Fuel cell system

Abstract

To provide a fuel cell system that can perform sufficient steam reforming even when using reforming water containing carbon dioxide. [Solution] The fuel cell system comprises a fuel cell 1, a fuel processing unit 2 that generates a hydrogen-containing fuel gas supplied to the fuel cell 1 by steam reforming, a reforming water storage unit 4 that stores water to be used as reforming water for steam reforming, a reforming water supply unit 5 that supplies the reforming water to the fuel processing unit 2, a recovered water recovery unit 6 that recovers water contained in the exhaust gas discharged from the fuel processing unit 2 as recovered water and supplies the recovered water to the reforming water storage unit 4 as reforming water, and a control unit C. The reforming water supply unit 5 has an adjustment mechanism that adjusts the flow rate of the reforming water flowing through the reforming water supply passage 51, and the control unit C calculates an estimated value of the carbon dioxide concentration in the reforming water based on temperature information related to the water as reforming water, and controls the adjustment mechanism based on the estimated value.

Description

Technical Field

[0001] The present invention relates to a fuel cell system.

Background Art

[0002] Conventionally, a fuel cell system including a fuel cell such as a solid oxide fuel cell (SOFC) has been known. In such a fuel cell system, an oxidant gas is supplied to the cathode of the fuel cell, and a fuel gas containing hydrogen is supplied to the anode, whereby a chemical reaction occurs between the cathode and the anode to generate electricity (see, for example, Patent Document 1). The fuel gas is generated by steam reforming a hydrocarbon gas such as methane.

[0003] Patent Document 1 discloses a fuel cell system that recovers water contained in off-gas (exhaust gas in Patent Document 1) discharged from a fuel cell as condensed water and uses the condensed water as reforming water (reforming water in Patent Document 1) for steam reforming of a raw fuel. Since the condensed water contains impurities such as sulfate ions in the air, the impurities are removed by an ion exchange resin.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By recovering and using the water generated by the power generation reaction of the fuel cell as condensed water as in the fuel cell system described in Patent Document 1, a water self-sufficient fuel cell system can be provided. From the viewpoint of establishing water self-sufficiency, it is desirable to recover the water contained in the exhaust gas discharged from the fuel processing unit that performs steam reforming as recovered water.

[0006] On the other hand, condensed water and recovered water contain impurities such as carbon dioxide, some of the exhaust gas components, and sulfate ions. Although these impurities are removed by ion exchange resins, carbon dioxide is easily soluble in water, and the concentration of carbon dioxide in the exhaust gas emitted from the fuel processing unit is high, so carbon dioxide tends to remain in the recovered water. When such recovered water with a high concentration of carbon dioxide is used as reforming water, when the reforming water vaporizes, the partial pressure of water vapor decreases by the partial pressure of carbon dioxide contained in the reforming water, which can prevent a sufficient reforming reaction and reduce the amount of hydrogen produced. As a result, the fuel cell may not be able to receive enough fuel gas, which could lead to fuel cell failure or reduced durability.

[0007] The present invention has been made in view of the above-mentioned problems, and its objective is to provide a fuel cell system that can perform sufficient steam reforming even when reforming water containing carbon dioxide is used. [Means for solving the problem]

[0008] The characteristic configuration of the fuel cell system according to the present invention is a fuel cell and A fuel processing unit that generates a hydrogen-containing fuel gas to be supplied to the fuel cell by steam reforming, A water storage unit for storing water used as reforming water in the aforementioned steam reforming, A water reforming supply unit that supplies the water reforming to the fuel processing unit, A water recovery unit recovers water contained in the exhaust gas discharged from the fuel processing unit as recovered water, and supplies the recovered water to the reforming water storage unit as water for reforming, It comprises a control unit and, The reforming water supply unit includes a reforming water supply channel connecting the reforming water storage unit and the fuel processing unit, an ion exchange resin for removing impurities contained in the water used as reforming water, and an adjustment mechanism for adjusting the flow rate of the reforming water flowing through the reforming water supply channel. The control unit calculates an estimated value of the carbon dioxide concentration in the water used for reforming based on temperature information relating to the water used for reforming, and controls the adjustment mechanism based on the estimated value.

