WO2015200824A1 - Optical detection of electrolyte concentrations - Google Patents

Abstract

Methods, systems and structures for monitoring or measuring unknown reactant concentrations in electrochemical flow systems are provided. Reactant concentrations may be measured optically by obtaining a digital image of an electrolyte sample, detecting color componenets in the digital image, and correlating the color component information with information corresponding to color components for samples of known concentrations.

Description

TITLE

OPTICAL DETECTION OF ELECTROLYTE CONCENTRATIONS

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit U.S. Provisional Patent Application No. 62/018150 entitled "OPTICAL DETECTION OF ELECTROLYTE CONCENTRATIONS," filed June 27, 2014.

FIELD OF THE INVENTION

[0002] This invention generally relates to reduction-oxidation (redox) flow batteries and more particularly to optically monitoring and characterizing reactant concentrations in liquid flow battery electrolytes.

BACKGROUND

[0003] Electrochemical flow systems, in which electrochemical reactants are dissolved in flowing liquid electrolytes, are used in many applications such as electroplating, chemical production, water processing, and energy storage (among others). Flow batteries are electrochemical flow systems configured for energy storage. In a flow battery, electrochemical reactants may be dissolved in liquid electrolytes (sometimes referred to generically as "reactants"), which flow or are pumped through reaction cells where electrical energy may be converted to or extracted from chemical potential energy in the reactants by way of reduction and oxidation reactions. In applications where megawatts of electrical energy are stored and discharged, a redox flow battery system may be expanded to the required energy storage capacity by increasing tank sizes. Flow batteries may be expanded to produce a larger output power by increasing the individual physical, material or parametric characteristics, number or size of electrochemical cells or cell blocks. A variety of flow battery chemistries and arrangements are known in the art.

[0004] In some redox flow battery systems based on the Fe/Cr redox couple, the catholyte (in the positive half-cell) contains FcC , FeCl₂ and HC1. The anolyte (in the negative half-cell) contains CrCla, CrCl₂ and HC1. Such a system is known as an "un-mixed reactant" system. In a "mixed reactant" system, the anolyte also contains FeCl₂, and the catholyte also contains CrCl₃. In an initial state of either case, the catholyte and anolyte may have equimolar reactant concentrations. [0005] The ability to rapidly and accurately monitor the state of charge or imbalance of electrolyte concentrations can provide advantages. Monitoring or measuring the state of charge and the imbalance of electrolytes however can be challenging.

BRIEF DESCRIPTION OF DRAWINGS

[0006] The accompanying drawings, which are incorporated herein and constitute part of this specification, illustrate exemplary embodiments of the invention, and together with the general description given above and the detailed description given below, serve to explain the features of the invention.

[0007] FIG. 1 is a schematic diagram illustrating an example electrochemical flow system including a rebalancing sub-system and an electrolyte monitoring sub-system.

[0008] FIG. 2 is a schematic cross-sectional diagram illustrating an electrolyte monitoring apparatus that may be used for optically determining a state-of-oxidation of one or more liquid electrolytes.

[0009] FIG. 3 is a schematic diagram illustrating a portion of an electrolyte monitoring apparatus including a plurality of light sources.

[0010] FIG. 4A is a process flow diagram illustrating a process for evaluating an electrolyte using an optical electrolyte monitoring apparatus.

[0011] FIG. 4B is a process flow diagram illustrating a process for evaluating an error in an imbalance determination using an optical electrolyte monitoring apparatus.

[0012] FIG. 5 is a schematic illustration of an example image having two separate electrolyte regions and a calibration region.

[0013] FIG. 6 is a schematic illustration of an example image having multiple electrolyte regions and multiple calibration regions corresponding to light sources of various colors.

[0014] FIG. 7 is a graph illustrating color component values at various states-of-oxidation of an example negative electrolyte.

[0015] FIG. 8 is a graph illustrating color component values at various states-of-oxidation of an example positive electrolyte.

[0016] FIG. 9 is a schematic diagram illustrating components of an electronic controller that may be used with some embodiments. DETAILED DESCRIPTION

[0017] Some redox flow batteries store electrical charge in one or more dissolved reactant species in liquid electrolytes. As flow battery electrolytes are charged and discharged, the quantity of reactant species in one or both electrolytes in a charged state may be unknown due to various factors as described further below. Various examples are provided below for reliably determining electrolyte state of charge (or state-of-oxidation) using relatively low-cost equipment. Various systems and methods below may be configured for determining electrolyte state of charge, state of oxidation, or imbalance by evaluating color component values obtained with a digital camera.

[0018] As used herein, the phrase "state of charge" and its abbreviation "SOC" refer to the ratio of stored electrical charges (measured in ampere-hour) to charge storage capacity of a complete redox flow battery system. In particular, the terms "state of charge' and "SOC" may refer to an instantaneous ratio of usable charge stored in the flow battery to the full ideal charge storage capacity of the flow battery system. In some embodiments, "usable" stored charge may refer to stored charge that may be delivered at or above a threshold voltage. In some embodiments, the ideal charge storage capacity may be calculated excluding the effects of unbalanced electrolytes.

[0019] As used herein the phrase "state of oxidation" and its abbreviation "SOO" refer to the chemical species composition of at least one liquid electrolyte. In particular, state of oxidation and SOO may refer to the proportion of reactants in the electrolyte that have been converted (e.g. oxidized or reduced) to a "charged" state from a "discharged" state. For example, in a redox flow battery based on an iron/chromium (Fe/Cr) redox couple, the state of oxidation of the catholyte (positive electrolyte) may be defined as the percent of total Fe which has been oxidized from the ferrous iron (Fe²⁺) form to the ferric iron (Fe³⁺) form' and the state of oxidation of the anolyte (negative electrolyte) may be defined as the negative percent of total Cr which has been reduced from the Cr³⁺ form to the Cr^2"*^" form.

[0020] The SOO may also be defined and expressed in terms of a concentration of one or more reactants in an electrolyte. For example, SOO may be expressed as a molar concentration (e.g., indicated as a number such as "#.#" followed by the letter "M" herein) of a charged reactant species (i.e., only the numerator of the ratios described above). In such cases, the SOO of the negative electrolyte may be given a negative sign or may be designated, for example, as "NegSOO." Likewise, the SOO of the positive electrolyte may be given a positive sign or may be designated, for example, as "PosSOO." In such cases, the sum of the negative electrolyte SOO and the positive electrolyte SOO may be equal to an electrolyte imbalance. Electrolytes may be described as un-balanced or as having an imbalance when a quantity of a charged active material in one electrolyte is greater than the quantity of the charged active material in the second electrolyte. For example, a positive imbalance exists between two flow battery electrolytes when the positive electrolyte contains a greater quantity of charged active material than a quantity of charged active material in the negative electrolyte. On the other hand, a negative imbalance exists between two flow battery electrolytes when the negative electrolyte contains a greater quantity of charged active material than a quantity of charged active material in the positive electrolyte.

