Working Inventory

In plain terms

A pressure vessel full of hydrogen cannot give all of it back. The offtaker needs gas above a minimum delivery pressure, so whatever remains below that pressure stays in the vessel. That remainder is cushion gas. What can be withdrawn between the maximum and minimum pressures is the working inventory.

For an ideal gas at constant temperature, stored mass is proportional to pressure, so:

Usable fraction ≈ (P_max − P_min) / P_max   (absolute pressures)

At high pressure, real-gas behaviour lowers this somewhat, so engineering models use a compressibility factor. In NREL's station analysis, raising storage pressure from 172 to 250 bar stores 38 % more hydrogen, less than the 45 % an ideal gas would give (Parks et al., 2014).

Why buffers exist. Solar and wind output varies by the hour and the season. Many offtakers, such as ammonia synthesis, need a steady flow. A buffer absorbs the mismatch: it fills when production exceeds demand and empties when it falls short.

Typical range

Usable fraction depends on the pressure window, which is set by storage type and the delivery pressure the offtaker needs, so it is calculated per design rather than taken from a general range. Minimum pressure is what caps delivery: in hydrogen fuelling stations, cascade banks need about 825 bar or more to fuel vehicles, and gas below that level cannot be dispensed from the bank (Parks et al., 2014).

Why it moves LCOH

Storage is costed on total capacity, but only the working inventory does useful work. A narrow pressure window means buying a lot of vessel for little usable gas. Raising maximum pressure adds compression cost; drawing storage below the delivery pressure needs a booster compressor downstream. The cheapest design balances the two.

Common mistake