Cabin thermal & humidity control
Removes metabolic and equipment heat and controls humidity in habitat air by circulating it across a condensing heat exchanger, holding temperature and dew point in the comfort/safety band. The condensed water (kilograms per crew per day) is captured and routed to water recovery, so humidity control is also a water-loop input. Reject heat goes to the thermal bus and ultimately the vacuum radiator. It guards against condensation damage and microbial growth as much as against discomfort.
Governing equations
Total cabin cooling load = sensible (temperature) + latent (moisture) heat, removed as the air gives up enthalpy across the condensing exchanger. The latent part is large because crew add water continuously. [1]
To dehumidify, the heat-exchanger surface must run below the cabin dew point so water condenses out — the defining requirement, and the reason a "thermal" system is also the humidity system. [2]
Latent water load per crew member (respiration + perspiration) — captured as condensate and sent to water recovery, a major reclaimed-water stream. [3]
All captured heat must ultimately leave the habitat — handed to the coolant/thermal bus and rejected by the vacuum radiator to the Martian environment. [2]
Key constants & quantities
| Symbol | Value | Units | Conditions | Description |
|---|---|---|---|---|
| Cabin temperature | 18–27 | °C | — | Crew comfort band the system holds against continuous internal heat gain.[4] |
| Relative humidity | 25–70 | % | — | Target humidity band — high enough for comfort/health, low enough to prevent condensation and microbial growth.[4] |
| Latent water / crew | 1.8–2.3 | kg/day | — | Respiration + perspiration water captured as condensate per crew member — a primary feed to water recovery.[3] |
| Metabolic heat / crew | 100–150 | W (avg, activity-dependent) | — | Sensible + latent heat each crew member adds to the cabin — multiplied across crew and equipment into the cooling load.[3] |
| Air circulation | 4–10 | cabin volumes / hour | — | Ventilation rate to avoid stagnant pockets (CO₂ buildup, condensation) — microgravity needs forced flow; Mars gravity helps but forced circulation is still required.[1] |
Operating envelope
Mass balance
Basis: crew of 4, steady-state thermal/humidity control (per day)
Inputs
| Cabin air (circulated) | 1 | continuous flow | [1] |
| Coolant (chilled loop) | 1 | loop | [2] |
| Fan + pump power | 5 | kWh/day | [3] |
- Cabin air (circulated): 4-10 volumes/h across the condensing exchanger.
- Coolant (chilled loop): Provides the sub-dew-point surface; rejects to thermal bus → vacuum radiator.
- Fan + pump power: Air-circulation fan and coolant pump for a crew of 4.
Direct power is just fans and pumps; the heat itself is moved, not created, and ultimately radiated. On cold Mars some reject heat is usefully recycled to keep the habitat warm — thermal control becomes heat MANAGEMENT, balancing rejection against recovery.
Variants & trade-offs
Condensing heat exchanger + slurper (baseline)
[1]Air driven across a chilled finned coil; condensate forms on the fins and is collected (in microgravity by a "slurper," on Mars assisted by gravity drainage) and routed to water recovery.
- Combines cooling and dehumidification in one unit
- Recovers latent water for the water loop — flight-proven
- Wet surfaces are microbial-growth sites — biofilm control needed
- Coolant loop and radiator dependency
When preferred: Primary habitat thermal/humidity control — the standard approach.
Desiccant dehumidification
[3]A regenerable desiccant wheel/bed adsorbs humidity, decoupling moisture removal from the cold coil — useful where condensation control or very low humidity is wanted.
- Avoids cold wet surfaces (microbial benefit); reaches low humidity
- Decouples latent from sensible load
- Regeneration energy; desiccant is a consumable/import
- Captured water needs a separate recovery path
When preferred: Low-humidity or condensation-sensitive zones; complements the condensing coil.
Heat-recovery (warm-side integration)
[2]On cold Mars, reject heat from cooling is fed back to warm other habitat volumes or processes rather than simply radiated away.
- Turns "waste" cooling heat into habitat heating — net energy win in a cold world
- Reduces both heating and radiator loads
- Added plumbing/control complexity; balances shifting loads
When preferred: Mars settlements where habitat heating demand can absorb the rejected heat.
