Track 6: Energy Conversion Technology and Development

Paper and Podium Presentation

AuthorsTitle
1J. N. Easley, C. DenbrockCONSIDERATIONS FOR CLOSED-LOOP BRAYTON POWER CYCLE FOR NUCLEAR THERMAL ROCKET WITH DECAY HEAT
2S. OritiDYNAMIC RPS PATH TO FLIGHT
3M. A. WhiteFLEXURE ISOTOPE STIRLING CONVERTOR (FISC) DEVELOPMENT PROGRESS
4B. SondelskiMASS OPTIMIZATION OF POWER SYSTEM FOR SPACE APPLICATIONS
5S. Wilson, S. OritiMATURATION OF DYNAMIC POWER CONVERTORS FOR RADIOISOTOPE POWER SYSTEMS
6G. Bruder, F. RitzertRADIOISOTOPE POWER GENERATION WITH THERMOACOUSTIC POWER SYSTEM (TAPS) TECHNOLOGY
7J. Collins, J. Gary Wood, J. Stanley, W. OttingSUNPOWER ROBUST STIRLING CONVERTOR (SRSC) PROJECT OVERVIEW
8C. G. MorrisonTEMPERATURE AND POWER SPECIFIC MASS SCALING FOR LEU CLOSED-CYCLE BRAYTON SYSTEMS FOR SPACE SURFACE POWER AND NUCLEAR ELECTRIC PROPULSION
9J. J. Breedlove, T. M. Conboy, M. V. ZagarolaTURBO-BRAYTON CONVERTER FOR RADIOISOTOPE POWER SYSTEMS

Lightning Talk

AuthorsTitle
1D. CheikhENHANCED THERMOELECTRIC PERFORMANCE OF RARE-EARTH TELLURIDE COMPOUNDS VIA BAND STRUCTURE ENGINEERING
2S. Kumar Mishra, A. Pente, A. Kumar, C. Prakash Kaushik, B. DikshitGRAPHENE SUPERLATTICE BASED THERMOELEMENT FOR RADIOISOTOPE THERMOELECTRIC GENERATOR
3A. LoHEAVY ION THERMIONIC ENERGY CONVERSION (HITEC): A DESIGN STUDY

Paper and Podium Presentation

CONSIDERATIONS FOR CLOSED-LOOP BRAYTON POWER CYCLE FOR NUCLEAR THERMAL ROCKET WITH DECAY HEAT

J. N. Easley, C. Denbrock jne87@msstate.edu | denbrock@umich.edu
A nuclear thermal rocket (NTR) is an example of a broader class of nuclear thermal propulsion (NTP) technologies. In future years, the nuclear thermal rocket will be a viable option for Mars and deep-space missions because of its high specific impulse (Isp) and efficient power conversion systems. One issue associated with NTP is cooling the reactor during post-thrust operations while in transit. One solution would be to incorporate a Brayton power cycle into the engine. The benefits of this are two-fold. Firstly, incorporating a power cycle would provide power to the habitat module without the need for solar panels (which become much less viable the farther out into space a spacecraft travels). Secondly, the Brayton cycle would act as a coolant system for the nuclear reactor during post-thrust operations and would eliminate the need to power down the reactor completely. Nuclear decay heat from the reactor could provide power for a time, but once the energy from decay heat drops below the energy needed to run the power cycle, the nuclear reactor can provide the additional energy needed to keep thermal power input into the cycle constant. This paper aims to investigate the benefits of incorporating a Brayton power cycle into the nuclear thermal rocket as well as model the reactor power input needed during this post-thrust transitional period.
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DYNAMIC RPS PATH TO FLIGHT

