Abstract:
A flow battery system includes a flow battery stack, a sensor and a coolant loop. The flow battery stack has an electrolyte solution flowing therethrough, and the sensor is in communication with the electrolyte solution. The coolant loop is in heat exchange communication with the electrolyte solution, wherein the heat exchange communication is selective based on an output from the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to PCT/US09/68681, U.S. patent application Ser. No. 13/084,156, U.S. patent application Ser. No. 13/023,101, and U.S. patent application Ser. No. 13/022,285, each of which is incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    This disclosure relates generally to a flow battery system and, more particularly, to a system and method for operating a flow battery system at an elevated temperature. 
         [0004]    2. Background Information 
         [0005]    A typical flow battery system includes a stack of flow battery cells, each cell having an ion-exchange membrane disposed between negative and positive electrodes. During operation, a catholyte solution flows through the positive electrode, and an anolyte solution flows through the negative electrode. The catholyte and anolyte solutions each electrochemically react in a reversible reduction-oxidation (“redox”) reaction. Ionic species are transported across the ion-exchange membrane during the reactions, and electrons are transported through an external circuit such as a power converter to complete the electrochemical reactions. 
         [0006]    An example of a pair of catholyte and anolyte solutions is a pair of vanadium/vanadium solutions. The vanadium catholyte solution typically includes a plurality of V 4+  and/or V 5+  ions. The vanadium anolyte solution typically includes a plurality of V 2+  and/or V 3+  ions. Ideally, the concentrations of these vanadium ion species should be as high as possible in order to minimize the size of the tank required for a given amount of energy storage; i.e., higher concentrations enable a flow battery system with a higher energy density. However, the concentrations are limited by the solubility of the vanadium salts in the solvent electrolyte, which is typically an aqueous acid such as sulfuric acid. Additionally, the solubility of these different vanadium salts (e.g., vanadium sulfates) vary with the temperature of the solution. The V 2+ , V 3+  and V 4+  salts are generally less acid soluble at lower temperatures. The V 5+  ions, on the other hand, are generally less acid soluble at higher temperatures. An additional complication is that the concentrations of the different oxidation states may vary with the state-of-charge (SOC) of the battery and, ideally, one would like the salts to remain in solution over a wide range of SOC (e.g., from 0 to 100% SOC, such that salt solubility does not limit the minimum or maximum SOC). For example, a typical electrolyte composition used in a vanadium redox battery system is an aqueous solution of approximately 1.5 to 2.0 molar (M) vanadium sulfate and 1.5 to 2.0 M sulfuric acid for both the anolyte and the catholyte. The anolyte and catholyte composition enables an operating range of approximately zero to forty degrees Celsius, with the lower temperature limit determined by the solubility of the V 2+ , V 3+  and V 4+  salts and the upper temperature limit determined by the solubility of the V 5+  salt. Vanadium flow battery systems, therefore, are typically operated within a relatively narrow temperature range (e.g., approximately zero and forty degrees Celsius) to prevent formation of metal salt precipitates. A wider temperature window would be beneficial since a lower minimum temperature would eliminate the need for “freeze” prevention measures and a higher maximum temperature can enable improved cell performance, as well as improved heat rejection to the environment (especially on hot days where ambient temperatures are close to, or may even exceed, forty degrees Celsius). 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic illustration of a flow battery system; 
           [0008]      FIG. 2  illustrates a flow battery stack included in the flow battery system illustrated in  FIG. 1 ; 
           [0009]      FIG. 3  illustrates a cross-section of a flow battery cell included in the flow battery system illustrated in  FIG. 1 ; 
           [0010]      FIG. 4  illustrates a cross-section of a reference cell included in the flow battery system illustrated in  FIG. 1 ; 
           [0011]      FIG. 5  is a flow diagram of a method for operating the flow battery system illustrated in  FIG. 1 ; and 
           [0012]      FIG. 6  is a flow diagram of a method for regulating temperatures of electrolyte solutions flowing through the flow battery system illustrated in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0013]      FIG. 1  illustrates a flow battery system  10 . The flow battery system  10  includes a first reservoir  12 , a second reservoir  14 , a first solution flow circuit  16 , a second solution flow circuit  18 , a plurality of coolant loops  20 ,  22  and  24 , a flow battery stack  26 , a power converter  28 , a reference cell  29 , a plurality of sensors  30 - 40  (see  FIGS. 1 and 4 ), and a controller  42 . 
