Patent Publication Number: US-6706433-B2

Title: Refuelable electrochemical power source capable of being maintained in a substantially constant full condition and method of using the same

Description:
This is a continuation of U.S. patent application Ser. No. 09/521,392, filed on Mar. 8, 2000, now U.S. Pat. No. 6,296,958 B1, which is hereby fully incorporated by reference herein as though set forth in full. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates, in general, to a refuelable electrochemical power source, and in particular, to a refuelable electrochemical power source capable of being maintained in a substantially full or maximum electroactivity condition. 
     2. Related Art 
     One of the more promising alternatives to conventional power sources in existence today is the metal/air fuel cell. These fuel cells have tremendous potential because they are efficient, environmentally safe, and completely renewable. Metal/air fuel cells can be used for both stationary and mobile applications, and are especially suitable for use in all types of electric vehicles. 
     Metal/air fuel cells and batteries produce electricity by electrochemically combining metal with oxygen from the air. Zinc, Iron, Lithium, and Aluminum are some of the metals that can be used. Oxidants other than air, such as pure oxygen, bromine, or hydrogen peroxide can also be used. Zinc/air fuel cells and batteries produce electricity by the same electrochemical processes. But zinc/air fuel cells are not discarded like primary batteries. They are not slowly recharged like secondary batteries, nor are they rebuilt like “mechanically recharged” batteries. Instead, zinc/air fuel cells are conveniently refueled in minutes or seconds by adding additional zinc when necessary. Further, the zinc used to generate electricity is completely recoverable and reusable. 
     The zinc/air fuel cell is expected to displace lead-acid batteries where higher specific energies are required and/or rapid recharging is desired. Further, the zinc/air fuel cell is expected to displace internal combustion engines where zero emissions, quiet operation, and/or lower maintenance costs are important. 
     In one example embodiment, the zinc “fuel” is in the form of particles. Zinc is consumed, which releases electrons to drive a load (the anodic part of the electrochemical process). Oxygen from ambient air accepts electrons from the load (the cathodic part). The overall chemical reaction produces zinc oxide, a non-toxic white powder. When all or part of the zinc has been consumed and, hence, transformed into zinc oxide, the fuel cell can be refueled by removing the reaction product and adding fresh zinc particles and electrolyte. The zinc oxide (ZnO) product is typically reprocessed into zinc particles and oxygen in a separate, stand-alone recycling unit using electrolysis. The whole process is a closed cycle for zinc and oxygen, which can be recycled indefinitely. 
     In general, a zinc/air fuel cell system comprises two principal components: the fuel cell itself and a zinc recovery apparatus. The recovery apparatus is generally stationary and serves to supply the fuel cell with zinc particles, remove the zinc oxide, and convert it back into zinc metal fuel particles. A metal recovery apparatus may also be used to recover zinc, copper, or other metals from solution for any other purpose. 
     The benefits of zinc/air fuel cell technology over rechargeable batteries such as lead-acid batteries are numerous. These benefits include very high specific energies, high energy densities, and the de-coupling of energy and power densities. Further, these systems provide rapid on-site refueling that requires only a standard electrical supply. Still further, these systems provide longer life potentials, and the availability of a reliable and accurate measure of remaining energy at all times. 
     The benefits over internal combustion engines include zero emissions, quiet operation, lower maintenance costs, and higher specific energies. When replacing lead-acid batteries, zinc/air fuel cells can be used to extend the range of a vehicle or reduce the weight for increased payload capability and/or enhanced performance. The zinc/air fuel cell gives vehicle designers additional flexibility to distribute weight for optimizing vehicle dynamics. 
     In a prior art zinc/air fuel cell of the assignee of the present application, Metallic Power, comprise a plurality of cells electrically coupled together in a serial fashion. Before use, each cell receives a mixture of zinc particles and electrolyte. The zinc particles in each cell form an anode bed. Each cell includes an electroactive zone where zinc undergoes electrolytic dissolution. In a traditional zinc/air fuel cell or “refuelable battery,” the bottom portion of a full anode bed is in the electroactive zone and immediately adjacent to a cathode. For an example of traditional zinc/air fuel cells, see U.S. Pat. No. 5,434,020 to Cooper and U.S. patent application Ser. No. 09/353,422 to Gutierrez, et al., filed Jul. 15, 1999. The zinc in the remaining upper portion is in an inactive zone called the hopper, and does not undergo electrodissolution. This zinc is regarded as fuel. Another approach of the prior art is to completely replace the zinc electrode between discharges. For example, see U.S. Pat. No. 5,441,820 to Evans, et al. and U.S. Pat. No. 5,196,275 to Goldman, et al. 
     In the electroactive zone, the zinc dissolves, the particles reduce in diameter and the particle bed collapses, allowing particles to fall from the hopper into the electroactive zone. When the cell is refueled, which typically occurs at the end of the discharge period when the zinc fuel level in each cell reaches the bottom of the hopper, zinc pellets are fluidized in a stream of electrolyte and pumped into a feed tube, which runs the length of the cell stack. The feed tube discharges directly into each cell until each hopper is full of zinc. The discharge and refueling phases occur sequentially. 
     There are several problems with this approach to cell filling. They are: 1) high electrolytic shunt currents flow between the cells via the feed tube during refueling, 2) the filling procedure requires careful balancing of flow rates and zinc volume fraction to maximize zinc transfer rates while avoiding channel blockage, 3) the relatively long filling times (ten to twenty minutes for a typical application) that result even from the best zinc transfer rates, 4) the loss of the hopper volumes within the cell stack as active, power-producing volume, 5) the need to measure the zinc level in every hopper to get a completely reliable measure of remaining fuel, and 6) the difficulty of mechanically sealing the feed tube against the cell walls sufficiently to prevent dendrite growth between filling procedures. 
