TITLE OF INVENTION

Fuel Cell System and Operation Method Thereof

TECHNICAL FIELD

[0001] The present invention relates to a fuel cell system and an operation method thereof, and particularly relates to a fuel cell system, which is capable of recovering steam contained in gas exhausted from the fuel cell to utilize it as water required for power generation of the fuel cell, and to an operation method thereof.

BACKGROUND ART

[0002] High-purity water containing an extremely lower level of impurity is required for the power generation of fuel cell. In the fuel cell system, for example due to such a reason, it is necessary in the system to recover the high-purity water, which has been utilized in the power generation, and to re-use the recovered water. For this reason, heat exchange of the exhaust gas from the fuel cell containing steam with a heat medium is conducted in a heat exchanger to condense the steam in the exhaust gas, and this condensed water is re-used for the power generation of the fuel cell. However, when the temperature of the heat medium is high, the condensation of the exhaust gas cannot be achieved, causing a problem that a recovery rate of the water degrades. To solve this problem, a method for cooling the heat medium is proposed.

[0003] For example, Patent Literature 1 discloses a combined system of a fuel cell apparatus and a hot water supply device. In such a system, heat exchange between an exhaust gas from a fuel cell and water from a hot water storage tank included in the hot water supply device is conducted in the heat exchanger. This cools and condenses steam in the exhaust gas to generate condensed water, which is recovered in the condensed water tank to be employed for the power generation. Meanwhile, the water in the hot water storage tank is warmed to be employed for the hot water supply. When the amount of the hot water supply from the hot water storage tank is small and the hot water temperature in the hot water storage tank is high in such a system, it is recognized that the heat exchange in the heat exchanger is not sufficiently achieved even if the water-level of the condensed water tank is high, and thus the water in the hot water storage tank has been cooled by a radiator before the heat exchange with the exhaust gas. [0004] Also, the fuel cell power system illustrated in Patent Literature 2 involves heat exchange between an exhaust gas from a fuel cell stack and water from a hot water tank in a condenser. Water in the hot water tank cools and condenses steam contained in this exhaust gas to generate condensed water, and this condensed water is recovered by the condenser. Meanwhile, water in the hot water tank is heated by the exhaust gas to be stored in the hot water tank. When the temperature of the water in the hot water tank is increased in this system, the water is cooled before the heat exchange with the exhaust gas by the radiator.

CITATION LIST

Patent Literature

[0005] Patent Literature 1 : Japanese Laid-Open Patent Application Publication No. 2001 -325982

Patent Literature 2: Japanese Laid-Open Patent Application Publication No. 2004-111208

SUMMARY OF INVENTION TECHNICAL PROBLEM

[0006] In the systems of the above-described Patent Literatures 1 and 2, the temperature of the water in the hot water storage tank (hot water tank) can be always maintained to be lower by employing the radiator. For this reason, the amount of steam, which is contained in the exhaust gas and is to be condensed with the above-described water, is increased, so that a larger amount of the condensed water can be utilized for the power generation of the fuel cell. However, this also requires providing of a radiator within the system, which results in an increase in the size of the system. In addition, increased product cost and running cost of the radiator causes increased cost of the whole system. Moreover, the electric power consumption during the actuation of the radiator decreases an energy efficiency of the entire system.

[0007] The present invention is made in order to solve the above described problems, and it is an object of the present invention to provide a fuel cell system capable of supplying water in a self-sustainable manner and an operation method thereof, while suppressing an increase in the size, an increase in the cost and a decrease in the energy efficiency.

SOLUTION TO PROBLEM

[0008] A fuel cell system according one aspect of the present invention comprises a fuel cell for generating electric power using a fuel gas and an oxidizing gas; a condensation heat exchanger for conducting heat exchange between an off-gas and a heat medium, the off-gas being at least one of an off-fuel-gas exhausted from the fuel cell and an off-oxidizing-gas; a first temperature detector for obtaining temperature of the heat medium before the heat exchange with the off-gas; a recovered water tank for recovering the condensed water obtained from the off-gas by the heat exchange with the heat medium; a water level detector for detecting a water level inside of the recovered water tank; and a controller, wherein the controller is configured to continue power generation of the fuel cell when the water level detected by the water level detector is equal to or higher than a first predetermined water level, even if the temperature of the heat medium obtained by the first temperature detector is equal to or higher than a first predetermined temperature during the power generation of the fuel cell, and wherein the controller is configured to stop the power generation of the fuel cell when the temperature of the heat medium is equal to or higher than the first predetermined temperature and when the detected water level is lower than the first predetermined water level.

