Production of green hydrogen and recovering energy from it are not the only challenges. If these operations are to be carried out as part of an ”energy storage project”, then obviously there must be a third challenge – namely, the storage of hydrogen itself. For well over a century, it has been known how to store hydrogen and other gases in steel cylinders . Everyone has certainly seen such cylinders, as they are used for many purposes, in hospitals, in welders’ shops, and in various other places. They are usually 10 inches in diameter and are 50 inches tall and have an internal volume of about 50 liters. Depending on the strength of the steel from which they are made, they can withstand a pressure of 200 to 400 bar, which corresponds to a gas volume of about 10 m3 to 20 m3 at normal pressure.
Pressure cylinders are, so far, the only possible fuel tanks in fuel cell electric vehicles (FCEVs). It is obvious that the reservoirs themselves should be as light as possible, but withstand the highest possible pressure. Conventional steel cylinders are far too heavy. The current state-of-the-arts solution to the problem are cylinders with walls made of carbon fiber composites, withstanding pressure up to 70 MPa (MegaPascals; 1 MPa 10 bar). At this pressure, 1 kg of hydrogen occupies a volume of approx. 26 liters. In Tyota Mirai, a state-of-the-arts FCEV, the two fuel reservoirs of total weight of 87.5 kg can store 5 kg of hydrogen. It means that these reservoirs can store hydrogen the weight of which is only about 5.7% of their own weight. In the so-called tube trailers, specially designed for transporting compressed hydrogen (with a capacity of typically 500 1000 kg of H2), the (H2 weight)/(tube weight) ratio may be better, even exceeding 10 %.
Another possible approach to the transport and storage issues is to use liquefied hydrogen. For liquefaction, hydrogen needs to be cooled to 20 K and this process requires a minimum of 15 MJ of energy per kilogram in an ideal process, and about three times as much using the best technology available today. Considering that the energy contained in hydrogen alone is 142.6 Mj / kg, around 30% is lost on the liquefaction alone. Liquid hydrogen has to be stored in special cryogenic tanks – currently, if the best available thermal insulation is used, the evaporation losses in such tanks can be as low as 0.5-1% per day. But one more problem is the density of liquid hydrogen, as low as 71kg/m3 – compared with 422 kg/m3 for liquid methane. Hydrogen’s heat of combustion is higher than that of methane by a factor of 2.57, but if we want to transport liquid methane and liquid hydrogen of the same energy content, the volume of the hydrogen tanks has to be 2.3 times larger than that of the methane tanks.
Nevertheless, hydrogen transport in liquid form appears to be one of the few options for long-distance shipping by sea. Japan, who already has an agreement with Australia and is counting on serious supplies of Australian hydrogen in the future, is very interested in this possibility. With this in mind, the first liquid hydrogen tanker was just launched at the Kawasaki shipyard. It will be used for testing the feasibility of this transportation mode.
Transportation of chemically converted hydrogen – this is a method that allows transporting hydrogen over long distances, or storing it for long periods of time without (in principle) any losses. For this, one needs a substance that acts as a ”carrier”, and allows hydrogen atoms to get attached to its molecules in a reversible way – i.e., such that the H atoms can be after some time relatively easily “disatached” from these molecules, leaving them unchanged.
One candidate for such a carrier that has recently attracted much interest, in particular in Australia, is nitrogen. To a single nitrogen atom one can attach three hydrogen atoms, making a molecule of ammonia, NH3. Australia has already much experience with synthesizing ammonia for other purposes – esprecially, for using it as a fertilizer.
But now, when the prospect of a very significant increase in green hydrogen production appears, the Australians also spot another possible application the use of ammonia as the “carrier” , which will greatly facilitate the export of “green hydrogen” to remote recipients (see Australia’s National Hydrogen Strategy, Page 40 and other pages alltogether, ammonia is mentioned in this document 36 times).
The heat of combustion of ammonia is 22.48 MJ/kg. The heat of combustion of hydrogen is 142.6 MJ/kg, over six times higher. But note that in 1 kg of ammonia there is only 3/17 kg of hydrogen. Burning such an amount of hydrogen would release 142.6 MJ/kg (3/17) kg = 25.16 MJ, which means that only 11% of energy originally contained in hydrogen is lost. But instead of burning ammonia (which itself can be used as a fuel), one can also “dis-attach” hydrogen atoms from nitrogen and continue to use pure hydrogen only.
One of the significant advantages of ammonia from the viewpoint of maritime transport is the ease of its liquefaction. To keep it liquefied, no low temperature is needed, only increased pressure: at 25 C, a pressure of about 10 bar is sufficient. Transport tanks must be therefore sturdy, but they do not need thermal insulation.
Another candidate for a hydrogen carrier – more specifially, for a “Liquid Organic Hydrogen Carrier” (LOHC) is toluene, C7H8. One of the not-so-nice applications of toluene is that it is used for the production of TNT, a powerful explosive that killed countless victims in the First and Second World Wars, and in many other wars. It is pleasant, therefore, that now it can be used in an absolutely peaceful role: three molecules of H2, six hydrogen atoms can be attached to one molecule of C7H8, yielding C7H14 which is known as “methylcyclohexane”. This product can be loaded into tankers and sent to the recipient where the 6 hydrogen atoms can be detached relatively easily. And toluene can be returned to the sender for reuse.
In this method, the molecular weight of C7H14 is 98, and the total weight of three H2 molecules is 6, so that the mass of hydrogen in the shipment is only 6/98 = 6.1% of the total mass of the substance shipped, which is perhaps not a spectacular proportion, given that in the case of ammonia this ratio is 3/17 = 17.6%. But the advantage is that methylcyclohexane can be transported at ambient temperature and ambient pressure – therefore, one can use any containers, even ordinary plastic barrels. Such transport has just arrived in recent days from Brunei to Kawasaki, Japan, covering a distance of 4,000 miles.