JP7176263B2 - FUEL CELL SYSTEM AND METHOD OF CONTROLLING FUEL CELL SYSTEM - Google Patents

Description

The present invention relates to a fuel cell system having an evaporator that evaporates raw fuel and a control method thereof.

Patent Document 1 discloses the following fuel cell using a hydrocarbon fuel such as methanol or gasoline as a fuel cell, which is provided with an evaporator or a vaporizer for evaporating the raw fuel before it is supplied to the fuel cell. A system is disclosed. By burning the remaining fuel in the off-gas discharged from the fuel cell in a combustor and supplying the resulting combustion gas to an evaporator, the raw fuel is heated and evaporated by heat exchange with the combustion gas. is.

JP 2003-197236 A (paragraph 0016)

Here, when the flow rates of the raw fuel and the oxidizing gas are reduced at a constant rate in response to changes in the load of the fuel cell, particularly when the load drops, the amount of heat possessed by the combustion gas supplied to the evaporator becomes insufficient. , there is a problem that the raw fuel cannot be sufficiently evaporated. If the vaporization of the raw fuel becomes insufficient, the raw fuel is supplied in a liquid state to the reformer provided downstream of the evaporator, or the unreformed gas is supplied to the fuel cell, resulting in reforming. This will cause deterioration of the catalyst provided in the reactor or fuel cell, especially the reformer.

The present invention provides a fuel cell system capable of supplying the evaporator with the amount of heat required to evaporate the raw fuel regardless of the load on the fuel cell and appropriately operating the evaporator, and a method of controlling the same. for the purpose.

In one aspect 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 the off-gas of the fuel cell, and an evaporator for evaporating the raw fuel of the fuel cell. The evaporator is connected to the combustor so that the raw fuel can be heated by the heat of the combustion gas of the residual fuel, the fuel supply device supplies the raw fuel to the evaporator, and the oxidant gas is supplied to the fuel cell. and a supply controller for controlling the fuel supply device and the oxidant gas supply device. In this embodiment, the supply control unit sets the flow rate of the raw fuel according to the required output of the fuel cell as the required flow rate, and controls the excess fuel flow rate, which is the ratio of the actual flow rate of the raw fuel to the required flow rate of the raw fuel, in response to the decrease in the load of the fuel cell. Let A be the exhaust heat flow of the combustion gas on the inlet side of the evaporator, B be the heat exchange efficiency between the raw fuel and the combustion gas in the evaporator, and C be the fuel evaporation heat flow required for evaporation of the raw fuel in the evaporator. , the excess fuel ratio is controlled so that A×B/C>1.

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

According to the present invention, it is possible to supply the evaporator with the amount of heat necessary for evaporating the raw fuel while minimizing the consumption of the raw fuel, regardless of the load of the fuel cell, so that the evaporator can be operated appropriately. be able to. Since the amount of heat given to the raw fuel is ensured, it is particularly effective when using a raw fuel that is relatively difficult to evaporate, such as an aqueous ethanol solution.

FIG. 1 is a schematic diagram showing the overall configuration of a fuel cell system according to one embodiment of the present invention. FIG. 2 is an explanatory diagram showing the operation at startup of the fuel cell system according to the above embodiment. FIG. 3 is an explanatory diagram showing normal operation of the fuel cell system according to the above embodiment. FIG. 4 is an explanatory diagram showing another example of normal operation of the fuel cell system according to the above embodiment. FIG. 5 is an explanatory diagram showing changes 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 heat flow according to parts of the fuel cell system. FIG. 7 is a flow chart showing the flow of power generation control during low-load operation of the fuel cell system. FIG. 8 is a flow chart showing specific contents of target excess air ratio setting processing in the same power generation control. FIG. 9 is an explanatory diagram showing the setting tendency of the amount of heat released from the fuel cell system (the amount of system heat released). FIG. 10 is an explanatory diagram showing the setting tendency of the excess fuel ratio correction value with respect to the outside air temperature. FIG. 11 is an explanatory diagram showing the setting tendency of the excess fuel ratio correction value with respect to the atmospheric pressure. FIG. 12 is an explanatory diagram showing the 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 the configuration of a fuel cell system S according to one embodiment of the invention.