[0009] Since the solubility of carbon dioxide in water increases or decreases with water temperature, an estimated value of the carbon dioxide concentration in the reforming water can be calculated based on temperature information related to the water used for reforming. In this configuration, the control unit controls the flow rate of the reforming water based on this estimated value. For example, if the carbon dioxide concentration in the reforming water is high, the flow rate of the reforming water circulating through the reforming water supply channel is increased to increase the amount of steam supplied to the fuel processing unit. As a result, even if the reforming water contains carbon dioxide, the amount of water necessary to properly perform steam reforming can be supplied to the fuel processing unit, thereby maintaining a constant hydrogen generation rate and suppressing fuel cell failures and a decrease in durability.

[0010] Another characteristic configuration is that the temperature information includes the temperature of the water stored in the water reforming reservoir. The control unit calculates the estimated value based on the temperature of the water stored in the water reforming reservoir.

[0011] With this configuration, the estimated carbon dioxide concentration is calculated based on the temperature of the water stored in the reforming water storage section. This allows for consideration of the temperature of other waters in addition to the recovered water, making it possible to adjust the flow rate of the reforming water supplied to the fuel processing section with greater precision.

[0012] Another characteristic configuration is that the temperature information includes the temperature of the recovered water. The control unit calculates the estimated value based on the temperature of the recovered water.

[0013] With this configuration, the adjustment mechanism can be controlled based on the estimated carbon dioxide concentration of the recovered water, which has the highest carbon dioxide concentration among the water used for reforming. This makes it possible to adjust the flow rate of reforming water supplied to the fuel processing unit with greater precision.

[0014] As another characteristic configuration, at least the reforming water storage section and the reforming water supply section are housed in a housing. The temperature information includes the internal temperature of the housing. The control unit calculates the estimated value based on the internal temperature.

[0015] The internal temperature of the housing is equivalent to the outside air temperature. Therefore, even without directly measuring the temperature of the water as reforming water, the adjustment mechanism can be controlled based on the temperature of the water estimated from the internal temperature of the housing, so that the configuration of the fuel cell system can be made simple.

[0016] As another characteristic configuration, it further includes a battery cooling water storage section for storing water as battery cooling water that recovers the heat of the exhaust gas. The temperature information includes the temperature of the battery cooling water after recovering the heat of the exhaust gas. The control unit calculates the estimated value based on the temperature of the battery cooling water after recovering the heat of the exhaust gas.

[0017] According to this configuration, since the estimated value is calculated based on the temperature of the battery cooling water after recovering the heat of the exhaust gas, it is not necessary to directly measure the temperature of the water as reforming water, and the configuration of the fuel cell system can be made simple.

[0018] As another characteristic configuration, the control unit calculates the estimated value based on the output of the fuel cell.

[0019] According to this configuration, since the estimated value is calculated based on the output of the fuel cell, it is possible to take into account the increase and decrease in the carbon dioxide concentration in the exhaust gas that increases and decreases according to the output, and it is possible to more accurately adjust the flow rate of the reforming water supplied to the fuel processing unit.

Brief Description of the Drawings

[0020] [Figure 1] It is a schematic configuration diagram showing a fuel cell system. [Figure 2] It is a diagram showing the state where water flows through a bifurcation. [Figure 3] It is a flowchart for explaining the control operation of the control unit. [Figure 4] It is a diagram showing the carbon dioxide concentration ratio with respect to the temperature of the recovered water.

Embodiments for Carrying Out the Invention

[0021] Hereinafter, embodiments of the fuel cell system according to the present invention will be described based on the drawings. However, it is not limited to the following embodiments, and various modifications are possible without departing from the gist thereof.

