Abstract:
A water supply device, which performs humidification and/or cooling of a fuel cell stack ( 2 ), is disclosed. The water supply device includes a water tank ( 25 ) for storing water, and a pump which sends water from the water tank ( 5 ) to the fuel cell stack ( 2 ), wherein one of a discharge port ( 39 ) and intake port ( 37 ) of the pump ( 26 ) is situated in the bottom portion of a pump chamber. The water supply device further includes a recirculation passage ( 28 ) which recirculates water between the water tank ( 25 ) and the fuel cell stack ( 2 ), and a compressor ( 20 ) which functions to supply water stored in the water tank to the pump by supplying air to the water tank. A controller ( 6 ) of the water supply device programmed to: command the compressor ( 20 ) to supply air to the water tank, when the fuel cell stack ( 2 ) is to be started up, and subsequently command the pump to start.

Description:
FIELD OF THE INVENTION  
       [0001]     This invention relates to a water supply device which supplies water to a fuel cell for the purpose of humidification and/or cooling. In particular, the water supply device aspirates water collected in a water storage tank and supplies this water to the fuel cell.  
       BACKGROUND OF THE INVENTION  
       [0002]     A water supply device disclosed in Tokkai 2002-81393, published by the Japan Patent Office in 2002, collects the water in the pump and passage in a water storage tank after the operation of the pump has finished. This prevents water remaining in the interior of a pump and passage from freezing.  
         [0003]     The pump of this water supply device of the prior art is a self-aspiration type pump and has an intake port situated coaxially with the rotation axis of a pump impeller which rotates in a horizontal plane and below the impeller. To aspirate the fluid in the pump, the water intake passage does not have a check valve and is always open. Consequently, after the pump has stopped operating, most of the water in the pump returns to the water storage tank.  
       SUMMARY OF THE INVENTION  
       [0004]     The water supply device generates an intake negative pressure of the pump by supplying water to the pump in advance. Therefore, when the pump starts, the pump impeller must be immersed in water. In a water supply device installed in a vehicle, if the vehicle inclines at an angle, the pump impeller cannot entirely be immersed in water. Therefore, the pump may not generate an effective intake negative pressure and may not start correctly.  
         [0005]     It is therefore an object of this invention to provide a water supply device for a fuel cell which can be applied to a vehicle which inclines according to a road surface.  
         [0006]     In order to achieve the above object, this invention provides a water supply device which performs humidification and/or cooling of a fuel cell stack. The water supply device comprises a water tank for storing water; a pump which sends water from the water tank to the fuel cell stack, wherein one of a discharge port and intake port of the pump is situated in the bottom portion of a pump chamber; a recirculation passage which recirculates water between the water tank and the fuel cell stack, wherein water leaves the water tank and flows through the pump and fuel cell stack to return to the water tank; a compressor which functions to supply water stored in the water tank to the pump by supplying air to the water tank; and a controller. The controller is programmed to command the compressor to supply air to the water tank so as to start an operation of the water supply device, and subsequently command the pump to start.  
         [0007]     This invention further provides a start method for starting the water supply device. The start method comprises commanding the compressor to supply air to the water tank whereby water stored in the water tank is supplied to the pump, and subsequently commanding the pump to start.  
         [0008]     The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]      FIG. 1  is a schematic view of a water supply device which supplies water to a fuel cell according to a first embodiment.  
         [0010]      FIG. 2  is a schematic cross-sectional view of a fuel cell stack.  
         [0011]      FIG. 3A  is a schematic plan view of a water supply pump, and  FIG. 3B  is a schematic side view of the water supply pump.  
         [0012]      FIG. 4  is a flowchart showing a control routine of the water supply device performed by a controller.  
         [0013]      FIG. 5  is a map showing a purged air amount relative to a fluid momentum (differential pressure) and an air purging time. The curve A is an isovalue curve for the purged air amount.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]     Referring to  FIG. 1 , a fuel cell system  1  comprises a fuel cell stack  2 , hydrogen supply line  3  which supplies hydrogen gas as fuel to the fuel cell stack  2 , air supply line  4  which supplies air (oxygen) as an oxidizing agent to the fuel cell stack  2 , and a water supply line  5  which supplies cooling water to cool and/or humidify the fuel cell stack  2 . Power generation by the fuel cell stack  2 , transport of hydrogen gas in the hydrogen supply line  3 , transport of the air in the air supply line  4  and transport of water in the water supply line are controlled by a controller  6 .  