S. Oriti salvatore.m.oriti@nasa.gov
NASA Glenn Research Center has been pursuing the development of dynamic power conversion for several decades. Candidate NASA missions involve mutli-year travel to far away destinations, or to extreme environments where sunlight does not exist. Human-base mission studies also show that power needs would be beyond the capabilities of solar energy conversion, and instead would require nuclear reactor energy sources, for which the thermal energy must be converted to electricity. Dynamic power conversion technology has developed sufficiently to make a sound engineering argument that it is suitable for these NASA missions. Dynamic conversion power sources have yet to be flown in space, and thus suffer a disadvantage owing to their lack of heritage data on flight missions. One of the largest obstacles for adoption is the uncertainty in reliability of a device with moving parts. However, significant progress has been made toward demonstrating the technology capable in all relevant environments, with the necessary long life. Another hurdle for adoption is the lack of mission, which would drive specific requirements, and provide a solid timeline for technology development endpoint. Until a mission is identified, an alternative approach is necessary to advance a dynamic power conversion system towards flight.
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FLEXURE ISOTOPE STIRLING CONVERTOR (FISC) DEVELOPMENT PROGRESS

M. A. White maury.white@amsc.com
For full papers, please enter your abstract here (250 words or less). For lightning talks, ignore this field. The AMSC Stirling development team (formerly Infinia Corporation) is developing an advanced Stirling convertor, designated as the Flexure Isotope Stirling Convertor (FISC), for high efficiency Radioisotope Power Systems (RPS). After a successful Phase I FISC design, a Phase II fabrication and test contract with NASA Glenn Research Center (GRC) is about 30% complete at this writing in late 2018. Two FISC demonstration units will be thoroughly evaluated in Phase 2, then delivered to GRC for a wide range of Phase 3 performance assessments and flight qualification tests. FISC is a direct derivative of Infinias Technology Demonstration Convertor (TDC) and Stirling Radioisotope Generator (SRG) development contract with DOE/GRC between the mid-1990s and 2005. All four TDC units assigned to long-term testing at GRC demonstrated exceptional life and reliability in excess of 12 years with no maintenance or degradation. FISC uses the same basic topology as the already robust TDC. Some changes were required to address significantly higher operating temperature environments, and additional changes will further improve design margins, robustness and efficiency.
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MASS OPTIMIZATION OF POWER SYSTEM FOR SPACE APPLICATIONS

B. Sondelski sondelski@wisc.edu
A Brayton cycle coupled to a direct cooled nuclear reactor is being designed and optimized for a space surface power application. Robust models for the various Brayton cycle components were developed and integrated. Separately, a nuclear reactor model which provides an optimized reactor mass for given flow conditions was developed. Then mass correlations for the size of the recuperative heat exchanger and radiator panel, along with the reactor mass model, were integrated with the cycle model, and an optimization algorithm was developed to find the least massive power system. The optimization routine is being used to explore the effects of turbine inlet temperature, cycle pressures, and other various cycle parameters on the full system mass.
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MATURATION OF DYNAMIC POWER CONVERTORS FOR RADIOISOTOPE POWER SYSTEMS

S. Wilson, S. Oriti scott.d.wilson@nasa.gov
Dynamic Radioisotope Power Systems (DRPS) are being developed by NASAs Radioisotope Power Systems (RPS) Program in collaboration with the U.S. Department of Energy (DOE) for space science and exploration missions. A development effort is currently underway to mature dynamic power convertors for infusion into a potential future flight generator. This convertor maturation effort was formulated by the RPS Program at NASA Headquarters and utilizes expertise from agencys technology and mission centers to support requirements development and technology assessments. The effort is being executed by Glenn Research Centers (GRC) DRPS Project and the Thermal Energy Conversion Branch. The Dynamic Power Convertor (DPC) contracts consist of three phases to enable design, fabrication, and independent assessment of prototypes after delivery to the government. The contracts are intended to gather data on candidate dynamic conversion technologies to fill knowledge gaps, support assessments of dynamic conversion technologies, and elicit generator requirements. The 110-watt Stirling Radioisotope Generator (SRG-110) and Advanced Stirling Radioisotope Generator (ASRG) flight development projects provided Stirling convertor demonstration units and engineering models to verify and validate convertors against performance specifications and mission requirements. These units utilize temperature resistant materials and non-contacting bearings to demonstrate wear-free, long-life operation. This maturation effort builds on past lessons-learned and new requirements focused on demonstrating convertor robustness to critical environments meant to stress key aspects of each convertor within the margins of the design. This effort seeks to realize the full potential of dynamic power conversion technologies for NASAs space science and exploration missions.
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RADIOISOTOPE POWER GENERATION WITH THERMOACOUSTIC POWER SYSTEM (TAPS) TECHNOLOGY

