Sodium-Sulfur Battery Energy Storage System Powers CNG Compression for New York Buses
5 May 2006
|The 3 compressors and battery system|
The New York Power Authority (NYPA) is installing a sodium-sulfur (NaS) battery energy storage system (BESS) at the Metropolitan Transportation Authority’s (MTA) Long Island Bus (LI Bus) subsidiary in Garden City.
The energy storage device, from NGK Ltd. in Japan, will power three 600 hp CNG compressors to fuel 220 CNG buses, and then recharge itself from the grid at night when utility electric rates are reduced. The BESS system, capable of discharging 1 MW of power for up to seven hours, is one of the first and largest technology NaS installations in the US.
LI Bus operates commercial bus routes linking Long Island and Queens communities to transportation hubs in New York City. The company had previously limited its refueling schedule, and the gas compressor’s operations, to over-night periods requiring a full maintenance crew and supervision.
Energy-storage is perfectly suited for this peak shaving application. We worked with LI Bus to identify the most environmentally responsible technology, which was also appropriate to meet its needs. The unit has it all; it’s emission-free, noiseless and, can improve power quality, as well as provide voltage support to the distribution system.—Shalom Zelingher, NYPA Chief Technology Development Officer
NaS storage systems—which NGK co-developed with Tokyo Electrical Power Company, and of which the company is currently the only vendor—are used primarily in Japan, and primarily for industrial load-levelling and power-quality applications.
The cells use molten sodium (Na) as an anode, molten sulfur (S) as a cathode, and a solid beta alumina ceramic (Al2O3) electrolyte.
The hermetically sealed battery is kept at approximately 300° C and is operated under the condition that the active materials at both electrodes are liquid and its electrolyte is solid.
|Operation of the NaS battery.|
During the discharge, sodium ions converted from sodium in the anode pass through solid electrolyte to reach the sulfur in the cathode. The electrons flow to outside circuits and the battery cell generates approximately 2 volts at about 300° C.
During discharge, sodium polysulfide is formed in positive electrode, with a corresponding decrease in sodium in the anode. During charging, the electric power supplied from outside forms sodium in the anode and sulfur in the cathode by following the reverse process of the discharge.
NaS cells feature low cost of materials (lower cost on a $/kW-yr basis than lead-acid batteries) and long cycle life, with high energy density (367 Wh/liter in early cells) and power.
|NaS Cell Module|
The battery energy storage system is a collection of more than 6,400 NaS battery cells grouped into 20 modules, each containing 320 individual cells.
NYPA attracted $1.9 million in funding from the New York State Energy Research and Development Authority, Electric Power Research Institute, Long Island Power Authority, Canadian Energy Association, American Public Power Association and several other electric utilities for this project. The Power Authority will finance the remaining $1.9 million in project costs through its energy services program, allowing the MTA-Long Island Bus to repay over several years.
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