     First, this prior art approach leads to high electrolytic shunt currents flowing through the feed tube between the cells that reduce the efficiency of the fuel cell and can lead to dendrite formation and catastrophic short-circuits between cells. Sealing each cell from the feed tube near the junction of the feed tube and each cell inlet after cell filling can reduce the shunt currents. However, the presence of zinc pellets in this area makes reliable sealing difficult. Furthermore, such a seal is a mechanical device that needs some form of motive power and the ability to open and close on the receipt of an electrical signal, adding to the cost and complexity of the fuel cell. 
     Second, the filling procedure for this approach is quite time-consuming and requires careful control of flow rates and zinc volume fraction, because the pellets must fill each cell in sequence and after filling, the cell inlet holes and the feeding tube must be cleared of zinc pellets. Additionally, the rate of pellet flow required to minimize cell filling time causes problems. A principal advantage of the zinc/air fuel cell is that it allows fast refueling. However, with the traditional approach, the rate of electrolyte flow and the volume fraction of pellets control the rate of filling. If the flow rate is too high, the pressure can damage the cells. If the pellet fraction is too high, the pellets can accumulate and block the passageways. 
     A further disadvantage with this traditional approach is that the fuel to be consumed is stored within the fuel cell, i.e., in the hopper. The hopper occupies a significant portion of the cell volume, effectively reducing the cell volume available for electrochemical activity. It is undesirable to put the hopper zone within the cell frame because the cell frame is an expensive part of the fuel cell system. 
     Thus, what is needed is an electrochemical power source that maximizes the electroactive portion of each cell by having the electroactive zone occupy substantially all of the available volume of each cell. The cell design should cause minimal shunt currents between cells, and allow the cell to be constantly maintained in a full condition with fuel particles to maximize electrochemical activity. The cell should not require complex filling procedures or long filling times. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention maximizes the electroactive zone in a metal/air fuel cell so that it occupies substantially the entire available space within the cell frame, inhibits the possibility of shunt currents between cells, and substantially maintains the fuel cell constantly full with metal fuel particles to maximize electrochemical activity. 
     The present invention provides a refuelable electrochemical power source having one or more cells adapted to employ particulate material electrodes. The one or more cells include a cell body defining an interior cell cavity, a flow path in the cell cavity through which particulate material and fluid flow, an electroactive zone occupying substantially all of the cell cavity except for the flow path, and a fluid mechanical device adjacent the flow path capable of substantially maintaining the electroactive zone in a constant full or maximum electroactivity condition. 
     Implementations of the above aspect of the invention may include any or all of the following. A plurality of cells, each cell including an elongated inlet conduit configured to reduce shunt currents between cells, wherein the plurality of cells includes an inlet manifold adapted to deliver particulate material and fluid to each cell via the elongated inlet conduits. Further, there is a fluid mechanical device that is located adjacent to or at least partially within the fluid flow path. The fluid mechanical device includes a plurality of baffles located at least partially within the fluid flow path, and the baffles are configured to direct particulate and fluid flow into the electroactive zone. The baffles are substantially vertically oriented and parallel with each other in the cell and each baffle terminates in a substantially wedge-shaped end located at least partially with the fluid flow path. The one or more cells including a plurality of cells, each cell including an elongated outlet conduit configured to reduce shunt currents between cells. Further, the plurality of cells includes an outlet manifold adapted to deliver particulate material and fluid not retained within the cells out of the cells via the elongated outlet conduits. The cell includes a bottom portion and a fluid outlet conduit in communication with the bottom portion for delivering the fluid and an electrochemical reaction product out of the cell. The one or more cells include a plurality of cells having a fluid outlet manifold in communication with the fluid outlet conduit of each cell for delivering the fluid and electrochemical reaction product out of the power source. The cell includes a cell cavity generally defining four quadrants, a first quadrant represents an upper corner of the cell cavity, a second quadrant represents the other upper corner of the cell cavity, a third quadrant represents a lower corner of the cell cavity below the first quadrant and a fourth quadrant represents the other lower corner of the cell cavity below the second quadrant, and the particulate material and the fluid flows into the first quadrant, the particulate and the fluid flows out of the second quadrant and the fluid without any significant amount of the particulate flows out of the fourth quadrant. The one or more cells together form a zinc/air fuel cell stack. 
     Another aspect of the invention involves a method of maintaining one or more cells of a refuelable electrochemical power source at a substantially constant full condition. The method includes providing a refuelable electrochemical power source having one or more electrochemical cells adapted to employ particulate material electrodes, the one or more cells including a cell body defining an interior cell cavity, a flow path in the cell cavity through which particulate material and fluid flow, an electroactive zone occupying substantially all of the cell cavity except for the flow path, and a fluid mechanical device adjacent the flow path adapted to maintain the electroactive zone in a substantially constant full, maximum electroactivity condition. The fluid mechanical device is further employed to circulate particulate material and fluid through the flow path in the one or more cells, past the fluid mechanical device, so that the electroactive zone of the one or more electrochemical cells is substantially maintained in a constant full or maximum electroactivity condition. 
