JP2020009723A - Fuel battery system and control method of the fuel battery system - Google Patents

Abstract

To enable supplying of a heat amount to be required for an evaporation independent of a load of a fuel battery to an evaporator.SOLUTION: A fuel battery system S comprises: a fuel battery stack 1; a combustor 4 that makes a residual combustion in an off gas of the fuel battery stack 1 to be combustion; an evaporator 4 to which a raw fuel of the fuel battery is connected so as to be heated by a heat quantity held by a fuel gas of the residual combustion to the combustor 4; and a controller 101 that controls a supply of the raw fuel and an oxidant gas to the fuel battery system S. As a requirement flow amount of a flow amount of the raw fuel in accordance with a request output P of the fuel battery stack 1, in a fuel excess rate η as a ratio of an actual flow amount to the requirement flow amount of the raw fuel, the control is made to the fuel excess rate η when an exhaust heat flow is A, a thermal conversion efficiency in the evaporator is B, a fuel evaporation heat flow is C so as to satisfy the following equation of A×B/C>1.SELECTED DRAWING: Figure 5

Description

  The present invention relates to a fuel cell system including an evaporator for evaporating raw fuel and a control method thereof.

  Patent Literature 1 discloses a fuel cell including a hydrocarbon-based fuel such as methanol or gasoline as a raw fuel, which includes an evaporator or a vaporizer for evaporating the raw fuel before supplying the fuel to the fuel cell. A system is disclosed. Combustion of residual fuel in off-gas discharged from a fuel cell in a combustor and supply of the resulting combustion gas to an evaporator to heat and evaporate the raw fuel by heat exchange with the combustion gas It is.

JP 2003-197236 A (paragraph 0016)

  Here, when the flow rates of the raw fuel and the oxidizing gas are reduced at a fixed rate in response to a change in the load of the fuel cell, particularly a decrease in the load, the amount of heat of the combustion gas supplied to the evaporator becomes insufficient. However, there is a problem that the raw fuel cannot be sufficiently evaporated. When the evaporation of the raw fuel becomes insufficient, the raw fuel is supplied in a liquid state to a reformer provided downstream of the evaporator, or an unreformed gas is supplied to the fuel cell, and the reforming is performed. This causes deterioration of a catalyst provided in a reactor or a fuel cell, particularly a reformer.

  The present invention provides a fuel cell system capable of supplying an amount of heat required for evaporation of raw fuel to an evaporator irrespective of the load of the fuel cell, and appropriately operating the evaporator, and a control method thereof. The purpose is to:

  According to one embodiment of the present invention, a fuel cell, a combustor connected to an anode exhaust passage and a cathode exhaust passage of the fuel cell, for burning remaining fuel in off-gas of the fuel cell, and an evaporator for evaporating raw fuel for the fuel cell An evaporator connected to the combustor so as to be able to heat the raw fuel by the amount of heat of the remaining fuel combustion gas, a fuel supply device for supplying the raw fuel to the evaporator, and an oxidizing gas supplied to the fuel cell. A fuel cell system is provided that includes an oxidizing gas supply device that supplies the fuel gas and a supply control unit that controls the fuel supply device and the oxidizing gas supply device. In the present embodiment, the supply control unit sets the flow rate of the raw fuel according to the required output of the fuel cell as a required flow rate, and sets The rate is assumed to be A, the exhaust heat flow of the combustion gas at the inlet side of the evaporator, the heat exchange efficiency between the raw fuel and the combustion gas in the evaporator as B, and the fuel evaporation heat flow required for the evaporation of the raw fuel in the evaporator as C. To control the excess fuel ratio so that A × B / C> 1.

  In another aspect, a method for controlling a fuel cell system is provided.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to supply the calorie | heat amount required for evaporation of a raw fuel to an evaporator regardless of the load of a fuel cell, minimizing consumption of a raw fuel, and operate an evaporator appropriately. be able to. Since the amount of heat applied to the raw fuel is ensured, it is particularly effective when the raw fuel is relatively hard to evaporate, such as an aqueous ethanol solution.

FIG. 1 is a schematic diagram showing an overall configuration of a fuel cell system according to one embodiment of the present invention. FIG. 2 is an explanatory diagram showing an operation at the time of startup of the fuel cell system according to the embodiment. FIG. 3 is an explanatory diagram showing an operation of the fuel cell system according to the embodiment in a normal state. FIG. 4 is an explanatory diagram showing another example of the operation of the fuel cell system according to the embodiment in a normal state. FIG. 5 is an explanatory diagram showing a change in the heat flow ratio (= exhaust heat flow / fuel evaporation heat flow) with respect to the output of the fuel cell. FIG. 6 is an explanatory diagram showing the relationship of the heat flow according to the parts of the fuel cell system. FIG. 7 is a flowchart showing the flow of power generation control during low-load operation of the fuel cell system. FIG. 8 is a flowchart showing specific contents of a target excess air ratio setting process in the power generation control. FIG. 9 is an explanatory diagram showing a setting tendency of the heat radiation amount (system heat radiation amount) of the fuel cell system. FIG. 10 is an explanatory diagram illustrating a setting tendency of the excess fuel ratio correction value regarding the outside air temperature. FIG. 11 is an explanatory diagram showing a setting tendency of the excess fuel ratio correction value with respect to the atmospheric pressure. FIG. 12 is an explanatory diagram showing a setting tendency of the oxidizing gas flow rate based on the excess fuel ratio.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Overall configuration of fuel cell system)
FIG. 1 schematically shows a configuration of a fuel cell system S according to an embodiment of the present invention.

  The fuel cell system S according to the present embodiment (hereinafter sometimes referred to as “fuel cell system” or simply “system”) S includes a fuel cell stack 1, fuel processing units 21 to 23, and an oxidizing gas heating unit 3. , A combustor 4 and a controller 101.