         [0015]     Referring to  FIG. 2 , the fuel cell stack  2  comprises a membrane electrode assembly  11  comprising a polymer electrolyte membrane, and a fuel electrode and oxygen electrode disposed on both sides of the polymer electrolyte membrane. The fuel cell stack  2  further comprises an oxygen electrode side collection plate  12  which forms a fluid passage for supplying air to the membrane electrode assembly  11  behind the oxygen electrode, and a fuel electrode side collection plate  13  which forms a fluid passage for supplying hydrogen to the membrane electrode assembly  11  behind the fuel electrode. The collection plates  12 ,  13  are formed from porous bodies. The membrane electrode assembly  11  and collection plates  12 ,  13  form an individual cell  10 . A cooling plate  15  which forms a cooling water passage  16  is disposed behind the fuel electrode side collection plate  13  via a humidifying water permeating plate  14  formed from a porous body. Humidifying water permeates into the permeating plate  14 . The fuel cell stack  2  is formed by laminating plural sets of the cells  10 , the humidifying water permeating plates  14  and the cooling plates  15 .  
         [0016]     Hydrogen from the hydrogen supply line  3  is supplied to the fuel electrode. Air from the air supply line  4  is supplied to the oxygen electrode. Due to the reaction between hydrogen and oxygen, the fuel cell generates power. Cooling water supplied from the water supply line  5  to the interior of the cooling plate  15  removes heat produced during power generation. Part of the cooling water supplied to the cooling plate  15  wets the humidifying water permeating plate  14  and fuel electrode side collection plate  13 , and is supplied to the fuel electrode. Hence, the polymer electrolyte membrane is humidified, and part of the cooling water evaporates in the hydrogen gas and air. At this time, the latent vaporization heat removes part of the reaction heat of the cell. Part of the cooling water passes through the cooling plate  15  to reach the oxygen electrode side collection plate  12  on the rear side, and removes part of the reaction heat of the cell by sensible heat. The remaining cooling water is discharged from the fuel cell stack  2 .  
         [0017]     By adjusting the pressure of the cooling water and thus by varying the differential pressure between the fuel gas and cooling water, the water amount supplied to the fuel electrode side as humidifying water can be adjusted. When the pressure of the cooling water is increased (i.e., the differential pressure is decreased), the water amount supplied as humidifying water increases. When the pressure of the cooling water is decreased (i.e., the differential pressure is increased), the water amount supplied as humidifying water can be reduced. Also, by adjusting the cooling water flowrate, the reaction heat of the cell removed by sensible heat can be adjusted.  
         [0018]     Referring again to  FIG. 1 , the air supply line  4  comprises a pipe  21  and a compressor  20 , and air compressed by the compressor  20  is sent to the fuel cell stack  2  via the pipe  21 .  
         [0019]     The water supply line  5  comprises a cooling water tank  25 , pump  26 , pressure control valve  27  and a heat exchanger, not shown, which are connected in series on a recirculation passage  28 . The cooling water tank  25  stores cooling water  24 . The recirculation passage  28  is connected to the cooling water passage  16  of the cooling plate  15 . The pump  26  supplies the cooling water  24  stored in the cooling water tank  25  to the cooling plate  15  of the fuel cell stack  2 . The cooling water returns from the cooling plate  15  of the fuel cell stack  2  to the cooling water tank  25  via the back pressure control valve  27  and heat exchanger. The open end on the discharge side of the recirculation passage  28  is situated inside the cooling water tank  25 . The controller  6  controls the rotation speed of the pump  26  by transmitting a rotation speed command value to the pump  26  and the pressure of the recirculation passage  28  by transmitting a pressure command value to the back pressure control valve  27 . A passage  29  branches off from the recirculation passage  28  immediately downstream of the pump  26 , and leads to the atmosphere. A shutoff valve  30  which is normally closed, but opened by the controller when the pump  26  starts, is disposed in the discharge passage  29 .  
         [0020]     An air inlet passage  31  which branches off from the pipe  21  of the air supply line  4 , is connected to the cooling water tank  25 . A shutoff valve  32  which is normally closed, whereof the opening and closing is controlled by the controller  6 , is disposed in the air inlet passage  31 . A shutoff valve  34  which is normally open, whereof the opening and closing is controlled by the controller  6 , is disposed in an atmosphere opening passage  33  which opens to the outside air at its end. When the shutoff valve  32  is opened after the compressor  20  of the air supply line  4  has been operated, compressed air is led into the cooling water tank  25  via the air inlet passage  31 . If the back pressure control valve  27  and shutoff valve  34  of the atmosphere opening passage  33  are closed during introduction of compressed air, the cooling water tank  25  is pressurized. When the cooling water tank  45  is pressurized, the stored cooling water  24  is pressurized by the compressed air, so that the cooling water flows out to the pump  26  via the recirculation passage  28 .  