     Implementations of the aspect of the invention described immediately above may include any or all of the following. Filling the electroactive zone of the one or more cells with particulate material and fluid before circulating the particulate material and fluid through the one or more cells. The one or more cells including a plurality of cells, each cell including an elongated inlet conduit configured to reduce shunt currents between cells and the plurality of cells include an inlet manifold adapted to deliver particulate material and fluid to each cell via the elongated inlet conduits. The fluid mechanical device is located at least partially within the fluid flow path and substantially circulating particulate material and fluid constantly through the flow path of the one or more cells includes circulating the particulate material and fluid past the fluid mechanical device. This causes the particulate material and fluid to flow into the electroactive zone of the cell so as to fill the electroactive zone and maintain the electroactive zone in a substantially constant full condition. The fluid mechanical device includes a plurality of baffles located at least partially within the fluid flow path, the baffles configured so as to direct particulate and fluid flow into any empty space between adjacent baffles. The mechanical device substantially circulates particulate material and fluid constantly through the one or more cells, which includes circulating the particulate material and fluid past the baffles. Thus, the particulate material and fluid is caused to move into any empty space between adjacent baffles so as to fill the electroactive zone and maintain the electroactive zone in a substantially constant full condition. Circulating the particulate material and fluid includes circulating the particulate material and fluid at a flow rate so that when a space between adjacent baffles becomes full with particulate material, the particulate material and fluid flow past this area without clogging the flow path and subsequently fill any downstream spaces between baffles in a similar manner. This occurs until all the spaces between baffles, and hence the entire cell, is full. This flow may preferably be continuous so as to maintain the electroactive zone of each cell in a substantially constant full condition, or the flow may be intermittent or periodic so as to periodically refill the electroactive zones. In either case, the cells may be discharged during this flow if desired. The baffles are preferably, but not necessarily, oriented substantially vertical and in parallel with each other in the cell and each baffle preferably terminates in a substantially wedge-shaped end located at least partially with the fluid flow path. The one or more cells are zinc/air cells. The particulate material is zinc pellets and the fluid is potassium hydroxide. The cell includes a cell cavity generally defining four quadrants, a first quadrant represents an upper corner of the cell cavity, a second quadrant represents the other upper corner of the cell cavity, a third quadrant represents a lower corner of the cell cavity below the first quadrant and a fourth quadrant represents the other lower corner of the cell cavity below the second quadrant. The particulate material and the fluid flows into the first quadrant, the particulate and the fluid flows out of the second quadrant and the fluid, without any significant amount of the particulate, flows out of the fourth quadrant. 
     A further aspect of the present invention involves a refuelable electrochemical power source including one or more electrochemical cells adapted to employ particulate material electrodes. The one or more cells include a cell body defining an interior cell cavity, a flow path in the cell cavity through which particulate material and fluid flow, an electroactive zone occupying substantially all of the cell cavity except for the flow path, and means adjacent the flow path for maintaining the electroactive zone in a substantially constant full, maximum electroactivity condition. 
     A still further aspect of the present invention involves an onboard refueling system for a portable electrical power source. The onboard refueling system includes a source of particulate material and fluid and a refuelable electrochemical power source adapted to produce electrical power. The power source is in communication with the source of particulate material and fluid. The power source includes one or more electrochemical cells adapted to employ particulate material electrodes. The one or more cells include a cell body defining an interior cell cavity, a flow path in the cell cavity through which particulate material and fluid from the particulate material and fluid source flow, an electroactive zone occupying substantially all of the cell cavity except for the flow path, and a fluid mechanic device adjacent the flow path capable of maintaining the electroactive zone in a substantially constant full, maximum electroactivity condition. At least one pump is interconnected with the source of particulate material and fluid and the refuelable electrochemical power source. The pump is capable of imparting a substantially constant flow or an intermittent or periodic flow of particulate material and fluid from the particulate material and fluid source to the one or more cells. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     The present invention is described with reference to the accompanying drawings, wherein: 
     FIG. 1 is a schematic illustration of a metal/air fuel cell system constructed in accordance with an embodiment of the invention. 
     FIG. 2 is a cross-sectional view of a metal/air fuel cell constructed in accordance with an embodiment of the invention, taken along line  2 — 2  of FIG.  1 . 
     FIG. 3 is an enlarged cross-sectional view of an embodiment of a plurality of baffles illustrated in FIG.  2 . 
     FIG. 4 is an enlarged cross sectional view of one of the baffles illustrated in FIGS. 2 and 3. 
     FIG. 5 is a cross-sectional view, similar to FIG. 2, of a metal/air fuel cell constructed in accordance with an additional embodiment of the invention. 
     FIG. 6 is a perspective view of a recycling/refueling system and an industrial cart incorporating a metal/air fuel cell system constructed in accordance with an embodiment of the invention. 
     FIG. 7 is a side elevational view of the industrial cart of FIG.  4 . 
     FIG. 8 is a cross-sectional view, similar to FIG. 3, of a fluid mechanical device capable of maintaining an electrochemical cell in a substantially full, maximum electroactivity condition in accordance with an additional embodiment of the invention. 
     FIG. 9 is a cross-sectional view, similar to FIG. 3, of a fluid mechanical device capable of maintaining an electrochemical cell in a substantially full, maximum electroactivity condition in accordance with an additional embodiment of the invention. 
     FIG. 10 illustrates an embodiment of the invention comprising a cartridge refuelable stationary or portable power source. 
     FIG. 11 illustrates a recycling unit for generating fuel cartridges for use with the power source of FIG.  10 . 
    
