Compression and storage

Compressor descriptions pair how the pressure is raised with how the unit is driven. Read this alongside the table.

Single stage, two stage

A large lift is done in steps rather than in one go. Single stage raises the pressure once, two stage in two steps

Single acting, double acting

A single acting piston compresses on one half of its travel, a double acting one on both. For the same size, double acting moves more per cycle

Air-driven

Compressed air pushes a large piston, and the small piston attached to it compresses the gas. There is no electric motor and no spark, but the site has to have a compressed air supply for it to run

Diaphragm

A metal diaphragm flexes back and forth to displace the gas. It is oil-free, the gas never touching lubricant, and no compressed air supply is needed to drive it

One HXB-V1 produces 500 litres an hour, while most compressors on the market are built to draw considerably more than that. So the compressor does not sit straight on the electrolyser outlet: a low pressure buffer goes in front of it to accumulate gas before it is pushed on. A single 50 L vessel holds about 0.55 Nm³ at 10 barg, which is usually enough to absorb the compressor cycling on and off. The outlet pressure setting is a separate check. HXB-V1 is set in software to either 0 or 10 barg, and at 0 none of the compressors in the table below will draw, because their minimum suction pressures run from 1.7 to 7 bar. If compression is going on the end, the outlet has to be set to 10 barg, and all three units in the table then meet the condition. The same manufacturer also lists models needing 14 or 17 bar, so choosing on type and discharge pressure alone will trip on suction. The flow reasoning here is for the 2 kW class. HXS-30 produces 7,500 litres an hour, fifteen times the scale, so both the compressor class and the buffer volume are set again from scratch. Send us the conditions and we will put a 30 kW configuration together separately.

You cannot use everything in a vessel. Once the pressure falls below what the point of use needs, what is left is stranded, so only the gas between the maximum and the minimum pressure is usable. We quote total and usable quantity separately, and both differ again from the water volume marked on the vessel. All three need to be read apart. This is also why "500 bar, 100 L" does not by itself say how many kilograms. Doubling the pressure roughly doubles the content, but at high pressure that proportionality breaks down and a real gas correction applies. Temperature, the minimum pressure, the gas left in the pipework and the filling profile all enter as well. Filling heats the gas for the same reason, so a gauge can read the target and still fall as it cools. Filling is complete at the pressure after the temperature settles. Where the scale calls for several vessels, they can be split into banks held at different pressures and drawn down lowest first. This is called a cascade. Used as one block, the usable quantity falls away sharply as pressure drops; split into banks, the low bank empties first and the high bank is kept for last, which raises overall utilisation. The cost is more valves and more control, so we look at whether the scale justifies it.

The gas leaving the electrolyser is saturated with water vapour, so drying has to be complete before the compressor. Leave drying until later and the compressor is compressing water along with the hydrogen.