SPECIAL Redox Flow Battery for Energy Storage Toshio SHIGEMATSU Renewable energies, such as solar and wind power, are increasingly being introduced as alternative energy sources on a global scale toward a low-carbon society. For the next-generation power system, which uses a large number of these distributed power generation sources, energy storage technologies will be indispensable. Among the energy storage technologies, battery energy storage technology is considered to be most viable. In particular, a redox flow battery, which is suitable for large scale energy storage, has currently been developed at various organizations around the world. This paper reviews the technical development of the redox flow battery. Keywords: redox flow battery, energy storage, renewable energy, battery, vanadium 1. Introduction developed for various applications to make effective use of their individual characteristics, and some of the develop- To realize a low-carbon society, the introduction of ments have been put into practical use. renewable energies, such as solar or wind power, is increas- An advantage of SMES is that they are high in energy ingly being promoted these days worldwide. A major chal- storage efficiency and can discharge a large amount of lenge presented by solar and wind power generators is power instantaneously, since they store electric energy as their fluctuation in output. If they are introduced in large it is. They are expected to be put into practical use in the numbers to the power system, problems, such as voltage near future, as electric power companies and national proj- rises, frequency fluctuations and surplus of the generated ects are conducting their verification tests. Chubu Electric power, are predicted to occur. As a solution to these prob- Power Co., Inc. is field-testing a 5 MVA SMES at a liquid- lems, energy storage technologies are attracting attention, crystal factory. This SMES, used for instantaneous voltage amongst which energy storage batteries are expected to sag compensation, is among the world’s largest. In other become indispensable for use. Various energy storage bat- countries, such as the United States, SMES is already com- teries are being developed and many application verifica- mercialized for the power system stabilization and for in- tion projects using such batteries are currently being stantaneous voltage sag compensation. Regarding flywheel promoted. Thus, expectations are growing for their prac- technologies, the Fusion Institute of the former Japan tical use to the power system in near future. The redox flow Atomic Energy Research Institute (currently the Japan (RF) battery, a type of energy storage battery, has been Atomic Energy Agency) has a flywheel power generator enthusiastically developed in Japan and in other countries with the world’s largest energy storage capacity (8 GJ or since its principle was publicized in the 1970s(1). Some such 2,200 kWh). The generator is used as a magnetic field coil developments have been put into practical use. This paper power supply. The Okinawa Electric Power Co., Inc, has a reviews the history of the RF battery’s development, along 23 MW flywheel for frequency regulation. The CAES is a with the status quo of its use. technology that compresses air and stores it in an under- ground hollow space, and generates power in combination with a gas turbine generator where necessary. It is in prac- tical use at some power stations in Germany and the United 2. The Necessity of Energy Storage and States. EDLC technology has a characteristic of instanta- Its Technologies in Practical Use(2) neous large output because, like capacitors, it charges and discharges electricity by absorbing and desorbing electric As an energy storage technology that has long been charge without any chemical reaction taking place. The used in the power system, pumped hydro energy storage is technology is also advantageous in that it is maintenance- widely known. Occupying about 10% of the total power free. Recently, EDLC products with large capacities that generation capacity, it functions as a load leveler; namely, can be used for electric-power facilities are commercially it levels the power load by storing power during off-peak available, and are also being used in such applications as hours and discharging it during peak hours. Variable-speed instantaneous voltage sag compensation, absorption of pumped hydro energy storage, which can vary the rotating regeneration energy and voltage regulators for electric- speed of a pump, is currently in practical use. Some railway, and natural-energy-generation output fluctuation pumped hydro systems have a sophisticated power system stabilization. stabilization function of frequency regulation or others. As Battery energy storage technology is superior in tech- other energy storage technologies, energy storage batter- nical integrity to the above energy storage technologies ies, superconducting magnetic energy storage (SMES), fly- and has excellent practicality because it can be installed wheels, compressed air energy storage (CAES), and electric and distributed in suburban areas. It is thus a highly prom- double-layer capacitors (EDLC) are known. They are being ising technology. 