SBIR-STTR Award

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.

We will build and test a stirling-cycle convertor for generating electrical power from the heat output of a radioisotope heat source (GPHS), addressing evolving NASA requirements for highly reliable, robust, and easily adaptable configurations for space-power applications. Our double-acting stirling cycle configuration combines a linear alternator with a moving piston/regenerator assembly into a self-contained module. A number of such modules can be connected together into several possible convertor layouts to scale power, achieve system redundancy and cancel vibration forces. This modular approach provides the system designer with unique packaging options not available with conventional stirling convertors. Our primary Phase II focus will be to build and test this core module within a simple three-module convertor configuration. The part count per module is low and the design is amenable to mass production manufacturing methods. An intrinsic feature within the thermodynamic circuit prevents catastrophic piston over-stroke in the event the electrical load is interrupted. A potentially transformational passive reciprocating hydrodynamic gas bearing suspends the moving piston within its cylinder, eliminating wear and providing a highly effective piston seal. An optional hydrodynamic spin bearing system is available as a backup.