Sodium Sulfur (NaS) Batteries were originally developed by Ford Motor Company in the 1960s and subsequently the technology was sold to the Japanese company NGK. NGK now manufactures the battery systems for stationary applications. The systems operate at a high temperature, 300 to 350 °C, which can be an operational issue for intermittent operation. Significant installations for energy storage have been used to facilitate distribution line construction deferral. The round trip efficiency is in the 90% range so provides an efficient use of energy.
How Sodium Sulfur Batteries Work
The active materials in a NaS battery are molten sulfur as the positive electrode and molten sodium as the negative. The electrodes are separated by a solid ceramic, sodium alumina, which also serves as the electrolyte. This ceramic allows only positively charged sodium-ions to pass through. During discharge electrons are stripped off the sodium metal (one negatively charged electron for every sodium atom) leading to formation of the sodium-ions that then move through the electrolyte to the positive electrode compartment. The electrons that are stripped off the sodium metal move through the circuit and then back into the battery at the positive electrode, where they are taken up by the molten sulfur to form polysulfide. The positively charged sodium-ions moving into the positive electrode compartment balance the electron charge flow. During charge this process is reversed. The battery must be kept hot (typically > 300 ºC) to facilitate the process (i.e., independent heaters are part of the battery system). In general Na/S cells are highly efficient (typically 89%).
NaS battery technology has been demonstrated at over 190 sites in Japan. More than 270 MW of stored energy suitable for 6 hours of daily peak shaving have been installed. In Abu Dhabi, fifteen NaS systems acting in coordination provide 108 MW / 648 MWh to defer fossil generation investment and provide frequency response and voltage control services.