  A fuel cell stack (hereinafter sometimes simply referred to as a “stack”) 1 is configured by stacking a plurality of fuel cells or fuel cell unit cells, and each fuel cell as a power generation source is, for example, a solid oxide type. It is a fuel cell (SOFC).

  The fuel processing units 21 to 23 process raw fuel as a primary fuel and convert it into fuel gas used for a power generation reaction in a fuel cell. The fuel processing units 21 to 23 are interposed in the anode supply passage 11 and receive supply of raw fuel. In the present embodiment, the raw fuel is a mixture of the oxygen-containing fuel and water, and is stored in the fuel tank 7 connected to the anode supply passage 11. As a raw fuel applicable to the present embodiment, a mixture of ethanol and water (that is, an aqueous ethanol solution) can be exemplified. In that case, the fuel gas is hydrogen obtained by reforming ethanol.

  The oxidizing gas heating unit 3 heats the oxidizing gas before supplying it to the fuel cell stack 1. The oxidizing gas heating section 3 is interposed in the cathode supply passage 12 and receives supply of the oxidizing gas. The oxidizing gas is, for example, air. By supplying air in the atmosphere to the cathode of the fuel cell, it is possible to supply oxygen used for the power generation reaction to the cathode.

Here, the reaction related to power generation at the anode and cathode of the solid oxide fuel cell can be expressed by the following equation.
Anode: 2H ₂ + 4O ^2- → 2H ₂ O + 4e ⁻ (1.1)
Cathode: O ₂ + 4e ⁻ → 2O ^2- … (1.2)

  The combustor 4 burns residual fuel in off-gas discharged from the fuel cell stack 1 to generate combustion gas. In the present embodiment, the combustor 4 is connected to the anode discharge passage 11exh and the cathode discharge passage 12exh, and receives the supply of the anode off-gas and the cathode off-gas through these passages 11exh and 12exh. The amount of heat of the combustion gas is supplied to the fuel processing units 21 to 23 and the oxidizing gas heating unit 3 and used for heating the raw fuel and the oxidizing gas.

  The configuration of the fuel cell system S will be further described. The fuel cell stack 1 includes, in an anode system, an anode supply passage 11 for supplying a fuel gas to an anode electrode of the fuel cell, and a power generation reaction discharged from the anode electrode. A cathode supply passage 12 for supplying an oxidant gas to the cathode of the fuel cell in the cathode system, and an anode discharge passage 11exh for flowing the anode off-gas after the power generation reaction discharged from the cathode. A cathode discharge passage 12exh for flowing a cathode off-gas.

  The fuel tank 7 and the fuel cell stack 1 are connected via an anode supply passage 11, and the evaporator 21, the fuel heat exchanger 22, and the reformer 21 are sequentially connected to the anode supply passage 11 from the upstream side in the flow direction. A porcelain 23 is interposed. Further, a branch fuel passage 11sub branches from the anode supply passage 11 on the upstream side of the evaporator 21, and the branch fuel passage 11sub is connected to the combustor 4. A first fuel injector 51 is provided in the anode supply passage 11 between the branch point of the branch fuel passage 11sub and the evaporator 21, and a second fuel injector 52 is provided in the branch fuel passage 11sub.

  The evaporator 21 receives a supply of an aqueous solution of ethanol as a raw fuel from the fuel tank 7, and heats and evaporates the aqueous solution to generate ethanol gas and water vapor.

  The fuel heat exchanger 22 receives the calorific value of the combustion gas from the combustor 4 and further heats the ethanol gas and the steam.

The reformer 23 incorporates a reforming catalyst and generates hydrogen from ethanol in a gaseous state by steam reforming. Steam reforming can be represented by the following equation.
C ₂ H ₅ OH + 3H ₂ O → 6H ₂ + 2CO ₂ (2)

  The oxidizing gas heating unit 3 is configured by an air heat exchanger, and heats the oxidizing gas flowing through the cathode supply passage 12 by heat exchange with the combustion gas supplied from the combustor 4 through the combustion gas passage 13. In the present embodiment, a blower or an air compressor 6 is provided near the open end of the cathode supply passage 12, and air in the atmosphere is sucked into the cathode supply passage 12 through the blower 6 as an oxidizing gas. The drawn air is heated from normal temperature (for example, 25 ° C.) when passing through the oxidizing gas heating unit 3 and is supplied to the fuel cell stack 1.

  The combustor 4 has a built-in combustion catalyst, receives supply of anode off-gas from the fuel cell stack 1, and generates combustion gas by catalytic combustion of residual fuel in the anode off-gas. The combustor 4 can receive, in addition to the anode off-gas, the supply of the aqueous solution of ethanol as the raw fuel through the branch fuel passage 11sub. In this case, when generating the combustion gas, the combustor 4 also combusts the ethanol in addition to the residual fuel. Let it. In the present embodiment, the combustor 4 and the evaporator 21 are connected via the combustion gas passage 13, while the fuel heat exchanger 22 and the reformer 23 are housed in the case L shared with the combustor 4 (case L is conceptually indicated by a two-dot chain line), and the heat quantity of the combustion gas is efficiently transmitted to the fuel heat exchanger 22 and the reformer 23 inside the case L.

  The controller 101 controls the supply of the raw fuel to the evaporator 21 and the combustor 4 and controls the supply of the oxidizing gas to the oxidizing gas heating unit 3 during the operation of the fuel cell system S. The controller 101 can be configured as an electronic control unit. The first fuel injector 51, the second fuel injector 52, and the blower 6 operate in response to a signal from the controller 101 to supply raw fuel to the evaporator 21 and the combustor 4 and to oxidize the oxidizing gas heating unit 3 Supply agent gas.

