JP2012178286A - Fuel cell system, operation method of fuel cell and method for estimating dryness degree of electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a fuel cell system capable of suppressing dryness of an electrolyte more suitably as compared with a conventional system, an operation method of a fuel cell and a method for estimating a dryness degree of the electrolyte.SOLUTION: A supply flow rate of hydrogen gas to be supplied to a fuel cell is temporarily increased (step S240). Then whether a temporary change (ΔP) of anode pressure loss when the supply flow rate of hydrogen gas is temporarily increased is zero or more is determined (step S250). When the ΔP is positive, the ΔP is detected (step S250, YES) and cathode back-pressure is set to P2 (step S260) to intensify dryness suppression of an electrolyte membrane.

Description

  The present invention relates to a fuel cell.

  Solid polymer fuel cells (hereinafter simply referred to as “fuel cells”) employ protons as charge carriers and solid polymer electrolyte membranes (hereinafter simply referred to as “electrolyte membranes”) as electrolytes that conduct protons. In many cases, the power generation capacity decreases unless the electrolyte membrane contains an appropriate amount of moisture. Therefore, as a technique for controlling the amount of water in the electrolyte membrane, the pressure loss (hereinafter also referred to as “anode pressure loss”) caused by the passage of fuel gas through the fuel cell increases the temperature (cell temperature) of the fuel cell. On the other hand, when it falls rapidly, it is estimated that "the electrolyte membrane was dried", and the technique about suppressing the drying of an electrolyte membrane based on this estimation (drying suppression) is disclosed (patent document 1). .

JP 2009-9891 A

  The prior art has a problem that the suppression of drying of the electrolyte membrane is not necessarily appropriate. This is because the change in the anode pressure loss with respect to the change in cell temperature may not be strongly correlated with the degree of drying of the electrolyte membrane, and the estimation that “the electrolyte membrane is dried” based on the above change is not accurate. Because it is possible.

  SUMMARY OF THE INVENTION In order to solve the above-described problems of the prior art, an object of the present invention is to provide a fuel cell system, a fuel cell operating method, and an electrolyte drying degree estimation method that can more appropriately suppress electrolyte drying.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

  Application Example 1 A fuel cell system that generates power using a fuel cell having an electrolyte that conducts a charge carrier, and that temporarily increases the supply flow rate of the fuel gas supplied to the fuel cell. And a change in pressure loss that occurs when the fuel gas passes through the fuel cell, based on a temporary change that is a change when the supply flow rate is temporarily increased by the supply flow rate increasing unit, A fuel cell system comprising a drying suppression unit that suppresses drying of the electrolyte.

  According to this application example, it is possible to suppress the drying of the electrolyte more appropriately than in the past. This is because the temporary change in the anode pressure loss when the supply flow rate is temporarily increased has a stronger correlation with the degree of drying of the electrolyte than the change in the anode pressure loss with respect to the change in the cell temperature.

  Application Example 2 The fuel cell system according to Application Example 1, wherein the drying suppression unit strongly suppresses drying as the temporary change value increases.

  When the anode pressure loss increases as the electrolyte is dried, the configuration as in this application example can be employed. Note that the value of the change can take a positive or negative value, and the magnitude comparison of values is performed in consideration of positive and negative rather than absolute value comparison.

  Application Example 3 The fuel cell system according to Application Example 1 or 2, wherein the drying suppression unit suppresses drying more strongly when the value of the temporary change is positive than when it is negative.

  According to this application example, the criterion for determining whether or not the drying suppression is to be strong is set at the zero point of the temporary change, so that the determination of the case classification by the drying suppression unit is simplified.

  Application Example 4 A detection unit that detects, based on a physical quantity acquired for operating the fuel cell, that the drying should be suppressed more weakly than when the value of the temporary change is positive, When the detection unit detects that drying should be suppressed more weakly than when the value of the temporary change is positive, the supply flow rate increase unit does not operate, and the drying suppression unit The fuel cell system according to application example 3 that suppresses drying more weakly than when the value of the temporary change is positive.

  According to this application example, the number of times of temporarily increasing the fuel gas supply flow rate can be reduced. The supply flow rate of the fuel gas is preferably set to a value required based on the operating state of the fuel cell, and can be brought close to the desired state by reducing the number of times of temporary increase.