[0022] FIG. 1 and FIG. 2 are diagrams showing the configuration of the fuel cell system of the present embodiment. Although details will be described later, FIG. 1 is a diagram showing a state where reforming water supply processing for supplying the water stored in the reforming water tank 4 to the fuel processing unit 2 is being performed. Also, in the state shown in FIG. 1, the fuel gas generated in the fuel processing unit 2 is supplied to the fuel cell 1, and power generation operation is also being performed in the fuel cell 1. FIG. 2 is a diagram showing a state where circulation processing for circulating the water stored in the reforming water tank 4 through the reforming water tank 4, the ion exchange resin 52, the battery cooling water tank 3, and the tank connection path 33 in this order is being performed. In FIGS. 1 and 2, the locations where fluids such as water are flowing are indicated by thick lines.

[0023] The fuel cell system includes a fuel cell 1, a fuel processing unit 2, a battery cooling water tank 3 (an example of a battery cooling water storage unit), a reforming water tank 4 (an example of a reforming water storage unit), a reforming water supply unit 5, a recovered water recovery unit 6, and a control unit C. The fuel cell 1 and the fuel processing unit 2 constitute a hot module.

[0024] The fuel cell 1 is, for example, a solid oxide fuel cell or a solid polymer fuel cell, and has an anode to which a fuel gas containing hydrogen is supplied and a cathode to which an oxidant gas containing oxygen is supplied. The fuel cell 1 converts chemical energy into electrical energy by reacting hydrogen and oxygen, and generates power.

[0025] The hydrogen-containing fuel gas supplied to the fuel cell 1 is produced by steam reforming in the fuel processing unit 2. For example, the fuel processing unit 2 is supplied with hydrocarbon gas such as methane and reforming water (steam), and hydrogen-containing fuel gas is produced by steam reforming of the hydrocarbon gas. The fuel gas produced in the fuel processing unit 2 is supplied to the fuel cell 1 via the fuel gas supply passage 21. In addition, exhaust gas is also generated in the fuel processing unit 2 by the combustion of hydrocarbon gas such as methane, and the exhaust gas is discharged via the exhaust gas flow passage 61. The exhaust gas contains carbon dioxide and water vapor.

[0026] The power generation reaction of the fuel cell 1 causes heat to be emitted from the fuel cell 1. The heat emitted from the fuel cell 1 is recovered by the battery cooling water circulating in the cooling water circulation path 31. In other words, in the fuel cell system of this embodiment, the fuel cell 1 is used as a heat source that can heat the water flowing through the cooling water circulation path 31 by generating heat (i.e., power generation operation).

[0027] The cooling water circulation path 31 is arranged in the following order: fuel cell 1, first heat exchange unit 7, battery cooling water tank 3, and first pump 32. The battery cooling water tank 3 stores water used as battery cooling water. The first pump 32 circulates the water used as battery cooling water in the cooling water circulation path 31. The operation of the first pump 32 is controlled by the control unit C.

[0028] Hot water circulating through the hot water circulation path 81 (described later) flows into the first heat exchange section 7, and heat exchange takes place in the first heat exchange section 7 between the hot water circulating through the hot water circulation path 81 and the battery cooling water circulating through the cooling water circulation path 31. Therefore, the battery cooling water circulating through the cooling water circulation path 31 flows into the fuel cell 1 to recover heat from the fuel cell 1, then flows into the first heat exchange section 7 to have heat removed by the hot water, flows into the battery cooling water tank 3 for storage, and then flows back into the fuel cell 1 by the first pump 32.

[0029] The hot and cold water circulation path 81 is a flow path through which hot and cold water circulates between the first heat exchange unit 7 and the hot water storage tank 8. The hot water storage tank 8 stores hot and cold water, and a second pump 82, installed between the hot water storage tank 8 and the first heat exchange unit 7 in the hot and cold water circulation path 81, circulates the hot and cold water stored in the hot water storage tank 8 through the hot and cold water circulation path 81. The hot and cold water that flows out of the hot water storage tank 8 and into the first heat exchange unit 7 absorbs heat from the battery cooling water and then returns to the hot water storage tank 8.

[0030] Therefore, the heat emitted from the fuel cell 1 during power generation is first transferred to the battery cooling water flowing through the cooling water circulation path 31, then transferred to hot water in the first heat exchange unit 7, and finally stored in the hot water storage tank 8. However, not all of the heat emitted from the fuel cell 1 is stored in the hot water storage tank 8; some is also stored in the battery cooling water tank 3. For example, the temperature of the battery cooling water stored in the battery cooling water tank 3 may be 60°C to 70°C. The operation of the second pump 82 is controlled by the control unit C.

