Hydrogen is the beacon of hope for the energy transition. Bill Gates likens the Element to a „Swiss Army Knife“ because of its versatility. Hydrogen can be used to store energy, as a fuel for cars and trucks, and as a fuel for power and heat generators, both in fuel cells and in turbines, combined heat and power plants or in condensing boilers. It is also praised for its high energy density: one kilogram of hydrogen contains as much energy as around 3.5 kilograms of natural gas. This naturally raises the question of why it should only now be used more intensively as an energy source.
Why hasn’t hydrogen been used for a long time?
Part of the answer is: because hydrogen does not occur in nature in its pure form. The element hydrogen occurring on earth – abbreviated as H2 in chemistry – is bound in molecules, especially in water molecules, chemically H2O. But natural gas, which mainly consists of methane, also contains a lot of hydrogen, because methane consists of a hydrocarbon, chemically CH4. Because hydrocarbons unfortunately have the property of releasing the carbon to the oxygen during combustion and thereby generating carbon dioxide, i.e. CO2, hydrogen in its pure form is required to produce CO2 > to use neutral energy. In order to use hydrogen, hydrogen – with green electricity – must first be generated, which is significantly more energy-intensive than promoting fossil fuels, which nature has already „produced“ over millions of years. Only under the condition that fossil fuels should no longer be used does the use of hydrogen become interesting.
Another part of the answer to why hydrogen isn’t already being used is that hydrogen is difficult to transport compared to other fuels. This seems to contradict the statement that hydrogen has a phenomenally high energy density: a substance with a high energy density should be easier to transport than a substance with a low energy density. But anyone who has ever packed a moving truck knows that not only the weight plays a role, but also the volume: if you only pack your moving boxes half full, you may not exceed the load capacity of the truck, but very quickly the volume of the loading area … And when it comes to energy density in relation to volume, hydrogen lags far behind other energy carriers. To transport 1,000 kilowatt hours (one megawatt hour) you only need 0.1 cubic meters for diesel, 90 cubic meters for natural gas and 282 cubic meters for hydrogen. A ship with a loading capacity of 250,000 cubic meters (equivalent to the capacity of a large LNG tanker) could therefore transport 2.6 million megawatt hours of diesel, 2,800 megawatt hours of natural gas or 885 megawatt hours of hydrogen. With all the advantages of hydrogen, there is no denying that transportation needs to be ingenious.
Improving transportability through compression
An obvious solution to reduce volume is: compression. Unlike liquids, gases can be compressed. In order to obtain acceptable room dimensions, gases are therefore usually transported under pressure. In regional natural gas distribution networks, natural gas is distributed at up to 70 bar, a natural gas filling station compresses the gas to up to 200 bar. In this way, a natural gas vehicle can carry around 14 kilograms of natural gas (which is significantly less than the tank volume of a petrol vehicle of around 40 kg). With a hydrogen vehicle, you have to operate with up to 700 bar in order to achieve acceptable tank dimensions. This requires complex and therefore expensive technology in the hydrogen filling stations. The high pressures also require very solid and heavy tanks. The tank of the hydrogen vehicle Toyota Mirai only holds 5 kilograms of hydrogen, which is enough for a range of 454 kilometers, but weighs an impressive 88 kilograms. The weight of the tank is about 18 times the content. With even higher tank volumes, the ratio improves somewhat, but the costs of hydrogen tanks are astronomical compared to petrol, heating oil or diesel tanks. Using tank trucks or tank train wagons to solve the transport problem is therefore not very promising. Hydrogen distribution by means of pipelines will always be significantly more economical, because here the transport capacity can be improved not only by increasing the pressure, but also by increasing the diameter of the pipe at comparatively low cost. However, pipelines cannot be laid across oceans – or only at great expense. Since hydrogen imports are also planned from countries such as Canada or South Africa, a way must be found to transport hydrogen with tankers. The transport problem cannot be solved by compression alone.
The problem is the physical state
The real problem is the physical state: hydrogen and natural gas are gaseous at normal ambient temperatures. The boiling point at which the substances change into a much more space-saving liquid state is -161°C for methane, the main component of natural gas. Hydrogen only becomes liquid at -253°C. For comparison: the absolute zero point, i.e. the lowest possible temperature that cannot be fallen below, is -273°C. Is it possible to cool gases that far? Ways have been found for natural gas so that sea transport of natural gas has been technically possible for a number of years: one then speaks of „Liquefied Natural Gas“ (LNG). However, the process is very energy-intensive, because the boiling temperature of -161°C not only has to be fallen below once, but also maintained throughout the entire transport, otherwise the natural gas would expand again and burst the LNG tanks. In turn, re-gasification requires energy because gas that expands absorbs heat from the environment. In order to prevent the LNG terminal from freezing, it must therefore be constantly heated. This effort is only justified because the volume of natural gas is reduced by a factor of 600 during liquefaction: to transport one megawatt hour of energy, you no longer need 90 cubic meters (as in the gaseous state), but only 0.15 cubic meters. That is still 50% more volume than is required for diesel transport, whereby diesel of course does not have to be compressed or cooled. In principle, the liquefaction of hydrogen would be conceivable with the same process, but due to the even lower boiling point of -253°C, it uses significantly more energy than natural gas, which has a negative impact on profitability.
