Darin L. Redinger

Developing a Free-Piston Stirling Convertor for advanced radioisotope space power systems

The Department of Energy (DOE) has selected Free-Piston Stirling Convertors as a technology for future advanced radioisotope space power systems. In August 2000, DOE awarded competitive Phase I, Stirling Radioisotope Generator (SRG) power system integration contracts to three major aerospace contractors, resulting in SRG conceptual designs in February 2001. All three contractors based their designs on the Technology Demonstration Convertor (TDC) developed by Stirling Technology Company (STC) for DOE. The contract award to a single system integration contractor for Phases II and III of the SRG program is anticipated in late 2001. The first potential SRG mission is targeted for a Mars rover. This paper provides a description of the Flight Prototype (FP) Stirling convertor design as compared to the previous TDC design. The initial flight prototype units are already undergoing performance tuning at STC. The new design will be hermetically scaled and will provide a weight reduction from approximately 4.8 kg to...

The New Generation Infinia Free-Piston Stirling Engine for Micro-CHP and Remote Power Applications

The advancement of free-piston Stirling engines in the last decade has made them attractive for use in the micro-CHP (combined heat and power) and remote power markets. Infinia Corporation, the leader in free-piston Stirling engine technology development, is the primary provider of high-efficiency, long-life Stirling engines and cryocoolers to U.S. government and commercial companies. This paper reviews Infinia’s free-piston Stirling technology development and latest technology advancements as they relate to the next generation Infinia 3.1-kW free-piston engine for micro-CHP and remote power applications. This new generation 3.1-kW Stirling engine inherits the key features of all Infinia free-piston engines – prodigious durability, low noise, and multi-fuels compatibility – while substantially reducing costs and increasing specific power. Many new technologies and features have been implemented to further improve the reliability and some design issues have been addressed to simplify system integration.

Preliminary Computational Fluid Dynamics Modeling of STC Stirling Engines

*† ‡ § Stirling Technology Company (STC) has developed a 55W-class Stirling Convertor technology-demonstration machine (RG-55), with capabilities suitable to meet existing specifications for a potential space-borne Stirling Radioisotope Generator. The RG-55 has proven to be very successful and robust, and has met ever changing performance targets designated by various potential customers. The RG-55 is the current baseline for a firstgeneration generator system, but there are also parallel efforts by STC and others to improve on the design with smaller, lighter, better performing machines. A recent STC conceptual design study is focused on improving power density and reducing size relative to the RG-55, with the introduction of a 70W-class Stirling Convertor. This new machine will incorporate low-mass packaging and has several improvements, including improved performance and simplified system interfaces. Computational fluid dynamics has been used as part of this new design, in an effort to improve thermodynamic modeling capabilities and to improve internal flow geometries for the new heater head. The fluid modeling described in this paper is very preliminary and no attempts were made to evaluate regenerator performance or heat exchanger performance. The current modeling effort specifically focused on developing simple models to help visualize entrance and exit losses and flow recirculation in variable-volume regions, in manifolds, and in gas ports.

Stirling convertor performance mapping test results

The Department of Energy (DOE) has selected Free-Piston Stirling Convertors as a technology for future advanced radioisotope space power systems. In August 2000, DOE awarded competitive Phase I, Stirling Radioisotope Generator (SRG) power system integration contracts to three major aerospace contractors, resulting in SRG conceptual designs in February 2001. All three contractors based their designs on the Technology Demonstration Convertor (TDC) developed by Stirling Technology Company (STC) for DOE. The contract award to a single system integration contractor for Phases II and III of the SRG program is anticipated in late 2001. The first potential SRG mission is targeted for a Mars rover. Recent TDC performance data are provided in this paper, together with predictions from Stirling simulation models.

