Abstract:
According to one embodiment of the present invention, the method for controlling the pump speed of a redox flow battery for transferring an electrolyte stored in an electrolyte tank to a cell stack comprises the steps of: measuring the input power and/or the output power of the redox flow battery; measuring the charging power and/or the discharging power of the redox flow battery; calculating the power loss of the redox flow battery by using the difference between the input power and the charging power, or the difference between the output power and the discharging power; and adjusting the pump speed according to the power loss.

Description:
BACKGROUND 
     1. Technical Field 
       [0001]    The present disclosure relates to a method and apparatus for controlling a pump speed of a redox flow battery. 
       2. Description of the Related Art 
       [0002]    General secondary batteries convert input electric energy to chemical energy and store the converted energy in a charge process, and convert the stored chemical energy to electric energy and output the converted electric energy in a discharge process. 
         [0003]    Redox flow batteries, like the general secondary batteries, convert input electric energy to chemical energy and store the converted energy in a charge process, and convert the stored chemical energy to electric energy and output the converted electric energy in a discharge process. However, the redox flow battery is different from the general secondary battery in that an electrolyte keeping energy is stored in a separate tank. Furthermore, in order to supply the electrolyte stored in the tank to a cell stack, a separate pump needs to be installed on a pipe connecting the tank to the electrolyte. 
         [0004]      FIG. 1  illustrates a configuration of a redox flow battery system according to the related art. Referring to  FIG. 1 , a redox flow battery may include a first tank  102  storing a first electrolyte, a second tank  104  storing a second electrolyte, and a cell stack  110  where charge and discharge of energy occur through an oxidation-reduction reaction using the first electrolyte and the second electrolyte transferred from the first tank  102  and the second tank  104 . Furthermore, in order to supply the electrolytes from the first tank  102  and the second tank  104  to the cell stack  110 , a first pump  106  is installed on a pipe connecting the first tank  102  to the cell stack  110 , and a second pump  108  is installed on a pipe connecting the second tank  104  to the cell stack  110 . 
         [0005]    To improve efficiency of the redox flow battery system having the configuration of  FIG. 1 , it is important to minimize power consumption by additional devices such as the first and second pumps  106  and  108 . In particular, the first and second pumps  106  and  108 , which are essential devices to supply the electrolyte, consume the largest portion of power among additional devices. Since a power loss in the cell stack  110  is affected by a flow rate of the supplied electrolyte, it may be a problem that the speed of the pump cannot be unilaterally decreased or increased. In general, when the flow rate of electrolyte supplied to the cell stack  110  increases due to an increase in the speeds of the first and second pumps  106  and  108 , while the power consumption of the first and second pumps  106  and  108  increases, the power loss in the cell stack  110  decreases. Reversely, while the power consumption of the first and second pumps  106  and  108  decreases, the power loss in the cell stack  110  increases. 
         [0006]    In consideration of the above, there has been a demand for an improved method of controlling a pump speed to improve power efficiency of the redox flow battery system. 
       SUMMARY 
       [0007]    It is an object of the present disclosure to maximize power efficiency of a redox flow battery system by controlling a pump speed and a supply amount of an electrolyte according thereto to minimize a power loss of the redox flow battery system. 
         [0008]    The technical problems of the present disclosure are not limited to the above-mentioned technical objects, and other technical objects which are not mentioned can be clearly understood by those skilled in the art from the following description. 
         [0009]    In accordance with one aspect of the present disclosure, a method of controlling a pump speed of a redox flow battery for transferring an electrolyte stored in an electrolyte tank to a cell stack, which includes measuring at least one of an input power and an output power of the redox flow battery, measuring at least one of a charging power and a discharging power of the redox flow battery, calculating a power loss of the redox flow battery by using a difference between the input power and the charging power or a difference between the output power and the discharging power, and adjusting the pump speed according to the power loss. 
         [0010]    The adjusting, of the pump speed may include increasing the pump speed, comparing a first power loss before the increasing of the pump speed and a second power loss after the increasing of the pump speed, and increasing the pump speed when the first power loss is greater than the second power loss, and decreasing the pump speed when the first power loss is less than or equal to the second power loss. 