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

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

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

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

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

The combustor 4 burns the remaining fuel in the off-gas discharged from the fuel cell stack 1 to generate combustion gas. In this embodiment, the combustor 4 is connected to the anode exhaust passage 11exh and the cathode exhaust passage 12exh, and is supplied with anode off-gas and cathode off-gas through these passages 11exh and 12exh. The amount of heat that the combustion gas has is supplied to the fuel processors 21 to 23 and the oxidizing gas heating unit 3 and used to heat the raw fuel and the oxidizing gas.

To further explain the configuration of the fuel cell system S, the fuel cell stack 1 includes, in the anode system, an anode supply passage 11 for supplying fuel gas to the anode of the fuel cell, and a gas after the power generation reaction discharged from the anode. and an anode discharge passage 11exh for flowing anode off-gas. In the cathode system, a cathode supply passage 12 for supplying an oxidant gas to the cathode of the fuel cell, and a post-power generation reaction exhausted from the cathode. and a cathode exhaust passage 12exh for flowing cathode off-gas.

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

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

The fuel heat exchanger 22 receives heat from the combustion gas from the combustor 4 and further heats the ethanol gas and water vapor.

The reformer 23 incorporates a reforming catalyst and generates hydrogen from ethanol in a gaseous state by steam reforming. Steam reforming can be expressed by the following equation.
C2H5OH ₊ 3H2O-> _{6H2 + 2CO2} ( 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 exchanging heat with the combustion gas supplied from the combustor 4 through the combustion gas passage 13 . In this embodiment, a blower or air compressor 6 is installed near the open end of the cathode supply passage 12, and air in the atmosphere is sucked into the cathode supply passage 12 via the blower 6 as the oxidant gas. The sucked air is heated from room temperature (for example, 25° C.) when passing through the oxidant gas heating section 3 and is supplied to the fuel cell stack 1 .

The combustor 4 incorporates a combustion catalyst, receives the supply of anode off-gas from the fuel cell stack 1, and generates combustion gas by catalytic combustion of the remaining fuel in the anode off-gas. In addition to the anode off-gas, the combustor 4 can also be supplied with an aqueous ethanol solution, which is the raw fuel, through the branch fuel passage 11sub. Let In this 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 accommodated in a case L shared with the combustor 4 (case L). L is conceptually indicated by a chain double-dashed 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 the supply of the oxidant gas to the oxidant gas heater 3 during operation of the fuel cell system S. Controller 101 may 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 the raw fuel to the evaporator 21 and the combustor 4 and to supply the oxidizing gas to the oxidizing gas heating section 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 motor generator. For example, the fuel cell system S can be applied to a drive system for a vehicle, charging a battery with electric power generated by the rated operation of the fuel cell stack 1, or supplying electric power according to the target driving force of the vehicle. It can be supplied from the battery and the fuel cell stack 1 to a motor generator for running.

(Control system configuration and basic operation)
In this embodiment, the controller 101 is configured as an electronic control unit comprising a microcomputer including a central processing unit (CPU), various storage units such as ROM and RAM, input/output interfaces, and the like. It controls the operation of various devices or parts required for the operation of the fuel cell system S, such as the fuel injector 52 and the blower 6 .

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

The stack temperature Tstk is an index indicating the temperature of the fuel cell stack 1 or the fuel cell. Let the temperature be the stack temperature Tstk.

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

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

The combustion gas flow rate Fc is the flow rate of the combustion gas supplied to the evaporator 21 . In this embodiment, the combustion gas flow rate sensor 204 is installed upstream of the evaporator 21, specifically in the combustion gas passage 13 between the oxidizing gas heating section 3 and the evaporator 21. Let the flow rate detected by is the combustion gas flow rate Fc.

The fuel temperature Tf is the temperature of the raw fuel supplied to the evaporator 21 . In this 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 taken as the fuel temperature Tf.

The combustion gas temperature Tc is the temperature of the combustion gas supplied to the evaporator 21 . In this embodiment, the combustion gas temperature sensor 206 is installed upstream of the evaporator 21, specifically in the combustion gas passage 13 between the oxidizing gas heating section 3 and the evaporator 21. The temperature detected by is taken as 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 performs start control for warming up the fuel cell stack 1 . Warming up the fuel cell stack 1 refers to raising the temperature of the fuel cell stack 1, which was at a low temperature (for example, normal temperature) during the stop, to its operating temperature.

