Fuel cell system

Abstract

To provide a fuel cell system capable of efficiently executing warming-up of a plurality of fuel cell stacks.SOLUTION: In a fuel cell system, the fuel cell system includes a plurality of fuel cell stacks, a cooling system, and a control part. In the fuel cell system, the plurality of fuel cell stacks include at least a first fuel cell stack, and a second fuel cell stack, and the cooling system includes a first cooling water piping, a second cooling water piping, a cooler, a first connection pipe, and a second connection pipe.SELECTED DRAWING: Figure 1

Description

Technical Field

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

Background Art

[0002] Various technologies have been proposed regarding fuel cells (FCs) as disclosed in Patent Document 1.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] In Patent Document 1, a fuel cell system is disclosed that includes a cooler for each of a plurality of fuel cell stacks individually. In the prior art, when warm - up is performed for each of a plurality of fuel cell stacks at sub - zero start - up, there is a risk of deterioration in fuel consumption.

[0005] The present disclosure has been made in view of the above circumstances, and the main object is to provide a fuel cell system capable of efficiently performing warm - up of a plurality of fuel cell stacks.

Means for Solving the Problems

[0006] That is, the present disclosure includes the following aspects. <1> A fuel cell system, wherein the fuel cell system includes a plurality of fuel cell stacks, a cooling system, and a control unit, the plurality of fuel cell stacks includes at least a first fuel cell stack and a second fuel cell stack, the cooling system includes a first cooling water pipe, a second cooling water pipe, a cooler, a first connection pipe, and a second connection pipe, The first cooling water pipe connects the cooler to the cooling water inlet of the first fuel cell stack and also connects the cooling water outlet of the first fuel cell stack to the cooler. The second cooling water pipe connects the cooler to the cooling water inlet of the second fuel cell stack and also connects the cooling water outlet of the second fuel cell stack to the cooler. The first connecting pipe has a first connecting valve. The first connecting pipe connects the first cooling water pipe and the second cooling water pipe, and when the first connecting valve is opened, it bypasses the cooler and enables the supply of cooling water from the first cooling water pipe to the second cooling water pipe. The second connecting pipe has a second connecting valve. The second connecting pipe connects the first cooling water pipe and the second cooling water pipe, and when the second connecting valve is opened, it bypasses the cooler and enables the supply of cooling water from the second cooling water pipe to the first cooling water pipe. When starting the fuel cell system below the freezing point, if the cooling water temperature of at least the first fuel cell stack among the plurality of fuel cell stacks is not below the freezing point, the control unit performs a normal start of the first fuel cell stack, and then opens the first connecting valve. When starting the fuel cell system below the freezing point, if the cooling water temperatures of all the fuel cell stacks among the plurality of fuel cell stacks are below the freezing point, the control unit compares the cooling water temperatures of the plurality of fuel cell stacks. If the cooling water temperature of the first fuel cell stack among the plurality of fuel cell stacks is the highest, the control unit performs a predetermined warm-up of the first fuel cell stack, and then when the cooling water temperature of the first fuel cell stack reaches a predetermined temperature or higher, the control unit opens the first connecting valve. If there is no temperature difference in the cooling water temperatures of the plurality of fuel cell stacks, the control unit performs a predetermined warm-up of the first fuel cell stack as determined in advance, and then when the cooling water temperature of the first fuel cell stack reaches a predetermined temperature or higher, the control unit opens the first connecting valve. A fuel cell system.

[0007] <2> When starting the fuel cell system below the freezing point, if the cooling water temperature of all the fuel cell stacks among the plurality of fuel cell stacks is below the freezing point and not less than -10°C, the control unit does not compare the cooling water temperatures of the plurality of fuel cell stacks, The control unit performs predetermined warm-up of the first fuel cell stack in advance, and then, when the cooling water temperature of the first fuel cell stack becomes 0°C or higher, the control unit opens the first connection valve. The fuel cell system according to <1>.

[0008] <3> The first connection pipe branches from the first cooling water pipe upstream of the cooler of the first cooling water pipe and merges with the second cooling water pipe downstream of the cooler of the second cooling water pipe. The fuel cell system according to <1> or <2>.

[0009] <4> The second connection pipe branches from the second cooling water pipe downstream of the cooler of the second cooling water pipe and merges with the first cooling water pipe upstream of the cooler of the first cooling water pipe. The fuel cell system according to any one of <1> to <3>.

