Novel Ring-Configuration Double-Acting Free-Piston Stirling Convertor

NASA SBIR-2015 Topic S3.01 seeks to evaluate and advance Stirling convertors as a potentially more efficient alternative to the radioisotope-heated thermoelectric power generators used on some U.S. space missions. Most free-piston Stirling (FPS) convertors are piston-displacer machines that use complex gas bearing/ flexure strategies to manage the wear of reciprocating and close-fitting internal components. The reliability of these mechanisms is critical to overall system performance and longevity. Converter Source, LLC has developed a double-acting free-piston Stirling arrangement that eliminates displacers and the problem of displacer seal wear while at the same time reducing the number of distinct parts required for the machine. The key design features of the new convertor most relevant to the space power application include: integrated alternator functionality within the piston assembly, eliminating connecting structures and buffer volumes with associated pressure walls, thereby reducing system mass compared to conventional technology, compatibility with simple lightweight hydrodynamic gas bearings to eliminate piston seal wear, dynamically-balanced radial layout that surrounds the heat source at the center, and heat rejection to ambient at the outer perimeter.

Space Power Generation - The proposed innovation has the potential to support space power generation applications in the 10's – 100's We power range using radioisotope heat sources. Multiple units could be stacked to provide increased power. Aided by its internal geometric simplicity, the ring-configuration double-acting free-piston Stirling convertor has the potential to enable more reliable and long-life convertor operation via less complicated and more robust hydrodynamic bearing approaches than are presently used by the other Stirling convertors. Space-based Cryocooling – The convertor could also be optimized to serve as a cryocooler for NASA applications that would benefit from a cooled central-bore and long-life operation. Possible examples might include: cryocooling of superconducting magnetic bearings in support of flywheel energy storage systems, flow-through fluid cooling, re-condensation of cryogenic gases for zero-boiloff fluid storage, sensor cooling, etc.

Power Generation - The FPS convertor has been proposed for solar thermal power generation applications in conjunction with thermal energy storage to provide fully-dispatchable power generation any time of day. The convertor has the potential to be a lower cost alternative to other Stirling machines and might find application as a generator using natural gas or renewable biofuels. Cryocooling – As a cryocooler, the proposed device might be optimized to cool high-temperature superconducting magnetic bearings (SMB) in industrial spindles and motors. The associated reduced frictional losses and reduced maintenance of these SMB systems combined with the lower cost of this cryocooler have the potential to enable the first economical widespread deployment of flywheel energy storage systems for portable and stationary applications – including grid stabilization, renewable energy firming, and uninterruptible power supplies. The cryocooler will also have potential to be applied for zero-boiloff re-condensation of volatile fuels and other substances.

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