Then, when the temperature of the fuel cell stack 1 reaches the operating temperature, the controller 101 terminates start-up control and shifts to normal power generation control. Under normal conditions, basically, the fuel cell stack 1 is operated at its rated point, and raw fuel is supplied to the evaporator 21 via the first fuel injector 52 at a flow rate required for rated operation. Here, rated operation of the fuel cell stack 1 means operation at the maximum power generation output of the fuel cell stack 1 .

2 and 3 show the operation of the fuel cell system S. FIG. FIG. 2 shows the operation at startup, and FIG. 3 shows the operation at normal time. In the figure, thick lines with arrows indicate passages with gas or liquid flow, and dotted lines indicate passages without flow.

At start-up ( FIG. 2 ), the first fuel injector 51 is stopped while the second fuel injector 52 is activated, and the raw fuel stored in the fuel tank 7 is injected into the combustor 4 via the second fuel injector 52 . supply to 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 oxidant gas heating section 3 . The air that has passed through the fuel cell stack 1 is supplied to the combustor 4 as an oxidant for the raw fuel through the cathode discharge passage 12exh. Combustion gas generated by combustion is supplied to the oxidant gas heating section 3 and the evaporator 21 through the combustion gas passage 13, while the heat of the combustion gas is transferred to the fuel heat exchanger 22 and the reformer 23 inside the case L. is transmitted to Therefore, at the time of start-up, the fuel cell stack 1 is heated by the radiant heat generated by the combustion of the raw fuel, and further heated by using the air heated by the oxidant gas heating unit 3 as a heat medium. Warmth is promoted. At 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, during normal operation ( FIG. 3 ), the second fuel injector 52 is stopped while the first fuel injector 51 is operated to supply raw fuel to the evaporator 21 via the first fuel injector 51 . At the same time, the blower 6 is operated to supply air, which is the oxidizing gas, to the fuel cell stack 1 via the oxidizing gas heating section 3 . The off-gas (anode off-gas, cathode off-gas) discharged from the fuel cell stack 1 after the power generation reaction is supplied to the combustor 4 through the anode discharge passage 11exh and the cathode discharge passage 12exh. The remaining fuel in the anode offgas is combusted in the combustor 4, and the resulting combustion gas is supplied to the oxidant gas heating section 3 and the evaporator 21 through the combustion gas passage 13. It is internally transmitted to the fuel heat exchanger 22 and the reformer 23 . As a result, the fuel processing units 21 to 23 and the oxidant gas heating unit 3 are heated, the evaporator 21 is maintained at a temperature at which the raw fuel (aqueous ethanol solution) can be evaporated, and the reformer 23 is kept at a temperature at which the raw fuel (ethanol solution) can be evaporated. ) is maintained at a temperature capable of reforming. Normally, 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 of the fuel cell stack 1 , in other words, the load of the fuel cell system 1 . If the remaining fuel in the anode offgas is insufficient to generate the amount of heat required to maintain the temperatures of the evaporator 21 and the reformer 23, the second fuel injector 52 is operated to supply the combustor 4 with the second It is also possible to supply raw fuel via fuel injectors 52 (FIG. 4).

Here, when the load of the fuel cell stack 1 is reduced during normal operation after warming up, for example, during steady operation in the low load range or when the output fluctuates to the low load side, the load of the fuel cell stack 1 is reduced. On the other hand, if the flow rates of the raw fuel and the oxidant gas are reduced at a constant rate, the heat flow that can be used as waste heat from the total energy input to the fuel cell system S decreases, and the heat flow that is supplied to the evaporator 21 The amount of heat in the combustion gas produced is insufficient for the heat flow required to evaporate the raw fuel. As a result, the raw fuel cannot be sufficiently vaporized, 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, causing the reforming. There is a concern that the catalyst provided in the qualityr 23 or the fuel cell stack 1 may be deteriorated. Here, as steady state operation in the low load region, the state of charge of the battery is sufficient and the fuel cell stack 1 does not require power generation at high or medium load. In the present embodiment, "heat flow" refers to the amount of heat [J/s] that passes through per unit time.