G. Bruder, F. Ritzert geoff.bruder@nirvana-es.com
Nirvana Energy Systems (NES) has pioneered and is commercializing an innovative ThermoAcoustic Power System (TAPS) based on technology developed by NASA and Xerox Palo Alto Research Center (PARC). The novel TAPS technology has no hot moving parts and incorporates well proven, reliable linear actuators in an engine based on the Stirling cycle. NES has designed, optimized, built and tested all sub-systems for reliability, ease of manufacturing and cost reduction over free-piston Stirling engines. The convertor is insensitive to radioisotope heat degradation, capable of 10+ years continuous operation, inexpensive to manufacture using well-established methods, and yields greater than 25% thermal to electrical efficiency all while being designed for a specific power greater than 30 We/kg. The NES Thermoacoustic Radioisotope Generator (TRG) represents the ultimate in remote power devices and is the next step toward reliable dynamic power conversion for space.
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SUNPOWER ROBUST STIRLING CONVERTOR (SRSC) PROJECT OVERVIEW

J. Collins, J. Gary Wood, J. Stanley, W. Otting josh.collins@ametek.com
In October 2017, the team of Sunpower and Aerojet Rocketdyne were awarded of a development contract by NASA GRC under NASAs ROSES 2016 Announcement of Opportunity. The contract is in support of the Radioisotope Power Source (RPS) offices goal of developing a robust, dynamic RPS for potential future flight missions. Sunpowers proposed design the Sunpower Robust Stirling Convertor (SRSC) implements robustness improvements identified in the point of departure, high performance free-piston Stirling convertor design the Advanced Stirling Convertor Engineering Unit (ASC-E3). New, key requirements for potential RPS missions have been added to project, and the expectation is the convertor produced in this contract will enable a 250W to 500W generator. Based on a modular design, the SRSC is predicted to enable a range of generators with variable levels of redundancy from 250W to 500W. This paper will focus on Phase II of the SRSC project, present the SRSC predicted performance, and highlight design improvements made to increase robustness while maintaining high performance and specific power.
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TEMPERATURE AND POWER SPECIFIC MASS SCALING FOR LEU CLOSED-CYCLE BRAYTON SYSTEMS FOR SPACE SURFACE POWER AND NUCLEAR ELECTRIC PROPULSION

C. G. Morrison c.morrison@usnc.com
The specific mass (or mass per unit power) is a fundamental performance metric in space power systems. For surface power, a low specific mass reduces launch costs and lander size. For nuclear electric propulsion, a low specific mass enables fast transit with within the solar system. Studies on specific mass have typically focused on point designs and have not adequately explored the design space and scaling of specific mass. Previous research by the author has studied closed cycle Brayton power conversion and has shown that specific mass is a strong function of temperature and power. This paper continues this research and explores the design space for radiatively-cooled closed nuclear Brayton systems with an emphasis on temperature and power. The resulting analyses show the scaling laws for specific mass.
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TURBO-BRAYTON CONVERTER FOR RADIOISOTOPE POWER SYSTEMS

J. J. Breedlove, T. M. Conboy, M. V. Zagarola jfb@creare.com
Creare has teamed with Aerojet Rocketdyne, Sest Incorporated (Sest), and the University of New Mexico Institute for Space and Nuclear Power Studies (UNM-ISNPS) to develop a turbo-Brayton power converter for future NASA missions that use radioisotope heat sources. NASA has considered the closed Brayton cycle attractive for space since the 1960s, and Creare has developed miniature Brayton technology for over 40 years. Key characteristics include high specific power, high efficiency, long-life operation without wear, undetectable vibration, and flexible packaging. Detailed design results indicate a 300 W e -class converter with a turbine inlet temperature of 730C will have a thermal-to-electric conversion efficiency of nearly 25% and a specific power greater than 20 W e /kg.
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Lightning Talk

