JP5646590B2 - Operation method of fuel cell power generation system - Google Patents

Description

  Embodiments described herein relate generally to a method for operating a fuel cell power generation system.

  In recent years, fuel cells that generate electricity by electrochemically reacting an oxidant (for example, air) and fuel (for example, a mixed gas containing hydrogen) have been in the limelight as a highly efficient power generation device that can reduce environmental impact. Research is underway to improve performance, reduce costs, and increase durability. There are many fields in which the application of this fuel cell is considered, but one of the fields where development is particularly active is home cogeneration plants.

  A household cogeneration plant is generally composed of a power generation unit, a heat utilization unit, and the like, and these units are connected by a hot water circulation line or the like for circulating hot water used for exhaust heat recovery.

  The power generation unit generally includes a fuel cell stack on which a unit cell formed by sandwiching an electrolyte, for example, a solid polymer film, between an oxidant electrode (cathode) and a fuel electrode (anode) is mounted. In addition, a fuel processing system that reforms raw fuel gas such as city gas or propane gas to generate hydrogen rich gas and supplies the hydrogen rich gas to the fuel electrode in the fuel cell stack is provided.

  Furthermore, the power generation unit includes an oxidant supply system that supplies air to the oxidant electrode in the fuel cell stack, a heat management system that collects exhaust heat from the battery and the fuel processing system and inputs the heat to the heat utilization unit, A power conversion device that converts a direct current output generated in the fuel cell stack into an alternating current that can be linked to a power system is provided.

  The energy conversion performed in the fuel cell stack is performed based on an electrochemical reaction in a unit cell represented by the following formulas (1) to (3), for example, in the case of a solid polymer type. In a fuel cell that operates at about 100 ° C. or less like a solid polymer type, a platinum catalyst or an alloy catalyst mainly composed of platinum is generally used for promoting the reaction. In this fuel cell, both the oxidant electrode and the fuel electrode are made of a conductive porous material that allows gas to pass through, and the catalyst is held on the surface so as to form a porous layer. On the other hand, this catalyst is in contact with the electrolyte.

Reaction at the fuel electrode: 2H ₂ → 4H ⁺ + 4e ⁻ (1)
Reaction at oxidant electrode: O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O (2)
Total reaction: 2H ₂ + O ₂ → 2H ₂ O (3)
Here, the voltage generated in the unit cell due to the electrochemical reaction is regulated by the Gibbs energy of the oxidation-reduction reaction at the electrode under operating conditions, and the theoretical upper limit is about 1.2V. Furthermore, due to the loss of activation polarization, concentration polarization, resistance, etc. accompanying the reaction, usually only a cell voltage of 1 V or less can be obtained.

  Therefore, the fuel cell stack is configured as a laminated battery by laminating a plurality of unit cells so as to be in electrical series and fastening them from both ends in order to obtain a required voltage. A fuel cell stack is a stack battery in which reaction gas, cooling water, a power tap, an instrumentation line, and the like are attached.

  A separator that prevents direct mixing of the oxidant and the fuel is inserted between the unit cells constituting the fuel cell stack. This separator is usually provided with a flow path for distributing oxidant and fuel to the unit cells.

  Since the unit cell is a conversion device that converts the energy drop (generated heat) that accompanies a chemical reaction into electrical energy, the energy drop that is not converted into electrical energy (for example, bound energy or lost energy due to polarization) is converted into heat. Released.

  Here, in order to prevent overheating in the unit cells and recover the released heat and effectively use it in the cogeneration plant, cooling plates are inserted between the plurality of unit cells. That is, the cooling water circulates through the fuel cell power generation system including the fuel cell stack through the cooling plate, thereby recovering heat generated in the cells. This cooling plate is usually integrated with a separator.

  By the way, the voltage generated in the unit cell decreases due to polarization. Polarization is roughly classified into activation polarization, resistance polarization, and diffusion polarization. It is important for cell design and operation control to control these polarizations and secure a necessary voltage. In particular, in the case of a polymer electrolyte fuel cell, it is important to humidify the polymer electrolyte membrane, which is an electrolyte, in a wet state and keep the resistance polarization low.

  However, the humidification method greatly affects the diffusion state of the reaction gas inside the cell. When the electrolyte membrane is in a wet state, the contribution of diffusion polarization in the voltage drop is relatively large. This diffusion polarization becomes more prominent as the reaction gas consumption rate due to the electrochemical reaction approaches the diffusion rate. Therefore, it is important to control factors that inhibit the diffusion of the reaction gas.

  In a polymer electrolyte fuel cell power generation system, as disclosed in, for example, Patent Document 1 and Patent Document 2, various fuel cell units are used for the purpose of maintaining a wet state of a solid polymer film as an electrolyte. A method of humidifying from the outside of the fuel cell stack or from the inside of the cell is proposed.