FIG. 5 shows changes in the heat flow ratio Rhf with respect to the output P of the fuel cell stack 1. FIG. Here, the heat flow ratio Rhf refers to the ratio of exhaust heat flow to fuel evaporation heat flow (=exhaust heat flow/fuel evaporation heat flow). The exhaust heat flow refers to the heat flow of the combustion gas on the inlet side or upstream side of the evaporator 21 , and the fuel evaporation heat flow refers to the heat flow required for vaporizing 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, and these can be calculated based on the state quantity of

As shown in FIG. 5, if the excess fuel ratio η is kept constant and the flow rates of the raw fuel and the oxidant gas are decreased at a constant rate with respect to the load decrease, the heat flow ratio Rhf gradually approaches 1 Decrease. As the heat flow ratio Rhf approaches 1, the exhaust heat flow tends to be insufficient with respect to the fuel evaporation heat flow. Insufficient exhaust heat flow means insufficient vaporization of raw fuel. When the vaporization of the raw fuel becomes insufficient, part of the raw fuel is supplied to the reformer 23 in a liquid state, or unreformed gas is supplied to the fuel cell stack 1 to reform the fuel. Degradation is caused to the catalyst provided in the vessel 23 or the fuel cell stack 1 .

In this embodiment, by increasing the excess fuel ratio η in response to a decrease in the load of the fuel cell stack 1, the heat flow necessary for evaporating the raw fuel to the evaporator 21 is increased regardless of the load on the fuel cell stack 1. supply. Specifically, with the flow rate of the raw fuel corresponding to the required output P of the fuel cell stack 1 as the required flow rate, the fuel Let A be the exhaust heat flow, let B be the heat exchange efficiency between the raw fuel and the combustion gas in the evaporator 21, and C be the fuel evaporation heat flow. to control.

FIG. 6 conceptually shows the relationship between the heat flows HF1 to HF6 in each part of the fuel cell stem S. As shown in FIG. 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) is greater than 1. The term “excess fuel ratio” refers to the ratio of the actual flow rate of the raw fuel to the required flow rate of the raw fuel when the flow rate of the raw fuel corresponding to the required output P of the fuel cell stack 1 is defined as the required flow rate. As will be described later, in this embodiment in which the required output P of the fuel cell stack 1 is 2.0 kW, the required output P (=2.0 kW) converted into flow rate corresponds to the required flow rate of the raw fuel. The ratio of the actual fuel flow rate (=2.4 kW) to the required fuel flow rate (=2.0 kW) corresponds to the "fuel surplus ratio."

References HF1 to HF6 shown in FIG. 6 indicate the following heat flows. As already mentioned, "heat flow" is the amount of heat that passes through a target site per unit time.
HF1: Heat flow of the raw fuel supplied to the evaporator 21 (fuel heat flow)
HF2: Heat flow required for evaporation of raw fuel in evaporator 21 (fuel evaporation heat flow)
HF3: heat flow absorbed in the reforming reaction (steam reforming) in the reformer 23 (absorbed heat flow)
HF4: heat flow possessed by combustion gas generated in combustor 4 (combustion gas heat flow)
HF5: Heat flow of combustion gas on inlet side of evaporator 21 (exhaust heat flow)
HF6: radiated heat flow according to the heat exchange efficiency ηvap in the 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 assumed to be 2.0 kW, and the amount of heat radiation from the fuel cell system S (hereinafter referred to as "the amount of system heat radiation") Q is assumed to be 0.2. The heat flows HF1 to HF6 at 3 kW are shown below. 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, etc. of the raw fuel. and

Fuel heat flow HF1=P×η=2.4 kW
Fuel vapor heat flow HF2 = HF1 x 0.2 = 0.48 kW
Absorption heat flow HF3 = HF1 x 0.1 = 0.24 kW
Combustion gas heat flow HF4=HF1+HF2+HF3-P-Q=0.82 kW
Exhaust heat flow HF5 = HF4 - HF3 = 0.58 kW
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 in which the heat flow ratio (=A×B/C) is greater than 1 regardless of the load of the fuel cell stack 1, or may be constant regardless of the load. B/C) may be increased for decreasing loads so that it is as small as possible while still exceeding unity.

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. Furthermore, α is a predetermined value that differs depending on the type or composition of the raw fuel.

The calculation of the fuel vaporization heat flow Qv is as follows when the raw fuel temperature Tfuel 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(Tfuel0)−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 vaporization heat flow Qv is supplied to the evaporator 21, in other words, depends on the flow rate Ffuel and temperature Tfuel of the raw fuel at the inlet side of the evaporator 21. FIG.