ENHANCED THERMOELECTRIC PERFORMANCE OF RARE-EARTH TELLURIDE COMPOUNDS VIA BAND STRUCTURE ENGINEERING

D. Cheikh Dean.A.Cheikh@jpl.nasa.gov
For full papers, please enter your abstract here (250 words or less). For lightning talks, ignore this field.
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GRAPHENE SUPERLATTICE BASED THERMOELEMENT FOR RADIOISOTOPE THERMOELECTRIC GENERATOR

S. Kumar Mishra, A. Pente, A. Kumar, C. Prakash Kaushik, B. Dikshit shaktimishra15@gmail.com
In the present days RTGs; major limitation is the lower conversion efficiency of segmented thermoelectric unicouple; which necessiates the development of technically suitable thermoelement with enhanced efficiency. The maximum conversion of thermal to electrical energy of the present day thermoelement is limited to 15 % only. This is attributed to its lower Seebeck coefficient and figure-of-merit. Graphene superlattice heterostructures based thermoelement is being studied to increase the conversion efficiencies significantly and seems to be promising candidate for replacement of the existing lower efficient thermoelement. This is possible by applying different electric potential on the top metallic gate electrodes periodically patterned over the h-BN encapsulated graphene superlattice which is deposited on a SiO2 substrate backed by a doped Si substrate acting as a back gate. The maximum Seebeck coefficient can be tuned by varying the gate electrodes potential. Initially the gate voltage is supplied from an external source and later switched over to Seebeck potential after reaching a stable voltage range. Transfer matrix approach was followed for theoretical calculations of the Seebeck coefficient. This also describes the feasibility of graphene superlattice element based RTGs followed by evaluation of its overall efficiency under the similar conditions with respect to temperature & surface area of heat source. Due to higher efficiency, this gives the possibility for an alternative radioisotope like easily available radio nuclides Am-241 and Sr-90 as compared to Pu-238; which is used in todays RTGs. This can be achieved without compromising with the power output & payload.  
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HEAVY ION THERMIONIC ENERGY CONVERSION (HITEC): A DESIGN STUDY

A. Lo loaustin09@berkeley.edu
The cesium vapor diode is the only demonstrable power producing thermionic energy conversion (TEC) device deployed in space to date. It remains a promising candidate for nuclear space power applications given its high power density and simplistic design. In the current state of the art, emission electrons ionize the Cs vapor which conducts the electrons across the interelectrode gap, losing 50% of their energy in the process. This fundamental physics drawback constrains a TECs design (interelectrode gap, wgap<0.5mm) and limits operating efficiencies (5.5%, TOPAZ reactor). The author proposes an alternative plasma ignition source for in-core nuclear reactor TECs: heavy ions from fissioning a cladless nuclear fuel element. This concept was explored briefly by General Motors and the Office of Naval Research in the 1960s and yielded promising results, yet was never pursued further by either group. This plasma ignition scheme would simultaneously enhance device efficiency (>30%) and relax design constraints (wgap>5mm) by raising the plasma Teand allowing for noble gas electron conduction instead of Cesium. LBNLs Nuclear Data Group is collecting dE/dx data in plasma targets of various compositions and ionizations using the 88-inch cyclotron as a source of low energy (~1MeV/nucleon), high charge-state heavy ions.This data will benchmark Michigan State Universitys Plasma Theory and Simulation Group PIC code to verify its applicability Heavy Ion Thermionic Energy Converter (HITEC) reactor design.This graduate work is being carried out in collaboration with Argonne National Laboratory, Michigan State University, and University of California Berkeley.
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