         [0014]    The first reservoir  12  contains a first electrolyte solution (e.g., a vanadium catholyte). The second reservoir  14  contains a second electrolyte solution (e.g., a vanadium anolyte). 
         [0015]    The first and second solution flow circuits  16  and  18  may each include a source conduit  44 ,  46 , a return conduit  48 ,  50 , a bypass conduit  52 ,  54  and a flow regulator  56 ,  58 , respectively. The flow regulator  56 ,  58  may include a variable speed pump  60 ,  62 , and an electronically actuated three-way valve  64 ,  66 , respectively. The pump  60 ,  62  and the valve  64 ,  66  are fluidly connected inline within the source conduit  44 ,  46 , respectively. The bypass conduit  52 ,  54  fluidly connects the valve  64 ,  66  to the return conduit  48 ,  50 , respectively. 
         [0016]    The coolant loops may include a first flow circuit coolant loop  20 , a second flow circuit coolant loop  22  and a stack coolant loop  24 . Each coolant loop  20 ,  22 ,  24  may include a first heat exchanger  68 ,  70 ,  72 , a second heat exchanger  74 ,  76 ,  78 , and a circulation pump  80 ,  82 ,  84 , respectively. The first and second heat exchangers and the circulation pump are fluidly connected in a closed loop. Each circulation pump circulates heat exchange fluid through its associated first and second heat exchangers in response to a respective circulation pump control signal. The heat exchange fluid can be water or an anti-freeze solution (e.g., ethylene glycol) or any other fluid with desirable properties (e.g., high heat capacity, low viscosity, etc.). One or more of the second heat exchangers  74 ,  76 ,  78  can be, for example, simple air-cooled radiators. Alternatively, no second heat exchange device is required if the first heat exchanger is cooled directly with air (e.g., a fan is used as the “circulation pump” and air is the heat exchange fluid). 
         [0017]      FIG. 2  illustrates the flow battery stack  26 , which includes one or more flow battery cells  92  and a stack manifold  96 . 
         [0018]      FIG. 3  illustrates a cross-section of one of the flow battery cells  92 . Each flow battery cell  92  includes a first current collector  98 , a second current collector  100 , a liquid-porous first electrode layer  102 , a liquid-porous second electrode layer  104 , and a separator  106 . The first electrode layer  102  may be a cathode, and the second electrode layer  104  may be an anode. The first electrode layer  102  may be coated with an acidic material (e.g., Nafion® polymer manufactured by DuPont of Wilmington, Del., United States) that at least partially impedes formation of precipitate within the first electrolyte solution. The separator  106  may be an ion-exchange membrane (e.g., Nafion® polymer membrane manufactured by DuPont of Wilmington, Del., United States), and is positioned between the electrode layers  102  and  104 . The electrode layers  102  and  104  are positioned between the current collectors  98  and  100 . 
         [0019]    Referring to  FIGS. 2 and 3 , the stack manifold  96  includes a first inlet  118 , first outlet  120 , a second inlet  122 , and a second outlet  124 . The first inlet  118  is fluidly connected to the first outlet  120  through the first current collector  98  and/or the first electrode layer  102  in each of the flow battery cells  92 . Similarly, the second inlet  122  is fluidly connected to the second outlet  124  through the second current collector  100  and/or the second electrode layer  104  in each of the flow battery cells  92 . 