[0010] <5> The first cooling water pipe has a first bypass pipe, The first bypass pipe has a first bypass valve, The first bypass pipe branches from the first cooling water pipe upstream of the cooler of the first cooling water pipe, merges with the first cooling water pipe downstream of the cooler of the first cooling water pipe, bypasses the cooler, The second cooling water pipe has a second bypass pipe, The second bypass pipe has a second bypass valve, The second bypass pipe branches from the second cooling water pipe upstream of the cooler of the second cooling water pipe, merges with the second cooling water pipe downstream of the cooler of the second cooling water pipe, and bypasses the cooler. The fuel cell system according to any one of <1> to <4>.

Advantages of the Invention

[0011] The fuel cell system of the present disclosure can efficiently warm up a plurality of fuel cell stacks.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Modes for Carrying Out the Invention

[0013] Hereinafter, embodiments according to the present disclosure will be described. Note that matters other than those specifically mentioned in this specification and necessary for the implementation of the present disclosure (for example, general configurations and manufacturing processes of fuel cell systems that do not characterize the present disclosure) can be grasped as design matters of those skilled in the art based on the prior art in the relevant field. The present disclosure can be implemented based on the content disclosed in this specification and common general knowledge in the relevant field. Also, the dimensional relationships (length, width, thickness, etc.) in the drawings do not reflect actual dimensional relationships. In the present disclosure, the reaction gas supplied to the anode of the fuel cell is a fuel gas (anode gas), and the reaction gas supplied to the cathode of the fuel cell is an oxidant gas (cathode gas). The fuel gas is mainly a gas containing hydrogen and may be hydrogen. The oxidant gas is a gas containing oxygen and may be oxygen, air, etc.

[0014] The present disclosure provides a fuel cell system, comprising: the fuel cell system includes a plurality of fuel cell stacks, a cooling system, and a control unit; the plurality of fuel cell stacks include at least a first fuel cell stack and a second fuel cell stack; the cooling system includes a first cooling water pipe, a second cooling water pipe, a cooler, a first connecting pipe, and a second connecting pipe; the first cooling water pipe connects the cooler to a cooling water inlet of the first fuel cell stack, and connects a cooling water outlet of the first fuel cell stack to the cooler; the second cooling water pipe connects the cooler to a cooling water inlet of the second fuel cell stack, and connects a cooling water outlet of the second fuel cell stack to the cooler; the first connecting pipe has a first connecting valve; the first connecting pipe connects the first cooling water pipe and the second cooling water pipe, and when the first connecting valve is opened, allows cooling water to be supplied from the first cooling water pipe to the second cooling water pipe, bypassing the cooler; the second connecting pipe has a second connecting valve; the second connecting pipe connects the first cooling water pipe and the second cooling water pipe, and when the second connecting valve is opened, allows cooling water to be supplied from the second cooling water pipe to the first cooling water pipe, bypassing the cooler; During a sub-freezing start-up of the fuel cell system, if the coolant temperature of at least the first fuel cell stack among the plurality of fuel cell stacks is not sub-freezing, the control unit performs a normal start-up of the first fuel cell stack, and then opens the first connecting valve; When the fuel cell system is started below freezing, if the coolant temperatures of all of the fuel cell stacks among the plurality of fuel cell stacks are below freezing, the control unit compares the coolant temperatures of the plurality of fuel cell stacks, When the cooling water temperature of the first fuel cell stack among the plurality of fuel cell stacks is the highest, the control unit performs a predetermined warm-up of the first fuel cell stack, and then, when the cooling water temperature of the first fuel cell stack reaches a predetermined temperature or higher, the control unit opens the first connection valve. When there is no temperature difference in the cooling water temperature of the plurality of fuel cell stacks, the control unit performs a predetermined warm-up of the first fuel cell stack as determined in advance, and then, when the cooling water temperature of the first fuel cell stack reaches a predetermined temperature or higher, the control unit opens the first connection valve, providing a fuel cell system.