  For example, a method is known in which oxidant gas supplied from an oxidant supply system is humidified by a humidifier or the like provided near the inlet of the fuel cell stack. In this example, humidified oxidant gas is introduced into the fuel cell stack. In addition, the fuel gas is mixed with the vapor that is supplied or generated in the process such as the reforming reaction in the fuel processing system and supplied to the fuel cell stack.

  In addition, the method of humidifying from the outside needs to secure energy for obtaining a vapor pressure corresponding to the dew point of the cell temperature. Therefore, the energy necessary for humidification is acquired while suppressing the influence on the system efficiency. For this reason, a method of exchanging heat of exhaust gas or cell inlet gas is adopted in the vicinity of the fuel cell stack outlet.

  However, even if the energy required for humidification can be acquired while suppressing the impact on system efficiency, the method of humidifying from the outside considers the condensation of humidified water after humidification and the uniform distribution between stacked cells. There is a need to. In addition, not only humidified water but also water generated by power generation in the oxidant electrode and water that passes through the electrolyte membrane with hydrogen ions from the fuel electrode toward the oxidant electrode, etc. There is a flow of water. Furthermore, there is a flow of water that diffuses according to the gradient of moisture content.

  Furthermore, heat exchange with the reaction gas introduced into the cell causes evaporation from the cell to the gas phase and condensation from the gas phase to the cell. Therefore, considering the heat generation of the cell itself, the water and heat in the cell interact with each other and exhibit complex behavior.

  The tendency inside the cell tends to be dry at the inlet of the cell and excessive moisture at the outlet. In the case where water stays beyond the appropriate range with respect to the gas diffusion layer of both electrodes in contact with the catalyst layer of the cell or the separator, the inhibition of gas diffusion becomes significant. This tendency of gas diffusion inhibition mainly occurs at the oxidizer electrode where water is enriched by generated water and accompanying water.

  Further, since the oxygen diffusion rate is slower than the hydrogen diffusion rate, the polarization of the oxygen electrode affects the performance of the entire cell.

  Regarding the condensation in the cell, the moisture in the reaction gas in the humidified cell is almost saturated humidity, and the moisture in the reaction gas is condensed in a low temperature portion in the cell. When this condensed water is generated in the gas groove and this condensed water cannot be removed, the groove is closed, and a reaction gas deficiency occurs downstream of the closed portion.

  In the cell in which the reaction gas deficiency occurs, the characteristics are rapidly deteriorated. Therefore, not only stable operation becomes difficult, but there is a possibility that the cell or the separator is permanently damaged due to local overheating due to local current concentration, corrosion of the carbon member, or the like.

  The above phenomenon is a serious problem in both the oxidant electrode and the fuel electrode. In the direct internal humidification method, the same phenomenon may occur when droplets stay in the groove due to variations in water supply or evaporation into the cell groove.

  Therefore, as disclosed in Patent Document 4, as a countermeasure against gas groove blockage due to non-humidification in the cells and retention of water droplets, a porous separator is arranged between the unit cells, and a part of the cooling water is made porous. Means have been proposed in which a quality separator is circulated and used to humidify the reaction gas.

  According to this method, by setting the pressure of the cooling system for circulating the cooling water to be lower than that of the reaction gas (including atmospheric pressure or less), water droplets staying in the groove can be sucked into the cooling system. It is possible to solve various problems caused by the blockage. Furthermore, since this method autonomously humidifies and absorbs moisture according to the local water balance in the cell plane, a uniform humidification state in the cell plane must be realized without risk of groove blockage. Can do.

  As described above, the flow path pattern on the separator, which plays an important role in water management and heat management of the cell, takes into account the temperature distribution, humidity distribution, pressure loss, and the joint position of the inlet or outlet along the flow path. It is decided.

JP 2001-176529 A JP 2003-142134 A JP 2001-176522 A Japanese National Patent Publication No. 11-508726 JP 2004-103456 A JP-A-6-68884 JP 2009-115649 A

  By the way, in order to put a household fuel cell power generation system into practical use, the fuel cell stack needs to supply a stable output for a long period of time in order to improve reliability. Therefore, the development for the purpose of preventing the deterioration of the cell characteristics is in progress for the cells in the fuel cell stack.

  As one of the measures for preventing the deterioration of cell characteristics, the latent heat cooling operation, that is, the cooling water flow rate is decreased so that the evaporation latent heat of the cooling water and the cell heat generation are balanced, thereby increasing the temperature of the fuel cell stack and A method for suppressing water condensation and suppressing long-term voltage drop has been considered.

  Further, by performing latent heat cooling, it is possible to reduce the water circulation flow rate required for cooling to about 1/10 of the conventional one. As a result, it is possible to reduce the size and power of the cooling water pump. Furthermore, exhaust heat recovery efficiency is improved by performing high-temperature operation, which contributes to improving the overall thermal efficiency of the entire system (the sum of power generation efficiency and exhaust heat recovery efficiency).