You can’t outsmart physics
Hydrogen is the lightest element: it has the atomic number 1 in the periodic table, has only one proton in the nucleus and one electron in the shell and is only 1/12 as heavy as a carbon atom. These physical parameters determine the basic properties of hydrogen, in particular that it occurs as a gas at normal ambient temperatures and that it has a high volume requirement, both of which are unfortunately very problematic properties for transport purposes. Because the chemical properties of hydrogen cannot be changed, transporting hydrogen requires a great deal of energy for compression and, if necessary, cooling. The question is whether it’s worth it. After all, energy sources are known that can be transported much more easily, especially fossil energy sources such as oil or natural gas. Perhaps hydrogen can be converted into an energy carrier that is easier to transport without losing the advantages of hydrogen.
Binding of hydrogen to carriers
There are several elements that are suitable for reacting with hydrogen. At first glance, a so-called methanation, in which hydrogen reacts with carbon dioxide to form methane and water, would be very practical. Since methane is the main component of natural gas, „artificial natural gas“ would have been produced with this process, which could be used in all natural gas-using aggregates without extensive conversion and transported with the known transport processes (LNG). However, these advantages are largely outweighed by the energy losses associated with this reaction, because a great deal of heat is released during methanation, which as waste heat losses has a negative impact on economics. The efficiency of methanation is given as around 80%, so 20% of the energy contained in the hydrogen is lost – and in the end a gas would still have to be transported.
Instead, a reaction that combines hydrogen (H2) with nitrogen (N) to produce ammonia (NH3) is often favored. Ammonia is a main component of fertilizer and the process for combining hydrogen with nitrogen to form ammonia was invented by the German chemists Fritz Haber and Carl Bosch at the beginning of the 20th century, which is why the technology is known as the „Haber-Bosch process“. Currently, the hydrogen used is usually obtained from natural gas, but green hydrogen could be used in production in the future. The Haber-Bosch process requires the use of energy, but only 60% of this is stored in the bonds of the ammonia. This method also has to contend with losses that affect the economy. On the other hand, ammonia is extremely easy to transport: the boiling point at normal pressure is only -33°C and at a pressure of just 9 bar the boiling point rises to +20°C – transporting it in the liquid state is therefore not very complex. Ammonia can therefore serve as a „transport medium“ for hydrogen.
But what happens when ammonia reaches its destination? If really pure hydrogen is needed, the ammonia has to be broken down back into its components, hydrogen and nitrogen, which in turn requires energy input that affects the economy. The required technology, a so-called „ammonia cracker“, also breaks down the ammonia into its components, hydrogen and nitrogen. However, these are then available as a gas mixture (so-called „forming gas“) – pure hydrogen, which could be used for feeding into a European transport network, would first have to be filtered out of the gas mixture in a further step. For many applications, a „recovery“ of the hydrogen is not necessary at all, because ammonia can also be used directly as a fuel. However, since the reactivity of the ammonia molecules is less than that of hydrogen, special conditions have to be created. In particular, continuous combustion cannot take place with ambient air because the oxygen content is too low: pure oxygen must be supplied. Ammonia can also be converted back into electricity in fuel cells, but must first be broken down by an „ammonia cracker“. The fuel cells themselves have electrical efficiencies of 52-57%. Overall, the undeniable transport advantages of ammonia come at a high price and key advantages of hydrogen must be sacrificed. In particular, the energy losses during conversion have a massive impact on profitability and would have to be overcompensated for when generating the green electricity used to produce hydrogen.
Since the transport of pure hydrogen is very expensive if it does not take place via pipelines and transport networks, further options were sought for docking hydrogen to carriers that have the desired transport properties. A liquid state of aggregation in the normal temperature range and the ability to absorb and release hydrogen were particularly important in the search. In addition to ammonia, organic compounds in particular have been identified as suitable, i.e. molecules based on carbon. So far these are „Toluene/Methylcyclohexane“, „N-Ethylcarbazole“, „Dibenzyltoluene“ and „Benzyltoluene“. Other molecules are under investigation. The collective term for suitable organic compounds is „Liquid organic hydrogen carriers“, abbreviated to LOHC. Terminals that can process LOHC are being planned. However, the technology suffers from the same disadvantages as ammonia: a double conversion of the hydrogen is required. When hydrogen is bound to LOHC, energy is lost, and when the LOHC is dehydrogenated, energy is lost again. This technology must also compensate for the economic disadvantages of conversion losses when generating green electricity.
There are various ways of transporting hydrogen, with the use of tanks being associated with major economic disadvantages. It is therefore to be expected that hydrogen will be transported via pipeline networks wherever possible. Tankers will probably only be used if it is possible to compensate for the economic disadvantages of this transport route through low production costs or if production connected to the distribution network cannot cover the demand covered by purchasing power. It is difficult to predict which technologies will be used and to what extent, since not only the technical efficiencies play a role, but in particular the comparative costs of green electricity generation in the country of generation and use.
But one thing is certain: a hydrogen economy will only establish itself in Germany and Europe if there is a well-developed transport and distribution network for hydrogen. Only then are projects for hydrogen production and use economically viable. Transport (and storage) using tanks, whether mounted on trucks or trains, is not justifiable for most projects. Of course, this (again) leads to a „chicken and egg problem“: should the hydrogen network be built first, so that there are incentives for hydrogen production and use? Or the other way around? On the one hand or the other, state start-up financing and corresponding incentives will certainly be necessary. It is therefore now the task of politicians to create suitable framework conditions.