[0015] In a fuel cell system in which two or more fuel cells are connected in parallel, if each fuel cell performs inefficient warm-up during sub-zero start-up of the fuel cell system, it will cause deterioration of fuel consumption at low temperatures. In the prior art, there is a bypass pipe for the cooler in the cooling system of each fuel cell, but there is no connecting pipe for the cooling system between the fuel cells, and the entire cooling system is connected through the cooler. If heat generated by one fuel cell is to be transmitted to another fuel cell through the cooling pipes of the entire cooling system, the heat capacity will become large, which is inefficient. The present disclosure relates to a piping configuration of a cooling system and a start-up method of a fuel cell system corresponding to sub-zero start-up in a fuel cell system in which two or more fuel cells are connected. In the present disclosure, one fuel cell is heated up by power generation, and the heat generated by one fuel cell is transmitted to another fuel cell. When the cooling water temperature of one fuel cell exceeds a predetermined temperature (for example, 30°C), the rotational speed of the water pump increases and at the same time the connection valve between the fuel cells is opened. When starting the fuel cell system, check the cooling water temperature of each fuel cell, and it may be started from the fuel cell with the higher cooling water temperature.

[0016] The fuel cell system of the present disclosure may be mounted on a mobile object such as a vehicle, or may be mounted on a stationary power generation system such as a generator that supplies power to an external device of the fuel cell system. The vehicle may be a fuel cell vehicle, etc. Examples of moving bodies other than vehicles include trains, ships, and aircraft. The fuel cell system of the present disclosure may also be mounted on a mobile object such as a vehicle that can run on power from a secondary battery. A mobile object and a stationary power generation system may be equipped with the fuel cell system of the present disclosure. The mobile object may have a drive unit such as a motor, an inverter, a hybrid control system, and the like. The hybrid control system may be capable of running a mobile object using both the output of the fuel cell and the power of the secondary battery.

[0017] The fuel cell system includes a plurality of fuel cell stacks, a cooling system, and a control unit. The fuel cell system may also include a fuel gas system, an oxidant gas system, and the like.

[0018] A fuel cell stack (stack) is a stack of a plurality of fuel cell unit cells (cells). In this disclosure, both cells and fuel cell stacks may be referred to as fuel cells. The number of cells stacked in the fuel cell stack is not particularly limited, and may be, for example, from 2 to several hundred. The fuel cell stack may have current collector plates, pressure plates, etc. at the ends in the stacking direction. The fuel cell system may include three or more fuel cell stacks, including at least a first fuel cell stack and a second fuel cell stack, and may include a number of fuel cell stacks such that the total output of the fuel cell stacks is approximately 1 MW.

[0019] The cell may have a power generation section. The shape of the power generation part may be rectangular in plan view. The power generation part may be a membrane electrode assembly (MEA) including an electrolyte membrane and two electrodes sandwiching the electrolyte membrane. The electrolyte membrane may be a solid polymer electrolyte membrane. Examples of the solid polymer electrolyte membrane include fluorine-based electrolyte membranes such as thin films of perfluorosulfonic acid containing moisture, and hydrocarbon-based electrolyte membranes. As the electrolyte membrane, for example, a Nafion membrane (manufactured by DuPont) may be used. One of the two electrodes is an anode (fuel electrode), and the other is a cathode (oxidant electrode). The electrode includes a catalyst layer and may optionally include a gas diffusion layer. The power generation part may be a membrane electrode gas diffusion layer assembly (MEGA). In this case, the cell may include a cathode separator, an anode separator, and a membrane electrode gas diffusion layer assembly disposed between the cathode separator and the anode separator. The membrane electrode gas diffusion layer assembly has an anode-side gas diffusion layer, an anode catalyst layer, an electrolyte membrane, a cathode catalyst layer, and a cathode-side gas diffusion layer in this order. The anode catalyst layer and the cathode catalyst layer are collectively referred to as the catalyst layer. The anode-side gas diffusion layer and the cathode-side gas diffusion layer are collectively referred to as the gas diffusion layer. The catalyst layer includes a catalyst, and the catalyst may include a catalyst metal that promotes an electrochemical reaction, an electrolyte having proton conductivity, and a carrier having electron conductivity. As the catalyst metal, for example, platinum (Pt) and alloys composed of Pt and other metals (for example, Pt alloys mixed with cobalt, nickel, etc.) can be used. The catalyst metal used as the cathode catalyst and the catalyst metal used as the anode catalyst may be the same or different. As the electrolyte, a fluorine-based resin or the like may be used. As the fluorine-based resin, for example, a Nafion solution or the like may be used. The catalyst metal is supported on a carrier, and in each catalyst layer, the carrier supporting the catalyst metal (catalyst-supporting carrier) and the electrolyte may be mixed. Examples of the carrier for supporting the catalyst metal include carbon materials such as generally commercially available carbon and the like. The gas diffusion layer may be a conductive member having pores or the like. Examples of the conductive member include carbon porous bodies such as carbon cloth and carbon paper, and metal porous members such as metal mesh and foamed metal. The cell may include a separator. The separator collects the current generated by power generation and functions as a partition. In the cell, usually, a pair of separators are arranged on both sides in the stacking direction of the power generation part so as to sandwich the power generation part. One of the pair of separators is an anode separator and the other is a cathode separator. The anode separator may have grooves serving as fuel gas flow paths on the surface on the power generation part side. The cathode separator may have grooves serving as oxidant gas flow paths on the surface on the power generation part side. The separator may have holes constituting a manifold such as supply holes and discharge holes for allowing a fluid to flow in the stacking direction of the cell. Examples of the separator include dense carbon obtained by compressing carbon to make it gas-impermeable, and press-molded metals (for example, iron, titanium, stainless steel, etc.). The cell may include an insulating resin frame disposed outside (outer periphery) in the plane direction of the membrane electrode assembly between the anode separator and the cathode separator. The resin frame is formed into a plate-like and frame-like shape using a thermoplastic resin, and seals between the anode separator and the cathode separator while holding the membrane electrode assembly in its central region. Examples of the resin that can be used for the resin frame include resins such as PE, PP, PET, and PEN. The resin frame may be a three-layer sheet composed of three layers with an adhesive layer disposed on the surface layer. The fuel cell stack may have gaskets, resin sheets, etc. between the cells in order to seal each gas.

