JP2011233439A - Operation control method and system for fuel cell - Google Patents

Abstract

A fuel cell operation control method and system by power load tracking capable of quickly following load fluctuation by controlling with feedforward control when the load fluctuation is large during operation of the fuel cell.
[Solution]
A fuel cell operation control method and system based on electric power load, in which the actual power output is Pac (n), the target power output is Pac (n + 1), the actual power output Pac (n) and the target When a difference between the power generation outputs Pac (n + 1) is ΔP and a certain threshold value P1 is determined, feedback control is performed when the absolute value of ΔP is equal to or less than P1, and feedforward control is performed when the absolute value of ΔP is greater than P1. A fuel cell operation control method based on power load tracking and a system therefor.
[Selection] Figure 3

Description

  The present invention relates to a fuel cell operation control method and system, and more particularly to a fuel cell operation control method and system based on power load tracking.

  In general, in a fuel cell system, the gas supply control device is slower to follow the fluctuation than the current control device. Therefore, if the power generation output is changed while giving instructions to both of them simultaneously, the current will be The gas flow rate increases after the value increases. For this reason, a fuel exhaustion state occurs in the cell stack, which causes deterioration.

  In order to avoid this, a technique has been adopted in which, when the output is increased, the output increase speed is suppressed and the current response speed is lowered so as to match the gas response speed. Specifically, a fuel cell system that collectively controls a power converter that converts direct current from a fuel cell into alternating current and a flow rate calculation control unit that controls the flow rate of various supply gases has been proposed. It has been proposed to detect a sudden increase in the load command value from the direct current output current of the fuel cell and output a suppression signal so as to keep the current increase rate of the fuel cell below the upper limit value (Patent Document 1).

  In addition, a fuel cell system is proposed that includes a response speed adjustment means that sends a gas flow rate setting signal instantly when the power set value increases, but the power change rate control unit does not respond until the gas flow rate reaches the target value. (Patent Document 2).

  Furthermore, in the fuel cell system comprising a solid oxide fuel cell that outputs direct current power and a power conversion system that converts direct current power from the fuel cell into alternating current power, the power conversion system includes a required alternating current from a load. When DC power is drawn from the fuel cell in accordance with the power, the DC power is drawn from the fuel cell at a constant rate of change in the DC output voltage from the fuel cell until the required AC power from the load changes. A characteristic solid oxide fuel cell system has been proposed (Patent Document 3).

Japanese Unexamined Patent Publication No. 7-57553 Japanese Patent Laid-Open No. 7-14598 JP 2008-84715 A

  In the above technique, the direct current output of the fuel cell is detected, and from there, feedback control is performed to calculate and instruct the required fuel gas and air flow rate. It will be late. Therefore, it is necessary to suppress the current change rate or always set the gas flow rate to a margin.

  In these technologies, the DC output current is detected, and the gas flow rate is calculated and instructed based on the instantaneous current value. Therefore, the gas flow rate is also instructed for the gradually increasing DC output current. It gradually increases step by step, leading to a decrease in the gas flow tracking speed. Due to the delay in following the load fluctuation, there is a problem in that efficiency, economic efficiency, and energy saving are reduced.

  Accordingly, the present invention provides a fuel cell operation control method by power load tracking that can quickly track load fluctuations by controlling with feedforward control when the load fluctuations are large when operating the fuel cell, and for the same The purpose is to provide a system.

  The present invention is (1) a fuel cell operation control method based on power load tracking, wherein the actual power generation output is Pac (n), the target power generation output is Pac (n + 1), and the actual power generation output Pac (n) is In this fuel cell operation control method, the feedback control used is combined with the feedforward control using the target power generation output Pac (n + 1) to follow the power load.

  The present invention is (2) a fuel cell operation control method based on power load tracking, in which the actual power generation output is Pac (n), the target power generation output is Pac (n + 1), and the actual power generation output Pac. When the difference between (n) and the target power generation output Pac (n + 1) is ΔP and a certain threshold value P1 is defined for ΔP, feedback control is performed when the absolute value of ΔP is equal to or less than P1, and the absolute value of ΔP is greater than P1. A fuel cell operation control method based on power load tracking is characterized in that when it is large, feedforward control is performed.

  The present invention is (3) a fuel cell operation control system based on power load tracking, wherein the actual power generation output is Pac (n), the target power generation output is Pac (n + 1), and the actual power generation output Pac (n) is The fuel cell operation control system is configured to follow the power load by combining the feedback control used and the feedforward control using the target power generation output Pac (n + 1).

