NAS battery supplied to MTA New York City Transit bus depot in the United States

February 4, 2009

The NAS battery has resulted in a significant reduction in the operating costs of electricity used for refueling natural gas buses.

NGK Insulators, Ltd. (President and CEO: Shun Matsushita; Head office: Nagoya) has supplied a 1-MW (megawatt) NAS (sodium-sulfur) battery system to the New York Power Authority (NYPA) 's customer, the MTA New York City Transit bus depot, resulting in reduced costs to the utility customer. Using these results as a stepping stone, NGK is expanding its marketing of the NAS battery throughout North America.

Project overview

Installation location

MTA New York City Transit bus depot (Long Island)

Rated output

1 MW


Electric Load leveling for a natural gas refueling bus depot

Start of operation

April, 2008


Cost reductions* from transferring electric load from daytime to inexpensive night time electric power rates and reduced labor costs for night-shift workers.

*Amount of reduction: Annual savings of US$46,000 in electric power rates and US$220,000 in labor costs of night-shift workers (values published by the NYPA)

The MTA owns 330 natural gas fueled buses used by 31 million passengers annually. The buses are refueled at the depots using electrically driven compressors for the natural gas. To minimize electric power costs, refueling has traditionally been done at night, when electric power rates are lower. This required the deployment of large numbers of night-shift workers at comparatively high hourly rates. In addition, daytime electric power had to be used for refueling in cases when refueling had not been completed the previous night. The NYPA introduced the NAS battery system to reduce operating costs at the MTA bus depot. By storing inexpensive nighttime electric power in the NAS battery and discharging the battery during the day, refueling can be concentrated during daylight hours. The NAS battery system has thus reduced electric power costs as well as the comparatively high labor costs for night-shift workers. This marks the first case in which a large-scale reduction in bus depot operating costs has been achieved via the NAS battery's load leveling function.

The NYPA, which has jurisdiction over New York State and is the largest state-owned electric authority in the United States, owns 6,000 MW of generating capacity, most from hydroelectric power plants (such as the Niagara Power Project). According to the NYPA, the current project, in which a NAS battery was installed at a customer's facility rather than the electric power supplier's facility, represents the first of its kind anywhere in the United States.

This large-scale storage battery installation project for a metropolitan customer has received significant interest from major North American electric utilities and public institutions. The United States Department of Energy, a private-sector electric utility in New York, a provincial electric utility in Canada, and several other entities have contributed more than 50% of the installation costs as part of this demonstration project.

New York State plans to introduce renewable energy sources such as wind and tidal power in the near future. Verifying the use of the latest storage battery technologies is an essential step in assessing their use for stabilizing the output of variable renewable electricity generation.

Following on from the successful deployment of NAS batteries to other major US utilities during the past three years, NGK is continuing its promotion of NAS batteries in North America.

Back to index