WO2005053070A1

WO2005053070A1 - Fuel cell system

Info

Publication number: WO2005053070A1
Application number: PCT/JP2004/015180
Authority: WO
Inventors: Yoshihito Kanno
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2003-11-04
Filing date: 2004-10-06
Publication date: 2005-06-09
Also published as: JP2005141943A

Abstract

The fuel cell system comprises a fuel cell, a state quantity measuring instrument, a gas supply equipment, and a controller. The state quantity measuring instrument is configured to measure a specific state quantity that has a correlation with an amount of water within the fuel cell. The gas supply equipment configured to deliver reaction gas to the fuel cell. The controller is configured to control the state quantity measuring instrument and the gas supply equipment. The controller has (a) a measurement control function of controlling the state quantity measuring instrument to measure the specific state quantity after a halt of generation by the fuel cell and (b) a drain control function of controlling the gas supply equipment to drain the water from the fuel cell, in response to the specific state quantity.

Description

DESCRIPTION Fuel cell system

Technical Field The present invention relates to a fuel cell system in which water forms in the course of generating electricity.

Background Art With fuel cells to date, the problem of damage occurring due to freezing of water within the fuel cell system when it is left exposed to outside temperatures below freezing after shutdown. To solve this problem, there has been proposed a technique that a water draining process is carried out in response to the shut down of the fuel cell system or the user instruction for a predetermined period of time when the outside temperature falls below freezing as disclosed in Japanese Patent Application No. Hei 11-273704. However, draining a fuel cell system at shutdown has the problem of freezing of water that forms within the fuel cell due to the drop in temperature of the fuel cell after shutdown. Such problems do not just occur when outside temperature drops below freezing! for example, a fuel cell can become blocked by water forming due to the drop in temperature of the fuel cell, and can hinder subsequent startup.

Disclosure of the Invention The object of the present invention is to provide a technique for use in a fuel cell system, to discharge water forming in a fuel cell due to the drop in temperature of the fuel cell after halting generation. In order to attain at least part of the above and the other related objects, there is provided a fuel cell system. The fuel cell system comprises a fuel cell, a state quantity measuring instrument, a gas supply equipment, and a controller. The state quantity measuring instrument is configured to measure a specific state quantity that has correlation with an amount of water within the fuel cell. The gas supply equipment is configured to deliver reaction gas to the fuel cell.