  The power generated by the fuel cell stack 1 can be used to charge a battery (not shown) or drive an external device such as an electric motor or a motor generator. For example, the fuel cell system S can be applied to a driving system for a vehicle, and charges a battery with power generated by rated operation of the fuel cell stack 1 or supplies power according to a target driving force of the vehicle. It can be supplied from the battery and the fuel cell stack 1 to a motor generator for traveling.

(Configuration and basic operation of control system)
In the present embodiment, the controller 101 is configured as an electronic control unit including a central processing unit (CPU), various storage units such as a ROM and a RAM, and a microcomputer having an input / output interface and the like. The operation of various devices and parts required for operation of the fuel cell system S, such as the fuel injector 52 and the blower 6, is controlled.

  The controller 101 includes, as information relating to the control of the fuel cell system S, a signal from the stack temperature sensor 201 for detecting the stack temperature Tstk, a signal from the fuel flow sensor 202 for detecting the fuel flow Ff, and air for detecting the air flow Fa. A signal from the flow rate sensor 203, a signal from the combustion gas flow rate sensor 204 for detecting the combustion gas flow rate Fc, a signal from the fuel temperature sensor 205 for detecting the fuel temperature Tf, and a signal from the combustion gas temperature sensor 206 for detecting the combustion gas temperature Tc. , A signal from the outside air temperature sensor 207 for detecting the outside air temperature Patm, a signal from the atmospheric pressure sensor 208 for detecting the atmospheric pressure Patm, and a signal from the system start switch 209.

  The stack temperature Tstk is an index indicating the temperature of the fuel cell stack 1 or the fuel cell. In the present embodiment, the stack temperature sensor 101 is installed near the cathode outlet of the fuel cell stack 1 and detected by the stack temperature sensor 201. The temperature is referred to as a stack temperature Tstk.

  The fuel flow rate Ff is a flow rate of the raw fuel supplied to the evaporator 21. In the present embodiment, a fuel flow sensor 202 is installed in the anode supply passage 11 on the upstream side of the evaporator 21, and the flow detected by the fuel flow sensor 202 is used as the fuel flow Ff.

  The air flow rate Fa is a flow rate of the oxidizing gas supplied to the fuel cell stack 1. In the present embodiment, an air flow sensor 203 is installed in the cathode supply passage 12 on the upstream side of the oxidizing gas heating unit 3, and the flow detected by the air flow sensor 203 is defined as the air flow Fa.

  The combustion gas flow rate Fc is a flow rate of the combustion gas supplied to the evaporator 21. In the present embodiment, a combustion gas flow sensor 204 is installed on the upstream side of the evaporator 21, specifically, in the combustion gas passage 13 between the oxidizing gas heating unit 3 and the evaporator 21. Is set as the combustion gas flow rate Fc.

  The fuel temperature Tf is the temperature of the raw fuel supplied to the evaporator 21. In the present embodiment, a fuel temperature sensor 205 is installed in the anode supply passage 11 on the upstream side of the evaporator 21, and the temperature detected by the fuel temperature sensor 205 is used as the fuel temperature Tf.

  The combustion gas temperature Tc is the temperature of the combustion gas supplied to the evaporator 21. In the present embodiment, a combustion gas temperature sensor 206 is installed on the upstream side of the evaporator 21, specifically, in the combustion gas passage 13 between the oxidizing gas heating unit 3 and the evaporator 21. Is the combustion gas temperature Tc.

  When a request to start the fuel cell system S is input from the system start switch 209, the controller 101 executes start control for warming up the fuel cell stack 1. The warming-up of the fuel cell stack 1 refers to raising the temperature of the fuel cell stack 1 that was at a low temperature (for example, normal temperature) during the stop to the operating temperature.

  Then, when the temperature of the fuel cell stack 1 reaches the operating temperature, the controller 101 ends the startup control and shifts to the normal power generation control. In normal times, basically, the fuel cell stack 1 is operated at its rated point, and the amount of raw fuel required for the rated operation is supplied to the evaporator 21 via the first fuel injector 52. Here, the rated operation of the fuel cell stack 1 refers to the operation of the fuel cell stack 1 at the maximum power generation output.

  2 and 3 show the operation of the fuel cell system S. FIG. 2 shows the operation at the time of startup, and FIG. 3 shows the operation at the time of normal operation. In the figure, a thick line with an arrow indicates a passage having a gas or liquid flowing, and a dotted line indicates a passage having no flow.

  At the time of startup (FIG. 2), the first fuel injector 51 is stopped, while the second fuel injector 52 is operated, and the raw fuel stored in the fuel tank 7 is supplied to the combustor 4 via the second fuel injector 52. To supply. On the other hand, air in the atmosphere is taken into the cathode supply passage 12 by the blower 6 and supplied to the fuel cell stack 1 via the oxidizing gas heating unit 3. The air that has passed through the fuel cell stack 1 is supplied to the combustor 4 as an oxidant of raw fuel through a cathode discharge passage 12exh. The combustion gas generated by the combustion is supplied to the oxidizing gas heating unit 3 and the evaporator 21 through the combustion gas passage 13, while the amount of heat of the combustion gas is reduced inside the case L by the fuel heat exchanger 22 and the reformer 23. Is transmitted to Therefore, at the time of startup, the fuel cell stack 1 is heated by the radiant heat generated by the combustion of the raw fuel, and further heated using the air heated by the oxidizing gas heating unit 3 as a heat medium, and the fuel cell stack 1 rises. Warmth is promoted. At the time of startup, the supply flow rate of the raw fuel (that is, the injection flow rate of the second fuel injector 52) is set based on the stack temperature Tstk.