  By adopting the porous separator also in the latent heat cooling operation, it becomes possible to uniformly supply water to the reaction surface of the cell and evaporate it, and to maintain the temperature and humidity distribution inside the battery uniformly.

  However, in order to use the porous separator under conditions where the cooling water flow rate is small, it is necessary to take measures against the gas mixed in the cooling water. During operation, the porous separator penetrates through the porous member and gas is mixed from the reaction gas flow path into the cooling water flow path. There is no problem if the gas mixed in the cooling water flow path is discharged by the water flow. However, when the gas mixed in the cooling water flow path accumulates inside the flow path, the cooling water flow path may be blocked. The blockage of the cooling water flow path causes local overheating and insufficient humidification due to a local shortage of water supply, and makes the operation of the fuel cell difficult. Further, the clogging of the cooling water flow path has a high possibility of adversely affecting the product life, such as irreversible performance degradation and electrolyte membrane damage.

  The main countermeasures against bubbles mixed into the cooling water flow path inside the battery include the following.

  (1) The minimum dimension of the groove is made larger than the bubble size when bubbles are discharged from the wall surface of the cooling water channel into the water flow.

  (2) In order to promote the discharge of the bubbles in the cooling water by buoyancy, the cooling water groove is formed in a straight line extending in the vertical direction, and the flow direction of the cooling water is the same as the buoyancy, that is, the upward flow.

  (3) To prevent coalescence of bubbles, the amount of bubbles mixed in is controlled so that the flow mode of the gas-liquid two-phase flow in the cooling water flow path composed of bubbles and cooling water becomes a bubble flow. Therefore, by reducing the pressure difference between the cooling water and the reaction gas at the separator inlet and the cooling water pressure loss in the separator, the pressure difference between the gas flow path and the cooling water flow path, that is, the gas passing through the porous member is reduced. Reduce the driving force to reduce the amount of mixed gas.

  (4) Forcibly removing bubbles by periodically increasing the cooling water flow rate.

  In order to carry out these countermeasures, a surpaintine type flow path as disclosed in Patent Document 5 or a vertical groove flow path as disclosed in Patent Document 6 is known.

  Since most of the surpaint tin flow channel is a horizontal groove, bubbles are eliminated by being pushed out by the water flow. Therefore, if the flow rate is lowered to perform latent heat cooling, it may not be possible to secure a flow rate necessary for bubble elimination. Further, since the number of flow paths is small, the area per flow path is large, and the frequency of mixing bubbles per flow path is increased. Furthermore, once the blockage occurs in the flow path, the influence range becomes large.

  Further, in the long-term operation, the possibility of bubbles being accumulated in the cooling water flow path is increased, so it is desirable to perform forced bubble removal by periodically increasing the cooling water flow rate. However, in this configuration, since the pressure loss of the flow path is large, the variation range of the pressure loss when the flow rate is increased becomes large. Therefore, there is a limit to downsizing the cooling water circulation pump as long as periodic bubble removal operation is performed. In addition, a large pressure loss also increases the amount of inflow of bubbles.

  On the other hand, as a method for solving the above problem, there is a method of adopting a vertical flow path. This is used for cells with a relatively small area and may be combined with a water permeable plate. Since the grooves are arranged in the vertical direction in this vertical flow path configuration, if the flow in the flow path is a bubble flow, the bubbles can be raised by buoyancy. Further, compared with the sir pain tin type, the flow path length is short and the number of parallel flow paths is large, so that the pressure loss is small and the pump can be downsized.

  However, in this configuration, there is a problem that non-uniformity in the flow rate distribution between the vertical flow paths increases as the flow velocity increases. This problem becomes conspicuous when the size of the cell is increased and the number of cooling water flow paths is increased. This non-uniform flow distribution is caused by the relationship between the change in static pressure accompanying branching and merging and the pressure loss at the horizontal part. Further, this non-uniformity becomes more prominent as the effects of pressure loss and gravity (water head) in the vertical flow path become relatively smaller compared to confluence and branching loss, and pressure loss in the horizontal portion. In the long-term operation, the possibility of bubbles accumulating in the cooling water flow path increases, so it is desirable to forcibly eliminate bubbles by periodically increasing the cooling water flow rate. However, in this configuration, even if the flow rate is increased, the flow rate does not increase in some of the flow paths. Therefore, it may be difficult to eliminate bubbles that accumulate in the portion.