10021] Although many of the embodiments herein are described with reference to an Fe/Cr flow battery chemistry, it should be appreciated with the benefit of the present disclosure that some embodiments are applicable to flow battery systems (and some hybrid flow battery systems) using other reactants.

[0022] In some embodiments, the state of oxidation of the two electrolytes may be changed or measured independent of one another. Thus, the terms "state of oxidation" and "SOO" may refer to the chemical composition of only one electrolyte, or of both electrolytes in an all-liquid redox flow battery system. The state of oxidation of one or both electrolytes may also be changed by processes other than desired charging or discharging processes. For example, undesired side reactions may cause oxidation or reduction of acti e species in one electrolyte without producing a corresponding reaction in the second electrolyte. Such side reactions may cause the respective SOCs of the positive and negative electrolytes to become unbalanced such that one electrolyte has a higher effective SOC than the other.

10023] For an Fe/Cr redox flow battery, the SOO of the positive electrolyte may be defined as the ratio of the concentration of Fe³⁺ in the electrolyte to the total concentration of Fe (i.e. the sum of Fe³⁺ and Fe²⁺ concentrations) in the electrolyte. Similarly, the SOO of the negative electrolyte is defined as the ratio of the concentration of Cr²⁺ in the electrolyte to the total concentration of Cr (i.e. the sum of Cr³⁺ and Cr ⁺ concentrations) and may be expressed as a negative number. In equation form, these are:

SOO_pos = Fe³⁺ / (Fe³⁺ + Fe²⁺) [1]

SOO_neg = - Cr²⁺ / (Cr²⁺ + Cr³⁺ [2]

[0024] In some embodiments, flow battery electrolytes may be formulated such that in both positive and negative electrolytes are identical in a fully discharged state. Such a system may be referred to as a "mixed reactant" system, an example of which is described in U.S. Patent No. 4,543,302 the contents of which are incorporated herein by reference. In some embodiments, a mixed reactant electrolyte that contains unequal concentrations of FeCl₂ and CrCl₃ in the initial electrolyte (fully discharged) can be used to minimize the inequality in concentrations of CrCl₂ and FeCl₃, and to mitigate ¾ evolution during operation of a flow battery system. One example of the composition in the fully discharged state is 1M FeCl₂/l.lM CrCl₃/2-3M HCl. In such embodiments, the concentration of CrCl₃ is intentionally made higher than that of FeCl₂ in an initially-prepared and fully-discharged electrolyte solution. Upon charge, the SOO of CrCl₂ will be lower than that of FeCl₃, thereby avoiding high SOO conditions at the Cr electrode where H₂ evolution is a greater problem. With this unequal mixed reactant, the Fe electrode can be charged to nearly 100% while the Cr electrode may be charged to a lower SOO.

[0025] The Fe ionic species (Fe³⁺, Fe²⁺) at the positive electrode may have a total concentration Fe_t = Fe³⁺ + Fe²⁺. Correspondingly, the Cr ionic species (Cr³⁺, Cr²*) at the negative electrode may have a total concentration Cr_t = Cr³⁺ + Cr²⁺. In embodiments of an unequal mixed reactant electrolyte, Fe_t does not equal Cr_t, and the concentration of ionic species Fe³⁺, Fe²⁺, Cr³⁺ and Cr^2"1" may vary widely with SOO.

[0026] The rate of H₂ evolution is exacerbated at more negative potentials, which occurs as the Cr electrode becomes more fully charged. During charge, the ratio of the concentration of Cr²* to the concentration of Cr³⁺ (i.e. Cr^24" / Cr³⁺) increases, which is reflected in the more negative potential of the Cr electrode. By adding excess Cr³⁺, the ratio of Cr ⁺ / Cr³⁺ will be effectively lowered and the potential of the Cr electrode will become less negative and H₂ evolution will thereby be mitigated.

[0027] For example, the maximum charge that can be inputted to a cell with a mixed reactant with unequal concentrations of FeCl₂ and CrCl₃ at 0% SOO (fully discharged) of 1M FeCl₂/l.lM CrCl₃/2M HCl is limited by the lower concentration of the electroactive species in the anolyte or catholyte. In this case, the lower concentration is 1M FeCl₂. The effect of excess CrCl₃ on SOO can be seen in the following example. During charge, if nearly the entire 1 FeCl₂ is oxidized to FeCl₃, then the PosSOO is nearly 100%. At the same time approximately the same amount (1M) of CrCl₃ is reduced to CrCl₂, making the NegSOO approximately 91% (1.0/1.1). In this example, the maximum SOO of the unequal mixed reactant composition is a function of the excess amount of CrCl₃ and the concentration of FeCl₂.

[0028] In some embodiments, an unequal mixed reactant may also provide advantages with respect to cell voltage. In an equal mixed reactant example, the cell voltage calculated using a Nernst potential relationship is 1.104 V for a cell containing equimolar mixed reactant (i.e. 1M FeCl₂/lM CrCl₃/lM HC1) that is charged to 90% SOO.

[0029] The equal mixed reactant example can be compared with a cell with an unequal mixed reactant containing an excess of Cr³⁺ with a composition of 1 M FeCla/l .1 M CrCl₃/l M HC1. When the PosSOO is 90% for the positive electrode (Fe electrode), the negative electrode (Cr electrode) NegSOO is 81.8% and the cell voltage is 1.084 V. By adding a slight excess of Cr³⁺, the cell voltage is lower by 20 mV and the SOO of the negative electrode is lower by about 8%. These two factors are beneficial for mitigating H₂ evolution at higher SOO, and may help enhance energy efficiency.

[0030] Similar advantages may be achieved in flow battery electrolytes based on other redox couples in which parasitic side-reactions become increasingly likely as one electrode approaches a high SOO.

[00311 In some embodiments, if flow battery electrolytes contain un-equal concentrations of total active materials, then a perfectly balanced pair of charged electrolytes will each contain equal amounts of both charged species (e.g., equal quantities of Fe³⁺ and Cr^2"1"), but the SOO of the two electrolytes will be different. For example, in an un-equal mixed reactant Fe/Cr flow battery, the total concentration of Fe may be less than the total concentration of Cr (e.g., total Fe = 1.3M and total Cr = 1.4M in some embodiments). In such a system, the absolute value of SOO of the negative electrolyte may be smaller than the absolute value of SOO of the positive electrolyte even when the charged species are in balance. For example, if Cr²⁺ and Fe³⁺ are both 0.7M, the SOO of the negative electrolyte is -0.7/1.4 = -0.50; The SOO of the positive electrolyte is 0.7/1.3 = 0.54.

