Sodium-sulfur battery

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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 fabricated from inexpensive materials. Because, however, 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. A suggested application is grid energy storage. A 6 MW, 48 MWh system has been installed at Tsunashima, Japan. Several other utilities are considering and implementing such a system.

Contents

[edit] Construction

The cell is usually made in a tall cylindrical configuration. The entire cell is enclosed by a steel casing that is protected from corrosion on the inside. Mostly, chromium and molybdenum are used for this purpose. This outside container serves as an electrode. The liquid sodium serves as the cathode. 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 sodium ion exchange membrane, which seletively conducts Na+. The cell becomes more economical with increasing size. In commercial applications the cells are arranged in blocks for better conservation of heat and are encased in a vacuum insulated box.


[edit] Operation

During the discharge phase, molten elemental sodium at the core serves as the anode, meaning that the Na donates electrons into the load-bearing wires. The sodium is separated by a beta-alumina solid electrolyte (BASE) cylinder from the container of sulfur, which is fabricated from an inert metal serving as the cathode. The sulfur is absorbed in a carbon sponge. BASE is a good conductor of sodium ions, but a poor 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 Sn2−, sodium polysulfide. The discharge process can be represented as follows:

2 Na + 4 S → Na2S4 Ecell ~ 2 V

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 sufficient to maintain operating temperatures and no external source is mostly required.[1]

[edit] Safety aspects

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

[edit] Applications

The first large-scale use of sodium-sulfur batteries was in the Ford "Ecostar" demonstration vehicle[2], an electric vehicle prototype that was demonstrated in 1991. The high temperature of sodium sulfur batteries present some difficulties for electric vehicle use, however, and with the development of other battery types better suited to automotive use, the Ecostar never went into production.

Sodium sulfur batteries are a possible energy storage application to support renewable energy plants, specifically wind farms and solar generation plants. In the case of a wind farm, the battery would 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.[3]

There is currently a demonstration project using NGK Insulators’ NAS battery at Japan Wind Development Co.’s Miura Wind Park in Japan.[4]

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

Xcel Energy has announced that it will be testing a Wind Farm energy storage battery based on 20-50kW Sodium-Sulfur batteries from NGK Insulators Ltd of Japan. The 80 tonne, 2 semi-trailer sized battery is expected to have 7.2MW hours of capacity at a charge and discharge rate of 1MW. [5]

[edit] Space Applications

Because of its high energy density, the NaS battery has been proposed for space applications[6][7]. Sodium sulfur cells can be made space qualified: a test sodium sulfur cell was flown on the space shuttle to demonstrate operation in space. The Sodium Sulfur flight experiment demonstrated a battery with a specific energy 150 Wh/kg (3 x nickel hydrogen battery energy density), operating at 350C. It was launched on the STS-87 mission in November 1997, and demonstrated 10 days of experiment operation in orbit[8].

[edit] See also

[edit] References

  1. ^ Taku Oshima, Masaharu Kajita, Akiyasu Okuno "Development of Sodium-Sulfur Batteries" International Journal of Applied Ceramic Technology Volume 1, Pages 269-276, 2004. doi:10.1111/j.1744-7402.2004.tb00179.x
  2. ^ Ford Ecostar EV, Ron Cogan
  3. ^ IEEE Spectrum: Taking Wind Mainstream
  4. ^ Japan for Sustainability - Japanese Companies Test System to Stabilize Output from Wind Power
  5. ^ Xcel Energy to trial wind power storage system - 04 Mar 2008 - BusinessGreen
  6. ^ A. A. Koenig and J. R. Rasmussen, "Development of a High Specific Power Sodium Sulfur Cell," IEEE 1990 available at IEEE Explore website
  7. ^ William Auxer, "The PB sodium sulfur cell for satellite battery applications," International Power Sources Symposium, 32nd, Cherry Hill, NJ, June 9-12, 1986, Proceedings Volume A88-16601 04-44 (Pennington, NJ, Electrochemical Society, Inc., 1986, p. 49-54).
  8. ^ NRL NaSBE Experiment, 1997 , see NRL page

[edit] External links

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