  Furthermore, in the operation at the plant, it is necessary to consider measures against bubbles mixed in the cooling water circulation system at a site other than the battery stack, in addition to the bubbles mixed inside the separator. By operating the porous separator with the cooling water pressure lower than the reaction gas pressure, the characteristics of the porous separator can be effectively utilized. However, in normal pressure operation, that is, when the reaction gas is introduced into the battery stack near atmospheric pressure, the cooling water pressure may be lower than atmospheric pressure. Therefore, there is a risk that the atmosphere leaks into the cooling water circulation system at a pipe connection portion or the like.

  Leak-in into the cooling water circulation system can be suppressed by appropriate construction and inspection of the piping system. However, there is also a demand for ensuring robustness against unforeseen circumstances during long-term operation at the site.

  In either of the two configurations described above, no consideration is given to suppressing the influence of bubbles at the inlet of the fuel cell stack.

  The present invention has been proposed in order to solve the above-mentioned problems, and its purpose is to suppress bubble inflow and eliminate bubbles in a latent heat cooling operation in an internal humidification cell using a porous separator, thereby cooling a wide range of cooling. It is to be able to operate with the amount of water.

In order to achieve the above object, a method for operating a fuel cell power generation system according to an embodiment of the present invention includes a unit cell formed by sandwiching an electrolyte between an oxidant electrode and a fuel electrode, and a cooling water supply unit that supplies cooling water. The cooling water flow path portion is formed so as to be supplied from the cooling water discharge portion and discharged from the cooling water discharge portion. The cooling water flow path portion is a porous material whose flow path surface is divided into a plurality of sections. each partition has a plurality of first grooves are formed in a flat row to each other, a first coolant passage in which the cooling water in these first groove is configured to flow upward, the The cooling water extends in a direction perpendicular to the first groove formed in each of the plurality of sections and flows in the grooves formed at both ends of the first groove so as to be connected to the first cooling water flow path. A second cooling water flow path configured as described above, and the second cooling water flow path An average air of a porous material that connects the cooling water supply section or the cooling water discharge section and has a third cooling water flow path configured to allow the cooling water to flow therethrough and constitutes the flow path surface. The hole diameter is d _P , the surface tension of the cooling water is σ , the contact angle of the cooling water at the contact surface between the cooling water and the first cooling water flow path is θ, the gravitational acceleration is g, and the density of the cooling water is ρ _L When the density of the gas mixed in the cooling water is ρ _G , the minimum dimension of the cross section perpendicular to the flow direction of the cooling water in the first cooling water channel is D _B = [4d _P σ sin θ / in _{g (ρ L -ρ G)]} 1/3 at defined values D operating method of the fuel cell power generation system comprising a fuel cell stack formed by alternately laminating a fuel cell separator larger, the more _B Supplying cooling water to the fuel cell separator at a constant flow rate A cooling water supply step; and a bubble discharging step of supplying the fuel cell separator with increasing or decreasing a flow rate of the cooling water at a predetermined time interval and discharging bubbles in the cooling water flow path section. .

  According to the embodiment of the present invention, in the internal humidification cell by the porous separator, the bubble inflow is suppressed and the bubbles are eliminated during the latent heat cooling operation, and the operation can be performed with a wide range of cooling water amount.

1 is a plan view showing an embodiment of a fuel cell stack according to the present invention. FIG. 2 is a sectional view taken along the line II-II of the fuel cell stack of FIG. 1. FIG. 3 is a partially enlarged sectional view of the vicinity of a cooling water supply unit in FIG. 2. FIG. 3 is a schematic vertical sectional view showing a wall surface of a cooling water passage in FIG. 2. It is a graph which shows the relationship between the bubble diameter and bubble rising speed in the fuel cell separator of FIG. 2 is a graph showing the relationship between the operation time of the fuel cell stack of FIG. 1 and the coolant flow rate. It is a graph which shows the relationship between the number of the 1st groove | channel of the fuel cell separator of FIG. 2, and a groove | channel flow volume. It is an elevation sectional view showing the conventional cooling water channel part. It is a graph which shows the result of having calculated the relationship between the number of the cooling water groove in the cooling water flow-path part of FIG. 8, and a groove flow volume by numerical analysis. FIG. 2 is an elevational sectional view showing an example in which the cooling water channel structure of FIG. 1 is applied to an internal manifold type fuel cell stack.

  Hereinafter, an embodiment of a fuel cell stack according to the present invention will be described with reference to the drawings. FIG. 1 is a plan view showing a part of the fuel cell stack of the present embodiment, and shows a state in which a unit cell 1 is sandwiched between two fuel cell separators 2.

  The fuel cell stack is composed of flat unit cells 1 standing substantially vertically and flat fuel cell separators 2 standing substantially vertically like this, alternately stacked in the horizontal direction. . The unit cell 1 has a structure in which a solid molecular electrolyte membrane 3 is sandwiched between an oxidant electrode 4 and a fuel electrode 5.

