consideration of variation in their value caused by factors, such as temperature.