  The present invention is (4) a fuel cell operation control system based on power load tracking, in which the actual power generation output is Pac (n), the target power generation output is Pac (n + 1), and the actual power generation output Pac. When the difference between (n) and the target power generation output Pac (n + 1) is ΔP and a certain threshold value P1 is defined for ΔP, feedback control is performed when the absolute value of ΔP is equal to or less than P1, and the absolute value of ΔP is greater than P1. A fuel cell operation control system based on electric power load is characterized in that when it is large, feedforward control is performed.

  According to the present invention, since the feedforward control is performed when the load fluctuation is large in the fuel cell, the load fluctuation can be quickly followed, and when the load fluctuation is small, the feedback control is performed. Can follow precisely. As a result, the electrical output tracking to the load fluctuation can be made faster than before, and the efficiency, economy and energy saving can be improved.

It is a figure explaining the structural example of the fuel cell system of this invention. It is a control block diagram of electric power load following concerning the present invention. It is a figure explaining the control flow of electric power load tracking concerning the present invention. It is a figure which shows the relationship between direct current output current and each flow volume of fuel gas, air, and reforming water. It is the figure which showed the IV characteristic example in a SOFC cell stack. It is a figure explaining the example of 4 systems as a structure of the fuel cell system of this invention. It is a figure explaining the process of the electric power load follow-up sequence which concerns on this invention. It is a figure explaining the process of the electric power load follow-up sequence which concerns on this invention. It is a figure explaining the process of the electric power load follow-up sequence which concerns on this invention. It is a figure explaining the process of the electric power load follow-up sequence which concerns on this invention. It is the figure which showed the process of FIGS. 7-10 collectively.

  In the present invention, feedforward control is incorporated by applying a characteristic known as “relationship between terminal voltage and current density of fuel cell” (IV characteristic) to power generation output control of the fuel cell system. By incorporating feedforward control, it is possible to suppress fuel depletion and to follow load fluctuations faster than before, thereby improving efficiency, economy, and energy saving.

  FIG. 1 is a diagram for explaining a configuration example of a solid oxide fuel cell system by power load tracking according to the present invention. The DC output power (current: Idc, voltage: Vdc) taken out from the cell stack (for example, SOFC cell stack) is converted into AC power by the DC converter and the voltage is adjusted by the DC converter. Power generation output (Pac). The power generation output (Pac) is connected to the grid received power (Pgrid) from the power company and supplied to the load (power load).

  Auxiliary drive power (Pbop) in the fuel cell system is covered by a part of the cell stack DC power. In the control unit, the cell stack DC output current, fuel gas, air, reformed water (reformed water = reformed water = reformed water) are determined from the cell stack DC output current (Idc), voltage (Vdc), various powers, temperatures, and auxiliary equipment states. The flow rate of water for generating steam for steam reforming the fuel gas to produce hydrogen is controlled.

  FIG. 2 is a control block diagram of the solid oxide fuel cell system based on power load according to the present invention. For the target power generation output [Pac (n + 1)], the feedback signal from the actual power generation output [Pac (n)] and the target DC output current required to output the target power generation output [Pac (n + 1)] Based on [Idc (n + 1)], a feedforward signal that predicts the flow rates of fuel gas, air, and reforming water in advance, the control amount to these accessories is calculated and a signal is output.

  FIG. 3 is a diagram for explaining a control flow of the solid oxide fuel cell system by power load tracking in the present invention. When the load following operation starts, first, the power load amount is measured from the actual power generation output [Pac (n)] and the system power reception power [Pgrid (n)] which is the power reception power from the power company, and the target power generation output [Pac (N + 1)] is calculated. The difference between the target power generation output [Pac (n + 1)] and the actual power generation output [Pac (n)] is ΔP [Expression (1)], and feedback control is performed according to the value of ΔP or feedforward control is performed. Determine whether.

  In the prior art, only feedback control is performed. On the other hand, the present invention is characterized in that feedforward control is incorporated together with feedback control. When ΔP is equal to or less than the threshold value P1, the target value [Idc (n + 1)] of the DC output current (Idc) taken out from the cell stack is held at the previous value [Equation (2)] with respect to the actual DC output current Idc (n). Or set to a value that is increased or decreased by a uniform constant current value (i) [equations (3-1), (3-2)], or by applying PID control and only ΔI that is a function of ΔP and a PID constant The changed value is set [formula (4)], and the instruction value for the DC output current (Idc) is updated.