[0032] Thus, the various embodiments include systems and methods for characterizing

concentrations of dissolved reactant species in flow battery electrolytes from a point of unknown concentration, including systems and methods for quantifying electrolyte imbalance. Although many of the embodiments are described with reference to Fe/Cr flow batteries, the same principles and concepts may also be applied to other flow battery chemistries.

[0033] The various embodiments are described in detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. References made to particular examples and implementations are for illustrative purposes, and are not intended to limit the scope of the invention or the claims.

[0034] As illustrated in FIG. 1, some embodiments of an electrolyte concentration monitoring system 100 may be integrated into a redox flow battery system 102. A redox flow battery system 102 such as that shown in FIG. 1 may comprise electrolyte tanks 104 fluidically joined to a flow battery stack assembly 106. In some embodiments, a redox flow battery system 102 may comprise four separate tank volumes that may be configured to keep charged electrolytes separated from discharged electrolytes. Such separated tank volumes may comprise four separate tanks or two tanks with dividers.

[0035] In some embodiments, a flow battery stack assembly 106 may comprise a plurality of electrochemical reaction cells configured for charging and discharging active species in the liquid electrolytes. Pumps 108 may be provided to pump electrolytes through the flow battery stack assembly 106 and any other connected systems, such as a rebalancing system 110 and/or an electrolyte concentration monitoring system 100. In some embodiments, the redox flow battery system 102 may be electrically connected to a power source 112 and/or an electric load 114. An electronic controller 116 may also be provided to control the operation of the redox flow battery system 102, including the operation of pumps, valves, electrical connections, or any other electronic or electromechanical component within the redox flow battery system 102. The concentration monitoring system 100 may be provided with sample quantities of the electrolytes through the operation of valves 188, and 189, which may be coupled to electrolyte inlets 184 and 186, where electrolytes are input into the flow battery stack assembly 106. Sample quantities of electrolytes may be provided to an SOO monitoring unit 120, which may be an optical monitoring unit as described in greater detail herein. The SOO monitoring unit 120 may be coupled to a concentration monitoring controller 198, which may contain digital and or analog electronics configured to operate the SOO monitoring unit 120 according to various control processes, such as the processes described in greater detail herein. The concentration monitoring controller 198 may also communicate with and/or be controlled by the electronic controller 116. In some embodiments, as the concentration is monitored in the SOO monitoring unit 120, sample quantities of electrolyte may be returned to the respective inlets 184 and 186 and new sample quantities may be input to the SOO monitoring unit 120. In other embodiments, electrolyte concentrations may be monitored continuously as electrolyte flows through one or more conduits of the SOO monitoring unit 120.

Iron / Chromium Flow Battery Electrochemistry

[0036] The valence state of the Fe and Cr ionic species in an Fe/Cr flow battery changes between charge and discharge. Information on the concentration of the ionic species may be needed to determine the state-of-charge (SOC) of the battery and the electrolyte balance of the anolyte and catholyte. In some embodiments, the electric potential of an Fe/Cr flow battery cell may be used to monitor the SOC of the battery. A higher voltage suggests that the battery SOC is higher. However, the voltage of a flow battery cell may be ambiguous in that there are four ionic species in an Fe/Cr flow battery (Cr²*, Cr³⁺, Fe³⁺ and Fe ⁺) that contribute to the cell voltage. In some embodiments, a more definitive measure of the SOC and concentration of the ionic species may be obtained by measuring the voltage of the anolyte and catholyte separately.

[0037] If charge and discharge are perfectly reversible, the cell is always in balance, with the same concentration of Fe³⁺ in the catholyte as Cr²⁺ in the anolyte. In reality, side reactions typically make the concentration of Fe³⁺ in the catholyte higher than that of Cr^2"1" in the anolyte. In this state, the system is said to be unbalanced and the energy storage capacity of the battery decreases. An unbalanced system must be appropriately rebalanced to regain the energy storage capacity. Insufficient rebalancing still leaves more Fe³⁺ in the catholyte than Cr²⁺ in the anolyte, leading to a condition that will be referred to herein as positive imbalance. Excessive rebalancing results in less Fe³⁺ than Cr²⁺ in the catholyte and anolyte respectively, leading to a condition that will be referred to herein as negative imbalance. In either case of positive or negative imbalance, the full capacity of the cell may not be realized.

[0038] In an ideal Fe/Cr redox flow battery, the overall electrochemical reaction during charging is:

Fe²⁺ + Cr³*→ Fe³⁺ + Cr²⁺ [3]

[0039] The Nernst equation gives the relationship between cell electric potential and electrolyte concentration.

Eceii = E° + (RT/nF)*ln([Fe³⁺] [Cr^MFe^Cr³*]) [4]

[0040] In some embodiments, if the cell does not suffer from H₂ evolution or other side reactions, then the concentrations of Fe³⁺ and Cr ⁺ may be equal, and the concentrations may be determined from the cell potential. However, with side reactions, the SOO of both catholyte and anolyte cannot be determined from cell potential measurement. To avoid issues related to the cell potential, separated half-cell redox potential measurements of the anolyte and catholyte may be made to determine the SOO of each electrolyte independently. Measuring the redox potential of the electrolyte may be carried out by using a reference electrode and an indicating electrode. In a measurement arrangement using both an indicating electrode and a reference electrode, the potential of the reference electrode is the same regardless of the concentration of various species in solution. But the potential of the indicating electrode varies linearly according to logarithmic relations Ln([Fe³⁺]/[Fe²⁺]) in the catholyte, and Ln([Cr²⁺]/[Cr³⁺]) in the anolyte. However, measurements obtained using reference electrodes in a redox flow battery are subject to several sources of error and may be subject to measurement uncertainty on the order of 10 mV or more. These and other limitations suggest that alternative methods of measuring the SOO of anolyte and/or catholyte of a flow battery may be needed.

Electrolyte Concentration Monitoring Apparatus

[0041] For various reasons it is desirable to determine the concentration of one or more reactants in an electrochemical flow system. For example, in order to control a rebalancing reaction so that it begins when desired and proceeds to the desired extent, it is important to know the concentrations of the charged form of the active species in the electrolytes (e.g., Fe³⁺ in the catholyte and Cr²* in the anolyte) at a particular time. As a result of side-reactions and other variations, such concentrations may be substantially unknown prior to beginning a measurement. It may be sufficient in some cases to know the difference between the concentration of charged reactants in catholyte, and that of charged reactants in the anolyte. In various embodiments, a difference of zero or substantially zero may be desired. In other embodiments, a positive or negative non-zero difference may be desired.