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

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

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

The calculation for determining the exhaust heat flow Qe from the viewpoint of enthalpy is based on the following equation.
Qe=Fexh×(α× _{H_co2} (Texh)+β× _{H_h2o} (Texh)+γ× _{H_o2} (Texh)+ζ× _{H_n2} (Texh))

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

Thus, the exhaust heat flow Qe corresponds to the flow rate Fexh and temperature Texh of the combustion gas on the inlet side of the evaporator 21 .

Here, the flow rate Fexh and the temperature Texh of the combustion gas correspond to "state quantities of the combustion gas".

(Explanation by flow chart)
7 and 8 are flow charts showing the content of power generation control (hereinafter referred to as "normal control") normally executed by the controller 101. FIG. FIG. 7 shows the overall flow of normal control, and FIG. 8 shows specific details of the target excess fuel ratio setting process in normal control.

In this embodiment, the controller 101 is programmed to perform 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, stack temperature Tstk, fuel flow rate Ff, air flow rate Fa, combustion gas flow rate Fc, fuel temperature Tf, combustion gas temperature Tc, outside air temperature Tatm, atmospheric pressure Tatm, and the like are read. When the fuel cell system S is for vehicle use and constitutes a drive source of a vehicle, in addition to the above, the accelerator opening APO, the vehicle speed VSP, and the like are read. The accelerator opening APO is an index indicating the required driving force of the vehicle.

In S102, it is determined whether or not the fuel cell system S has been warmed up. If the warm-up has been completed, the process proceeds to S103, and if the warm-up has not been completed and is still being warmed up, the normal control is terminated and a separately set warm-up control is executed. Whether or not the warm-up of the fuel cell system S is 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 drive 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 or not the required output P of the fuel cell stack 1 is in the 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 is in the low load range, the process proceeds to S105; Execute the processing that is set separately for the region.

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

At S106, a target excess fuel ratio ηfuel is calculated.

In S107, a fuel flow rate Ff corresponding to the target excess fuel ratio ηfuel is calculated. The fuel flow rate Ff is calculated by calculating the fuel heat flow HF1 (=P×η) and converting it into the raw fuel flow rate Ff by 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 at the flow rate Ff to the evaporator 21 via the first fuel injector 51 .
Ff=(P×η)/LHV (4)
In formula (4), LHV represents the lower heating value of the raw fuel.

In S108, the air flow rate Fa is calculated. In this embodiment, the calculation of the air flow rate Fa is performed based on the following equation representing the heat balance in the combustor 4 . The following equation gives the lower limit flow rate of the oxidant gas that can keep the temperature of the combustor 4 below its upper limit temperature as the air flow rate Fa. Therefore, this lower limit flow rate is set to the air flow rate Fa during low-load operation. The air flow rate Fa set in this manner tends to increase, for example, as the outlet temperature Tout rises. As a result, it is possible to avoid an excessive temperature rise of the combustor 4 while minimizing the energy spent for supplying the oxidant gas (air). The outlet temperature Tout can be substituted with the stack temperature Tstk.
Hf/(Cpf×Ff+Cpa×Fa)<(Tmax−Tout) (5)

The symbols used in equation (4) indicate the following.
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

The controller 101 then sends a signal corresponding to the air flow rate Fa to the blower 6 to supply the oxidant gas (that is, air) at the flow rate Fa to the oxidant gas heater 3 via the blower 6 .

Moving to FIG. 8, in S201, the basic excess fuel ratio ηbase is calculated. The basic excess fuel ratio ηbase is given by equation (3), and in the present embodiment, is set in advance corresponding to the required output P in the low load range, and is calculated as a smaller value as the required output P increases. be.

In S202, an excess fuel rate correction value (hereinafter referred to as "outside temperature correction value") HOSa for 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 amount of heat released from the combustion gas decreases as the outside air temperature Tatm rises, and the heat flow that can be used for exhaust heat, that is, the exhaust heat flow HF5 increases.

In S203, an excess fuel rate correction amount (hereinafter referred to as "atmospheric pressure correction value") HOSb for 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 as the atmospheric pressure Patm rises, and when the raw fuel is ethanol aqueous solution, the methane in the combustion gas increases as the atmospheric pressure Patm rises, so the volumetric flow rate of the combustion gas increases. This is because the heat exchange efficiency in the evaporator 21 decreases. In other words, the increase in the exhaust heat flow HF5 compensates for the decrease in heat exchange efficiency due to the change in the composition of the combustion gas.