  FIG. 4 is a diagram showing the relationship between the DC output current and the flow rates of fuel gas, air, and reformed water. 4A is a relationship diagram of the fuel gas flow rate (Qf) with respect to the DC output current (Idc), FIG. 4B is a relationship diagram of the air flow rate (Qa) with respect to the fuel gas flow rate (Qf), and FIG. These are the relationship diagrams of the reforming water flow rate (Qw) with respect to the fuel gas flow rate.

  Each flow rate of fuel gas, air, and reformed water is set so that all flow rate values are uniformly determined when the DC output current Idc is determined from the relationship diagram shown in FIG. The flow rate of gases is also changed. In the state of output increase where the DC output current Idc increases, the current change precedes and the gas flow rate change is delayed, so there is a possibility that the fuel will run out. This can be avoided by setting it as small as possible.

  When ΔP is larger than the threshold value P1, a target value [Idc (n + 1)] of the DC output current Idc is first calculated from an IV characteristic equation (Equation 8) described later. Next, it is determined whether the output increases or decreases from the positive / negative of ΔP. In the case of output reduction, the instruction value for the DC output current (Idc) taken out from the cell stack is updated to the target value Idc (n + 1), and the flow rates of fuel gas, air, and reformed water are also shown in the relationship diagram of FIG. The follow-up change is made to a value suitable for the DC output current Idc.

  On the other hand, in the case of an output increase, the flow rates of fuel gas, air, and reformed water that match the target value Idc (n + 1) of the DC output current Idc extracted from the cell stack are calculated from the relationship diagram of FIG. The flow rate instruction of the kind is performed in advance. Thereafter, the instruction value for the DC output current Idc is updated to Idc (n + 1) while observing the actual flow rates of the fuel gas and air. After updating the instruction value to Idc in accordance with the above procedure according to the value of ΔP, the process returns to the initial power generation output (Pac) and system received power (Pgrid) measurement, and proceeds to ΔP calculation again.

  Next, a method of deriving the target value [Idc (n + 1)] to the DC output current Idc extracted from the cell stack when ΔP is large (feed forward control) will be described.

  FIG. 5 is a diagram showing an example of IV characteristics in the SOFC cell stack. In general, the IV characteristic is a complicated one that changes depending on the fuel gas flow rate, the air flow rate, the cell stack temperature, and the degree of cell stack deterioration. However, by approximating this characteristic to a simpler expression, it can be used for feedforward control. FIG. 5 shows an approximation to a linear expression as an example.

  The value obtained by subtracting the activation overvoltage from the open circuit voltage of the cell stack (Vovv) is relatively small in variation of the value with respect to the state change of the cell stack, and the value is different for each cell stack type and each fuel type. An almost fixed value can be determined. Therefore, the DC output current [Idc (n)] and the output voltage [Vdc (n)] of the cell stack at the time immediately before the output fluctuation are measured, and a straight line (Equation 5) having Vocv as a y-intercept through this point is measured. The IV characteristic approximate line at this point is used. R represents the electric resistance in the cell stack.

  After the output fluctuation, the cell stack temperature also changes and the electric resistance [R (n)] also changes. However, since this change is sufficiently slow with respect to the change in the electric output, the IV characteristic immediately after the electric output fluctuation is Approximately by Equation 6. By inserting a relational expression between AC output and DC output (Expression 7) and a relational expression of DC output current / voltage / output (Expression 8) into Expression 6, the target power generation output [Pac (n + 1)] is changed to the target. It is possible to create an equation (Equation 9) for obtaining the DC output current [Idc (n + 1)]. Note that ηinv is the conversion efficiency in the inverter, and the conversion efficiency in the DC converter is assumed to be almost 100%.

  Formula 5 is an example of the IV characteristic approximation. In addition, for example, the IV characteristic of the cell stack is measured in advance for each of several temperatures, stored as a relationship diagram, and an appropriate IV characteristic is obtained from the measured value of the cell stack temperature at the moment of output fluctuation. It is also possible to calculate the target value of Idc.

<Power load tracking sequence process>
Hereinafter, the process of the power load follow-up sequence according to the present invention will be further described. FIG. 6 is a configuration example of a fuel cell system to which this sequence can be applied. In this system, four cell stacks and four DC converters are provided, and the DC current in each cell stack is controlled in each DC converter. Each process is shown in FIGS. 7-10, and they are collectively shown in FIG. 7-11, code | symbol S1-S21 is corresponded to each step 1-step 21 in the following.

1. Press the start button.
2. Start the start mode.
3. Measure temperature and voltage values during start-up mode.
4). When the temperature and voltage values become values that can be moved to the load following mode, they are moved to the load following mode.