ADVANTAGEOUS EFFECTS OF INVENTION

[0009] The present invention has the configuration described above, and achieves advantages that it is possible to provide a fuel cell system capable of supplying water in a self-sustainable manner and an operation method thereof, while suppressing an increase in the size, an increase in the cost and a decrease in the energy efficiency.

[0010] The above object and other objects, advantages and features of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

[0011] [Fig. 1] Fig. 1 is a functional block diagram, showing a configuration of a fuel cell system according to Embodiment 1 of the present invention.

[Fig. 2] Fig. 2 is a flow chart, showing an example of an operation method for the fuel cell system of Fig. 1.

[Fig. 3] Fig. 3 is a flow chart, showing condensed water amount determination process in the operation method for the fuel cell system of Fig. 2. DESCRIPTION OF EMBODIMENTS

[0012] According to a first aspect of the present invention, a fuel cell system comprises a fuel cell for generating electric power using a fuel gas and an oxidizing gas; a condensation heat exchanger for conducting heat exchange between an off-gas and a heat medium, the off-gas being at least one of an off-fuel-gas exhausted from the fuel cell and an off-oxidizing-gas; a first temperature detector for obtaining temperature of the heat medium before the heat exchange with the off-gas; a recovered water tank for recovering the condensed water obtained from the off-gas by the heat exchange with the heat medium; a water level detector for detecting a water level inside of the recovered water tank; and a controller, wherein the controller is configured to continue power generation of the fuel cell when the water level detected by the water level detector is equal to or higher than a first predetermined water level, even if the temperature of the heat medium obtained by the first temperature detector is equal to or higher than a first predetermined temperature during the power generation of the fuel cell, and wherein the controller is configured to stop the power generation of the fuel cell when the temperature of the heat medium is equal to or higher than the first predetermined temperature and when the detected water level is lower than the first predetermined water level.

[0013] According to a second aspect of the present invention, a method of operating a fuel cell system comprises: generating electric power in a fuel cell by using a fuel gas and an oxidizing gas; conducting heat exchange between an off-gas and a heat medium in a condensation heat exchanger, the off-gas being at least one of an off-fuel-gas exhausted from the fuel cell and an off-oxidizing-gas; and recovering the condensed water obtained from the off-gas in a recovered water tank by the heat exchange with the heat medium; wherein the power generation of the fuel cell is continued when a water level inside of the recovered water tank is equal to or higher than a first predetermined water level, even if a temperature of the heat medium before the heat exchange with the off-gas is equal to or higher than a first predetermined temperature, and wherein the power generation of the fuel cell is stopped when the temperature of the heat medium is equal to or higher than the first predetermined temperature and when the water level inside of the recovered water tank is lower than the first predetermined water level.

[0014] Preferred embodiments of the present invention will be described in reference to the annexed figures. Hereinafter, same reference numeral is assigned to same or corresponding element in all diagrams, and repetitive description is not presented.

(Embodiment 1)

(Configuration of Fuel Cell System)

[0015] Fig. 1 is a functional block diagram, showing a configuration of a fuel cell system according to Embodiment 1. As shown in Fig. 1, a fuel cell system 10 is provided together with a hot water system 1 1, which stores heat generated during power generation, to constitute a cogeneration system. Alternatively, the hot water supply system 11 may be omitted and the fuel cell system 10 may be employed alone.

[0016] The fuel cell system 10 comprises a fuel cell 12 for generating electric power using a fuel gas and an oxidizing gas. The fuel cell 12 is typically constituted of, for example, polymer electrolyte fuel cell (PEFC) or solid oxide fuel cell (SOFC) or the like. The fuel cell 12 is connected to an oxidizing gas supply device such as a blower and the like (not shown), and the oxidizing gas such as air or oxygen is supplied from the oxidizing gas supply device. Also, the fuel cell 12 is connected to a reformer 16 via a fuel gas path 14, and is connected to a condensation heat exchanger 20 via an off-gas path 18.

