Quick answer: Size a BESS container dehumidifier from the moisture that enters and must be removed, not from the container's length. The inputs are: free air volume, outdoor design condition (humid season), air leakage, any ventilation, door openings for service, the pull-down needed after doors close, and the target dew point (below the coldest internal surface). For a well-sealed container, the steady leakage load is often only a few litres per day, while door openings and the recovery after service can be as large or larger; size for steady load plus recovery within an agreed time, at the container's internal condition.
Quick answer
- Load = leakage + ventilation + door events + pull-down after service.
- Target: dew point a margin below the coldest surface (cooling plates, walls at night).
- "20 ft" or "40 ft" is not a capacity class; free air volume and leakage are.
The inputs
| Input | Why it matters |
|---|---|
| Free air volume (m³) | Container volume minus racks and equipment; sets pull-down mass |
| Outdoor design condition | Humid-season temperature and RH (moisture content g/kg) |
| Internal temperature profile | Cooling setpoints, coolant temperatures, night cooling |
| Air leakage | Door seals, cable entries, vents; often 0.05–0.3 air changes per hour for sealed containers (verify) |
| Ventilation | Any required outdoor air (by design or for safety systems) |
| Door openings | Frequency, duration, number of doors |
| Moisture already inside | After shipping, construction or long standby |
| Control target | Dew point (or RH at temperature) with a margin to the coldest surface |
| Product performance | Dehumidifier capacity at the container's actual condition |
| Installation | Mounting, drainage, power (AC or DC), controls |
ASHRAE Guideline 10 addresses interactions affecting indoor environmental conditions, including humidity (ASHRAE).
Worked example (illustrative)
Container: free air volume about 40 m³. Outdoor design: 35 °C, 80 % RH (about 28.9 g/kg). Target: dew point 10 °C (about 7.6 g/kg), so each kg of outdoor air entering brings about 21.3 g of water to remove.
| Load | Assumption | Water |
|---|---|---|
| Leakage | 0.1 ACH = 4 m³/h | about 0.1 kg/h ≈ 2.3 L/day |
| Door events | One service visit replaces about twice the free volume (80 m³) | about 2.0 L per visit |
| Pull-down after service | Bring 40 m³ from outdoor to target | about 1.0 L, e.g. within 2 hours = 0.5 L/h |
| Ventilation | None in this example | — |
Selection: steady load about 2.3 L/day plus recovery capacity of about 0.5 L/h after service. A compressor cabinet unit rated 6–10 L/day at 30 °C / 80 % RH could cover this, provided its capacity at the container's internal condition is checked; Peltier units suit only smaller or very tight enclosures. If the container has continuous ventilation, its airflow usually dominates and must be added.
Check the target against cold surfaces
If liquid-cooling plates can run at 15 °C, a 10 °C dew point gives a 5 K margin. If plates can go to 12 °C, lower the target or limit coolant temperatures. See charge–discharge condensation risk and condensation in battery enclosures.
Practical sizing worksheet
- Free air volume: ___ m³
- Outdoor design: ___ °C / ___ % RH → ___ g/kg
- Target dew point: ___ °C → ___ g/kg
- Δx = (2) − (3)
- Leakage: ___ ACH × volume × 1.15 kg/m³ × Δx = ___ kg/h
- Ventilation: ___ m³/h × 1.15 × Δx = ___ kg/h
- Door events: ___ m³ per visit × 1.15 × Δx = ___ kg per visit
- Pull-down: volume × 1.15 × Δx ÷ recovery hours = ___ kg/h
- Design capacity at internal condition ≥ (5) + (6) + recovery (8), plus margin
(1.15 kg/m³ is the approximate density of warm air; use site values at altitude.)
Verification after selection
- Log internal dew point and coldest surface temperature for a full humid week, including a service visit.
- Check recovery time after doors close.
- Confirm drainage and alarms.
- See the BESS commissioning checklist.
Our energy storage dehumidifiers and electrical cabinet dehumidifiers include compressor units of 4–10 L/day and Peltier units of 0.5–1 L/day.
FAQ
Frequently asked questions
What size dehumidifier does a 40 ft BESS container need?
It depends on leakage, ventilation, door openings and the target dew point, not the length. Sealed containers often need a few litres per day plus recovery capacity after service.
Why is door opening so important?
Each service visit can bring in as much moisture as a day or more of leakage.
Should the target be RH or dew point?
Dew point, set a margin below the coldest internal surface.
Sources
- ASHRAE Guideline 10 (addendum, 2023).
- ASHRAE Handbook—Fundamentals, Psychrometrics.
