L2-ESS-THRM ESSENTIAL POWER ENERGY Level 2 · hardware

Thermal Energy Storage

Phase-Change Thermal Battery System (PCTBS)

The Phase-Change Thermal Battery System (PCTBS) is a thermal energy storage assembly that absorbs high-temperature surplus heat—including 20 to 30 kWt of nuclear reactor waste heat—into metallic or salt-based phase-change materials melting between 400°C and 800°C. Storing over 200 kWh-thermal as latent heat of fusion, the system discharges 3 to 8 kWt during the 14-day lunar night for direct habitat heating, equipment freeze prevention, or electrical conversion at 10 to 30 percent efficiency. Vacuum-insulated containment supplemented by lunar regolith exploits the vacuum environment's lack of convective heat transfer, restricting parasitic losses to under 0.5 percent per day across more than 50,000 melt-freeze cycles.

Thermal energy storage using phase-change materials to capture waste heat and solar thermal energy for later retrieval as heat or electrical conversion

Purpose

Absorb and store thermal energy from reactor waste heat, solar thermal collectors, and other heat-producing systems during surplus periods, then release it during lunar night for habitat heating, process heat, or thermoelectric conversion to electricity

Context

L2-ESS-THRM complements the electrochemical storage by utilizing a different energy domain (thermal vs. electrical). The nuclear reactor produces ~20-30 kWt of waste heat that can be captured in phase-change materials rather than radiated away. During lunar night, this stored heat can warm habitats, prevent equipment freezing, or be converted back to electricity via thermoelectric generators. Phase-change materials offer high energy density per unit mass, zero self-discharge over long dormancy, and extremely long cycle life since they involve no electrochemical degradation.

Principles

▸Phase-change materials store energy as latent heat of fusion at a nearly constant temperature
▸Metallic PCMs (e.g., aluminum-silicon alloys) offer very high volumetric energy density and thermal conductivity
▸Salt-based PCMs (e.g., LiF-based eutectics) provide high gravimetric energy density at moderate temperatures
▸Thermal-to-electric conversion via thermoelectric generators or Stirling engines recovers stored thermal energy as electricity
▸Heat exchangers transfer thermal energy between PCM storage, heat transport fluid, and end-use loads

Typical implementations

▸Metallic PCM thermal batteries for spacecraft (Al-Si eutectic, Tm ~577C, ~400 kJ/kg)
▸NASA Glenn Research Center thermal energy storage for Stirling power conversion
▸LiF-CaF2 eutectic PCM (Tm ~767C, ~520 kJ/kg) for high-temperature storage
▸Sodium heat pipes for thermal transport from reactor to storage
▸Thermoelectric generators (TEGs) using skutterudites or half-Heusler alloys

Lunar considerations

▸Vacuum environment means no convective heat loss; insulated storage tanks retain heat very efficiently
▸Reactor waste heat (~20-30 kWt) is available continuously and would otherwise be radiated to space
▸Thermal storage can directly heat the habitat during lunar night without electrical conversion losses
▸Regolith insulation can supplement engineered thermal insulation for storage tanks
▸No atmosphere means thermal radiator area must be sized for worst-case rejection during discharge to electrical loads
▸Long dormancy periods: solidified PCM retains latent energy indefinitely until melting point is reached again

Specifications

Functional

primary function	Store thermal energy in phase-change materials and deliver it as heat or convert to electricity on demand
inputs	Waste heat from L1-PWR nuclear reactor via heat transport loop, Solar thermal energy from concentrated solar collectors (during lunar day), Charge commands from L2-ESS-MGMT
outputs	Thermal energy to L1-TCS for habitat heating and equipment thermal conditioning, Electrical power via thermoelectric/Stirling conversion to L1-PDM (secondary output), Temperature, charge state, and health telemetry to L2-ESS-MGMT
thermal capacity kwh t	200+ kWh-thermal total stored
charge rate kwt	5-10 kWt from reactor waste heat loop
discharge rate kwt	3-8 kWt for heating loads
electrical conversion efficiency percent	10-20% (thermoelectric), up to 30% (Stirling)
pcm cycle life	>50,000 melt-freeze cycles (metallic PCMs effectively unlimited)
operating temperature c	400-800 (PCM melt range, depending on material selection)
thermal loss rate percent day	<0.5% per day with vacuum-insulated containment

Physical

materials	Al-Si eutectic phase-change material (~577C melt point), LiF-CaF2 eutectic PCM (~767C melt point), Inconel or Haynes alloy containment vessels (high-temperature corrosion resistance), Multi-layer insulation (MLI) for vacuum thermal isolation, Sodium or NaK heat pipe wicks and envelopes, Skutterudite or BiTe thermoelectric generator modules
temperature range c	400 to 800 (internal PCM operating); external vault near ambient lunar
radiation	Shielded within vault; minimal impact on passive thermal storage materials
dust	Sealed system; no direct exposure
vacuum	Beneficial for thermal insulation; MLI performs optimally in hard vacuum

Operational

power consumption w	100
thermal range c	400, 800
lifetime years	100
mtbf hours	500000

Interfaces

Provides

→Active Thermal Control Systemthermalessential
Stored thermal energy for habitat heating and equipment thermal conditioning during lunar night
→Power Distribution & Managementpoweressential
Electrical power from thermal-to-electric conversion (thermoelectric/Stirling), secondary power path
→Battery/Storage Management Systemdataessential
PCM temperature, melt fraction (charge state), heat exchanger performance, and fault telemetry

Requires

←Power Generation Systemthermalessential
Reactor waste heat via heat transport loop for PCM charging
←Active Thermal Control Systemthermalessential
Heat transport fluid loop interface for thermal energy delivery to end-use loads
←Battery/Storage Management Systemdataessential
Charge/discharge commands, thermal scheduling directives, and mode control
←Primary Structuremechanicalcritical
High-temperature containment vault, structural support for heavy PCM tanks, and thermal isolation mounts
←Power Distribution & Managementpoweressential
Electrical power for pumps, valves, control electronics, and thermoelectric module parasitic loads