         [0011]    The adjusting of the pump speed may include decreasing the pump speed, comparing a first power loss before the decreasing of the pump speed and a second power loss after the decreasing of the pump speed, and decreasing the pump speed when the first power loss is greater than the second power loss, and increasing the pump speed when the first power loss is less than or equal to the second power loss. 
         [0012]    The adjusting of the pump speed may include maintaining an existing pump speed when the power loss is less than or equal to a preset first lost power, and allowing a change of the pump speed only when the power loss is greater than or equal to a preset second lost power. 
         [0013]    The adjusting of the pump speed may include terminating charging or discharge of the redox flow battery when the pump speed reaches a preset upper limit speed, and maintaining an existing pump speed when the pump speed reaches a preset lower limit speed and the power loss decreases less than a preset third reference power. 
         [0014]    In accordance with another aspect of the present disclosure, an apparatus for controlling a pump speed of a redox flow battery for transferring an electrolyte stored in an electrolyte tank to a cell stack, which includes a measuring unit measuring at least one of an input power and an output power of the redox flow battery and measuring at least one of a charging power and a discharging power of the redox flow battery, a calculation unit calculating a power loss of the redox flow battery by using a difference between the input power and the charging power or a difference between the output power and the discharging power, and a controller adjusting the pump speed according to the power loss. 
         [0015]    The controller may increase the pump speed, compare a first power loss before the increasing of the pump speed and a second power loss after the increasing of the pump speed, increase the pump speed when the first power loss is greater than the second power loss, and decrease the pump speed when the first power loss is less than or equal to the second power loss. 
         [0016]    The controller may decrease the pump speed, compare a first power loss before the decreasing of the pump speed and a second power loss after the decreasing of the pump speed, decrease the pump speed when the first power loss is greater than the second power loss, and increase the pump speed when the first power loss is less than or equal to the second power loss. 
         [0017]    The controller may maintain an existing pump speed when the power loss is less than or equal to a preset first lost power, and allow a change of the pump speed only when the power loss is greater than or equal to a preset second lost power. 
         [0018]    The controller may terminate charging or discharge of the redox flow battery when the pump speed reaches a preset upper limit speed, and maintain an existing pump speed when the pump speed reaches a preset lower limit speed and the power loss decreases less than a preset third reference power. 
         [0019]    According to the present disclosure, power efficiency of a redox flow battery system may be maximized by controlling a pump speed and a supply amount of an electrolyte according thereto to minimize a power loss of the redox flow battery system. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  illustrates a configuration of a redox flow battery system according to the related art. 
           [0021]      FIG. 2  illustrates a configuration of a redox flow battery system and a pump speed controller according to an embodiment. 
           [0022]      FIG. 3  is a block diagram for explaining a state of charge (SOC) change of an electrolyte input to a unit cell and an electrolyte output from the unit cell. 
           [0023]      FIG. 4  is a graph showing a cell voltage curve of a redox flow battery according to a difference in the SOC between the electrolyte input to a unit cell and the electrolyte output from the unit cell. 
           [0024]      FIG. 5  is a graph showing a stack power loss, a pump power consumption amount, and a system power loss according to a pump speed. 
           [0025]      FIG. 6  is a flowchart of a method of controlling a pump speed of a redox flow battery, according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    The above objects, features and advantages will become apparent from the detailed description with reference to the accompanying drawings. Embodiments are described in sufficient detail to enable those skilled in the art in the art to easily practice the technical idea of the present disclosure. Detailed descriptions of well-known functions or configurations may be omitted in order not to unnecessarily obscure the gist of the present disclosure. Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Throughout the drawings, like reference numerals refer to like elements. 
         [0027]      FIG. 2  illustrates a configuration of a redox flow battery system and a pump speed controller according to an embodiment. Referring to  FIG. 2 , a redox flow battery according to an embodiment may include a first tank  202  storing a first electrolyte, a second tank  204  storing a second electrolyte, a cell stack  210  where charge and discharge of power occur through an oxidation-reduction reaction using the first electrolyte and the second electrolyte transferred from the first tank  202  and the second tank  204 . The cell stack  210  may include a plurality of cells. 