The controller is configured to control the state quantity measuring instrument and the gas supply equipment. The controller has (a) a measurement control function of controlling the state quantity measuring instrument to measure the specific state quantity after a halt of generation by the fuel cell and (b) a drain control function of controlling the gas supply equipment to drain the water from the fuel cell, in response to the specific state quantity. According to the fuel cell system of the invention, it is possible to drain water from the fuel cell with reference to a specific state quantity correlated with the amount of water within the fuel cell, this amount being measured after generation by the fuel cell has halted, whereby it is possible to drain water that has formed within the fuel cell due to the drop in temperature of the fuel cell after the fuel cell has been shut down. As a state quantity correlated with the amount of water within the fuel cell, there could be employed any of various state quantities, such as fuel cell temperature, absolute humidity, internal resistance, or cumulative value of electrical current of the fuel cell from startup until halting generation. In the fuel cell system herein, following arrangements are possible wherein the gas supply equipment comprises a purge gas supply equipment configured to deliver purge gas to the fuel cell, the purge gas being for expelling the water from the fuel cell, wherein the controller is configured to control the gas supply equipment to deliver the purge gas to the fuel cell, in response to the specific state quantity! or the gas supply equipment comprises a gas-liquid separator configured to separate and collect the water from reaction gases discharged from the fuel cell, and have a discharge valve capable of open-close actuation for discharging the collected water, wherein the controller is configured to control the gas supply equipment to actuate the discharge valve between open and closed positions, in response to the specific state quantity. In the fuel cell system herein, there is a possible arrangement wherein the drain control function includes^: ftrl) a damage probability assessment function of assessing a probability of damage to the fuel cell resulting from freezing of water inside the fuel cell, and (b-2) a function of controlling the gas supply equipment such that the water is drained from the fuel cell in response to an assessment that the damage is probable. This arrangement enables to reduce the damage to the fuel cell due to freezing of water which has formed within the fuel cell due to the drop in temperature of the fuel cell after generation is halted. By way of specific example, in the fuel cell system herein, there could easily be implemented an arrangement wherein the specific state quantity is a state quantity that has a correlation with temperature of the fuel cell, and the damage probability assessment function assesses the probability of damage to the fuel cell depending on whether the temperature as the specific state quantity has reached a predetermined threshold value in proximity to 0 degrees Celsius. In the fuel cell system herein, there is a possible arrangement wherein the drain control function includes^" (b^_3) a flooding assessment function of assessing whether a performance degradation of the fuel cell exceeds a predetermined threshold value, the performance degradation being caused by a blockage by the water inside the fuel cell, and (b-4) a function of controlling the gas supply equipment such that the water is drained from the fuel cell in response to the assessment that the performance degradation exceeds the predetermined threshold value. This arrangement enables reduce the flooding to occur which chokes the fuel with water formed within the fuel cell due to the drop in temperature of the fuel cell after a halt of power generation. This enables reduction of impaired startup due to the flooding. By way of specific example, in the fuel cell system herein, there could easily be implemented an arrangement wherein the specific state quantity includes^ a state quantity that has a correlation with a temperature of the fuel cell when electricity generation by the fuel cell is halted, a state quantity that has a correlation with an absolute humidity when electricity generation by the fuel cell is halted, and a state quantity that has a correlation with a temperature at the time of a final measurement, wherein the flooding assessment function is configured to assess whether the performance degradation exceeds the predetermined threshold value in response to the three state quantities. In the fuel cell system herein, the specific state quantity may include a state quantity that at a minimum has correlation with the temperature of the fuel cell or with absolute humidity. State quantities correlated with fuel cell temperature could include, for example, coolant outlet temperature in a cooling system (not shown) provided to the fuel cell system, or the temperature of the fuel cell per se. State quantities correlated with absolute humidity could include, for example, relative humidity inside the fuel cell. State quantities correlated with both temperature and absolute humidity could include, for example, internal resistance of the fuel cell. The invention could be reduced to practice in various embodiments other than the fuel cell system set forth hereinabove, such as a fuel cell system control method or a computer program.

Brief Description of the Drawings Fig. 1 is a block diagram showing the arrangement of a fuel cell system 100 in an embodiment of the invention! Fig. 2 is a flowchart showing the specifics of the drain control process in the embodiment of the invention! and Fig. 3 is a block diagram showing the arrangement of a fuel cell system 100a in a variation of the invention.

Best Mode for Carrying Out the Invention The fuel cell system of the invention is discussed below in detail with reference to a preferred embodiment with the accompanied drawings.

