CGA’s H-4 Publication Defines Processes and Terms Associated with Hydrogen Fueling Technologies

November 4, 2021

The use of hydrogen has been expanding, particularly for fuel cell electric vehicles (FCEVs). The growing use of hydrogen means that terms that were previously found only in specialized fields are now becoming part of the mainstream vernacular. Other terms may have developed meanings that differ from those used in ordinary, non-hydrogen applications.

In April 2020, the Compressed Gas Association (CGA) released the 3rd edition of CGA H-4, Terminology Associated with Hydrogen Fuel Technologies. This safety publication covers technologies and terminology that applies to hydrogen fuel production, storage, transport, and use, and provides a single source of uniform terminology for hydrogen fuel technologies. This publication will be useful to those involved with the production, storage, transport, and use of hydrogen, as well as regulators, and codes and standards developers.

CGA H-4 consists of two major sections:

  • Hydrogen technology descriptions
  • Glossary of hydrogen and fueling terms

Section 1: Hydrogen Technology Descriptions

The technology descriptions section of CGA H-4 provides an overview of the ways in which hydrogen is produced, stored, transported, and used.

Hydrogen Production

Typical methods used to produce hydrogen include:

  • Steam reformation is the process of using steam to reform (sometimes called “cracking”) methane (natural gas) and other hydrocarbons into hydrogen and carbon monoxide. This section of CGA H-4 includes a schematic detailing this process.
  • Partial oxidation is a similar process that reacts hydrocarbons or hydrogen-containing compounds with oxygen to produce hydrogen and carbon monoxide.
  • Autothermal reformation also produces hydrogen and carbon monoxide by using heat from partial oxidation reactions to sustain the steam reforming reactions.
  • Gasification produces hydrogen by injecting steam into a hot bed of coal.
  • Electrolysis uses electricity to separate water molecules into gaseous hydrogen and oxygen.
  • Photoelectrolysis uses solar energy to produce hydrogen and oxygen from water.

Hydrogen Transportation and Storage

Once hydrogen has been produced and purified, it then must be transported to customer (user) sites where it will be stored before being used. There are five techniques described in CGA H-4:

  • Compressed gas hydrogen storage and transportation
  • Cryogenic liquid hydrogen storage and transportation
  • Reversible metal hydride
  • Chemical hydride
  • Adsorption

We provide details about each technique below.

Compressed Gas Hydrogen Storage and Transportation

Transporting and storing hydrogen in gaseous form is one of the simplest methods. However, gas hydrogen must be compressed to high pressures, as much as 15,000 psi, to have usable amounts of hydrogen storage. Hydrogen’s light weight means that the majority of the weight is found in the storage or transportation container, rather than in the stored hydrogen itself.

The four typical methods of transporting and storing gaseous hydrogen are listed below:

  • Cylinders are portable high pressure gas containers that are cylindrical in shape and come in a variety of sizes. The U.S. Department of Transportation (DOT) regulates cylinders in the United States, while Transport Canada (TC) regulates them in Canada.
  • Tube trailers are large high pressure gas containers that are mounted on a trailer chassis for transport on the highway. Tube trailers may be delivered and used at a customer location or can be used to fill customer receivers.
  • Receivers are large high pressure gas containers similar to those on tube trailers, but which are permanently installed at a location. Receivers are used for storage only and can be refilled by tube trailers or from a high pressure liquid hydrogen pumping system.
  • Pipelines are large, often interstate piping systems used to deliver gas hydrogen. DOT and TC regulate these pipelines.

Cryogenic Liquid Hydrogen Storage and Transportation

Transporting and storing hydrogen in a liquid form, is much more efficient than transportation and storage in high pressure gas form. For example, a gallon of liquid hydrogen converts to 848 standard cubic feet (SCF) of usable gas hydrogen, while a gallon of high pressure gas hydrogen (2640 psi) converts to 24 SCF of usable gas hydrogen.