         [0021]     Referring to  FIG. 3 , the pump  26  is a volute pump wherein an impeller  41  of the pump  26  is rotated by a drive motor  36  installed horizontally with its drive axis being horizontal. The impeller  41  is a rotating member which, by its rotation, forces fluid towards the outside of the radial direction relative to the rotation axis. An intake port  37  is situated in the middle part of a substantially cylindrical pump chamber (or impeller chamber)  38  in which the impeller is housed and rotates, and an air discharge port  39  is situated in the bottom portion of the pump chamber  38 . The recirculation passage  28  extends from the bottom portion of the pump chamber  38 . As a result, when the pump  26  has stopped, water present in the pump chamber  38  flows out from the discharge port  39  which opens out at a lower position in the pump chamber  38  which has a substantially cylindrical shape, and does not remain in the pump chamber  38 .  
         [0022]     Referring to the flowchart of  FIG. 4 , the control routine of the water supply device performed by the controller  6  will now be described. The controller  8  comprises a microcomputer having a central processing unit (CPU), read-only memory (ROM), random access memory (RAM) and input/output interface (I/O) interface.  
         [0023]     A startup switch  40  of the fuel cell system is switched ON/OFF by an operator, and is electrically connected to the controller  8  to send an ON/OFF signal to the controller  8 . When the startup switch  40  is switched OFF, the controller  8  performs control to stop the fuel cell system  1  in response to the OFF signal. When the fuel cell system  1  has stopped, the controller  8  performs control to stop the pump  26  and compressor  20 , and open the back pressure control valve  27 . The positions of the pump  26  and fuel cell stack  2  are higher than the position of the cooling water tank. Therefore, the cooling water in the fuel cell stack  2  returns to the cooling water tank  25  via the open back pressure control valve  27 . In this way, damage to the pump  26  due to freezing of water at low temperature can be prevented.  
         [0024]     If the position of the pump  26  is higher than the position of the fuel cell stack  2 , cooling water in the pump chamber  38  and cooling water in the recirculation passage  28  between the pump  26  and cooling water tank  25  returns to the cooling water tank  25  via the cooling water passage  16  of the fuel cell stack  2 . If the position of the pump  26  is lower than the position of the fuel cell stack  2 , the shutoff valve  30  in the discharge passage  29  is opened to permit efficient discharge of cooling water in the lower part of the pump chamber and in the recirculation passage  28  between the fuel cell stack  2  and pump  26 . In this way, when the fuel cell system  1  has stopped, no water remains in the pump  26  and fuel cell stack  2 . Subsequently, the shutoff valve  30  in the discharge passage  29  is closed, the shutoff valve  34  in the atmosphere opening passage  33  remains open, and the shutoff valve  32  in the air inlet passage  31  remains closed.  
         [0025]     If the intake port  37  of the pump  26  is situated in the bottom portion of the pump chamber  38 , all the cooling water in the recirculation passage  28  up to the fuel cell stack  2 , including that in the pump chamber  38 , can be made to flow back into the cooling water tank  25 .  
         [0026]     When the fuel cell stack is to be started up, the startup switch  40  is switched ON. When the startup switch  40  is switched ON, the controller  6  starts the operation of the water supply device according to the control routine shown in the flowchart of  FIG. 4 . The controller  6  executes the control routine as a program or programs.  
         [0027]     First, in a step S 1 , the back pressure control valve  27  and shutoff valve  34  in the atmosphere opening passage  33  are closed, and the compressor  20  of the air supply line  4  is started. Due to the closure of the back pressure control valve  27  and shutoff valve  34 , communication between the cooling water tank  25 , the fuel cell stack  2  and the atmosphere is shut off. Compressed air from the compressor  20  is supplied to the fuel cell stack  2  via the pipe  21 , and supplied to the passage of the oxygen electrode side collection plate  12 .  
         [0028]     In a step S 2 , the shutoff valve  32  of the air inlet passage  31  is opened, and the shutoff valve  30  of the discharge passage  29  is opened. Due to the opening of the shutoff valve  32 , compressed air from the compressor  20  is introduced to the cooling water tank  25 , and the pressure in the cooling water tank  25  rises. Due to the internal pressure, the liquid surface of the cooling water  24  falls, and the stored cooling water  24  flows into the pump  26  via the recirculation passage  28 . Simultaneously, due to the opening of the shutoff valve  30  in the discharge passage  29 , air which was left in the pump chamber  38  is discharged in a short time to the atmosphere (or outside air) via the discharge passage  29 , and the pressure on the discharge side of the pump  26  falls. Due to the pressure difference between the cooling water tank  25  and discharge side of the pump  26 , the cooling water  24  in the cooling water tank  25  flows into the pump  26  via the recirculation passage  28 . In this case, the cooling water  24  is supplied to the pump chamber  38  regardless of whether the intake port  37  of the pump  26  is situated in the middle, upper part or lower part of the pump chamber  38 , and regardless of the posture of the vehicle or the inclination of a road surface on which the vehicle is standing.  