[0042] FIG. 2 schematically illustrates an optical SOO meter 200 for use in optically determining the SOO of an electrolyte. An optical SOO meter 200 may include one or more light sources 210, 212, 214 arranged to direct light through a section of a conduit 220, 222 that contains a sample of electrolyte to be monitored. The system 200 may also include a digital camera 230 arranged such that light from the light source (or sources) 210, 212, 214 passing through the electrolyte conduit (or conduits) 220, 222 may be received at the digital camera 230. The digital camera 230 may capture an image of the received light. The captured image may be analyzed by a computer 240 in order to determine an electrolyte concentration as described below.

[0043] In some cases, a collimator 250 may be provided to surround a region between the digital camera 230 and the conduit 220, 222 to isolate the light passing through the sample(s) from any ambient light. In some cases, an isolation plate 260 may be provided to further isolate one or more desired regions of the conduit or conduits 220, 222. The collimator 250 and isolation plate 260 may be made in any suitable shape and size as needed. For example, the collimator 250 may be a tube with a cylindrical, rectangular or other cross-sectional shape. Similarly, the isolation plate 260 may be a circular, rectangular or otherwise shaped plate with circular, rectangular or otherwise shaped holes for allowing light to pass through. The collimator 250 and the isolation plate 260 may generally be made of any suitable material that is generally optically opaque or otherwise configured to block external light at least within the range wavelengths of light used in obtaining a measurement as described below. In some cases, a lens 270 (or multiple lenses) may also be provided between the digital camera 230 and the sample conduit(s) 220, 222 in order to suitably focus the camera such that the image of the conduit(s) falls on the capturing mechanism, which may be a digital capturing mechanism such as a CCD or other similar image capturing mechanism.

[0044] In some embodiments, a light diffuser 280 may be positioned between the light source(s) 210, 212, 214 and the electrolyte conduit(s) 220, 222. The light diffuser may be made of any suitable material, such as frosted glass or translucent plastic with optical properties sufficient to diffuse light from the light source(s) in order to illuminate the sample window with substantially even light (i.e., minimizing bright spots and dark spots). In some embodiments, a diffuser material may be selected to filter out one or more wavelengths of light that may be undesirable for a particular application.

[0045] In some embodiments, the conduit sections 220, 222 adjacent the light source(s) 210, 212, 214 may have substantially flat or planar walls substantially perpendicular to a line between the light source(s) 210, 212, 214 and the digital camera 230. For example, the conduit sections 220, 222 may have rectangular or square cross sections. Alternatively, the conduit sections may have one or more curved side walls. In some cases, the conduit sections may have a cylindrical cross-section in examples in which the refraction of light through the curved walls is found to enhance or to not significantly interfere with the measurements. The optical SOO meter of FIG. 2 is shown with two electrolyte conduit sections 220, 222. In alternative embodiments, an optical SOO meter may be configured with only one electrolyte conduit, or with three or more electrolyte conduits. For example, a single electrolyte conduit may be used for evaluating a single electrolyte in isolation. In another example, a single electrolyte conduit may be used for evaluating a mixed-electrolyte solution (e.g., as described in further detail below) either alone or in combination with conduits for evaluating positive and negative electrolytes.

[0046] In some cases, the light sources 210, 212, 214 may include a calibration light source 212, and one or more electrolyte light sources 210, 214 adjacent the electrolyte conduit(s) 220, 222. In some cases, the calibration light source 212 may be identical to the electrolyte light source (s) 210, 214. If a system is to be configured for evaluating only a single liquid electrolyte, then only a single conduit 220 may be necessary. Similarly, in some configurations, a single light source may be arranged to transmit light through two or more electrolyte conduit sections. Use of the calibration light source is described below.

[0047] In some embodiments, the isolation plate 260 may include a positive electrolyte opening 262, a negative electrolyte opening 266, and a calibration opening 264. While the apparatus shown in FIG. 2 and FIG. 3 includes openings 262, 266 and conduits 220, 222 for both positive and negative electrolytes, the apparatus may also be configured for evaluating only a single liquid electrolyte, or three or more electrolytes.

[0048] In some embodiments, the light sources 210, 212, 214 may be "white" LEDs, incandescent lights, fluorescent lights or other white light sources. In some cases, a single white light source may illuminate all three openings in the isolation plate 260. Alternatively, multiple white light sources may be used. As used herein, a "white" light source may refer to a broad or full spectrum light source containing as close to a full spectrum of light wavelengths in the visible and near visible spectrum as possible.

[0049] As shown in FIG. 3, in some embodiments, the optical SOO meter 200 may be constructed with multiple light sources adjacent each electrolyte conduit 220, 222, the openings 262, 266 and the calibration opening 264. For example, if RGB color components are to be used, red 215a, 215b, 215c, green 216a, 216b, 216c and blue 217a, 217b, 217c light source (e.g., LEDs) may be provided for each electrolyte instead of or in addition to one or more white light sources. In such cases, the apparatus may include a first set of the red 215a, the green 216a, and the blue 217a light sources for a first electrolyte, a second set of the red 215b, the green 216b, and the blue 217b light sources for a second electrolyte, and a third calibration set of the red 215c, the green 216c, and the blue 217c light sources, for a total of nine light sources in the illustrated example. Separate red, green and blue light sources may provide better isolation of the absorption bands than "white" light sources. In some cases, one or more white light sources may be used in addition to red, green and blue light sources. Light sources of other colors may also be used instead of or in addition to red, green, blue or white light sources.

[0050] In some embodiments, the digital camera 230 may be a common "webcam" or other digital camera connected to a computer 240. Any computer with sufficient processing and data storage capabilities may be used. For example, the computer 240 may comprise a laptop computer, a tablet computer, a mobile phone, a computer based on a microprocessor breakout board such as a Raspberry Pi, a BeagleBone Black (http:/ beagleboard.org), a Minnowboard Max (made by Intel), or any other computer capable of interfacing with the digital camera and performing other steps as described in various examples herein, such as capturing images, evaluating images, performing mathematical calculations and comparing calculation results to a table of stored values.

