Load levelling cells

Redox flow batteries, under development since the early 1970s, are stUl of interest primarily for utility load leveling applications (77). Such a battery is shown schematically in Figure 5. Unlike other batteries, the active materials are not contained within the battery itself but are stored in separate tanks. The reactants each flow into a half-ceU separated one from the other by a selective membrane. An oxidation and reduction electrochemical reaction occurs in each half-ceU to generate current. Examples of this technology include the iron—chromium, Fe—Cr, battery (79) and the vanadium redox cell (80). 

The conductivity of gelled electrolytes is determined primarily by the liquid and salt components. High liquid content, of the order of 40 percent, is required to attain conductivities comparable with those of the corresponding liquid electrolyte. At these liquid loading levels there is often insufficient mechanical strength, and although this effect may not be noticeable on 1-2 cm2 laboratory cells, it is certainly evident on scale-up [111]. Polymer blends such as PEO-MEEP are much more mechanically stable than MEEP itself and more conductive than PEO but there is little overall improvement of the room tern-... 

MgO, Y2O3, and AIN powder-based separators are substantially less expensive. Cell assembly with such powder separators is simple and amenable to mass production. Although powder separators were shown to maintain their integrity and conform to small dimensional changes of electrodes, their porosity ( 50%) is considerably lower than that of BN felt and this drawback can limit their application to stationary load-leveling systems operating at low current densities. ... 

By far the largest sector of the battery industry worldwide is based on the lead-acid aqueous cell whose dominance is due to a combination of low cost, versatility and the excellent reversibility of the electrochemical system, Lead-acid cells have extensive use both as portable power sources for vehicle service and traction, and in stationary applications ranging from small emergency supplies to load levelling systems. In terms of sales, the lead-acid battery occupies over 50% of the entire primary and secondary market, with an estimated value of 100 billion per annum before retail mark-up. 

Fig. 8.17 Schematic diagram of a sodium-sulphur cell for load levelling applications...

The use of two redox couples as active components has been proposed for cells designed for load levelling, bulk energy storage and more recently for electric vehicles. Such systems have inherently low energy densities and... 

R. Yeo, Application of Perfluorosulfonated Polymer Membranes in Fuel Cells, Electrolysis, and Load Leveling Devices, in Perfluorinated Ionomer Membranes, A. Eisenberg and H.L. Yeager (eds), ACS Symposium Series Number 180, American Chemical Society, Washington, DC, pp. 453-473 (1982). 

During the tests, the stack and SRU were instrumented to record voltage on each cell and to measure temperatures at different positions to highlight thermal gradients if present. Mechanical load was applied through weights external to the furnace. The load level was adapted to the stack/SRU design. 

A further possible and important application for the Na/S and ZEBRA cells is in load levelling . By this is meant the storage of electrical energy from power supply generators when demand is relatively low and its release into the supply network (the grid ) at peak demand times. In this way the generators can be run continuously at their optimal efficiency speeds. 

---