Synthetic fuels: Are they carbon-neutral?
E-fuels are seen as a beacon of hope for a climate-neutral future – particularly in areas where direct electrification is difficult. They can be produced from renewable energy sources and make use of existing infrastructure, which reduces the transition costs for the system as a whole
But beyond application scenarios and discussions, a fundamental question arises: just how efficient are e-fuels from a physical perspective?
The answer lies in the laws of thermodynamics. Any conversion of energy involves losses – particularly when electrical current is converted several times, stored and finally converted back into motion or heat. It is precisely this conversion pathway that is at the heart of efficiency considerations when it comes to e-fuels.
This article provides a step-by-step explanation of how e-fuels are produced, what energy losses occur along the process chain, and why efficiency plays a key role in assessing their use. The aim is to provide a physically sound analysis that clearly illustrates what actually happens in terms of energy when e-fuels are used.
What are e-fuels? Definition, production and applications
E-fuels (electric fuels) are synthetic, liquid or gaseous energy carriers produced using electricity from renewable sources. The starting point is usually water, which is split into hydrogen and oxygen via electrolysis. The hydrogen produced is then processed further with CO₂ – for example, from industrial exhaust gases or the ambient air – to produce synthetic fuels such as e-methane, e-methanol or synthetic diesel and kerosene.
A key feature of e-fuels is that they contain chemically stored energy and can be stored, transported and used in a similar way to fossil fuels. This means they can be used in existing internal combustion engines, turbines or heating systems without requiring any fundamental changes to the infrastructure. Typical areas of application include aviation and shipping, certain industrial applications, and existing vehicle fleets where direct electrification is technically difficult.
From a physical perspective, however, e-fuels are not a primary energy source, but rather an energy conversion product in which electrical energy is converted into chemical energy through a series of process steps – an aspect that plays a key role in assessing their efficiency.
The energy pathway of e-fuels: from electricity to liquid fuel
The energy pathway for e-fuels begins with electricity, ideally from renewable energy sources. This electricity is first used in electrolysis to split water into hydrogen and oxygen. Conversion losses occur as early as this first step, as some of the electrical energy used is lost as heat.
In the next step, the hydrogen produced is further processed with CO₂ in a synthesis plant. These processes are also energy-intensive and result in further losses in efficiency. The end products are liquid or gaseous synthetic fuels.
This is followed by the processing, storage and transport of the e-fuels. These steps result in further energy losses, for example through compression, cooling or logistics. When the fuel is finally used – for example in an internal combustion engine or a heating system – chemical energy is once again converted into mechanical work, again with limited efficiency.
The energy pathway for e-fuels thus involves several successive conversion stages: from electricity to hydrogen, from hydrogen to synthetic fuel, and finally to usable energy. From a physical perspective, it is precisely this chain that is crucial for assessing efficiency, as each conversion inevitably involves energy losses.
Electrolysis, synthesis, combustion: where do the greatest losses occur?
Energy losses along the value chain in the production of e-fuels are a key consideration in assessing efficiency. Even during the production of hydrogen via electrolysis, a significant proportion of the electrical energy used is lost: typical electrolytic systems achieve efficiencies of around 60–70%, which means that 30% or more of the electrical energy is lost as waste heat.
In the next step – the synthesis of hydrogen with CO₂ to produce liquid fuels – further significant amounts of energy are lost. Studies suggest that the efficiency of the Fischer-Tropsch synthesis or comparable power-to-liquid processes is often also only in the range of 60–70%.
When the losses from electrolysis and synthesis are taken into account, often only a relatively small proportion of the electrical energy originally used remains as chemically stored fuel energy. Estimates suggest that, following these process stages, less than half of the energy remains in the synthetic fuel (around 44%).
Depending on the application of e-fuels, further reductions in efficiency – some of them significant – come into play. Take, for example, combustion losses in internal combustion engines: despite technical optimisations, modern petrol or diesel engines usually achieve an efficiency of only 30%.