Optical Electrolyte Concentration Determination

[0051] FIG. 4 A is a process flow diagram illustrating an embodiment process 400 for determining an electrolyte concentration using an optical monitoring system with features as described herein. As shown in connection with the embodiment process 400 for determining an electrolyte SOO, in block 410 a volume of liquid electrolyte may be directed into or through an electrolyte evaluation conduit. The volume of liquid may be based on pumping or other flow from a source of the liquid electrolyte in which the concentration is to be determined. The conduit may be transparent or at least partially transparent to allow light to pass from the light source or sources through the electrolyte sample and into the body of an SOO detector. In block 420, the evaluation conduit and the electrolyte sample within the conduit may be illuminated with a light source or with multiple light sources. In block 430, at least one image of light transmitted through the conduit and the electrolyte sample may be captured, such as using a digital camera. In block 440, color component values from the image may be obtained, such as by analyzing the digital image captured in block 430 to extract the color component values. In block 450, the color component values may be optionally mathematically transformed, such as by adding, subtracting, multiplying or dividing two or more color component values with one another or with other factors to enhance a measurement signal, to obtain a transformed measurement value. In block 460, the measurement value may be compared with calibration values, or with other reference values, to determine a state-of-oxidation of the evaluated electrolyte. In block 470, the operations in block 410, 420, 430, 440, 450, and 460 may be repeated for one or more additional liquid electrolytes. In embodiments, the operations may be performed in parallel for electrolytes, such as a positive electrolyte and a negative electrolyte. In block 480, when operations in block 410, 420, 430, 440, 450, and 460 have been performed for at least both a positive electrolyte and a negative electrolyte a degree of imbalance between the SOOs of positive and negative electrolytes may be determined.

[0052] In some embodiments, electrolyte may be continuously flowed through the conduit(s) while images are captured in block 430. In other embodiments, electrolyte may sit stagnant in the conduit(s) while one or more images are captured in block 430. In some examples, a single still image may be captured in block 430 from which all needed color component information may be obtained. In such cases, the captured image may contain multiple regions that may be evaluated separately and in combination to obtain the desired color component information.

[0053] In a further embodiment method 401 , as illustrated in FIG. 4B, a degree of imbalance may be determined based on mixed electrolytes, either as an alternative to the embodiment method 400 or as a way of determining an error value, or for other purposes. In block 411, equal quantities of a first liquid electrolyte and a second liquid electrolyte may be mixed. In block 413, the mixed electrolytes may be directed into an evaluation conduit. The conduit may be transparent or partially transparent to allow light from a light source or sources to pass through the mixed electrolyte sample in the conduit. In block 421, the evaluation conduit and the mixed electrolyte sample within the conduit may be illuminated with a light source or with multiple light sources. In block 431, at least one image of light transmitted through the conduit and the mixed electrolyte sample may be captured, such as using a digital camera. In block 441, color component values from the image may be obtained, such as by analyzing the digital image captured in block 431 to extract the color component values. In block 451 , the color component values may be optionally mathematically transformed, such as by adding, subtracting, multiplying or dividing two or more color component values with one another or with other factors to enhance a measurement signal, to obtain a transformed measurement value. In block 461, the measurement value may be compared with calibration values, or with other reference values, such as known color component values corresponding to known imbalances of the mixed electrolyte solution. In block 471, a degree of state-of-oxidation imbalances of the evaluated mixed electrolyte may be determined. In block 481, the determined imbalance from block 471 may be optionally compared with the imbalance determined in block 480 of the embodiment method 400. Such comparison may be used to confirm the degree of imbalance determined in block 480, to confirm a degree of error between the imbalances, and so on. In embodiments, the embodiment method 401 may be used as an alternative manner of determining the degree of imbalance between the positive and negative electrolytes.

[0054] FIG. 5 illustrates one possible image layout comprising multiple sections that may be evaluated. The image 500 of FIG. 5 is based on an apparatus such as the optical SOO meter 200shown in FIG. 2 that uses one light source 210, 214 adjacent each electrolyte conduit220, 222 and a single light source 212 adjacent to the calibration opening 264 in the isolation plate 260. [0055] A captured still digital image 500 such as that shown in FIG. 5, may be divided into multiple measurement regions and at least one calibration region. For example, the image of FIG. 5 may include a first electrolyte measurement region 510 corresponding to the first electrolyte window 262 in the isolation plate 260 through which light from the illuminated electrolyte conduit 220 may pass. The image 500 may also include a second electrolyte measurement region 520 corresponding to the second electrolyte window 266 in the isolation plate 260 through which light from the illuminated electrolyte conduit 222 may pass. The image 500 may also include a calibration region 530 corresponding to the calibration opening 264 in the isolation plate 266. The image regions correspond to light that has passed through the electrolyte in the regions and may contain information as to the concentrations of the various species within the electrolyte.

[0056] As described above with reference to FIG. 3, instead of or in addition to a single white light source for illuminating the electrolyte conduit(s) and calibration section, some embodiments of an optical SOO evaluation device may comprise multiple light sources configured to produce light of one or more particular colors (e.g., RGB, CMYK, etc.). In some cases, the colors of light sources may be selected to correspond to color components to be detected in captured images as described below.

[0057] An image captured using an optical SOO evaluation device with multiple colored light sources may have multiple regions corresponding to light from each colored light source that has passed through either air in the case of the calibration region, or respective electrolytes for each of the positive electrolyte measurement region, the negative electrolyte measurement region. For example, as shown in FIG. 6 an image 600 may include a section for a first color, such as a red light section 610a corresponding to light that has passed through electrolyte in a corresponding conduit and the first electrolyte window 262, a red light section 610b corresponding to light that has passed through electrolyte in a corresponding conduit and the second electrolyte window 266, and a red light section

610c corresponding to light that has passed through the calibration window 264. The image 600 may further include a section for a second color, such as a green light section 620a corresponding to light that has passed through electrolyte in a corresponding conduit and the first electrolyte window 262, a green light section 620b corresponding to light that has passed through electrolyte in a corresponding conduit and the second electrolyte window 266, and a green light section 620c corresponding to light that has passed through the calibration window 266. The image 600 may further include a section for a third color, such as a blue light section 630a corresponding to light that has passed through electrolyte in a corresponding conduit and the first electrolyte window 262, a blue light section 630b corresponding to light that has passed through electrolyte in a corresponding conduit and the second electrolyte window 266, and a blue light section 630c corresponding to light that has passed through the calibration window 264.

10058] In some cases, the various image sections to be evaluated may be identified in advance as pixel regions for a particular apparatus. The locations of the pixel regions corresponding to each section may be mapped for the particular apparatus and stored for use in evaluating each of the sections as described below.

