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.