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Dynamic Thermal Energy Conversion

NASA advances Stirling power systems to deliver efficient, long-life energy for deep-space and planetary missions, expanding science and exploration impact.

Overview
Stirling Engine Systems
Fission Surface Power
Aircraft Thermal Management
Contact

Overview

Dynamic thermal energy conversion uses moving components to generate electrical power from thermal energy. Unlike passive systems such as radioisotope thermoelectric generators (RTGs), dynamic systems incorporate mechanical motion, enabling significantly higher efficiencies — often approaching 30%, or up to six times greater than their passive counterparts. These systems typically use Stirling or Brayton engines, both of which convert thermal energy into reciprocating mechanical motion. An attached alternator then converts this mechanical energy into electrical power.

Dynamic conversion technologies can be applied in scenarios requiring either short-term or continuous power. Fuel sources may include the decay of radioisotopes for low-power systems or nuclear fission for higher-power applications. Some systems can also use waste heat from other onboard processes. Due to their high efficiency and durability, dynamic energy conversion systems are strong candidates for supporting long-duration space exploration and surface operations.

Stirling Engine Systems

Stirling Research Lab

NASA’s Glenn Research Center in Cleveland is advancing free-piston Stirling dynamic power convertors to support future Radioisotope Power Systems (RPS) and Fission Power Systems (FPS). Working with industry partners, Glenn engineers have helped develop reliable, robust, and highly efficient Stirling prototypes. These convertors undergo rigorous testing to simulate the extreme environments expected during space missions, including random vibration to mimic launch conditions and centrifuge testing to replicate the forces experienced during descent and landing on planetary surfaces.

One notable prototype is the Technology Demonstration Convertor (TDC), developed by Stirling Technology Company (also known as Infinia). The TDC generates 55 watts of electricity at 25% conversion efficiency. Between 2000 and 2006, 16 TDC prototypes and two engineering units were built under a flight contract. These units use flexure bearings to maintain non-contacting clearances, enabling wear-free operation. Three TDCs have each operated maintenance-free for over 17 years. One of them, TDC #13, is the longest-running heat engine in the world.

Metallic 55-watt Stirling convertor with cylindrical housing, wiring, and vertical mounting for long-duration space power testing.
The 55-watt Technology Demonstration Convertor (TDC), developed to validate long-duration Stirling power conversion. The TDC is an early-generation free-piston Stirling convertor designed to demonstrate high-efficiency energy conversion for space missions. It features non-contacting flexure bearings for maintenance-free operation.
NASA

The Advanced Stirling Convertor (ASC) was developed by Sunpower Incorporated to produce 80 watts of electricity with a conversion efficiency of 40%. Between 2007 and 2015, 17 ASC prototypes and 20 engineering units were built. These convertors use gas bearings to maintain non-contacting running clearances, allowing for wear-free operation. The longest-running unit, ASC-0 #3, has operated maintenance-free for more than 14 years. Some units were disassembled and inspected to identify design improvements that enhance thermal stability and increase resilience under test conditions exceeding operational limits.

Metallic 80-watt Stirling convertor with flanged housing, designed for high-efficiency, wear-free space power generation.
The 80-watt Advanced Stirling Convertor (ASC), designed for high-efficiency space power applications using gas bearing technology. The ASC uses gas bearings to maintain non-contacting motion of internal components, enabling long-duration operation with minimal wear. It was developed as a next-generation solution for dynamic radioisotope power systems.
NASA

The Sunpower Robust Stirling Convertor (SRSC) was developed by Sunpower Incorporated to produce 60 watts of electricity with 26% conversion efficiency. Based on the Advanced Stirling Convertor (ASC) design, the SRSC incorporates key improvements informed by lessons learned during ASC development. Enhancements include stiffer bearings, a more durable regenerator to reduce debris risk, and a passive collision-prevention system for temporary electrical load loss.

Five SRSC prototypes were produced between 2019 and 2025. The leading unit has operated for more than 2.5 years. This design has reached Technology Readiness Level (TRL) 5, having successfully completed qualification-level random vibration tests up to 7.7 grms (root-mean-square acceleration) and static acceleration testing up to 22.5 g.