    
     In the figures, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     All of the examples of the present invention presented herein are associated with a zinc/air fuel cell and system. It is important to note, however, that the present invention can be applied to other types of power sources, fuel cells, and/or batteries. For example, the present invention may be applied to other metal/air fuel cells, such as, but without limitation, aluminum/air fuel cells, lithium/air fuel cells or iron/air fuel cells. More generally, the invention could be applied to any electrochemical device using solid particulate matter as an oxidant or reductant, or systems with an oxidant other than air, such as oxygen, hydrogen peroxide, bromine, or liquids with the ability to carry a dissolved oxidant. The present invention can also be applied to metal recovery systems. An example of a metal recovery system is a system that recovers zinc from aqueous solution by applying electricity. Accordingly, the examples used herein depicting zinc/air fuel cells should not be construed to limit the scope and breadth of the present invention. 
     With reference to FIG. 1, a zinc/air fuel cell system  100  constructed in accordance with an embodiment of the invention will now be described. The basics of a zinc/air fuel cell system, which should be sufficient background for the reader to understand the present invention, have been described above in the background of the invention. However, additional background on zinc/air fuel cell systems can be found in U.S. patent application Ser. No. 09/449,176, filed on Nov. 24, 1999, entitled “System and Method for Preventing the Formation of Dendrites in a Metal/Air Fuel Cell, Battery or Metal Recovery Apparatus,” U.S. Pat. No. 5,952,117, filed on Oct. 24, 1996, entitled “Method and Apparatus for Refueling an Electrochemical Power Source,” and U.S. patent application Ser. No. 09/353,422, filed on Jul. 15, 1999, entitled “Fuel Particle Feeding Apparatus for Electrochemical Power Source and Method of Making the Same,” all of which are hereby incorporated herein by reference in their entirety as though set forth in full. 
     With reference to FIGS. 1 and 2, the zinc/air fuel cell system  100  includes a zinc fuel tank  102 , a zinc/air fuel cell stack or power source  104 , an electrolyte management unit  106 , a piping system  108 , one or more pumps  110 , and one or more valves (not shown) that define a closed flow circuit for the circulation of zinc particles and electrolyte during fuel cell operation. 
     Zinc pellets in a flow medium such as concentrated potassium hydroxide (KOH) electrolyte solution are located in the zinc fuel tank  102 . In another implementation, the particles may be a type of metal other than zinc such as and without limitation, aluminum (aluminum/air fuel cell), lithium, (lithium/air fuel cell), iron (iron/air fuel cell) or a particulate material other than metal that can act as an oxidant or reductant. In another implementation, the flow medium may be a fluid, i.e., liquid or gas, other than an electrolyte. The zinc fuel tank  102  and the rest of the system  100  may be part of an integral assembly. Alternatively, the zinc fuel tank  102 , the electrolyte management unit  106 , or a combination of these and/or other system components, may be a separate, detachable part of system  100 . 
     The zinc and electrolyte solution are pulsed or continuously fed from the zinc fuel tank  102 , through the piping system  108 , and into an inlet manifold  112  of the cell stack  104 . Piping system  108  may include one or more fluid connecting devices, e.g., tubes, conduits, elbows, etc., for connecting the components of the system  100 . 
     The power source  104  includes a stack of one or more bipolar cells  114 , each generally defining a plane and coupled together serially. Five such cells  114  are shown in FIG. 1 for illustrative purposes, however, the number of cells  114  in the power source  104  may vary depending on the desired application of the power source  104 . Each cell  114  has a total open circuit potential of M volts. The power source  104  has an open-circuit potential P equal to M volts×N cells, where N is the number of cells in the power source  104 . 
