A 1050 K Stirling space engine design

As part of the NASA CSTI High Capacity Power Program on Conversion Systems for Nuclear Applications, Sunpower, Inc. completed for NASA Lewis a reference design of a single-cylinder free-piston Stirling engine that is optimized for the lifetimes and temperatures appropriate for space applications. The NASA effort is part of the overall SP-100 program which is a combined DOD/DOE/NASA project to develop nuclear power for space. Stirling engines have been identified as a growth option for SP-100 offering increased power output and lower system mass and radiator area. Superalloy materials are used in the 1050 K hot end of the engine; the engine temperature ratio is 2.0. The engine design features simplified heat exchangers with heat input by sodium heat pipes, hydrodynamic gas bearings, a permanent magnet linear alternator, and a dynamic balance system. The design shows an efficiency (including the alternator) of 29 percent and a specific mass of 5.7 kg/kW. This design also represents a significant step toward the 1300 K refractory Stirling engine which is another growth option of SP-100

Free-piston Stirling engine conceptual design and technologies for space power, phase 1

As part of the SP-100 program, a phase 1 effort to design a free-piston Stirling engine (FPSE) for a space dynamic power conversion system was completed. SP-100 is a combined DOD/DOE/NASA program to develop nuclear power for space. This work was completed in the initial phases of the SP-100 program prior to the power conversion concept selection for the Ground Engineering System (GES). Stirling engine technology development as a growth option for SP-100 is continuing after this phase 1 effort. Following a review of various engine concepts, a single-cylinder engine with a linear alternator was selected for the remainder of the study. The relationships of specific mass and efficiency versus temperature ratio were determined for a power output of 25 kWe. This parametric study was done for a temperature ratio range of 1.5 to 2.0 and for hot-end temperatures of 875 K and 1075 K. A conceptual design of a 1080 K FPSE with a linear alternator producing 25 kWe output was completed. This was a single-cylinder engine designed for a 62,000 hour life and a temperature ratio of 2.0. The heat transport systems were pumped liquid-metal loops on both the hot and cold ends. These specifications were selected to match the SP-100 power system designs that were being evaluated at that time. The hot end of the engine used both refractory and superalloy materials; the hot-end pressure vessel featured an insulated design that allowed use of the superalloy material. The design was supported by the hardware demonstration of two of the component concepts - the hydrodynamic gas bearing for the displacer and the dynamic balance system. The hydrodynamic gas bearing was demonstrated on a test rig. The dynamic balance system was tested on the 1 kW RE-1000 engine at NASA Lewis

Sage Simulation Model for Technology Demonstration Convertor by a Step-by-Step Approach

The development of a Stirling model using the 1-D Saga design code was completed using a step-by-step approach. This is a method of gradually increasing the complexity of the Saga model while observing the energy balance and energy losses at each step of the development. This step-by-step model development and energy-flow analysis can clarify where the losses occur, their impact, and suggest possible opportunities for design improvement

Reliability Demonstration Approach for Advanced Stirling Radioisotope Generator

Developed for future space missions as a high-efficiency power system, the Advanced Stirling Radioisotope Generator (ASRG) has a design life requirement of 14 yr in space following a potential storage of 3 yr after fueling. In general, the demonstration of long-life dynamic systems remains difficult in part due to the perception that the wearout of moving parts cannot be minimized, and associated failures are unpredictable. This paper shows a combination of systematic analytical methods, extensive experience gained from technology development, and well-planned tests can be used to ensure a high level reliability of ASRG. With this approach, all potential risks from each life phase of the system are evaluated and the mitigation adequately addressed. This paper also provides a summary of important test results obtained to date for ASRG and the planned effort for system-level extended operation

A Design of a Modular GPHS-Stirling Power System for a Lunar Habitation Module

Lunar habitation modules need electricity and potentially heat to operate. Because of the low amounts of radiation emitted by General Purpose Heat Source (GPHS) modules, power plants incorporating these as heat sources could be placed in close proximity to habitation modules. A design concept is discussed for a high efficiency power plant based on a GPHS assembly integrated with a Stirling convertor. This system could provide both electrical power and heat, if required, for a lunar habitation module. The conceptual GPHS/Stirling system is modular in nature and made up of a basic 5.5 KWe Stirling convertor/GPHS module assembly, convertor controller/PMAD electronics, waste heat radiators, and associated thermal insulation. For the specific lunar application under investigation eight modules are employed to deliver 40 KWe to the habitation module. This design looks at three levels of Stirling convertor technology and addresses the issues of integrating the Stirling convertors with the GPHS heat sources assembly using proven technology whenever possible. In addition, issues related to the high-temperature heat transport system, power management, convertor control, vibration isolation, and potential system packaging configurations to ensure safe operation during all phases of deployment will be discussed