[0020] The cooling system supplies cooling water as a cooling medium to the fuel cell. The cooling water includes water, ethylene glycol, etc., and may be a mixture thereof. The cooling system has a first cooling water pipe, a second cooling water pipe, a cooler, a first connecting pipe, and a second connecting pipe, and may be provided with a cooling water pump, a reserve tank, an ion exchanger, an intercooler, etc. as necessary. The first cooling water pipe and the second cooling water pipe are collectively referred to as the cooling water pipe. The first connecting pipe and the second connecting pipe are collectively referred to as the connecting pipe.

[0021] The first cooling water pipe connects the cooler to the cooling water inlet of the first fuel cell stack and connects the cooling water outlet of the first fuel cell stack to the cooler. The first cooling water pipe may have a first bypass pipe. The first bypass pipe may have a first bypass valve. The first bypass pipe branches from the first cooling water pipe upstream of the cooler of the first cooling water pipe, merges with the first cooling water pipe downstream of the cooler of the first cooling water pipe, and may bypass the cooler. The first bypass valve performs a flow path switching to switch whether the cooling water discharged from the first fuel cell stack flows to the cooler or to the first bypass pipe. The first bypass valve may be provided with an electric motor such as an electric actuator for performing the flow path switching. The first cooling water pipe may have a first cooling water inlet valve. The first cooling water inlet valve may be arranged downstream of the cooler, may be arranged upstream of the confluence with the first bypass pipe of the first cooling water pipe, and may be arranged upstream of the branch to the first connecting pipe of the first cooling water pipe.

[0022] The second cooling water pipe connects the cooler to the cooling water inlet of the second fuel cell stack and connects the cooling water outlet of the second fuel cell stack to the cooler. The second cooling water pipe may have a second bypass pipe. The second bypass pipe may have a second bypass valve. The second bypass pipe branches from the second cooling water pipe upstream of the cooler of the second cooling water pipe, merges with the second cooling water pipe downstream of the cooler of the second cooling water pipe, and may bypass the cooler. The second bypass valve performs a flow path switching to switch whether the cooling water discharged from the second fuel cell stack flows to the cooler or to the second bypass pipe. The second bypass valve may include an electric motor such as an electric actuator for performing the flow path switching. The second cooling water pipe may have a second cooling water inlet valve. The second cooling water inlet valve may be arranged downstream of the cooler, may be arranged upstream of the confluence portion of the second cooling water pipe with the second bypass pipe, and may be arranged upstream of the branching portion of the second cooling water pipe to the second connecting pipe.