     Each cell  114  includes an air positive electrode  132  that occupies an entire surface or side of the cell  114  and a zinc negative electrode current collector  134  that occupies an opposite entire surface or side of the cell  114 . A porous and electrically conductive substance may be inserted between the electrodes  132 ,  134  of adjacent cells  114  so that air can be blown therethrough for supplying oxygen to each air positive electrode  132 . The bipolar stack  104  may be created by simply stacking the cells  114  such that the negative electrode current collector  134  of each cell is in physical contact with the positive electrode surface  132  of the adjacent cell  114 , with the porous and electrically conductive substance therebetween. As stated, this series connection provides a total open circuit potential (between the first negative electrode  134  and the last positive electrode  132 ) of P volts. In this fashion, extremely compact high voltage bipolar stacks  104  can be built. Further, because no wires are used between the cells  114  and the electrodes  132 ,  134  comprise large surface areas, the internal resistance between the cells  114  is extremely low. 
     The inlet manifold  112  preferably runs through the cells  114  of the power source  104 , perpendicular to the planes defined by the cells  114 . The inlet manifold  112  distributes fluidized zinc to the cells  114  via inlet conduits or cell filling tubes  116 . Each inlet conduit  116  lies within its respective cell  114 . 
     The zinc particulates and electrolyte flow through a flow path  115  in each cell  114 , generally within the plane of the cell  114 . The method of delivering particles to the cells  114  is a flow-through method. A dilute stream of pellets in flowing KOH electrolyte is delivered to the flow path  115 , at the top of the cell  114  via the conduit  116 . The stream flows through the flow path  115 , across the zinc particle bed, and exits on the opposite side of the cell  114  via outlet tube  118 . Some of the pellets in the stream are directed by baffles  140  into electroactive zone  119 . Pellets that remain in the flow stream are removed from the cell  114 . This flow-through method (along with baffles  140 ) allows the electroactive zone  119  to occupy substantially all of the cell cavity and remain substantially constantly filled with zinc particles. As a result, the electrochemical potential of each cell  114  is maximized per cell cavity volume. 
     The pumps  110  may be used to control the flow rate of the electrolyte and zinc through the system  110 . The proper flow rate through the system  100  is important. If the flow rate is too high, the pressure can damage the cells  114 . If the flow rate is too low, it may be difficult to maintain the cells  114  in a substantially constant full condition. The particle fraction in the electrolyte is also important. If the particle fraction is too high, the particles may accumulate and block the passageways of the system  100 . If the pellet fraction is too low, it may be difficult to maintain the cells  114  in a substantially constant full condition. Because there is a substantially constant flow of electrolyte and zinc across the top of the cells  114 , the pellet fraction is diluted compared to pellet fractions used in zinc/air fuel cell systems in the past. This helps to inhibit accumulation and blockage in the passageways and reduces shunt currents between cells  114 . One way of controlling the pellet fraction in the electrolyte is with a screening or filtering mechanism (not shown) on the outlet of the zinc/air fuel tank  102 . The flow rate of the zinc and electrolyte through the system  100  and the zinc pellet fraction should be sufficient to maintain maximum electrochemical activity in the cell  114 . 
     The flow rate going into each individual cell is determined by two flow requirements: 
     a) The flow rate Q e  through the particle bed necessary for the electrochemical reaction to take place. Q e  is determined experimentally and can vary according to the electrical performance of the pellet bed. 
     b) The flow rates Qsi necessary to keep the pellets fluidized throughout the conduits or channels both internal and external to the cell. Qsi is called the Stokes flow rate and, in the ith conduit, is defined via the classical expression for minimal fluidization velocity Us by Stokes. Us is a function of the pellet diameter, the density of the pellet material and both the viscosity and density of the carrier fluid. In the case of the present work, the fluid is preferably potassium hydroxide. Stokes equation is:                  U   S     =       [       (     4   3     )                     (       gD   p     f     )                     (         ρ   p     -     ρ   f         ρ   p       )       ]     2       ,           (   1   )                         
     where D p  is the diameter of the particle, ρ p  the density of the particle, ρ f  the density of the fluid, g is the gravitational constant of 9.81 m/s 2 , and f is the friction factor that depends on the Reynolds Number. 
     The local stokes flow rate Qs is defined as: 
     
