Method for estimating the performance of a fuel cell

Abstract

A method for estimating a performance of a fuel cell, comprising: Estimating a prediction current at a predetermined voltage in a control unit based on a present current-voltage characteristic of a fuel cell while increasing a temperature of the fuel cell; estimating a first power based on the estimated prediction current and the predetermined voltage after the prediction current estimation step; Estimating a second power based on a cell voltage ratio while estimating the first power; and Calculating an available power of the fuel cell based on the first power and the second power after the estimation step of the first power and the estimation step of the second power have been performed.

Description

BACKGROUND1. Area

[0001] The present invention relates to a method for estimating a power of a fuel cell, which estimates in real time an available power of the fuel cell to determine whether a cold start can be completed while increasing the temperature of the fuel cell for the cold start. 2. Description of the state of the art

[0002] The statements in this section merely provide background information concerning the present disclosure and cannot constitute prior art.

[0003] When water inside a fuel cell freezes due to low temperature, a temperature raising process is necessary to ensure fuel cell performance.

[0004] Fig. 1 is a graph illustrating a variation in power output of a fuel cell as the temperature of the fuel cell is increased according to the prior art. With reference to Fig. 1. At the beginning of the temperature-raising process, the fuel cell stack under a low temperature condition outputs the power shown by the points on line 1. During the temperature-raising process, the stack voltage gradually increases at a nearly constant current as shown by line 2. Then, when the temperature-raising process is completed, the power is represented by the points on line 3. In other words, when the fuel cell with the stack power shown by line 1 cannot output the power required to drive a vehicle, the temperature-raising process continues until the fuel cell reaches the stack power shown by line 3.

[0005] However, the fuel cell's power during the temperature increase process is low compared to the power required to power a vehicle. This is because the stack's power drops below the voltage conditions required for normal operation of high-voltage components when current is applied to power the vehicle.

[0006] In other words, the temperature-boosting operation must continue until the fuel cell output reaches the power required to operate the vehicle. However, since the technology for detecting the degree of temperature rise or available fuel cell power during the temperature-boosting operation is insufficient, ignition start may be excessively delayed until the temperature is sufficiently elevated.

[0007] The foregoing is merely intended to aid in understanding the background of the present disclosure and is not intended to place the present disclosure within the scope of the prior art already known to those skilled in the art.

[0008] DE 10 2009 007 168 A1 relates to a method for calculating the maximum net power for a fuel cell system based on an online polarization curve estimation. In particular, an algorithm is described for determining the maximum net power available from a fuel cell stack as the stack degrades over time, using an adaptive online estimation of a stack's polarization curve.

[0009] KR 10 2013 0 124 790 A relates to an operating method for a fuel cell system, and in particular to an operating method for pressurizing a fuel cell system that can improve the efficiency of a fuel cell system and maintain the voltage stability of a stack. In particular, a cell stack is evaluated by considering cell voltages. SUMMARY

[0010] Accordingly, the present disclosure has been made in consideration of the above problem, and an object of the present disclosure is to provide a method for estimating a power of a fuel cell for minimizing an ignition start delay, which enables completion of a cold start at an appropriate time by estimating an available power of the fuel cell based on a current-voltage characteristic and a cell voltage ratio while raising the temperature of the fuel cell for the cold start.

[0011] To achieve the above object, a method for estimating a power of a fuel cell according to the present invention may include: estimating a prediction current at a predetermined voltage in a control unit based on a current current-voltage characteristic of a fuel cell while increasing a temperature of the fuel cell; estimating a first power based on the estimated prediction current and the predetermined voltage after estimating the prediction current; estimating a second power based on a cell voltage ratio while estimating the first power; and calculating an available power of the fuel cell based on the first power and the second power after estimating the first power and estimating the second power are performed.

[0012] Further, determining whether an engine start is a cold start is included before estimating the prediction current, and if it is determined that the engine start is a cold start, estimating the prediction current may be performed after a temperature increase operation of the fuel cell is performed.

[0013] The predetermined voltage is a reference voltage suitable for operating high-voltage components of a vehicle.

[0014] The prediction current is estimated based on a current-voltage gradient predetermined by the existing current-voltage characteristic of the fuel cell while the temperature of the fuel cell is increased.

[0015] The cell voltage ratio is a ratio of a lowest voltage of several cells to an average voltage of the several cells and increases with an increase in a voltage of a cell outputting the lowest voltage.

[0016] The second power increases with an increase in the cell voltage ratio.

[0017] The second power is estimated as a positive value when the cell voltage ratio is equal to or greater than a first reference value, as zero when the cell voltage ratio is equal to or greater than a second reference value and less than the first reference value, and as a negative value when the cell voltage ratio is less than the second reference value.