  Note that the physical quantity that is used as a detection criterion by the detection unit may be, for example, the temperature of the fuel cell, the electrical resistance value of the fuel cell, and the change in anode pressure loss over time. It is desirable to set a standard for these physical quantities so that it is possible to estimate that “they should be suppressed more weakly than when the pressure loss increases” in consideration of the influence other than the degree of drying of the electrolyte.

  Application Example 5 When the detection unit detects that the drying suppression unit is weaker than the case where the value of the temporary change is positive and the drying should be suppressed, the temporary change of the The fuel cell system according to application example 4, which suppresses drying with the same strength as when the value is negative.

  According to this application example, “when the value of the temporary change is weaker than when the value is positive, drying should be suppressed”, when determined by the drying suppression unit, and when determined by the determination unit Since the drying is suppressed with the same strength, the configuration can be simplified.

The same effects as those of the application example described above can also be obtained by the following method.
[Application Example 6] A fuel cell operating method for generating power using a fuel cell having an electrolyte that conducts charge carriers, which is a change in pressure loss caused when fuel gas passes through the fuel cell, A method of operating a fuel cell that suppresses drying of the electrolyte based on a change when the supply flow rate of the fuel gas is temporarily increased.

  Application Example 7 A method for estimating the degree of dryness of an electrolyte used in a fuel cell to conduct charge carriers, which is a change in pressure loss that occurs when the fuel gas passes through the fuel cell. A method for estimating the degree of dryness of the electrolyte, which estimates the degree of dryness of the electrolyte based on a change when the supply flow rate of the battery is temporarily increased.

1 is a schematic diagram of a fuel cell vehicle 20. The flowchart which shows a drying suppression process. The graph which shows a time-dependent change of the cathode back pressure, cell temperature, anode pressure loss, and internal resistance value of a fuel cell by execution of a drying suppression process.

  An embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram schematically showing a fuel cell vehicle 20 to which the present invention is applied.

[1. Hardware configuration]
As shown in the figure, in the fuel cell vehicle 20, a fuel cell system 30 and a motor 170 for driving the front wheels of the fuel cell vehicle 20 are mounted on a vehicle body 22. The fuel cell system 30 is for supplying electric power to the front-wheel drive motor 170 and the like, and includes a fuel cell 100, a hydrogen gas supply system 120, an air supply system 140, a cooling system 160, a secondary battery 172, and a DC. A DC converter 174 is provided.

  The fuel cell 100 is configured by laminating a cell including a membrane electrode assembly (MEA) in which both electrodes of an anode and a cathode are joined to both sides of an electrolyte membrane, and includes a front wheel FW and a rear wheel RW. The vehicle body 22 is disposed under the floor. The fuel cell 100 generates power by an electrochemical reaction between hydrogen supplied from the hydrogen gas supply system 120 and oxygen in the air supplied from the air supply system 140, and drives the motor 170 with the generated power. The current generated by the power generation of the fuel cell 100 is measured by the current sensor 102, and the measurement result is output from the current sensor 102 to the control device 200 described later.

  The hydrogen gas supply system 120 includes a hydrogen gas tank 110, a hydrogen supply path 121, a supply opening / closing valve 124, a pressure reducing valve 125, and a hydrogen supply device 126. The hydrogen gas tank 110 stores hydrogen gas therein, and the hydrogen supply path 121 is a gas path that connects the hydrogen gas tank 110 and the fuel cell 100. On the other hand, the hydrogen supply device 126 supplies the hydrogen gas in the hydrogen gas tank 110 to the fuel cell 100 via the hydrogen supply path 121. The supply opening / closing valve 124 has a function of opening / closing the hydrogen supply path 121, and the pressure reducing valve 125 has a function of reducing the pressure in the hydrogen supply path 121.

  The hydrogen gas supply system 120 further includes a hydrogen gas circulation path 122, a discharge path 123, a hydrogen gas circulation pump 127, and a hydrogen gas flow rate sensor 128. The discharge path 123 is a path for releasing hydrogen gas (anode off gas) that has not been consumed by the fuel cell 100 to the atmosphere. On the other hand, the hydrogen gas circulation path 122 is a path for circulating the anode off-gas back to the hydrogen supply path 121, and this circulation is performed by the hydrogen gas circulation pump 127. The hydrogen gas flow rate sensor 128 is a sensor that measures the flow rate of the circulating hydrogen gas, and the discharge opening / closing valve 129 is a valve that opens and closes a path between the hydrogen gas circulation path 122 and the discharge path 123. The discharge path 123 is also used as a path for discharging air in the air supply system 140 described later.

