Today, hydrogen is transported from the point of production to the point of use via pipeline, over the road in cryogenic liquid trucks or gaseous tube trailers, or by rail or barge. Read on to learn more about current hydrogen delivery and storage technologies.
Approximately 700 miles of hydrogen pipelines are currently operating in the United States (compared to more than one million miles of natural gas pipelines nationwide). Owned by merchant hydrogen producers, these pipelines are located where large hydrogen users, such as petroleum refineries and chemical plants, are concentrated (for example, in the Gulf Coast region).
Transporting gaseous hydrogen via existing pipelines is currently the lowest-cost option for delivering large volumes of hydrogen. The high initial capital costs of new pipeline construction, however, constitute a major barrier to expanding hydrogen pipeline delivery infrastructure. Research is also focused on overcoming other technical concerns related to pipeline transmission, including the potential for hydrogen to embrittle the steel and welds used to fabricate the pipelines; the need to control hydrogen permeation and leaks; and the need for lower cost, more reliable, and more durable hydrogen compression technology.
One possibility for rapidly expanding the hydrogen delivery infrastructure is to adapt part of the natural gas delivery infrastructure to accommodate hydrogen. Converting natural gas pipelines to carry a blend of natural gas and hydrogen (up to about 20% hydrogen) may require only modest modifications to the pipeline; converting existing natural gas pipelines to deliver pure hydrogen may require more substantial modifications. Current research and analyses are examining both approaches.
Another possible delivery process involves producing a liquid hydrogen carrier at a central location, pumping it through pipelines to distributed refueling stations, and processing the carrier on-site to produce hydrogen for dispensing at the station. Ethanol, made from renewable resources with near-zero net greenhouse gas emissions, is among the hydrogen carriers under consideration. Liquid hydrogen carriers offer the potential of using existing pipeline and truck infrastructure technology for hydrogen transport.
Trucks, Railcars, Ships, and Barges
Trucks, railcars, ships, and barges can be used to deliver compressed hydrogen gas, cryogenic liquid hydrogen, or novel hydrogen liquid or solid carriers.
Compressed Hydrogen Gas
Today, compressed hydrogen can be shipped in tube trailers at pressures up to 3,000 psi (about 200 bar). This method is expensive, however, and it is cost-prohibitive for distances greater than about 200 miles. Researchers are investigating technology that might permit tube trailers to operate at higher pressures (up to 10,000 psi), which would reduce costs and extend the utility of this delivery option.
Currently, for longer distances, hydrogen is transported as a liquid in super-insulated, cryogenic tank trucks. Gaseous hydrogen is liquefied (cooled to below -253°C/-423°F) and stored at the liquefaction plant in large insulated tanks. The liquid hydrogen is then dispensed to delivery trucks and transported to distribution sites where it is compressed and vaporized to a high-pressure gaseous product for dispensing. Over long distances, trucking liquid hydrogen is more economical than trucking gaseous hydrogen because a liquid tanker truck can hold a much larger mass of hydrogen than a gaseous tube trailer. But it takes energy to liquefy hydrogen—using today's technology, liquefaction consumes more than 30% of the energy content of the hydrogen and is expensive. In addition, some amount of stored hydrogen will be lost through evaporation, or "boil off" of liquefied hydrogen, especially when using small tanks with large surface-to-volume ratios. Research to improve liquefaction technology, as well as improved economies of scale, could help lower costs (today's liquefaction units are small to meet minimal demand).
Additional research and analyses are needed to investigate novel liquid or solid hydrogen carriers—which store hydrogen in some other chemical state, rather than as free molecules—for use in delivery. Potential carriers include metal hydrides, carbon or other nanostructures, and reversible hydrocarbons or other liquids, among others in the early stages of research. Using such novel carriers would constitute a significant departure from the way transportation fuels are delivered today.
A national hydrogen infrastructure could require geologic (underground) bulk storage to handle variations in demand throughout the year. In some regions, naturally occurring geologic formations, such as salt caverns and aquifer structures, might be used, while in other regions, specially engineered rock caverns are a possibility. Geologic bulk storage is common practice in the natural gas industry. But the properties of hydrogen are different from natural gas—hydrogen molecules are much smaller than natural gas, for example—and further research is needed to evaluate the suitability of geologic storage for hydrogen and to ensure proper engineering of the storage site and hydrogen containment.
Interface with Vehicles
The technologies used for storing hydrogen on-board vehicles will also affect the design and selection of a hydrogen delivery system and infrastructure. To maximize the overall energy efficiency, the system must avoid any unnecessary energy-intensive stages and match the delivery technology with the on-board vehicle storage technology.