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3. Battery Energy Storage Table 1 shows the varieties of energy storage batteries and their individual characteristics(3). Among them, lead acid batteries have the longest history and are extremely common for use in automobiles. In some overseas coun- tries, such as the United States, Germany and Puerto Rico, lead acid batteries have been used as energy storage facili- ties as several aged application examples indicate. In Puerto Rico, 20 MW (40 minutes) lead acid batteries were introduced to regulate frequency and to provide spinning reserve. With the recent increase in demand for energy storage batteries, not only lead acid batteries but also vari- ous other types of batteries are being enthusiastically de- veloped for practical applications worldwide, to make the best use of the characteristics of the individual batteries. Among them, sodium sulfur (NaS) batteries have excellent features, such as high energy density and superior charge/ discharge efficiency, and have been used in many practical applications in Japan and in other countries. They are used not only for load leveling in power substations and indus- trial plants but also for use in combination with solar and wind power generation. At the Futamata Wind Power Sta- tion in Aomori Prefecture, Japan, 34 MW NaS batteries are installed along with 51 MW wind power generation facilities. 4. Redox Flow Battery for Energy Storage The word redox is a combination of, and thus stands for, reduction and oxidation. A redox battery refers to an electrochemical system that generates oxidation and reduc- tion between two active materials, forming a redox system, on the surface of inactive electrodes (the electrodes them- selves do not change). A redox flow (RF) battery has the electrolyte including these active materials in external con- tainers, such as tanks, and charges and discharges electric- ity by supplying the electrolyte to the flow type cell by pumps or other means. 4-1 Principle, configuration and characteristics of RF batteries (1) Principle and configuration of an RF battery As shown in Fig. 1, an RF battery consists mainly of a cell where the redox reaction occurs, positive and negative electrolyte tanks in which active material solution is stored, pumps and piping that circulate the electrolyte from the tanks to the cell. It is interconnected with the AC power system via an AC/DC converter. Power station Charge Discharge AC/DC converter +− Load V2+/V3+ Electrolyte tank Pump Cell Table 1. Energy Storage Batteries VO2+/VO2+ Electrolyte tank Pump VO2+ V2+ H+ VO2+ V3+ Electrode Separating membrane e- e- Fig. 1. Principle and Configuration of an RF Battery Metal ions that change valence can be used in a redox system; however, in light of such factors as energy density and economy, the iron (Fe2+/Fe3+)–chromium (Cr3+/Cr2+) system and the vanadium (V2+/V3+–VO2+/VO2+) system are considered feasible redox systems. The V–V system is espe- cially advantageous because it uses the same metal ions at both the positive and negative electrodes and the battery capacity does not decrease even when the positive and neg- ative electrolytes are mixed each other through the mem- brane, while in the case of a system using two different metal ions, such as iron and chromium, the battery capac- ity decreases if the two electrolytes are mixed. The V–V sys- tem is thus currently widely developed around the world. Battery variety Redox flow NaS Lead acid Lithium ions Nickel hydride Zinc bromide Active material (positive/negative) V ions/V ions S/Na Lead dioxide/Lead Metal compound oxides containing Li ions/Carbon Nickel oxyhydroxide/ Hydrogen-storing alloy Br/Zn Theoretical energy density (Wh/kg) 100 786 167 392~585 225 428 Open-circuit voltage/cell (V) 1.4 2.1 2.1 3.6~3.8 1.2 1.8 Operating temperature (°C) Room temperature About 300 Room temperature Room temperature Room temperature Room temperature Major accessory Circulation pump Heater Not in particular Not in particular Not in particular Circulation pump Characteristics • Independently designable output and capacity • Easily measurable state of charge • Reusable electrolyte • Most common storage batteries • Useful under high tempera- tures • Many records of use such as in cars and UPS • Many records of use as small batteries • Many records of use as small batteries SEI TECHNICAL REVIEW · NUMBER 73 · OCTOBER 2011 · 5PDF Image | Redox Flow Battery for Energy Storage
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