Transportation and storage of hydrogen

Fossil fuels power most present-day infrastructure, but when combusted they emit pollutants, most notably carbon dioxide. This is a greenhouse gas (GHG), which has been linked to global warming and climate change.

Conversely, combusting hydrogen produces innocuous water vapor and some nitric oxide (NOx), without emitting carbon dioxide or other pollutants, like sulfur dioxide (SOx). In addition, hydrogen is compatible in many existing natural gas turbines with internal combustion engines which can operate on hydrogen, natural gas, or a blend of both. However, hydrogen is a dangerous substance when handled improperly.

First, its molecules are the smallest of any element, so leakage - which poses a fire and explosion hazard - from tanks and pipelines is a serious concern. Special consideration must be given to the materials and techniques used to seal these systems, such as fittings, gaskets, valves and other sealing devices. Environmental monitoring devices, such as flame detectors and combustible gas detectors - or inline devices, such as pressure and temperature transmitters - must be applied to detect abnormal events, like loss of containment. Because hydrogen is diatomic, newer technologies, such as infrared-based gas detectors often used in natural gas applications, cannot be used for hydrogen gas detection.

Leakages are primarily a result of embrittlement, which most frequently forms as steel and other metals absorb hydrogen atoms. These atoms can recombine to form hydrogen molecules that diffuse throughout the metal and form bubbles that weaken the material, causing embrittlement and cracking, even at ambient temperature. It is therefore critical to mitigate these issues by specifying proper materials based on the application.

Safe hydrogen storage is a key enabler for the advancement of hydrogen and fuel cell technologies.

Hydrogen can be physically stored as a compressed gas or cryogenic liquid. Compressed gaseous hydrogen is typically held in tanks at 350-700 bar (5,000-10,000 psi). Fully liquid hydrogen can be stored at approximately -253 °C (-423 °F), whereas cryo-compressed hydrogen can be stored at approximately -233 °C (-387 °F). Gaseous storage has lower equipment requirements and is significantly more economical, but liquid storage has its advantages, primarily much higher energy storage density

Liquid hydrogen has long been used as rocket fuel for space launches. In space, it has been stored as a compressed gas or cryogenic liquid in cylinders, tubes and spherical tanks. In gaseous form, hydrogen is typically stored in cylinders. However, spherical tanks are preferred for storing liquid hydrogen to minimize surface area, which correlates directly to heat transfer from the environment.

Hydrogen can also be stored in material-based systems on the surfaces of solids (adsorption) or within them (absorption). These procedures are being developed to meet fuel density requirements and increase process safety because they reduce the potential for leaks and uncontrolled combustion.

Safety measures for all hydrogen storage systems include:

• Locate storage in well-ventilated, non-smoking outdoor areas away from structures, vehicles, heat, sparks and open flames
• Never drag, roll, slide or drop storage containers
• Use only spark-proof tools and explosion-proof equipment for handling hydrogen
• Ground all equipment and piping
• Check for leaks in hydrogen systems regularly using soapy water, and never with a flame

High-density hydrogen storage requirements pose significant challenges for transportation systems. The energy density of hydrogen is much lower than that of gasoline, so larger tanks are required to store the same quantity of energy. Generally speaking, vehicular hydrogen tanks are larger than natural gas variants and able to withstand higher pressure.

These added space requirements detract from a vehicle’s functionality to comfortably transport people and objects in a space-efficient manner, and extra weight adversely affects the distance a vehicle can travel with a set amount of energy. Additionally, hydrogen fuel cells occupy more space than combustion engines, add weight and introduce another potential source of leakage.

Hydrogen-powered cars and trucks are available, but the number of hydrogen fueling stations globally is limited. This makes them impractical for most people, especially everyday consumers. As hydrogen infrastructure continues to develop, however, this reality could change down the road.

Despite these drawbacks, hydrogen-powered cars and long-haul trucks have significant advantages compared to electric vehicles. They can be refueled in minutes as opposed to hours and the stored energy does not degrade over time. Energy storage density is much higher than batteries, by a factor of over 100, making fuels much lighter and more compact than batteries. Finally, the materials needed to produce modern batteries, especially lithium, is in short supply, while the materials needed to produce hydrogen fuel cells are abundant.

As gaseous hydrogen is produced, it can be consumed locally, compressed and piped to nearby storage tanks, compressed and filled into cylinders for transit, or liquified for improved storage density or long-range transport. Hydrogen transportation typically occurs via pipeline, truck, rail, or ship. Pipelines are most often used between nearby production facilities and consumers, and over wider geographies where stable long-term demand is anticipated.

Over short distances, truck transit is most common, either in elongated high-pressure cylinders stacked on a tube trailer, or in liquid hydrogen tankers at cryogenic temperatures. Railcars are used to haul liquid hydrogen over medium distances, while ships take on heavy payloads for long-range transport.

Research continues in the quest to develop viable compact hydrogen storage systems that are safe for use in both vehicles and fixed installations. In conjunction with viable hydrogen production, developments in transportation and storage will help propel the hydrogen economy.

As industries strive to reduce carbon emissions by implementing hydrogen and other alternative fuels in their infrastructure, proper training is of paramount importance to ensure safe engineering, installation, operation and maintenance of these systems.