Sodium Sulfur (NaS) batteries are high capacity battery systems developed for electric power applications. A NaS battery consists of liquid (molten) sulfur at the positive electrode and liquid (molten) sodium at the negative electrode as active materials separated by a solid beta alumina ceramic electrolyte. The electrolyte allows only the positive sodium ions to go through it and combine with the sulfur to form sodium polysulfides.
2Na + 4S = Na2S4
During discharge, as positive Na+ ions flow through the electrolyte and electrons flow in the external circuit of the battery producing about 2 volts. This process is reversible as charging causes sodium polysulfides to release the positive sodium ions back through the electrolyte to recombine as elemental sodium. This hermetically sealed battery is kept at approximately 300 oC and is operated under conditions such that the active materials at both electrodes are liquid and the electrolyte is solid. At this temperature, since both active materials react rapidly and because the internal resistance is low, the NAS battery performs well. Because of reversible charging and discharging the NAS battery can be used continuously.
NaS battery cells are efficient ( about 89%) and have a pulse power capability over six times their continuous rating (for 30 seconds). This attribute enables the NaS battery to be economically used in combined power quality and peak shaving applications.
Only one company is producing NaS batteries, NGK, of Japan.
The two largest NGK NAS® battery systems were installed since 2003 and are now in operation. One is located at Hitachi, Ltd.’s Automotive Systems Group. The system has enabled Hitachi to significantly reduce electricity costs through load leveling (using electricity supplied by the grid at night during the daytime). Consisting of four 2MW NAS® batteries, this system has the same output wattage as an 8MW system installed at the Morigasaki Water Reclamation Center operated by the Bureau of Sewerage, Tokyo Metropolitan Government. Both systems have an energy capacity of approximately 58MWh, making them the largest energy storage battery systems in the world. These NAS® battery systems also provide backup power supplies in the event of power outages or momentary voltage sags, contributing to more reliable operations at customer sites.
NaS battery technology has been used at over 30 sites in Japan totaling more than 28 MW with stored energy suitable for 8 hours daily peak shaving. Combined power quality and peak shaving applications in the U.S. market are under evaluation.