How to Specify a Hydrogen Storage Tank for a Fuel-Cell UAV
For a hydrogen-powered UAV, the storage tank is the single component that limits flight time, payload, and range. Get the specification wrong and the airframe won’t fly. This guide walks through the parameters drone integrators need to define before issuing an RFQ.
Why hydrogen storage is the limiter
A fuel-cell UAV converts compressed hydrogen and ambient oxygen into electricity. Flight time is essentially:
Flight time ≈ (H2 mass) × (energy density) × (fuel-cell efficiency) / (UAV power demand)
You can’t increase fuel-cell efficiency much (~50–55% is the ceiling for current PEM systems). You can’t reduce power demand without removing payload. The lever you control is how much hydrogen you can carry — which is set by the tank.
The six parameters that matter
1. Hydrogen mass required
Start from the mission. For a typical small UAV (5–25 kg gross weight) running on a fuel cell, hydrogen consumption is roughly 40–120 g per hour depending on power demand. Multiply by your target flight time:
- 1 hour mission: 40–120 g H2
- 2 hour mission: 80–240 g H2
- 4 hour mission: 160–480 g H2
Add a 20–30% reserve. This is your design hydrogen mass.
2. Working pressure (300 / 500 / 700 bar)
Hydrogen is stored compressed. Higher pressure means more mass per litre of tank volume:
- At 300 bar: ~24 g H2 per litre
- At 500 bar: ~37 g H2 per litre
- At 700 bar: ~46 g H2 per litre
700 bar gives ~93% more H2 per litre than 300 bar — but at the cost of heavier tank walls. For most UAV missions 300–500 bar is the sweet spot. 700 bar is reserved for missions where volume is constrained more than mass.
3. Tank capacity (litres)
Capacity = (H2 mass / pressure-density). Worked example for a 2-hour mission at 100 g/h:
- Required H2: 200 g + 25% reserve = 250 g
- At 300 bar: 250 / 24 ≈ 10 L tank
- At 500 bar: 250 / 37 ≈ 7 L tank
- At 700 bar: 250 / 46 ≈ 5.5 L tank
For sub-1-hour missions on small UAVs, the HDRX-030 (3L, 300 bar) covers most cases. Larger missions need custom designs.
4. Tank mass (the key trade-off)
Mass is what makes the difference between a Type 4 COPV and a metal cylinder. For a 3L 300 bar cylinder:
- All-metal: ~5–6 kg
- Type 3 (aluminium-lined): ~2.5 kg
- Type 4 (polymer-lined CFRP): ~1.3 kg (HDRX-030)
That 4-kg savings versus all-metal is approximately 5–10 minutes of additional flight time for a typical small UAV, or equivalent payload uplift.
5. Mounting and form factor
Cylinder dimensions matter for airframe integration. Standard cylindrical COPVs are common, but custom geometry is available for tight envelopes (conformal tanks, shorter aspect ratios, or spherical geometry where layout permits). When you RFQ, specify:
- Maximum diameter
- Maximum length
- Mounting style: end-boss, mid-strap, custom bracket
- Thread / valve interface (M18, M25, AN, custom)
6. Cycle life and qualification
How many fill/empty cycles will the tank see in service? For commercial UAV operations expect 1,000–5,000 cycles over the airframe lifetime. MEYER Type 4 COPVs are designed for 10,000+ cycles with No Limited Lifespan (NLL).
If your operation requires civil aviation certification (e.g. EASA SC-VTOL or FAA Part 135), specify that upfront — it adds qualification testing requirements (drop test, fire test, gunfire test) that affect lead time.
Don’t forget the regulator
The tank stores hydrogen at high pressure. The fuel cell consumes it at low pressure (typically 0.5–1 bar). You need a high-pressure regulator between them. For drone applications the HDRX-R450 is purpose-built for this duty: 450 bar inlet, 0–10 bar outlet, hydrogen-embrittlement-resistant materials.
RFQ checklist
When you contact a COPV manufacturer, have these answers ready:
- UAV class and gross weight
- Mission profile (flight time, power demand)
- Required H2 mass (with reserve)
- Working pressure (300 / 500 / 700 bar)
- Volume envelope (max D × L)
- Target tank mass (kg)
- Mounting / interface preferences
- Annual production volume
- Certification requirements (if any)
With those, an experienced supplier can give you a feasibility answer in days, not weeks.