         [0029]     In a step S 3 , it is determined whether or not an elapsed time T after the step S 2  was executed, has reached a predetermined time T 0 . The step S 2  is repeated until the predetermined time T 0  has elapsed. If the predetermined time T 0  has elapsed, the routine proceeds to the step S 4 . The predetermined time T 0  is a sufficient time for the cooling water  24  to fill the pump chamber  38  of the pump  26 , and signifies a suitable air purging time (i.e., operating time of the compressor  20 ).  
         [0030]     Referring to  FIG. 5 , the determination of the predetermined time T 0  will be described in more detail.  FIG. 5  shows a purged air amount relative to the fluid momentum (differential pressure) and air purging time. Herein, the differential pressure is the difference between the air pressure introduced to the cooling water tank  25  and the pressure of the discharge port of the pump (atmospheric pressure level). The purged air amount is approximately equivalent to the amount of cooling water sent from the cooling water tank  25  to the pump  26  and discharge passage  29 . The fluid momentum increases with increase of the water supply pressure of the compressor  20 . The air purging time is the time for cooling water to be sent from the cooling water tank  25  to the pump  26  and discharge passage  29  by the operation of the compressor  20  during this interval. As the fluid momentum and air purging time increase, the purged air amount increases. The predetermined time T 0  (suitable air purging time) is determined such that the air amount to be purged is larger than the total volume of the recirculation passage  28  from the cooling water tank  25  to the pump  26  and the pump chamber  38 . For example, if the total volume is A 1 , the differential pressure and predetermined time T 0  are determined as ΔP 1  and T 1  in the shaded region  101  of the figure. The shaded region  101  is situated above the curve A 1 . However, the differential pressure and predetermined time T 0  must be set so that the air amount to be purged does not exceed the stored water amount in the cooling water tank  25 , and the air pressure introduced to the cooling water tank  25  does not exceed the maximum supply pressure of the compressor  20 .  
         [0031]     The map of  FIG. 5  may be stored in a memory of the controller  6 . The controller  6  may compute the differential pressure based on a pressure command value (or rotation speed command value) sent to the compressor  20 , and may determine the predetermined time T 0  by referring to a map based on the computed differential pressure and the total volume of the recirculation passage  28  from the cooling water tank  25  to the pump  26  and the pump chamber  38  measured beforehand by experiment.  
         [0032]     In a step S 4 , the pump  26  is started. Next, in a step S 5 , the shutoff valve  32  of the air inlet passage  31  and the shutoff valve  30  of the discharge passage  29  are closed. Next, in a step S 6 , the back pressure control valve  27  of the recirculation passage  28  and shutoff valve  34  of the atmosphere opening passage  33  are opened. This completes the startup control of the pump  26 .  
         [0033]     In the step S 3 , cooling water is supplied during the predetermined time T 0 , so the pump chamber  38  of the pump  26  is filled with cooling water. In the step S 4 , the controller  6  starts the pump  26 , and the impeller  41  discharges cooling water. In the step S 5 , the air inlet passage  31  and discharge passage  29  are closed. In the step S 6 , the atmosphere opening passage  33  is in communication, and the back pressure control valve  27  is opened, so the interior of the cooling water tank  25  is at atmospheric pressure. The cooling water discharged from the pump  26  flows into the cooling water passage  16  of the fuel cell stack  2  via the recirculation passage  28 . Subsequently, the cooling water discharged from the cooling water passage  16  is returned to the cooling water tank  25  via the recirculation passage  28  and the back pressure control valve  27 . The pressure of the cooling water in the water passage  16  is controlled by opening adjustment of the back pressure control valve  27  by the controller  6 .  
         [0034]     In this way, when the fuel cell system is started up, by introducing an air pressure to the cooling water tank  25 , a differential pressure is generated between the cooling water tank  25  and the discharge port of the pump  26 . The air in the pump chamber  38  is discharged downstream of the pump chamber  38  by the cooling water introduced to the pump chamber  38  regardless of the inclination of the vehicle. As a result, the pump starts rapidly. The compressor  20  which introduces the air supplied to the oxygen electrode of the fuel cell stack  2 , introduces air to the cooling water tank  25 , so the fuel cell system has a simple construction.  
         [0035]     In the aforesaid embodiment, the discharge port  39  of the pump  26  included in a water supply device was situated lower than the intake port  37 . However, as long as one of the intake port  37  and discharge port  39  is situated in the bottom portion of the pump chamber  38  which has a substantially cylindrical shape, water in the pump chamber  38  can be discharged when the pump  26  has stopped. Also, in the aforesaid embodiment, the drive axis of the pump  26  used in the water supply device was horizontal, but the drive axis may be oriented in a vertical direction.  
         [0036]     The entire contents of Japanese Patent Application P2003-347073(filed Oct. 6, 2003) are incorporated herein by reference.  
         [0037]     Although the invention has been described above by reference to certain embodiments of the invention, the invention is not limited to the embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings. The scope of the invention is defined with reference to the following claims.