Sodium-sulfur battery

A sodium-sulfur battery is a type of battery constructed from sodium (Na) and sulfur (S). This type of battery exhibits a high energy density, high efficiency of charge/discharge (89—92%), long cycle life, and is made from inexpensive, non-toxic materials. However, the operating temperature of 300 to 350 °C and the highly corrosive nature of sodium make it suitable only for large-scale non-mobile applications. A suggested application is grid energy storage. A 6 MW, 48 MWh system has been installed at Tsunashima, Japan.

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Construction

The cell is usually made in a tall cylindrical configuration. The entire cell is enclosed by an inert metal (such as galvanized iron, stainless steel, graphite rod or platinum wire) container and sealed at the top with an airtight alumina lid. The cell becomes more economical with increasing size. In commercial applications the cells are arranged in blocks for better conservation of heat.

Operation

During the discharge phase, molten metalic sodium at the core acts as the anode, separated by a beta-alumina solid electrolyte (BASE) cylinder from a sulfur container made from an inert metal acting as the cathode. The sulfur is absorbed in a carbon sponge. Alumina is a good conductor of sodium ions but a bad conductor of electrons, avoiding self-discharge. When sodium gives off an electron, the Na⁺ ion migrates to the sulfur container. The electron travels through the molten sodium to the contact and through the electric load to the sulfur container. Here, the electron reacts with sulfur to form S⁻, which then forms sodium polysulfide. As the cell discharges the sodium level drops. During the charging phase the reverse process takes place. Once running, the heat produced by charging and discharging cycles is enough to maintain operating temperatures and no external source is required.

Safety aspects

Pure sodium presents a hazard because it spontaneously burns on contact with water so the system must be protected from moisture. In modern NaS cells, sealing techniques make fires unlikely.

Renewable energy applications

Sodium sulfur batteries are emerging as a possible energy storage application to support renewable energy plants, specifically wind farms and solar generation plants. In the case of a wind farm, there can be a need to store energy during times of high wind but low power demand. This stored energy can then be discharged from the batteries during peak load periods. In addition to this power shifting, it is likely that sodium sulfur batteries can be used throughout the day to assist in stabilizing the power output of the wind farm during wind fluctuations. These types of batteries present an option for energy storage in locations where other storage options are not feasible due to location or terrain constraints. Pumped-storage hydroelectricity facilities require a lot of space and a significant water resource. Compressed air energy storage (CAES) requires some type of geologic feature for storage.^[1]

There is currently a demonstration project using NGK Insulators’ NAS battery at Japan Wind Development Co.’s Miura Wind Park in Japan.^[2]

Japan Wind Development is also developing a 51 MW wind farm that will incorporate a 34 MW sodium sulfur battery system.^[3]

See also

• Molten salt battery

References

1. ^ http://www.spectrum.ieee.org/jun06/3544
2. ^ http://www.japanfs.org/db/1843-e
3. ^ http://www.renewable-energy-world.com/display_article/308327/121/CRTIS/none/none/A-call-for-back-up:-How-energy-storage-could-make-a-valuable-contribution-to-renewables/

http://www.aep.com/newsroom/newsreleases/default.asp?dbcommand=displayrelease&ID=956 First US Utility application at American Electric Power