  On the other hand, in the normal state (FIG. 3), the second fuel injector 52 is stopped, while the first fuel injector 51 is operated, and the raw fuel is supplied to the evaporator 21 via the first fuel injector 51. At the same time, the blower 6 is operated to supply air, which is an oxidizing gas, to the fuel cell stack 1 via the oxidizing gas heating unit 3. The off-gases (anode off-gas, cathode off-gas) discharged from the fuel cell stack 1 after the power generation reaction are supplied to the combustor 4 through the anode exhaust passage 11exh and the cathode exhaust passage 12exh. The remaining fuel in the anode off-gas is burned in the combustor 4 and the resulting combustion gas is supplied to the oxidizing gas heating unit 3 and the evaporator 21 through the combustion gas passage 13, while the calorific value of the combustion gas is less than that of the case L. The heat is internally transmitted to the fuel heat exchanger 22 and the reformer 23. Thereby, the fuel processing units 21 to 23 and the oxidizing gas heating unit 3 are heated, the evaporator 21 is maintained at a temperature at which the raw fuel (ethanol aqueous solution) can be evaporated, and the reformer 23 is heated at the raw fuel (ethanol). ) Is maintained at a temperature at which it can be reformed. In normal times, the supply flow rate of the raw fuel (that is, the injection flow rate of the first fuel injector 51) is set based on the output required for the fuel cell stack 1, in other words, the load of the fuel cell system 1. If the remaining fuel alone in the anode off-gas is not enough to generate the heat required to maintain the temperatures of the evaporator 21 and the reformer 23, the second fuel injector 52 is operated, and the second combustor 4 Raw fuel can also be supplied via the fuel injector 52 (FIG. 4).

  Here, when the operation is performed with the load of the fuel cell stack 1 reduced during normal operation after warm-up, for example, during a steady operation in a low load region or when the output fluctuates to a low load side, the load on the fuel cell stack 1 decreases. On the other hand, assuming that the flow rates of the raw fuel and the oxidizing gas are reduced at a fixed rate, of the entire energy input to the fuel cell system S, the heat flow available as exhaust heat is reduced and supplied to the evaporator 21. The amount of heat of the combustion gas is insufficient for the heat flow required for evaporating the raw fuel. As a result, the raw fuel cannot be sufficiently evaporated, and the raw fuel is supplied to the reformer 23 in a liquid state, or the unreformed gas is supplied to the fuel cell stack 1. It is feared that the catalyst provided in the fuel container 23 or the fuel cell stack 1 may be deteriorated. Here, a case where the state of charge of the battery is sufficient and the fuel cell stack 1 does not need to generate power under a high load or a medium load as the steady operation in the low load region can be exemplified. In the present embodiment, “heat flow” refers to the amount of heat [J / s] that passes per unit time.

  FIG. 5 shows a change in the heat flow ratio Rhf with respect to the output P of the fuel cell stack 1. Here, the heat flow ratio Rhf refers to a ratio of the exhaust heat flow to the fuel evaporation heat flow (= exhaust heat flow / fuel evaporation heat flow). The exhaust heat flow refers to a heat flow of the combustion gas at the inlet side or the upstream side of the evaporator 21, and the fuel evaporation heat flow refers to a heat flow required for evaporating the raw fuel in the evaporator 21. The exhaust heat flow correlates with the combustion gas flow rate Fc and the combustion gas temperature Tc, and can be calculated based on these state quantities. The fuel evaporation heat flow correlates with the fuel flow rate Ff and the fuel temperature Tf. Can be calculated based on the state quantity of

  As shown in FIG. 5, assuming that the excess fuel ratio η is constant and the flow rates of the raw fuel and the oxidizing gas are reduced at a constant rate with respect to the decrease in the load, the heat flow ratio Rhf gradually increases toward 1. Decrease. As the heat flow ratio Rhf approaches 1, the exhaust heat flow tends to be insufficient for the fuel evaporation heat flow. Insufficient exhaust heat flow means insufficient evaporation of the raw fuel. When the evaporation of the raw fuel becomes insufficient, a part of the raw fuel is supplied to the reformer 23 in a liquid state, or the unreformed gas is supplied to the fuel cell stack 1, and the reforming is performed. This causes the catalyst provided in the reactor 23 or the fuel cell stack 1 to deteriorate.

  In the present embodiment, the excess fuel ratio η is increased in response to a decrease in the load on the fuel cell stack 1, so that the evaporator 21 can supply the heat flow required for evaporating the raw fuel irrespective of the load on the fuel cell stack 1. Enable supply. Specifically, the flow rate of the raw fuel according to the required output P of the fuel cell stack 1 is set as a required flow rate, and the ratio of the actual flow rate to the required flow rate of the raw fuel for a decrease in the load on the fuel cell stack 1 Assuming that the excess ratio η is A, the heat flow of exhaust gas is A, the heat exchange efficiency between the raw fuel and the combustion gas in the evaporator 21 is B, and the heat flow of fuel evaporation is C, the heat flow ratio = A × B / C becomes larger than 1. Control.

  FIG. 6 conceptually shows the relationship between the heat flows HF1 to HF6 in each part of the fuel cell stem S. Although not limited to this, in the present embodiment, 1.2 is adopted as the excess fuel ratio η at which the heat flow ratio (= A × B / C) becomes larger than 1. The “excess fuel ratio” refers to the ratio of the actual flow rate to the required flow rate of the raw fuel when the flow rate of the raw fuel according to the required output P of the fuel cell stack 1 is set as the required flow rate. As described later, in the present embodiment in which the required output P of the fuel cell stack 1 is 2.0 kW, a value obtained by converting the required output P (= 2.0 kW) into a flow rate corresponds to the required flow rate of the raw fuel. The ratio of the actual flow rate (= 2.4 kW) to the required fuel flow rate (= 2.0 kW) corresponds to the “fuel excess rate”.

