Thermal-to-Electrical Converter
Free-Piston Stirling Engine with Linear Alternator
Free-piston Stirling engine (FPSE) with linear alternator that converts stored thermal energy from the PCM tank (via L3-ESS-THRM-HEX hot side) into ~5 kWe of electrical power for the Ark DC bus when needed during lunar night discharge events.
Purpose
Provide an alternative electrical generation path that draws on the abundant stored thermal energy rather than on chemical reactants, augmenting the RFC and battery storage with a different physics for resilience and lifetime balance.
Context
Hot end interfaced with L3-ESS-THRM-HEX (discharge side); cold end rejected to L1-TCS deep-space radiator. Power output through rectifier to L1-PDM. Coordinated with L2-ESS-MGMT dispatch. Leverages same Stirling heritage as L2-PWR-NUC's reactor converters.
Principles
- ▸Free-piston Stirling engine (FPSE) has no mechanical seals/bearings — gas-bearing-supported piston/displacer in helium working gas
- ▸Linear alternator integrated with piston converts reciprocating motion directly to AC; rectifier produces DC bus output
- ▸Carnot efficiency limit (1 – T_cold/T_hot) bounds maximum efficiency; practical FPSE achieves 25–35% of input thermal energy as electricity
- ▸Hot-side temperature 800 °C and cold-side 100 °C give η_Carnot ≈ 65%, η_practical ≈ 30%
- ▸Hermetic helium charge maintained for decades by all-welded construction
- ▸Power output controlled by stroke amplitude — proportional to load
- ▸Sunpower / Infinia / NASA / DOE Stirling generations 1–6 demonstrate this technology
Typical implementations
- ▸NASA Advanced Stirling Convertor (ASC) for MMRTG replacement — 80 We, >100,000 hr life
- ▸Sunpower SunMaxx Stirling generators (terrestrial)
- ▸Infinia Corporation 1 kWe Stirling generators
- ▸NASA Kilopower 10 kWe with Stirling converters (KRUSTY heritage)
- ▸Lockheed Martin Sunflower concept (CSP + Stirling)
- ▸DOE / NASA Free-Piston Stirling Engine (FPSE) development line
Lunar considerations
- ▸Cold-side radiator size dominated by lunar day-night temperature swing; PSR-adjacent siting helps
- ▸Long-life gas-bearings demonstrate >100,000 hr operation in flight-relevant tests
- ▸Stirling reliability favored over turbines for unattended operation (no fluid film bearings to maintain)
- ▸Vibration counter-balanced by paired opposed-piston configuration
- ▸Helium leak rate must be vanishingly low — all-welded titanium construction
- ▸Compatible with same control architecture as reactor Stirling — software re-use
Specifications
Functional
| primary function | Convert stored heat to electrical power on demand |
| inputs | Heat from L3-ESS-THRM-HEX (hot side, 600–800 °C), Cold-side rejection to L1-TCS radiator (100 °C), Helium working gas charge (sealed at manufacture), Stroke setpoint from L3-ESS-THRM-CTRL |
| outputs | AC electrical output rectified to DC for L1-PDM (~5 kWe), Cold-side waste heat to radiator, Telemetry: piston position, alternator current, hot/cold temperatures |
| rated electrical output kwe | 5 |
| hot side temperature c | 600, 800 |
| cold side temperature c | 80, 120 |
| thermal to electric efficiency percent | 30 |
| operating frequency hz | 60 |
| design lifetime hours | 100000 |
| working fluid | Helium at 100 bar charge |
Physical
| mass kg | 60 |
| dimensions | 600 × 400 × 400 mm |
| materials | Inconel 718 hot-end heater head, Stainless 316 cold-end heat sink, Titanium pressure vessel, Samarium-cobalt linear alternator magnets (high-T stable), Iron-cobalt alternator core laminations, Gas-bearing materials (no lubrication) |
| operating temperature c hot | 800 |
| operating temperature c cold | 100 |
| vibration qual grms | 12 |
| ultimate load g | 6 |
Operational
| power consumption w aux | 50 |
| thermal range c | 50, 800 |
| lifetime years | 15 |
| mtbf hours | 100000 |
| design starts min | 5000 |
Interfaces
Provides
- 5 kWe DC electrical output
- Cold-side waste heat at 100 °C (~12 kW thermal)
- Operating telemetry
Requires
- Hot-side heat input ~17 kW thermal
- Cold-side radiator interface
- Stroke/load setpoint
Cite this entry
Lunar Ark Codex. "Thermal-to-Electrical Converter" (L3-ESS-THRM-CONV). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-ESS-THRM-CONV
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.