The active materials in a Na/S 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%). Higher efficiency cells can be designed and built but increase the battery’s cost.
Na/S 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. The largest Na/S installation is a 34-MW, 245-MWh unit for wind stabilization in Northern Japan. The demand for Na/S batteries as an effective means of stabilizing renewable energy output and providing ancillary services is expanding. U.S. utilities have deployed 9 MW for peak shaving, backup power, firming wind capacity, and other applications. Projections indicate that development of an additional 9 MW is in-progress. Several projects are also under development in Europe and Japan.
Updated April 2010