Cylindrical 60-watt Stirling convertor with flanged ends and polished housing, designed for robust space power generation.
The 60-watt Sunpower Robust Stirling Convertor (SRSC), developed to enhance durability and fault tolerance for long-duration space power applications.
NASA

Stirling Generators

Researchers at NASA Glenn have advanced Stirling-based radioisotope power systems (RPS) for many years. Stirling RPS offer several advantages over legacy systems. With a conversion efficiency of approximately 20%, more than three times higher than previous NASA RPS (~6%), these systems require less radioisotope fuel and produce less waste heat.

Stirling RPS also exhibit zero mechanical degradation; power output declines only due to the natural radioactive decay of the fuel. For example, plutonium-238 (Pu-238) has a half-life of 88 years, while americium-241 (Am-241) has a half-life of 432 years. With convertors designed for multi-decade service life, Stirling RPS could support missions lasting significantly longer than any flown to date.

Another benefit is flexibility: a single Stirling RPS design can support a variety of mission types, including deep-space vacuum environments and planetary surface operations. Recent research has focused on a multi-hundred-watt configuration using redundant convertors. This design allows one or more convertor failures without reducing the total power delivered to the spacecraft.

In 2022, NASA Glenn collaborated with the U.S. Department of Energy (DOE) and Aerojet Rocketdyne to complete a one-year effort resulting in a generator design comprising six General-Purpose Heat Source (GPHS) modules and eight Sunpower Robust Stirling Convertors (SRSCs). The system produces more than 350 watts of electrical power (We). The generator’s specific power could be increased by using multilayer insulation instead of solid insulation in vacuum-only environments.

NASA has also built and tested generator-like hardware to demonstrate system-level redundancy. One prototype, the Stirling generator concept shown below, integrates four convertors and a Pu-238 heat source and has achieved 17% efficiency in early lab testing. In another effort, NASA collaborated with the University of Leicester to develop and test a Stirling RPS concept powered by Am-241. The result, “Stirling generator designed for use with an americium-241 heat source,” shown further down, demonstrated 16% conversion efficiency.

A 3D model of a Stirling generator with four convertors arranged around a central core for space power system applications.
A Stirling generator concept illustrating a multi-convertor configuration for space power systems, showing a central core surrounded by four integrated convertors and radiators for heat rejection. This conceptual design demonstrates redundancy and scalability for deep-space or planetary surface missions.
Aerojet Rocketdyne

SRSC-Based Generator Statistics

ParameterMultimissionVacuum Only
Design Life> 17 years> 17 years
Number of GPHS Modules66
Power Output (BOL)354 We356 We
Power Output (EODL)297 We298 We
Conversion Efficiency24%24%
BOL Specific Power3.2 We/kg4.0 We/kg
EOL Specific Power2.6 We/kg3.3 We/kg
Mass112 kg90 kg
Two NASA engineers test an electrically heated Stirling generator in a lab, collecting data from a table-mounted power system.
Tina Wozniak and Sal Oriti, engineers at NASA’s Glenn Research Center in Cleveland, operate an electrically heated Stirling generator during laboratory testing, collecting performance data and verifying thermal and electrical output. The test article simulates mission-like operating conditions for space-rated power conversion hardware.
NASA
Stirling generator with four convertors mounted on a lab platform, designed for use with a plutonium-238 heat source.
A Stirling generator designed for use with a plutonium-238 heat source, developed by engineers at NASA’s Glenn Research Center in Cleveland. The system shown includes four Stirling convertors integrated around a central core, mounted on a vibration-isolated test platform, and connected to diagnostics and control instrumentation for system-level performance evaluation.
NASA
Laboratory setup of an americium-241 Stirling generator with a test enclosure, a vacuum system, and power instrumentation.
A Stirling generator designed for use with an americium-241 heat source, developed collaboratively by engineers at NASA’s Glenn Research Center in Cleveland and the University of Leicester. The test configuration includes a vacuum system, power instrumentation, and a compact thermal enclosure, enabling efficiency characterization of americium-based radioisotope power systems under controlled laboratory conditions.
NASA

