The Department of Energy (DOE) and NASA have identified Stirling Radio-isotope Generators (SRGs) as a candidate power system for use on long-duration, deep-space science missions and Mars rovers (ref. 1). One development planned for an upgraded SRG (known as the ASRG) is to increase efficiency by increasing the overall operating temperature of the system. However, increasing the operating temperature of the convertor puts increased demands on all hot-end parts, including the regenerator. To meet these needs, researchers at the NASA Glenn Research Center developed and demonstrated a high-temperature regenerator.
Fecralloy (UKEA) regenerator developed for the Advanced Stirling Convertor Program.
The regenerator consists of a sintered compact of very fine random fibers that form a structure which is approximately 90-vol% porous. The purpose of the regenerator is to increase the efficiency of the convertor by storing and releasing thermal energy to the working gas during the Stirling cycle. Two problems have historically plagued sintered-fiber-type regenerators: surface contamination and fiber shedding. Any compromise of the surface (by oxidation for example) degrades the performance of the regenerator. The other major failure mode is from release of the fibers into the convertor. These fibers can shed for a number of reasons, but the primary cause is from improper processing. The release of fibers can be catastrophic if they find their way into one of the many tight clearances between the moving parts of the convertor.
Micrograph showing the fiber structure of the regenerator.
Historically, the regenerator has been fabricated from stainless steel. Shedding has always been an issue with the sintered fiber design, but surface degradation was of secondary concern because of the relatively low temperatures (650 °C or lower). However, with the higher temperatures used in the advanced Stirling designs, both shedding and oxidation have become serious concerns.
An extensive examination of Stirling convertors that had been in operation for extended periods of time revealed that oxidation had indeed occurred. A material selection program identified Fecralloy as an alternative regenerator material. Stainless steel regenerators are consumed in just a few hundred hours at 850 °C, but Fecralloy regenerators show a stable structure even after 5000 hr.
A comprehensive process development study also was performed, with the primary goal of producing a regenerator with increased resistance to fiber shedding. The processing method that was eventually developed included both optimized thermomechanical processing as well as postmachining cleaning methods to remove any liberated fibers. In the end, the new material and techniques have all but eliminated the fiber-shedding problem that plagued past designs of both stainless and Fecralloy regenerators.
Success of this program was confirmed by the adoption of these regenerators by members of the industry team, which included the Department of Energy, Lockheed Martin, Rocketdyne, and Sunpower Inc. To date, Glenn has supplied over 50 regenerators to the advanced Stirling development effort. The regenerator design and fabrication procedure have proven to be so successful that Sunpower has decided to incorporate them into their commercial systems. Glenn recently hosted representatives of Sunpower for a 2-day training course. Glenn continues to provide technical support to Sunpower as they develop in-house capabilities to produce these regenerators and incorporate them into both present and future Government and commercial applications.
Find out more about this research:
Glenn’s Thermal Energy Conversion Branch:
http://www.grc.nasa.gov/WWW/TECB/
Glenn’s Power and In-Space Propulsion:
http://www.grc.nasa.gov/WWW/5000/pep/
Last updated: December 14, 2007
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