[0023] The first connecting pipe has a first connecting valve. The first connecting pipe connects the first cooling water pipe and the second cooling water pipe. When the first connecting valve is opened, the first connecting pipe bypasses the cooler and enables the supply of cooling water from the first cooling water pipe to the second cooling water pipe. When the first connecting valve is closed, the first connecting pipe does not supply cooling water from the first cooling water pipe to the second cooling water pipe. The connection location of the first connecting pipe between the first cooling water pipe and the second cooling water pipe is not particularly limited. From the viewpoint of efficiently warming up a plurality of fuel cell stacks, it may branch from the first cooling water pipe upstream of the cooler of the first cooling water pipe and merge with the second cooling water pipe downstream of the cooler of the second cooling water pipe. The branching portion of the first connecting pipe from the first cooling water pipe may be upstream of the branching portion of the first bypass pipe from the first cooling water pipe. The confluence portion of the first connecting pipe to the second cooling water pipe may be downstream of the confluence portion of the second bypass pipe to the second cooling water pipe. The confluence portion of the first connecting pipe to the second cooling water pipe may be downstream of the branching portion of the second connecting pipe from the second cooling water pipe. The first connection valve performs a flow path switching to switch the cooling water discharged from the first fuel cell stack to flow to the cooler, the first fuel cell stack, or the second cooling water pipe. The first connection valve may include an electric motor such as an electric actuator for performing the flow path switching.

[0024] The second connection pipe has a second connection valve. The second connection pipe connects the first cooling water pipe and the second cooling water pipe. When the second connection valve is opened, the second connection pipe bypasses the cooler and enables the supply of cooling water from the second cooling water pipe to the first cooling water pipe. When the second connection valve is closed, the second connection pipe does not supply cooling water from the second cooling water pipe to the first cooling water pipe. The connection location between the first cooling water pipe and the second cooling water pipe of the second connection pipe is not particularly limited. From the perspective of efficiently warming up a plurality of fuel cell stacks, it may branch from the second cooling water pipe downstream of the cooler of the second cooling water pipe and merge with the first cooling water pipe upstream of the cooler of the first cooling water pipe. The branch portion of the second connection pipe from the second cooling water pipe may be downstream of the confluence portion of the second bypass pipe to the second cooling water pipe. The confluence portion of the second connection pipe to the first cooling water pipe may be upstream of the branch portion of the first bypass pipe from the first cooling water pipe. The confluence portion of the second connection pipe to the first cooling water pipe may be downstream of the branch portion of the first connection pipe from the first cooling water pipe. The second connection valve performs a flow path switching to switch the cooling water discharged from the second fuel cell stack to flow to the cooler, the second fuel cell stack, or the first cooling water pipe. The second connection valve may include an electric motor such as an electric actuator for performing the flow path switching.

[0025] The cooling water pump circulates the cooling water for cooling the fuel cell and adjusts the flow rate of the cooling water supplied to the fuel cell. The reserve tank is a tank that temporarily stores the cooling water overflowing from the cooling water pipe whose internal pressure has increased due to the temperature rise of the cooling water. The cooler is arranged on the cooling water pipe and cools the cooling water. Examples of the cooler include a radiator and the like. The cooler may be shared by the first cooling water pipe and the second cooling water pipe. The cooler may be individually provided for the first cooling water pipe and the second cooling water pipe. That is, the first cooling water pipe may be connected to the first cooler, and the second cooling water pipe may be connected to the second cooler.

[0026] The oxidant gas system supplies an oxidant gas containing oxygen to the fuel cell and adjusts the flow rate of the oxidant gas. The oxidant gas system may include an oxidant gas supply means, an oxidant gas pipe, an inlet side sealing valve at the oxidant gas inlet of the fuel cell, an outlet side sealing valve at the oxidant gas outlet of the fuel cell, and the like. The oxidant gas supply means may be an air compressor or the like.

[0027] The fuel gas system supplies a fuel gas containing hydrogen necessary for the power generation of the fuel cell to the fuel cell and adjusts the flow rate of the fuel gas. The fuel gas system may include a fuel gas tank, a fuel gas inlet valve, an injector, a gas-liquid separator, an exhaust and drainage valve, an ejector for fuel gas circulation, a fuel gas pump for fuel gas circulation, and a fuel gas pipe and the like.

[0028] The fuel cell system may include a secondary battery. The secondary battery may be any rechargeable battery. Examples include conventionally known secondary batteries such as nickel-metal hydride secondary batteries and lithium-ion secondary batteries. Further, the secondary battery may include a power storage element such as an electric double layer capacitor. The secondary battery may be configured by connecting a plurality of them in series. The secondary battery supplies power to an air compressor or the like. The secondary battery may be rechargeable from an external power source of the fuel cell system such as a household power supply, for example. The secondary battery may be charged by the output of the fuel cell. The charging and discharging of the secondary battery may be controlled by a control unit.