[0017] The reformer 16 is a steam-reforming type reformer, and for example, a steam reforming system and an automatic thermal system and the like is adopted for the reformer 16. The reformer 16 reforms a raw material gas with water (hereinafter, referred to as "reforming water") to generate a fuel gas containing plenty of hydrogen. This fuel gas containing steam is supplied to the fuel cell 12 through the fuel gas path 14. Also, a raw material supply device (not shown) such as for example, a cylinder or a tank filled with the raw material gas, a supply infrastructure for the raw material gas and the like is connected to the reformer 16, and a raw material gas is supplied from the raw material supply device.

This raw material gas contains an organic compound as a major constituent, containing hydrocarbon composed of at least carbon and hydrogen. Alternatively, in place of the reformer 16, a fuel gas supply device (not shown) such as a hydrogen tank and the like may be directly connected to the fuel cell 12.

[0018] An off-gas is introduced from the fuel cell 12 to the condensation heat exchanger 20 through an off-gas path 18. The off-gas is an exhaust gas exhausted from the fuel cell 12, and contains steam.

The off-gas is composed of a fuel gas exhausted from the fuel cell 12 (hereinafter referred to as "off-fuel -gas") and an oxidizing gas exhausted from the fuel cell 12 (hereinafter referred to as "off-oxidizing-gas"). The off-fuel-gas may be combusted in a combustor (not shown) through an off-fuel-gas path (not shown) after it passes through the condensation heat exchanger 20. Also, the off-oxidizing-gas may be released to atmosphere through an off-oxidizing-gas path (not shown) after it passes through the condensation heat exchanger 20. The off-gas may contain only one of the off-fuel-gas and the off-oxidizing-gas, or otherwise may contain a combustion exhaust gas which is a gas generated after the off-fuel-gas has been combusted in the combustor (not shown), in addition to the off-fuel-gas and/or the off-oxidizing-gas. In such a case, a plurality of heat exchangers including a condensation heat exchanger for the off-fuel-gas and a condensation heat exchanger for the off-oxidizing-gas may be provided.

[0019] The condensation heat exchanger 20 is connected to a heat medium circulating path 24, in addition to the off-gas path 18, so that a heat medium is introduced through the heat medium circulating path 24. Heat exchange among these off-gases and the heat medium is conducted in the condensation heat exchanger 20. The high temperature off-gas is cooled by the low temperature heat medium by this heat exchange, such that the steam in the off-gas is cooled to a condensation temperature to generate water (hereinafter referred to as "condensed water"). This condensation heat exchanger 20 is connected to a recovered water tank 28 through a water recovery path 26.

[0020] The condensed water is introduced from the condensation heat exchanger 20 through the water recovery path 26 to the recovered water tank 28. The recovered water tank 28 is provided with a purifier (not shown) such as an ion exchange resin for deionization. Alternatively, the purifier may be provided outside of the water recovery path 26. In such a case, the condensed water purified by the purifier is stored in the recovered water tank 28.

[0021] The recovered water tank 28 is provided with a water level detector 30 for detecting the water level inside of the tank. Typically, a general float type level sensor, a compression type water level sensor and the like are available to be employed as the water level detector 30. This recovered water tank 28 is connected to the reformer 16 through a water supply path 32, and thus the condensed water in the recovered water tank 28 is supplied to the reformer 16 as the reforming water. Alternatively, the condensed water in the recovered water tank 28 may be utilized as water that is required for the power generation of the fuel cell 12, besides the reforming water. For example, the recovered water tank 28 may be connected to the fuel cell 12 through the water supply path 32, and the condensed water in the recovered water tank 28 may be utilized as the cooling water for the fuel cell 12.