         [0020]      FIG. 4  illustrates a cross-section of the reference cell  29 . The reference cell  29  is a flow battery cell that includes a plurality of sensors  34 - 40 . The reference cell  29  includes a first current collector  108 , a second current collector  110 , a liquid-porous first electrode layer  112 , a liquid-porous second electrode layer  114 , and a separator  116 . The first electrode layer  112  may be a cathode, and the second electrode layer  114  may be an anode. The first electrode layer  112  may be coated with an acidic material (e.g., Nafion® polymer manufactured by DuPont of Wilmington, Del., United States) that at least partially impedes formation of precipitate within the first electrolyte solution. The separator  116  may be an ion-exchange membrane (e.g., a Nafion® polymer membrane manufactured by DuPont of Wilmington, Del., United States), and is positioned between the electrode layers  112  and  114 . The electrode layers  112  and  114  are positioned between the current collectors  108  and  110 . 
         [0021]    Referring to  FIGS. 1 and 4 , the sensors  30 - 40  may include a first reservoir temperature sensor  30 , a second reservoir temperature sensor  32 , a first cell temperature sensor  34 , a second cell temperature sensor  36 , a precipitate sensor  38  and a state-of-charge sensor  40 . The first reservoir temperature sensor  30  senses the fluid temperature within the first reservoir  12 , and the second reservoir temperature sensor  32  senses the fluid temperature within the second reservoir  14 . The precipitate sensor  38  may include an optical detector that detects precipitate within an electrolyte solution based on, for example, the color of a dye mixed within the solution or the color of the precipitate that is likely to form (e.g., V 5+  can react with water to form vanadium pentoxide, V 2 O 5 , which is orange, whereas V 5+  sulfate is yellow). The state-of-charge sensor  40  may include a voltmeter that determines a state-of-charge of ions within an electrolyte solution based on open cell voltage (OCV). The term “state-of-charge” is used herein to describe a ratio of (i) a quantity of ions within a volume of a solution having a relatively high charge (e.g., V 5+  and/or V 2+ ) to (ii) a quantity of ions within the volume of the solution having a relatively low charge (e.g., V 4+  and/or V 3+ ). 
         [0022]    Referring to  FIG. 4 , the first cell temperature sensor  34 , the precipitate sensor  38 , and the state-of-charge sensor  40  are disposed with the first current collector  108 . The second cell temperature sensor  36  is disposed with the second current collector  110 . 
         [0023]    Referring again to  FIG. 1 , the source conduit  44  fluidly connects the first reservoir  12  to the flow battery stack  26 , via the first stack manifold inlet  118 . The return conduit  48  fluidly connects the flow battery stack  26 , via the first stack manifold outlet  120 , to the first reservoir  12 . The source conduit  46  fluidly connects the second reservoir  14  to the flow battery stack  26 , via the second stack manifold inlet  122 . The return conduit  50  fluidly connects the flow battery stack  26 , via the second stack manifold outlet  124 , to the second reservoir  14 . The reference cell  29  is connected in line with the return conduit  48 , via the first current collector  108  and/or the first electrode layer  112  (see  FIG. 4 ). The reference cell  29  is connected in line with the return conduit  50 , via the second current collector  110  and/or the second electrode layer  114  (see  FIG. 4 ). 
         [0024]    The first heat exchanger  68  is fluidly connected inline within the source conduit  44  between the pump  60  and the valve  64 , which thereby places the coolant loop  20  in heat exchange communication with the first electrolyte solution. The first heat exchanger  70  is fluidly connected inline within the source conduit  46  of the second solution flow circuit  18  between the pump  62  and valve  66 , which thereby places the coolant loop  22  in heat exchange communication with the second electrolyte solution. The first heat exchanger  72  is thermally connected to the flow battery stack  26  and, thus, to each of the flow battery and reference cells  92  and  94  (see  FIG. 2 ). The first heat exchanger  72  therefore places the coolant loop  24  in heat exchange communication with both the first and second electrolyte solutions. 