         [0028]    Furthermore, to supply the electrolytes from the first tank  202  and the second tank  204  to the cell stack  210 , a first pump  206  is installed on a pipe connecting the first tank  202  to the cell stack  210 , and a second pump  208  is installed on a pipe connecting the second tank  204  to the cell stack  210 . In the present disclosure, when speeds of the first and second pumps  206  and  208  increase, a flow rate of the electrodes supplied from the first and second tanks  202  and  204  to the cell stack  210  increases. Reversely, when speeds of the first and second pumps  206  and  208  decrease, the flow rate of the electrodes supplied from the first and second tanks  202  and  204  to the cell stack  210  decreases. 
         [0029]    Referring back to  FIG. 2 , according to the present embodiment, a pump speed controller  212  may include a measurement unit  214 , a calculation unit  216 , and a controller  218 . 
         [0030]    The measurement unit  214  is connected to the redox flow battery and may measure or calculate input power, output power, charging power, and discharging power of the redox flow battery. In the present disclosure, when the redox flow battery operates in a charge mode, the input power may be defined to be a total amount of power supplied to the redox flow battery during charging, and the charging power may be defined to be power actually charged in the battery. Furthermore, when the redox flow battery operates in a discharge mode, the output power may be defined to be a total amount of power discharged from the redox flow battery to the outside during discharge, and the discharging power may be defined to be an amount of power that is actually discharged from the battery. In the present disclosure, the charging power or the discharging power signify power reflecting only a state of charge (SOC) change, excluding a power loss due to the internal resistance of the redox flow battery and additional devices such as pumps. 
         [0031]    Furthermore, the measurement unit  214  may measure amounts of a voltage and current applied to the cell stack  210 , an open-circuit voltage of the cell stack  210 , speeds or power consumption amounts of the first and second pumps  206  and  208 . 
         [0032]    The calculation unit  216  may measure a power loss of a redox flow battery by using the input power and the charging power, or the output power and the discharging power, which are measured by the measurement unit  214 . 
         [0033]    The controller  218  may control the speeds of the first and second pumps  206  and  208  of the redox flow battery according to the power loss calculated by the measurement unit  214 . The flow rates of the electrolytes flowing into the cell stack  210  from the first and second tanks  202  and  204  are adjusted according to the pump speed control of the controller  218 . 
         [0034]    In the following description, a method of controlling a pump speed of a redox flow battery according to an embodiment is described in detail with reference to  FIG. 2 . 
         [0035]    The power loss of an overall redox flow battery system of  FIG. 2  may be determined according to a difference between the input power supplied to the redox flow battery from the outside and the charging power that is actually charged in the redox flow battery, or a difference between the output power discharged from the redox flow battery to the outside and the discharging power discharged from the redox flow battery. When the redox flow battery operates in the charge mode, the input power may be defined to be a total amount of power supplied to the redox flow battery during charging, and the charging power may be defined to be power actually charged in the battery. Furthermore, when the redox flow battery operates in the discharge mode, the output power may be defined to be a total amount of power discharged from the redox flow battery to the outside during discharge, and the discharging power may be defined to be an amount of power that is actually discharged from the battery. 
         [0036]    In other words, the power loss of the redox flow battery system during charging may be defined by Equation 1 below. 
         [0000]      System Power Loss ( P )=Input Power ( B )−Charging power ( A )=Stack Power Loss (Power loss occurred in a stack)+Additional Device Power Consumption (Amount of power consumption by an additional device such as a pump)   [Equation 1]
 
         [0037]    Furthermore, the power loss of the redox flow battery system during discharge may be defined by Equation 2 below. 
         [0000]      System Power Loss ( P )=Discharging power ( C )−Output Power ( D )=Stack Power Loss+Additional Device Power Consumption   [Equation 2]
 
         [0038]    In the redox flow battery system of  FIG. 2 , Input Power (B) or Output Power (D) may be measured by the measurement unit  214 . Charging power (A) or Discharging power (C) may be obtained from a product of an open-circuit voltage of the cell stack  210  measured by the measurement unit  214  and current applied to the cell stack  210 , as expressed below. 
