Distribution of hydrogen

For industrial purposes hydrogen is distributed as either gas or liquid, while the solid state is not often used. Most commonly hydrogen is distributed in its gaseous state under pressure in conventional gas cylinders for which there is a worldwide and extended distribution system. The investments in this technique have been significant and the structure is highly developed. Besides there is international experience of the transport of hydrogen in pipelines, and the transport of liquid hydrogen is also being considered.

If hydrogen is to be a major contributor to the energy system, very large quantities must be distributed and for that reason and from a technical point of view, pipeline transport is the best solution. From a cost-efficient point of view it is, however, questionable whether pipeline distribution will be cheaper if investments are to be made in a distribution pipeline network.

Storage of hydrogen

Hydrogen can be stored in all three phases: gaseous, solid or liquid. In the solid state, however, it appears not as pure hydrogen, but as chemical compounds. Storage of pressurized gaseous hydrogen in steel cylinders is and has been used for several years in many countries, but storage of hydrogen in steel cylinders in the quantities required for seasonal and daily adjustments of the load of a 100 – 300 MW power station, is unrealistic. This kind of storage may however be interesting as a fuel for transport.

Many geological structures make it possible to store hydrogen in underground caverns, as is the case with natural gas. The high diffusivity of hydrogen in comparison with methane has only little effect in terms of leakage. This utilization of caverns for underground storage of natural gas is widely used in several countries. The physical and chemical differences between hydrogen and methane do not allow methane to be directly replaced by hydrogen in caverns, but the technique is the same in principle. However, different demands are made on materials and the stability of the caverns, and the storage requirements will also be different.

Liquid hydrogen is already used in several countries, and is stored in vacuum insulated spherical tanks. The volume of the biggest tanks that are available at NASA is about 4000 m³ (radius 10 m), but it is considered possible to construct considerably larger tanks, up to 15,000 m³ (radius 15 m) containing about 12.10⁶ Nm³. The storage capacity of liquid hydrogen is thus sufficient to meet the day-and-night requirements of a power station with a capacity of some hundred MW.

The storage of hydrogen in the solid state can take place by the formation of chemical compounds between metals and hydrogen (metal hydrides). This is an equilibrium reaction with high sensitivity to pressure and temperature, and involves reaction heat. For almost all metals, certainly those of practical interest, the reaction is exothermic, i.e. heat is developed on formation of the hydride and conversely the same quantity must be added during degassing of the hydride.

Hydrogen for electricity production or heating

Hydrogen can be converted to electricity and heat by gas engines, gas turbines or fuel cells. Gas engines have a higher efficiency than the smaller gas turbines. On the other hand the waste heat from a gas turbine is hotter, giving a better potential for generating high pressure steam.

Internal combustion engines are the clear winners in the CHP technology for smaller outputs up to a couple of MW, while the big gas turbines are more suitable at the other end of the scale, i.e. over 10 MW.

Hydrogen for vehicles

The utilization of hydrogen for transport purposes has been the subject of several studies, and internationally there has been considerable experimental research into the field at several car factories. In the former USSR experiments have been carried out with hydrogen as fuel for a big airplane. Car factories have primarily been working with burning hydrogen in internal combustion engines, but the use of fuel cells have also been considered, although to a small extent.

The use of combustible gas for internal combustion engines has been known for many years. The gas used has been mostly propane, but lately experiments have also been made with natural gas. The problems with the use of hydrogen as a propellant compared to the well-known kinds of gas are small. Only minor technical alterations of the engine are necessary.

In principle only water is produced by the combustion of hydrogen and water is not polluting. In practice, however, an unacceptably large amount of nitrogen oxides is produced. Therefore the key players of the automobile industry are working very intensively to reduce these NO_x values.