       
           Qs =( Us*A ),  (2)  
       
     
     where A is the local cross-sectional area. 
     The flow rate Qsi that is necessary to keep the pellets fluidized throughout the ith conduit or channel is equal to: 
     
       
           Qsi =( Us* Amaxi ),  (3)  
       
     
     where Amaxi is the maximum cross-sectional area in the ith conduit. Also, we define Uf as the velocity of the fluid. For proper fluidization, Q must in all conduits and channels be chosen such that the flow velocity is greater than the Stokes velocity at all points: 
     
       
         ( Uf/Us )&gt;1 everywhere.  (4)  
       
     
     The most stringent requirement on the flow rate is determined at the widest point in each conduit, where the velocity is slowest. If relation (4) holds there, it will hold at all other points in the conduit. This gives the requirement on the flow rate Qi through each conduit or channel to keep the pellets fluidized throughout the conduit or channel: 
     
       
           Qi &gt;( Us*Amaxi )  (5)  
       
     
     The minimum total flow rate into each cell will therefore depend on the number and configuration of channels and must be sufficient to ensure that the flow rate through the particle bed exceeds Q e  and the flow rate through each conduit or channel satisfies relation (5). 
     The pellet volume fraction Xp is the volume of pellets divided by the sum of the volume of liquid and the volume of pellets. In the flow path region it should be in the range 0.001&lt;Xp&lt;0.6, but preferably in the range 0.01&lt;Xp&lt;0.2. This fraction should be small enough to minimize the interactions between pellets so as to ensure they obey Stokes law and will be always fluidized, but large enough to ensure adequate particle fill rates for the cell without requiring unduly high liquid flow rates and pumping power. 
     As the zinc particles dissolve in the electroactive zone  119  of the cell  114 , soluble zinc reaction product, zincate, is produced. The zincate passes through a screen mesh or filter  122  near a bottom  123  of the cell  114  and is washed out of the active area of cell  114  with electrolyte that also flows through the cell  114  and filter  122 . The screen mesh or filter  122  causes the electrolyte that exits the cell  114  to have a negligible amount of or no zinc particles. The flow of electrolyte through the cell  114  not only removes the soluble zinc reaction product and, therefore, reduces precipitation of discharge products in the electroactive zone  119 , it also removes unwanted heat, helping to prevent the cell  114  from overheating. 
     The electrolyte exits the cell  114  and cell stack  104  via an electrolyte outlet conduit  128  and electrolyte outlet manifold  130 , respectively. The electrolyte is drawn into the electrolyte management unit  106  though the piping system  108 . A pump (not shown) may be used to draw electrolyte into the electrolyte management unit  106 . The electrolyte management unit  106  is used to remove the zincate and/or heat from the electrolyte so that the same electrolyte can be added to the zinc fuel tank  102  for zinc fluidization purposes. The electrolyte management unit  106 , like the zinc fuel tank  102 , may be part of an integral assembly with the rest of the system  100  or it may be a separate, detachable part of the system  100 . 
     A constant supply of oxygen is required for the electrochemical reaction in each cell  114  to take place. To this end, one embodiment of the system  100  may include a plurality of air blowers  124  and air outlet  126  on the side of cell stack  104  in order to supply a flow of air (comprising the oxygen required for the electrochemical reaction) to the air cathodes of each cell  114 . A porous substance such as a nickel foam may be disposed between each cell  114  to allow the air to reach the air cathode of each cell and flow through the cell stack  104 . In another implementation, an oxidant other than air such as, but without limitation, pure oxygen, bromine, or hydrogen peroxide, may be supplied to each cell  114  for the electrochemical reaction to take place. 
     With reference to FIG. 2, the flow into and out of the cell  114  may be described using a quadrant system, i.e., quadrants 1-4, of the cell cavity. Quadrant 1 or first quadrant represents an upper corner of the cell cavity, quadrant 2 or second quadrant represents the other upper corner of the cell cavity, quadrant 3 or third quadrant represents a lower corner of the cell cavity below quadrant 1, and quadrant 4 or fourth quadrant represents the other lower corner of the cell cavity below quadrant 2. In a preferred embodiment of the invention, zinc particulate and electrolyte enters the cell cavity via the inlet conduit  116  into quadrant 1 and exits the cell cavity via the outlet conduit  118  from quadrant 2. Electrolyte and reaction product preferably exit the cavity via electrolyte outlet conduit  128  from quadrant 4. Based on results of experiments and computational modeling, the inventors determined that this combination worked well for optimizing fluid circulation in the cell  114 . Although quadrants 1-4 are illustrated in an upper left hand corner, upper right hand corner, lower left hand corner, and lower right hand corner, respectively, in an alternative embodiment of the invention the quadrants may be assigned different quadrant numbers, e.g., quadrants 1-4 may represent an upper right hand corner, upper left hand corner, lower right hand corner, and lower left hand corner. 
     The manifolds  112 ,  120 ,  130  may run perpendicular to the planes defined by the cells  114  of the stack  104  in order to communicate the cells  114  with the manifolds  112 ,  120 ,  130  via the respective conduits. It will be readily apparent to those skilled in the art that any or all of the manifolds may take any well-known manifold configuration known in the art other that that shown and described herein and may be located outside of the cells  114 . Further, although the inlets of the manifolds  112 ,  120 ,  130  are shown at one end of the stack  104 , implementations of the stack  104  may include the inlets of any or all of the manifolds at locations other than that shown and described herein. 
     With reference specifically to FIGS. 2 and 3, the zinc/air fuel cell  114  will now be described in more detail. The zinc/air fuel cell  114  is defined by a fuel cell frame or body  136 . The fuel cell body  136  generally defines a cell cavity  137 . The fuel cell cavity  137  communicates with the inlet manifold  112  via the cell filling tube  116 . Electrolyte and zinc are introduced into the cell filling tube  116  and flow into the cell cavity  137  through a cell inlet  138 . The electrolyte and zinc flow through the flow path  115 , across the top of the zinc pellet bed. As the electrolyte and zinc flow across the top of the zinc pellet bed, some zinc particles are directed from the flow stream to the pellet bed in the electroactive zone  119 . The remaining pellets exit with the electrolyte though the outlet conduit  118  and outlet manifold  120 , and return to the zinc tank  102 . The cell filling tube  116  is narrow and sufficiently long, about 15 cm in a preferred embodiment, to reduce total shunt currents to less than 10 amps. This substantially minimizes electrical conductivity and parasitic current leakage between the cells  114  through the electrolyte. This eliminates the need for a sealing device between the manifold  112  and the cell inlet  138 . When the cell  114  is not in use and the pumps  110  are not operating, the feed tube  116  will drain, breaking the shunt current flow path. 
