Green ammonia (NH3) is produced by synthesising green hydrogen (H2) and nitrogen (usually using the Haber-Bosch process). Ammonia, a pungent-smelling, colourless, poisonous gas that is easily soluble in water, causes tears, irritates the respiratory tract when inhaled and has a suffocating effect, can be liquefied more easily than H2 (-33°C for NH3 versus -253°C for H2) and can therefore be transported and stored under more moderate conditions. As ammonia has been used on an industrial scale for decades - annual production approx. 200 million tonnes, mainly for fertiliser production - the worldwide transport and storage of ammonia is technologically mature and safe. The volumetric hydrogen density of ammonia is approx. 1.7 times higher than that of liquid H2 - consequently, the energy expenditure for transport is reduced compared to H2 transport - especially as the liquefaction and storage of ammonia is less energy-intensive compared to H2 as lower pressures and more "conventional" temperatures are sufficient.

From the current perspective, ammonia is the most cost-effective, liquid hydrogen carrier for long-distance transport - especially by sea. Ammonia can be burnt directly in gas turbines and marine engines, eliminating the need for energy-intensive reconversion to H2 for this application and allowing ammonia to be used as a fuel. If ammonia has to be split back into hydrogen and nitrogen at its destination (ammonia cracking), high temperatures (600-900 °C) are required for this process - and consequently a lot of energy. At 5.2 kWh/kg, the calorific value of ammonia is significantly lower than that of petrol (11.4 kWh/kg) and even lower than that of hydrogen (33.33 kWh/kg).

Despite the advantages mentioned, the question of the energy balance for the provision of H2 using ammonia as an H2 carrier arises - in other words, how much energy is required to produce 33.33 kWh in the form of H2, "convert" it into ammonia in the next step, transport it by sea to Europe and then crack it. The energy required per process step is as follows:

This means that approx. 124.5 kWh, either as electricity/heat/fuel, are required to generate 33.33 kWh of H2 (calorific value), to transport it by sea in the form of ammonia from the ammonia production site to the port of landing and to enable the return journey of the transport ship. This calculation assumes that the electrolysis plant is operated on the site of the ammonia production facility. It is also assumed that the ammonia production plant is located directly at the harbour, meaning that no intermediate transport is required between the ammonia production plant and the loading facility at the harbour.

As the energy required for ammonia production amounts to approx. 2 % of global energy production, it would make sense to replace the H2 previously produced mainly from natural gas by means of steam reformation with green H2 - thus minimising the high CO2 emissions associated with this.