  The hydrogen gas supply system 120 is further provided with a pressure loss sensor 190 in the hydrogen gas path. The pressure loss sensor 190 measures the pressure difference (anode pressure loss) caused by passing through the fuel cell 100 with respect to the pressure difference immediately before and after the supply to the fuel cell 100 with respect to the hydrogen gas.

  On the other hand, the air supply system 140 includes a compressor 130, an air supply path 141, a discharge flow rate adjustment valve 143, a humidifier 145, and an air flow rate sensor 147. The air supply system 140 is a path connecting the atmosphere and the cathode of the fuel cell 100, and this path supplies the air taken in from the atmosphere by the compressor 130 to the fuel cell 100 through the air supply path 141. The humidifier 145 is disposed between the compressor 130 and the fuel cell 100 on the air supply path 141, and humidifies the air supplied to the fuel cell 100. On the other hand, the discharge flow rate adjusting valve 143 is disposed on a path connecting the hydrogen gas supply system 120 and the discharge path 123, and discharge pressure (cathode back pressure) of air (cathode off-gas) not consumed by the fuel cell 100. And a valve for adjusting the discharge amount.

  The air discharged from the discharge flow rate adjusting valve 143 flows into the discharge path 123 described above through the humidifier 145. This humidifier 145 is configured as a gas-liquid separator. That is, the humidifier 145 separates the moisture from the cathode off gas, mixes the separated moisture with the air in the air supply path 141, and increases the amount of moisture to be separated and mixed as the cathode back pressure increases. It is configured as follows. Therefore, the humidity of the air supplied to the fuel cell 100 can be adjusted by adjusting the cathode back pressure by the discharge flow rate adjusting valve 143. The humidity adjustment of the supply air is used in the drying suppression control described later.

  On the other hand, the cooling system 160 includes a radiator 150, a cooling water circulation path 161, a bypass 162, a three-way flow rate adjustment valve 163, a cooling water circulation pump 164, and a temperature sensor 166. The cooling water circulation path 161 is a path for circulating cooling water between the fuel cell 100 and the radiator 150, and this circulation is performed by the cooling water circulation pump 164. The cooling water circulating in this way absorbs heat in the fuel cell 100 and dissipates heat in the radiator 150, thereby cooling the fuel cell 100 (reducing the cell temperature). The bypass 162 is a path for allowing the coolant that has flowed out of the fuel cell 100 to flow into the fuel cell 100 again without passing through the radiator 150. On the other hand, the three-way flow rate adjustment valve 163 is a valve that adjusts the flow rate of the cooling water passing through the bypass 162.

  Next, the electric system will be described. In addition to the fuel cell 100, the electrical system includes the secondary battery 172 and the DC-DC converter 174 described above. The secondary battery 172 is connected to the fuel cell 100 via the DC-DC converter 174, and can charge the power generated by the fuel cell 100 or supply the charged power to other devices. A remaining capacity detection sensor 176 is connected to the secondary battery 172. The DC-DC converter 174 performs power generation control of the fuel cell 100, charge / discharge control of the secondary battery 172, and voltage application to the motor 170.

  The control device 200 includes a so-called microcomputer provided with a CPU, ROM, RAM, and the like. The control device 200 acquires signal inputs from the accelerator 180 and the various sensors described above, and controls the DC-DC converter 174 and other various valves and devices as described above based on the acquired signals. Thus, the fuel cell system 30 is operated.

[2. Drying suppression process]
The above-described various controls by the control device 200 are realized by executing corresponding processes (programs), but description of known processes is omitted, and the drying suppression process will be described below. The drying suppression process is a process that is repeatedly executed mainly by the control device 200 while the power generation by the fuel cell system 30 is being performed with the aim of suppressing the drying of the electrolyte membrane that constitutes a part of the fuel cell 100. .

