Future liquid & gaseous energy carriers: A key prerequisite for carbon-free mobility

Abstract

Effective and affordable protection of the climate represents one of the utmost challenges of the current time. The overall reduction of greenhouse gases is a fundamental mission for all associated stakeholders. The transport sector covers nearly a quarter, accurately 24 %, of the global CO2 emissions – therefore the potential lever for reductions and optimizations is correspondingly huge. In order to meet the ambitious targets, like the European Green Belt initiative, cross-sectoral, multi-disciplinary and holistic measures are mandatory. An increased emphasis on sustainability throughout the society will not only drive the use of sustainable raw materials, it will also motivate and stimulate a closed loop economy for non-renewable materials, esp. for so-called critical materials, such as rare earths, whose supply is potentially threatened by political conflicts, increased use or by restrictions in trade or mining. As refining such materials from alloys is sometimes even very energy-intensive, Europe’s need for energy will likely not become smaller but rather greater, despite all activities aiming at better overall energy efficiency. Consequently, it is predicted that major markets in Europe will continue to import large amounts of energy from abroad. This paper in hand shares ongoing research activities and latest project results on sustainable, carbon-neutral mobility using green electricity to produce renewable fuels. This approach represents a complementary way to shift traffic to electrically powered propulsion systems. In view of the real-world market requirements, exist still many applications in the transport sector, where purely electric propulsion systems under all environmental conditions do not offer a suitable solution. This applies at least to the heavier CV’s and to long-distance operation as well as to large non-road mobile machinery. For such applications, gaseous and liquid fuels with their high energy densities will remain the prime choice for the near future. In the mid-term future, some of these energy carriers will be produced cost-efficiently from renewable energy sources in worldwide areas with the most favourable climatic conditions. Thus, besides powertrain electrification, either H2-operated propulsion systems, incl. H2-powered ICE’s but as well newly designed Power-to-X (PtX) fuels from renewable electricity and CO2 from various sources, so called e-fuels, are a highly attractive alternative to ensure mobility with a closed carbon cycle. Synthetic e-fuels also show considerable potential for solving the classic trade-off’s between CO2 and pollutant emissions of ICE based powertrains. Consequently, they provide a worthwhile solution for a clean and sustainable mobility in the next decades. However, to achieve a short-term reduction of CO2 emissions of the transportation sector, these fuels must be compatible with the technology of the current vehicle fleet and the existing re-fueling infrastructure, and, ideally, they should be miscible with fossil fuels. Fischer-Tropsch (FT) products for example are very similar to petroleum-based fuels and meet the latter requirement so that they can be mixed with conventional fuels without any problems. However, despite the principal similarity to fossil Diesel fuel, major differences in the combustion behaviour of FT-Fuels can arise. The presentation concludes with a summary on functional benefits and associated cost estimations.