Liquid hydrogen is transported in specially designed trailers and railcars, then stored in liquid tanks.

  • A liquid trailer has a double-walled tank consisting of an inner container, an outer casing, and a vacuum and multilayed, insulated annular section between the container and casing. The tank typically contains up to 17,000 gallons of hydrogen and has a maximum working pressure of 150 to 175 psi.
  • Hydrogen railcars use the same design principle as that of liquid trailers but are transported via rail. They are used much less frequently than liquid trailers and hold up to 28,000 gallons of hydrogen.
  • A liquid hydrogen tank is similar to a container on a liquid trailer but is used for storage only. CGA H-4 includes a system flow diagram showing how liquid hydrogen is converted to gaseous hydrogen, regulated to the customer’s required pressure, and delivered to the customer use point.

The nature of liquid hydrogen (and other cryogenic fluids) makes for a bit of a challenge deciding to what percentage of its capacity to fill a storage container. An empty space called “ullage” provides room for liquid expansion to avoid overfilling the container, operating the pressure relief devices, and venting hydrogen. Appendix A gives a detailed description of the variables that affect the optimum sizing of the ullage space. Too small an ullage space can lead to hydrogen venting, while too large an ullage space leads to wasted space where hydrogen could have been stored.

Other Hydrogen Storage Methods

Hydrogen can also be stored via these methods:

  • Reversible metal hydride is a general term used to describe a class of materials that can reversibly react with hydrogen gas to form a metal-hydrogen compound. The hydrogen-absorbing material is typically a metal alloy.
  • In a chemical hydride system, gaseous hydrogen is generated from a hydrogen-containing compound through a chemical reaction.
  • In adsorption methods of hydrogen storage, hydrogen molecules are stored in an adsorbent material through a process known as physisorption.

Uses for Hydrogen

Although hydrogen has many uses, CGA H-4 focuses on hydrogen use in fuels cells and combustion, as described below:

Fuel cells

  • A fuel cell is an electrochemical device that uses the strong attraction of hydrogen and oxygen to its advantage to produce electrical energy. Electrical energy is produced by breaking down hydrogen into protons and electrons and using the electrons to produce power.
  • Fuel cell systems combine a fuel cell stack with supporting components such as valves, fans, manifolds, and electrical controls.
  • There are several fuel cell technologies that combine hydrogen and oxygen in a fuel cell:
  1. Proton exchange membrane (PEM) fuel cells
  2. Direct methanol fuel cells
  3. Alkaline fuel cells
  4. Phosphoric acid fuel cells
  5. Molten carbonate fuel cells
  6. Solid oxide fuel cells

CGA H-4 contains a schematic of a PEM fuel cell.


Hydrogen can also be used as the fuel in an internal combustion engine (ICE) either as a pure gas or mixed with natural gas (or other hydrocarbon fuels):

  • Hydrogen has been shown to reduce exhaust emissions under controlled conditions when added to natural gas in a blended fuel ICE application.
  • With properly designed engine control systems, ICEs can operate at high efficiency on hydrogen and present a very low-emission profile.
  • Hydrogen and oxygen are used in specialty combustion applications such as burners and torches to allow for a high temperature thermal processing environment that is free from carbon and sulfur contamination.

Section 2: Terms Associated with Hydrogen

The second part of CGA H-4 consists of a glossary of 118 terms, commonly used for hydrogen. This glossary provides an essential reference tool for those needing an understanding of hydrogen production, transportation, storage, and use terminology. The glossary supplements the information in the technology section.

Learn More About CGA’s Expanding Role in the Hydrogen Energy Revolution

Members of the Compressed Gas Association have long been global leaders in the production, storage, distribution, and application of hydrogen and other industrial gases.

In addition to our existing library of hydrogen standards, we are bringing our unique expertise in hydrogen to the development of new standards for emerging applications in the hydrogen-powered transportation space.

A complete list of CGA’s hydrogen-related safety standards, as well as details about our Hydrogen Initiative, may be viewed here.