JP5646221B2 - Fuel cell system and operation method thereof - Google Patents

Description

  The present invention relates to a fuel cell system that realizes a rated operation state and a low-load operation state, and a method for operating the fuel cell system.

  A fuel cell is a device that generates electricity and heat by an electrochemical reaction between hydrogen in fuel gas and oxygen in air. In addition to automobile and portable applications, development of fuel cell systems for stationary use at home and the like is underway. A household fuel cell system is a cogeneration system that supplies electricity and hot water to users. That is, at the same time as supplying electricity obtained by power generation, the generated heat is recovered by heat exchange to warm the secondary cooling water and provide it as hot water.

  FIG. 5 is a graph showing a general fluctuation in daily household electric power demand and a general operation method of a household fuel cell system. Household power demand fluctuates with the time of day. In general, as shown by a solid line A in FIG. 5, the demand for power is high in the morning and evening, and the amount of electricity used is reduced in the daytime and at night. In the fuel cell system, the operation load is generally changed according to the power demand. As shown by the broken line B in FIG. 5, the fluctuation of the operating load is not continuous, but is performed by switching between the rated output and the operation at a constant low output of about 1/4 to 1/2 of the rated output. That is, when the power demand is equal to or higher than the rated output, the rated output operation is performed, and when the power demand is less than the rated output, the low load operation is performed at a constant low output of about 1/4 to 1/2 of the rated output.

  The cooling water introduced into the fuel cell stack is heated by heat generated by the fuel cell reaction. The high-temperature cooling water discharged from the fuel cell stack heats the secondary cooling water through the heat exchanger. The heated secondary cooling water is stored in a hot water storage tank and supplied to the home as hot water.

  During low-load operation, heat generated from the fuel cell stack is small, so the temperature of the cooling water coming out of the fuel cell stack is lower than that during rated operation. Therefore, the heat exchange efficiency in the above-mentioned heat exchanger is lower than that during rated operation. For example, in Patent Document 1, when the heat exchange efficiency is low, a bypass is provided on the fuel cell cooling water side to suppress the heat supply amount and to improve the heat utilization rate. In Patent Document 2, the flow rate of gas supplied to the fuel cell stack during low-load operation is reduced, the amount of water in the fuel cell is increased, and the fuel cell stack is protected and its characteristics are restored.

JP 2007-123032 A JP 2008-2108050 A

During low-load operation in the above-described fuel cell system, the heat exchange efficiency in the heat exchanger described above decreases, and the hot water supply capability decreases. In the technique disclosed in the above-mentioned Patent Document 1 or 2, the problem is that it does not become a measure for increasing the heat exchange efficiency. The present invention was made to solve the above-described problem, and It aims at improving the heat exchange efficiency at the time of low load operation.

In order to achieve the above object, one aspect of a fuel cell system according to the present invention includes a fuel cell stack, an air supply device that supplies air to the fuel cell stack, and an air flow rate that controls the air supply flow rate. In the fuel cell system comprising a control unit, when the ratio of the air consumption consumed by power generation of the fuel cell stack to the air supply flow rate is defined as an air utilization rate, the air flow control unit When the stack is operating in a normal state, the air utilization rate in the low load operation state where the fuel cell stack is operated at a predetermined output lower than the rated output is higher than the air utilization rate in the rated operation state. as such, it is intended to control so as to change the flow rate of air in accordance with the switching of the rated operating condition and the low load operating state, characterized by

Another aspect of the fuel cell system according to the present invention includes a fuel cell stack, a cooling water supply device that supplies cooling water to the fuel cell stack, and a cooling water flow rate control unit that controls the flow rate of the cooling water. The cooling water supply device includes a cooling water line that circulates the cooling water flowing through the fuel cell stack, and heat of the cooling water that is provided in the cooling water line and flows through the fuel cell stack. a heat exchanger which releases the waste heat recovery line for recovering the heat released from the heat exchanger, wherein the cooling water flow rate control unit, in the fuel cell stack is a predetermined lower than the rated output output flow rate of the cooling water in the low load operation state to drive the, so that less than the flow rate of the cooling water of the rated operating conditions, according to the switching of the rated operating condition and the low load operation state It serial and controls to change the flow rate of the cooling water, characterized by.

