Hydrogen Compression

In plain terms

Hydrogen is the lightest molecule there is. That makes it hard to compress: centrifugal machines struggle to build pressure with so little mass, and the small molecule leaks through seals other gases would not pass.

Compression is done in stages. Each stage raises pressure by a limited ratio, then an intercooler removes the heat of compression before the next stage. Cooler gas takes less work to compress, and temperature limits protect valves and seals.

The three main hydrogen compressor types:

• Reciprocating (piston): the workhorse for high pressure and moderate flow.
• Diaphragm: a metal diaphragm separates gas from oil, giving oil-free, high-purity output at small to medium flow.
• Centrifugal: suited to very large flows and low pressure ratios, such as pipelines, but needs many stages for hydrogen.

bar(a) or bar(g). Always state the basis. Absolute pressure, bar(a), is gauge pressure, bar(g), plus atmospheric pressure. Mixing the two at low pressure can distort pressure ratios and stage count.

Typical range

Specific compression energy depends on inlet and outlet pressure, flow and machine type. US DOE figures (Program Record #9013, 2009):

The gap between the theoretical and practical figures is machine efficiency, intercooling and real-gas behaviour.

Why it moves LCOH

For an ideal gas, compression work scales with the logarithm of the pressure ratio. Going from 1 to 30 bar(a) therefore takes about as much work as going from 30 to 900 bar(a). Real hydrogen is less compressible than an ideal gas at high pressure, so the upper range costs somewhat more. A pressurised electrolyzer skips that low-pressure range and can remove whole stages, machines and their power demand.

Common mistake