[0022] In order to circulate the heat medium between the hot water tank 36 and the condensation heat exchanger 20, the heat medium circulating path 24 is connected to these equipment. As the heat medium, water in the hot water tank 36 (hereinafter referred to as "water for hot water supply") may be employed, or a liquid except this water for hot water supply, such as an antifreeze solution and the like and a gas may also be employed. The heat medium circulating path 24 is provided with a first temperature detector 34 at a downstream side of the hot water storage tank 36 and at an upstream side of the condensation heat exchanger 20. The first temperature detector 34 detects the temperature of the heat medium discharged from the hot water tank 36 before entering the condensation heat exchanger 20. Since this detected temperature is a temperature of the heat medium before the heat exchange with the off-gas (hereinafter referred to as "first temperature of heat medium"), the first temperature of this heat medium is directly derived from the detected temperature by the first temperature detector 34.

[0023] The hot water tank 36 is included in the hot water supply system 11. The hot water tank 36 is connected to a water-supply installation (not shown) such as the tap water through a water supply path 37 to store the water for hot water supply fed from the water-supply installation. Also, the condensation heat exchanger 20 is connected to the hot water tank 36 through the heat medium circulating path 24 to introduce the heat medium of high temperature that is increased when passing through the condensation heat exchanger 20. Further, a hot water path 38 is connected to the hot water tank 36, so that the water for hot water supply, which has been heated with the high temperature heat medium, is supplied to the customers from the hot water tank 36 through the hot water path 38.

[0024] Here, as described above, the first temperature detector 34 is provided in the heat medium circulating path 24 at the downstream side of the hot water storage tank 36 and at the upstream side of the condensation heat exchanger 20. For this reason, the temperature of the heat medium before the heat exchange with the off-gas is directly derived on the basis of the detected temperature from the first temperature detector 34. If the temperature of the heat medium before the heat exchange with the off-gas is indirectly derived by the detected temperature of the first temperature detector 34, other configurations may alternatively be adopted. For example, a configuration for providing the first temperature detector 34 inside of the hot water tank 36, in particular, in the lower portion inside of the hot water storage tank, may be adopted. In such a case, the temperature of the water for hot water supply inside of the hot water tank 36 is detected by the first temperature detector 34. Here, heat exchange is conducted between the heat medium and the water for hot water supply inside of the hot water tank 36 either in the case where the heat medium is the water for hot water supply or in the case where the heat medium is not the water for hot water supply. For this reason, the temperature of the heat medium discharged from the lower portion inside of the hot water storage tank is substantially equivalent to the temperature of the water for hot water supply in the lower portion inside of the hot water storage tank. Therefore, if the temperature change of the heat medium from the hot water tank 36 to the condensation heat exchanger 20 is figured out in advance, the temperature of the heat medium before entering the condensation heat exchanger 20 can be predicted on the basis of the detected temperature by the first temperature detector 34. As described above, the first temperature of the heat medium before entering the condensation heat exchanger 20 (more specifically, temperature of the heat medium before the heat exchange with the off-gas) can be intermittently derived from the temperature of the water for hot water supply inside of the hot water tank 36 detected by the first temperature detector 34.

[0025] The controller 40 is connected to the respective components of the fuel cell system 10 with signal lines, and controls these components by transmitting and receiving signals to and from the respective components. For example, the controller 40 controls the power generation of the fuel cell 12 and the stopping thereof on the basis of the measured values from the water level detector 30 and the first temperature detector 34.

[0026] The controller 40 may be constituted by a microcontroller, and may be constituted by a micro processing unit (MPU), a programmable logic controller (PLC), a logic circuit or the like.

[0027] (Operation of Fuel Cell System)

Operations of the fuel cell system 10 configured as described above will be described in reference to Fig.

1. The reforming water is supplied to the reformer 16 from the recovered water tank 28, and a raw material gas is supplied in the reformer 16. Then, the raw material gas is reformed with the steam of the reforming water in the reformer 16 to generate a fuel gas. This fuel gas is supplied to the fuel cell 12, and the oxidizing gas is also supplied to the fuel cell 12. Then, the fuel cell 12 causes a reaction for generating electric power by using the fuel gas and the oxidizing gas, and the generated electric power is supplied to customers.