One aspect of the operation method of the fuel cell system according to the present invention is an operation method of a fuel cell system including a fuel cell stack and an air supply device that supplies air to the fuel cell stack, When the ratio of the air consumption consumed by the power generation of the fuel cell stack to the supply flow rate is defined as the air utilization rate, a predetermined output lower than the rated output when the fuel cell stack is operating in a normal state The air supply flow rate is changed according to the switching between the rated operation state and the low load operation state so that the air utilization rate during low load operation is higher than the air utilization rate during rated operation. It is characterized by controlling.

Another aspect of the operation method of the fuel cell system according to the present invention is an operation method of a fuel cell system including a fuel cell stack and a cooling water supply device that supplies cooling water to the fuel cell stack. The cooling water supply device includes a cooling water line that circulates the cooling water flowing through the fuel cell stack, a heat exchanger that is provided in the cooling water line and releases heat of the cooling water flowing through the fuel cell stack, A waste heat recovery line that recovers the heat released from the heat exchanger , and the cooling water flow rate during the low load operation that operates at a predetermined output lower than the rated output is the cooling water during the rated operation. Control is performed so as to change the flow rate of the cooling water in accordance with switching between the rated operation state and the low-load operation state so as to be less than the flow rate.

  ADVANTAGE OF THE INVENTION According to this invention, the heat exchange efficiency at the time of low load operation | movement of a fuel cell system can be improved.

1 is a system configuration diagram showing a first embodiment of a fuel cell system according to the present invention. It is a graph which shows the relationship between the supply air flow rate by the 1st Embodiment of this invention and a prior art, and a generated electric current. It is a graph showing the effect of the 1st Embodiment of this invention, Comprising: It is a graph which shows the relationship between cooling water exit temperature and cathode exit temperature, and an air utilization factor. It is a system configuration | structure figure which shows 2nd Embodiment of the fuel cell system which concerns on this invention. It is a graph showing the fluctuation | variation of the general household electric power demand one day, and the general operating method of a household fuel cell system.

  Hereinafter, embodiments of a method for operating a fuel cell system according to the present invention will be described with reference to the drawings.

[First Embodiment]
First, the first embodiment will be described with reference to FIG. FIG. 1 is a system configuration diagram showing a first embodiment of a fuel cell system according to the present invention.

  The fuel cell system according to the first embodiment includes a fuel cell stack 11, an air supply device 60 that supplies air to the fuel cell stack 11, and an air flow rate control unit 51. The air supply device 60 includes an air supply line 41 and an air blower 42. The supply air flow rate from the air blower 42 is adjusted by a signal from the air flow rate control unit 51.

  A cooling water line 21 that circulates cooling water is connected to the fuel cell stack 11. A heat exchanger 23 and a cooling water pump 22 are arranged in the cooling water line 21.

  The cooling water introduced into the fuel cell stack 11 by the cooling water line 21 absorbs heat generated by the fuel cell reaction in the fuel cell stack 11, becomes a high temperature, is discharged from the fuel cell stack 11, and enters the heat exchanger 23. be introduced. The high-temperature cooling water is heated by the heat exchanger 23 to the secondary cooling water, and then the pressure is raised by the cooling water pump 22 and sent to the fuel cell stack 11 again.

  A waste heat recovery line 31 that circulates the secondary cooling water is connected to the heat exchanger 23. A hot water storage tank 33 and a waste heat recovery line pump 32 are connected to the waste heat recovery line 31. The secondary cooling water is heated by the high-temperature cooling water from the fuel cell stack 11 in the heat exchanger 23, stored in the hot water storage tank 33, pressurized by the waste heat recovery line pump 32, and sent to the heat exchanger 23 again. It is done.

  The flow rate of air supplied to the fuel cell stack 11 is determined according to the generated current. The air flow rate necessary for power generation is obtained by the following equation (1).