  Hereinafter, the drying suppression process will be described with reference to FIGS. 2 and 3. FIG. 2 is a flowchart showing the drying suppression process, FIG. 3A is a graph showing the change over time in the cathode back pressure, and FIG. 3B shows the change over time in the anode pressure loss, the cell temperature, and the internal resistance value of the fuel cell 100. It is a graph. Each graph shown in FIG. 3 schematically shows an example in which drying is suppressed by executing a drying suppression process after the cell temperature has increased due to high-speed climbing of the fuel cell vehicle 20 and the electrolyte membrane has been dried. Is.

  First, the cathode back pressure is set to the pressure P1 by adjusting the valve opening degree of the discharge flow rate adjusting valve 143 (step S210) (see FIG. 3A). When the cathode back pressure is the pressure P1, normal drying suppression (weak drying suppression) is performed. Next, it is determined whether both “cell temperature is less than threshold Th (T)” and “internal resistance value is less than threshold Th (R)” are satisfied (step S220). The internal resistance value is obtained from the voltage applied to the fuel cell 100 by the DC-DC converter 174 and the current value acquired from the current sensor 22. Note that the threshold Th (T) and the threshold Th (R) are desirably values that ensure that sufficient power generation capacity can be obtained by normal drying suppression, and the cell temperature and voltage-current characteristics in a normal operation state. It is determined based on (V-I characteristics) and the like.

  In step S220, whether the conditions of both the cell temperature and the internal resistance value are satisfied is determined in order to estimate whether or not the drying suppression should be strengthened by estimating the degree of drying of the electrolyte membrane. . If both of the above conditions are satisfied, the degree of drying of the electrolyte membrane is low, and it can be estimated that normal drying suppression is sufficient. Therefore, when the fuel cell 100 is operated at a normal load, if both the cell temperature and the internal resistance value continue to be less than the threshold value, the determination in step S220 is YES, and normal drying control is performed. Repeat (step S210). The state during this period is shown from time zero to t1 in FIG.

  On the other hand, if at least one of the cell temperature and the internal resistance value is equal to or higher than the threshold value (step S220, NO), the dry state of the electrolyte membrane is estimated more accurately and it is determined whether or not strong dry suppression should be entered. The process moves to

  When the traveling temperature of the fuel cell vehicle 20 shifts to a high load such as high-speed climbing or the like, the cell temperature or the internal resistance value becomes equal to or higher than the threshold value (step S220, NO), standby for a predetermined time (step S230). After that, the hydrogen supply device 126 is controlled to temporarily increase the supply flow rate of hydrogen gas (step S240). In this embodiment, the standby time is about 20 seconds, and the increase in the supply flow rate of hydrogen gas is 30% of the supply flow rate before the increase, and is performed for 2 seconds.

  Subsequently, it is determined whether or not the temporary change (ΔP) in the anode pressure loss when the hydrogen gas supply flow rate temporarily increases is greater than or equal to zero (step S250). When the supply flow rate of hydrogen gas temporarily increases, a temporary change appears in the anode pressure loss accordingly. This temporary change has a strong correlation with the degree of drying of the electrolyte membrane. Specifically, when ΔP is negative (temporary decrease in pressure loss), the electrolyte membrane is not dried, and when ΔP is positive (temporary increase in pressure loss), the electrolyte membrane is dried. Can be estimated.

  This relationship has been confirmed by experiments, but can be explained as follows. That is, it is considered that ΔP becomes negative when the hydrogen supply flow rate is temporarily increased because water droplets adhering to the hydrogen gas flow path are blown off and removed. That is, it is considered that sufficient moisture exists in the electrolyte membrane, and it is not necessary to increase the degree of drying of the electrolyte membrane.

  On the other hand, the reason why ΔP becomes positive is considered to be due to an increase in moisture contained in the hydrogen gas due to a drying suppression effect due to an increase in the flow rate of the hydrogen gas. That is, it is considered that the hydrogen gas is so dry that there is room for humidification to some extent, and thus the electrolyte membrane is dry.

  Therefore, if the determination in step S250 is NO, that is, if ΔP is negative, it is determined that normal drying suppression is sufficient, and the process returns to step S210 and the above steps are repeated. The state during this period is shown from time t1 to time t2 in FIG.