All in all, this means that the energy ultimately available as mechanical power in a vehicle or machine is often only a small fraction of the original electrical input. On average, this amounts to just 15% of the initial renewable electricity
Der Wirkungsgrad von E-Fuels versus E-Autos
In contrast, with direct electrification – for example, in battery-electric vehicles – a large proportion of the electrical energy is retained within the system: efficiency analyses often put the efficiency of electric cars at 70–80%, as charging, storage and motor operation involve significantly fewer conversion losses than the e-fuel chain. Factors to consider here include distribution losses in the electricity grid, and the efficiency of the battery and electric motor.
The same applies when comparing the operation of heating systems based on e-fuel combustion and heat pumps. The latter can provide more than ten times the amount of heat using the same amount of primary energy.
A comparison of the two approaches reveals a clear difference: whilst a significant proportion of the energy is lost during the production process for e-fuels, direct electrification uses electricity in a much more direct and efficient way. In concrete terms, this means that the same amount of renewable energy can power around five times as many battery-powered vehicles as those running on e-fuels, or heat an area 10 times larger.
These physically based differences in efficiency affect not only overall energy requirements but also the cost-benefit analysis for various applications – a key consideration when evaluating e-fuels against direct electrification solutions.
Carbon footprint and efficiency: Why the two must be considered together
The assessment of e-fuels must not be limited solely to their carbon footprint, but must always be considered alongside their energy efficiency. Whilst e-fuels can be carbon-neutral in terms of their carbon footprint if they are produced exclusively using renewable electricity and biogenic or air-captured CO₂, Yet this is precisely where the connection lies: the lower the efficiency of an energy source, the more renewable electricity is required to provide the same amount of energy – and the greater the demand for land, infrastructure and resources.
From a physical perspective, low overall efficiency means that a significant proportion of the primary energy used is lost before it is actually utilised. Even where production is theoretically carbon-neutral, this results in scarce renewable energy being used less effectively. A good carbon footprint alone therefore says little about how effectively an energy source is utilised within the overall system.
Efficiency and carbon footprint are therefore inextricably linked: the more efficient a process is, the lower the energy consumption and the easier it is to decarbonise. Only by considering both factors together can we make a scientifically sound assessment of the contribution e-fuels can make to the energy system – and where their limitations lie.
However, one must also take the following point into account: energy is not lost during the conversion process. It is released in the form of thermal energy. Recovering this energy naturally improves the overall efficiency of the system significantly and should, of course, be taken into account when planning such a plant.
Conclusion: What physics tells us about the efficiency of e-fuels for transport
From a physical perspective, e-fuels are a clear example of how electrical energy can be converted into chemical energy, albeit at the cost of significant conversion losses. A large proportion of the energy originally used is lost along the process chain, from electrolysis through synthesis to use in internal combustion engines or heating systems. The end result is a comparatively low overall efficiency, which is significantly lower than that achieved through the direct use of electricity.
The laws of thermodynamics make one thing clear: these losses are not avoidable, but are inherent to the system. Every additional conversion stage further reduces the proportion of usable energy. This is precisely why a physical analysis shows that e-fuels must be viewed critically where efficient electrical alternatives are available.
That does not mean, however, that they have no useful role to play in the energy transition. On the contrary. The further the expansion of renewable energy progresses, the more surplus electricity is produced in the summer; once all battery storage systems have been filled for short-term storage, this surplus electricity is ideally suited for producing hydrogen and e-fuels. As mentioned at the outset, there are some sectors, particularly in agriculture and aviation, that cannot currently be electrified. Only time will tell whether this will change at some point. Until then, e-fuels are an important component of the future energy mix.
Overall, it is clear that e-fuels are not a question of technical feasibility, but of energy efficiency. The laws of physics provide a clear basis for assessment here: the scarcer renewable energy becomes, the more important it is to use it as directly and efficiently as possible. E-fuels can therefore only be useful in selected applications – not because of their efficiency, but despite their efficiency losses.
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