(Air flow rate) = (Current) × (Number of cells) /4F×22.4/0.21/ (Air utilization rate)
[NL / sec] (1)
However, F is a Faraday constant, 22.4 [L / mol] is the volume [L] of 1 mol of ideal gases in a standard state, and 0.21 is the oxygen concentration in the air. The air utilization rate is a ratio of air consumed by power generation in the air supplied to the fuel cell stack 11. Since the current value is different between the rated output operation and the low load operation, the flow rate of air introduced into the fuel cell stack 11 is controlled by a signal from the air flow rate control unit 51 when the operation state is switched. As a method of controlling the air flow rate, there is a method of changing the opening degree of a valve (not shown) provided in the air supply line 41 in addition to a method of changing the rotational speed of the air blower 42.

  In the present embodiment, (the air utilization rate during rated operation) <(air utilization rate during low-load operation).

  FIG. 2 is a graph showing the relationship between the supply air flow rate and the generated current according to the first embodiment and the prior art. In the prior art, as indicated by the solid line C, the supply air flow rate is proportional to the generated current. The electrical output is given by the product of the generated current and the voltage. In the present embodiment, the air utilization rate is set to a value different from the rated operation. As shown at a point D in FIG. 2, the supply air flow rate is set to a value smaller than a value proportional to the generated current during the low load operation. That is, in the present embodiment, the operation is performed under the condition that there is less excess air than during the rated operation during the low load operation.

  The air utilization rate during rated operation is generally 30 to 60%. During operation of the fuel cell stack 11, water is generated at the cathode by a power generation reaction. The phenomenon in which the generated water hinders the diffusion of the reaction gas and the voltage decreases is called flooding. In general, operating at a low air utilization rate is to prevent flooding by increasing the amount of excess air and increasing the amount of oxygen supplied or increasing the amount of water produced. It is because it can do. The amount of generated water is proportional to the current flowing through the fuel cell stack 11. Therefore, during low load operation, the current is low, the amount of generated water is reduced, and flooding is unlikely to occur. Therefore, it is possible to stably operate at a high air utilization rate during low load operation and higher than during rated operation.

  In the present embodiment configured as above, the amount of heat taken out from the fuel cell stack 11 by air can be reduced. Therefore, the operating temperature of the fuel cell stack 11 can be kept high even during low load operation. Thereby, the temperature of the cooling water coming out of the fuel cell stack 11 can be kept high, and the heat recovery efficiency in the heat exchanger 23 can be increased.

  In particular, by setting the air utilization rate during low-load operation to 80 to 90%, it is possible to increase the effect of increasing the heat recovery efficiency. As described above, the air utilization rate during rated operation is generally 30 to 60%, but it is possible to increase the air utilization rate during low-load operation. As the air utilization rate is higher, the effect of keeping the temperature of the fuel cell stack 11 high by reducing the heat brought out by excess air can be obtained. On the other hand, if the air utilization rate exceeds 90%, there is a risk of voltage drop due to flooding even during low load operation. Therefore, it is desirable that the air utilization rate is 80 to 90%.

  FIG. 3 shows the effect of this embodiment. FIG. 3 is a graph showing the relationship between the cooling water outlet temperature from the fuel cell stack 11 (shown by the solid line E), the air exhaust gas temperature at the cathode outlet (shown by the dotted line F), and the air utilization rate. It can be seen that as the air utilization rate increases, both temperatures increase. Therefore, the effect of increasing the heat exchange efficiency of the fuel cell system as described above can be obtained.

[Second Embodiment]
Next, a second embodiment of the operation method of the fuel cell system according to the present invention will be described with reference to FIG. FIG. 4 is a system configuration diagram showing a second embodiment of the fuel cell system according to the present invention. In addition, the same code | symbol is attached | subjected to the structure same as 1st Embodiment, and the overlapping description is abbreviate | omitted.

  In the present embodiment, the flow rate of the cooling water introduced into the fuel cell stack 11 is reduced during low load operation. When shifting from the rated operation to the low-load operation, a signal is sent from the cooling water flow rate control unit 61 to the cooling water pump 22 to reduce the liquid feeding amount of the cooling water pump 22.