[0031] Next, the reforming water tank 4 will be described. The reforming water tank 4 stores water used as reforming water for steam reforming in the fuel processing unit 2. Condensed water recovered from the fuel cell 1 via the condensate recovery path 9 flows into the reforming water tank 4. The condensed water is water contained in the off-gas, which includes fuel gas (anode off-gas) and oxidizer gas (cathode off-gas) that were not used for power generation, and also contains water produced by the power generation reaction.

[0032] The water treatment tank 4 is equipped with a temperature measuring unit T for measuring the temperature of the water stored in the water treatment tank 4. The temperature measuring unit T may be a contact-type thermometer or a non-contact-type thermometer.

[0033] The water stored in the reforming water tank 4 is supplied to the fuel processing unit 2 by the reforming water supply unit 5. The reforming water supply unit 5 includes a reforming water supply passage 51 connecting the reforming water tank 4 and the fuel processing unit 2, an ion exchange resin 52, a third pump 53 (an example of an adjustment mechanism), and a first on-off valve 11. The reforming water supply unit 5 is connected to the part of the reforming water tank 4 other than the bottom 4a. More specifically, the reforming water supply unit 5 is connected to the side 4b of the reforming water tank 4.

[0034] The ion exchange resin 52 is installed on the reformed water supply channel 51 and is located upstream of the third pump 53 in the flow of water used for reforming (hereinafter, the upstream and downstream sides in the flow of water used for reforming will simply be referred to as the upstream side and the downstream side). The ion exchange resin 52 removes impurities contained in the water used for reforming (for example, ionic substances such as salts and ammonia that are ionized and dissolved in the water). The water that flows out of the ion exchange resin 52 is supplied to the fuel treatment section 2 as reformed water.

[0035] The third pump 53 adjusts the flow rate of water flowing through the water treatment supply channel 51. The operation of the third pump 53 is controlled by the control unit C. A first on-off valve 11 is located downstream of the third pump 53, and the flow rate of water treatment water flowing through the water treatment supply channel 51 can also be adjusted by adjusting the opening degree of the first on-off valve 11. The operation of the first on-off valve 11 is controlled by the control unit C.

[0036] The reforming water supplied to the fuel processing unit 2 is vaporized by the combustion heat of the burner, etc., into water vapor, which is then mixed with hydrocarbon gas such as methane. The hydrocarbon gas is steam reformed by the water vapor to become a fuel gas containing hydrogen, which is supplied to the fuel cell 1 via the fuel gas supply line 21. At this time, the fuel processing unit 2 also generates exhaust gas containing carbon dioxide produced by the combustion of hydrocarbons such as methane.

[0037] The exhaust gas discharged from the fuel processing unit 2 also contains water vapor. Therefore, the fuel cell system according to this embodiment is equipped with a water recovery unit 6 that recovers the water vapor contained in the exhaust gas as recovered water. The water recovery unit 6 has an exhaust gas flow passage 61 through which the exhaust gas flows, a second heat exchange unit 62 provided on the exhaust gas flow passage 61, and a water recovery path 63 that branches off from the exhaust gas flow passage 61 downstream of the second heat exchange unit 62. The water recovery path 63 is connected to a water reforming tank 4 downstream.

[0038] A heat exchange medium, such as cooling water, flows into the second heat exchange section 62 to cool the exhaust gas, and heat is recovered from the exhaust gas through heat exchange between the exhaust gas and the heat exchange medium. The exhaust gas that flows into the second heat exchange section 62 is cooled by the heat being absorbed by the heat exchange medium and discharged outside the system as exhaust gas. During this cooling process, the water vapor contained in the exhaust gas condenses into recovered water, which flows through the recovered water recovery path 63 and is sent to the reforming water tank 4. In other words, the reforming water tank 4 is supplied with water for reforming, including condensed water recovered from the off-gas discharged from the fuel cell 1, as well as recovered water recovered from the exhaust gas discharged from the fuel processing section 2.

