Lee S. Mason

Stirling Technology Development at NASA GRC

The Department of Energy, Stirling Technology Company (STC), and NASA Glenn Research Center (GRC) are developing a free-piston Stirling convertor for a high-efficiency Stirling Radioisotope Generator (SRG) for NASA Space Science missions. The SRG is being developed for multimission use, including providing electric power for unmanned Mars rovers and deep space missions. NASA GRC is conducting an in-house technology project to assist in developing the convertor for space qualification and mission implementation. Recent testing of 55-We Technology Demonstration Convertors (TDC’s) built by STC includes mapping of a second pair of TDC’s, single TDC testing, and TDC electromagnetic interference and electromagnetic compatibility characterization on a non-magnetic test stand. Launch environment tests of a single TDC without its pressure vessel to better understand the convertor internal structural dynamics and of dual-opposed TDC’s with several engineering mounting structures with different natural frequencies h...

A Comparison of Brayton and Stirling Space Nuclear Power Systems for Power Levels from 1 Kilowatt to 10 Megawatts

An analytical study was conducted to assess the performance and mass of Brayton and Stirling nuclear power systems for a wide range of future NASA space exploration missions. The power levels and design concepts were based on three different mission classes. Isotope systems, with power levels from 1 to 10 kilowatts, were considered for planetary surface rovers and robotic science. Reactor power systems for planetary surface outposts and bases were evaluated from 10 to 500 kilowatts. Finally, reactor power systems in the range from 100 kilowatts to 10 megawatts were assessed for advanced propulsion applications. The analysis also examined the effect of advanced component technology on system performance. The advanced technologies included high temperature materials, lightweight radiators, and high voltage power management and distribution.

Early Results from Solar Dynamic Space Power System Testing

A government/industry team designed, built and tested a 2-kWe solar dynamic space power system in a large thermal vacuum facility with a simulated Sun at the NASA Lewis Research Center. The Lewis facility provides an accurate simulation of temperatures, high vacuum and solar flux as encountered in low-Earth orbit. The solar dynamic system includes a Brayton power conversion unit integrated with a solar receiver which is designed to store energy for continuous power operation during the eclipse phase of the orbit. This paper reviews the goals and status of the Solar Dynamic Ground Test Demonstration project and describes the initial testing, including both operational and performance data. System testing to date has accumulated over 365 hours of power operation (ranging from 400 watts to 2.0-W(sub e)), including 187 simulated orbits, 16 ambient starts and 2 hot restarts. Data are shown for an orbital startup, transient and steady-state orbital operation and shutdown. System testing with varying insolation levels and operating speeds is discussed. The solar dynamic ground test demonstration is providing the experience and confidence toward a successful flight demonstration of the solar dynamic technologies on the Space Station Mir in 1997.

Status of Brayton Cycle Power Conversion Development at NASA GRC

The NASA Glenn Research Center is pursuing the development of Brayton cycle power conversion for various NASA initiatives. Brayton cycle power systems offer numerous advantages for space power generation including high efficiency, long life, high maturity, and broad salability. Candidate mission applications include surface rovers and bases, advanced propulsion vehicles, and earth orbiting satellites. A key advantage is the ability for Brayton converters to span the wide range of power demands of future missions from several kilowatts to multi-megawatts using either solar, isotope, or reactor heat sources. Brayton technology has been under development by NASA since the early 1960’s resulting in engine prototypes in the 2 to 15 kW-class that have demonstrated conversion efficiency of almost 30% and cumulative operation in excess of 40,000 hours. Present efforts at GRC are focusing on a 2 kW testbed as a proving ground for future component advances and operational strategies, and a 25 kW engine design as a ...

800 hours of operational experience from a 2 kWe Solar Dynamic system

From December 1994 to September 1998, testing with a 2 kWe Solar Dynamic power system resulted in 33 individual tests, 886 hours of solar heating, and 783 hours of power generation. Power generation ranged from 400 watts to over 2kWe, and SD system efficiencies have been measured up to 17 per cent, during simulated low-Earth orbit operation. Further, the turbo-alternator-compressors successfully completed 100 start/stops on foil bearings. Operation was conducted in a large thermal/vacuum facility with a simulated Sun at the NASA Lewis Research Center. The Solar Dynamic system featured a closed Brayton conversion unit integrated with a solar heat receiver, which included thermal energy storage for continuous power output through a typical low-Earth orbit. Two power conversion units and three alternator configurations were used during testing. This paper will review the test program, provide operational and performance data, and review a number of technology issues.

Realistic Specific Power Expectations for Advanced Radioisotope Power Systems

Radioisotope power systems are being considered for a wide range of future NASA space science and exploration missions. Generally, radioisotope power systems offer the advantages of high reliability, long life, and predictable power production regardless of operating environment. Previous radioisotope power systems, in the form of radioisotope thermoelectric generators, have been used successfully on many NASA missions including Apollo, Viking, Voyager, and Galileo. NASA is currently evaluating design options for the next generation of radioisotope power systems. Of particular interest is the use of advanced, higher efficiency power conversion to replace the previous thermoelectric devices. Higher efficiency reduces the quantity of radioisotope fuel and potentially improves the radioisotope power systems specific power (watts per kilogram). Power conversion options include segmented thermoelectric, Stirling, Brayton, and thermophotovoltaic. This paper offers an analysis of the advanced 100 W-class radioisotope power system options and provides credible projections for specific power. Based on the analysis presented, radioisotope power system specific power values greater than 10 W/kg appear unlikely.