[0059] Any particular region of the image may be evaluated by selecting one or more pixels within the chosen image region and obtaining numeric values representing color components of the selected image pixels. In some cases, two or more pixels within a single region may be selected, and color component values may be obtained for each pixel. Color component values from the multiple pixels within a region may then be aggregated by any suitable method such as calculating a simple average, a sum, a weighted average, selecting a maximum value, selecting a minimum value, or generating a histogram of color component values or otherwise.

[0060] In some cases, color component values may be red, green, and blue (RGB) components of the selected image pixel(s). Each of the RGB color components may be represented as a value

corresponding to the resolution, such as a value from zero to 255 for 2 6 color resolution. In other cases, color components may be cyan, magenta, yellow and black (CMYK), or components of another color model such as HSL (hue, saturation, lightness), HSV (hue, saturation, value), HSB (hue, saturation, brightness), HSI (hue, saturation, intensity), or others. In some cases, one or more light sources may be selected to correspond to one or more of the color component values to be obtained. Alternatively, color component values need not necessarily correspond with light source colors. For example, CMYK color component values may be used in combination with red, green, blue and/or white light sources. Depending on characteristics of the electrolyte to be evaluated, light sources of any color may be used. For example light sources may include, red, pink, orange, green, blue, purple, violet, white, infrared or ultraviolet LEDs or other light sources.

[0061] In some embodiments, in addition to capturing a single still image, multiple images, or video frames, may be captured at slightly different times. Color component values from one or more pixels from multiple images captured at different times may be aggregated by any suitable method such as calculating a simple average, a sum, a weighted average, a maximum, a minimum, a histogram, or otherwise. In some cases, multiple images may be obtained at different times while electrolytes flow through the conduit sections.

[0062] In some embodiments, the calibration image region(s) may be used during initial system setup to adjust camera variables such as exposure, gain, brightness, contrast, color intensity, white balance, etc. to establish baseline color component values for the light in the calibration image region(s). For example, for a pure white color in an image region, the RGB color components should have red, green and blue values of 255 each. In some cases, one or more camera settings may be adjusted until the calibration image portion has the expected pure white color component values. In other cases, the calibration portion may simply be used to determine offsets for one or more color component values in the positive or negative image portion. For example, if the calibration image portion has average RGB values of red = 250, green = 255, blue = 255, then measured RGB values of the positive and negative electrolyte portions may be adjusted by adding 5 to the red values measured for the positive and negative electrolyte image sections. Similarly, baseline values and/or adjustments may be applied to calibration sections using colored LEDs, such as separate red, green and blue LEDs.

[0063] During operation, when color component values have been obtained for a particular region under evaluation (e.g., the positive electrolyte image region or regions), the color component values obtained for those regions may be transformed. In some embodiments, color component values may be transformed by performing mathematical operations on the obtained color component values in order to amplify a signal of a desired measurement. For example, if green and blue color values are known to be positively correlated with SOO of a negative flow battery electrolyte, while a red color value is known to show a negligible change over a substantial range of SOO, then adding the green and blue color values may amplify the correlation power of the measurement relative to either color value alone. Alternatively, color component values may be divided or multiplied with one another or with other values. For example, one or more color component values may be weighted by multiplying the color component value by a weighting factor (e.g., an integer or real number constant greater than zero). Such weighted color component values may then be added, subtracted, averaged, or otherwise combined.

[0064] FIG. 7 illustrates an example graph 700 showing red, green and blue color component values at various SOO values of a negative mixed reactant Fe/Cr electrolyte with a composition of 1.4M total Cr and 1.3M total Fe in 1.0N HC1. The values of FIG. 7 are associated with white LED light sources for the electrolyte and for a calibration light as described herein. In the example shown in FIG. 7, the red color component values 710 are nearly zero for all values of SOO, while both the green color component values 720 and the blue color component values 730 increase with increasing SOO. Thus, in the illustrated example, increasing SOO for a negative electrolyte may be positively correlated with the green 720 and blue 730 values, and negligibly correlated with the red value 710. In embodiments therefore, a useful metric for determining SOO for a negative electrolyte may be a sum of blue and green values (G+B) 740. It should be noted that in the graph of FIG. 7, the values in the (G+B) 740 dataset are shifted down by a value of 220 (i.e., G+B-220) in order to keep the entire dataset within the 255 scale of the RGB color system. This subtraction/shift is typically not necessary for measurement purposes, and is done here for simplicity of illustration. Therefore, an approximate negative electrolyte SOO may be determined by calculating a sum of blue and green pixel color components and comparing the resulting B+G value 740 with B+G values for electrolytes of known SOO.

[0065] In some cases, B+G values (or other measurement values) may be obtained for known electrolyte SOO values, and such measurement values may be stored in a lookup table that may be accessed by a computer or controller as described in further detail below.

[0066] FIG. 8 is a graph 800 of color component values and SOO illustrating another example. The example graph of FIG. 8 shows red, green and blue color component values plotted at various SOO levels for a positive mixed reactant Fe/Cr electrolyte with a composition of 1.4M total Cr and 1.3M total Fe in 1.0N HCL. The values of FIG. 8 are associated with white LED light sources for the electrolyte and for a calibration light as described herein. In the example shown in FIG. 8, the blue color component values 830 are nearly zero for all values of SOO, the green color component values 820 decrease with increasing SOO, and the red color component values 810 increase with increasing SOO. Thus, in the illustrated example, increasing SOO of a positive electrolyte may be positively correlated with the red value, negatively correlated with the green value, and negligibly correlated with the blue value. Thus, in some cases a useful transformation may be to take a difference between green and red values (G-R). Therefore, an approximate positive electrolyte SOO may be determined by calculating a difference between green and red pixel color components and comparing the resulting G- R value with G-R values for electrolytes of known SOO.

[0067] In some cases, G-R values (or other measurement values) may be obtained for known electrolyte SOO values, and such measurement values may be stored in a lookup table that may be accessed by a computer or controller. After obtaining a measurement value for an electrolyte sample of unknown SOO (e.g., G-R or B+G in the examples above), the controller/computer may look up the measurement value in the lookup table to obtain a corresponding SOO value. If the measurement value lies in between values in the lookup table, the computer/controller may be configured to select an SOO value based on a nearest measurement value, or based on a linear or non-linear interpolation between adjacent values. Alternatively, a mathematical curve may be fit to the empirical measurement value vs. SOO curve, and a mathematical equation describing the curve may be used to calculate an SOO based on a measurement value. Similar measurement value calculations may be developed for other combinations of color component values or for individual color component values based on identified correlations.