  The fuel cell separator 2 configured to sandwich the unit cell 1 is formed of a porous material, and includes a fuel gas flow path portion 6, an oxidant gas flow path portion 7, and a cooling water flow path portion 8. Yes. A fuel gas passage 11 through which a fuel gas such as hydrogen flows is formed in the fuel gas passage portion 6. The oxidant gas flow path portion 7 is formed with an oxidant gas flow path 12 through which an oxidant gas such as air flows. The cooling water channel portion 8 is formed with a cooling water channel 13 through which the cooling water flows. The surface of the fuel cell separator in which the cooling water channel 13 is formed is defined as a channel surface.

  The fuel cell separator 2 has a structure in which a cooling water channel portion 8 is sandwiched between a fuel gas channel portion 6 and an oxidant gas channel portion 7. Note that an oxidant gas flow path portion 7 is in contact with the oxidant electrode 4 side of the unit cell 1, and a fuel gas flow path portion 6 is in contact with the fuel electrode 5 side.

  The energy conversion performed in the fuel cell stack is performed based on an electrochemical reaction in the unit cell 1 represented by the formulas (1) to (3) in the solid polymer form, for example. At this time, since a part of the cooling water flowing through the cooling water flow path 13 is used for the electrochemical reaction of the above formulas (1) to (3), the cooling water flowing along the reaction cooling water flowing direction 40 shown in FIG. Distribute the route.

  FIG. 2 is a sectional view taken along the line II-II of the cooling water flow path portion 8 of the fuel cell separator 2 arranged in the fuel cell stack of FIG.

  The cooling water flow path portion 8 is provided with a cooling water supply portion 15 that supplies cooling water from the outside of the fuel cell separator 2 and a cooling water discharge portion 16 that discharges the cooling water to the outside of the fuel cell separator 2. . In the assembled state of the fuel cell stack, the cooling water supply unit 15 is disposed below and the cooling water discharge unit 16 is disposed above.

  The cooling water flow path portion 8 is formed with a plurality of first grooves 21 that are arranged in parallel substantially over the entire surface along the surface on which the cooling water flow path portion 8 is formed. The first groove 21 is formed to extend in the vertical direction when the fuel cell stack is assembled. The first groove 21 is formed so that cooling water flows. A flow path group composed of the plurality of first grooves 21 is defined as a first cooling water flow path 22.

  The first cooling water channel 22 is divided into four sections, for example, a first section 31, a second section 32, a third section 33, and a fourth section 34. In each of the four sections 31, 32, 33, 34, approximately the same number of first grooves 21 are arranged.

  The cooling water supply unit 15 is formed with a communication groove 17 that distributes the cooling water to the four sections 31, 32, 33, and 34.

  In each of the first cooling water flow paths 22 divided into four sections 31, 32, 33, and 34, the end of the plurality of first grooves 21 on the cooling water inlet side is perpendicular to the direction in which the first grooves 21 extend. Grooves are formed so that the cooling water flows in the direction, that is, in the horizontal direction. Similarly, a groove is formed at the end on the cooling water outlet side so that the cooling water flows in the horizontal direction. These grooves are defined as an inlet-side second cooling water channel 23 and an outlet-side second cooling water channel 24. In addition, the inlet side 2nd cooling water flow path 23 and the exit side 2nd cooling water flow path 24 are arrange | positioned at the four divisions 31, 32, 33, and 34, respectively.

  Further, a flow path connecting the communication groove portion 17 arranged in the cooling water supply unit 15 and the inlet side second cooling water flow path 23 is formed, and this flow path is defined as an inlet side third cooling water flow path 25. On the other hand, a flow path connecting the cooling water discharge portion 16 and the outlet side second cooling water flow path 24 is formed, and this flow path is defined as an outlet side third cooling water flow path 26.

  Here, the flow of the cooling water in the fuel cell separator 2 will be described.

  The cooling water is supplied from the cooling water supply unit 15 to the fuel cell separator 2. The cooling water supplied from the cooling water supply unit 15 is distributed to the inlet-side third cooling water channel 25 arranged in each of the four sections 31, 32, 33, 34 by the communication groove 17. Cooling water flowing through the inlet-side third cooling water channel 25 flows into the inlet-side second cooling water channel 23 and is distributed to the first cooling water channel 22 having the plurality of first grooves 21. The inlet-side second cooling water channel 23 is used as a distribution header that distributes the cooling water supplied from the inlet-side third cooling water channel 25 to the first cooling water channel 22.

  The cooling water that has flowed through the first cooling water flow path 22 merges at the outlet side second cooling water flow path 24, flows through the outlet side third cooling water flow path 26, and is discharged from the cooling water discharge unit 16. The outlet-side second cooling water channel 24 is used as a merging header that combines the cooling water supplied from the first cooling water channel 22 and supplies it to the outlet-side third cooling water channel 26.