         [0025]    The power converter  28  may be a two-way power inverter or a two-way DC/DC converter connected to a DC bus (not shown). The power converter  28  is electrically connected to the flow battery stack  26 , and in particular, to the first and second current collectors  98  and  100 , in each of the flow battery cells  92 . Alternatively, the power converter  28  may be electrically connected to the current collectors on the opposite ends of the stack  26 . 
         [0026]    The controller  42  can be implemented using hardware, software, or a combination thereof. The hardware can include, for example, one or more processors, analog and/or digital circuitry, etc. The controller  42  is in signal communication (e.g., hardwired or wirelessly connected) with each of the sensors  30 - 40  (see  FIGS. 1 and 2 ), each flow regulator  56 ,  58  via its associated pump  60 ,  62  and valve  64 ,  66 , each coolant loop  20 ,  22 ,  24  via its associated circulation pump  80 ,  82 ,  84 , and the power converter  28 . 
         [0027]      FIG. 5  illustrates a method for operating the flow battery system  10  over a wide range of temperatures (e.g., between ˜10° C. to ˜80° C.). Referring to  FIGS. 1-5 , in step  500 , the controller  42  signals the pump  60  and valve  64  in the first flow regulator  56  to circulate the first electrolyte solution between the first reservoir  12  and the flow battery stack  26 . The controller  42  also signals the pump  62  and valve  66  in the second flow regulator  58  to circulate the second electrolyte solution between the second reservoir  14  and the flow battery stack  26 . As the electrolyte solutions are circulated through the flow battery system  10 , they are typically heated, for example, by heat generated as a byproduct of (i) inefficiencies in the flow battery cells  92  (e.g., ohmic losses) and/or (ii) operating the pumps  60  and  62  and/or other components of the flow battery system  10 . 
         [0028]    In step  502 , the controller  42  selectively controls the heat exchange communication and, thus, a rate of heat exchange between one or more of the coolant loops  20 ,  22  and  24  and the first and/or second electrolyte solutions based on cell temperature signals respectively provided by the first and/or second cell temperature sensors  34  and  36 . In particular, the controller  42  controls one or more of the coolant loops  20 ,  22  and  24  to maintain temperatures of the first and/or second electrolyte solutions within the flow battery and reference cells  92  and  29  in a desired operating range (e.g. between approximately 40° and 80° C.). The controller  42 , for example, may allow the electrolyte solutions to be heated, by the heat generated from inefficiencies in the flow battery cells  92  and/or operation the components of the flow battery system  10 , when the cell temperature signals indicate that the electrolyte solution temperatures within the cells  92  and/or  29  are approaching or are at a lower threshold (e.g., ≦40° C.). If necessary, waste heat from other sources such as the power converter  28 , or even devices outside of the flow battery system  10  (e.g., local power generation devices) may be used to heat the electrolyte solutions, either directly or indirectly (e.g., by heating one of the second heat exchange devices). In contrast, the controller  42  may signal one or more of the circulation pumps to operate its respective coolant loop to cool the electrolyte solutions when the cell temperature signals indicate that the electrolyte solution temperatures within the cells  92  and/or  29  are approaching or are at an upper threshold (e.g., ≧80° C.). 
         [0029]    Maintaining the temperatures of the electrolyte solutions between approximately 40° and 80° C. may enable the flow battery system  10  to charge and/or discharge at relatively high current densities (e.g., &gt;100-200 mA/cm 2 ). Operating at such high current densities may enable the flow battery system  10  to store or deliver higher power than operating the system  10  at low current densities, which may be desirable during some periods of the day or year. Operating at such high current densities may also decrease system runtime, and thereby enable the flow battery system to quickly meet fluctuating energy demands. The decreased runtime may also enable the flow battery to complete its charging or discharging before precipitation occurs. In addition, maintaining the temperatures above ˜40° C. decreases the performance requirement of the coolant loop, relative to a coolant loop maintaining an electrolyte solution temperature below 40° C., which has less of a temperature difference relative to ambient temperature. Maintaining the temperatures above ˜40° C. may also improve the performance of the cells, which may thereby increase flow battery system efficiency. 
