The Advanced Stirling Convertor (ASC) is being developed by Sunpower, Inc. for NASA’s Glenn Research Center (GRC) with critical technology support tasks led by GRC. The goal of the ASC project, which is funded by NASA’s Science Mission Directorate, is to develop a highly efficient, low mass, reliable power convertor for future Radioisotope Power Systems (RPS). The high efficiency is a requirement for future missions in order to minimize the fuel needed.
The ASC technology has evolved through progressive convertor builds and successful testing to demonstrate high conversion efficiency (38% greater than 80 W output power), low mass (1.3 kg), hermetic sealing, launch vibration simulation, Electromagnetic Interference (EMI) characterization, and extended operation. The GRC and Sunpower team delivered on-schedule in October 2007, two ASC-E convertors and a spare to the Department of Energy (DOE) and Lockheed Martin Space Systems Company for integration onto the Advanced Stirling Radioisotope Generator Engineering Unit (ASRG EU).
The ASC consists of a Free-Piston Stirling engine integrated with a linear alternator to produce electricity. It is sized for the thermal output of a single General Purpose Heat Source (GPHS) module. To date, Sunpower has produced five generations of ASC-related hardware evolving the technology progressively with each build. The ASC-E2, shown on left with external interfaces, is based on the heritage ASC-E convertors.
A simplified layout of the ASC is shown below. The key technologies in this convertor that enable high efficiency and low mass are the hydrostatic gas bearings, a moving-magnet linear alternator, high-frequency operation, high-temperature hot end materials and fabrication processes, and high-temperature, high-porosity regenerators. The gas bearings for the ASC reside within the piston and are charged from the engine working space via a check valve within the piston. The displacer is sprung by means of a single planar spring located outboard from the working space and alternator.
The use of gas bearings and the outboard displacer spring allows for a very compact working space of the machine which in turn helps lead toward high convertor efficiency. The ASC is similar in design configuration and size to thousands of commercial terrestrial cryocoolers in the field that were manufactured by Sunpower or Sunpower licensees lending confidence to the reliability of the ASC. Further, Sunpower cryocooler technology has flown, demonstrating reliable operation in space applications. One in particular continues service aboard NASA’s RHESSI satellite, launched on February 5, 2002.
After Sunpower successfully demonstrated the performance of the ASC technology with earlier convertor tests, Lockheed Martin Space Systems Company, DOE’s system integration contractor, initiated the incorporation of the ASC technology into the generator design, naming it the ASRG EU. It was anticipated that the high efficiency and low mass of the ASC would increase the specific power of the generator from 3.5 W/kg for the initial design using a different Stirling convertor to about 7 W/kg for a flight ASRG configured similarly to the EU. It is estimated that greater than 8 W/kg specific power is possible in an optimized ASRG design.
Regarding the earlier ASC builds (ASC-1 shown on right), due to the scope of the project at the time, minimal consideration of generator system design interfaces and integration requirements were included in the design. Earlier builds were all designed as developmental laboratory units. The adoption of the ASC on to the ASRG EU allowed an opportunity to significantly advance the technology by inclusion of integration interfaces and system requirements flowed-down from the generator design. Integrated into the ASRG, the Advanced Stirling Convertor could enable NASA missions as early as 2013 while minimizing the fuel requirements, and maximizing the specific power.
The ASC performance and low mass have been successfully demonstrated by Sunpower. NASA GRC in-house technology tasks are focused on enhancing reliability, reducing risk, and preparing for flight implementation.