[0039] Furthermore, downstream of the third pump 53 and upstream of the first on-off valve 11, a branch channel 54 branches off from the reformed water supply channel 51. The branch channel 54 is connected to the battery cooling water tank 3 downstream, and a second on-off valve 12 is located in the middle of the branch channel 54. The operation of the second on-off valve 12 is controlled by the control unit C. As shown in Figure 1, when the first on-off valve 11 is closed, the reformed water flows only through the branch channel 54, and as shown in Figure 2, when the second on-off valve 12 is open, the reformed water that has flowed out of the ion exchange resin 52 flows through the branch channel 54 and into the battery cooling water tank 3.

[0040] Furthermore, the battery cooling water tank 3 and the reforming water tank 4 are connected by a tank connection passage 33. The water stored in the battery cooling water tank 3 (battery cooling water) flows through the tank connection passage 33 and into the reforming water tank 4. For example, when water stored in the reforming water tank 4 is to flow into the battery cooling water tank 3, the third pump 53 operates to deliver a certain volume of water over a certain period of time. Therefore, the operating period of the third pump 53 and the volume of water delivered by the third pump 53 are proportional.

[0041] The recovered water recovery unit 6 recovers water from water vapor contained in the exhaust gas discharged from the fuel processing unit 2. This exhaust gas contains carbon dioxide, and since carbon dioxide is easily soluble in water, the carbon dioxide concentration in the recovered water may be higher than that of other water used for reforming. Although other water used for reforming also contains dissolved carbon dioxide from the air due to contact with air, in the following explanation, it will be assumed that almost no carbon dioxide is dissolved. If recovered water with a high carbon dioxide concentration is used as reforming water, the carbon dioxide contained in the reforming water will be supplied to the fuel processing unit 2. When such reforming water vaporizes in the fuel processing unit 2, not only water vapor but also carbon dioxide is generated, and the amount of water vapor available for reforming is reduced compared to when only water vapor is generated. In other words, the partial pressure of water vapor is lowered due to the presence of carbon dioxide. As a result, the fuel processing unit 2 cannot carry out a sufficient reforming reaction, the amount of hydrogen generated decreases, and a sufficient amount of hydrogen-containing fuel gas cannot be supplied to the fuel cell 1, which may lead to failure or reduced durability of the fuel cell 1.

[0042] It is known that the solubility of carbon dioxide in water changes with water temperature. That is, the solubility of carbon dioxide increases at lower water temperatures and decreases at higher water temperatures. The solubility of carbon dioxide in water is represented by a solubility curve based on Henry's Law. Therefore, the carbon dioxide concentration in water can be estimated based on the water temperature. In this embodiment, the control unit C is configured to calculate an estimated value of the carbon dioxide concentration in the reforming water based on information (temperature information) regarding the temperature of the water used as reforming water, and to adjust the amount of reforming water supplied to the fuel processing unit 2 based on the estimated value. Information regarding the temperature of the water used as reforming water includes the temperature of the water stored in the reforming water tank 4 measured by the temperature measurement unit T, and the average value of that temperature at predetermined intervals. Information regarding the temperature of the water used as reforming water may be stored in a memory unit (not shown). The memory unit may also store correlation formulas showing the correlation between carbon dioxide concentration and water temperature, and control tables described later.

[0043] The control unit C is composed of a microcontroller including a processor and semiconductor memory. The control unit C controls the third pump 53 and the first on-off valve 11 by referring to information regarding the temperature of the water used for reforming and correlation formulas stored in the memory unit.

