System with water reservoir

Abstract

When the pressure reducing section and the water holding section are connected to evaporate the water in the flow passage, the heat of evaporation can cool the surrounding water, causing it to freeze. [Solution] A system having a water holding section capable of holding water therein comprises: a pressure reducing section that reduces pressure to a pressure lower than atmospheric pressure; a flow path that can connect the water holding section and the pressure reducing section; a pressure reducing speed adjusting section provided in the flow path that, when the water holding section and the pressure reducing section are connected by the flow path, makes the pressure reducing speed on the water holding section side slower than the pressure reducing speed on the pressure reducing section side; and a heating section that heats the flow path between the pressure reducing speed adjusting section and the pressure reducing section.

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to a system having a water holding portion capable of holding water therein. [Background technology]

[0002] In a fuel cell stack that generates electricity by reacting hydrogen and oxygen, water is produced during the power generation process. If the operation of the fuel cell stack is stopped while water remains in the stack, it becomes difficult to start the operation in a sub-freezing environment due to the frozen remaining water.

[0003] To address this issue, Patent Document 1 discloses a scavenging method for a fuel cell system. According to this scavenging method, when the fuel cell stack is stopped, the oxidant gas flow path in the fuel cell stack is connected to a pressure reducing section that has been previously reduced to a pressure lower than atmospheric pressure, and residual water in the oxidant gas flow path is removed by evaporation. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-97993 Summary of the Invention [Problem to be solved by the invention]

[0005] When the water in the flow passages is evaporated by the above-mentioned pressure reduction, the surrounding water is cooled by the heat of evaporation, and the water may freeze in the fuel cell stack or the flow passages. Such freezing may lead to clogging of the flow passages or malfunction of the system. Furthermore, this problem can occur in systems that include a water storage unit, not just fuel cell stacks. [Means for solving the problem]

[0006] This specification discloses a system including a water retention unit capable of retaining water therein. The system includes a pressure reducing unit that reduces pressure to a pressure lower than atmospheric pressure, a flow path that can connect the water retention unit and the pressure reducing unit, a pressure reducing rate adjusting unit provided in the flow path that, when the water retention unit and the pressure reducing unit are connected via the flow path, makes the pressure reducing rate on the water retention unit side slower than the pressure reducing rate on the pressure reducing unit side, and a heating unit that heats the flow path between the pressure reducing rate adjusting unit and the pressure reducing unit.

[0007] According to the above configuration, the flow path that can connect the water holding portion and the pressure reducing portion is provided with a pressure reducing rate adjusting portion, and a heating portion that heats the flow path between the pressure reducing rate adjusting portion and the pressure reducing portion. As a result, when the water holding portion and the pressure reducing portion are connected, it is possible to prevent the water from freezing due to the heat of evaporation described above on either the water holding portion side or the pressure reducing portion side of the flow path. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a diagram schematically illustrating a fuel cell system according to an embodiment. [Figure 2] FIG. [Figure 3] FIG. 10 is a diagram simply illustrating that the depressurization speed differs between the upstream side and the downstream side of the depressurization speed adjusting unit. [Figure 4] 10A and 10B are graphs showing the time it took for water in a test container to freeze. [Figure 5] FIG. 10 is a diagram showing a heated portion according to a modified example. DETAILED DESCRIPTION OF THE INVENTION

[0009] The present embodiment will be described with reference to the drawings. Each drawing is merely an example, and the present embodiment is not limited to the contents shown in the drawings. Also, since each drawing is an example, some parts may be omitted.

[0010] FIG. 1 shows a schematic diagram of a fuel cell system 40 according to this embodiment. The fuel cell system 40 is an example of a system having a water retention unit capable of retaining water therein. The fuel cell system 40 is applied to, for example, a fuel cell vehicle. The fuel cell system 40 includes a fuel cell stack 20. The fuel cell stack 20 is an example of a water retention unit. In this specification, the water retention unit refers to a part, device, unit, etc. that can at least temporarily retain water generated by its own function or water obtained from the outside. In FIG. 3, the water retention unit is indicated by the reference numeral 20.

[0011] The fuel cell system 40 includes a hydrogen supply source 21 that supplies hydrogen as fuel gas, a hydrogen supply flow path 22 that is a flow path for supplying hydrogen from the hydrogen supply source 21 to the fuel cell stack 20, and a hydrogen discharge flow path 23 that is a flow path for discharging hydrogen from the fuel cell stack 20. The hydrogen supply source 21 is, for example, a tank filled with hydrogen. The hydrogen supply flow path 22 is provided with a hydrogen inlet valve 24 that is located between the hydrogen supply source 21 and the fuel cell stack 20 and opens and closes the flow path. The hydrogen discharge flow path 23 is provided with a hydrogen outlet valve 25 that opens and closes the flow path.