     In the past, because electrolytic shunt currents flow between the cells via the manifold tube, each cell was sealed from the manifold tube after cell filling to reduce the shunt currents. Not only was it difficult to build a reliable sealing method that would close in the presence of zinc pellets, but closing the cell off to the manifold tube inhibits having a substantially constant zinc and electrolyte flow across the top of the zinc pellet bed to maintain the full bed height. The addition of the long, thin cell filling tube  116  between the inlet manifold  112  and cell inlet  138  reduces shunt currents to acceptable level without the need for a sealing device, allowing substantially constant zinc and electrolyte flow for maintaining full bed height and, hence, full electrochemical activity in the anode bed. 
     With reference specifically to FIG. 3, the baffles  140  are a fluid mechanical device  117  adapted to maintain the electroactive zone of each cell  114  in a substantially constant full, maximum electroactivity condition. In a preferred embodiment, the baffles  140  are fixed near a top  142  of the cell  114 , substantially vertically oriented and parallel with each other. However, the baffles need not be vertically oriented or parallel with each other. Additionally, each baffle  140  preferably terminates in a substantially wedge-shaped end  141  located at least partially within the fluid flow path  115 . The baffles  140  are sized, shaped and spaced, i.e., configured, to create fluid stream vortices between the baffles  140  if an empty or partially empty pellet drop zone, i.e., the space between and/or below the baffles  140 , exists. 
     These vortices between the baffles  140  cause some of the electrolyte and zinc particles to flow down between the baffles  140 , filling the empty pellet drop zone between and/or below the baffles  140 . The baffles  140  may extend down through the electroactive zone, even to the bottom of the cell  114  if desired. Once the pellet level in a pellet zone, e.g., zone A or B, between any two adjacent baffles  140  is filled to the top of the baffles  140 , i.e. to a top  144  of the electroactive zone  119 , the pellets no longer accumulate and move into the next baffle zone, e.g., zone C. In this way, the whole upper surface of the pellet bed is maintained at the fill line  144  by providing a flow of electrolyte and zinc pellets through the flow path  115  and over the ends  141  of the baffles  140 . Thus, the baffles  140  are a fluid mechanic device capable of maintaining the electroactive zone  119  in a substantially constant full condition. 
     Because the zinc pellets can be maintained at the fill line  144 , the electroactive zone  119  of the fuel cell  114  can also be located up to this point, making the electroactive zone  119  of the fuel cell  114  substantially larger per cell volume than has been available in the past. It is important to note that other fluid mechanic devices, including other baffle configurations, can be used for this same purpose. With reference to FIG. 8, an example of an alternative embodiment of a fluid mechanic device capable of maintaining the electroactive zone  144  in a substantially constant full condition is shown. In this embodiment, staggered pegs  212  replace the baffles  140 . With reference to FIG. 9, an example of an alternative embodiment of a fluid mechanic device capable of maintaining the electroactive zone  144  in a substantially constant full condition is shown. In this embodiment, a helical “knockout” coil  214  serves the same function as the baffles  140 . The most important function of these embodiments of the fluid mechanic device is to create a difference between the fluid velocity in the flow path  115  (relatively high velocity) and the electroactive zone  119  (relatively low velocity) while enabling particles to travel from the flow path  115  into the electroactive zone  119 . This allows particles in the flow path  115  to fill the electroactive zone  119  without high velocity fluid propelling a substantial number of particles back out of the electroactive zone  119 . This function is not essential for an acceptable fluid mechanic device, but it is preferred. 
     The flow rate U (see FIG. 3) of pellets is designed to maintain the pellet bed at a substantially constant optimum height, i.e., the fill line  144 , under electrodissolution conditions. However, even when the fuel cells  114  generate the maximum amount of current, the rate of decrease in the pellet bed height is very slow. Accordingly, if the flow rate U through the flow path  115  is substantially constant, the rate of pellet feeding is low. Consequently, the electrolyte flow rate and the pellet volume fraction are relatively low. In an alternative implementation, the pellet bed may be maintained substantially constant at the fill line  144  by a periodic or intermittent flow, e.g., pulsed flow, of pellets and electrolyte through the one or more cells  114 . In this implementation, the pellet and electrolyte flow may also be substantially constant to ensure that fill rate is proportional to the rate of decrease in the pellet bed height caused by electrodissolution. 
     With reference to FIG. 4, an enlarged view of a single baffle  140  is shown. Other important variables or baffle design parameters for maintaining the pellet bed at an optimum height are the height h of the baffles  140 , the width w of the baffles  140 , the separation distance s between adjacent baffles  140 , the angle of inclination α of the baffles  140 , and the radius of curvature φ of the border of attack at the top of the baffles  140 . In one embodiment, the angle α is the angle between the long dimension of a baffle and the direction of fluid flow in the flow path adjacent said baffle. The variables involved in the proper design of the baffles  140  (height, width separation and top edge shape) are ruled by the proper detachment of the boundary layer that is needed at the top of the baffle  140 . When the boundary layer detaches, it creates down stream a vortex of the dimensions of the confined space. The flow between the baffles  140  is usually referred to as cavity flow. The circulating velocity within the vortex depends, among many things, on the main flow velocity in the flow path and on the shape of the edge of the baffle  140 . Sharp edges cause fast detachments with resulting fast vortex velocities. Smooth edges (airfoil like) cause smooth detachments and very low velocity vortices down stream. 
     The detachment of a boundary layer can generally be described by Blasius&#39;s and Prantl&#39;s equations together. Therefore, the design parameters of the baffles  140  strictly emanate from the fundamental equations of boundary layer detachment. The separation between baffles  140  can be calculated using Seban&#39;s equation for a given flow and a given fluid. Each alternative embodiment of the fluid mechanic device (such as shown in FIGS. 8 and 9) is also ruled by the same equations and a new set of heights, separations and shapes will arise from the equations applied to each problem in particular. As a rule of thumb, vortices have a size given by the physical space that confines them (Prandtl&#39;s mixing length). Typically, boundary layers reattach at approximately 6 perturbation heights away from the point of detachment. 
     The design parameters for the baffles (s, w, h, α, φ) can be expressed in dimensionless form as follows: 
     