         [0000]      Charging power ( A )=Open-Circuit Voltage (Ocv)×Current ( I )   [Equation 3]
 
         [0000]      Discharging power ( C )=Open-Circuit Voltage (Ocv)×Current ( I )   [Equation 4]
 
         [0039]    The cell stack  210  according to the present embodiment may include a plurality of unit cells. When an open-circuit voltage (cell voltage) with respect to specific unit cells of all unit cells forming the cell stack  210  is measured and used, a charging power (A) may be defined as follows. 
         [0000]      Charging power ( A )=Open-Circuit Voltage (Ocv)×Current ( I )×{Number of cells serially connected in a stack (ST)/Number of serially connected cells subject to OCV measurement ( SI )}  [Equation 5]
 
         [0040]    For example, when power of 100 W is supplied from the outside during the charge of the redox flow battery system of  FIG. 2 , the current of the cell stack  210  is 10 A, the open-circuit voltage of one unit cell of the cell stack  210  including ten unit cells is 0.8V, System Power Loss (P) may be calculated as follows. 
         [0000]        P=B−A= 100 W−(0.8 V×10 A×(10/1))=20 W   [Equation 6]
 
         [0041]    In other words, in this case, an amount of 20 W corresponds to the power loss consumed due to heat generation or operation of additional devices such as the first and second pumps  206  and  208 . 
         [0042]      FIG. 3  is a block diagram for explaining an SOC change of an electrolyte input to a unit cell and an electrolyte output from the unit cell. 
         [0043]      FIG. 3  illustrates a unit cell  302  forming the cell stack  210 . The electrolytes stored in the first and second tanks  202  and  204  are input to the unit cell  302  by the first and second pumps  206  and  208 , and are output from the unit cell  302  after completing an oxidation-reduction reaction. In this state, assuming that an SOC of the input electrolyte is x %, an SOC of the output electrolyte is (x+Δx)%. A difference Δx between the SOC of the electrolyte input to the unit cell  302  and the SOC of the electrolyte output from the unit cell  302  may be expressed as follows. 
         [0000]      Δx ∝(Current applied to a unit cell/Pump speed)   [Equation 7]
 
         [0044]    For example, when the redox flow battery system operates in the charge mode, if the SOC of the electrolyte input to the unit cell  302  reflects the state of the electrolyte that is currently charged, the SOC of the electrolyte output from the unit cell  302  reflects power needed for charging. Assuming that the electrolyte is charged by applying a certain amount of current to the unit cell  302 , if the first and second pumps  206  and  208  supply the electrolyte to the unit cell  302  at a sufficiently high speed, an SOC difference between the input electrolyte and the output electrolyte may be close to zero (0), and as the speeds of the first and second pumps  205  and  208  get lower, the SOC difference between the input electrolyte and the output electrolyte may increase further. 
         [0045]      FIG. 4  is a graph showing a cell voltage curve of a redox flow battery according to the SOC difference between the electrolyte input to a unit cell and the electrolyte output from the unit cell. In  FIG. 4 , when a difference between the SOC of the electrolyte input to the unit cell  302  and the SOC of the electrolyte output from the unit cell  302  is 0%, 16%, or 32%, the tendency of a cell voltage curve of the redox flow battery is illustrated. In  FIG. 4 , the horizontal axis denotes the SOC of the redox flow battery and the vertical axis denotes the amount of a voltage needed to charge the battery at each SOC. 
         [0046]    In  FIG. 4 , assuming that the curve on which the SOC difference between the input electrolyte and the output electrolyte is 0% is regarded to be a current electrolyte state, it may be seen that, as the SOC difference gradually increases to 16% and 32%, that is, as the pump speed decreases, a more amount of a voltage is needed for charging at the same SOC. 
         [0047]    As a result, in addition to the power loss due to the resistance of the unit cell  302  that is proportional to the current applied to the unit cell  302 , an additional power loss due to the pump speed occurs. Referring to  FIG. 4 , it may be seen that an increase of a necessary voltage according to the pump speed, that is, the power loss of the redox flow battery system, is maximum at opposite ends (0% and 100%) of the SOC, and minimum in the middle. Furthermore, as the pump speed increases, the power loss decreases. 
         [0048]    The power consumption of the pumps according to the increase of the pump speed is proportional to a product of a differential pressure and a flow rate and the differential pressure is proportional to the flow rate, and thus the power consumption is proportional to the square of the flow rate. 