[0012] The fuel cell system 40 includes an air supply flow path 27, which is a flow path for supplying air, which is an oxidant gas, to the fuel cell stack 20, and an air discharge flow path 28, which is a flow path for discharging air from the fuel cell stack 20. The air supply flow path 27 is provided with an air pump 26 for sending out air, and an air inlet valve 29, which is located between the air pump 26 and the fuel cell stack 20 and opens and closes the flow path.

[0013] An air outlet valve 30 that opens and closes the air discharge flow path 28 is provided in the air discharge flow path 28. The air discharge flow path 28 branches off at a position closer to the fuel cell stack 20 than the air outlet valve 30. This branched flow path is called a branched discharge flow path 31. The branched discharge flow path 31 is connected to a pressure reduction section 33 on the downstream side. The section of the air discharge flow path 28 that branches off from the fuel cell stack 20 to the branched discharge flow path 31 and the branched discharge flow path 31 are examples of "flow paths that can connect the water retention section and the pressure reduction section." A check valve 37 is provided in the section of the air discharge flow path 28 that branches off to the branched discharge flow path 31 to prevent backflow of exhaust gas.

[0014] The fuel cell stack 20 generates electricity by utilizing a chemical reaction between hydrogen and oxygen. The electricity generated by the fuel cell stack 20 is used, for example, to drive a load 14 or to charge a secondary battery (not shown). The load 14 is a motor for driving an automobile or various auxiliary machinery. The fuel cell system 40 also has a control device 13 that controls the entire fuel cell system 40, including the load 14. The control device 13 can control the operation of each of the hydrogen inlet valve 24, hydrogen outlet valve 25, air pump 26, air inlet valve 29, air outlet valve 30, branch outlet valve 32, pressure reduction speed adjustment unit 34, heating unit 35, and vacuum pump 38.

[0015] A fuel cell stack 20 is constructed by stacking a large number of fuel cell cells 11 and electrically connecting them in series. FIG. 1 shows a simplified view of some of the fuel cell cells 11. FIG. 2 shows a cross section of one fuel cell 11. As is known, the fuel cell 11 has a membrane electrode assembly 12 in which an anode electrode, i.e., a hydrogen electrode catalyst layer 2, and a cathode electrode, i.e., an oxygen electrode catalyst layer 3, are bonded to both sides of an electrolyte membrane 1. The membrane electrode assembly 12 is called an MEA (Membrane Electrode Assembly).

[0016] Two gas diffusion layers, a hydrogen gas diffusion layer 4 and an oxygen gas diffusion layer 5, are bonded to each of both sides of the membrane electrode assembly 12. Furthermore, the integrated structure of the membrane electrode assembly 12, hydrogen gas diffusion layer 4, and oxygen gas diffusion layer 5 is sandwiched between a hydrogen separator 6 and an oxygen separator 7 from the anode side and the cathode side. The hydrogen separator 6 and the oxygen separator 7 form a hydrogen flow path 8 and an air flow path 9, respectively, between themselves and the gas diffusion layers 4, 5. Furthermore, a cooling water flow path 10 for flowing cooling water is formed on the surface of the hydrogen separator 6 opposite the surface forming the hydrogen flow path 8, and on the surface of the oxygen separator 7 opposite the surface forming the air flow path 9.

[0017] When air and hydrogen are supplied to the fuel cell stack 20 from the air supply flow path 27 and the hydrogen supply flow path 22, gases are introduced into the air flow path 9 and the hydrogen flow path 8 in each fuel cell 11, and the above-mentioned chemical reaction occurs at both electrodes of the membrane electrode assembly 12. Air and hydrogen not used in the reaction are discharged from the air discharge flow path 28 and the hydrogen discharge flow path 23. As simply shown by dashed lines in FIG. 1 , in the fuel cell stack 20, the hydrogen supply flow path 22 and the hydrogen discharge flow path 23 are connected by the hydrogen flow path 8, and the air supply flow path 27 and the air discharge flow path 28 are connected by the air flow path 9. In the air flow path 9, water is produced by a chemical reaction during power generation.