[0029] The fuel cell system includes a control unit. The control unit may control the oxidant gas system, the fuel gas system, the cooling system, etc. and control the entire fuel cell system. The control unit physically has, for example, a processing unit such as a CPU (Central Processing Unit), a ROM (Read Only Memory) that stores control programs and control data processed by the CPU, a storage device such as a RAM (Random Access Memory) that is primarily used as various working areas for control processing, and an input / output interface, and may be an ECU (Electronic Control Unit) or the like.

[0030] During sub-freezing start-up of the fuel cell system, if the coolant temperature of at least the first fuel cell stack among the plurality of fuel cell stacks is not sub-freezing, the control unit performs a normal start-up of the first fuel cell stack and then opens the first connecting valve, and at this time, the control unit may also open the second connecting valve. In the present disclosure, opening the first connection valve means allowing the cooling water to flow from the first cooling water pipe through the first connection pipe to the second cooling water pipe. In the present disclosure, opening the second connection valve means allowing the cooling water to flow from the second cooling water pipe through the second connection pipe to the first cooling water pipe.

[0031] When the fuel cell system is started below freezing, if the coolant temperatures of all of the fuel cell stacks are below freezing, the control unit compares the coolant temperatures of the fuel cell stacks. When the coolant temperature of the first fuel cell stack among the plurality of fuel cell stacks is the highest, the control unit performs a predetermined warm-up of the first fuel cell stack. Thereafter, when the coolant temperature of the first fuel cell stack reaches or exceeds a predetermined temperature, the control unit opens the first connecting valve. At this time, the control unit may also open the second connecting valve. If there is no temperature difference between the coolant temperatures of the multiple fuel cell stacks, the control unit performs a predetermined warm-up of the first fuel cell stack. Thereafter, when the coolant temperature of the first fuel cell stack reaches or exceeds the predetermined temperature, the control unit opens the first connection valve. At this time, the control unit may also open the second connection valve. The specified temperature is not particularly limited and may be 0°C or higher.

[0032] When starting the fuel cell system below the freezing point, if the cooling water temperatures of all the fuel cell stacks among the plurality of fuel cell stacks are below the freezing point and -10°C or higher, the control unit may not compare the cooling water temperatures of the plurality of fuel cell stacks. In the above case, the control unit may perform predetermined warm-up of a predetermined first fuel cell stack, and then, when the cooling water temperature of the first fuel cell stack reaches 0°C or higher, the control unit may open the first connection valve. At this time, the control unit may also open the second connection valve together. When starting the fuel cell system below the freezing point, if the cooling water temperatures of all the fuel cell stacks are higher than -10°C, since there is almost no difference in the startability of the fuel cell stacks, it is not necessary to determine the fuel cell stack to be started, and predetermined warm-up of a predetermined first fuel cell stack may be performed.

[0033] The predetermined first fuel cell stack may be the least deteriorated fuel cell stack among the plurality of fuel cell stacks. The degree of deterioration of the fuel cell stack may be determined based on the total operating time of the fuel cell stack, the voltage of the fuel cell stack, etc. The fuel cell stack with the shortest total operating time may be the least deteriorated fuel cell stack. The fuel cell stack with the highest voltage may be the least deteriorated fuel cell stack. When cooling systems with different lengths of cooling water pipes are combined, the fuel cell stack of the cooling system with less cooling water volume may be used as the predetermined first fuel cell stack.

[0034] The fuel cell system may be provided with a temperature sensor. The temperature of the cooling water may be measured by the temperature sensor. The temperature of the cooling water may be the temperature of the cooling water on the inlet side of the fuel cell stack, or the temperature of the cooling water on the outlet side of the fuel cell stack. The control unit may determine whether the temperature of the cooling water measured by the temperature sensor is below the freezing point at the start-up of the fuel cell system or at all times.