[0068] In some cases, if light sources of various colors are used, it may be desirable to only seek a color component corresponding to a light source associated with a particular image segment. For example, if an image has a segment associated with a red light source, then it may be desirable to seek only red color component values for that segment. Similarly, green color component values may be sought for image segments corresponding to green light sources, and blue color component values may be sought for image segments corresponding to blue light sources.

[0069] In alternative embodiments, multiple color component values may be obtained for an image segment illuminated by only a single-color light source. For example, red, green and blue color components might be obtained for an image segment illuminated by a red light source. As another example, red, green and blue color components may be obtained for an image segment illuminated by a purple light source.

[0070] Similar techniques may be applied to any electrolyte composition based on measurable correlations between color component values and SOO for a particular electrolyte composition.

Optical Electrolyte Imbalance Determination

[0071] In some cases, substantially equal volumes of positive and negative mixed-reactant electrolytes may be mixed and directed into the electrolyte conduit. Mixing equal volumes of the positive and negative electrolytes will tend to cause the "charged" reactant species to react with one another, effectively self-discharging one another. If, prior to mixing, the electrolytes contain equal quantities of charged reactants (i.e., if the electrolytes are balanced), then after mixing equal volumes, the mixed solution should have an SOO of zero. However, if either electrolyte has a greater quantity of charged reactant than the other before mixing, then the mixed solution will have an SOO that represents the imbalance between the electrolytes. Thus, after mixing equal quantities of the electrolytes, the SOO of the mixed solution may be determined using the systems and methods described herein. The resulting SOO may be interpreted and stored as the imbalance between the electrolytes.

[0072] In some cases, a positive imbalance may be distinguished from a negative imbalance based on the color component values. In such embodiments, it may be desirable to obtain calibration data describing color component values at a range of both negative and positive imbalances.

[0073] In some embodiments, an optical SOO evaluation system may include imaging regions for a positive electrolyte, a negative electrolyte and a mixed electrolyte solution. In use, such a system may determine SOO values for each of the positive, negative and mixed electrolyte solutions by obtaining and evaluating color component values as described above. In some cases, a calculated imbalance value obtained by subtracting the measured negative SOO value from the measured positive SOO value may be compared with a measured imbalance value obtained by evaluating color component values of the mixed electrolyte solution. Comparing the calculated and measured imbalance values may comprise calculating a difference, an average, a range or other values.

Electronic Control Components

[0074] FIG. 9 is a schematic block diagram illustrating an embodiment electronic controller 1000 for controlling light sources and/or image evaluation for an optical SOO meter 200 such as those described herein. In embodiments, an exemplary hardware implementation may include a controller, processor, ASIC or similar component, or may be a computer or computing system. In the illustrated examples, the electronic controller 1000 may be implemented with a bus architecture, represented generally by a bus 1002. The bus 1002 may include any number of interconnecting buses and bridges depending on the specific application of the electronic controller 1000 and the overall system constraints. The bus 1002 may be one or more digital data lines, analog lines, control lines, or combinations of the aforementioned lines. The bus 1002 links together various circuits including one or more processors, represented generally by a processor 1004. In addition to the bus 1002, various analog components may be coupled to the processor 1004 and other system components through an analog bus 1003. The analog bus 1003 may include analog data and control lines including lines from sensors and other measurement components. In some embodiments, the processor 1004 and possibly other system components may be coupled through the bus 1002 to a computer-readable media, represented generally by a computer-readable medium 1006. The computer readable medium 1006 may be a non-transitory computer readable medium such as a storage device or the like, that may contain instructions 1014, which when read and executed by the processor 1004 may cause the processor to perform operations as described herein, such as operations in connection with performing an embodiment method.

[0075] The bus 1002 (and the analog bus 1003) may also link various other circuits such as timing sources, peripherals, sensors, and power management circuits, which are well known in the art, and therefore, will not be described any further. A bus interface 1008 provides an interface between the bus 1002 and the optical SOO meter 200. Depending upon the nature of the apparatus, a user interface 1012 (e.g., keypad, display, speaker, microphone, joystick, etc.) may also be provided.

[0076] The processor 1004 may responsible for managing the bus 1002 and general processing, including the execution of software or instructions 1014 stored on the computer-readable medium

1006. The software, when executed by the processor 1004, causes the electronic controller 1000 to perform the various functions described above, such as capturing an image via the digital camera, storing the image, obtaining color component values from the image, calculating measurement values from the color component values, and determining an SOO value and/or an electrolyte imbalance value. The computer-readable medium 1006 may also be used for storing data that is manipulated by the processor 1004 when executing software or instructions 1014. For example, the computer readable medium 1006 may be used to store a lookup table relating SOO to one or more measurement values.

[0077] In some embodiments, analog electronics 1016 may also be joined to the bus 1002 by an analog-to-digital converter (and in some embodiments a digital-to-analog converter) 1018. Analog electronics 1016 may be provided to perform various analog functions such as voltage regulation, electric current measurement, current regulation, analog control, or other functions.

[0078] In embodiments described herein, examples of systems, devices, and methods for quantifying and monitoring electrolyte imbalances have been described with reference to an exemplary flow battery system using Fe/Cr flow battery chemistry and the attendant equations and relations. However, it should be understood that the same principles and concepts may be applied to any flow battery or hybrid flow battery chemistry without departing from the spirit of the invention.

[0079] Further, embodiments of redox flow battery cells, stack assemblies and systems described herein may be used with any electrochemical reactant combinations that include reactants dissolved in an electrolyte, such as assemblies and systems wherein determining the concentration of reactants is desired. One example may include a stack assembly containing the vanadium reactants V(II)/V(IH) or V²⁺ /V³⁺ at the negative electrode (anolyte) and V(IV)/V(V) or V⁴⁺ /V⁵⁺ at the positive electrode (catholyte). The anolyte and catholyte reactants in such a system are often dissolved in sulfuric acid or a mixture of sulfuric acid and hydrochloric acid. This type of battery is often called the all- vanadium battery because both the anolyte and catholyte contain vanadium species. Other combinations of reactants in a flow battery that may utilize the features and advantages of the systems described herein include Sn (anolyte)/Fe (catholyte), Mn (ano!yte)/Fe (catholyte), V (anolyte)/Fe (catholyte), V

(anolyte)/Ce (catholyte), V (anolyte)/Br₂ (catholyte), Fe (anolyte)/Br₂ (catholyte), and S (anolyte)/Br₂ (catholyte). In each of these example chemistries, the reactants are present as dissolved ionic species in the electrolytes, which permits the advantageous use of configured cascade flow battery cell and stack assembly designs in which cells may have different physical, chemical or electrochemical

configurations and properties along the cascade flow path (e.g. cell size, type of membrane or separator, type and amount of catalyst, etc.). A further example of a workable redox flow battery chemistry and system is provided in U.S. Patent No. 6,475,661, the entire contents of which are incorporated herein by reference. Many of the embodiments herein may be applied to so-called "hybrid" flow batteries (such as a zinc/bromine battery system) which use only a single flowing electrolyte.