       
           s ′=( s/D ),  (6)  
       
     
     where D, the hydraulic diameter of the flow path, is equal to four times the average area of the flow path cross-section divided by the average perimeter of the flow path cross-section; 
     
       
           w =( w*n/L ),  (7)  
       
     
     where n is the number of baffles and L is the length of the flow path region; 
     
       
           h ′=( h/H ),  (8)  
       
     
     where H is the cell height excluding the flow path; and 
     
       
         φ′=(φ /w ).  (9)  
       
     
     where φ′ is the radius of curvature of the tip nearest the flow path of a baffle divided by the width of a baffle. 
     The best ranges for these dimensionless parameters are: 
     0.1&lt;s′&lt;30 and preferably 1&lt;s′&lt;3; 
     0&lt;w′&lt;0.95 and preferably 0.05&lt;w′&lt;0.3; 
     0&lt;h′&lt;1 and preferably 0.001&lt;h′&lt;0.2; 
     Arcos (s/h)≦α≦90° and preferably α=90°; and 
     0&lt;φ′&lt;0.25 and preferably 0&lt;φ′&lt;0.01. 
     In FIG. 4, the border of attack is shown to be on the leading (upstream) face  145  of the baffle  140 , but it may also be on the trailing (downstream) face  146  or anywhere in between. 
     With reference to FIG. 5, a zinc/air fuel cell  150  constructed in accordance with an alternative embodiment of the invention will now be described. The zinc/air fuel cell  150  is similar to the zinc/air fuel cell  114  described above, except the electrolyte outlet conduit  128  and manifold  130  are replaced with an electrolyte inlet conduit  152  and manifold  154 . In this embodiment, zinc particulate and electrolyte enters the cell cavity via the inlet conduit  116  into quadrant 1 and exits the cell cavity via the outlet conduit  118  from quadrant 2. Electrolyte supplied by the electrolyte management unit  106  enters quadrant 3 of the fuel cell cavity via the manifold  154  and the electrolyte inlet conduit  152 . The electrolyte, with reaction product, flows up and across the cell cavity and exits with the zinc and electrolyte flow via the outlet conduit  118  from quadrant 2. This embodiment of the zinc/air fuel cell is advantageous because for moderate flow rates and high cell electrical discharge rates, it may be advantageous for electrolyte to flow upward through the electroactive zone. 
     In an alternative embodiment of the invention, the conduit  152  and manifold  154  described above with respect to FIG. 5, may be used to remove electrolyte and reaction product from the fuel cell cavity at quadrant 3 instead of supplying electrolyte to the fuel cell cavity at this location. This would cause the supplied electrolyte to flow downwards, into quadrant 3, and out the conduit  152 . In this embodiment of the invention, zinc particulate and electrolyte would still enter the cell cavity via the inlet conduit  116  into quadrant 1 and exit the cell cavity via the outlet conduit  118  from quadrant 2. A screen or filter (not shown) similar to that described above with respect to FIGS. 1 and 2 would preferably be located near the bottom  122  of the cell  150  to prevent zinc particulate from exiting the cell  150  with the electrolyte. Other embodiments, with different locations and configurations of particle and fluid inlets and outlets, are possible. The essential elements of any useful embodiment are that it allows for fluid flow through substantially all of the electroactive zone to remove heat and reaction product, and that it allows for the introduction of fresh zinc particles into the electroactive zone. 
     With reference to FIGS. 6 and 7, an industrial cart  200  is equipped with the metal/air fuel cell system  100  for powering an electric motor  202  that drives the cart  200 . It will be readily understood by those skilled in the art that the industrial cart  200  is just one of numerous portable or movable electrically powered devices that the metal/air fuel cell system  100  may be used to power. Other examples include, but without limitation, portable or stationary electrical generators, lift trucks, floor sweepers and scrubbers, automobiles, golf carts, electrically-powered transportation vehicles, auxiliary power units, and commercial lawn and garden equipment. 
     Fuel tank  102  may be refilled at a recycling/refueling system  210 . This can generally be accomplished with very high zinc transfer rates, for example by using a high velocity, high zinc volume-fraction flow through a large diameter hose, compared to the much slower transfer rates achievable by refilling a plurality of cell hoppers, as in the prior art. During refueling, the spent zinc, which has been converted to zinc oxide (ZnO) in the electrolyte management unit  106 , is transferred to the zinc recycling/refueling system  210 , as shown in FIG.  6 . The recycling/refueling system  210  uses electrolysis to convert the zinc oxide back to zinc metal in pellet form using AC electricity from an electrical outlet, e.g., an electrical wall outlet. The pellets are stored in a tank in the recycling/refueling system  210 , and, when required, are pumped in a stream of flowing electrolyte into the fuel tank  102  of the zinc/air fuel cell system  100 . This may be accomplished either simultaneously with the zinc oxide removal from the electrolyte management unit  106 , or in sequence with the removal. Further, this may be accomplished when the fuel tank  102  is mounted to the fuel cell (as a permanent, integral component or otherwise), or when the fuel tank is detachable from the fuel cell (see U.S. Pat. No. 5,952,117. 
     With reference to FIG. 10, a cartridge refuelable mobile or stationary power source  300  in accordance with the subject invention is illustrated. In this embodiment, fuel tank  102  and/or electrolyte management unit  106  form a detachable cartridge  301   a  which, when exhausted, can be replaced by a regenerated cartridge  301   b  provided by recycling unit  305  as shown in FIG.  11 . The recycling unit  305  can then be used to regenerate the exhausted cartridge  301   a  for later use by the power source  300 . 
     While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.