Correction: Contribution to a standardized economic and ecological assessment methodology for e-fuel production in Germany

The transport sector worldwide still does not meet the international obligations to become climate neutral. One of the countries lacking behind their set goals is Germany. In 2021, the German transport sector contributed to 19.4 % of Germany's greenhouse gas emissions (German Environment Agency, 2022). In order to reach the maximum target of 85 Mt CO2 equivalents by 2030, emissions need to be reduced by about 79 Mt CO2 equivalents compared to 2020. Also, other European countries do have a long way to go, to reach net zero emissions in the transport sector as depicted in Figure 1.Figure 1: Share of renewable energy in the transport sector of selected European countries (adapted from (Eurostat, 2019)).In order to explore pathways for the de-fossilization of the transport sector on national and global level, the German federal ministry for economic affairs and climate action (BMWK) launched the initiative "energy transition in the transport sector: sector coupling through the use of e-fuels" (EiV) in 2017 with a total funding of 99 M€ and 16 projects (BMWK, 2017). Since each project started with its own goals to find a solution for a certain part of the transport sector, it was not ensured whether their analysis concerning costs or environmental impacts would be comparable. To propose the most promising pathways for the short, mid and long term, these pathways needed to be assessed technically, economically and ecologically. Here, a standardized methodology and a framework of basic assumptions, based on previous work (e.g. Albrecht et al., 2017), had to be provided. This was done by the authors within the additional project "accompanying research on the EiV" (BEniVer) (Energiesystem-Forschung, 2021).A comparable analysis of power-to-x production (PtX) routes is only possible if the underlying assumptions and methodologies are consistent and are reported. However, due to different levels of detail and assumption, this is often not the case as illustrated by the following examples.In the absence of all technical information, reproduction of results is limited: In Gorre et al. (2020) information about the catalyst, used for methanation, is missing. Furthermore, the compressors are only defined by a share of total energy consumption. The target purity of the product is not reported, as also in Moioli and Schildhauer (2022) for their methanol synthesis. Tremel et al. (2015) provides neither flow diagram nor simulation data. Furthermore, information on the thermodynamic model used, the purity of the product and the reactor model is missing. Adnan and Kibria (2020) do not specify product purity. Even though many economic values in Gorre et al. (2019) are listed in a very good manner, not mentioning the product's purity and assuming that both, different electrolysis technologies and also different methanation processes had "minor technological differences," makes their technical insight questionable.Incomparable approaches and assumption in economic analysis prevent comparability of results beyond the certain publication. Even though there are good examples for economic analysis, such as for Parigi et al. (2019), Dahiru et al. (2022) state in their review on techno-economic analysis of powerto-X production: "[…] there is not yet any globally accepted method for TEA due to lack of consistency and comparability between different studies." For instance, Adnan and Kibria (2020) do not report hydrogen costs, annual operating time, or base year for the synthetic methanol production. Schemme et al. (2020) perform an analysis with a consistent system boundary and the same assumption for internal comparability. However, e.g. the data for the CO2 costs and the amount of energy required to extract the CO2 have been taken from different sources and thus are inherently inconsistent. Furthermore, the factors underlying the techno-economic analysis (TEA), e.g. Lang factors, are missing. Peters et al. (2019) give no information on plant operating time and capital costs.Even in ecological analysis, data are often neither collected uniformly nor based on the same assumption: In a meta-study of various publications on the life cycle assessment (LCA) of synthetic fuels, Kigle et. al. stated: "Despite the large number of publications and normalized LCA standards, comparison of results between different fuels across different studies proves difficult" (translated) (Kigle et al., 2019 p. 7). The greenhouse gas (GHG) emissions for methane fuel, for example, range from just under 200 g CO2-equivalent km -1 (CO2-eq. km -1 ) for Wettstein (2018) to 650 g CO2-eq. km -1 for Reiter and Lindorfer (2015). A significant influencing factor is the GHG intensity of the electricity supply and the associated share in the synthetic fuels. Wettstein (2018) consider a photovoltaic (PV)plant as the power source, Reiter and Lindorfer (2015) assume the electricity mix. The choice of the system boundary, to which functional unit the results refer, also influences the comparability of the results, as well as to which process possible captured CO2 is attributed (Kigle et al., 2019). The publication of the methodology guideline for LCA of e-fuels was already a step further (Pichlmaier et al., 2021). Also, the work of Müller et al. (2020) gives a good overview over LCA-guidelines. But still, an LCA can be done following the standards but still not be comparable, if basic assumptions are not defined identical and the methodology is not set.In order to address the non-comparability, a comprehensive summary of necessary criteria for TEA and LCA was presented by Langhorst et al. (2022). They require, that "the target audience for the study shall be stated". Which seems to imply different qualities of scientific work for different audiences. This should not be done. Due to their focus on using the definitions of LCA for TEA, the definitions for economic analysis are sometimes not sufficient. For instance, in order to define the products, they require to state the intended application. This is not necessary for the calculation of costs and environmental impacts of the production. Even if they wanted the application of the product to define its quality, they do not define to which extend the application should be stated. So, if, for instance, methanol as fuel for cars would be the application, it could be used in an internal combustion engine or in a methanol fuel-cell. Between both cases, the water content can differ a lot. Similarly, they state, that the functional unit of the product should be defined. Here also, the quality of that definition is not stated. So, if, for instance, the functional unit would be 1 kg synthetic natural gas for heating, the composition or the lower heating values are still not defined. Natural gas has a lower heating value of 8.9 or 12.5 kWh/kg in Europe, and about 13.1 kWh/kg in the USA (Fluessiggas1, 2023) (The Engineering Tool Box, 2024). So, even if the functional unit is defined, the results are not comparable, if a standard is not defined. Better is a definition by composition (pressure and temperature) of the product. Also, they did not the net production costs as economic They only focus on which on whether the for product be for there is no for renewable fuels. Furthermore, they did not define the methodology of So, if analysis are based on their different methodologies of calculation can be methodologies can differ a for in So, if the calculation methodology is not defined, the results are not comparable. Also, they for different qualities of as well as based on different standards, are not same for et al. 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