Stirling Isotope Power Systems for Stationary and Mobile Lunar Applications

The NASA Exploration Systems Architecture Study (ESAS) places a significant emphasis on the development of a wide range of capabilities on the lunar surface as a stepping-stone to further space exploration. An important aspect of developing these capabilities will be the availability of reliable, efficient, and low-mass power systems to support both stationary and mobile applications. One candidate system to provide electrical power is made by coupling the General Purpose Heat Source (GPHS) with a high-performance Stirling convertor. In this paper we explore the practical power range of GPHS/Stirling convertor systems all with conductively coupled hot-end designs for use on the lunar surface. Design and off-design operations during the life of the convertor are studied in addition to considering these varying conditions on system. Unique issues concerning Stirling convertor configurations, integration of the GPHS with the Stirling convertor, controller operation, waste heat rejection, and thermal protection are explored. Of particular importance in the evaluation process is a thorough understanding of the interactions between the wide range of unique lunar environments and the selection of key systems operating characteristics and the power systems design. Additionally, as power levels rise the interface between the GPHS and Stirling and the Stirling and the radiator begins to dominate system mass and material selection becomes more important

Lunar Surface Stirling Power Systems Using Isotope Heat Sources

For many years, NASA has used the decay of plutonium-238 (Pu-238) (in the form of the General Purpose Heat Source (GPHS)) as a heat source for Radioisotope Thermoelectric Generators (RTGs), which have provided electrical power for many NASA missions. While RTGs have an impressive reliability record for the missions in which they have been used, their relatively low thermal to electric conversion efficiency and the scarcity of plutonium-238 (Pu-238) has led NASA to consider other power conversion technologies. NASA is considering returning both robotic and human missions to the lunar surface and, because of the long lunar nights (14.75 Earth days), isotope power systems are an attractive candidate to generate electrical power. NASA is currently developing the Advanced Stirling Radioisotope Generator (ASRG) as a candidate higher efficiency power system that produces greater than 160 W with two GPHS modules at the beginning of life (BOL) (~32% efficiency). The ASRG uses the same Pu-238 GPHS modules, which are used in RTG, but by coupling them to a Stirling convertor provides a four-fold reduction in the number of GPHS modules. This study considers the use of americium-241 (Am-241) as a substitute for the Pu-238 in Stirling- convertor-based Radioisotope Power Systems (RPS) for power levels from tens of watts to 5 kWe. The Am-241 is used as a substitute for the Pu-238 in GPHS modules. Depending on power level, different Stirling heat input and removal systems are modeled. It was found that substituting Am-241 GPHS modules into the ASRG reduces power output by about one-fifth while maintaining approximately the same system mass. In order to obtain the nominal 160 W of electrical output of the Pu-238 ASRG requires 10 Am-241 GPHS modules. Higher power systems require changing from conductive coupling heat input and removal from the Stirling convertor to either pumped loops or heat pipes. Liquid metal pumped loops are considered as the primary heat transportation on the hot end and water pumped loop/heat pipe radiator is considered for the heat rejection side for power levels above 1 kWe

Long-Lived Venus Lander Conceptual Design: How To Keep It Cool

Surprisingly little is known about Venus, our neighboring sister planet in the solar system, due to the challenges of operating in its extremely hot, corrosive, and dense environment. For example, after over two dozen missions to the planet, the longest-lived lander was the Soviet Venera 13, and it only survived two hours on the surface. Several conceptual Venus mission studies have been formulated in the past two decades proposing lander architectures that potentially extend lander lifetime. Most recently, the Venus Science and Technology Definition Team (STDT) was commissioned by NASA to study a Venus Flagship Mission potentially launching in the 2020- 2025 time-frame; the reference lander of this study is designed to survive for only a few hours more than Venera 13 launched back in 1981! Since Cytherean mission planners lack a viable approach to a long-lived surface architecture, specific scientific objectives outlined in the National Science Foundation Decadal Survey and Venus Exploration Advisory Group final report cannot be completed. These include: mapping the mineralogy and composition of the surface on a planetary scale determining the age of various rock samples on Venus, searching for evidence of changes in interior dynamics (seismometry) and its impact on climate and many other key observations that benefit with time scales of at least a full Venus day (Le. daylight/night cycle). This report reviews those studies and recommends a hybrid lander architecture that can survive for at least one Venus day (243 Earth days) by incorporating selective Stirling multi-stage active cooling and hybrid thermoacoustic power

Investigation of Liquid Metal Heat Exchanger Designs for Fission Surface Power

Fission surface power is an option for future Moon and Mars surface missions. High power nuclear reactor heated Stirling convertors are an option to provide reliable power for long duration outpost operations. This report investigates various design approaches for the liquid metal to acceptor heat exchange and clarifies the details used in the analysis

Free-Piston Stirling Power Conversion Unit for Fission Surface Power, Phase I Final Report

This report summarizes the design of a 12 kW dual opposed free-piston Stirling convertor and controller for potential future use in space missions. The convertor is heated via a pumped NaK loop and cooling is provided by a pumped water circuit. Convertor efficiency is projected at 27 percent (AC electrical out/heat in). The controller converts the AC electrical output to 120 Vdc and is projected at 91 percent efficiency. A mechanically simple arrangement, based on proven technology, was selected in which the piston is resonated almost entirely by the working space pressure swing, while the displacer is resonated by planar mechanical springs in the bounce space