Symbols HF1 to HF6 shown in FIG. 6 indicate the following heat flows. As described above, the “heat flow” is the amount of heat that passes through a target portion per unit time.
HF1: Heat flow (fuel heat flow) of raw fuel supplied to evaporator 21
HF2: Heat flow required for evaporation of raw fuel in evaporator 21 (fuel evaporation heat flow)
HF3: Heat flow (absorption heat flow) absorbed by reforming reaction (steam reforming) in reformer 23
HF4: Heat flow of combustion gas generated in combustor 4 (combustion gas heat flow)
HF5: heat flow (exhaust heat flow) of the combustion gas at the inlet side of the evaporator 21
HF6: released heat flow according to heat exchange efficiency ηvap in evaporator 21

  Under the condition that the excess fuel ratio η is 1.2, the required output P of the fuel cell stack 1 is 2.0 kW, and the amount of heat radiation (hereinafter referred to as “system heat radiation”) Q from the fuel cell system S is 0. The respective heat flows HF1 to HF6 when the power is set to 3 kW are as follows. Here, the fuel evaporation heat flow HF2 and the absorption heat flow HF3 can be estimated in advance based on the type, flow rate, temperature, and the like of the raw fuel. In the present embodiment, respectively, 20% and 10% of the fuel heat flow HF1 are used. And

Fuel heat flow HF1 = P × η = 2.4 kW
Fuel evaporation heat flow HF2 = HF1 × 0.2 = 0.48 kW
Absorption heat flow HF3 = HF1 × 0.1 = 0.24 kW
Combustion gas heat flow HF4 = HF1 + HF2 + HF3-P-Q = 0.82kW
Exhaust heat flow HF5 = HF4-HF3 = 0.58kW
Released heat flow HF6 = HF5 (1-ηvap)

Here, the heat flow ratio (= A × B / C) satisfies the following relationship.
{1.2−Q / (P × η) −1 / η} × ηvap / 0.2> 1 (3)

  The excess fuel ratio η may be constant regardless of the load within a range where the heat flow ratio (= A × B / C) is larger than 1 irrespective of the load of the fuel cell stack 1, or the heat flow ratio (= A × B / C) may be increased in response to a decrease in load so that the ratio becomes as small as possible while exceeding 1.

The fuel evaporation heat flow HF2 (= Qv) can be obtained as follows.
Qv = Ffuel × LHV × α

  Ffuel indicates the flow rate of the raw fuel supplied to the evaporator 21, and LVH indicates the lower heating value of the raw fuel. Further, α is a predetermined value that varies depending on the type or composition of the raw fuel.

The calculation of the fuel evaporation heat flow Qv is as follows when the temperature Tfuel of the raw fuel is considered.
Qv = Ffuel × LHV × α × (1 + β × (Tfuel0−Tfuel) / (Tfuel0−25))

  Tfuel0 indicates the vaporization temperature of the raw fuel, and β is a predetermined value that varies depending on the type or composition of the raw fuel.

The calculation for obtaining the fuel evaporation heat flow Qv from the viewpoint of enthalpy is based on the following equation.
Qv = Ffuel × (H_h ₂ o (Tfuel 0) −H_h ₂ ol (Tfuel))

H_h ₂ o indicates the enthalpy of water (gas), and H_h ₂ ol indicates the enthalpy of water (liquid).

  Thus, the fuel evaporation heat flow Qv is supplied to the evaporator 21, in other words, according to the flow rate Ffuel and the temperature Tfuel of the raw fuel at the inlet side of the evaporator 21.

  Here, the flow rate Ffuel of the raw fuel and the temperature Tfuel correspond to the “state quantity of the raw fuel”.

On the other hand, the exhaust heat flow HF5 (= Qe) can be obtained as follows.
Qe = Fexh × Cp_e × (Texh−T0)

  Fexh indicates the flow rate of the combustion gas flowing on the inlet side of the evaporator 21, and Cp_e indicates the specific heat according to the ratio of the air and the raw fuel supplied to the system (air-fuel ratio λ). Further, Texh indicates the temperature of the combustion gas at the inlet side of the evaporator 21, and T0 indicates the outside air temperature.

The calculation for obtaining the exhaust heat flow Qe from the viewpoint of enthalpy is based on the following equation.
Qe = Fexh × (α × H_co ₂ (Texh) + β × H_h ₂ o (Texh) + γ × H_o ₂ (Texh) + ζ × H_n ₂ (Texh))

α, β, γ, and ζ are coefficients corresponding to the composition of the combustion gas at the time of complete combustion, and can be calculated from the flow rate of air and the temperature of the combustion gas. H indicates the enthalpy of each component of the combustion gas, H_co ₂ indicates the enthalpy of carbon dioxide, H_h ₂ o indicates the enthalpy of water (gas), H_o ₂ indicates the enthalpy of oxygen, and H_n ₂ indicates the enthalpy of oxygen. Shows the enthalpy of

  As described above, the exhaust heat flow Qe depends on the flow rate Fexh and the temperature Texh of the combustion gas on the inlet side of the evaporator 21.

  Here, the flow rate Fexh of the combustion gas and the temperature Texh correspond to the “state quantity of the combustion gas”.

(Explanation by flowchart)
7 and 8 are flowcharts showing the contents of power generation control (hereinafter, referred to as “normal control”) executed by the controller 101 in a normal state. FIG. 7 shows the overall flow of the normal control, and FIG. 8 shows the specific contents of the target excess fuel ratio setting process in the normal control.