  On the other hand, when drying of the electrolyte membrane proceeds under conditions such as continuing high-load operation, it is desirable to increase drying suppression. In this case, the determination in step S250 is YES, and the cathode back pressure is set to pressure P2 (> pressure P1) by adjusting the valve opening of the discharge flow rate adjusting valve 143 (step S260). When the cathode back pressure is increased, the amount of humidification by the humidifier 145 is increased, and strong drying suppression is executed.

  After step S260, the process returns to step S230, and steps S230 to S260 are repeated for a period in which ΔP is zero or more. That is, the cathode back pressure is maintained at the pressure P2, and strong drying suppression is continued. The state during this period is shown from time t2 to time t3 in FIG.

  Thereafter, when drying of the electrolyte membrane is suppressed by strong drying suppression or a decrease in cell temperature, and ΔP turns negative, this is detected (step S250, NO), and the process returns to step S210. That is, the cathode back pressure is set to the pressure P1, and then it is determined whether the cell temperature and the internal resistance value are less than the threshold (step S220). Therefore, during the period in which the cell temperature or the internal resistance value is equal to or higher than the threshold value (step S220, NO) and ΔP is negative (step S250, NO), the supply flow rate of hydrogen gas continues to increase temporarily, and the anode pressure loss against this increases. Normal drying suppression is executed while monitoring the temporary change (ΔP). The state during this period is shown from time t3 to time t4 in FIG.

  Thereafter, when the cell temperature and the internal resistance value decrease and fall below the threshold value due to the end of the high-speed climbing of the fuel cell vehicle 20, this is detected (step S220, YES), and the process returns to step S210. Therefore, since the process after step S230 is not performed, the supply flow rate of hydrogen gas is not temporarily increased while continuing normal drying suppression. The state during this period is shown after time t4 in FIG. This figure shows an example in which the cell temperature has previously fallen below the threshold Th (T), and then the internal resistance value has fallen below the threshold Th (R) at time t4.

  As explained above, the degree of drying of the electrolyte membrane is estimated using the behavior of the anode pressure loss when the hydrogen gas is temporarily increased, and the drying is suppressed based on the estimation. It is possible to control the strength of drying suppression.

  Further, since the determination and processing (steps S230 to S260) for strong drying suppression is started based on the conditions for the cell temperature and the internal resistance value, a temporary increase in the hydrogen gas supply flow rate or detection of anode pressure loss is detected.・ There is no need to always make decisions. That is, a rough estimation is made using the cell temperature and the internal resistance value, which are physical quantities acquired for the operation of the fuel cell 100, and based on the estimation, the hydrogen gas supply amount is temporarily increased and the accuracy is increased. By making a good estimate, it has a structure that takes advantage of both advantages.

  Further, since the hydrogen gas supply flow rate is temporarily increased every predetermined time (step S230), the hydrogen gas supply amount is not increased unnecessarily. In addition, by performing after a predetermined time, the influence of the temporary increase in the hydrogen gas supply flow rate on each other is reduced, and the correlation between ΔP and the degree of drying of the electrolyte membrane can be maintained. In addition, since the determination criterion for ΔP is simple such as zero or more or negative, the determination is simple and reliable.

[3. Correspondence between embodiment and application example]
Steps S210, S250, and S260 correspond to software for realizing the drying suppression unit, step S220 as the detection unit, and step S240 as software for realizing the supply flow rate increasing unit.

[4. Other Embodiments]
The present invention is not limited to the embodiment described above, and can be implemented in various forms within the scope not departing from the gist of the invention. For example, additional components in the embodiment can be omitted from the embodiment. The additional components referred to here are elements corresponding to matters not specified in the substantially independent application example. In addition, for example, the following embodiments can be considered.

-Any of the following may be sufficient as the method of drying suppression. (A) Humidifying hydrogen gas (b) Humidifying both cathode gas and hydrogen gas (c) Increasing the supply amount of hydrogen gas (d) Decreasing the supply pressure of hydrogen gas (e) Decreasing the supply amount of air (F) Increasing the supply pressure of air (g) The cooling system 160 increases the degree of cooling.

