The Tokyo Electric Power Co. (TEPCO) has established large-scale power generation facilities, including base-load nuclear power plants and high-efficiency thermal peak-load generators, to supply the peak summer demand, which exceeds 60 GW.

The utility provides electrical energy to Japan's capital city, Tokyo, an area of dense domestic and industrial customers. To meet the 60-GW summer demand, TEPCO has installed load-leveling technology in the form of pumped storage hydroelectric plants to improve system utilization, but there is now a shortage of suitable sites for these peak-load generation plants.

One solution to the problem of supplying peak loads is electrochemical batteries, which store electricity in the form of chemical energy. After performing long-term research, TEPCO has succeeded in developing and commercializing high-capacity, long-life sodium-sulfur (NAS) batteries. Already connected and in operation on the utility's distribution network, the NAS battery systems offer the same operational capabilities as pumped storage hydroelectricity.

Furthermore, the high-speed load-following capabilities of NAS batteries can serve as a countermeasure against infrequent power disturbances such as momentary outages and voltage sags. NAS batteries are expected to not only improve power quality, but also to play important roles in stabilizing the operation of independent small-scale networks and using renewable energy resources such as wind and solar power.

Overview of NAS Battery System

TEPCO, with manufacturer NGK Insulators, began researching NAS batteries in 1983, resulting in the development of a beta alumina single-cell battery. NAS battery cells comprise a sodium negative electrode, a sulfur positive electrode and a solid electrolyte made of beta alumina. Beta alumina is a fine ceramic material with a property that allows sodium ions to pass through the material (meaning it is a sodium-ion conductive material). The battery is charged and discharged as electrical potential causes the sodium ions to move between the negative and positive electrodes through the beta alumina solid electrolyte.

The attributes of NAS battery technology are as follows:

  • Financial benefits from mass production as raw materials are abundant and high-volume ceramic manufacturing is a proven technology.

  • NAS batteries require about one-third of the space required by alternative commercial options such as lead-acid batteries.

  • NAS Applications on Distribution Networks

    Because a NAS battery does not self-discharge, the loss of charge during storage and periods of standby is minimal.

  • NAS batteries have a long life span of about 15 years and superior life-cycle characteristics, with about 4500 daily discharge cycles.

  • Load Leveling

    The batteries are 70% to 80% energy efficient, and the hermetically sealed cells release no emissions.

  • Minimal maintenance is required as there are few auxiliaries and no moving parts.

  • Because it is designed with integral thermal management to maintain an internal operating temperature of about 300°C (572°F), the NAS battery system is insensitive to ambient environmental temperatures.

  • Emergency Power System

    Although the sodium and sulfur are within hermetically sealed cells, these hazardous materials need to be handled in accordance with regulations.

The NAS battery system is composed of the battery, an ac-dc converter, the power conversion system (PCS) and a control unit. Battery modules rated at 50 kW (power) and 360 kWh (energy) are connected in series-parallel arrays to the PCS, providing the electrical interface with the distribution network. Typically, the integrated NAS system is connected to a 6600-V ac distribution network through a transformer. Manufactured by several companies including Toshiba, Meidensha and Takaoka, the PCS is used for both charging and discharging.

Standby Power System

Initially, TEPCO installed the NAS battery systems in distribution substations. In 2002, it began installing these systems on customers' premises. By the end of 2008, there were 99 installations with a cumulative capacity of 180 MW. In the majority of cases, customers lease an NAS battery system from TEPCO, which remains responsible for the installation, monitoring and maintenance of the unit. In TEPCO's service area, the largest unit installed is 4000 kW, which is able to discharge 28,800 kWh per cycle.

Although designed primarily for load leveling, NAS battery systems offer additional benefits, including improved network reliability and power quality.

Renewable Energy Control

Load leveling is affected by operating the NAS battery system automatically on a daily cycle of charging at nighttime and discharging during the daytime. This operation mode is performed by configuring an output pattern for each time interval corresponding to the difference in tariff rates applicable during the day and night periods. Automatic operation is typically conducted on a programmed basis scheduled a year in advance.

The load-leveling operation was performed at each customer's site, and the combined performance during 2006 was an annual discharge of 200 GWh, which equates to the output of a small hydroelectric power plant. Similarly, in the summer of 2007, the recorded discharge output was 160 MW during the peak-load daytime period (1 p.m. to 4 p.m.), effectively shifting the peak demand on TEPCO's distribution network.

These statistics confirm that NAS battery systems can store excess off-peak energy at diverse locations on the distribution network, and release or discharge the stored energy during the peak-load daytime period. In addition to the load-leveling function, the NAS battery system can improve network reliability by supplying stored electrical energy during outages and power quality by mitigating voltage sags.

The Future

In addition to load leveling, NAS systems can be programmed to store electric energy for emergencies (for example, 10% of full capacity while in the load-leveling mode). In the event of a network outage, the system automatically supplies the load, fulfilling the role of an emergency generator or emergency power system (EPS) during the outage. To compensate for sudden load changes, the design of the NAS battery system was modified to simulate the characteristics of a generator by improving the current level of the ac-dc converter and the response speed between charge and discharge.