  A part of the cooling water that has flowed into the first cooling water flow path 22 moves through the path along the reaction cooling water flow direction 40, and the reaction of the equations (1) to (3) Evaporates in the oxidant gas flow path 12 together with the water generated in the cell.

  The lengths of the first cooling water flow paths 22 arranged in the four sections 31, 32, 33, and 34 and the cross-sectional dimensions of the cross section perpendicular to the flow direction of the cooling water are formed to be equal. Similarly, the lengths and cross-sectional dimensions of the inlet-side and outlet-side second cooling water channels 23 and 24 are the same.

  About the groove | channel which comprises the inlet side and outlet side 3rd cooling water flow paths 25 and 26, it is comprised so that the total value of the length of the water inflow part of each division and a water outlet part may become equal in a flow-path surface.

Furthermore, the smallest dimension of the cross section of the first cooling water passage 22 is greater than the value defined by D _B of the formula (4), it is desirable that the two more times.

Here, d _P is the average pore diameter of the porous material constituting the channel surface, σ is the surface tension of the cooling water, θ is the contact angle of the cooling water at the contact surface between the cooling water and the cooling water channel 13, and g is the acceleration of gravity. , Ρ _L is the density of the cooling water, and ρ _G is the density of the gas mixed in the cooling water.

D _B = [4d _P σ sin θ / g (ρ _L −ρ _G )] ^1/3 (4)
In this embodiment, _{D B} is about 0.3 mm, the smallest dimension of the first coolant passage 22 is about 0.7 mm. Further, the cross-sectional area of the inlet-side and outlet-side second cooling water flow paths 23, 24 is larger than the cross-sectional area of the first cooling water flow path 22. In the present embodiment, it has about twice the cross-sectional area of the first cooling water channel 22. On the other hand, the cross-sectional areas of the inlet side and outlet side third cooling water passages 25 and 26 are substantially equal to those of the first cooling water passage 22.

  FIG. 3 is a partially enlarged sectional view in the vicinity of the cooling water supply unit 15 of the fuel cell separator 2 shown in FIG. 2, and the flow path shape in the cooling water supply unit 15 with respect to bubbles 42 flowing from the outside of the fuel cell stack. This shows the effect of.

  Moreover, the cooling water supply part 15 changes the width | variety and pitch of a flow path with the communication groove part 17 as a boundary. The width and pitch of the flow path are equal to those of the first cooling water flow path 22 on the upstream side of the communication groove 17, and the width dimension of the flow path is equal to that of the inlet-side second cooling water flow path 23 on the downstream side of the communication groove 17. In the cooling water discharge part 16, the width of the flow path is made equal to the outlet side second cooling water flow path 24.

  When bubbles 42 larger than the internal flow path enter from the outside of the fuel cell stack, particularly when the flow velocity is small as in the latent heat cooling operation, a part of the flow path of the bubble 42 cooling water supply unit 15 is blocked. There is a case where it stays. Even in such a case, the presence of the communication groove portion 17 allows the cooling water to flow into all the internal flow paths from the unblocked flow path.

  Next, referring to FIG. 4 and FIG. 5, the effect of the channel size on the inflow of the bubbles 42 from the porous wall surface inside the channel will be described.

  FIG. 4 is an elevational sectional view showing the bubbles 42 in the cooling water channel of the fuel cell separator 2. FIG. 5 is a graph showing the bubble rising speed with respect to the bubble diameter.

  The gas that has permeated the porous vertical wall becomes a bubble 42 on the wall surface in contact with the liquid, and when the bubble 42 grows and becomes large to a certain extent, it separates from the wall surface due to the action of buoyancy. Bubbles 42 separated from the wall surface rise in the liquid as single bubbles. The above equation (4) is given as a function of the interfacial tension and buoyancy between the wall surface, cooling water, and gas.

  Once the air bubble 42 leaves the wall surface, it rises due to buoyancy. The magnitude of this buoyancy depends only on the pressure change of the head corresponding to the ascent distance. For this reason, if the bubbles 42 do not coalesce, the change in the size of the bubbles 42 is slight. In the case of a normal riser tube, the condition for the coalescence of the bubbles 42 is when the void ratio exceeds about 25%. In this embodiment, it rises as a single bubble 42 without coalescence.

  As shown in FIG. 5, the rising speed of the single bubble 42 in the vertical tube tends to increase as the diameter of the bubble 42 increases. However, as the bubble diameter approaches the channel cross-sectional dimension Dp, it is decelerated under the influence of the wall surface. In this embodiment, since the cross-sectional dimension Dp of the first cooling water channel is 0.7 mm with respect to the bubble diameter of 0.3 mm to 0.4 mm, the bubble rising speed due to buoyancy is maximized, and the discharge efficiency of the bubbles 42 is Highest.