[0018] The available power of the fuel cell is a sum of the first power and the second power.

[0019] According to the method for estimating a power of a fuel cell configured as described above, when an available power of the fuel cell is estimated, a start of ignition can be completed at an appropriate time, whereby an ignition start delay can be minimized.

[0020] Likewise, if an available power of the fuel cell is estimated by considering a cell voltage ratio as well as a current-voltage characteristic while the temperature of the fuel cell is increased, the available power of the fuel cell can be accurately estimated to calculate the time when a start of the ignition is completed.

[0021] Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. FIGURES

[0022] In order that the disclosure may be well understood, various forms thereof will now be described, given by way of example, with reference to the accompanying figures, in which: Fig. 1 is a graph illustrating a variation in a power output of a fuel cell while increasing the temperature of the fuel cell, according to the prior art; Fig. 2 is a flowchart illustrating a method for estimating a performance of a fuel cell; Fig. 3 to 4 are graphs for illustrating a method for estimating a first power in a method for estimating a power of a fuel cell; and Fig. 5 is a graph illustrating a method for estimating a second power in a method for estimating the power of a fuel cell. Fig. 6 is a block diagram showing an apparatus for estimating a performance of a fuel cell. DETAILED DESCRIPTION

[0023] Hereinafter, a method for estimating a performance of a fuel cell according to an embodiment of the present invention will be described with reference to the accompanying figures. Fig. 2 is a flowchart illustrating a method for estimating a performance of a fuel cell according to an embodiment of the present disclosure; Fig. 3 to 4 are graphs for illustrating a method for estimating a first power in a method for estimating a power of a fuel cell according to an embodiment of the present disclosure; Fig. 5 is a graph illustrating a method for estimating a second power in a method for estimating a power of a fuel cell according to an embodiment of the present disclosure; and Fig. 6 is a block diagram illustrating an apparatus for estimating a performance of a fuel cell according to an embodiment of the present disclosure.

[0024] Referring to the Fig. 2 to 6, a method for estimating a power of a fuel cell may include: estimating a prediction current at a predetermined voltage in a control unit based on a current-voltage characteristic while increasing the temperature of the fuel cell (S120); estimating a first power based on the estimated prediction current and the predetermined voltage after estimating the prediction current S120 (S130); estimating a second power based on a cell voltage ratio while estimating the first power at S130 (S140); and calculating an available power of the fuel cell based on the first power and the second power after estimating the first power S130 and estimating the second power S140 are performed (S150).In this case, the predetermined voltage is a reference voltage suitable for driving high-voltage components of a vehicle, and the prediction current is estimated based on a current-voltage gradient predetermined by the existing current-voltage characteristic of the fuel cell while increasing the temperature of the fuel cell.

[0025] Here, as in Fig. 6, the control unit is connected to the fuel cell for sensing the present current-voltage characteristic of a fuel cell while increasing a temperature of the fuel cell, thereby estimating the prediction current, the first power, the second power, and the available power of the fuel cell in the predetermined voltage.

[0026] First, before estimating the prediction current (S120) by the control unit, a determination is made as to whether an engine start is a cold start (S100). If it is determined in S100 that the engine start is a cold start, an estimation of the prediction current (S120) may be performed after a temperature increase operation of the fuel cell (S110) is performed.

[0027] At S100, whether the engine start is a cold start is determined based on the coolant temperature sensed by the control unit via a temperature sensor. If the coolant temperature is equal to or below a predetermined temperature, since it is determined that the engine is in a cold start condition, the temperature increase operation in which hydrogen and air are supplied to the fuel cell is executed (S110). On the other hand, if the coolant temperature is above the predetermined temperature, since it is determined that no cold start condition exists, a normal start operation is executed (S105).

[0028] According to the prior art, the performance of a fuel cell changes depending on the temperature increase process during a cold start. At the beginning of ignition start, a low voltage-to-current ratio is output. Afterward, the fuel cell's characteristic changes to output a high voltage at a certain current, and the voltage-to-current ratio is output at a high level upon completion of the temperature increase process.

[0029] Referring to Fig. 3. When the engine is in a cold start state, a change in the performance of the fuel cell stack corresponding to the temperature increase process is checked by the control unit. A bottom line indicates the performance of the fuel cell stack at the beginning of the ignition start, and a top line indicates the performance of the fuel cell stack at the end of the temperature increase process. There is a difference in absolute voltage level between the two lines, but the lines are similar to each other in terms of the voltage gradient, which changes with the increase or decrease of the voltage. The present invention sets the gradient as a predetermined current-voltage gradient.