[0044] The specific control flow of the control unit C will be explained using Figure 3. When the fuel cell system is in operation (Yes in S01), the control unit C first determines the amount of reforming water to be supplied to the fuel processing unit 2 (target amount), and controls the third pump 53 and the first on-off valve 11 to set the amount of reforming water flowing through the reforming water supply passage 51 to the target amount (S02). The temperature measurement unit T also measures the temperature of the water stored in the reforming water tank 4 (S03). The measured value is preferably stored in the memory unit. The control unit C then estimates the concentration of carbon dioxide dissolved in the reforming water from the information regarding the temperature of the water used as reforming water (for example, the temperature of the water stored in the reforming water tank 4) and a correlation formula showing the correlation between carbon dioxide concentration and water temperature, and calculates the estimated value. Then, if the amount of water supplied to the fuel processing unit 2 in the reforming water, calculated from the estimated value, is greater or less than the amount of water in the reforming water that does not contain dissolved carbon dioxide, the control unit C determines that a correction is necessary (Yes in S04) and calculates the corrected flow rate. The corrected flow rate refers to the flow rate of reforming water that can supply a sufficient amount of water to the fuel processing unit 2 for steam reforming, even if it contains carbon dioxide. The control unit C increases or decreases the output of the third pump 53 to control the flow rate of reforming water circulating in the reforming water supply passage 51 to the corrected flow rate (S05). If the carbon dioxide concentration of the reforming water is low and the amount of water in the reforming water is approximately the same as the amount of water in the reforming water that does not contain dissolved carbon dioxide, the control unit C determines that correction is unnecessary (No. in S04) and maintains the flow rate (S06). The control unit C controls the flow rate of reforming water until the operation of the fuel cell system is stopped. The determination of the target water amount (S02) may be performed each time the output of the fuel cell 1 fluctuates, or it may be performed at predetermined intervals.

[0045] Furthermore, the control unit C may perform control using a control table instead of using a correlation formula that shows the correlation between carbon dioxide concentration and water temperature. The control table contains a correction coefficient for setting the amount of water in the reformed water to the target amount, and a corrected flow rate obtained by multiplying the correction coefficient by the target amount, for each temperature of the reformed water. The correction coefficient may be determined from the correlation formula between carbon dioxide concentration and water temperature. For example, the control unit C compares the corrected flow rate at the temperature measured by the temperature measurement unit T with the target amount. If the corrected flow rate is the same as the target amount, it determines that no correction is necessary and maintains the flow rate. If the corrected flow rate is greater than or less than the target amount, it determines that a correction is necessary and controls the amount of water in the reformed water to become the corrected flow rate.

[0046] In this way, the control unit C controls the third pump 53 and the first on-off valve 11 based on information regarding the temperature of the water used for reforming, thereby supplying a sufficient amount of water to the fuel processing unit 2 for the reforming reaction, even if the reforming water contains carbon dioxide.

[0047] Furthermore, in this embodiment, the carbon dioxide concentration in the reformed water is estimated from the temperature of the water stored in the reformed water tank 4. Therefore, even if there is a difference between the temperature of the recovered water flowing through the recovered water recovery channel 63 and the temperature of the water stored in the reformed water tank 4, the carbon dioxide concentration is estimated based on the temperature of the water when the recovered water supplied to the reformed water tank 4 and the stored water are mixed, allowing for more accurate control of the reformed water flow rate. For example, if the temperature of the water stored in the reformed water tank 4 is higher than the temperature of the recovered water, the temperature of the recovered water supplied to the reformed water tank 4 will rise, which may decrease the solubility of carbon dioxide and thus the carbon dioxide concentration. Therefore, by performing control based on the temperature of the water stored in the reformed water tank 4, it becomes possible to take into account the temperature difference between the recovered water and the water stored in the reformed water tank 4.

[0048] [Second Embodiment] The fuel cell system according to the second embodiment will be described with reference to Figure 4. In the second embodiment, the control unit C controls the third pump 53 and the first on-off valve 11 based not only on information regarding the temperature of the water used for reforming, but also on the output of the fuel cell 1. The other configurations are the same as in the first embodiment, so their description will be omitted.

