Ammonia is vital for global food production. Around 175 million tonnes are produced every year using unsustainable processes contribution 1.8% of global CO2 emissions. To meet global emission targets ammonia needs to become green.
The contribution of ammonia to a sustainable future doesn't end there. Ammonia produced from renewable sources can act as a much needed energy store. Green ammonia promises to be the lowest cost fuel, which can be generated where renewable resources are abundant and transported, stored and reconverted where and when demand is high.
Using local renewables is a good first choice. The cost and availability of land limits how much of our local demands can be met, especially during periods of high demand, such as on winter evenings.
Using ammonia as a globally tradable energy vector allows to locate renewable generation where they cost the least, cause the least environmental and biodiversity impact, and pose less competition with food production. Unpopulated and arid regions can produce renewable energy for hydrogen and ammonia production at scale, to be shipped where it is needed, such as urban and industrial centres.
Of the known technology options, ammonia has the least cost for high volume and high distance transferBased on: IEA (2018) and IRENA (2020)
Among the main alternatives to transporting large volumes of energy over large distances are:
They all may have a role to play, but each has some distinct disadvantages to ammonia as a bulk energy vector.Wires and pipelines
Grids and interconnects can reduce volatility regionally and improve asset utilisation.
International transmission lines are politically fraught and lock the investing parties into a bilateral dependency. Distance increases cost, while proximity increases the risk of temporal correlations in demand and supply. Weather systems extend beyond the connecting points may experience power shortages at the same time.
Export regions may experience higher energy prices as a result of such connections, making them politically unattractive.
The physical presence of sensitive infrastructure can make them vulnerable to attack or sabotage.Shipping hydrogen
Hydrogen has to be liquefied before it is viable for bulk transport by ship. This process is energy intensive and expensive at both ends. Storing hydrogen in large volumes is also more problematic than for ammonia. The small size of H₂ molecules make them prone to leakage, which could itself cause global warming (the global warming potential of hydrogen is estimated to be as high as 5.8 by Derwent (2001). Unless salt caverns happen to be available on site, hydrogen storage can limit the tradable volumes or significantly increase the cost.Biomass
Expanding biomass production to the scale required for international energy needs would be disastrous for deforestation and biodiversity. Per unit land, producing ammonia from solar energy produces higher yields and less environmental damage.
It is most cost-effective to store ammonia in insulated tanks at -33℃ and atmospheric pressure. (Salmon, 2021)
In its liquid state ammonia has an energy density of 13 MJ/l. This is less than LNG (27 MJ/l), but more than liquid hydrogen (8.5 MJ/l LHV) or Li-Ion batteries (~2 MJ/l).
Storing ammonia at higher pressures would require more costly pressure vessels. Some boil-off (~0.05% per day) needs to be recondensed with a refrigerator (Al-Breiki, 2020).
As a gas, yes. Gaseous ammonia is flammable, toxic if inhaled and causes severe skin burns and eye damage. It may explode if heated. It can be very toxic to aquatic life. See Ammonia Safety Sheet
Ammonia is shipped around the world in large volumes and safety procedures and regulations are in place.
The safety properties for liquid ammonia promise to be significantly better than the gas, and for solid ammonia better still. However, research is urgently needed to quantify and assess evaporation behaviour and safety implications of cold ammonia.
Ammonia is made from hydrogen (and nitrogen) and this process has a cost. This makes ammonia more expensive than hydrogen at the point of generation.
However, hydrogen is more expensive to transport and store in bulk. If large volumes need to be moved over large distances or stored for long durations, then ammonia is likely to be the cheapest long term option.
The shipping costs are significantly lower than for hydrogen, which requires high pressures or extreme temperatures. The majority of costs is therefore largely independent of the distance. Once loaded on a vessel ammonia can be taken anywhere around the world at low costs.Source: Royal Society (2020) / IEA (2018)
|Lower Heating Value||5.2 kWh/kg|
|Latent heat of vaporisation||1.37 MJ/kg|
|Heat capacity||4.7 kJ/kg/K|
|Latent heat of fusion||0.3 MJ/kg|
|Heat capacity||4.9 kJ/kg/K|
Overall efficiency can range anywhere from 17% to 39%, depending on the conversion chain (Royal Society, 2020). Improvements in efficiency are still possible in electrolysis (currently around 70% (Stolten, 2016)), the Haber Bosch process and in Solid Oxide Fuel Cells.
For future applications current efficiencies can already produce competitive energy prices.
N. Salmon, R. Banares-Alcantara, and R. Nayak-Luke. (2021). Optimization of green ammonia distribution systems for intercontinental energy transport. iScience.
Al-Breiki, M., and Bicer, Y. (2020). Technical assessment of liquefied natural gas, ammonia and methanol for overseas energy transport based on energy and exergy analyses. Int. J. Hydrogen Energy 45, 34927–34937.
IEA (2018). The future of hydrogen. Technical report, International Energy Agency.
IRENA (2020). Green hydrogen cost reduction: Scaling up electrolysers to meet the 1.5℃ climate goal. Technical report, International Renewable Energy Agency, Abu Dhabi
The Royal Society. (2020). Ammonia: zero-carbon fertiliser, fuel and energy store. Policy briefing, The Royal Society.
S. Giddey, S. P. S. Badwal, C. Munnings, and M. Dolan. (2017). Ammonia as a renewable energy transportation media. ACS Sustainable Chemistry & Engineering, 5(11):10231–10239
Stolten, D. (2016). Hydrogen Science and Engineering: Materials, Processes, Systems and Technology. John Wiley & Sons. p. 898. ISBN 9783527674299.
Derwent, R.G., et al. (2001). Transient behaviour of tropospheric ozone precursors in a global 3-D CTM and their indirect greenhouse effects. Climatic Change 49, 463-487.