Lunar Thermosiphon System (LTS): A Passive Organic Rankine Cycle for In-Situ Lunar Power Generation

Description This preprint introduces the Lunar Thermosiphon System (LTS)—a novel passive power-generation architecture designed for sustained lunar surface operations. The system exploits the extreme and persistent thermal gradient between sunlit regolith (~400 K) and permanently shadowed crater interiors (~100 K) near the lunar poles to produce continuous electrical power without nuclear fuel. LTS operates as a gravity-assisted Organic Rankine Cycle (GA-ORC) using a binary ammonia–water working fluid derived from Kalina cycle principles. By combining solar-heated evaporator arrays deployed on crater rims with radiatively cooled condenser networks suspended into shadowed regions, the system converts natural lunar temperature differences directly into turbine shaft work and electricity. A detailed thermodynamic and fluid-dynamic model is developed, accounting for reduced lunar gravity, two-phase flow behavior, pressure losses, cryogenic freezing risks, radiative heat rejection, and passive circulation requirements. Performance estimates indicate that a 30% ammonia mixture operating near 35 bar can achieve multi-kilowatt continuous output with radiator-limited scaling, offering a potential alternative to radioisotope thermoelectric generators and large solar-plus-storage installations for polar lunar outposts. The paper further examines material selection, vacuum tribology, micrometeoroid protection, radiation effects, start-up from frozen conditions, system control strategies, economic considerations, and phased implementation from Earth-based testing to lunar pilot deployment. This work is intended to stimulate discussion and early-stage feasibility assessment of non-nuclear baseload lunar power systems that leverage in-situ environmental conditions rather than imported energy sources.