[0030] Consequently, the present invention can improve the performance curve defined by the line in the center of Fig. 3, by applying the current-voltage gradient, which is predetermined by the current current and voltage, estimating while raising the temperature of the fuel cell. In this case, the predicted current that can output the predetermined voltage corresponding to the current power of the fuel cell can be estimated from the power curve. The first power of the available power of the fuel cell performing the temperature-raising operation can be estimated by multiplying the estimated predicted current by the predetermined voltage.

[0031] Fig. Fig. 4 shows a variation in a performance of the fuel cell in which the temperature increase operation is longer than in the embodiment of Fig. 3. In the power curve of the fuel cell, which is shown in the center of Fig. 4, the voltage-to-current ratio is compared to Fig. 3 higher. Likewise, since the prediction current estimated from the predetermined voltage is higher, the fuel cell stack can output the predetermined voltage corresponding to the predetermined voltage corresponding to the prediction current.

[0032] Additionally, the present invention can improve the accuracy of the available power of the fuel cell by estimating the second power based on the cell voltage ratio. Here, the cell voltage ratio is a ratio of the lowest voltage among multiple cells to the average voltage of the multiple cells, and the cell voltage ratio increases as the voltage of a fuel cell outputting the lowest voltage increases. Likewise, the second power increases as the cell voltage ratio is higher.

[0033] In other words, if the fuel cell is frozen at the beginning of the temperature-raising process, there may be a cell with relatively high performance and a cell with relatively low performance due to uneven freezing. When the temperature-raising process is performed, large amounts of heat are generated in the low-performance cell, and the cell's performance is steadily restored to normal. At the end of the temperature-raising process, the cell voltage ratio is close to 1.

[0034] With reference to Fig.5. If the cell voltage ratio is equal to or greater than a first reference value, the second power is estimated as a positive value; if the cell voltage ratio is equal to or greater than a second reference value and less than the first reference value, the second power is estimated as zero (0); and if the cell voltage ratio is less than the second reference value, the second power is estimated as a negative value. If the practically estimated first power is increased or decreased according to the cell voltage ratio, the accuracy of the first power may be reduced.

[0035] The available power of the fuel cell can be calculated by adding the first power and the second power by the control unit. In summary, the available power can be accurately estimated by adding the second power, which is obtained by considering cell voltage uniformity from the cell voltage ratio, to the first power estimated from the current-voltage characteristic of the fuel cell while increasing the temperature of the fuel cell.

[0036] According to the method for estimating a power of the fuel cell configured as the above description, it is possible to estimate the available power of the fuel cell, whereby starting of the ignition can be completed at an appropriate time and unnecessary delay can be minimized.

[0037] Likewise, when estimating the available power of the fuel cell, if a cell voltage ratio as well as the current-voltage characteristic is taken into account, the available power of the fuel cell can be determined more accurately, making it possible to calculate an appropriate termination time.

Claims

[1] A method for estimating a performance of a fuel cell, comprising: Estimating a prediction current at a predetermined voltage in a control unit based on a present current-voltage characteristic of a fuel cell while increasing a temperature of the fuel cell; estimating a first power based on the estimated prediction current and the predetermined voltage after the prediction current estimation step; Estimating a second power based on a cell voltage ratio while estimating the first power; and Calculating an available power of the fuel cell based on the first power and the second power after the estimation step of the first power and the estimation step of the second power have been performed. [2] The method according to claim 1, further comprising: Determining whether an engine start is a cold start before the prediction current estimation step, wherein, when it is determined that the engine start is a cold start, the prediction current estimation step is performed after a temperature increase operation of the fuel cell is performed. [3] The method of claim 1, wherein the predetermined voltage is a reference voltage suitable for operating high voltage components of a vehicle. [4] The method according to claim 1, wherein the prediction current is estimated based on a current-voltage gradient predetermined by the present current-voltage characteristic of the fuel cell while increasing the temperature of the fuel cell. [5] The method according to claim 1, wherein the cell voltage ratio is a ratio of a lowest voltage of a plurality of cells to an average voltage of the plurality of cells and increases with an increase in the voltage of the cell outputting the lowest voltage. [6] The method of claim 5, wherein the second power increases with an increase in the cell voltage ratio. [7] The method according to claim 5, wherein the second power is estimated as a positive value when the cell voltage ratio is equal to or greater than a first reference value, as zero when the cell voltage ratio is equal to or greater than a second reference value and less than the first reference value, and as a negative value when the cell voltage ratio is less than the second reference value. [8] The method of claim 1, wherein the available power of the fuel cell is a sum of the first power and the second power.

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