Heat Rejection Concepts for Brayton Power Conversion Systems

This paper describes potential heat rejection design concepts for closed Brayton cycle (CBC) power conversion systems. Brayton conversion systems are currently under study by NASA for Nuclear Electric Propulsion (NEP) applications. The Heat Rejection Subsystem (HRS) must dissipate waste heat generated by the power conversion system due to inefficiencies in the thermal-to-electric conversion process. Space Brayton conversion system designs tend to optimize at efficiencies of about 20 to 25% with radiator temperatures in the 400 to 600 K range. A notional HRS was developed for a 100 kWe-class Brayton power system that uses a pumped sodium-potassium (NaK) heat transport loop coupled to a water heat pipe radiator. The radiator panels employ a sandwich construction consisting of regularly-spaced circular heat pipes contained within two composite facesheets. Heat transfer from the NaK fluid to the heat pipes is accomplished by inserting the evaporator sections into the NaK duct channel. The paper evaluates various design parameters including heat pipe diameter, heat pipe spacing, and facesheet thickness. Parameters were varied to compare design options on the basis of NaK pump pressure rise and required power, heat pipe unit power and radial flux, radiator panel areal mass, and overall HRS mass.

Performance and Operational Characteristics for a Dual Brayton Space Power System With Common Gas Inventory

This paper provides an analytical evaluation on the operation and performance of a dual Brayton common gas system. The NASA Glenn Research Center in-house computer program Closed Cycle System Simulation (CCSS) was used to construct a model of two identical 50 kWe-class recuperated closed-Brayton-cycle (CBC) power conversion units that share a common gas inventory and single heat source. As operating conditions for each CBC change, the total gas inventory is redistributed between the two units and overall system performance is affected. Several steady-state off-design operating points were analyzed by varying turbine inlet temperature and turbo-alternator shaft rotational speed to investigate the interaction of the two units.

A Solar Dynamic Power Option for Space Solar Power

Lee S. MasonNational Aeronautics and Space AdministrationGlenn Research CenterCleveland, Ohio 44135ABSTRACTA study was performed to determine the potentialperformance and related technology requirements ofSolar Dynamic power systems for a Space Solar Powersatellite. Space Solar Power is a concept where solarenergy is collected in orbit and beamed to Earthreceiving stations to supplement terrestrial electric powerservice. Solar Dynamic systems offer the benefits ofhigh solar-to-electric efficiency, long life with minimalperformance degradation, and high power scalability.System analyses indicate that with moderate componentdevelopment, SD systems can exhibit excellent massand deployed area characteristics. Using the analysesas a guide, a technology roadmap was generated whichidentifies the component advances necessary to makeSD power generation a competitive option for the SSPmission.INTRODUCTIONThe Space Solar Power (SSP) concept represents anattempt to provide environmentally benign, terrestrialelectric power with positive economic rate-of-return. Inorder to satisfy these goals, the solar power generationsystem must 1) provide very high power levels to thebeam power source to maximize delivered power toEarth, 2) be very lightweight to control launch costs, and3) be extremely long-lived to minimize maintenance andupkeep requirements. One representative SSPconfiguration has a power generation requirement of 1.6Gigawatts (GW), a power system specific power goal of1000 W/kg, and system lifetimes of 20 years [1]. Thisrepresentative configuration would be located in mediumEarth orbit (approximately 12000 km), although optionsin geosynchronous Earth orbit are also beingconsidered. The representative system would deliver400 MW to the Earth receiving station.Power system options for SSP include photovoltaic (PV)arrays and Solar Dynamic (SD) power systems. A widerange of PV cell technologies are being consideredincluding thin-film blankets and multi-junction crystallinecells. SD heat engine technology options includeBrayton, Stirling, and Rankine. This study assumes theuse of closed Brayton cycle (CBC) conversion based onits potential for high power and its relative technicalmaturity. Figure 1 shows a potential layout for a SSPSD power module utilizing a refractive fresnelconcentrator and Brayton conversion.

Design and Off-Design Performance of 100 kWe-Class Brayton Power Conversion Systems

Abstract The NASA Glenn Research Center in-house computer model Closed Cycle Engine Program (CCEP) was used to explore the design trade space and off-design performance characteristics of 100 kWe-class recuperated Closed Brayton Cycle (CBC) power conversion systems. Input variables for a potential design point included the number of operating units (1, 2, 4), cycle peak pressure (0.5, 1, 2 MPa), and turbo-alternator shaft speed (30, 45, 60 kRPM). The design point analysis assumed a fixed turbine inlet temperature (1150 K), compressor inlet temperature (400 K), helium-xenon working-fluid molecular weight (40 g/mol), compressor pressure ratio (2.0), recuperator effectiveness (0.95), and a Sodium-Potassium (NaK) pumped-loop radiator. The design point options were compared on the basis of thermal input power, radiator area, and mass. For a nominal design point with defined Brayton components and radiator area, off-design cases were examined by reducing turbine inlet temperature (as low as 900 K), reducing shaft speed (as low as 50% of nominal), and circulating a percentage (up to 20%) of the compressor exit flow back to the gas cooler. The off-design examination sought approaches to reduce thermal input power without freezing the radiator.