[0028] Portions of the off-fuel-gas and the off-oxidizing-gas, which are not utilized in the power generation reaction of the fuel cell 12, are exhausted as the off-gas from the fuel cell 12. This off-gas is introduced to the condensation heat exchanger 20 in the state in which it contains the remained steam of reforming water that has not been consumed during the reforming reaction in the reformer 16 and the steam generated in the power generation reaction in the fuel cell 12.

[0029] In the condensation heat exchanger 20, high temperature off-gas is cooled with the heat medium introduced through the heat medium circulating path 24. In such an occasion, the steam in the off-gas is condensed, and the condensed water is stored in the recovered water tank 28. The water level inside of this recovered water tank 28 is detected by the water level detector 30, and this detected water level is output to the controller 40 so that the amount of the condensed water in the recovered water tank 28 is suitably managed. Also, the condensed water in the recovered water tank 28 is supplied to the reformer 16 as the reforming water. This allows the amount of water required for the power generation in the fuel cell system 10 (in this case, reforming water) to be covered by the condensed water recovered in the fuel cell system 10. Therefore the fuel cell system 10 is able to supply the water in a self-sustainable manner.

[0030] Meanwhile, the water for hot water supply from the water-supply installation is stored in the hot water tank 36, and the low temperature heat medium is introduced from the lower portion inside of the hot water storage tank to the condensation heat exchanger 20 along to the heat medium circulating path 24. The temperature of the heat medium before introduced to this condensation heat exchanger 20 is detected by the first temperature detector 34, and then is output to the controller 40, so that the temperature of the heat medium before the heat exchange with the off-gas is managed. Then, the low temperature heat medium is heated by the high temperature off-gas in the condensation heat exchanger 20, and the heat medium of increased temperature is returned from the condensation heat exchanger 20 to the hot water tank 36.

[0031] The high temperature heat medium enters the upper portion of the hot water tank 36 to warm the water for hot water supply in the hot water tank 36. This provides the exhaust heat generated in the fuel cell system 10 such as the exhaust heat of the off-gas and the like to the water for hot water supply, and the water for hot water supply having gained a certain heat amount is stored in the hot water tank 36 to achieve the heat storage in the hot water tank 36. Then, the hot water supply system 11 supplies the high temperature water for hot water supply in the upper portion of the hot water tank 36 from the hot water path 38 to the customers as desired.

[0032] (Operation Method of Fuel Cell System)

The operation method of the fuel cell system 10 configured as described above will be described with reference to Fig. 1 to Fig. 3. Fig. 2 is a flow chart, showing an example of an operation method for the fuel cell system 10. Fig. 3 is a flow chart, showing condensed water amount determination process in the operation method for the fuel cell system 10 of Fig. 2.

[0033] For example, as the accumulation of heat is progressed in the hot water tank 36, the temperature of the water for hot water supply in the hot water tank 36 is increased, and the first temperature of the heat medium entering the condensation heat exchanger 20 is also increased. This causes insufficient cooling of the steam in the off-gas with the heat medium in the condensation heat exchanger 20, such that an amount of generated condensed water becomes less. Therefore, there causes a possibility of a deficiency of the amount of the water required for the power generation. For this reason, as shown in Fig. 2, the controller 40 obtains the temperature detected by the first temperature detector 34, and the first temperature of the heat medium before the heat exchange with the off-gas is derived on the basis of this detected temperature (step S10).

[0034] The controller 40 determines whether or not the first temperature is equal to or higher than a first predetermined temperature (step Sll). This first predetermined temperature is the temperature of the heat medium, at which the steam in the off-gas cannot be sufficiently condensed in the condensation heat exchanger 20, and for example, is set to 40 to 50 degrees C. If the first temperature of the heat medium is lower than the first predetermined temperature (step Sll : NO), the heat medium can condense the steam in the off-gas in the condensation heat exchanger 20, and thus the controller 40 returns to the processing of the step S 10.

[0035] On the other hand, if the first temperature of the heat medium is equal to or higher than the first predetermined temperature (step SI 1 : YES), this means that the heat storage in the hot water tank 36 is completed. In such a case, there is a possibility that the heat medium cannot sufficiently condense the steam in the off-gas in the condensation heat exchanger 20 and thus the required amount of the water for the power generation in the fuel cell 12 cannot be covered. Therefore, a condensed water amount determination process is conducted in order to confirm whether or not the amount of the condensed water in the recovered water tank 28 satisfies the required amount of the water for the power generation of the fuel cell 12 and whether or not the power generation is continued (step SI 2).

