The term "well-to-tank" (WTT) describes the entire upstream chain of energy sources—that is, all processes from the extraction of primary energy through processing to the delivery of fuel into a vehicle’s tank or storage system. As such, WTT constitutes the first part of the well-to-wheel (WTW) analysis.
While Tank-to-Wheel (TTW) measures emissions during actual driving, WTT shows the climate and environmental impacts that occur even before a liter of diesel, a kilogram of hydrogen, or a kilowatt-hour of electricity can be used in the vehicle.
Overview of the process steps – from the pre-train (WTT) to train operations (TTW).
The Significance of the WTT Approach
Taking WTT into account is crucial for businesses, policymakers, and researchers, as it allows for a realistic assessment of the climate impact of various energy sources. Without this component, a large portion of indirect emissions would be overlooked.
Practical relevance:
Transparent carbon footprint: WTT highlights the fact that fuels and electricity can generate significant emissions even before they are used.
Comparison of energy sources: Upstream emissions vary significantly—for example, between fossil diesel, biofuels, green hydrogen, and electricity from renewable sources.
Policy regulation: WTT emissions are increasingly being incorporated into subsidy programs, carbon pricing, and regulatory frameworks.
Gaseous energy sources: production, liquefaction, pipeline transport.
Electricity: Electricity mix in each country, losses during transmission and storage.
Comparing alternative fuels: Green hydrogen performs significantly better in the WTT assessment than fossil hydrogen, while biofuels yield very different results depending on the feedstock.
Assessing supply chains: Logistics companies include WTT emissions in their Scope 3 emissions inventories to accurately reflect their entire transport chain.
Challenges and Limitations
Calculating WTT emissions is complex because numerous factors must be taken into account:
Regional differences: Diesel in Europe may have different upstream emissions than diesel in Asia.
Technology and process dependence: The CO₂ factor for hydrogen varies depending on the production method (electrolysis vs. steam reforming).
Data quality and transparency: Reliable or up-to-date data is not always available for the entire supply chain.
This creates uncertainties that can make it difficult to compare different energy sources.