  If the coolant flow rate is reduced, the amount of heat taken out from the fuel cell stack 11 can be reduced, and the fuel cell stack 11 can be kept at a high temperature. At that time, the effect of keeping the temperature of the cooling water coming out of the fuel cell stack 11 high and keeping the heat exchange efficiency in the heat exchanger 23 high can be obtained.

[Other Embodiments]
Each embodiment described above is merely an example, and the present invention is not limited thereto. For example, the supply air flow rate control of the first embodiment and the cooling water flow rate control of the second embodiment may be combined. In this case, the air flow rate control unit 51 of the first embodiment and the cooling water flow rate control unit 61 of the second embodiment may be separate devices, but one control device has both control functions. May be.

DESCRIPTION OF SYMBOLS 11 ... Fuel cell stack 21 ... Cooling water line 22 ... Cooling water pump 23 ... Heat exchanger 31 ... Waste heat recovery line 32 ... Waste heat recovery line pump 33 ... Hot water storage tank 41 ... Air supply line 42 ... Air blower 51 ... Air flow rate Control unit 60 ... air supply device 61 ... cooling water flow rate control unit

Claims (5)

In a fuel cell system comprising a fuel cell stack, an air supply device that supplies air to the fuel cell stack, and an air flow rate control unit that controls the supply flow rate of the air,
When the ratio of the air consumption consumed by the power generation of the fuel cell stack to the air supply flow rate is defined as an air utilization rate, the air flow control unit is operated when the fuel cell stack is operating in a normal state. In the rated operation state and the low load operation state, the air utilization rate in the low load operation state in which the fuel cell stack is operated at a predetermined output lower than the rated output is higher than the air utilization rate in the rated operation state. The fuel cell system is characterized in that control is performed so as to change the air supply flow rate in accordance with the switching .   2. The fuel cell system according to claim 1, wherein the air flow rate control unit controls an air supply flow rate so that the air utilization rate in the low-load operation state is 80 to 90%. . In a fuel cell system comprising a fuel cell stack, a cooling water supply device that supplies cooling water to the fuel cell stack, and a cooling water flow rate control unit that controls the flow rate of the cooling water,
The cooling water supply device includes a cooling water line that circulates the cooling water flowing through the fuel cell stack, a heat exchanger that is provided in the cooling water line and that releases heat of the cooling water flowing through the fuel cell stack, and a waste heat recovery line for recovering the heat released from the heat exchanger,
The cooling water flow rate control unit is configured such that the flow rate of the cooling water in a low load operation state where the fuel cell stack is operated at a predetermined output lower than a rated output is smaller than the flow rate of the cooling water in the rated operation state. In addition, the fuel cell system is characterized in that control is performed to change the flow rate of the cooling water in accordance with switching between the rated operation state and the low load operation state . A method for operating a fuel cell system comprising a fuel cell stack and an air supply device for supplying air to the fuel cell stack,
When the ratio of air consumption consumed by power generation of the fuel cell stack to the air supply flow rate is defined as an air utilization rate, the fuel cell stack is lower than the rated output when operating in a normal state. Change the air supply flow rate according to the switching between the rated operation state and the low-load operation state so that the air utilization rate during low-load operation that operates at a specified output is higher than the air utilization rate during rated operation. And a control method for the fuel cell system. An operation method of a fuel cell system comprising a fuel cell stack and a cooling water supply device for supplying cooling water to the fuel cell stack,
The cooling water supply device includes a cooling water line that circulates the cooling water flowing through the fuel cell stack, a heat exchanger that is provided in the cooling water line and that releases heat of the cooling water flowing through the fuel cell stack, and a waste heat recovery line for recovering the heat released from the heat exchanger,
For switching between the rated operation state and the low load operation state so that the flow rate of the cooling water during low load operation that operates at a predetermined output lower than the rated output is smaller than the flow rate of cooling water during the rated operation. A control method for changing the flow rate of the cooling water according to the control is provided.

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