[0018] The pressure reduction unit 33 is a pressure reduction chamber that is reduced in pressure by a vacuum pump 38, and is capable of reducing the pressure in the pressure reduction chamber and in the flow paths communicating with the pressure reduction chamber to a pressure lower than atmospheric pressure. A pressure reduction speed adjustment unit 34 is provided in the branch discharge flow path 31 at a predetermined position upstream of the pressure reduction unit 33, i.e., on the fuel cell stack 20 side. The pressure reduction speed adjustment unit 34 is provided in the branch discharge flow path 31 downstream of the branch outlet valve 32. When the fuel cell stack 20 and the pressure reduction unit 33 are connected by a flow path, the pressure reduction speed adjustment unit 34 can make the pressure reduction speed upstream of the pressure reduction speed adjustment unit 34, i.e., on the fuel cell stack 20 side, slower than the pressure reduction speed downstream of the pressure reduction speed adjustment unit 34, i.e., on the pressure reduction unit 33 side.

[0019] The pressure reduction speed adjustment unit 34 is configured by, for example, a valve that can adjust the flow rate of the branch discharge flow path 31. The valve serving as the pressure reduction speed adjustment unit 34 is, for example, a variable throttle valve. The valve serving as the pressure reduction speed adjustment unit 34 is, for example, an electrically driven valve such as a solenoid valve or an electric valve. Furthermore, the branch discharge flow path 31 is provided with a heating unit 35 that heats the flow path between the pressure reduction speed adjustment unit 34 and the pressure reduction unit 33.

[0020] The flow path of the branched discharge flow path 31 that is heated by the heating unit 35 is referred to as the heated portion 36. Simply speaking, the flow path between the pressure reduction rate adjustment unit 34 and the pressure reduction unit 33 may be regarded as the heated portion 36. Various heating methods by the heating unit 35 are conceivable. The heating unit 35 may be, for example, a heater that contacts the heated portion 36, or may be a heating means by induction heating (IH). Alternatively, at least the portion of the branched discharge flow path 31 between the pressure reduction rate adjustment unit 34 and the pressure reduction unit 33 may be a double pipe, with the inner pipe being the heated portion 36 and the outer pipe being the heating unit 35. In this case, the heating unit 35 heats the heated portion 36 by flowing a heating fluid (e.g., hot water) between the outer pipe and the inner pipe. The hot water may be a refrigerant for regulating the temperature of other parts, such as the fuel cell stack 20.

[0021] Furthermore, the heating unit 35 may be configured to use, as a heat source, heat generated by power loss when the depressurization speed adjustment unit 34, which is an electrically driven valve, is driven in response to instructions from the control device 13, to heat the heated unit 36. In this case, the valve constituting the depressurization speed adjustment unit 34 may be a normally closed valve, and heat generated when the valve is energized and opened may be supplied to the heating unit 35. In either case, the heating unit 35 heats the heated unit 36 ​​to a temperature equal to or higher than the freezing point of water.

[0022] Next, the processing executed by the control device 13 in this embodiment will be described. However, a description of the control relating to the supply and discharge of hydrogen will be omitted here. The control device 13 starts operation of the vacuum pump 38 at a certain timing while the fuel cell stack 20 is continuing to operate to generate electricity, and reduces the pressure in the pressure reducing section 33. At this time, the air inlet valve 29 and the air outlet valve 30 are open, and the branch outlet valve 32 is closed. The control device 13 stops operation of the vacuum pump 38 after confirming that the pressure in the pressure reducing section 33 has been reduced to a predetermined vacuum pressure or lower. The vacuum here refers to a space state in which the pressure is reduced below atmospheric pressure, and the predetermined vacuum pressure is a pressure that can sufficiently suck in the gas in each fuel cell 11 of the fuel cell stack 20.

[0023] Thereafter, when the control device 13 detects an instruction to stop the power generation operation of the fuel cell stack 20, for example, a stop signal from the ignition switch of the fuel cell vehicle, the control device 13 stops the air pump 26 and stops the supply of air to the fuel cell stack 20 through the air supply passage 27. The control device 13 then closes the air inlet valve 29 and the air outlet valve 30. After that, the control device 13 opens the branch outlet valve 32, and further opens the depressurization speed adjustment unit 34 so that its opening degree becomes a predetermined opening degree, and starts heating the heated portion 36 by the heating unit 35. However, heating by the heating unit 35 may be started before the branch outlet valve 32 is opened.

[0024] As a result, the air flow path 9 of each fuel cell 11 in the fuel cell stack 20 communicates with the pressure reduction section 33, which has been previously reduced in pressure. Because the air inlet valve 29 and the air outlet valve 30 are closed, the air in the air flow path 9 is sucked into the lower-pressure pressure reduction section 33, reducing the pressure. As a result, the boiling point of water drops, and the moisture that has been generated in the air flow path 9 evaporates.