A. Arrangement of Fuel Cell System in Embodiment of the Invention FIG. 1 is a block diagram showing the arrangement of a fuel cell system 100 in an embodiment of the invention. Fuel cell system 100 comprises a fuel cell 10, an air supply system 30 for supplying air as the oxidant gas to fuel cell 10, a hydrogen gas circulating system 20 for circulating hydrogen gas as the fuel gas to fuel cell 10, a hydrogen gas supply system 40 for supplying hydrogen gas to hydrogen gas circulating system 20, a temperature sensor 70s and fuel cell thermometer 70 for measuring the temperature of the fuel cell 10, and a controller 50. The controller 50 controls the air supply system 30, hydrogen gas supply system 40, hydrogen gas circulating system 20, and fuel cell thermometer 70. Fuel cell 10 is a fuel cell of solid polymer electrolyte type having a stack structure composed of a plurality of individual fuel cells (not shown) stacked up. Inside each of the individual fuel cells (not shown) are a fuel cell internal air flow passage 35 and a fuel cell internal hydrogen flow passage 25. The air supply system 30 is a system for supplying humidified air to fuel cell internal air flow passages 35. Air supply system 30 comprises a blower 31 for drawing in outside air, a humidifier 39 for humidifying the drawn in air, a humidified air supply line for delivering humidified air to fuel cell internal air flow passage 35, and an exhaust line for discharging air from fuel cell internal air flow passage 35. The hydrogen gas supply system 40 comprises a hydrogen tank 42 for storing hydrogen gas, and a hydrogen valve for controlling the supply of hydrogen gas to the hydrogen gas circulating system 20. The hydrogen gas circulating system 20 comprises a circulating pump 28 for circulating hydrogen gas in the interior of the hydrogen gas circulating system 20, a hydrogen gas supply line 24 for supplying to the fuel cell internal hydrogen flow passage 25 the hydrogen gas pumped by the circulating pump 28, an exhaust gas line for supplying to a gas-liquid separator 29 hydrogen gas containing water, and a gas-liquid separator 29 for separating the water from the hydrogen gas and supplying the hydrogen gas to the circulating pump 28. In the embodiment, the hydrogen gas circulating system 20 corresponds to the "gas supply equipment" recited in the claims. FIG. 2 is a flowchart showing the specifics of the drain control process in the embodiment of the invention. The drain control process routine is run when a halt generation instruction is input to the fuel cell system 100. In the embodiment, this drain control process is only carried out on the hydrogen gas circulating system 20. The reason that the drain control process is not carried out on the air supply system 30 is that when a halt generation instruction is issued to the fuel cell system 100, water vapor which could cause liquid water to form is expelled by means of supplying air via a bypass line (not shown), without passing through humidifier 39. On the other hand, the reason that expulsion of water vapor using hydrogen gas is not carried out in the hydrogen gas circulating system 20 in a manner analogous to the air supply system 30 is that it is desirable to hold down discharge of hydrogen gas, which ties in to fuel costs. In Step Si 10, controller 50 halts generation by fuel cell 10. Specifically, generation is halted by halting operation of the hydrogen gas supply system 40 and the hydrogen gas circulating system 20. Additionally, operation of the air supply system 30 is halted as well, once water vapor has been expelled in the manner described above. In the embodiment, halting operation of the hydrogen gas circulating system 20 corresponds to "halting generation by the fuel cell" recited in the claims. Controller 50 also stores in memory (not shown) the halt time temperature tO, which is the measured temperature of the fuel cell 10 at the time that generation is halted. This measurement is made by the fuel cell thermometer 70, using temperature 70s. In Step S120, controller 50 goes into standby for a given time period. By so doing, subsequent processes (Step S130 -Step S180) can be executed at temporal intervals so as to reduce power consumption. In Step S130, controller 50 measures the temperature of the fuel cell 10. By so doing it is possible to measure the temperature of the fuel cell 10 whose temperature has dropped over given time period. On the basis of this drop in temperature, it is possible to estimate the amount of liquid water formed by means of condensation of water vapor present within the fuel cell 10. In Step S140, controller 50, on the basis of this amount of water, predicts the occurrence of flooding. Herein, "flooding" refers to a phenomenon whereby excess liquid water forming in the catalyst bed (not shown) of fuel cell 10 prevents the reaction gases from circulating through the catalyst bed, thereby diminishing fuel cell performance. Predicting the occurrence of flooding involves predicting whether the drop in performance of the fuel cell 10 due to flooding is excessive, on the assumption that generation by the fuel cell 10 has commenced at the point in time of temperature measurement. In the embodiment, this prediction is made on the basis of a prepared map (not shown) representing estimated amount of liquid water present within fuel cell 10 after halting generation, and the relationship between liquid water present within a fuel cell 10 and the drop in performance of the fuel cell 10. The amount of liquid water present within fuel cell 10 can be estimated as the total of the amount of liquid water present in the fuel cell 10 at the time generation is halted, and the amount of liquid water forming through internal condensation as the temperature of the fuel cell 10 drops. The estimate of liquid water forming through internal condensation is carried out depending on the temperature differential between the halt time temperature tO and measured temperature. This is because internal condensation occurs due to a drop in saturated water vapor pressure resulting from the drop in temperature of the fuel cell 10. Controller 50 estimates the extent of drop in performance of fuel cell 10 on the basis of the amount of liquid water within the fuel cell 10 estimated in this manner, and the map described earlier. By comparing the extent of this performance drop with a predetermined threshold value, the controller 50 then decides whether flooding has occurred. If as a result of this decision it is determined that flooding has occurred, the process advances to Step S150! if it is determined that flooding has not occurred, the process advances to Step S160. In Step S150, the controller 50 carries out a purging process to prevent flooding. The purging process to prevent flooding is a process that involves halting operation of the circulating pump 28 for a given time interval, and then opening a drain valve 29V in order to drain liquid water from the fuel cell 10. In this purging process, the operating time of the circulating pump 28 is set such that the amount of water inside the fuel cell 10 when generation by the fuel cell 10 is restarted is an appropriate amount. Following is description of the specific details of the purging process to prevent flooding. When circulating pump 28 operates, reaction gases containing liquid water are supplied to the gas-liquid separator 29. The gas-liquid separator 29 separates and collects the gas-liquid separator 29 from the reaction gases containing liquid water. The controller 50 drains the collected water by opening drain valve 29V. Finally, the controller 50 stores in memory (not shown) information (a flag) indicating that the purging process to prevent flooding has been completed. By means of this process, it is possible to avoid subsequent startup of the fuel cell system 100 from being hampered by flooding. In Step S160, the controller 50 additionally estimates the probability of freezing. Probability estimation of freezing is carried out depending on the temperature of the fuel cell 10 at the time of measurement. Specifically, the probability of damage to the fuel cell by freezing is determined by whether the temperature of the fuel cell 10 has reached a predetermined threshold value in proximity to 0 degrees Celsius (for example, 5 degrees Celsius or 3 degrees Celsius). The purpose of the predetermined threshold value in proximity to 0 degrees Celsius is so that draining may be carried out before the liquid water freezes. If as a result of the decision it is determined that there is a probability of freezing due to further drop in temperature, the process advances to Step S170! if determined that there is no probability of freezing, the process advances to Step S180. In Step S170, the controller 50 carries out a purging process to prevent freezing. This purging process to prevent freezing is a process substantially identical to the purging process to prevent flooding. However, in this purging process, water is drained to a sufficient extent by the circulating pump 28, so as to avoid the fuel cell 10 from being damaged by frozen water if the temperature of the fuel cell 10 should go below freezing. Once the purging process to prevent freezing is completed, the controller 50 stores in memory (not shown) information (a flag) indicating that the purging process to prevent freezing has been completed. The above process is carried out until both the purging process to prevent flooding and the purging process to prevent freezing are completed (Step S180). In the event that both are completed, since the risk of freezing-induced damage or flooding of fuel cell 10 is sufficiently small, the fuel cell system 100 is shut down completely. In this way, in the embodiment, the fuel cell 10 is drained in the event that flooding or freezing is predicted as a result of a flooding or freezing prediction decision, making it possible to avoid hindered startup due to flooding, or damage to the fuel cell 10 due to freezing. It is not necessary to always carry out both prediction and process determination for flooding and freezing! an arrangement whereby at least one of these is carried out is possible. An identical process may be used for both the purging process to prevent flooding and purging process to prevent freezing.