[0035] FIG. 1 is a system configuration diagram showing an example during normal operation of the fuel cell system of the present disclosure. The fuel cell system of the present disclosure includes a first fuel cell stack 10, a second fuel cell stack 20, and a cooling system. Although not shown, the fuel cell system may include a control unit, a fuel gas system, an oxidant gas system, and the like. The cooling system includes a first cooling water pipe 11, a second cooling water pipe 21, a cooler 30, a first connecting pipe 12, a second connecting pipe 22, a first bypass pipe 13, a first connecting valve 14, a first bypass valve 15, a second bypass pipe 23, a second connecting valve 24, a second bypass valve 25, a first cooling water pump 16, a second cooling water pump 26, a first cooling water inlet valve 17, and a second cooling water inlet valve 27. During normal operation of the fuel cell system, the first cooling water inlet valve 17 and the second cooling water inlet valve 27 are opened. The first connecting pipe 12 side of the first connecting valve 14 is closed, and the upstream and downstream sides of the first cooling water pipe 11 are opened. The first bypass pipe 13 side of the first bypass valve 15 is closed, and the upstream and downstream sides of the first cooling water pipe 11 are opened. The second connecting pipe 22 side of the second connecting valve 24 is closed, and the upstream and downstream sides of the second cooling water pipe 21 are opened. The second bypass pipe 23 side of the second bypass valve 25 is closed, and the upstream and downstream sides of the second cooling water pipe 21 are opened.

[0036] FIG. 2 is a system configuration diagram showing an example at the start-up below the freezing point of the fuel cell system of the present disclosure. In FIG. 2, the same components as those in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted. When starting the fuel cell system below freezing point, close the first cooling water inlet valve 17 and the second cooling water inlet valve 27. Close the downstream side of the first cooling water pipe 11 of the first connection valve 14, and open the upstream side of the first cooling water pipe 11 and the side of the first connection pipe 12. Close the downstream side of the first cooling water pipe 11 of the first bypass valve 15, and open the upstream side of the first cooling water pipe 11 and the side of the first bypass pipe 13. Close the downstream side of the second cooling water pipe 21 of the second connection valve 24, and open the upstream side of the second cooling water pipe 21 and the side of the second connection pipe 22. Close the downstream side of the second cooling water pipe 21 of the second bypass valve 25, and open the upstream side of the second cooling water pipe 21 and the side of the second bypass pipe 23.

[0037] Figure 3 is a system configuration diagram showing another example of starting the fuel cell system of the present disclosure below freezing point. In Figure 3, the same components as in Figure 1 are labeled with the same numbers, and their descriptions are omitted. The fuel cell system shown in Figure 3 has a first cooler 31 and a second cooler 32 instead of the cooler 30.

[0038] Figure 4 is a flowchart showing an example of the control of the fuel cell system of the present disclosure. The control unit determines whether the cooling water temperatures of the two fuel cell stacks are below freezing point. If the cooling water temperatures of neither of the two fuel cell stacks are below freezing point, the control unit starts the two fuel cell stacks normally. If the cooling water temperature of one of the first fuel cell stacks among the two fuel cells is not below freezing point, the control unit starts the first fuel cell stack normally, and then opens the first connection valve of the first connection pipe of the first fuel cell stack to supply cooling water to the second cooling water pipe. At this time, the control unit may also open the second connection valve. If the cooling water temperatures of both of the two fuel cell stacks are below freezing point, the control unit determines whether there is a temperature difference between the cooling water temperatures of the two fuel cell stacks. When there is no temperature difference in the cooling water temperature between the two fuel cell stacks, the control unit performs predetermined warm-up of the first fuel cell stack. Then, when the cooling water temperature of the first fuel cell stack reaches a predetermined temperature or higher, the control unit opens the first connection valve and supplies cooling water to the second cooling water pipe. At this time, the control unit may also open the second connection valve. When there is a temperature difference in the cooling water temperature between the two fuel cell stacks and the cooling water temperature of the first fuel cell stack is higher than that of the second fuel cell stack, the control unit performs predetermined warm-up of the first fuel cell stack. Then, when the cooling water temperature of the first fuel cell stack reaches a predetermined temperature or higher, the control unit opens the first connection valve and supplies cooling water to the second cooling water pipe. At this time, the control unit may also open the second connection valve.

[0039] FIG. 5 is a flowchart showing another example of the control of the fuel cell system of the present disclosure. The control unit determines whether the cooling water temperatures of the two fuel cell stacks are below the freezing point. If the cooling water temperatures of both of the two fuel cell stacks are below the freezing point among the two fuel cell stacks, the control unit determines whether the cooling water temperature of the two fuel cell stacks is below the freezing point and not lower than -10°C. When the cooling water temperatures of both of the two fuel cell stacks are below the freezing point and not lower than -10°C, the control unit does not compare the cooling water temperatures of the two fuel cell stacks, and the control unit performs predetermined warm-up of the first fuel cell stack. Then, when the cooling water temperature of the first fuel cell stack reaches 0°C or higher, the control unit opens the first connection valve and supplies cooling water to the second cooling water pipe. At this time, the control unit may also open the second connection valve.

