Description: 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.

Benefits: Space Power Generation - The proposed innovation has the potential to support space power generation applications in the 75-500 W electrical power range using thermal input from one or more radioisotope heat sources, with waste heat radiated to space. Overall conversion efficiency is projected to be around 31% with a 640C heat source and 60C radiator. Other heat source/sink options and temperatures are possible depending on convertor efficiency requirements. Cooling - The convertor is a reversible heat engine and can be run backwards to produce cooling in the cryogenic temperature range (50-100 K) from electrical input. By introducing staging lower temperatures are possible. NASA cooling applications include direct cooling of space sensors, vapor re-liquefaction for zero-boiloff fluid storage or cooling superconducting magnetic bearings in support of flywheel energy storage systems.

Our 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. The redundant convertor configurations could be beneficial for terrestrial remote power applications requiring high reliability (e.g. navigation or communications equipment in off-grid areas). Operated as a cryocooler, the convertor could cool high-temperature superconducting magnetic bearings in industrial spindles and motors. The ability to cool a central load and reject heat at the periphery is ideal for zero-boiloff re-condensation of liquid nitrogen, volatile fuels and other substances. The core hydrodynamic bearing technology could be applied to linear free-piston compressors for domestic refrigeration. The Department of Energy Office recently issued a new Roadmap report which prioritized accelerating the commercialization of high-efficiency appliance technologies. This Roadmap ranked the development of advanced compressor technologies for refrigerators and freezers as having the highest overall importance and potential impact.