  Next, the operation of the combination of the first cooling water channel 22, the inlet side and outlet side second cooling water channels 23, 24, and the inlet side and outlet side third cooling water channels 25, 26 will be described.

  The bubbles 42 rise up the first cooling water flow path 22, hit the outlet side second cooling water flow path 24, pass through the outlet side third cooling water flow path 26 as they are, and are discharged to the outside of the fuel cell separator 2. The outlet side second cooling water channel 24 and the outlet side third cooling water channel 26 are horizontal channels. However, since the flows of the first cooling water channel 22 merge, the flow rates of the outlet side second cooling water channel 24 and the outlet side third cooling water channel 26 are faster than those of the first cooling water channel 22. Therefore, it is possible to quickly discharge the bubbles 42 to the outside.

  The speed at which the bubbles 42 are discharged depends on the number of first grooves 22 arranged in the first cooling water flow path 22 communicated by the outlet-side second cooling water flow path 24, that is, the number of first grooves 22 per section. It is determined.

  The combination of the dimensions of the inlet-side and outlet-side second cooling water flow paths 23, 24 and the first cooling water flow path 22 affects the flow distribution between the first grooves 21 of the first cooling water flow path 22 in the compartment. That is, the pressure loss in the first cooling water flow path 22 is the pressure loss in the inlet side and outlet side second cooling water flow paths 23, 24 and the first cooling water flow path 22 and the inlet side and outlet side second cooling water flow paths 23, As the effect of pressure loss at the time of branching or merging with 24 decreases, the non-uniformity in the flow distribution between the first grooves 21 of the first cooling water passage 22 increases.

  The pressure loss of the first cooling water channel 22 is limited in reducing the channel cross-sectional dimension Dp from the viewpoint of the bubble rising speed. Further, the flow rates of the inlet-side and outlet-side second cooling water channels 23 and 24 need to be as large as possible from the viewpoint of bubble elimination. Therefore, both of increasing the cross-sectional dimensions of the inlet-side and outlet-side second cooling water channels 23 and 24 and reducing the number of branches, that is, the number of the first grooves 21 of the first cooling water channel 22 per section, are both. It is necessary to optimize so as to satisfy.

  In the present embodiment, the cross-sectional area of the inlet-side and outlet-side second cooling water flow paths 23, 24 is approximately twice the cross-sectional area of the first cooling water flow path 22.

  In order to make the flow distribution between the sections 31, 32, 33, and 34 in the flow path surface uniform, it is necessary to optimize the dimensions of the inlet side and outlet side third cooling water flow paths 25 and 26. In the present embodiment, the lengths of the inlet-side and outlet-side third cooling water channels 25, 26 are made uniform between the sections 31, 32, 33, 34, and the inlet side and the pressure loss of the first cooling water channel 22 By making the pressure loss of the outlet side third cooling water passages 25 and 26 larger, the uniformity is achieved.

  FIG. 6 is a graph showing the relationship between the operation time of the fuel cell stack and the coolant flow rate in the present embodiment.

  In spite of measures for designing the fuel cell separator 2 as described above, bubbles 42 may accumulate in the cooling water flow path 13 during long-term operation. In order to forcibly eliminate the bubbles 42 accumulated in the cooling water supply unit 15, for example, as shown in FIG. 6, the cooling water flow rate is periodically increased from the flow rate Q1 to the flow rate Q3 to forcibly eliminate the bubbles. You may go.

  FIG. 7 is a graph showing the relationship between the numbers assigned to the plurality of first grooves 21 of the fuel cell separator 2 and the flow rate of the cooling water flowing through the first grooves 21 corresponding to the numbers. 8 and 9 are comparative examples of FIGS. 1 and 7.

  FIG. 8 is an elevational sectional view showing the conventional cooling water flow path 110 of the conventional fuel cell separator. The conventional cooling water flow path section 110 is provided with a cooling water supply section 15 that supplies cooling water from the outside of the conventional fuel cell separator, and a cooling water discharge section 16 that discharges the cooling water to the outside.

  On the surface on which the conventional cooling water flow path portion 110 is formed, a plurality of cooling water grooves 120 through which the cooling water flows are arranged substantially in parallel along this surface. The cooling water groove 120 is formed to extend in the vertical direction when the fuel cell stack is assembled. It is formed so that cooling water flows through the cooling water groove 120.

  FIG. 9 shows the amount of cooling water flowing in each cooling water groove 120 when the flow rate is increased in the configuration shown in FIG. 8 by numerical analysis. This conventional configuration has a problem that the non-uniform flow rate between the cooling water grooves 120 increases as the flow velocity increases.

  As shown in FIG. 9, the flow rate Q1, the flow rate Q2, and the flow rate Q3 indicate the cooling water flow rate per separator, and the flow rate Q2 and the flow rate Q3 are 2.5 times and 5 times the flow rate Q1, respectively. is there. As shown in FIG. 9, it can be seen that the flow rate distribution between the cooling water grooves 120 is remarkably non-uniform as the flow rate increases.

