Appendix V

Hydrogen Compression

Let us recall that hydrogen is not an ideal gas. For an ideal gas the relationship between pressure and density is  P = rRT in other words the pressure is directly related to the temperature T. In the case of hydrogen, a real gas, compressibility starts to affect the relation, which becomes P = ZrRT, including a compressibility factor Z which flattens out the P(r) curve.

 

A long tradition is for storing hydrogen in metallic cylinders of various sizes. The most common steel cylinders are made of stainless steel with added chromium, nickel or molybdenum.

 

A more modern design utilises the properties of composites, for example carbon fibre reinforced composites.

 

Cylinder-based compressors do most of the compression. The heart of a hydrogen compressor is often a hydraulically driven intensifier, which consists of motive cylinder coupled with two gas cylinders. The most common ones on the market are either single or double stage compressors. The main disadvantage is tied to the use of lubricating oils in these compressors and has often led to contamination of the hydrogen stored by oil and other impurities. We will discuss the potential of metal hydride based compression in the chapter on solid-state storage.