Reference Publications

Small-Scale Stirling Engines

  • Small nuclear power systems using Stirling technology could provide long-duration electric power for probes, landers, rovers, and communication repeaters on the Moon or Mars.
  • These systems would convert heat from the decay of radioisotope fuel into usable electricity to support spacecraft mobility, science instruments, and communications.
  • Each system would use a single convertor and balancer assembly to convert heat from an isotope source and eliminate residual dynamic disturbance.
  • Small Stirling RPS could achieve 15% to 25% conversion efficiency, producing 20–50 watts of usable electrical power.
Concept image of compact 1-watt Stirling generator with a linear alternator and labeled thermal interfaces for LWRHU source.
A 1-watt Stirling generator concept designed for use with a light-weight radioisotope heater unit (LWRHU) heat source. The concept illustrates the potential for scalable, low-power radioisotope systems in space missions requiring minimal electrical output.
NASA
A scalable Stirling generator concept with 40-, 80-, and 160-Wₑ versions using modular convertors and a commercial heat source.
A scalable Stirling generator concept ranging from 40 to 160 watts electrical (Wₑ), designed for use with a commercial heat source. The modular design supports mission flexibility and redundancy for a range of space or terrestrial applications.
NASA

Reference Publications

Stirling Simulators

NASA Glenn is developing replicable Stirling simulator hardware to support the rapid development of Stirling engine controllers and power systems. These simulators streamline power system testing by eliminating the need for engine warm-up and cool-down cycles, and they are immune to damage during electronics debugging.

Stirling simulators accurately emulate the electrical and dynamic behavior of actual engines, including terminal characteristics, large-signal gas dynamics, and small-signal electrical parasitics. Successful integration with simulator hardware has consistently yielded performance results nearly identical to those achieved with operational heated engines.

Designs are available to help industry partners quickly adopt this simulation capability.

A block diagram of NASA’s Stirling simulator system and rack setup for dynamic space power conversion testing.
A diagram of NASA’s in-house Stirling simulator system for dynamic power conversion testing. The schematic shows a hardware-in-the-loop (HIL) configuration used to emulate Stirling convertor behavior. It includes a signal generator, AC power supply, and an alternator emulator. A labeled 24U rack on the right highlights the electronics box, HIL system, and multiple power supplies used in integrated testing of spaceflight power systems.
NASA
Photo and diagram of Stirling simulator hardware and a circuit model showing winding impedance and alternator components.
An in-house rack-mounted Stirling simulator and electrical analog models. The photo shows NASA hardware featuring coil-based components simulating a Stirling alternator. Adjacent diagrams illustrate the winding impedance model and an electrical circuit with resistors (R) and inductors (L) representing alternator behavior under operational loads.
NASA

Reference Publications

Controller Work

NASA Glenn develops in-house Stirling controllers for dynamic space power generation systems, including radioisotope power systems (RPS) and fission surface power (FSP). Multiple breadboard hardware units are built to support rapid design iteration, enabling timely development of robust flight-ready controllers.

NASA also collaborates with external partners to deliver Stirling controller technologies for a range of projects supporting both NASA and the U.S. Department of Defense.

Side-by-side view of NASA’s C-NAC and DiSC Stirling engine controller boards, showing analog and digital control designs.
A comparison of NASA’s analog and digital Stirling engine controllers. The image shows two circuit boards used to control dynamic power conversion systems. On the left is the Capless NASA Analog Controller (C-NAC), which features a simplified, capacitor-free analog architecture for robust operation. On the right is the Digital Stirling Controller (DiSC), which incorporates microprocessor control, SMA connectors, and software-configurable components to support adaptive power system control.
NASA

Reference Publications

Fission Surface Power

Power Conversion

The power conversion subsystem is a central component in systems that use fission surface power (FSP) and nuclear electric propulsion (NEP), transforming a reactor’s thermal energy into usable electrical power for surface operations or spacecraft propulsion. Efficient and reliable energy conversion is essential for enabling long-duration missions.

NASA Glenn is developing advanced high-efficiency power conversion technologies, such as free-piston Stirling and closed Brayton cycle convertors, to support these systems. Work focuses on improving thermal-to-electric conversion efficiency, enhancing long-term reliability, and reducing mass to enable sustainable missions to the Moon and Mars.

A technician inspects the 12.5-kW Component Test Power Convertor with a spherical core and attached fluid and electrical lines.
The 12.5-kilowatt (kW) Component Test Power Convertor (CTPC) developed for high-power dynamic energy conversion research. This system was used to evaluate power generation and thermal performance of advanced closed Brayton cycle technologies.
NASA

Thermal Transport

NASA Glenn researchers are advancing thermal management technologies for applications such as fission surface power (FSP), nuclear electric propulsion (NEP), and radioisotope power systems. Key technologies include high-temperature alkali metal heat pipes, which transfer heat between critical FSP or NEP subsystems, and advanced radiators that reject excess heat from power conversion systems.