  In the present embodiment, the controller 101 is programmed to execute the normal control at predetermined time intervals, and the target excess fuel ratio setting process is configured as a subroutine (S106) of the basic routine shown in FIG.

  In S101, the outputs of various sensors are read. Specifically, the stack temperature Tstk, the fuel flow rate Ff, the air flow rate Fa, the combustion gas flow rate Fc, the fuel temperature Tf, the combustion gas temperature Tc, the outside air temperature Tatm, the atmospheric pressure Tatm, and the like are read. When the fuel cell system S is mounted on a vehicle and constitutes a drive source of the vehicle, the accelerator opening APO, the vehicle speed VSP, and the like are read in addition to the above. The accelerator opening APO is an index indicating the required driving force of the vehicle.

  In S102, it is determined whether the warm-up of the fuel cell system S has been completed. If the warm-up has been completed, the process proceeds to S103. If the warm-up has not been completed, and if the warm-up is still in progress, the normal control is terminated and the separately set warm-up control is executed. Whether or not the warm-up of the fuel cell system S has been completed can be determined based on the stack temperature Tstk.

  In S103, the required output P of the fuel cell stack 1 is calculated. When the fuel cell system S constitutes the driving source of the vehicle, the required output P can be calculated based on the accelerator opening APO and the vehicle speed VSP.

  In S104, it is determined whether the required output P of the fuel cell stack 1 is in a low load range. If the required output P is equal to or less than the predetermined load indicating the upper limit of the low load range and the load is in the low load range, the process proceeds to S105. If not, the process after S105 is not performed. Execute the processing set separately for the area.

  In S105, the system heat radiation amount Q is calculated. In the present embodiment, the calculation of the system heat radiation amount Q is performed with reference to a tendency map shown in FIG. 9 in which the system heat radiation amount Q is allocated according to the outside air temperature Patm and the atmospheric pressure Patm. The system heat radiation amount Q tends to increase with a decrease in both the outside air temperature Patm and the atmospheric pressure Patm.

  In S106, the target excess fuel ratio ηfuel is calculated.

In S107, a fuel flow rate Ff according to the target excess fuel ratio ηfuel is calculated. The fuel flow rate Ff is calculated by calculating a fuel heat flow rate HF1 (= P × η) and converting it to a raw fuel flow rate Ff according to the following equation. The controller 101 transmits a signal corresponding to the fuel flow rate Ff to the first fuel injector 51, and supplies the raw fuel having the flow rate Ff to the evaporator 21 via the first fuel injector 51.
Ff = (P × η) / LHV (4)
In the equation (4), LHV indicates a lower heating value of the raw fuel.

In S108, the air flow rate Fa is calculated. In the present embodiment, the calculation of the air flow rate Fa is performed based on the following equation representing the heat balance in the combustor 4. From the following equation, the lower limit flow rate of the oxidizing gas that can suppress the temperature of the combustor 4 to the upper limit temperature or lower is given as the air flow rate Fa. Therefore, this lower limit flow rate is set to the air flow rate Fa during the low load operation. The air flow rate Fa set in this manner has a tendency to increase, for example, with an increase in the outlet temperature Tout. Thus, it is possible to avoid an excessive rise in the temperature of the combustor 4 while minimizing the energy consumed for supplying the oxidizing gas (air). The outlet temperature Tout can be substituted by the stack temperature Tstk.
Hf / (Cpf × Ff + Cpa × Fa) <(Tmax−Tout) (5)

The codes used in equation (4) indicate the following, respectively.
Tout: outlet temperature of fuel cell stack 1 Cpf: specific heat of raw fuel Cpa: specific heat of air Tmax: upper limit temperature of combustor 4

  Then, the controller 101 transmits a signal corresponding to the air flow rate Fa to the blower 6 and supplies the oxidizing gas (that is, air) at the flow rate Fa to the oxidizing gas heating unit 3 via the blower 6.

  Referring to FIG. 8, in S201, a basic fuel excess ratio ηbase is calculated. The basic fuel excess ratio ηbase is given by Expression (3). In the present embodiment, the basic fuel excess ratio ηbase is set in advance in correspondence with the required output P in a low load range, and is calculated as a smaller value as the required output P increases. You.

  In S202, an excess fuel ratio correction value (hereinafter, referred to as “outside air temperature correction value”) HOSa regarding the outside air temperature Tatm is calculated. The outside air temperature correction value HOSa is calculated as a smaller value as the outside air temperature Tatm is higher. This is because the heat release amount from the combustion gas decreases as the outside air temperature Tatm increases, and the heat flow available for exhaust heat, that is, the exhaust heat flow HF5 increases.

  In S203, an excess fuel ratio correction amount (hereinafter referred to as “atmospheric pressure correction value”) HOSb related to the atmospheric pressure Patm is calculated. The atmospheric pressure correction value HOSb is calculated as a larger value as the atmospheric pressure Patm is higher. This is because the composition of the combustion gas changes due to an increase in the atmospheric pressure Patm, and when the raw fuel is an aqueous ethanol solution, the methane in the combustion gas increases as the atmospheric pressure Patm increases. This is because the heat exchange efficiency in the evaporator 21 decreases. That is, the decrease in the heat exchange efficiency due to the change in the composition of the combustion gas is compensated for by the increase in the exhaust heat flow HF5.

In S204, the basic excess fuel ratio ηbase is corrected by the outside air temperature correction value HOSa and the atmospheric pressure correction value HOSb, and the target excess fuel ratio fuel is calculated.
ηfuel = ηbase × HOSa × HOSb (6)

  In the present embodiment, the fuel cell stack 1 constitutes a “fuel cell”, the combustor 4 constitutes a “combustor”, the evaporator 21 constitutes an “evaporator”, and the first fuel injector 51 constitutes “fuel”. A supply device ”is configured, a blower 6 configures an“ oxidizing gas supply device ”, and the controller 101 configures a“ supply control unit ”.