[0049] The amount of fuel gas required for the power generation reaction varies depending on the output of fuel cell 1, and therefore the amount of exhaust gas generated in the fuel processing unit 2 also varies. For example, the lower the output of fuel cell 1, the less fuel gas is required, and the less exhaust gas, i.e., carbon dioxide, is generated, thus reducing the carbon dioxide concentration in the recovered water. For this reason, the correlation equation showing the correlation between carbon dioxide concentration and water temperature differs for each output of fuel cell 1, as shown in Figure 4. Figure 4 is a diagram showing the ratio of recovered water temperature and carbon dioxide concentration for each output of fuel cell 1. Three output levels for fuel cell 1 are shown as examples: rated power output, intermediate power output, and minimum power output. The carbon dioxide concentration ratio is shown with the carbon dioxide concentration at the minimum power output and the condensed water temperature at 25°C set to 100%. The control unit C can more accurately control the flow rate of the reforming water by calculating an estimated value of the carbon dioxide concentration using the correlation equation for each output. Note that a control table may also be created for each output of fuel cell 1.

[0050] [Other Embodiments] (a) In the above embodiment, the information regarding the temperature of the water used for reforming includes the temperature of the water stored in the reforming water tank 4. However, it may also include the temperature of the recovered water flowing through the recovered water recovery channel 63, and the control unit C may control the flow rate of the reforming water based on the temperature of the recovered water and a correlation formula or the like. Therefore, a temperature measuring unit may be installed on the recovered water recovery channel 63.

[0051] By controlling the flow rate of the reformed water based on the temperature of the recovered water with the highest carbon dioxide concentration, the carbon dioxide concentration in the recovered water can be estimated more accurately. Furthermore, it is possible to calculate the carbon dioxide concentration of the water when the water stored in the reformed water tank 4 and the recovered water are mixed, using the carbon dioxide concentration determined from the temperature of the recovered water and the amount of water in the reformed water tank 4. This allows for more precise control of the flow rate of the reformed water.

[0052] (b) In the above embodiment, the exhaust gas is cooled by the second heat exchange unit 62. The fuel cell system may also include a temperature measuring unit that recovers heat from the exhaust gas and measures the temperature of the heat exchange medium flowing out of the second heat exchange unit 62. Since the temperature of the recovered water can be estimated from the temperature of the heat exchange medium after the heat from the exhaust gas has been recovered, the control unit C controls the flow rate of the reformed water by referring to a correlation formula or a control table, using the temperature of the heat exchange medium after the heat from the exhaust gas has been recovered as information regarding the temperature of the water used as reformed water. This makes it possible to estimate the temperature of the recovered water without placing a temperature measuring unit in the recovered water recovery unit 6, and the device configuration can be simplified. The correlation formula or control table may be created based on the temperature of the heat exchange medium after the heat from the exhaust gas has been recovered.

[0053] (c) The second heat exchange unit 62 may be located on the cooling water circulation path 31. That is, heat exchange may occur between the battery cooling water circulating in the cooling water circulation path 31 and the exhaust gas. The second heat exchange unit 62 is preferably located upstream of the fuel cell 1. A temperature measuring unit is preferably provided downstream of the second heat exchange unit 62 in the cooling water circulation path 31 to measure the temperature of the battery cooling water after the heat from the exhaust gas has been recovered. Since the temperature of the recovered water can be estimated from the temperature of the battery cooling water after the heat from the exhaust gas has been recovered, the control unit C controls the flow rate of the reforming water by referring to a correlation formula or a control table, using the temperature of the battery cooling water after the heat from the exhaust gas has been recovered as information regarding the temperature of the water used as reforming water. This makes it possible to estimate the temperature of the recovered water without having to place a temperature measuring unit in the recovered water recovery unit 6, thus simplifying the device configuration. The correlation formula or control table may be created based on the temperature of the battery cooling water after the heat from the exhaust gas has been recovered.

[0054] (d) At least the water treatment tank 4 and the water treatment supply unit 5 may be housed in the enclosure. The enclosure may also be equipped with a temperature measuring unit for measuring the internal temperature of the enclosure. The internal temperature of the enclosure is equivalent to the ambient temperature and is stored in the memory unit as information regarding the temperature of the water used for treatment. Since there is a correlation between the internal temperature of the enclosure and the temperature of the recovered water, the control unit C can estimate the temperature of the recovered water from the internal temperature of the enclosure and control the flow rate of the water treatment by referring to a correlation formula or a control table. Furthermore, since the flow rate of water flowing through the water treatment supply line 51 can be controlled by the ambient temperature, the device configuration can be simplified. The correlation formula or control table may be created based on the internal temperature of the enclosure. The water treatment tank 4 and the water treatment supply unit 5 may also be integrally formed inside the enclosure.