[0025] 3, the decompression rate adjuster 34 reduces the decompression rate upstream of the decompression rate adjuster 34 compared to the decompression rate downstream of the decompression rate adjuster 34. Therefore, upstream of the decompression rate adjuster 34, the rate at which heat flows in from the surrounding components is faster than the rate at which heat is lost as water evaporates due to decompression and the surrounding water is cooled, thereby preventing water from freezing. In order to accurately prevent water from freezing upstream of the decompression rate adjuster 34, the opening of the valve that constitutes the decompression rate adjuster 34 is adjusted so that the decompression rate upstream of the decompression rate adjuster 34 is 10 kPa / s or less, for example.

[0026] Furthermore, downstream of the depressurization rate adjustment unit 34, the heated portion 36 is heated by the heating unit 35. Therefore, downstream of the depressurization rate adjustment unit 34, the rate at which heat flows in from the heating unit 35 is faster than the rate at which heat is removed as the water evaporates due to depressurization and the surrounding water is cooled, and as a result, water is prevented from freezing. In order to accurately prevent water from freezing downstream of the depressurization rate adjustment unit 34, the heat output of the heating unit 35 is adjusted to, for example, 8 [W / g] or more relative to the amount of water discharged from the fuel cell stack 20.

[0027] When the pressure in the air flow path 9 of the fuel cell stack 20 becomes approximately equal to the pressure in the pressure reducing section 33, the control device 13 again closes the pressure reduction speed adjusting section 34 and the branch outlet valve 32, and opens the air outlet valve 30. The control device 13 then opens the air inlet valve 29 and operates the air pump 26. This introduces air into the fuel cell stack 20, which has been depressurized, to purge the air flow path 9, and the pressure reduction purge process ends.

[0028] FIG. 4 shows graphs 50 to 56, which show the test results of the time required for liquid water in a test container to freeze as the water evaporates when the pressure inside the test container is reduced. In each of graphs 50 to 56, the time required for water to freeze under certain standard conditions (standard time) is set to 1, and the vertical axis shows the ratio of the time required for water to freeze under the test conditions to the standard time (freezing time ratio). The larger the freezing time ratio, the more difficult it is to evaluate freezing. For example, if the freezing time ratio exceeds 10, it is evaluated as requiring a sufficiently long time for freezing, meaning that freezing is not actually a problem.

[0029] Graph 50 shows the test results corresponding to each test condition with different depressurization rates. The depressurization rate here refers to the rate at which the pressure inside the test container decreases. Graph 50 shows that, generally, the slower the depressurization rate, the longer the time required for freezing. Graph 51 shows the test results corresponding to each test condition with different initial liquid water volumes. The initial liquid water volume is the amount of water sealed in the test container at the start of the test. Graph 51 shows that, generally, the greater the initial liquid water volume, the longer the time required for freezing.

[0030] Graph 52 shows the test results corresponding to each test condition with different initial water temperatures. The initial water temperature is the temperature of the water sealed in the test container at the start of the test. Graph 52 shows that, generally, the higher the initial water temperature, the faster the water evaporates, and therefore the shorter the time required for freezing. Graph 53 shows the test results corresponding to each test condition with different heat capacities of the base used to hold the water in the test container. Graph 53 shows that the smaller the heat capacity, the quicker the water cools, and so there is a slight tendency for the time required for freezing to be shorter.

[0031] Graph 54 shows the test results corresponding to each test condition with different internal volumes of the test container. Graph 54 shows that whether the internal volume is large or small has almost no effect on the time required for freezing. Graph 55 shows the test results corresponding to each test condition with different ultimate pressure. The ultimate pressure is the pressure reached inside the test container after decompression. Graph 55 shows that, generally, the higher the ultimate pressure, the more the amount of evaporation is suppressed, and therefore the longer the time required for freezing.

[0032] Graph 56 shows the test results corresponding to each test condition where the heating power of the heater incorporated in the base was varied. Graph 56 shows that the greater the heating power of the heater, the greater the evaporation amount, and once a certain threshold was exceeded, all of the water evaporated before freezing occurred. Considering these test results, it can be said that the time it takes for water to freeze in a reduced pressure environment is generally highly dependent on the decompression rate, the ultimate pressure, and the heating power. Based on this perspective, the present embodiment is provided with a decompression rate adjuster 34 and a heater 35.

[0033] Thus, according to this embodiment, the system having a water holding section capable of holding water therein comprises a pressure reduction section 33 that reduces the pressure to a pressure lower than atmospheric pressure, a flow path that can connect the water holding section and the pressure reduction section 33, a pressure reduction speed adjustment section 34 provided in the flow path that, when the water holding section and the pressure reduction section 33 are connected by the flow path, makes the pressure reduction speed on the water holding section side slower than the pressure reduction speed on the pressure reduction section 33 side, and a heating section 35 that heats the flow path between the pressure reduction speed adjustment section 34 and the pressure reduction section 33.