(Explanation of effects)
The fuel cell system S according to the present embodiment is configured as described above, and the operation and effect obtained by the present embodiment will be described below.

  First, assuming that the flow rate of the raw fuel according to the required output P of the fuel cell stack 1 is a required flow rate (for example, 2.0 kW), the actual flow rate with respect to the required flow rate of the raw fuel against the decrease of the load or the required output P Assuming that the excess fuel ratio η, which is a ratio of (for example, 2.4 kW), the exhaust heat flow is A, the heat exchange efficiency in the evaporator 21 is B, and the fuel evaporation heat flow is C, A × B / C> 1. The excess fuel ratio η was controlled. Accordingly, the evaporator 21 can be supplied with the amount of heat necessary for evaporating the raw fuel while suppressing the consumption of the raw fuel as much as possible regardless of the load of the fuel cell system S, and the evaporator 21 can be operated appropriately. Can be. Since the amount of heat applied to the raw fuel is secured in this manner, it is particularly effective when the aqueous ethanol solution is used as the raw fuel as in the present embodiment.

  Here, by increasing the excess fuel ratio η with respect to the decrease in the required output P of the fuel cell stack 1, the amount of heat provided to the evaporator 21 by the combustion gas becomes insufficient for the amount of heat required for evaporation of the raw fuel. In this case, by increasing the exhaust heat flow HF5, the shortage of heat can be compensated, and the operation of the evaporator 21 can be optimized.

  Second, based on the required output P of the fuel cell stack 1, the outside temperature Tatm, and the atmospheric pressure Patm, for example, the excess fuel ratio η can be calculated by the equation (3), thereby calculating the excess fuel ratio η in advance. In addition to the above, it is possible to employ an appropriate fuel excess ratio η according to the actual outside temperature Tatm or the like during the actual operation.

  Thirdly, by detecting the outlet temperature Tout of the fuel cell stack 1 and increasing the flow rate of the oxidizing gas with respect to the rise of the outlet temperature Tout, the combustor 4 is appropriately cooled and the combustor 4 is excessively cooled. Heating can be avoided. Then, from the viewpoint of the heat balance in the combustor 4, the lower limit flow rate of the oxidizing gas can be set by calculating the flow rate of the oxidizing gas by the equation (5), thereby reducing the energy consumed for supplying the oxidizing gas. can do.

  Fourth, by setting a fuel excess ratio correction value (outside air temperature correction value) HOSa according to the outside air temperature Tatm, and correcting the excess fuel ratio η with the outside air temperature correction value HOSa, It is possible to set the excess fuel ratio η in consideration of a decrease in the amount of heat released from the combustion gas. Specifically, the consumption of raw fuel can be suppressed by reducing the excess fuel ratio with respect to the rise in the outside air temperature Tatm.

  Fifth, by setting an excess fuel ratio correction value (atmospheric pressure correction value) HOSb according to the atmospheric pressure Patm and correcting the excess fuel ratio η with the atmospheric pressure correction value HOSb, The excess fuel ratio η can be set in consideration of a decrease in heat exchange efficiency using the combustion gas as a heat medium. Specifically, by increasing the excess fuel ratio η with respect to the increase in the atmospheric pressure Patm, it is possible to compensate for the decrease in the heat exchange efficiency and promote the evaporation of the raw fuel.

  Further, when the atmospheric pressure Patm is high and the absorption heat flow HF3 in the reformer 23 is large in the reformer 23 disposed upstream of the evaporator 21 with respect to the flow of the combustion gas, the exhaust heat is increased due to the increase in the absorption heat flow HF3. The shortage generated in the heat flow HF5 can be compensated for by increasing the fuel heat flow HF1, and the evaporation of the raw fuel can be promoted.

  Sixth, the control of the excess fuel rate η is performed by detecting the flow rate Ff or the temperature Tf of the raw fuel on the upstream side of the evaporator 21 and reflecting at least one of these state quantities in the control of the excess fuel rate η. Is more appropriate, and the amount of heat necessary for the evaporation of the raw fuel can be more reliably supplied to the evaporator 21. Then, the flow rate Fc or the temperature Tc of the combustion gas on the upstream side of the evaporator 21 is detected, and the control of the excess fuel rate η is preferably performed by reflecting at least one of these state quantities in the control of the excess fuel rate η. Can be encouraged.

  In the present embodiment, from the viewpoint of the heat balance in the combustor 4, the air flow rate Fa is calculated by the equation (5) (S108). However, the calculation of the air flow rate Fa is not limited to this. It is also possible to perform the search by using the table data having the tendency shown in FIG. 12, thereby simplifying the calculation of the air flow rate Fa. Here, in response to the increase in the excess fuel ratio η (the target excess fuel ratio ηfuel), the air flow rate Fa is increased to avoid an excessive rise in the temperature of the combustor 4 and to suppress the deterioration of the catalyst provided in the combustor 4. It is possible to

  Further, in this case, in addition to increasing the excess fuel ratio η in response to the decrease in the required output P of the fuel cell stack 1, the flow rate Fa of the oxidizing gas supplied to the fuel cell system S and the flow rate of the raw fuel Since the ratio to Ff (= Fa / Ff) is increased, not only the evaporator 21 but also the combustor 4 can be appropriately operated, and the operation of the fuel cell system S at the time of low load operation can be improved. It is possible to stabilize the whole.

  As described above, the embodiment of the present invention has been described. However, the above embodiment is only a part of the application example of the present invention, and the technical scope of the present invention is limited to the specific configuration of the above embodiment. It is not the purpose. Various changes and modifications can be made to the above embodiment within the scope of the matters described in the claims.