B. Variations: While the invention has been described hereinabove in terms of an embodiment, the invention is not limited to the embodiment herein, and could be reduced to practice in various ways without departing from the scope and spirit thereof. The following variations are possible, for example.

B-l: In the embodiment hereinabove, the draining process of the invention is applied to the hydrogen gas circulating system 20! however, the invention could be applied to the air supply system 30, or to both the hydrogen gas circulating system

20 and the air supply system 30. In this case, the hydrogen gas circulating system

20 and air supply system 30 would correspond to the "gas supply equipment" recited in the claims. It should be noted that since expelling water vapor by supply hydrogen gas to the hydrogen gas circulating system 20 leads to a drop in fuel consumption, the invention is more advantageously implemented in the hydrogen gas circulating system 20 than in the air supply system 30.

B-2: In the embodiment hereinabove, in the draining process of the invention, actuation of the circulating pump 28 is followed by opening of the drain valve 29V! however, and arrangement is which only actuation of the circulating pump 28 is executed would be acceptable instead. This is because water can be drained from fuel cell 10 even where the circulating pump 28 is simply actuated. "Draining water from the fuel cell" as recited in the claims can include both arrangements wherein only actuation of the circulating pump 28 is executed, and arrangements wherein actuation of the circulating pump 28 and opening of the drain valve 29V are executed.

B"3^: In the embodiment hereinabove, there is not provided a mechanism for supply the fuel cell with purge gas, i.e. gas used to expel water from the fuel cell interior! however, the invention could be implement in a fuel cell system 100a furnished with a purge gas supply equipment 27 as shown in the Variation (FIG. 3). With this arrangement, after the drain valve 29V has been opened by controller 50a, the purge gas supply equipment 27 is actuated at the same time that the circulating pump 28 is actuated by controller 50 in the embodiment.

