Product Water Recovery and Conditioning Loop
Closed-Loop Water Reservoir, Deionizer, and Quality Monitor
Closed-loop water system that captures liquid water produced by the fuel cell, removes residual ionic contamination and dissolved gases, stores it in an insulated reservoir, and returns it to the electrolyzer — closing the H2O ↔ H2+O2 cycle of the regenerative fuel cell.
Purpose
Achieve a hermetic, low-loss water inventory loop so the RFC requires only periodic make-up water (from L1-WTR) for radiation-induced loss, leakage, and venting, rather than continuous resupply.
Context
Sits between L3-ESS-FC-STACK (water producer) and L3-ESS-FC-ELEC (water consumer). Interfaces with L1-WTR for make-up water from lunar ISRU ice. Monitors for any contamination that could poison the electrolyzer or fuel cell electrodes.
Principles
- ▸Stoichiometric water balance: every kg of H2 cycled produces/consumes 9 kg of water; for 100 kg H2 inventory, ~900 kg water cycling annually
- ▸DI water purity must exceed 1 MΩ·cm resistivity for PEM electrolyzer operation; mixed-bed ion-exchange resins polish recycled water
- ▸Dissolved-gas removal (degasser) prevents gas pocket formation in electrolyzer cells
- ▸Conductivity, resistivity, and TOC (total organic carbon) sensors monitor water quality in real time
- ▸Closed-loop water systems on ISS achieve <1% loss per year using catalytic and ion-exchange polishing
- ▸Reservoir size set by the larger of: peak transient demand mismatch and 1–2 days of operating buffer
Typical implementations
- ▸ISS Water Processor Assembly (WPA) — closed-loop water management with multi-filtration
- ▸Apollo fuel cell water tanks — heritage closed-loop water system
- ▸Shuttle Orbiter potable-water reservoirs supplied by fuel cells
- ▸Lunar Pathfinder and Artemis Foundational Surface Habitat (FSH) water reuse concepts
- ▸Terrestrial PEM electrolyzer water polishing units (Veolia, Suez)
Lunar considerations
- ▸ISRU water from lunar ice may contain dissolved gases, sulfur compounds, and dust particles — multi-stage purification required
- ▸1/6 g gravity reduces buoyancy-driven mixing but does not eliminate it; pumps required for circulation
- ▸Freeze protection: reservoir must remain above 0 °C during all operating modes (heated jacket or thermal interface to L1-TCS)
- ▸Radiolysis from cosmic radiation slowly decomposes water into H2 + O2 over years — small replenishment from ISRU expected
- ▸Long-term ion-exchange resin life under radiation is uncertain — periodic replacement via L1-MNT recommended
- ▸Microbial growth in stagnant water lines mitigated by UV sterilization and low-residence-time design
Specifications
Functional
| primary function | Capture, purify, store, and recirculate water in the RFC loop |
| inputs | Product water from L3-ESS-FC-STACK (~6 g/s peak during discharge), Make-up water from L1-WTR (~1 kg/day to compensate radiolysis), Power from L1-PDM (pumps, sensors), Thermal interface from L1-TCS (freeze protection) |
| outputs | Polished DI water to L3-ESS-FC-ELEC at controlled flow rate, Water-quality telemetry (resistivity, TOC, dissolved oxygen), Reservoir level and temperature telemetry, Vent gas (dissolved gas degasser output) to safe vent |
| reservoir volume l | 50 |
| feed water resistivity mohm cm min | 5 |
| max total organic carbon ppb | 50 |
| make up rate kg per day max | 5 |
| internal water loop loss percent per year | 1 |
| freeze protection temperature c min | 5 |
Physical
| mass kg | 25 |
| dimensions | 400 × 400 × 600 mm (reservoir + polishing skid) |
| materials | 316L stainless steel reservoir, Mixed-bed ion-exchange resin cartridges (replaceable), PTFE / PFA tubing for chemical inertness, Diaphragm metering pumps (rad-tolerant brushless motors), UV-C LED sterilizer (germicidal), Foam / vacuum insulation |
| operating temperature c | 5, 60 |
| vacuum compatibility | Hermetically sealed loop |
Operational
| power consumption w | 30 |
| thermal range c | 5, 60 |
| lifetime years | 15 |
| mtbf hours | 100000 |
| resin replacement interval years | 5 |
Interfaces
Provides
- Polished DI water at controlled flow
- Water quality and reservoir state
Requires
- Product water capture
- ISRU-derived make-up water (~1 kg/day)
- DC power for pumps and sterilizer (~30 W)
- Heat injection to maintain >5 °C during shutdown
Cite this entry
Lunar Ark Codex. "Product Water Recovery and Conditioning Loop" (L3-ESS-FC-WTR). Retrieved 10 September 2026, from https://lunarark.com/entry/L3-ESS-FC-WTR
Licensed CC-BY-SA 4.0. You may reuse and adapt this entry with attribution, under the same licence.