TEPCO used a 2-MW EPS to confirm that network stability was maintained following the energization of a 185-kW induction motor and that the system continued to supply other loads. The result demonstrated that the NAS battery system has the ability to be considered an EPS on an interconnected network and that its high-speed control characteristics are superior to those of a conventional emergency generator.

NAS battery systems also can function as an emergency power supply, available to mitigate voltage sags. The time interval of switching on the battery, basically the time it takes the sensor to detect the voltage sag (less than 80% of system voltage), is around 4.6 msec.

Within this time interval, there is no effect on the load. TEPCO has nine NAS battery systems installed with voltage sag prevention, and its records indicate that voltage sags have been prevented for at least 150 lightning incidents. Initially, thyristor switches and vacuum circuit breakers were used as the high-speed switch current installations, but recently these have been replaced by gate turn-off (GTO) switches that enable the switching time intervals to be reduced to 2 msec to 3 msec.

TEPCO is now considering the NAS battery systems for network stabilization applications on interconnected dispersed generators on the distribution network. TEPCO, aided by government funding, has already undertaken tests to stabilize and control the load variations on an isolated island network and a mainland hybrid wind-NAS generation facility.

The electricity network on Hachijo Island is supplied by internal combustion engine generators in addition to abundant natural resources, including geothermal and wind energy. As a part of a government-subsidized project, a 400-kW NAS battery system was installed adjacent to an existing 500-kW wind turbine generator. An investigation of the NAS system's capability to suppress fluctuations in wind generator output has been conducted since 2001. The results over the years confirm that wind generator output fluctuations can be effectively suppressed by the NAS battery's high-speed charge/discharge control system.

Based on the successful results at Hachijo Island, Japan Wind Development Co. has launched a government-funded hybrid wind-NAS project at Rokkasho Village in North Japan. The key components of this project are 34 1.5-MW wind turbine units (51 MW) and 17 2-MW NAS battery systems (34 MW). This project will verify that large-scale wind generator output variations can be stabilized by the NAS battery system and demonstrate that energy generated by wind during the night can be stored for dispatch at constant power during the daytime. When the anticipated operational performance of the Rokkasho project is proven in terms of stabilizing and controlling wind power generation, TEPCO expects that hybrid wind-NAS systems will be widely deployed.

Companies mentioned in this article:

TEPCO has developed and demonstrated that NAS battery energy-storage technology has advanced significantly and now offers an exciting new energy-storage component for distribution networks. Previously, system planners were locked into an equation that required power generation to coincide with consumption, but this is no longer necessary.

The functionality combined with the high-speed response of the NAS battery system is not limited to load leveling as it has features useful for the stability of renewable energy resource output. It is impossible to control the output of the wind generator, and sudden changes in output adversely affect the voltage, frequency and overall power quality. These problems can be overcome by the NAS battery system, which also offers the potential to store energy during off-peak periods for dispatch at times of peak demand.

TEPCO is confident the proven durability, reliability, stability and performance of the NAS battery system will ensure this versatile tool is used for the efficient use of distribution networks with dispersed generation.


Kouji Tanaka (tanaka.kouji@tepco.co.jp) graduated from Waseda University in Japan in 1981 and started his engineering career in Tokyo Electric Power Co. (TEPCO) in the substation construction and maintenance department. In 1992, Tanaka moved to TEPCO's Ultra-High Voltage (UHV) Construction Centre for the design of UHV substations before joining the Research & Development Centre for the development of the NAS battery in 1997. Since 2001, Tanaka has been working in the corporate marketing and sales department in the energy storage group. Tanaka has been a member of the Institute of Electrical Engineers of Japan since 1981 and a member of CIGRÉ since 2007.

Jun Yoshinaga (yoshinaga.jun@tepco.co.jp) received a master's degree in engineering in 1994 in Osaka University. Since 1994, he has been working at Tokyo Electric Power Co. as an engineer in the distribution department. Since 2003, he has been dealing with technical issues on the distribution system, such as the interconnection of dispersed generation, power quality and earth systems. He has been a member of the Institute of Electrical Engineers of Japan since 1998 and the secretary of CIGRÉ's SC C6 Japanese National Committee since 2003.

Naoki Kobayashi (naoki.kobayashi@tepco.co.jp) graduated from Waseda University in Japan in 1997. Since 1997, he has been working at Tokyo Electric Power Co. as an engineer in the distribution department. Since 2003, he has been dealing with technical issues on the distribution system, such as power quality, the interconnection of dispersed generation and network protection. He has been a member of the Institute of Electrical Engineers of Japan since 2002.

Meidensha www.meidensha.co.jp

Japan Wind Development Co. www.jwd.co.jp

NGK Insulators www.ngk.co.jp

Tokyo Electric Power Co. www.tepco.co.jp

Toshiba www.toshiba.com