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

In this embodiment, the fuel cell stack 1 constitutes a “fuel cell”, the combustor 4 constitutes a “combustor”, the evaporator 21 constitutes a “vaporizer”, and the first fuel injector 51 constitutes a “fuel The blower 6 constitutes the "oxidant gas supply device", and the controller 101 constitutes the "supply control section".

(Description of Action and Effect)
The fuel cell system S according to this embodiment is configured as described above, and the actions and effects obtained by this embodiment will be described below.

First, assuming that the flow rate of the raw fuel corresponding to the required output P of the fuel cell stack 1 is the required flow rate (for example, 2.0 kW), the actual flow rate of the raw fuel with respect to the required flow rate of the raw fuel with respect to the decrease in the load or the required output P is (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. Excess fuel ratio η is controlled. This makes it possible to supply the evaporator 21 with the amount of heat necessary for evaporating the raw fuel while minimizing the consumption of the raw fuel, regardless of the load of the fuel cell system S, so that the evaporator 21 can be operated appropriately. can be done. Since the amount of heat imparted to the raw fuel is ensured in this way, it is particularly effective when using an aqueous ethanol solution 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 given to the evaporator 21 by the combustion gas becomes insufficient for the amount of heat necessary for evaporating the raw fuel. In this case, by increasing the exhaust heat flow HF5, the insufficient amount of heat can be compensated for, and the optimization of the operation of the evaporator 21 can be promoted.

Second, based on the required output P of the fuel cell stack 1, the outside air temperature Tatm, and the atmospheric pressure Patm, the excess fuel ratio η can be calculated in advance by, for example, equation (3). In addition, it is possible to adopt an appropriate excess fuel ratio η according to the actual outside air temperature Tatm and the like during 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 in the outlet temperature Tout, the combustor 4 is appropriately cooled, and the combustor 4 is cooled excessively. It becomes possible to avoid temperature rise. From the viewpoint of the heat balance in the combustor 4, the flow rate of the oxidant gas is calculated by the formula (5), thereby enabling the setting of the lower limit flow rate of the oxidant gas and reducing the energy spent on supplying the oxidant gas. can do.

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

Fifth, by setting the excess fuel ratio correction value (atmospheric pressure correction value) HOSb according to the atmospheric pressure Patm and correcting the excess fuel ratio η by the atmospheric pressure correction value HOSb, It is possible to set the excess fuel ratio η in consideration of the decrease in heat exchange efficiency using 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.

Furthermore, in the reformer 23 arranged upstream of the evaporator 21 with respect to the flow of the combustion gas, when the atmospheric pressure Patm is high and the absorption heat flow HF3 in the reformer 23 is large, the increase in the absorption heat flow HF3 causes exhaust gas The shortage in the heat flow HF5 can be compensated for by an increase in the fuel heat flow HF1, and the vaporization of the raw fuel can be promoted.

Sixth, 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 ratio η, the control of the excess fuel ratio η is made more appropriate, and the amount of heat necessary for evaporating the raw fuel can be supplied to the evaporator 21 more reliably. Also, by detecting the flow rate Fc or the temperature Tc of the combustion gas on the upstream side of the evaporator 21 and reflecting at least one of these state quantities in the control of the excess fuel ratio η, it is possible to control the excess fuel ratio η. It is possible to promote

In the present embodiment, the air flow rate Fa was calculated from the equation (5) from the viewpoint of the heat balance in the combustor 4 (S108), but the calculation of the air flow rate Fa is not limited to this. It is also possible to search from table data having the tendency shown in 12, thereby simplifying the calculation of the air flow rate Fa. Here, by increasing the air flow rate Fa with respect to the increase in the excess fuel ratio η (target excess fuel ratio ηfuel), excessive temperature rise of the combustor 4 is avoided and deterioration of the catalyst provided in the combustor 4 is suppressed. It is possible to

Furthermore, in this case, in addition to increasing the excess fuel ratio η with respect to the decrease in the required output P of the fuel cell stack 1, the flow rate Fa of the oxidant gas supplied to the fuel cell system S 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 during low-load operation can be improved. It is possible to stabilize the whole.

