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AF073-107




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Basic Information:

Title:	Sulfur Hexafluride (SF6) Replacement or Reduction in high voltage switchgear and airborne radar
Program:	SBIR
Technology Area:	Materials/Processes
Open Date:	7/19/2007
Close Date:	9/19/2007

Description:

Sulfur Hexafluride (SF6) is the electric power industrys presently preferred gaseous dielectric (besides air) and is present in airborne military radar systems, such as airborne warning and control systems (AWACs) for military radar applications. However, SF6 is also a greenhouse gas with high global warming potential (GWP). SF6 can escape through seals, especially in older equipment, and is vented in airborne radar systems where it then becomes a source of green house gas emissions. The military could possibly substitute with a lower GWP replacement gas or use a mixture of SF6 with another gas such as nitrogen. The military standards currently use pure SF6 and no alternatives have been proposed. SF6 is a man-made gas. Its basic physical and chemical properties, behavior in various types of gas discharges, and uses by the electric power industry have been broadly investigated. In its normal state, SF6 is chemically inert, non-toxic, non-flammable, non-explosive, and thermally stable. It is a strong electronegative (electron attaching) gas both at room temperature and at temperatures well above ambient, which principally accounts for its high dielectric strength and good arc-interruption properties. The breakdown voltage of SF6 is nearly three times higher than air at atmospheric pressure. Furthermore, it has good heat transfer properties and it readily reforms itself when dissociated under high gas-pressure conditions in an electrical discharge or an arc. Most of its stable decomposition byproducts do not significantly degrade its dielectric strength and are removable by filtering. It produces no polymerization, carbon, or other conductive materials used in electrical equipment at temperatures up to about 200 degrees Celsius. Besides its good insulating and heat transfer properties, SF6 has a relatively high pressure when contained at room temperature. The pressure required to liquefy SF6 at 21 degree C is about 2100 kPa; its boiling point is reasonably low, 63.8 degree C, which allows pressures of 400 kPa to 600 kPa (4 to 6 atmospheres) to be employed in SF6-insulated equipment. It is easily liquefied under pressure at room temperature allowing for compact storage in gas cylinders. It presents no handling problems, is readily available, and up until recently has been reasonably inexpensive. The electric power industry has become familiar and experienced with using SF6 in electrical equipment. However, SF6 has some undesirable properties: it forms highly toxic and corrosive compounds when subjected to electrical discharges (e.g., S2F10, SOF2); nonpolar contaminants (e.g., air, CF4) are not easily removed from it; its breakdown voltage is sensitive to water vapor, conducting particles, and conductor surface roughness; and it exhibits non-ideal gas behavior at the lowest temperatures that can be encountered in the environment, i.e., in cold climatic conditions (about 50 degree C), SF6 becomes partially liquefied at normal operating pressures (400 kPa to 500 kPa). Sulfur hexafluoride is also an efficient infrared (IR) absorber and due to its chemical inertness, is not rapidly removed from the earth's atmosphere. Both of these latter properties make SF6 a potent greenhouse gas, although due to its chemical inertness (and the absence of chlorine or bromine atoms in the SF6 molecule) it is benign with regard to stratospheric ozone depletion.

Objective:

Reduce or replace SF6 gas with an alternate material that produces the same or better results without the harmful effects on the environment.

Phase I:

Build on existing knowledge to research full replacement of SF6 or replace with a lower ratio SF6 mixture to find best method(s). Replacement gas must preform as well or better than current SF6 gas. Present findings showing feasibility to replace SF6 gas.

Phase II:

Work with the government on approval of SF6 replacement (Full or mixture) by showing same or better results can be achieved with the results of adequate research and develop/manufacture of replacement gas.

Commercialization Potential:

Military application: SF6 is present in airborne military radar systems, such as airborne warning and control systems (AWACs) for military radar applications. Commercial application: SF6 is the electric power industrys preferred gaseous dielectric (besides air). As it is used so extensively, there would be great demand for a environmentally friendly, low-cost alternative.

References:

1. http://www.epa.gov/highgwp/electricpower-sf6/pdf/new_report_final.pdf 2. http://www.ornl.gov/sci/eere/PDFs/CCTP_Wkshp_Rpt_6-28Final.pdf

SBIR Keywords

SF6, Sulfur Hexafluride, Greenhouse gas, dielectric

TechMatch Keyword(s):

Aviation Technology

Sensors

Radar

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