Electromobility has great potential to reduce energy consumption and emissions – provided the CO2 intensity of electricity production decreases and the vehicle fleet is heavily penetrated with battery-electric drives. Analyses of publicly available data from reputable institutions such as the International Energy Agency (IEA) and the World Bank, in conjunction with forecasts for the development of road traffic, show that even with 25 percent battery-electric vehicles in 2035, around 26,000 TWh of energy from fossil sources will be needed. To minimize their share as much as possible, regenerative fuels of synthetic or biological origin are required.
Focus on Hydrogen, Methanol, and Ammonia
In addition to biological fuels and synthetic diesel and gasoline, which can be mixed into fossil fuels without restriction or can also substitute them, hydrogen, methanol, and ammonia are three promising renewable fuels that play a central role in the discussion about the future of road traffic and the transport sector in general. But which of these fuels is particularly promising? The criteria are primarily their properties during combustion in an internal combustion engine, i.e., emission and efficiency levels, storage and transportability, energy expenditure during production, and their costs. However, the efficiency of the entire chain, from the provision of renewable electricity to the implementation of the generated synthetic energy carrier in the internal combustion engine, is of decisive importance.
A study by IAV, presented at the 46th Vienna Motor Symposium, examined the life cycle of these fuels. The goal is to compare the efficiency from energy generation to use in the vehicle between hydrogen, methanol, and ammonia and to optimize the entire production process and their use based on this data. In addition, the data obtained can also support decision-making for the selection of one of these energy carriers.
The basis of the study is a physico-chemical digital twin of the production processes of all three fuels, developed at IAV. In addition to each individual production step, from hydrogen production with various electrolysis processes, nitrogen separation for ammonia production, to synthesis processes for the production of ammonia and methanol, different carbon separation processes as well as transport options and storage technologies are also analyzed in detail.
To evaluate and optimize the usage phase of the fuels, specific phenomenological combustion models for the three aforementioned fuels were developed at IAV. With their help, it is possible to define optimal configurations of internal combustion engines, whereby the energy content of the fuels can be converted into mechanical energy at the wheel as efficiently as possible.
Efficiency Comparison
The study shows that an engine running on ammonia achieves a significantly higher efficiency compared to those running on methanol and hydrogen. This is due to the specific fuel properties. With ammonia, the engine achieves a maximum effective peak efficiency of about 48 percent, which is above the level of diesel engines. When running on methanol, the same 6-cylinder commercial vehicle engine with 12l displacement achieves about 47 percent. Despite many efficiency-enhancing properties, such as high laminar flame speed, an engine with direct injection and premixed combustion can only achieve an efficiency of about 41 percent with hydrogen.
The three very different efficiency potentials now raise the question of whether the fuel that achieves the highest efficiencies during use can also be produced, transported, and distributed with the least effort.
Assuming production and use of the three fuels in Germany, e.g., Berlin, it shows that the production of hydrogen from renewable electricity to provision at the gas station is the most efficient. With an optimized overall process, an efficiency of 52 percent is achieved, while ammonia can be produced with about 44 percent and methanol with about 43 percent efficiency. Including the engine efficiencies, hydrogen and ammonia each achieve about 21 percent overall efficiency and methanol about 20 percent.
Location-dependent Efficiency of Renewable Fuels
Since it is foreseeable that there will not be enough regenerative electricity production in Germany, energy imports from regions with high regenerative electricity potential are inevitable.
If the production location, with the same usage location, is now moved from Berlin to Agadir in Morocco, a completely different ranking results than with local production. Hydrogen can only be produced and used with an efficiency of about 17 percent, while ammonia and methanol can each increase by about 1 percentage point. This is due to the high energy required for the liquefaction of hydrogen. Whenever there is no pipeline connection, this liquefaction must be chosen for ship transport. Thus, there are significant differences depending on the production location, and this also applies to the use of various electrolysis or carbon separation processes. Detailed results can be read in the publication by IAV.
Even if the differences shown appear small, they mean a significant difference in the number of plants built for the production of green electricity. Since these bring a considerable CO2e potential with them during their construction, the production location is of decisive importance. Since the so-called capacity factors of plants for the production of renewable energy in regions with high regenerative energy potential are about two to three times higher than in Central Europe, the same amount of energy or fuels can be produced with half or a third of the number of plants.
The CO2e reduction potential due to fewer plant constructions means an immense speed advantage in the transformation towards the use of renewable energies in the transport sector. Conclusion: For the fastest reduction of greenhouse gas emissions in the transport sector, especially in road traffic, synthetic fuels are comprehensively and urgently needed.
Our expert on the topic
Marc Sens
marc.sens@iav.de
LinkedIn