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. During discharge, as positive Na+ ions flow through the electrolyte and electrons flow in the external circuit of the battery producing voltage. 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.
A sodium sulfur battery is a type of molten-salt battery constructed from liquid sodium (Na) and sulfur (S). This type of battery has a high energy density, high efficiency of charge/discharge (89–92%) and long cycle life, and is fabricated from inexpensive materials. However, because of the operating temperatures of 300 to 350 °C and the highly corrosive nature of the sodium polysulfides, such cells are primarily suitable for large-scale non-mobile applications such as grid energy storage.
The cell is usually made in a tall cylindrical configuration. The entire cell is enclosed by a steel casing that is protected, usually by chromium and molydenum, from corrosion on the inside. This outside container serves as the positive electrode, while the liquid sodium serves as the negative electrode. The container is sealed at the top with an airtight alumina lid. An essential part of the cell is the presence of a BASE (beta-alumina solid electrolyte) membrane, which selectively conducts Na+. The cells are arranged in blocks for better conservation of heat and are encased in a vacuum-insulated box.
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. (The Energy Blog, 1/18/2006, Wikipedia, Electricity Storage Association)
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