-The application is not limited to being mounted on an automobile, but may be anything such as mounting on a transport device such as a two-wheeled vehicle or a home generator.
-There are various ways to control the drying suppression based on ΔP. For example, when ΔP is zero, normal drying suppression may be performed. Or the reference | standard for switching the strength of drying suppression may be a positive value or a negative value instead of the zero point as in the embodiment. Alternatively, a plurality of determination criteria may be prepared, and drying suppression to the extent corresponding to the magnitude of ΔP may be performed by switching the strength of drying suppression in three or more stages. Specifically, the drying suppression strength may be determined based on a phenomenon in which ΔP becomes smaller as drying progresses further after ΔP becomes positive.

-It is also conceivable that the relationship between ΔP and the degree of drying of the electrolyte membrane differs from that in the embodiment by using other than hydrogen as the fuel gas. In such a case, the relationship between ΔP and the strength of drying suppression may be appropriately determined according to the actual situation.

In the above-described embodiment, after the determination that the cell temperature or the internal resistance value is less than the threshold value (step S220, NO) and after the determination that ΔP is negative (step S250, NO), the cathode back pressure is all reduced. Although weak drying suppression is performed by setting to the pressure P1, drying suppression may be performed with different strengths.
-Step S220 may be omitted and the hydrogen gas supply flow rate may be temporarily increased whenever the fuel cell is in operation.

DESCRIPTION OF SYMBOLS 20 ... Fuel cell vehicle 22 ... Car body 30 ... Fuel cell system 100 ... Fuel cell 102 ... Current sensor 110 ... Hydrogen gas tank 120 ... Hydrogen gas supply system 121 ... Hydrogen supply path 122 ... Hydrogen gas circulation path 123 ... Release path 124 ... For supply Open / close valve 125 ... Pressure reducing valve 126 ... Hydrogen supply device 127 ... Hydrogen gas circulation pump 128 ... Hydrogen gas flow rate sensor 129 ... Release open / close valve 130 ... Compressor 140 ... Air supply system 141 ... Air supply path 143 ... Discharge flow rate adjustment valve 145 ... Humidifier 147 ... Air flow sensor 150 ... Radiator 160 ... Cooling system 161 ... Cooling water circulation path 162 ... Bypass 163 ... Three-way flow control valve 164 ... Cooling water circulation pump 166 ... Temperature sensor 170 ... Motor 172 ... Secondary battery 174 ... DC-DC Converter 176 ... Remaining capacity detection sensor 180 ... Accelerator 190 ... Pressure loss sensor 200 ... Control device

Claims (7)

A fuel cell system that generates power using a fuel cell having an electrolyte that conducts charge carriers,
A supply flow rate increase unit for temporarily increasing the supply flow rate of the fuel gas supplied to the fuel cell;
Based on the change in pressure loss that occurs when the fuel gas passes through the fuel cell, and the change that occurs when the supply flow rate is temporarily increased by the supply flow rate increasing unit, the electrolyte A fuel cell system comprising a drying suppression unit that suppresses drying of the fuel. The fuel cell system according to claim 1, wherein the drying suppression unit strongly suppresses drying as the temporary change value increases. The fuel cell system according to claim 1, wherein the drying suppression unit suppresses drying more strongly when the temporary change value is positive than when it is negative. A detection unit for detecting, based on a physical quantity acquired for operating the fuel cell, that the drying should be suppressed more weakly than when the value of the temporary change is positive;
When the detection unit detects that drying should be suppressed more weakly than when the value of the temporary change is positive, the supply flow rate increase unit does not operate, and the drying suppression unit The fuel cell system according to claim 3, wherein drying is suppressed more weakly than when the value of the temporary change is positive. When the detection unit detects that the drying suppression unit is weaker than the case where the temporary change value is positive and the drying should be suppressed, the temporary change value is negative. The fuel cell system according to claim 4, wherein drying is suppressed with the same strength as the case. A method of operating a fuel cell that generates electricity using a fuel cell having an electrolyte that conducts charge carriers,
A change in pressure loss that occurs when fuel gas passes through the fuel cell, and suppresses drying of the electrolyte based on a change when the supply flow rate of the fuel gas is temporarily increased. how to drive. A method for estimating the degree of drying of an electrolyte used in a fuel cell to conduct charge carriers,
The degree of dryness of the electrolyte is estimated based on the change in pressure loss that occurs when the fuel gas passes through the fuel cell and when the supply flow rate of the fuel gas is temporarily increased. Drying degree estimation method.

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