Explanation of Reference Numerals

[0040] 10 First fuel cell stack 11 First cooling water pipe 12 First connection pipe 13 First bypass pipe 14 First connection valve 15 First bypass valve 16 First cooling water pump 17 First cooling water inlet valve 20 Second fuel cell stack 21 Second cooling water pipe 22 Second connection pipe 23 Second bypass pipe 24 Second connection valve 25 Second bypass valve 26 Second cooling water pump 27 Second cooling water inlet valve 30 Cooler 31 First cooler 32 Second cooler

Claims

Claim 1 A fuel cell system comprising: the fuel cell system having a plurality of fuel cell stacks, a cooling system, and a control unit; the plurality of fuel cell stacks including at least a first fuel cell stack and a second fuel cell stack; the cooling system having a first cooling water pipe, a second cooling water pipe, a cooler, a first connecting pipe, and a second connecting pipe; the first cooling water pipe connecting the cooler to a cooling water inlet of the first fuel cell stack and connecting a cooling water outlet of the first fuel cell stack to the cooler; the second cooling water pipe connecting the cooler to a cooling water inlet of the second fuel cell stack and connecting a cooling water outlet of the second fuel cell stack to the cooler; the first connecting pipe having a first connecting valve; the first connecting pipe connecting the first cooling water pipe and the second cooling water pipe and, when the first connecting valve is opened, bypassing the cooler to enable supply of cooling water from the first cooling water pipe to the second cooling water pipe; the second connecting pipe having a second connecting valve; the second connecting pipe connecting the first cooling water pipe and the second cooling water pipe and, when the second connecting valve is opened, bypassing the cooler to enable supply of cooling water from the second cooling water pipe to the first cooling water pipe; when starting the fuel cell system below the freezing point, if the cooling water temperature of at least the first fuel cell stack among the plurality of fuel cell stacks is not below the freezing point, the control unit performs normal start-up of the first fuel cell stack and then opens the first connecting valve; when starting the fuel cell system below the freezing point, if the cooling water temperatures of all the fuel cell stacks among the plurality of fuel cell stacks are below the freezing point, the control unit compares the cooling water temperatures of the plurality of fuel cell stacks; if the cooling water temperature of the first fuel cell stack among the plurality of fuel cell stacks is the highest, the control unit performs predetermined warm-up of the first fuel cell stack, and then, when the cooling water temperature of the first fuel cell stack reaches a predetermined temperature or higher, the control unit opens the first connecting valve; When there is no temperature difference in the cooling water temperature of the plurality of fuel cell stacks, the control unit performs predetermined warm-up of the first fuel cell stack, and then, when the cooling water temperature of the first fuel cell stack reaches or exceeds a predetermined temperature, the control unit opens the first connection valve, a fuel cell system.

2. When starting below the freezing point in the fuel cell system, if the cooling water temperatures of all the fuel cell stacks among the plurality of fuel cell stacks are below the freezing point and not lower than -10°C, the control unit does not compare the cooling water temperatures of the plurality of fuel cell stacks, the control unit performs predetermined warm-up of the first fuel cell stack, and then, when the cooling water temperature of the first fuel cell stack reaches or exceeds 0°C, the control unit opens the first connection valve, the fuel cell system according to claim 1.

3. The first connection pipe branches from the first cooling water pipe upstream of the cooler of the first cooling water pipe and merges with the second cooling water pipe downstream of the cooler of the second cooling water pipe, the fuel cell system according to claim 1.

4. The second connection pipe branches from the second cooling water pipe downstream of the cooler of the second cooling water pipe and merges with the first cooling water pipe upstream of the cooler of the first cooling water pipe, the fuel cell system according to claim 1.

5. The first cooling water pipe has a first bypass pipe, the first bypass pipe has a first bypass valve, the first bypass pipe branches from the first cooling water pipe upstream of the cooler of the first cooling water pipe and merges with the first cooling water pipe downstream of the cooler of the first cooling water pipe, bypassing the cooler, the second cooling water pipe has a second bypass pipe, the second bypass pipe has a second bypass valve, the second bypass pipe branches from the second cooling water pipe upstream of the cooler of the second cooling water pipe and merges with the second cooling water pipe downstream of the cooler of the second cooling water pipe, bypassing the cooler, the fuel cell system according to claim 1.

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