[0034] According to the above configuration, when the water holding section and the pressure reducing section 33 are connected, freezing of water can be prevented both upstream and downstream of the pressure reducing rate adjusting section 34, that is, in the entire range from the air flow path 9 to the pressure reducing section 33. Furthermore, evaporation of water due to pressure reduction reduces the amount of water in the air flow path 9, making it possible to suppress corrosion within the fuel cell 11 and freezing in sub-zero environments when the fuel cell stack 20 is out of operation, thereby improving product durability and improving start-up performance in sub-zero environments. Furthermore, temperature reduction using the heat of evaporation can improve product durability.

[0035] Furthermore, according to this embodiment, the water holding unit may be the fuel cell stack 20 that generates electricity through a chemical reaction between fuel gas and oxidant gas. This makes it possible to achieve the above-described effects in a configuration in which the fuel cell stack 20 is used as the water holding unit. However, the water holding unit is not limited to the fuel cell stack 20. The water holding unit may be, for example, a gas-liquid separator that recovers water in the piping system, or a water tank that stores water in the piping system.

[0036] Furthermore, according to this embodiment, the decompression speed adjustment unit 34 may be configured with a valve capable of adjusting the flow rate of the flow path. With this configuration, the decompression speed adjustment unit 34 can easily and accurately suppress the decompression speed on the water retention unit side to the required decompression speed.

[0037] Furthermore, according to this embodiment, the heat source of the heating unit 35 may be an electrically driven valve that constitutes the depressurization speed adjustment unit 34. With this configuration, the heating unit 35 can perform heating by effectively utilizing heat generated by power loss when the depressurization speed adjustment unit 34 is driven.

[0038] FIG. 5 shows a heated section 36 according to a modified example. On the moon's surface, there are 15 nights with temperatures of approximately -173°C, followed by 15 days with temperatures of approximately 127°C. Therefore, assuming that the system according to this embodiment is used in a lunar environment, it is important that the heated section 36 does not clog even if ice 60 grows during the 15 nights. Based on this perspective, in this modified example, the heated section 36, which is heated by the heating section 35 in the flow path that can connect the water storage section and the pressure reduction section 33, has a volume 15 times or more the amount of water discharged from the water storage section per day. With this configuration, even if water is discharged from the water storage section at night and accumulates in the heated section 36, and is cooled by the ambient temperature on the moon and freezes, the heated section 36 will not clog for 15 days. The ice 60 can then melt and evaporate during the daytime ambient temperature.

[0039] The decompression section 33 may be a space decompressed by the vacuum pump 38, or may be, for example, a part of outer space, or the lunar surface environment.

[0040] Although specific examples of the technology disclosed in this specification have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and variations of the specific examples exemplified above. Furthermore, the technical elements described in this specification or drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technology exemplified in this specification or drawings simultaneously achieves multiple objectives, and achieving one of those objectives itself has technical utility. [Explanation of symbols]

[0041] 9: Air flow path, 11: Fuel cell, 13: Control device, 20: Fuel cell stack, 22: Hydrogen supply flow path, 23: Hydrogen discharge flow path, 27: Air supply flow path, 28: Air discharge flow path, 31: Branch discharge flow path, 32: Branch outlet valve, 33: Pressure reduction section, 34: Pressure reduction speed adjustment section, 35: Heating section, 36: Heated section, 40: Fuel cell system

Claims

1. A system including a water holding unit capable of holding water therein, a pressure reducing section that reduces the pressure to a pressure lower than atmospheric pressure; a flow path that can connect the water holding portion and the pressure reducing portion; a pressure reduction rate adjusting unit provided in the flow path, the pressure reduction rate adjusting unit setting the pressure reduction rate on the water retention unit side to be slower than the pressure reduction rate on the pressure reduction unit side when the water retention unit and the pressure reduction unit are connected by the flow path; a heating unit that heats the flow path between the pressure reduction speed adjustment unit and the pressure reduction unit.

2. 2. The system according to claim 1, wherein the water holding unit is a fuel cell stack that generates electricity through a chemical reaction between a fuel gas and an oxidant gas.

3. The system according to claim 1 , wherein the pressure reduction speed adjusting unit is configured by a valve capable of adjusting the flow rate of the flow path.

4. The system according to claim 1 , wherein the heat source of the heating unit is an electrically driven valve constituting the pressure reduction speed adjusting unit.

5. The system according to claim 1 , wherein the heated portion of the flow path heated by the heating portion has a volume at least 15 times the volume of water discharged per day from the water holding portion.

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