4 · Redox Flow Battery for Energy Storage 3. Battery Energy Storage 4-1 Principle, configuration and characteristics of RF batteries Table 1 shows the varieties of energy storage batteries (1) Principle and configuration of an RF battery and their individual characteristics(3). Among them, lead As shown in Fig. 1, an RF battery consists mainly of a acid batteries have the longest history and are extremely cell where the redox reaction occurs, positive and negative common for use in automobiles. In some overseas coun- electrolyte tanks in which active material solution is stored, tries, such as the United States, Germany and Puerto Rico, pumps and piping that circulate the electrolyte from the lead acid batteries have been used as energy storage facili- tanks to the cell. It is interconnected with the AC power ties as several aged application examples indicate. In system via an AC/DC converter. 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- Charge Discharge ous other types of batteries are being enthusiastically de- Power Load veloped for practical applications worldwide, to make the station AC/DC converter + − best use of the characteristics of the individual batteries. Cell Among them, sodium sulfur (NaS) batteries have excellent + 2+ features, such as high energy density and superior charge/ VO2 V + 2+ 2+ 3+ discharge efficiency, and have been used in many practical VO2 /VO H+ V /V Electrolyte e- e- Electrolyte applications in Japan and in other countries. They are used tank tank not only for load leveling in power substations and indus- VO2+ V 3+ trial plants but also for use in combination with solar and Electrode Separating wind power generation. At the Futamata Wind Power Sta- Pump membrane Pump tion in Aomori Prefecture, Japan, 34 MW NaS batteries are Fig. 1. Principle and Configuration of an RF Battery installed along with 51 MW wind power generation facilities. Metal ions that change valence can be used in a redox 4. Redox Flow Battery for Energy Storage system; however, in light of such factors as energy density and economy, the iron (Fe2+/Fe3+)–chromium (Cr3+/Cr2+) 2+ 3+ 2+ + The word redox is a combination of, and thus stands system and the vanadium (V /V –VO /VO2 ) system are for, reduction and oxidation. A redox battery refers to an considered feasible redox systems. The V–V system is espe- electrochemical system that generates oxidation and reduc- cially advantageous because it uses the same metal ions at tion between two active materials, forming a redox system, both the positive and negative electrodes and the battery on the surface of inactive electrodes (the electrodes them- capacity does not decrease even when the positive and neg- selves do not change). A redox flow (RF) battery has the ative electrolytes are mixed each other through the mem- electrolyte including these active materials in external con- brane, while in the case of a system using two different tainers, such as tanks, and charges and discharges electric- metal ions, such as iron and chromium, the battery capac- ity by supplying the electrolyte to the flow type cell by ity decreases if the two electrolytes are mixed. The V–V sys- pumps or other means. tem is thus currently widely developed around the world. Table 1. Energy Storage Batteries Battery variety Redox flow NaS Lead acid Lithium ions Nickel hydride Zinc bromide Active material Lead Metal compound oxides Nickel oxyhydroxide/ V ions/V ions S/Na Br/Zn (positive/negative) dioxide/Lead containing Li ions/Carbon Hydrogen-storing alloy Theoretical energy 100 786 167 392~585 225 428 density (Wh/kg) Open-circuit 1.4 2.1 2.1 3.6~3.8 1.2 1.8 voltage/cell (V) Operating Room Room Room Room Room About 300 temperature (°C) temperature temperature temperature temperature temperature Major accessory Circulation pump Heater Not in particular Not in particular Not in particular Circulation pump • Independently • Most common • Many records • Many records • Many records designable output storage batteries of use such as of use as small of use as small and capacity • Useful under in cars and batteries batteries Characteristics • Easily measurable high tempera- UPS state of charge tures • Reusable electrolyte SEI TECHNICAL REVIEW · NUMBER 73 · OCTOBER 2011 · 5 The electrode reaction of the vanadium system can be Frame expressed by the following formulae: Serial connection • Positive electrode 2+ + VO (tetravalent) + H2O ⇆ VO2 (pentavalent) + 2H+ + e−: E0 = 1.00 V (1) • Negative electrode Separating membrane V 3+ (trivalent) + e− ⇆ V 2+ (bivalent): E0 = −0.26 V (2) Electrode In these formulae, the reaction from left to right rep- resents the reaction during charging.