Storing hydrogen: composite tanks at 700 bar
A hydrogen car carries its fuel at 700 times atmospheric pressure in a tank a fraction the weight of steel. How carbon-fibre pressure vessels work, why the fibre angle is the design, and what 'Type IV' means.
The tank is the hard part
Hydrogen has wonderful energy per kilogram and terrible energy per litre — it is the lightest, least dense gas there is. To carry a useful amount in a car you have to squeeze it to around 700 bar, seven hundred times atmospheric pressure.
A steel tank strong enough to hold that would be far too heavy to be practical. So the enabling technology of hydrogen mobility is not really the fuel cell — it is a pressure vessel light enough to carry the fuel at all. That vessel is a composite.
How a COPV works
A carbon-fibre-overwrapped pressure vessel, or COPV, splits the job in two. A thin inner liner is only a gas barrier: it holds the hydrogen in but carries almost none of the load. Around it, a thick shell of carbon fibre wound in resin carries all of the pressure.
That division is the trick. The carbon does the structural work at a fraction of steel's weight, while the liner simply stops the small, mobile hydrogen molecules leaking through the wall. Neither could do the job alone.
Fibre angle is the design
A cylinder under internal pressure feels twice as much stress around its circumference — the hoop direction — as it does along its length. So the fibre is not wound at random. Nearly circumferential hoop windings resist the large hoop stress; shallower helical windings carry the axial load and wrap over the domed ends.
The winding pattern is engineered to match the stress field, the same fibre-angle tailoring as a flat laminate but wrapped onto a body of revolution. It is filament winding, and it is why the angle, not just the amount, of fibre is the design.
Type I to Type V
Tanks are graded by how much composite has replaced metal. Type I is an all-metal cylinder. Type III uses a thin metal liner under a full carbon overwrap. Type IV uses a plastic liner — polyethylene or nylon — as barrier only, fully overwrapped with carbon, and it is the standard for 700-bar automotive hydrogen. Type V is linerless, all composite, the lightest and newest, still solving the problem of hydrogen slowly permeating through the bare wall.
The energy branch of Make shows this ladder as a bar that fills with composite from Type I to Type V — the whole history of how the hydrogen tank got light.
Why provenance matters here
A tank that fails at 700 bar fails violently, so COPVs are among the most rigorously tested structures ever made — proven by burst tests, tens of thousands of pressure cycles, and even gunfire tests, built up a validation pyramid much like an aircraft's.
It is a vivid case of this site's core idea: when the stakes are this high, a material property is only as trustworthy as its source and its test. The design-allowables and test-pyramid articles explain the discipline that stands behind a number you would bet a life on.
1. In a carbon-fibre-overwrapped pressure vessel (COPV), what does the inner liner do?
2. Why are the fibres wound at specific angles rather than randomly?
3. What defines a 'Type IV' hydrogen tank?