5). Step 1 (S1): In the load follow-up mode of Step 1, “received power Pgrid (n)”, which is the grid received power at the present time, is measured. Since the received power Pgrid (n) is purchased from an electric power company, it is desirable that the received power Pgrid (n) be as small as possible when the fuel cell is generating power.
6). Step 2 (S2): Next, “power generation output Pac (n)”, which is an actual power generation output at the present time, is measured.

  7). Step 3 (S3): Next, “target power generation output Pac (n + 1)” is calculated. This calculation can be expressed as “Pac (n + 1) = Pgrid (n) + Pac (n) −Poffset” using “received power Pgrid (n)” and “power generation output Pac (n)”. Here, Poffset is a small value of about 50 W, for example. For example, a small value of about 50 W is set to prevent power from flowing backward from the fuel cell side to the power company grid side. Note that Poffset = 0 may be set when power is allowed to flow backward from the fuel cell side to the power company grid side.

8). Step 4 (S4): Next, a difference “ΔP” [= Pgrid (n) −Poffset] between “target power generation output Pac (n + 1)” and “power generation output Pac (n)” is calculated. Based on the value of ΔP, it is determined what kind of control is performed.
9. Step 4 (S4) right: The absolute value of ΔP | ΔP | is very small, but less than P0 (for example, less than 20 W), it means that the amount of change in the power load is very small. Yes. In this case, the process returns to step 1 without changing any DC output current or gas flow rate. This is a control dead zone provided to prevent excessive fluctuations in the power generation output.
10. Step 4 (S4) / Lower: When the absolute value | ΔP | of ΔP is medium, that is, P0 or more and less than P1 (for example, 100 W), the power load is changed, but the amount of change is relatively small. Represents. In this case, the follow-up accuracy is prioritized over the follow-up speed of the output, and the power generation output by the fuel cell is controlled by feedback control that is generally performed conventionally.

  11. Step 5 (S5): It is determined whether ΔP is positive or negative. Step 6 (S6) or step 7 (S7) is determined depending on whether it is positive or negative.

12 Step 6 (S6): When ΔP is positive, the target DC output current Idc (n + 1) is obtained by adding a constant current amount i (for example, 4 mA) to the actual DC output current Idc (n).
13. Step 7 (S7): When ΔP is negative, the target DC output current Idc (n + 1) is obtained by subtracting a constant current amount i (for example, 4 mA) from the actual DC output current Idc (n).

  14 Step 8 (S8): Based on this target DC output current Idc (n + 1), the target fuel gas flow rate is determined from FIG. 4 (a), the target air flow rate is determined from FIG. 4 (b), and FIG. The target reforming water flow rate is determined from (c).

  15. Step 9 (S9): Next, the target DC output current value is instructed to the DC converter connected to each cell stack, and the power generation output is changed. Specifically, the DC output currents Idc1 to Idc4 of each cell stack are updated to Idc (n + 1).

  16. Step 10 (S10): Next, the auxiliary machinery is instructed so that the fuel gas flow rate, the air flow rate, and the reforming water flow rate become the target fuel gas flow rate, the target air flow rate, and the target reforming water flow rate, respectively. After completing the flow of current, fuel gas, air, and reforming water to the auxiliary equipment, return to Step 1.

17. Step 4 (S4), left: When the absolute value | ΔP | of ΔP is large, that is, P1 (for example, 100 W) or more, it indicates that the power load is greatly changed. In this case, there is a problem that control takes too much time if the power generation output of the fuel cell is controlled only by feedback control, which is conventionally performed. For this reason, feed forward control is performed in such a case. In other words,
18. Step 11 (S11): DC output currents Idc1 to Idc4 of each cell stack are measured.
19. Step 12 (S12): DC output voltages Vdc1 to Vdc4 of each cell stack are measured.

  20. Step 13 (S13): Consider each cell stack as a single cell, and calculate only the internal resistances R1 to R4 of each cell stack.

  21. Step 14 (S14): the internal resistances R1 to R4 of each cell stack calculated in Step 14, the constant Vocv determined by the stack type and fuel type, the coefficients α and β of the DC / AC conversion characteristics in the inverter, and the auxiliary power during power generation From Pbop, the target DC output current Idc (n + 1) is calculated using the target DC output current calculation formula.

  22. Step 15 (S15) Based on this target DC output current Idc (n + 1), the target fuel gas flow rate is determined from FIG. 4 (a), the target air flow rate is determined from FIG. 4 (b), and FIG. The target reforming water flow rate is determined from c).

