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3. FLOW CELLS In any type of electrochemical system based on flowing the reactants through a porous electrode structure,thereareanumberoftradeoffs. These are associated with shunt currents, pumping power, cell performance and reactant inventory. At very low flow rates, the condition may be reached where the reactants in the solutions are fully depleted just as they leave the cell. This is referred to as the stoichiometric flow rate. At these flow rates, a very large change in the Nernstian correction(log +3 +2 +2 +5 (Cr )/(Cr ) x CFe )/(Fe ) ) takes place withits associated reduction in cell voltage. Even at flow rates above stoichioraetric, poor performance may result if diffusion limiting currents are approached. On the other hand, high flow rates are associated with some compromise between high pumping power require- ments and high shunt current losses. Some of the information required to assess these various ramifi- cations can be obtained from a performance map of a singleflowcell. Figure5showsthecellperfor- mance map of a single flow cell. Figure 5 shows the cell performance of one cell of a five cell stack operated at steady state conditions at various current densities and flow rates. At high flow rates, the separation of the lines is a function of the internal resistance of the cell. At the lower flow rates, Nernstiancorrectionsalsomustbeconsidered. The stoichiometric flow rate at each current density for the condition of the test (50% DOD) is noted on the plot. The cell used in these studies was the standard 2 0.33 ft si:e. In this cell configuration (Fig. 2) narrow inlet and outlet slits connect the main mani- folds to the secondary manifolds at the base of the electrodes. Thesecondarymanifoldallowsthein- coming liquid to spread out along the width of the electrode prior to its flow through the carbon felt electrode placed in the cavity between the membranes and the bipolar plate. This sheet flow proceeds from the bottom of the cell up to the top where another secondary manifold connects through the exit port to the exit manifold. Considering pumping requirements, Figure 6 gives the relationship between pressure dropandflowrate. Superimposedontheplateare' thedimensionsandplanformofthecellused. At Table 2 - Summary of Data-Stack No. 7 several points the actual ideal pump powers are 2 noted. Thiscellwhenoperatedat50amps/ft delivers about 15 watts. The pumping requirements represent a minor efficiency penalty. 4. SHALL STACKS Advances in membranes, electrodes and flow field design all come together at the small stack level of testing. Thisworkwasstartedinthespringof1978, with what were called iron/iron cells. These five- 2 cellstacksof0.33ft cellswereoperatedwith ferrous/ferric solutions at both electrodes. Since this redox couple is fully reversible without the need for any catalyst, these cells were used to ex- plore mass transport effects caused by variations inflowrateandflowfieldconfiguration. Itwas from this type of testing that the selection of sheet flow was made. As techniques were developed to produce large scale gold-lead catalyzed carbon felt electrodes, five-cell iron/chromium Redox stacks weretested. Table2listssomeexperimentaldata gathered from development stack No. 7. Number of Cells Flow Type Fe-Electrode Cr-Electrode Cell Resistivity MembraneResistivity V(discharge)/V(charge) .86,.82,.70 at 10,20,30 asf at 50% DOD Stack Power Charge/Discharge Cycles Arap-hr efficiency (Goal-95%)* Watt-hr efficiency (Goal-75%)* *At 20 amps/ft E= 2 5 (.33 ft active area) Sheet .125" Carbon Felt .125" Carbon Felt, Au-Pb catalyzed . 2 .0050fl-Ft 2 .0026n-Ft (in-lNHCl) 50 watt/ft at 75 amps/ft + 70 96-99% 70-80% The polarization curves on charge and discharge for this stack are plotted in Figure 7. The perfor- mance necessary in order for this type of hardware to meet the anticipated requirements for solar appli- cationsis0.9volts/cellat50amps/ft at50%depth of discharge. To meet this goal, a further reduction in the overall cell resistance is still required. The.major part of the internal resistance still resides in the membrane. To the basic hardware used in stack No. 7, several features were added that resulted in what is referred to as a full function Redox stack (Fig.2). OPEN CIRCUIT VOLTAGE CELL - The open circuit voltage of a redox half cell is given by the simple relationship: _o,RT,. IReactant nF |_Product J For the situation in which the iron and chromium redox couples are involved in a complete cell reaction in chloride solutions (where complexes are formed) the equation 32 (Cr*) (Fe+)1 E = 1.075 - .059 log (Cr+2) (Fe+3)J is approximately correct. The value of 1.075 is experimentally obtained from measurement of the open circuitvoltageat50%stateofcharge. OnceE°is known, the state of charge of the system is related in a simple manner to the open circuit voltage. REBALANCE CELL - In common with other battery systems, repeated cycling of Redox systems will re- sult in the reactants becoming out of balance in terms of capacity. Because the solutions are common to all cells, the imbalance in Redox systems occurs at the system level rather than at the individual single cell level as is the case with traditional batterysystems. Ifchromousionswereallowedto be oxidized with air, or if, during the charge cycle, hydrogen is evolved instead of chromic ions being reduced, the reactant solutions would become out of balance. Ifthisout-of-balanceconditionwereto be uncorrected, a permanent loss of usable system capacity would result. The rebalance cell (Fig. 1), a unique feature in Redox technology, consumes ferric ions from the circulating iron solution and hydrogen gas either from the ullage over the chromium solution or from an external hydrogen source. The reactions taking place within the rebalance cell have the net effect of returning the Redox solutions to electro- chemical balance.PDF Image | RECENT ADVANCES IN REDOX FLOW CELL STORAGE SYSTEMS
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