The vast majority of the hydrogen produced today is transported only a short distance before use. Short-distance distribution is by pipeline; similar to the method used for natural gas. At present, long-distance distribution is primarily in liquefied form in large tanks. Both options pose certain technical challenges. Techniques for central bulk storage are also important for the distribution infrastructure. (Small-scale storage techniques for the point of end use are discussed in the next section.) If fuel use of hydrogen is to be expanded significantly, a shift of emphasis seems likely, from tanker to pipeline.

Transport in Liquid Form

     At atmospheric pressure, liquid hydrogen (known as LH₂) boils at 20°K (-423°F), making liquefaction, storage, and distribution challenging. Liquefaction is also very energy-intensive. Nevertheless, greatly reduced space requirements compared with gaseous hydrogen make the use of LH₂ an attractive option in some cases.

     Hydrogen is usually liquefied in a complex, multi-stage process that includes the use of liquid nitrogen and a sequence of compressors. Detailed procedures are required throughout the process to control the proportions of the two types of hydrogen molecule, known as ortho and para. If this were not done, ortho hydrogen in the distribution and storage tanks would slowly but spontaneously convert to para hydrogen over a period of days or weeks, releasing enough heat to revaporize most of the liquid.

     There are over 10,000 bulk shipments of LH₂ per year in the United States, to over 300 locations; NASA is by far the largest customer. Three main techniques are used for transportation: barges, truck trailers, and railcars. All these vehicles carry the hydrogen in pressurized, vacuum-insulated tanks, holding tens, or hundreds of thousands of gallons (3500-70,000 kg).

     The cost of distribution in tanks is likely to remain higher for LH₂ than for other liquid fuels such as gasoline. This is because hydrogen takes up several times more space than an energy-equivalent amount of other fuels. It also requires special insulating equipment to keep it liquid.

Gaseous Distribution

     Compared with the hundreds of thousands of miles of existing natural gas network, the hydrogen pipeline system is very small, totaling only about 460 miles. Air Products and Chemicals, Inc., has two gaseous hydrogen pipelines in the United States, one near Houston and one in Louisiana. Their total length is approximately 110 miles, and they carry an average of 190,000 kilograms of hydrogen per day to more than 20 customers at refineries and chemical plants. Air Products also operates a 30-mile, 50,000-kg/day pipeline in the Netherlands. Praxair, Inc. operates pipelines near Houston and in Indiana, totaling 160 miles and delivering about 200,000 kg/day to refineries, chemical plants, and steel manufacturers. Several other shorter lines deliver "over the fence" to individual industrial customers.

     If the use of hydrogen pipelines were to be expanded, possible embrittlement problems would have to be considered. Pipes and fittings can become brittle and crack as hydrogen diffuses into the metal of which they are made. The severity of this problem depends on the type of steel and weld used and the pressure in the pipeline. The technology is available to prevent embrittlement, but depending on the configuration being considered, distribution costs may be affected.

     The capacity of a given pipeline configuration to carry energy is somewhat lower when it carries hydrogen than when it carries natural gas. In a pipe of a given size and pressure, hydrogen flows about three times faster, but since it also contains about three times less energy per cubic foot, a comparable amount of energy gets through the pipe. Since compressors operate on the volume of a gas, however, not its energy content, the capacity of the compression stations (on an energy basis) is about one third less with hydrogen. In a pipeline system optimized to carry hydrogen, the pipe's dimensions and the size and spacing of the compressors would be changed to accommodate these factors. All told, transmission costs might be about 50 percent higher than for natural gas.

     Some segments of the small hydrogen pipelines mentioned above were originally designed to carry natural gas. Could the existing natural gas networks be used to carry hydrogen on a larger scale, even though they are not optimized to do so? This question requires further study (see "Recent Legislation", below) but probably each pipeline segment would have to be considered on a case-by-case basis. Some steels and welds would be compatible, but others might be subject to embrittlement, particularly the welds in older segments. Compressors would generally have to be refitted with new seals and valves. Department of Transportation safety standards for hydrogen and natural gas pipelines are the same.

Mass Storage Systems

      Any large-scale hydrogen distribution system must address the problem of bulk storage, to provide a buffer between production facilities and fluctuations in demand. Low-cost and efficient bulk storage techniques are a major research goal.

     One can store hydrogen as either a gas or a liquid. The most widely studied options for storing gaseous hydrogen are underground caverns and depleted underground natural gas formations. Although hydrogen is more prone to leak than most other gases, leakage has been shown not to be a problem for these techniques. For example, town gas (a mixture containing hydrogen) has been stored successfully in a cavern in France, and helium, which is even more leak-prone than hydrogen, has been stored in a depleted natural gas field near Amarillo, Texas. The energy consumed in pumping gas in and out of such storage facilities may be significant, however. Aboveground storage tanks at high pressure are another option.

      A certain amount of gaseous storage can be achieved by allowing modest pressure changes in the distribution pipeline system. In the case of natural gas, this technique is used to help manage transient demand fluctuations, such as the morning and evening peaks in residential demand in urban areas. Though the same technique might be useful for hydrogen, its potential is limited, particularly if the hydrogen is to be produced from intermittent sources such as solar or wind.

      Storage in liquid form uses tanks similar to those used for liquid hydrogen distribution. Kennedy Space Center uses an 850,000-gallon sphere near the launch pad, and can transfer fuel from this tank to the space shuttle at up to 10,000 gallons per minute. Storage at liquefier plants is in vacuum-insulated spherical tanks that usually hold about 400,000 gallons. The energy required for liquefaction may not be a barrier if the hydrogen is to be transported as a liquid anyway, or if the end-use application requires its fuel to be in liquid form.