         [0030]    Each of the flow battery cells  92  are operated at a certain current density to store energy or discharge energy from the first and second electrolyte solutions, which are maintained at the desired temperature. In step  504 , the controller  42  selects the current density at which the cells  92  are operated by signaling the power converter  28  to exchange (i.e., provide or receive) electrical current with the each of the flow battery cells  92  at a rate that corresponds to the desired current density. Alternatively, instead of controlling the rate of charge or discharge at a constant current, the rate may be controlled by controlling the power delivered to or released by the cells  92 , by controlling the stack voltage, or by some combination thereof. 
         [0031]    In step  506 , the controller  42  selectively controls the heat exchange communication between the coolant loops  20 ,  22 ,  24  and the electrolyte solutions to regulate the temperatures of the electrolyte solutions, within the flow battery cells, based on (i) the sensed amount of precipitate formed in the first and/or second electrolyte solutions, and/or (ii) the sensed state-of-charge of ions within the first and/or second electrolyte solutions. 
         [0032]      FIG. 6  illustrates a method for regulating the temperature of the electrolyte solutions. Referring to  FIGS. 1-4  and  6 , in step  600 , the controller  42  receives signals from the precipitate sensor  38  and/or state-of-charge sensor  40  indicative of a quantity of precipitate and a state-of charge of ions in the first and/or second electrolyte solutions. The term “state-of-charge” is used herein, as indicated above, to describe a ratio of (i) a quantity of ions within a volume of a solution having a relatively high charge (e.g., V 5+ ) to (ii) a quantity of ions within the volume of the solution having a relatively low charge (e.g., V 4+ ). 
         [0033]    In step  602 , the controller  42  controls the stack coolant loop  24  and/or the flow circuit coolant loops  20  and  22  to cool the first and/or second electrolyte solutions when the quantity of precipitate and/or state-of-charge in the first and/or second electrolyte solution is greater than certain threshold values. The controller  42 , for example, may control the first flow circuit coolant loop  20  to cool the first electrolyte solution, independent of the second electrolyte solution, when the quantity of precipitate in the first electrolyte solution is greater than the threshold value. The controller  42  may also control the first flow circuit coolant loop  20  to cool the first electrolyte solution to a relatively low temperature (e.g., ˜40° C.) as the state-of-charge of the ions in the first electrolyte solution increases to a relatively high state-of-charge (e.g., where the majority of the ions are V 5+ ). The controller  42  therefore controls the first flow circuit coolant loop  20  to cool the first electrolyte solution as the flow battery system  10  is being charged. In step  604 , on the other hand, the controller  42  may allow the first electrolyte solution to be heated, by heat generated from inefficiencies in the flow battery cells  92  and/or operation the components of the flow battery system  10 , to a relatively high temperature (e.g., ˜65°-80° C.) as the state-of-charge of the ions in the first electrolyte solution decreases to a relatively low state-of-charge (e.g., where the majority of the ions are V 4+ ). The first electrolyte solution therefore is heated as the flow battery system  10  is being discharged. Alternatively, the controller  42  may control one or more of the coolant loops  20 ,  22  and  24  to cool both electrolyte solutions as the state-of-charge of the ions in the first electrolyte solution increase, or allow both electrolyte solutions to be heated as the state-of-charge of the ions in the first electrolyte solution decreases. 
         [0034]    Referring again to  FIGS. 1-5 , in step  508 , the controller  42  determines whether the flow battery system  10  is charged or discharged based on the sensed state-of-charge of ions within the first electrolyte solution. The controller  42  may determine that the flow battery system  10  is discharged, for example, when the sensed state-of-charge is below a lower threshold (e.g., approximately five to ten percent of the ions in a vanadium catholyte are V 5+  ions). The controller  42  may determine that the flow battery system  10  is charged, on the other hand, when the sensed state-of-charge is above an upper threshold (e.g., approximately ninety to ninety-five percent of the ions in a vanadium catholyte are V 5+  ions). If the controller  42  determines that the flow battery system is not yet charged or discharged, method steps  500 - 508  are repeated. If the controller  42  determines that the flow battery system is charged or discharged, however, the method moves to step  510 . 