[0080] The foregoing description of the various embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein, and instead the claims should be accorded the widest scope consistent with the principles and novel features disclosed herein.

[0081] In particular, materials and manufacturing techniques may be employed as within the level of those with skill in the relevant art. Furthermore, reference to a singular item, includes the possibility that there are plural of the same items present. More specifically, as used herein and in the appended claims, the singular forms "a," "and," "said," and "the" include plural referents unless the context clearly dictates otherwise. As used herein, unless explicitly stated otherwise, the term "or" is inclusive of all presented alternatives, and means essentially the same as the commonly used phrase "and/or." Thus, for example the phrase "A or B may be blue" may mean any of the following: A alone is blue, B alone is blue, both A and B are blue, and A, B and C are blue. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as "solely," "only" and the like in connection with the recitation of claim elements, or use of a "negative" limitation. Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

Claims

What is claimed is:

1. A method of determining a state-of-oxidation of a flowing electrolyte, comprising:

directing a quantity of a first liquid electrolyte through a first conduit, wherein the first conduit is at least partially transparent;

transmitting light from a first light source through the first conduit and the first liquid electrolyte in a direction transverse to a liquid flow direction;

capturing a first digital image of the light transmitted through the first conduit and the first liquid electrolyte;

obtaining first color component values from the first digital image;

comparing the first color component values obtained from the first digital image with color component values for samples of known states-of-oxidation of the first liquid electrolyte; and

determining a first state-of-oxidation value of the first liquid electrolyte based on the comparison.

2. The method of claim 1, wherein the first conduit has a rectangular cross-section.

3. The method of claim 1, further comprising transmitting light from a second light source through a calibration window, capturing a second digital image of the light transmitted through the calibration window, obtaining calibration color component values from the second digital image and adjusting the first color component values prior to determining a first state-of-oxidation value.

4. The method of claim 1 , wherein the first light source produces white light.

5. The method of claim 1, wherein the first light source produces light that is substantially from one member of the group consisting of: red light, green light, and blue light.

6. The method of claim 1 , further comprising simultaneously transmitting light from a plurality of light sources through the first conduit and the first liquid electrolyte, and obtaining one or more digital images of the light transmitted by each of the light sources through the first conduit and the first liquid electrolyte.

7. The method of claim 6, wherein digital images of the light transmitted by each of the light sources through the first conduit and the first liquid electrolyte comprise pre-determined regions of a single digital image.

8. The method of claim 6, wherein the plurality of light sources correspond to color components of the first color component values.

9. The method of claim 1 , further comprising:

directing a quantity of a second liquid electrolyte through a second conduit, wherein the second conduit is at least partially transparent;

transmitting light from a second light source through the second conduit and the second liquid electrolyte in a direction transverse to a liquid flow direction;

capturing a second digital image of the light transmitted through the second conduit and the second liquid electrolyte;

obtaining second color component values from the second digital image;

comparing the second color component values with color component values for samples of known states-of-oxidation;

determining a second state-of-oxidation value of the second liquid electrolyte based on the comparison; and

obtaining an electrolyte imbalance value between the first liquid electrolyte and the second liquid electrolyte by subtracting the second state-of-oxidation value from the first state-of-oxidation value.

10. The method of claim 9, further comprising:

mixing a quantity of the first liquid electrolyte with an equal quantity of the second liquid electrolyte to produce a mixed solution, and directing the mixed solution through a third conduit, wherein the third conduit is at least partially transparent;

transmitting light from a third light source through the third conduit and the mixed solution in a direction transverse to a liquid flow direction;

capturing a third digital image of the light transmitted through the third conduit; obtaining third color component values from the third digital image;

comparing the third color component values with color component values for samples of known states-of-oxidation representing positively and negatively unbalanced electrolytes; and

determining a second imbalance value of the mixed solution based on the comparison; and comparing the second imbalance value with the first imbalance value to determine an error value between the first imbalance value and the second imbalance value.

11. The method of claim 1 , further comprising calculating a sum or a difference of at least two color component values prior to determining a state-of-oxidation value or determining an imbalance value.

12. A method of determining a charge imbalance between a positive electrolyte and a negative electrolyte:

mixing a quantity of the positive liquid electrolyte with an equal quantity of the negative liquid electrolyte to produce a mixed solution, and directing the mixed solution through a conduit;

transmitting light from a light source through the conduit in a direction transverse to a liquid flow direction;

capturing a digital image of the light transmitted through the conduit;

obtaining color component values from the digital image;

determining an imbalance value of the mixed solution by comparing the color component values with color component values for samples of known negative and positive imbalance.

13. The method of claim 12, wherein the first conduit has a rectangular cross-section.

14. The method of claim 12, further comprising transmitting light from a second light source through a calibration window, capturing a second digital image of the light transmitted through the calibration window, obtaining calibration color component values from the second digital image and adjusting the first color component values prior to determining a first state-of-oxidation value.

15. The method of claim 12, wherein the first light source produces white light.

16. The method of claim 12, wherein the first light source produces light that is substantially from one member of the group consisting of: red light, green light, and blue light.

17. The method of claim 12, further comprising simultaneously transmitting light from a plurality of light sources through the first conduit and the first liquid electrolyte, and obtaining one or more digital images of the light transmitted by each of the light sources through the first conduit and the first liquid electrolyte. 8. The method of claim 17, wherein digital images of the light transmitted by each of the light sources through the first conduit and the first liquid electrolyte comprise pre-determined regions of a single digital image.

19. The method of claim 17, wherein the plurality of light sources correspond to color components of the first color component values.

20. The method of claim 12, further comprising:

directing a quantity of a second liquid electrolyte through a second conduit, wherein the second conduit is at least partially transparent;

transmitting light from a second light source through the second conduit and the second liquid electrolyte in a direction transverse to a liquid flow direction;

capturing a second digital image of the light transmitted through the second conduit and the second liquid electrolyte;

obtaining second color component values from the second digital image;

comparing the second color component values with color component values for samples of known states-of-oxidation;

determining a second state-of-oxidation value of the second liquid electrolyte based on the comparison; and

obtaining an electrolyte imbalance value between the first liquid electrolyte and the second liquid electrolyte by subtracting the second state-of-oxidation value from the first state-of-oxidation value.