Sodium Sulfur (NAS) Batteries

Executive Summary

Sodium Sulfur (NaS) Batteries were originally developed by Ford Motor Company in the 1960’s and subsequently the technology was sold to the Japanese company NGK. NGK now manufactures the battery1. Two or more electricAn adjective meaning “needing electricity to operate” such as electric motor or wire. IEEE: Containing, producing , arising from, actuated by or carrying electricity. cells connected together electrically. In common usage, the term “battery” is also applied to a single cell, such as a household battery. 2. A system comprised of identical electrochemical cells. 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 energy1. Energy is the potential of a physical system to perform work. (A common unit of work is foot-pound—the amount of energy needed to lift one pound up a distance of one foot.) Energy exists in several forms such as electromagnetic radiation ... storage to facilitate distributionThe practice of and infrastructure for distribution of electricity to end-users by utilities. Typical voltages range from 12 to 138 kiloVolts (kV) line construction deferral. The round trip efficiencyThe amount of energy that a storage system can deliver relative to the amount of energy injected into the system during the immediately preceding charge. (Also referred to as efficiency.) is in the 90% range so gives an efficient use of energy.

Discussion

The active materials in a Na/S battery are molten sulfur as the positive electrodeAn electrical conductor through which an electric current enters or leaves a conducting medium, whether it be an electrolytic solution, solid, molten mass, gas, or vacuum. For electrolytic solutions, many solids, and molten masses, an electrode is an... and molten sodium as the negative. The electrodes are separated by a solid ceramic, sodium alumina, which also serves as the electrolyteFor electrochemical batteries; A chemical compound which, when fused or dissolved in certain solvents, usually water, will conduct an electric current. All electrolytes in the fused state or in solution give rise to ions which conduct the electric.... This ceramic allows only positively charged sodium-ions to pass through. During dischargeThe process of extracting stored energyThe energy available in the storage system to perform physical work through the conversion of its chemical or mechanical energy, stated in kWh or MWh. from the storage system. 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 chargeThe process of injecting energy to be stored into the storage system. 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%).

Conclusion

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. The largest NaS installation is a 34-MW, 245-MWh unit for wind stabilization in Northern Japan. The demand1. The rate at which electric energy is delivered to or by a system or part of a system, generally expressed in kilowatts or megawatts, at a given instant or averaged over any designated interval of time. 2. The rate at which energy is being used by... for NaS 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 powerThe rate at which energy is generated, converted, transmitted, distributed or delivered., firming wind capacityThe rate at which equipment can either generate, convert or transfer energy., and other applications. Projections indicate that development of an additional 9 MW is in-progress.