         [0035]    In step  510 , the controller  42  selectively controls the heat exchange communication between one or more of the coolant loops  20  and  22  and the first and/or second electrolyte solutions based on signals provided by the first and second reservoir temperature sensors  30  and  32 . In particular, the controller  42  controls the coolant loops  20  and  22  and the flow regulators  56  and  58  to maintain temperatures of the first and/or second electrolyte solutions within the reservoirs  12  and  14  within a storage temperature range (e.g., between approximately 10° and 40° C.). The controller  42 , for example, may signal the valves  64  and  66  and pumps  60  and  62  to circulate the electrolyte solutions between the first and second reservoirs  12  and  14  and the first and second flow circuit coolant loops  20  and  22  through the bypass conduits  52  and  54 , rather than the return conduits  48  and  50 . The controller  42  may then signal the circulation pumps  80  and  82  in the flow circuit coolant loops  20  and  22  to operate each respective coolant loop to cool the electrolyte solutions to a temperature between approximately 10° and 40° C. 
         [0036]    In some embodiments, the controller  42  may control the coolant loops to cool one or both of the solutions to temperatures between approximately 10° and 40° C., and control the flow regulators to circulate the solutions through the flow battery stack to dissolve precipitate that has formed within one or more of the cells  92  and  94 . In other embodiments, the first and/or second reservoirs  12  and  14  may each include an agitator that assists in dissolving precipitates that have formed in the respective electrolyte solution. 
         [0037]    In some embodiments, the first and second flow circuit coolant loops  20  and  22  may be respectively disposed with the first and second reservoirs  12  and  14 . In other embodiments, two or more of the coolant loops  20 ,  22  and  24  may be configured as a single coolant loop having a plurality of fluidly connected first heat exchangers  68 ,  70  and  72 . In still other embodiments, the flow battery system  10  may include either the stack coolant loop  24  or the first and second flow circuit coolant loops  20  and  22 . In still other embodiments, one or more of the coolant loops may be thermally connected to a heat source (e.g., the power converter or a local power producing device such as a fuel cell). 
         [0038]    One of ordinary skill will recognize that the cooling and/or heating of the electrolyte solutions may be automatically regulated in various ways. In some embodiments, for example, the precipitate sensor  38  may include a pair of pressure sensors rather than the optical sensor. The pressure sensors may be operated together, for example, to detect precipitate within the first electrolyte solution based on pressure drop across the reference cell  94 , or across the entire flow battery stack  26 . 
         [0039]    In some embodiments, the respective temperature, precipitate and/or state-of-charge sensors  34 - 40  may be disposed within the flow battery stack  26  where, for example, the reference cell  29  is configured within the flow battery stack  26 . In other embodiments, the state-of-charge sensors  34 - 40  may be disposed outside of the reference cell, for example, in the return conduits  48  and/or  50 . 
         [0040]    In some embodiments, the first and/or second reservoirs  12  and  14  may each include a source reservoir and a return reservoir, such that the flow battery system  10  may operate in an open loop. Such an open loop system may enable the solutions in the source and return reservoirs to be maintained in different temperature ranges. In one embodiment, the source and return reservoirs may be configured as two separate tanks. In another embodiment, the source and return reservoirs may be disposed within the same tank, and may be separated by a divider (e.g., walls of a plastic bladder). In other embodiments, the first reservoir  12  and  14  may include a plurality of acidic ion-exchange beads for impeding precipitation of, for example, V 5+  therewithin. 
         [0041]    While various embodiments of the flow battery system have been disclosed, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope of the flow battery system. Accordingly, the flow battery system is not to be restricted except in light of the attached claims and their equivalents.