HYDROGEN STORAGE

A technical appraisal of hydrogen storage options.

If renewable energy sources are to be exploited then an efficient energy storage system must be integrated to meet a variable demand; therefore in the application of solar energy it is necessary to store excess energy in the form of hydrogen.

Hydrogen has similarities to some of our current fuels; particularly natural gas yet for the acceptance of hydrogen as an energy provider there must be an understanding, in terms of safety, of both the physical and chemical properties.

Negative aspects of hydrogen and possible risks involved with its usage are primarily because hydrogen is colourless, odourless, tasteless and non-toxic under normal conditions. Hydrogen is potentially explosive due to the wide limits on the explosive ranges (4-75 vol.-% hydrogen); it has an extremely low ignition energy, a low viscosity, high combustion and detonation velocities, all of which are contributory factors to the hazards associated with hydrogen.

Hydrogen however has several beneficial properties that increase its potential as a fuel. This includes hydrogen's low density of 0.08Kg /m2 (under normal conditions) and a high ignition temperature of 585oC. Due to its low-density any leakage of hydrogen would disperse quickly, minimising the risk of an explosion. A hydrogen flame also emits minimal radiation due to its small heating value per volume. 

Presently hydrogen can be stored in three forms; gaseous, liquid or as a solid combined with a metal hybrid. The most suitable storage method is dependant upon safety aspects, environmental issues, economic criteria and the end-use of hydrogen.

The most commonly used and simplest method is to store hydrogen in its natural form as a gas. Storage of gaseous hydrogen is primarily limited by volume considerations as a result of hydrogen's low density, as even at high-pressure very large volumes are required resulting in high material costs. Today gaseous hydrogen is stored in prodominaly in steel cylinders at a pressure of 150-200 bar and at an ambient temperature of approximately 298K. The most common conventional steel gaseous cylinders contain a volume of 40 litres and a pressure of 150 bar and in the last decade significant progress has been made in a move towards lightweight cylinders using chrome-molybdenum steel. Frequently the individual cylinders are clustered together to be filled and released through just one valve and the cylinders are interconnected through high-pressure tubing, a grouping of cylinders also allows for financial savings.

Significant amounts of hydrogen can also be stored within high-pressure storage tanks, that can be situated above ground or underground which is similar to the storage of natural gas. The construction material properties in above ground storage impose limitations on the quantity of gaseous hydrogen that can be stored and the hydrogen can be stored at an increased pressure in an underground pressure tank or underwater tank. 

3. LIQUID HYDROGEN

Hydrogen stored in a liquid form is substantially more compressed than in gaseous form and superficially it appears an appealing means of energy storage but there are various contributory negative factors. Primarily that the liquidification requires a large expenditure of energy and secondly, through the use of insulation, liquid hydrogen must be continually kept at a low temperature (<20K). There are risks associated with this constant low temperature, due to the high expansion ratio of liquid hydrogen to gaseous hydrogen. If there was a warming of liquid hydrogen extremely high pressures could accumulate and result in damage or an explosion.

Liquid hydrogen must be stored in cryogenic tanks, which is a well-established technique. There are three ways that heat transfer occurs from the external air to the liquid hydrogen; conduction, convection and radiation. Heat transfer increases with external surface area therefore the majority of cryogenic tanks are spherical or cylindrical to minimise the surface area. Most liquid hydrogen storage tanks are also double walled tanks that are insulated with an evacuated layer of Perlite insulation.

4. METAL HYDRIDE

Another means of hydrogen storage is that of metal hydride storage, this method is a relatively new method, the advantage of the system is that hydrogen being stored as a metal at low pressure and that it stores more volume than compressed gaseous or liquid hydrogen. The basic principle is that certain metals alloys absorb hydrogen to form a metal hydride. The hydrogen is absorbed at a lower temperature and when the hydrogen gas is needed the gas is recaptured by lowering the pressure below, or raising the temperature of the metal hydride above the absorption process. The hydrating alloy is stored in stainless steel tubes and according to the Van't Hoff equation hydrogen reacts with the metal alloy granules in the tank at a specific temperature and pressure. The steel tubes are surrounded by a water jacket that facilitates heat exchange during absorption and desorption of hydrogen gas. The need for a compressor is eliminated in the hydride system as the hydriding and dehydriding reactions occur at specified temperature and pressure depending on the electrolyser specifications.

5. CONCLUSION

There are several important characteristics that the metal storage material must posses and these include; economic feasibility, minimal weight and reasonable storage capacity, it must also have good reversibility and be able to undergo a frequent number of charge-discharge cycles. There is a wide range of alloys that are suitable for hydrogen storage depending on the conditions required these include hydrides of magnesium alloys, LaNi5 based alloys and FeTi based alloys. The main problem with the metal hydride storage system is their high cost.

The foregoing above appraisal has confirmed that if necessary precautions are taken then hydrogen can be safely stored in gaseous, liquid and solid form, although one of the key issues appears to be ventilation. With reference to this particular system for an autonomous dwelling the preferred storage option appears to be storage in compressed gaseous form either in a series of clustered cylinders or tanks. It should be noted that there are mature systems in place to allow for the safe storage of compressed gases for domestic use.