Compression and storage

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

Suction range

The first figure is the pressure below which the compressor will not run; the second is the most it can contain. Read the first against your electrolyser outlet; the second only matters when compressors are staged one behind another

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. Gas that leaks past the seals is carried out through a vent, and on hydrogen that vent has to be piped to a ventilated area

Modular air-driven booster

Rather than one machine doing the whole lift, a module per stage is racked into a 19-inch frame and stacked. A higher target adds a module; more flow adds another of the same stage alongside. It only lifts as far as what is fitted, so the maximum discharge in the table is the figure with the final stage on the rack. The configuration is set out under "How a HULC is configured" below

Oil-free dry-running piston · hermetic magnetic coupling

The piston runs dry, with no lubricant, and the motor shaft never enters the gas section; the force crosses as magnetism. With no shaft penetration there is nothing to leak past and no vent to pipe. Units built this way tend to ask for a higher suction pressure, so check first whether a stage is needed ahead of it

What an air-driven booster can reach is the drive air pressure multiplied by the booster ratio, so every maximum discharge figure in the table assumes drive air near the top of its range. The 310 bar for AG-30 is with 10.3 bar air; on a 7 bar utility line, which is what most plants run, the same unit stalls at about 210 bar. If you are filling into the 200 bar range there is almost no margin. Check the pressure of the air on site before you look at models. Air flow matters as much. The manufacturer rates AG-30 performance at 100 psi and 50 SCFM, about 85 Nm³/h. That is 85 Nm³/h of air to move 0.5 Nm³/h of hydrogen, which calls for a separate 10 kW class air compressor. HXB-V1 itself is 2 kW, so the moment a booster goes on the end the largest electrical load on site is the air compressor rather than the electrolyser. If the pressure is right but the flow is short, the booster does not stop, it slows down, which is harder to spot than a failure. Where there is no compressed air supply, or this condition is too much, look at the electric drive side.

Air drive pressure

Maximum discharge in the table assumes this near the top of its range. A 7 bar line lowers the discharge in proportion

Air drive flow

AG-30 is rated at 100 psi and 50 SCFM, about 85 Nm³/h. Short of that the booster slows

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 the four air-driven units then meet the condition. The two REJOOL rows ask for 20 bar and 25 bar of suction respectively and will not draw unless a stage is put ahead of them. The same manufacturer also lists models needing 14 or 17 bar, so choosing on type and discharge pressure alone will trip on suction. Where you have to run at 0 barg the table has no answer, because it currently carries no compressor that draws from sub-atmospheric upwards. Send us the conditions for that case and we will find and configure one separately. 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. AG-30 in the table has a ceiling of 2 Nm³/h and cannot take the 7.5 Nm³/h that HXS-30 makes. At the 30 kW scale, read the flow ceiling column before the discharge pressure. 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.

Compression ratio is the absolute discharge pressure divided by the absolute inlet pressure. The larger it is, the hotter the gas gets. A bicycle pump warms in your hand for the same reason, and in a compressor that heat reaches the valves and seals first. So when the lift is large it is not done in one go but split into stages. How much one stage can take differs by machine. An air-driven booster strokes slowly and has time to cool between strokes, so a single stage covers a wide span: that is how the AG-30 in the table goes from 7 bar to 310 bar in one stage. An electrically driven reciprocating machine runs without pause and cannot be asked for the same ratio in one stage. Two stages is the common answer. The Terek GU-GTD-60 in the table is two-stage double acting, and splitting one stage into two is where the temperature falls hardest. Past that, each further stage buys less and less while the modules and the money keep adding up. So stacking stages does not solve it on its own. From the atmospheric outlet of an unpressurised electrolyser up to 700 bar is a ratio of about 690, and even split in two the theoretical discharge sits in the 480 °C range. At that point the cheaper move is not another stage but more pressure upstream — which is what the chart below is about. Splitting does not add flow. Stacking stages takes the gas higher; sending more of it means putting another of the same stage alongside. Swap the two and the quotation comes out wrong.

What a compressor has to cover is not the target pressure but the ratio between the target and the electrolyser outlet. So even a little pressure made upstream lightens everything bolted on behind it. The chart above is that difference: for the same 700 bar, 691× from atmospheric, 64× from 10 barg, 19× from 35 barg. Temperature is not the only thing that falls. From atmospheric an ordinary two-stage machine cannot carry the span and more stages have to be stacked, while at 35 barg a single two-stage unit reaches 700 bar. There is less machinery to buy, less compressed air to supply if the drive is pneumatic, and less duty on the seals that get replaced. That is why the electrolyser outlet pressure is worth settling before the compressor is chosen — and why, when an HXB-V1 offers 0 or 10 barg and compression is planned, we say to set it at 10. It is not free, though. Running an electrolyser at higher pressure sends more hydrogen across the membrane, so the hydrogen content on the oxygen side rises and the membrane carries more load. How far to push it has to stay inside what the stack allows, and we look at that figure together with the stack conditions.

Plan around the price of the machine alone and the site comes up short. Five things sit outside the figures in the table. With air drive the compressed air plant is the big one. The AG-30 reaches its rated performance on 100 psi at 50 SCFM, about 85 Nm³/h, which calls for a 10 kW class air compressor of its own — more electricity than the electrolyser itself. A low-pressure buffer is separate too. The electrolyser delivers steadily while the compressor starts and stops, so a vessel in front has to absorb the difference. One 50 L cylinder is usually enough. Vent piping is its own cost. An air-driven unit is built to carry away the gas that slips past the seals, and that line has to be run somewhere safe. The oil-free magnetically coupled machines have no such line. Seals and packings are consumables, particularly on the oil-free machines, and they are replaced on a schedule. The interval and the cost move with the model and the duty, so we have not fixed a figure here — once the shortlist is down we confirm it with the manufacturer and pass it on. The rack or cabinet is sometimes outside the price. The HULC is a 19-inch rack module and the manufacturer literature says the cabinet is ordered separately. On top of all this come the high-pressure gas permits and the installation, which belong to whoever puts the plant in.

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.