Failure modes
| Mode | Cause | Detection | Mitigation |
|---|---|---|---|
| Microbial / mold growth on wet surfaces[1] | Condensing surfaces and humid pockets are perfect microbial habitat; biofilm fouls coils and degrades air quality. | Microbial sampling, coil inspection, odor/air-quality monitoring. | Antimicrobial coil surfaces, biocide in coolant/condensate, adequate circulation to prevent stagnant humid pockets, periodic cleaning. |
| Condensation pooling / equipment damage[2] | Poor condensate capture or cold spots (thermal bridges) let water pool on or in equipment and structure. | Humidity sensors, cold-spot IR survey, leak/moisture detection. | Effective condensate collection, eliminate cold bridges, vapor barriers, adequate dehumidification capacity. |
| Circulation fan failure → stagnant pockets[1] | Loss of forced air circulation lets CO₂ and humidity stratify into dangerous local pockets (acute in low/microgravity). | Airflow monitoring, distributed CO₂/humidity sensors. | Redundant fans, distributed circulation, monitoring for stagnant zones. |
| Coolant loop / radiator loss[2] | Loss of the chilled coolant or radiator capacity removes the cold surface; cabin heats and humidity rises. | Coolant temperature/flow, radiator performance monitoring. | Redundant coolant loops, radiator margin, load-shed plan; ties to the vacuum-radiator and thermal-bus design. |
| Over-drying[4] | Excessive dehumidification drops humidity too low — crew discomfort, static, mucous-membrane and respiratory irritation. | Humidity monitoring against the lower comfort bound. | Humidity control band with a floor, modulating dehumidification, humidity add-back if needed. |
Mars adjustments
Gravity helps condensate management[1]
Impact: Unlike microgravity (which needs "slurpers" to corral floating condensate), Mars 0.38 g lets condensate drain — simplifying collection and routing to water recovery versus the ISS.
Mitigation: Gravity-drained condensate collection; simpler than orbital designs but still positively managed.
Heat rejection vs heat retention balance[2]
Impact: In a -60 °C world the habitat usually needs heating, so "cooling" the cabin often means moving heat to where it's wanted, not dumping it. Thermal control becomes a recovery/balance problem, not pure rejection.
Mitigation: Heat-recovery integration to the thermal bus; radiate only genuine surplus via the vacuum radiator.
Humidity control is a water-loop input[3]
Impact: The ~2 kg/crew·day of captured condensate is a major reclaimed-water stream — THC is not just comfort, it is a front-end of water recovery, tightly coupled to the water loop.
Mitigation: Route condensate to water-recovery; account it in the settlement water balance.
Microbial control matters more long-term[1]
Impact: A permanent settlement runs for years; wet surfaces that an ISS expedition tolerates become serious biofilm/mold reservoirs over long durations.
Mitigation: Antimicrobial surfaces, biocide management, design for cleanability, monitoring as a long-duration health metric.
Greenhouse coupling[5]
Impact: Agricultural zones transpire large amounts of water; their humidity/thermal control overlaps with crop water recovery, linking THC to the agriculture and bioregenerative loops.
Mitigation: Integrate greenhouse condensate recovery with cabin THC and the water loop; manage the much higher greenhouse humidity load.
Alternatives & substitutes
Passive cooling to the Mars environment[2]
- A cold ambient can passively pull heat through the envelope — partial free cooling
- Uncontrolled; can't manage humidity; risks over-cooling at night
When preferred: Supplemental heat rejection only; never humidity control.
bioregenerative-life-support (plant transpiration balance)[5]
- Plant transpiration and uptake participate in the water/humidity balance
- Adds humidity (transpiration) rather than removing it; needs its own condensate capture
When preferred: Greenhouse zones, where transpiration recovery is itself a water source — coupled to, not replacing, THC.
Requires
References
- (1998). Living Together in Space: The Design and Operation of the Life Support Systems on the International Space Station. NASA Marshall Space Flight Center, NASA/TM-98-206956. NASA/TM-98-206956. — NASA Baseline Values & Assumptions (BVAD); LiOH, amine, and zeolite scrubber trade study.
- (2017). Fundamentals of Heat and Mass Transfer, 8th Edition. John Wiley & Sons. ISBN 978-1-119-32042-5. — Standard undergraduate / engineering reference for heat transfer: Stefan-Boltzmann radiation, conduction, convection.
- (2018). Life Support Baseline Values and Assumptions Document (BVAD). NASA Johnson Space Center. NASA/TP-2015-218570/REV1. — The authoritative ECLSS reference: crew metabolic rates, consumable mass balances, atmosphere/water/waste loop sizing, and life-support technology trades.
- (2023). NASA Space Flight Human-System Standard, Volume 2: Human Factors, Habitability, and Environmental Health. NASA. NASA-STD-3001 Vol. 2 Rev. C. — Cabin CO₂ partial-pressure limits; crew habitat environmental health standard.
- (2010). MELiSSA: The European project of closed life support system. Gravitational and Space Biology, 23(2), 3-12. — ESA Micro-Ecological Life Support System Alternative project — closed-loop bioregenerative life support architecture; mature analog for Mars closed-loop ECLSS + agriculture.