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RECENT ADVANCES IN REDOX FLOW CELL STORAGE SYSTEMS L. H. Thaller NASA-Lewis Research Center Cleveland, Ohio 44135 ABSTRACT Recent dramatic advances made in membrane and electrocatalyst technologies as they relate to the iron chromium Redox system have brought this energy storage concept to the point where complete systems have been designed, built and tested. Several novel features have been conceived and incorporated into complete Redox systems that greatly enhance its abilitytobekeptinproperchargebalance,tobe capable of internal voltage regulation, and in general be treated as a true multi-cell electrochemical system rather than an assembly of single cells that are wired together. The paper will cover the recent data pertaining to the system-related features as well as the state ofthetechnologyoftheRedoxmembranes. Thetech- nology status as it relates to the two application areas of storage for solar photovoltaic/wind and distributed energy storage for electricutility applications will also be addressed. The cost and life advantages of Redox systems will also be covered. FIVE YEARS AGO the concept of an electrochemical storage system based on two fully soluble redox couplesseparatedbyanionexchangemembranewas presented at the 9th IECEC meeting (1*). In con- ceptual form, the use of tanks to store redox fluids and pumps to circulate the stored reactants through the stack of individual Redox flow cells wasdes- cribed. Thepotentialadvantagesofthisconcept, in which the storage portion of the overall system can be sized independently of the power-related portion of the system and in which no solid electrode reactants are present, were outlined. At the present time, ion exchange membranes meeting the extremely stringent selectivity and area resistivity requirements are well on their way to being fully developed. Redox couples (Fe'f2/Fe+3, Cr+2/Cr+3) when used with suitably catalyzed elec- trode structures have been developed that possess such very high exchange currents that complete Redox. cells,operate reversibly out to current densities in the hundreds of amperes per square foot. The hard- ware used for testing membranes and electrodes has grown from single cells with an active area of 0.0156 ft2 to multi-cell stacks where the active area of each cell is 0.33 ft . Currently the ferrous/ferric redox couple and the chromous/chromic redox couple are being used as the positive and negative reactants, respectively. These solutions are one molar chlorides of the respective cations in two normal hydrochloric acid supporting electrolyte. The membranes and electro- catalysts were developed under contract by Ionics, Incorporated of Katertown, MA, and Giner, Incor- porated of Kaltham, MA, respectively. The list of advantages for Redox systems has grown as the Redox technology has matured from the conceptual stage to the present status of actual working systems with predictable performance, high reliability, stable operation and long life. Appli- cations for this technology in the near-term include 'Numbers in parentheses designate References at end of paper. energy storage for stand-alone village power systems that use solar photovoltaic or wind energy as their primary energy source. As the technology advances further, distributed energy storage installations in electric utility service will become possible. The current cost projections for this technology appear very attractive compared with lead acid batteries. Moreover, its most attractive features are related to the ease with which Redox technology may be handled at the total system level. The near-term application for NASA-Redox energy storage systems will be small village power systems where solar arrays or wind turbines would provide the power but where storage is required for night-time or periods of low sun or wind levels. This application (2) is characterized by having small power require- ments (5 to 25 kW) but where rather long durations of dependence on the storage devices is required (30 to 60hours). Thistypeofapplicationisidealfor Redox systems since long storage times require only expanded reactant fluid and storage tank volumes. The basic fluid schematic for a full-function Redox system is shown in Figure 1. It should be noted that an open circuit cell and a rebalance cell have been added to the basic Redox system described inearlierpresentations. Thefunctionoftheopen circuit voltage cell is to provide a continuous direct reading of the state-of-charge or the depth- of-discharge of the entire Redox system. As its name implies, this cell is never placed under load but simply reads the voltage of the redox solutions that are flowing through it. The cell itself may be located hydraulically in parallel with the flow to the other cells in the stack if the state of charge of the incoming solutions is required or in series downstream of the stack if the state of charge of the exitingsolutionsisbeingmonitored. Thevoltage of the open circuit voltage cell is approximately This voltage may be directly related to the state of . charge of the system as a whole. The function of the rebalance cell is to allow the state of charge of the negative reactants to be maintained at exactly the same state of charge as the positive reactants. This cell can correct for the cumulative effects of; (1) coevolution of hydrogen, hydrogen at the chromium electrodes during the charge portion of the cycle, (2) any chemical reduction of hydrogen ion by chromous ions, (3) any air intrusion into the system reactant solutions causing air oxidation of either some of the ferrous orchromousions. Itshouldbenotedthatwhether the small amounts of hydrogen from within'the cell are being used for rebalancing or hydrogen from an external supply is being consumed in a rebalancepro- cess, it is being performed at the system level with- out any need to take the actual Redox stack out of service. The Redox stack is sized to produce the desired system output. Typically, these cells are connected in a bipolar manner with inlet and exit manifolds used to supply reactant flow to each of the cells whichareconnectedinparallelhydraulically. The use of fully soluble reactants permits the added featureoftrimcellstobeemployed. Forinstance, a fourteen cell stack may have the last three cells equipped with current leads so that either twelve, thirteen, or the full fourteen cells may be con- nected in series. This feature permits a type of internal voltage control while at the same time does not leave different parts of the system at different states of charge.PDF Image | RECENT ADVANCES IN REDOX FLOW CELL STORAGE SYSTEMS
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