S: fuel cell system 1: fuel cell stack 21: evaporator 22: fuel heat exchanger 23: reformer 3: oxidant gas heating unit (air heat exchanger)
DESCRIPTION OF SYMBOLS 4 ... Combustor 51 ... 1st fuel injector 52 ... 2nd fuel injector 6 ... Blower 7 ... Fuel tank 11 ... Anode supply passage 12 ... Cathode supply passage 11exh ... Anode discharge passage 12exh ... Cathode discharge passage 101 ... Controller

Claims (10)

A fuel cell,
A combustor that is connected to the anode discharge passage and the cathode discharge passage of the fuel cell and burns residual fuel in off-gas of the fuel cell;
An evaporator for evaporating raw fuel of the fuel cell before supply to the fuel cell, wherein the evaporator is connected to the combustor so that the raw fuel can be heated by a calorific value of the combustion gas of the remaining fuel. An evaporator,
A fuel supply device for supplying the raw fuel to the evaporator;
An oxidizing gas supply device that supplies an oxidizing gas to the fuel cell;
A supply control unit that controls the fuel supply device and the oxidant gas supply device,
With
The supply control unit sets a flow rate of the raw fuel according to a required output of the fuel cell as a required flow rate, and is a ratio of an actual flow rate of the raw fuel to the required flow rate with respect to a decrease in the load of the fuel cell. The excess fuel ratio is defined as A, the exhaust heat flow of the combustion gas at the inlet side of the evaporator, the heat exchange efficiency between the raw fuel and the combustion gas in the evaporator as B, and the raw fuel in the evaporator as B. The fuel evaporation heat flow required for evaporation of C is controlled as C, and the excess fuel ratio is controlled so that A × B / C> 1.
Fuel cell system. The fuel cell system according to claim 1, wherein
The supply control unit includes:
Has means for acquiring the outside air temperature and atmospheric pressure,
Based on the required output of the fuel cell, the outside air temperature and the atmospheric pressure, the excess fuel ratio is calculated,
For the fuel supply device, output a signal to supply the raw fuel at a flow rate according to the excess fuel rate,
Fuel cell system. The fuel cell system according to claim 1, wherein
The supply control unit includes:
Means for acquiring a state quantity of the raw fuel including at least one of a flow rate and a temperature of the raw fuel on the inlet side of the evaporator,
The control of the excess fuel ratio reflects the state quantity of the raw fuel,
For the fuel supply device, output a signal to supply the raw fuel at a flow rate according to the excess fuel rate,
Fuel cell system. The fuel cell system according to claim 1, wherein
The supply control unit includes:
Means for acquiring a state quantity of the combustion gas including at least one of a flow rate and a temperature of the combustion gas on the inlet side of the evaporator,
In controlling the excess fuel ratio, the state quantity of the combustion gas is reflected,
For the fuel supply device, output a signal to supply the raw fuel at a flow rate according to the excess fuel rate,
Fuel cell system. A fuel cell,
A combustor that is connected to the anode discharge passage and the cathode discharge passage of the fuel cell and burns residual fuel in off-gas of the fuel cell;
An evaporator for evaporating raw fuel of the fuel cell before supply to the fuel cell, wherein the evaporator is connected to the combustor so that the raw fuel can be heated by a calorific value of the combustion gas of the remaining fuel. An evaporator,
A fuel supply device for supplying the raw fuel to the evaporator;
An oxidizing gas supply device that supplies an oxidizing gas to the fuel cell;
A supply control unit that controls the fuel supply device and the oxidant gas supply device,
With
The supply control unit sets a flow rate of the raw fuel according to a required output of the fuel cell as a required flow rate, and sets an excess fuel ratio, which is a ratio of an actual flow rate to the required flow rate of the raw fuel, to a load of the fuel cell. Increase against a decrease in
Fuel cell system. The fuel cell system according to any one of claims 1 to 5,
The supply control unit increases the flow rate of the oxidizing gas with respect to an increase in the outlet temperature of the fuel cell,
Fuel cell system. The fuel cell system according to any one of claims 1 to 5,
The supply control unit increases the excess fuel ratio with respect to a decrease in the load of the fuel cell, and increases a ratio of the flow rate of the oxidizing gas to the flow rate of the raw fuel.
Fuel cell system. The fuel cell system according to claim 1, wherein:
The supply control unit includes:
It has a means to get the outside temperature,
Reducing the excess fuel rate with respect to the rise in the outside air temperature;
Fuel cell system. The fuel cell system according to claim 1, wherein:
The supply control unit includes:
Has means to obtain atmospheric pressure,
Increasing the excess fuel ratio with respect to the increase in the atmospheric pressure;
Fuel cell system. A control method for a fuel cell system including an evaporator that evaporates a raw fuel of a fuel cell before supplying the fuel to the fuel cell,
Supplying the raw fuel to the evaporator;
Burning the remaining fuel in the off-gas of the fuel cell,
Supplying the combustion gas of the residual fuel generated by the combustion to the evaporator,
In the evaporator, the raw fuel is heated by heat exchange with the combustion gas,
In supplying the raw fuel, a flow rate of the raw fuel according to a required output of the fuel cell is set as a required flow rate, and a ratio of an actual supply flow rate of the raw fuel to the required flow rate with respect to a decrease in the load of the fuel cell. Where A is the heat flow of the combustion gas at the inlet side of the evaporator, B is the heat exchange efficiency between the raw fuel and the combustion gas in the evaporator, and The heat flow required for fuel evaporation is defined as C, and the excess fuel ratio is controlled so that A × B / C> 1.
A control method for a fuel cell system.

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