Although the embodiments of the present invention have been described above, the above embodiments merely show a part of the application examples of the present invention, and the technical scope of the present invention is limited to the specific configurations of the above embodiments. not on purpose. Various changes and modifications can be made to the above-described embodiment within the scope of 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)
4 Combustor 51 First fuel injector 52 Second 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 connected to the anode exhaust passage and the cathode exhaust passage of the fuel cell and configured to burn residual fuel in the offgas of the fuel cell;
An evaporator for evaporating the raw fuel of the fuel cell before being supplied to the fuel cell, the evaporator being connected to the combustor so that the raw fuel can be heated by the amount of heat of the combustion gas of the residual fuel. an evaporator;
a fuel supply device for supplying the raw fuel to the evaporator;
an oxidant gas supply device that supplies an oxidant 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 the flow rate of the raw fuel according to the required output of the fuel cell as a required flow rate, and the ratio of the actual flow rate of the raw fuel to the required flow rate of the raw fuel with respect to the decrease in the load of the fuel cell. Let A be the exhaust heat flow of the combustion gas on the inlet side of the evaporator, B be the heat exchange efficiency between the raw fuel and the combustion gas in the evaporator, and B be the raw fuel in the evaporator. where C is the fuel evaporation heat flow required for the evaporation of the fuel excess ratio of A × B / C>1,
fuel cell system. The fuel cell system according to claim 1,
The supply control unit
having means for obtaining outside temperature and atmospheric pressure;
calculating the excess fuel ratio based on the required output of the fuel cell, the outside temperature and the atmospheric pressure;
outputting a signal to the fuel supply device to supply the raw fuel at a flow rate corresponding to the excess fuel ratio;
fuel cell system. The fuel cell system according to claim 1,
The supply control unit
means for acquiring a state quantity of the raw fuel including at least one of flow rate and temperature of the raw fuel on the inlet side of the evaporator;
Reflecting the state quantity of the raw fuel in the control of the excess fuel ratio,
outputting a signal to the fuel supply device to supply the raw fuel at a flow rate corresponding to the excess fuel ratio;
fuel cell system. The fuel cell system according to claim 1,
The supply control unit
means for acquiring a state quantity of the combustion gas including at least one of flow rate and temperature of the combustion gas on the inlet side of the evaporator;
Reflecting the state quantity of the combustion gas in the control of the excess fuel ratio,
outputting a signal to the fuel supply device to supply the raw fuel at a flow rate corresponding to the excess fuel ratio;
fuel cell system. a fuel cell;
a combustor connected to the anode exhaust passage and the cathode exhaust passage of the fuel cell and configured to burn residual fuel in the offgas of the fuel cell;
An evaporator for evaporating the raw fuel of the fuel cell before being supplied to the fuel cell, the evaporator being connected to the combustor so that the raw fuel can be heated by the amount of heat of the combustion gas of the residual fuel. an evaporator;
a fuel supply device for supplying the raw fuel to the evaporator;
an oxidant gas supply device that supplies an oxidant 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 uses the flow rate of the raw fuel corresponding to the required output of the fuel cell as the required flow rate, and calculates the excess fuel ratio, which is the ratio of the actual flow rate of the raw fuel to the required flow rate, as the 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 oxidant gas in response to an increase in 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 and increases the ratio of the flow rate of the oxidant gas to the flow rate of the raw fuel in response to a decrease in the load of the fuel cell.
fuel cell system. The fuel cell system according to any one of claims 1 to 7,
The supply control unit
having means for obtaining the outside temperature;
reducing the excess fuel ratio with respect to the increase in the outside air temperature;
fuel cell system. The fuel cell system according to any one of claims 1 to 8,
The supply control unit
having means for obtaining atmospheric pressure;
increasing the excess fuel ratio with respect to the increase in atmospheric pressure;
fuel cell system. A control method for a fuel cell system comprising an evaporator that evaporates raw fuel for a fuel cell before being supplied to the fuel cell,
supplying the raw fuel to the evaporator;
Burning the remaining fuel in the off-gas of the fuel cell,
supplying combustion gas of the remaining fuel generated by the combustion to the evaporator;
heating the raw fuel by heat exchange with the combustion gas in the evaporator;
When supplying the raw fuel, the ratio of the actual supply flow rate to the required flow rate of the raw fuel with respect to the decrease in the load of the fuel cell, with the flow rate of the raw fuel corresponding to the required output of the fuel cell as the required flow rate. Let A be the heat flow of the combustion gas on the inlet side of the evaporator, B be the heat exchange efficiency between the raw fuel and the combustion gas in the evaporator, and B be the raw material in the evaporator. The heat flow required for fuel vaporization is 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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