Structural and Thermal Analysis of Infinia Corporation Stirling Convertors

Free-piston Stirling machines offer high efficiency, long life, quiet operation and long durability. They are the primary candidates for micro-CHP (combined heat and power) and remote power markets. They are also potential candidates for future deep space missions. To ensure the long durability of the Stirling engines, Infinia has worked with NASA Glenn Research Center and Allied Signal to develop a comprehensive set of FEA design criteria and standards. The design standard Infinia is using is ASME pressure vessel design-code compliant. It also combines the material testing results, as well as other aerospace companies’ past practical experiences. This paper describes the detailed FEA analysis process at Infinia for Stirling pressure vessel and component design. This paper also illustrates the path to achieving long durability without sacrificing the system efficiency.

Stirling Convertor Performance Mapping Test Results for Future Radioisotope Power Systems

Long‐life radioisotope‐fueled generators based on free‐piston Stirling convertors are an energy‐conversion solution for future space applications. The high efficiency of Stirling machines makes them more attractive than the thermoelectric generators currently used in space. Stirling Technology Company (STC) has been performance‐testing its Stirling generators to provide data for potential system integration contractors. This paper describes the most recent test results from the STC RemoteGen™ 55 W‐class Stirling generators (RG‐55). Comparisons are made between the new data and previous Stirling thermodynamic simulation models. Performance‐mapping tests are presented including variations in: internal charge pressure, cold end temperature, hot end temperature, alternator temperature, input power, and variation of control voltage.

Continuing Development for Free‐Piston Stirling Space Power Systems

Long‐life radioisotope power generators based on free‐piston Stirling engines are an energy‐conversion solution for future space applications. The high efficiency of Stirling machines makes them more attractive than the thermoelectric generators currently used in space. Stirling Technology Company (STC) has been developing free‐piston Stirling machines for over 30 years, and its family of Stirling generators is ideally suited for reliable, maintenance‐free operation. This paper describes recent progress and status of the STC RemoteGen™ 55 W‐class Stirling generator (RG‐55), presents an overview of recent testing, and discusses how the technology demonstration design has evolved toward space‐qualified hardware.

Low -Mass Stirling Convertor Assembly Progress Update

Infinia is developing the next generation of space‐ready Stirling Convertor Assemblies. Infinia has previously proposed a Low‐Mass Stirling Convertor Assembly (SCA) design employing a flux‐concentrating, moving‐iron linear alternator. This paper describes further development of that proposed machine, including additional improvements and advancements. One significant change is a new, lighter‐weight moving‐magnet alternator design. Infinia has shown progress in the low mass design’s development and testing: namely in the area of flexure spring rates, welding techniques on the flange, and a change to the alternator configuration. Progress has been made with the flat‐top heater head design and its capabilities, as well. The changes described in this paper will significantly reduce the mass and increase the power density of the low‐mass design.

Operation and Control of Dual Opposed Stirling Machines

Free-piston Stirling-based generators offer an alternative to the traditional radioisotope thermoelectric generators currently used in certain space power applications. Stirling machines are dynamic power conversion devices that employ axially-moving components and generate a resulting vibration force. Rather than transferring this vibration force to the spacecraft, Stirling designers seek to minimize the influence of vibration on sensitive instruments and payloads. Operating two Stirling machines in opposed, axial alignment allows the net vibration force from these machines to be significantly reduced. This paper discusses some of the methods used to optimize the vibration reduction for opposed Stirling machines. Sources for operating mismatches between machines are discussed, including potential methods to reduce their effects. Both electrical and physical methods for vibration fine-tuning are discussed.

Practical Applications of Stirling Dynamic Modeling

Free -Piston Stirling machines are usually of the beta or gamma configuration: employing a piston and a displacer to interact with the internal working fluid. These two moving components, and additionally, the external casing of the machine and other externally mounted moveable components comp ri se a multi -degree of freedom in a physically -coupled dynamic system. Modeling details of these systems have been described by the authors and by several others, in many previous publications. This paper describes some practical applications of these models to evaluate operating scenarios in various system configurations. Different software applications for modeling these dynamics are discussed in this paper and example simulation results are shown for several operating scenarios.