[0036] In the condensed water amount determination process shown in Fig. 3, the controller 40 at first obtains the detected water level from the water level detector 30 to derive the water level in the recovered water tank 28 (step S20). Next, the controller 40 determines whether or not the water level inside of the recovered water tank 28 is equal to or higher than the first predetermined water level (step S21). This first predetermined water level is determined to be, for example, a water level inside of the recovered water tank 28 corresponding to the sufficient amount of the water for continuing the rated operation for a predetermined time, even if the amount of the recovered condensed water is zero. If the water level in the recovered water tank 28 is equal to or higher than the first predetermined water level (step S21 : YES), the power generation in the fuel cell system 10 can be conducted with the condensed water in the recovered water tank 28 even if additional condensed water cannot be further recovered. Therefore, the controller 40 supplies the condensed water in the recovered water tank 28 to the reformer 16 as the reforming water to continue the power generation in the fuel cell system 10 (step S22).

[0037] On the other hand, if the water level in the recovered water tank 28 is lower than the first predetermined water level (step S21 : NO), the amount of the condensed water in the recovered water tank 28 is small, and thus this condensed water cannot cover the amount of water required for the power generation in the fuel cell system 10. Therefore, the controller 40 stops the power generation in the fuel cell system 10 (step S23).

[0038] According to the fuel cell system 10 of the above-described configuration, if the water level of the recovered water tank 28 is high even when the first temperature of the heat medium before the heat exchange with the off-gas in the condensation heat exchanger 20 is high, the condensed water stored in the recovered water tank 28 can cover the amount of water required for the power generation in the fuel cell 12. Thus, the fuel cell system 10 is able to supply the water in a self-sustainable manner. Also, since this configuration does not require providing of a radiator for cooling the heat medium in the fuel cell system 10, an increase in the size of the fuel cell system 10, an increase in the cost and a decrease in the energy efficiency can be suppressed.

[0039] Further, if the water level of the recovered water tank 28 is equal to or higher than the first predetermined water level, even when the first temperature of the heat medium reaches the first predetermined temperature, the water level of the recovered water tank 28 reaches the first predetermined water level, and therefore the power generation of the fuel cell 12 can be continued. This can reduce the number of times of stop and start-up of the power generation in the fuel cell 12, and large amount of the energy consumption at the start-up of the fuel cell system 10 can be avoided. Therefore, the decrease in the energy efficiency in the fuel cell system 10 due to the repetition of the start-up and the stopping can be avoided. Here, when the power generation is continued based on the determination that the water level of the recovered water tank 28 is equal to or higher than the first predetermined water level (step S22), the controller 40 may conduct the processing starting from the step S10 (or the condensed water amount determination process starting from the step S20) again after a predetermined time is passed. This allows the fuel cell system 10 to operate in response to further changes in the first temperature of the heat medium or the water level of the recovered water tank 28 after it is once decided to continue the power generation (step S22).

[0040] Many improvements and alternative embodiments of the present invention are apparent for a person having ordinary skills in the art from the above-described descriptions. Consequently, the above descriptions should be construed as an illustration only, and shall set forth for the purpose of providing the best mode contemplated by the inventor of carrying out the present invention to a person having ordinary skills in the art. It will be recognized that the detailed configurations and/or functions may be substantially changed without parting from the spirit and scope of the invention.

INDUSTRIAL APPLICABILITY

[0041] The fuel cell system of the present invention is useful in providing the fuel cell system, which is capable of supplying water in a self-sustainable manner while suppressing an increase in the size, an increase in the cost and a decrease in the energy efficiency, and the operation method thereof. REFERENCE SIGNS LIST

[0042]

10 fuel cell system

12 fuel cell

20 condensation heat exchanger

20a first condensation heat exchanger (condensation heat exchanger)

20b second condensation heat exchanger (condensation heat exchanger)

24 heat medium circulating path

28 recovered water tank

30 water level detector

34 first temperature detector

40 controller