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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 To realize a low-carbon society, the introduction of renewable energies, such as solar or wind power, is increas- ingly being promoted these days worldwide. A major chal- lenge presented by solar and wind power generators is their fluctuation in output. If they are introduced in large numbers to the power system, problems, such as voltage rises, frequency fluctuations and surplus of the generated power, are predicted to occur. As a solution to these prob- lems, energy storage technologies are attracting attention, amongst which energy storage batteries are expected to become indispensable for use. Various energy storage bat- teries are being developed and many application verifica- tion projects using such batteries are currently being promoted. Thus, expectations are growing for their prac- tical use to the power system in near future. The redox flow (RF) battery, a type of energy storage battery, has been enthusiastically developed in Japan and in other countries since its principle was publicized in the 1970s(1). Some such developments have been put into practical use. This paper reviews the history of the RF battery’s development, along with the status quo of its use. 2. The Necessity of Energy Storage and Its Technologies in Practical Use(2) As an energy storage technology that has long been used in the power system, pumped hydro energy storage is widely known. Occupying about 10% of the total power generation capacity, it functions as a load leveler; namely, it levels the power load by storing power during off-peak hours and discharging it during peak hours. Variable-speed pumped hydro energy storage, which can vary the rotating speed of a pump, is currently in practical use. Some pumped hydro systems have a sophisticated power system stabilization function of frequency regulation or others. As other energy storage technologies, energy storage batter- ies, superconducting magnetic energy storage (SMES), fly- wheels, compressed air energy storage (CAES), and electric double-layer capacitors (EDLC) are known. They are being developed for various applications to make effective use of their individual characteristics, and some of the develop- ments have been put into practical use. An advantage of SMES is that they are high in energy storage efficiency and can discharge a large amount of power instantaneously, since they store electric energy as it is. They are expected to be put into practical use in the near future, as electric power companies and national proj- ects are conducting their verification tests. Chubu Electric Power Co., Inc. is field-testing a 5 MVA SMES at a liquid- crystal factory. This SMES, used for instantaneous voltage sag compensation, is among the world’s largest. In other countries, such as the United States, SMES is already com- mercialized for the power system stabilization and for in- stantaneous voltage sag compensation. Regarding flywheel technologies, the Fusion Institute of the former Japan Atomic Energy Research Institute (currently the Japan Atomic Energy Agency) has a flywheel power generator with the world’s largest energy storage capacity (8 GJ or 2,200 kWh). The generator is used as a magnetic field coil power supply. The Okinawa Electric Power Co., Inc, has a 23 MW flywheel for frequency regulation. The CAES is a 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 States. EDLC technology has a characteristic of instanta- neous large output because, like capacitors, it charges and discharges electricity by absorbing and desorbing electric charge without any chemical reaction taking place. The technology is also advantageous in that it is maintenance- free. Recently, EDLC products with large capacities that can be used for electric-power facilities are commercially available, and are also being used in such applications as instantaneous voltage sag compensation, absorption of regeneration energy and voltage regulators for electric- railway, and natural-energy-generation output fluctuation stabilization. Battery energy storage technology is superior in tech- nical integrity to the above energy storage technologies and has excellent practicality because it can be installed and distributed in suburban areas. It is thus a highly prom- ising technology. 4 · Redox Flow Battery for Energy StoragePDF Image | Redox Flow Battery for Energy Storage
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