Thermal Feasibility of a Hybrid MMRTG–Stirling Generator: A Three-Dimensional Finite-Volume Study of Fin Waste-Heat Extraction

Title: Thermal Feasibility of a Hybrid MMRTG–Stirling Generator: A Three-Dimensional Finite-Volume Study of Fin Waste-Heat Extraction Abstract:The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) converts about 5.5% of its roughly 2 kW thermal output into electricity and rejects the rest through eight radial aluminium fins. A hybrid concept would harvest part of this waste heat with heat pipes feeding a Stirling converter. We test this concept with a custom three-dimensional finite-volume conjugate heat-transfer model (conduction in Al-6063 fins, full T^4 radiation to a 4 K sink) of a fin carrying a heat-pipe block, for two root conditions: a fixed 1750 W flux (Case B) and a fixed 450 K root temperature (Case A). With the Stirling hot head at 430 K, the premise of the concept, the heat pipes do not extract heat but inject it: QHP = -311 W for four pipes in Case B, because the fins operate at 354–388 K. The break-even hot-head temperature is 357.5 K. For hot-head temperatures of 300–340 K, the four specified heat pipes carry 69–217 W; assuming a Stirling efficiency of 39% of Carnot and a 280 K cold end, this corresponds to only 5–7 We of additional electrical power and 0.2–0.7 m² of extra radiator. Case A would reject 2.9 kW, more than the heat source supplies, and is therefore retained only as a sensitivity case. The solver reproduces a 1D fin solution within 0.4% and is mesh-independent to a grid convergence index below 0.8%. Within the stated model assumptions, the 430 K premise is infeasible and the specified collector yields a marginal gain.