Claims

CLAIMS:

1. A fuel cell system comprising: a fuel cell! a state quantity measuring instrument configured to measure a specific state quantity that has a correlation with an amount of water within the fuel cell! a gas supply equipment configured to deliver reaction gas to the fuel cell! and a controller configured to control the state quantity measuring instrument and the gas supply equipment, wherein the controller has: (a) a measurement control function of controlling the state quantity measuring instrument to measure the specific state quantity after a halt of generation by the fuel cell! and (b) a drain control function of controlling the gas supply equipment to drain the water from the fuel cell, in response to the specific state quantity.

2. The fuel cell system according to claim 1, wherein the gas supply equipment comprises a purge gas supply equipment configured to deliver purge gas to the fuel cell, the purge gas being for expelling the water from the fuel cell, wherein the controller is configured to control the gas supply equipment to deliver the purge gas to the fuel cell, in response to the specific state quantity.

3. The fuel cell system according to claim 1 wherein the gas supply equipment comprises a gas-liquid separator configured to separate and collect the water from reaction gas discharged from the fuel cell, and having a discharge valve capable of open-close actuation for discharging the collected water, wherein the controller is configured to control the gas supply equipment to actuate the discharge valve between open and closed positions, in response to the specific state quantity.

4. The fuel cell system according to any of claims 1 to 3, wherein the drain control function includes: (b" l) a damage probability assessment function of assessing a probability of damage to the fuel cell resulting from freezing of water inside the fuel cell, and (b"2) a function of controlling the gas supply equipment such that the water is drained from the fuel cell in response to an assessment that the damage is probable.

5. The fuel cell system according to claim 4, wherein the specific state quantity is a state quantity that has a correlation with temperature of the fuel cell, and the damage probability assessment function assesses the probability of damage to the fuel cell depending on whether the temperature as the specific state quantity has reached a predetermined threshold value in proximity to 0 degrees Celsius.

6. The fuel cell system according to any of claims 1 to 5, wherein the drain control function includes: (b-3) a flooding assessment function of assessing whether a performance degradation of the fuel cell exceeds a predetermined threshold value, the performance degradation being caused by a blockage by the water inside the fuel cell, and (b-4) a function of controlling the gas supply equipment such that the water is drained from the fuel cell in response to the assessment that the performance degradation exceeds the predetermined threshold value.

7. The fuel cell system according to claim 6, wherein the specific state quantity includes: a state quantity that has a correlation with a temperature of the fuel cell when electricity generation by the fuel cell is halted, a state quantity that has a correlation with an absolute humidity when electricity generation by the fuel cell is halted, and a state quantity that has a correlation with a temperature at the time of a final measurement, wherein the flooding assessment function is configured to assess whether the performance degradation exceeds the predetermined threshold value in response to the three state quantities.

8. The fuel cell system according to any of claims 1 to 7 wherein the specific state quantity includes a state quantity having at least one of a correlation with the temperature of the fuel cell and a correlation with absolute humidity.

9. A control method for a fuel cell system, the method comprising the steps of: (a) providing a fuel cell, a state quantity measuring instrument configured to measure a specific state quantity that has a correlation with an amount of water within the fuel cell, and a gas supply equipment configured to deliver reaction gas to the fuel cell! (b) controlling the state quantity measuring instrument to measure the specific state quantity after a halt of generation by the fuel cell! and (d) controlling the gas supply equipment to drain the water from the fuel cell, in response to the specific state quantity.

10. A computer program product for causing a computer to control a fuel cell system comprising a fuel cell, a state quantity measuring instrument configured to measure a specific state quantity that has a correlation with an amount of water within the fuel cell, and a gas supply equipment configured to deliver reaction gas to the fuel cell, the computer program product comprising: a computer readable medium! and a computer program stored on the computer readable medium, the computer program comprising: a first program for causing the computer to control the state quantity measuring instrument to measure the specific state quantity after a halt of generation by the fuel cell! and a second program for causing the computer to control the gas supply equipment to drain the water from the fuel cell, in response to the specific state quantity.