  23. Step 16 (S16): It is determined whether ΔP is positive or negative. Step 19 (S19) or step 17 (S17) is determined depending on whether it is positive or negative.

  24. Step 17 (S17): When ΔP is negative, it means that the power load is greatly reduced. In order to follow the power generation output, it is necessary to reduce the current and various gas flow rates. Therefore, in order to avoid a fuel exhaustion state in the cell stack, the DC output current is first reduced, and then the flow rates of fuel gas, air, and reformed water are reduced. Since the target DC output current has already been calculated in step 14, this current value is instructed to the DC converter connected to each cell stack to change the power generation output. Specifically, the DC output currents Idc1 to Idc4 of each cell stack are updated to Idc (n + 1).

  25. Step 18 (S18): After reducing the DC output current in Step 17, the flow rates of various gases are reduced. Since the target flow rates of various gases have already been calculated in step 15, so that the fuel gas flow rate, air flow rate, and reforming water flow rate become the target fuel gas flow rate, target air flow rate, and target reforming water flow rate, respectively. Give instructions to auxiliary equipment. After completing the flow of current, fuel gas, air, and reforming water to the auxiliary equipment, return to Step 1.

  26. Step 19 (S19): When ΔP is positive, it means that the power load is greatly increased, and in order to follow the power generation output, it is necessary to increase the current and various gas flow rates. Therefore, in order to avoid a fuel exhaustion state in the stack, first, the flow rate of fuel gas, air, or reformed water is increased, and then the DC output current is increased. Since the target flow rates of various gases have already been calculated in step 15, so that the fuel gas flow rate, air flow rate, and reforming water flow rate become the target fuel gas flow rate, target air flow rate, and target reforming water flow rate, respectively. Give instructions to auxiliary equipment.

  27. Step 20 (S20): An instruction to increase the flow rates of various gases is given in Step 19, but the flow rates of the fuel gas, air, and reformed water actually change until these gases are sufficiently distributed to the cell stack. In general, some time difference occurs. If the DC output current is increased before a sufficient amount of gas reaches the cell stack, a fuel depleted state occurs in the stack. Therefore, the flow rate and supply pressure of fuel gas, air, and reforming water are confirmed, and an instruction to increase the DC output current is put on standby until it sufficiently increases. If it can be confirmed that the flow rate and supply pressure of the fuel gas, air, and reforming water have increased sufficiently, the process proceeds to step 21, and if the increase in the flow rate and pressure is not sufficient even after waiting for a certain time (for example, 5 seconds), The power generation output increase in this cycle is abandoned, and the process returns to step 1 without increasing the DC output current.

  28. Step 21 (S21): After confirming that the flow rate and supply pressure of fuel gas, air, and reforming water have increased sufficiently in step 20, the DC output current is increased. Since the target DC output current has already been calculated in step 14, this current value is instructed to the DC converter connected to each cell stack to change the power generation output. Specifically, the DC output currents Idc1 to Idc4 of each cell stack are updated to Idc (n + 1). After updating the DC output current, return to Step 1.

S1 to S21 Step 1 to Step 21

Claims (4)

  A fuel cell operation control method based on power load tracking, wherein actual power generation output is Pac (n), target power generation output is Pac (n + 1), feedback control using actual power generation output Pac (n), target An operation control method for a fuel cell, which performs power load tracking in combination with feedforward control using a power generation output Pac (n + 1).   A fuel cell operation control method based on power load tracking, in which the actual power generation output in the fuel cell is Pac (n), the target power generation output is Pac (n + 1), the actual power generation output Pac (n) and the target power generation output When the difference of Pac (n + 1) is ΔP and a certain threshold value P1 is defined for ΔP, feedback control is performed when the absolute value of ΔP is less than or equal to P1, and feedforward control is performed when the absolute value of ΔP is greater than P1. A method for controlling the operation of a fuel cell by following a power load.   A fuel cell operation control system based on power load, wherein actual power generation output is Pac (n), target power generation output is Pac (n + 1), feedback control using actual power generation output Pac (n), target An operation control system for a fuel cell, characterized in that power load tracking is performed in combination with feedforward control using a power generation output Pac (n + 1). An operation control system for a fuel cell by following an electric power load, in which the actual power output is Pac (n), the target power output is Pac (n + 1), the actual power output Pac (n) and the target power output When the difference of Pac (n + 1) is ΔP and a certain threshold value P1 is defined for ΔP, feedback control is performed when the absolute value of ΔP is less than or equal to P1, and feedforward control is performed when the absolute value of ΔP is greater than P1. An operation control system for a fuel cell by following a power load, characterized by comprising:

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