         [0049]      FIG. 5  is a graph showing a stack power loss, a pump power consumption amount, and a system power loss according to the pump speed. In  FIG. 5 , the horizontal axis denotes the pump speed, and the vertical axis denotes the power loss or power consumption amount. Referring to  FIG. 5 , the stack power loss shows a decreasing tendency according to an increase in the pump speed, and the pump power consumption shows an increasing tendency according to an increase in the pump speed. Consequently, the system power loss of a redox flow battery system according to the pump speed may be expressed by a graph having one minimum value as shown in  FIG. 5 . As such, it may be seen that a specific pump speed at which the system power loss becomes minimum exists, and the pump speed does not proportional to the SOC of the redox flow battery. Accordingly, by increasing or decreasing the pump speed while measuring or calculating the system power loss in a periodic or non-periodic manner, the pump may be operated at an optimal speed at which the system power loss becomes minimum. 
         [0050]      FIG. 6  is a flowchart of a method of controlling a pump speed of a redox flow battery, according to an embodiment. 
         [0051]    First, when a redox flow battery system is in operation, the calculation unit  216  calculates a first power loss P 1  at a current pump speed of the redox flow battery system ( 602 ). As described above, the power loss of the redox flow battery system is defined to be a difference between the input power and the charging power during charging or a difference between the output power and the discharging power during discharge. 
         [0052]    Then, the controller  218  changes the pump speed to a preset value ( 604 ), and after the pump speed is increased, the calculation unit  216  measures a second power loss P 2  of the system. 
         [0053]    After the pump speed is changed, when the first power loss P 1  before the pump speed change and the second power loss P 2  after the pump speed change are calculated, the controller  218  compares the size of the first power loss P 1  with the size of the second power loss P 2  ( 608 ). Then, the controller  218  controls the pump speed according to a result of the comparison of the first power loss P 1  and the second power loss P 2  ( 610 ). 
         [0054]    If the controller  218  increases the pump speed in the operation  604  of  FIG. 6 , in the operation  610 , the controller  218  continuously increases the pump speed when the first power loss P 1  is greater than the second power loss P 2 , and decreases the pump speed when the first power loss P 1  is less than or equal to the second power loss P 2 . 
         [0055]    If the controller  218  decreases the pump speed in the operation  604  of  FIG. 6 , in the operation  610 , the controller  218  continuously decreases the pump speed when the first power loss P 1  is greater than the second power loss P 2 , and increases the pump speed when the first power loss P 1  is less than or equal to the second power loss P 2 . 
         [0056]    The controller  218  compares the first power loss P 1  and the second power loss P 2  as described above and repeatedly performs the pump speed increasing or decreasing process according to the comparison result, thereby controlling the pump speed to minimize the power loss of the redox flow battery system. 
         [0057]    In the present embodiment, the power loss measurement and the pump speed control of the redox flow battery system, as illustrated in  FIG. 6 , may be repeatedly performed according to a preset time interval. Furthermore, the time interval may be freely changed according to a degree of a change in the power loss or the pump speed. 
         [0058]    Furthermore, although not illustrated in  FIG. 6 , in another embodiment, the method may further include an operation of maintaining the pump speed, without change, when the measured power loss P 1  or P 2  is less than or equal to a preset first reference power. Furthermore, in another embodiment, the method may further include an operation of allowing a change of the pump speed only when the measured power loss P 1  or P 2  is greater than or equal to a preset second reference power. 
         [0059]    Furthermore, although not illustrated in  FIG. 6 , in another embodiment, the method may further include an operation of terminating the charge or discharge of the redox flow battery when the pump speed reaches a preset upper limit speed. Furthermore, in another embodiment, the method may further include an operation of maintaining the pump speed when the measured power loss P 1  or P 2  decreases less than a preset third reference power and when the pump speed reaches a preset lower limit speed. 
         [0060]    The present disclosure described above may be variously substituted, altered, and modified by those skilled in the art to which the present inventive concept pertains without departing from the scope and spirit of the present disclosure. Therefore, the present disclosure is not limited to the above-mentioned exemplary embodiments and the accompanying drawings.