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NASA Glenn Research Center : Ansoft's Maxwell 3D
The Department of Energy (DOE), NASA Glenn Research Center (Cleveland, OH), and Stirling Technology Company (STC) of Kennewick, WA are developing a free-piston Stirling convertor for a Stirling Radioisotope Generator (SRG). The SRG is being developed for multi-mission use to provide electric power for potential NASA deep space missions and power for unmanned Mars rovers, especially for missions of long duration. Currently, the SRG is being evaluated as a high-efficiency power source alternative to existing Radioisotope Thermoelectric Generators (RTGs).
NASA engineer Steven M. Geng uses Maxwell 3D Field Simulator software from Ansoft Corporation (Pittsburgh, PA) to perform electromagnetic modeling. "Our work focuses on determining whether or not a design will work successfully and survive the expected harsh environmental operating conditions," says Geng. The software uses electromagnetic field simulation to accurately predict product performance from physical design information. Using technology specifically designed for electromagnetic analysis, the Maxwell 3D Field Simulator allows Geng to evaluate various 3D geometries, materials, and excitation levels to evaluate linear alternator designs.
Geng stated that the convertor is also being designed to survive a high-radiation environment such as for a potential mission to Europa (a moon of Jupiter). "As a result," adds Geng, "we are evaluating the organic materials in the convertor to make sure that these meet the requirements for radiation, performance, and life.
"Glenn engineers are conducting a variety of in-house tasks to provide data in developing the Stirling convertor for readiness for space qualification and mission implementation. Our work helps determine if a design will perform and function properly with the capability of surviving in a deep space or Mars surface environment. As part of this effort, a 55-watt Stirling convertor has been characterized for ElectroMagnetic Interference and Compatibility (EMI/EMC). It is very important that the EMI emitted from the convertor is kept below levels that could interfere with any science experiments. In addition, the convertor has successfully passed launch environment random vibration testing at workmanship, flight acceptance, and qualification test levels while operating at full stroke and full power," adds Geng. Performance verification and performance mapping of dual-opposed Stirling convertors has been completed over a range of hot-end and cold-end temperatures and strokes at the Glenn Stirling Research Laboratory. Endurance testing is expected to be initiated in the future.
NASA Glenn materials and structures researchers are currently evaluating the convertor heater head to investigate its potential for long life. Heater head life is a critical element for achieving the 100,000+ hour life of the convertor. Glenn is also evaluating the magnets used in the convertor's linear alternator in both short-term and long-term aging tests. The tests are being conducted with margin on the magnet operating temperature and in a demagnetization field to quantify any potential magnet degradation with time and temperature. These results will be used to project magnet characteristics over the mission lifetimes.
"Using Maxwell 3D Field Simulator, we studied the magnets and the alternator to estimate the magnets margin to demagnetization," says Geng. "We wanted to determine the degree to which the magnets are stressed in the alternator."
The 55-watt Stirling convertor that is being developed is about the size of a football. Its efficiency ranges between 25-28%. The unit is being developed for long life. A similar 10-watt Stirling convertor has endured over 65,000 hours of life testing. "These convertors will be able to operate for over 10 years on deep space missions, adds Geng.
He says that the Maxwell 3D software helped him and his colleagues investigate new methodologies to improve designs, as well as analyze existing designs. He started using the software several years ago and says that it does not take long to learn to use the product. "I had the linear alternator model up and running quickly. It took a little longer to develop a macro to validate the model with experimental data."
Geng has taken several training classes at Ansoft in Pittsburgh. He adds that the customer support group is outstanding.
For more information about Ansoft and Maxwell 3D, visit www.ansoft.com. For more information about NASA Glenn and the Glenn technology development for the Stirling Radioisotope Generator, visit www.grc.nasa.gov and www.grc.nasa.gov/WWW/tmsb. More information on NASA Glenn's use of Ansoft's Maxwell 3D software can be found in the following references by Geng, S.M., et al: "A 3-D Magnetic Analysis of a Linear Alternator for a Stirling Power System," in Proceedings from the 35th Intersociety Energy Conversion Engineering Conference, Las Vegas, NV, AIAA-2000-2838, NASA TM-2000-210249, 2000, and "A 3-D Magnetic Analysis of a Stirling Convertor Linear Alternator Under Load," in Proceedings from the 36th Intersociety Energy Conversion Engineering Conference, Savannah, GA, IECEC2001-CT-34, NASA TM-2001-211084, 2001.
Author: Laura Carrabine