These technologies are tested and evaluated in a range of conditions, including thermal-vacuum environments, microgravity, radiation exposure, and long-duration operations, to ensure performance and reliability for future space missions.

A Glenn engineer configures thermocouples on a heat pipe setup for space thermal testing at NASA Glenn.
Greeta Thaikattil, an engineer at NASA’s Glenn Research Center in Cleveland, configures a thermocouple array on a heat pipe experiment assembly. This setup is used to monitor temperature distribution during testing of thermal transport systems for space applications.
NASA
Engineer adjusts equipment for a sodium heat pipe test inside a large metal vacuum chamber with control panels and insulated wiring.
HX5 engineer Jim Sanzi prepares a high-temperature sodium heat pipe test in a vacuum chamber facility at NASA’s Glenn Research Center in Cleveland.
NASA

Reference Publications

Radiation Effects

Radiation is ubiquitous in space. In addition to natural background radiation, some NASA missions will induce harsh radiation. NASA is advancing technology for a nuclear reactor that would provide surface power for human space exploration. This reactor will irradiate electronics, sensors, structural materials, heat-rejection mechanisms, power conversion systems, and most other technologies flown on the mission.

The effects of radiation on the various systems associated with this thermal energy source need to be understood to ensure a successful mission. To investigate radiation effects, NASA has conducted irradiation experiments, used radiation transport and neutron transmutation models, assessed potential testing facilities, and more. This work is conducted in close collaboration with the U.S. Department of Energy.

NASA engineers and a graduate student stand beside reactor equipment and thermocouples, preparing for heat pipe tests in a high-bay lab environment.
Left to right: NASA engineers Greeta Thaikattil and Tyler Steiner, with University of Tennessee graduate student Luke Hansen, prepare for heat pipe irradiation testing at The Ohio State University Research Reactor.
NASA
Color map of a reactor model showing varying neutron flux, with red for high flux and blue for low, across structural components.
Modeled neutron flux distribution in a test reactor environment, with color gradients representing intensity variation across components.
NASA

Reference Publications

Aircraft Thermal Management

Thermal Recovery Energy Efficient System (TREES)

The Thermal Recovery Energy Efficient System (TREES), developed at NASA Glenn, demonstrates how thermoacoustic waves can be used to manage heat. Aircraft, spacecraft, and power systems often generate high-grade waste heat, which can pose challenges for thermal control.

TREES harnesses this thermal energy and uses oscillating pressure waves to convert high-grade waste heat into low-grade waste heat. This process critically reduces the burden on heat rejection systems and improves overall energy efficiency.

A long laboratory testbed with coiled metal tubing and components mounted on a frame, which is part of the TREES thermal management system.
The Thermal Recovery Exergy Efficient System (TREES) assembly used to manage high-grade thermal energy through the use of acoustic pressure waves.
NASA

Reference Publications

Strayton

Diagram of combined Stirling and Brayton thermodynamic cycle with labeled heat and work flows, plotted on temperature-entropy axes.
The Strayton (Stirling + Brayton) thermodynamic cycle, temperature (T) versus entropy (S). C: Compressor; H.S.: Heat source; PIC: Pressure of flow into compressor; POC: Pressure out of compressor; QIN: Heat energy into engine; QREGEN: Heat transferred in the recuperator; QREJ: Heat energy rejected out of engine; T: Turbine; TIC: Temperature of flow into compressor; TIT: Turbine inlet temperature; TOC: Temperature of flow out of the compressor; TOT: Temperature of flow out of the turbine; TQIN: Temperature of the heat into the engine; TREJ: Temperature of heat rejected from engine; WC: Work required for the compressor; and WT: Work produced by the turbine.
A diagram showing integration of Brayton and acoustic Stirling technologies into a Strayton system for energy conversion.
The Strayton concept integrates acoustic Stirling and Brayton thermodynamic cycles to leverage commercially available components for efficient thermal energy conversion. This system combines acoustic loop Stirling technology with Brayton-based compressors and turbines to maximize performance across a range of power applications.
NASA

Reference Publications

Contact

Area of ExpertiseNameEmail
Stirling Engine SystemsScott Wilsonscott.d.wilson@nasa.gov
Fission Surface PowerSalvatore Oritisalvatore.m.oriti@nasa.gov
Aircraft Thermal ManagementLuis Rodriguezluis.a.rodriguez-1@nasa.gov