  On the other hand, as shown in FIG. 7, even if the cooling water flow rate is increased or decreased, the flow rate distribution between the first grooves 21 of the first cooling water channel 22 is made uniform compared to the conventional configuration. It is possible to reliably eliminate the bubbles 42.

  Further, since the pressure loss is smaller than that of a conventional sirpaine tine type flow path and the pressure loss change with respect to the flow rate change can be reduced, the cooling water pump can be reduced in size and power.

  Therefore, according to the separator 2 of the fuel cell according to the present embodiment and the fuel cell stack using the same, air bubbles into the fuel cell stack can be obtained while using the porous separator that is particularly excellent in uniform humidification and avoiding the groove blockage. It is possible to suppress the deposition of 42.

  Accordingly, the latent heat cooling operation can be continued stably, which greatly contributes to the improvement of product value.

  As mentioned above, although embodiment which concerns on this invention was described, these are only illustrations, Comprising: This invention is not limited to these. The configuration illustrated in the above embodiment is not limited to the shape shown in FIG. 1. For example, as shown in FIG. 10, the cooling water flow path structure shown in FIG. 1 can be applied to an internal manifold type fuel cell stack.

  Moreover, in the said embodiment, although the cooling water flow path 13 of the fuel cell separator 2 was divided | segmented into the four divisions 31, 32, 33, 34, it is not restricted to this but can be divided | segmented into plurality.

  In addition, the fuel gas flow path portion 6, the oxidant gas flow path portion 7, and the cooling water flow path portion 8 of the fuel cell separator 2 of the above embodiment each form an independent layer to form a three-layer structure. However, it is not limited to this. For example, the fuel gas channel 11 may be formed on one side of one flat plate, and the cooling water channel 13 may be formed on the opposite side.

  DESCRIPTION OF SYMBOLS 1 ... Unit cell, 2 ... Fuel cell separator, 3 ... Solid molecular electrolyte membrane, 4 ... Oxidant electrode, 5 ... Fuel electrode, 6 ... Fuel gas flow path part, 7 ... Oxidant gas flow path part, 8 ... Cooling water flow 11: Fuel gas flow path, 12 ... Oxidant gas flow path, 13 ... Cooling water flow path, 15 ... Cooling water supply section, 16 ... Cooling water discharge section, 17 ... Communication groove section, 21 ... First groove, 22 ... 1st cooling water flow path, 23 ... Inlet side 2nd cooling water flow path, 24 ... Outlet side 2nd cooling water flow path, 25 ... Inlet side 3rd cooling water flow path, 26 ... Outlet side 3rd cooling water flow path, 31 ... 1st 1 section, 32 ... 2nd section, 33 ... 3rd section, 34 ... 4th section, 40 ... reaction water flow direction for reaction, 42 ... bubbles, 110 ... conventional cooling water flow path section, 120 ... cooling water groove

Claims (1)

A unit cell formed by sandwiching an electrolyte between an oxidant electrode and a fuel electrode;
A cooling water flow path portion configured to supply cooling water from the cooling water supply section and discharge from the cooling water discharge section, the cooling water flow path portion being a porous material on the flow path surface, and having a plurality of compartments is divided into, first these compartments plurality of first grooves each are configured to have formed in a flat row to each other, the cooling water to these first grooves flows upward The cooling water flows through a cooling water flow path and a groove formed in each of both ends of the first groove extending in a direction perpendicular to the first groove formed in each of the plurality of sections, and the first A second cooling water channel configured to be connected to the cooling water channel, the second cooling water channel and the cooling water supply unit or the cooling water discharge unit are connected, and the cooling water is circulated. A third cooling water flow path, and a flat surface of a porous material constituting the flow path surface. The pore diameter d _P, the surface tension of the coolant sigma, the contact angle of the cooling water on the contact surfaces of the first coolant passage and the coolant theta, a gravitational acceleration g, the density of the cooling water ρ _{When L} and the density of the gas mixed in the cooling water are ρ _G , the minimum dimension of the cross section perpendicular to the flow direction of the cooling water in the first cooling water channel is D _B = [4d _P σ sin θ a fuel cell separator is greater than _{/ g (ρ L -ρ G)} ] value _{D B} defined by ^1/3,
In a method of operating a fuel cell power generation system comprising a fuel cell stack formed by alternately laminating
A cooling water supply step of supplying cooling water to the fuel cell separator at a constant flow rate;
A bubble discharging step of supplying the fuel cell separator with increasing or decreasing a flow rate of cooling water at a predetermined time interval, and discharging bubbles in the cooling water flow path section,
A method for operating a fuel cell power generation system, comprising:

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