[0055] (e) The control unit C may change the method for estimating the carbon dioxide concentration depending on the temperature of the water stored in the water treatment tank 4 and the temperature of the recovered water. Specifically, if the temperature of the recovered water is approximately the same as the temperature of the water stored in the water treatment tank 4, the carbon dioxide concentration of the treated water may be estimated from the temperature of the recovered water and the amount of water stored in the water treatment tank 4. If the temperature of the recovered water is different from the temperature of the water stored in the water treatment tank 4, the carbon dioxide concentration of the treated water may be estimated from the temperature of the water stored in the water treatment tank 4.

[0056] (f) In the above embodiment, the control unit C controls the flow rate of water flowing through the water reforming supply channel 51 by increasing or decreasing the output of the third pump 53, but the flow rate may also be controlled by adjusting the opening degree of the first on-off valve 11.

[0057] Furthermore, the configurations disclosed in the above-described embodiments (including other embodiments, the same applies hereinafter) can be applied in combination with configurations disclosed in other embodiments, as long as no inconsistencies arise. In addition, the embodiments disclosed herein are illustrative, and the embodiments of the present invention are not limited thereto, and can be modified as appropriate without departing from the object of the present invention. [Industrial applicability]

[0058] The present invention can be used in a fuel cell system that includes a fuel cell and a fuel processing unit that generates a hydrogen-containing fuel gas supplied to the fuel cell by steam reforming. [Explanation of Symbols]

[0059] 1:Fuel cell 2: Fuel Processing Unit 3: Battery cooling water tank (battery cooling water storage section) 4: Water treatment tank (water treatment storage section) 5: Water Reformation Supply Department 6: Water recovery unit 51: Water supply channel for water treatment 52: Ion exchange resin 53: Second pump (adjustment mechanism) C: Control Department

Claims

1. Fuel cells and A fuel processing unit that generates a hydrogen-containing fuel gas to be supplied to the fuel cell by steam reforming, A water storage unit for storing water used as reforming water in the aforementioned steam reforming, A water reforming supply unit that supplies the water reforming to the fuel processing unit, A water recovery unit recovers water contained in the exhaust gas discharged from the fuel processing unit as recovered water, and supplies the recovered water to the reforming water storage unit as water for reforming, It comprises a control unit and, The reforming water supply unit includes a reforming water supply channel connecting the reforming water storage unit and the fuel processing unit, an ion exchange resin for removing impurities contained in the water used as reforming water, and an adjustment mechanism for adjusting the flow rate of the reforming water flowing through the reforming water supply channel. The control unit calculates an estimated value of the carbon dioxide concentration in the water used for reforming based on temperature information relating to the water used for reforming, and controls the adjustment mechanism based on the estimated value.

2. The aforementioned temperature information includes the temperature of the water stored in the water reforming reservoir. The fuel cell system according to claim 1, wherein the control unit calculates the estimated value based on the temperature of the water stored in the reforming water storage unit.

3. The temperature information includes the temperature of the recovered water. The fuel cell system according to claim 1, wherein the control unit calculates the estimated value based on the temperature of the recovered water.

4. At least the water reforming storage unit and the water reforming supply unit are housed in the casing. The temperature information includes the internal temperature of the housing, The fuel cell system according to claim 1, wherein the control unit calculates the estimated value based on the internal temperature.

5. The system further includes a battery cooling water storage unit that stores water to be used as battery cooling water for recovering heat from the exhaust gas, The temperature information includes the temperature of the battery cooling water after the heat from the exhaust gas has been recovered. The fuel cell system according to claim 1, wherein the control unit calculates the estimated value based on the temperature of the battery cooling water after recovering the heat from the exhaust gas.

6. The fuel cell system according to any one of claims 1 to 5, wherein the control unit calculates the estimated value based on the output of the fuel cell.

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