Legislation and public debate on reducing CO2 emissions in the transport sector have centered almost exclusively on nationwide use of battery electric vehicles (BEVs). There are areas of application, however, for which pure battery electric powertrains are not a suitable solution, e.g. for long-distance and heavy-goods transport. Due to their high energy density, liquid and gas fuels will remain the fuel of choice in these areas for a long while yet. This is particularly true for Europe, which will still remain heavily dependent on importing chemical sources of energy. In the future, some of these energy sources will be produced in regions where renewable energy sources are consistently available in large quantities. E-fuels – synthetic fuels made from renewable electricity and carbon dioxide (CO2) – represent a very attractive option here for powering mobility using a closed carbon cycle.
Significant reduction of CO2 emissions
Through the Paris Agreement, as well as the social policy goal of combating climate change, all sectors are committed to significantly reducing their CO2 emissions. The electric power supply sector is supposed to become entirely CO2 neutral by 2050. The transport sector is meant to reduce its carbon footprint by at least 80 percent as compared to 1990 – this despite an ever-increasing volume of transport. All conceivable climate-friendly solutions for passenger and goods transport also urgently need to be implemented. Some of these goals are reflected in the increasingly strict CO2 limits for newly registered vehicles in most countries. On December 17, 2018, the European Parliament and the Council of the European Union decided to reduce the fleet CO2 limits for newly registered vehicles again. CO2 emissions from cars are supposed to be reduced another 37.5 percent between 2021 and 2030. This translates to 59 g/km, which is equivalent to about 2.5 l of fossil fuel. When it comes to greenhouse gas reduction in the private transport sector in particular, one technical solution pops up more than most in the public and media debate: BEVs. The number of BEVs will indeed rise dramatically over the next decade. The starting level is still relatively low, however, with BEVs, plug-in vehicles, and range-extender vehicles currently making up roughly 2 percent of newly registered cars. The market penetration of BEVs depends heavily on various local legal parameters and government subsidies, and thus varies significantly in global terms.
The majority of all cars sold in 2030 (close to 90 percent) will still have a combustion engine for various reasons. The most frequent reasons cited for deciding not to buy an electric vehicle are the purchase price, the limited range, long charging times, and inadequate development of the charging infrastructure. In addition to this, only 2.5?–?5 percent of Europe’s vehicle fleet is replaced each year. As such, it will take several more years for BEVs and fuel cell vehicles to have significant market penetration. Relying solely on the growth of the electric fleet to achieve the ambitious CO2 targets is thus not an option. Instead, other effective technologies also need to be used. Using CO2-neutral synthetic fuels (e-fuels) can help in addition to electrifying powertrains and improving the efficiency of combustion engines.
It can be presumed that electrification will provide the largest contribution to CO2 reduction. In this context, electrification means not only pure electric vehicles, but also fuel cell vehicles, hybridizing combustion-powered powertrains, and powertrains with range extenders. An additional 24 percent reduction can be achieved by increasing efficiency through weight, friction, and enhanced aerodynamics as well as changing the modal split (shifting goods to rail). The remaining 31 percent needs to be covered by CO2-neutral fuels, as seen in Figure 2.
Looking at the energy situation in Europe and particularly in Germany, it is clear that the task of creating a system based 100 percent on renewable electricity poses a major challenge. Since renewable power (especially in Europe) is highly volatile and is not easy to store, grid expansion and necessary reserve capacities could involve very high investment costs in the future. The goal of using 100 percent renewable electric energy also means Germany would have to increase its production of renewable energy at least threefold compared to today’s production.
In addition to energy consumption, there is also significantly higher demand for energy for various industrial applications, the heating of buildings, and transport networks, currently total 2,600 TWh per year in Germany. In order to meet these needs using energy from renewable sources, Germany will have to start importing renewable energy on a large scale (Figure 3). Since some of the transmission paths are long, directly importing electric energy is only technically feasible to a certain extent, however. As a result, electric energy harnessed overseas using solar and wind power will need to be converted into chemical sources of energy by means of power-to-fuel. For regions with shorter distances between the production site and consumers, hydrogen or substitute natural gas could also be used as a carrier and transported via pipeline. Conversion into methanol or even Fischer-Tropsch products is a more sensible approach for more remote production facilities. Overall, Germany needs to import up to 29 percent of its energy requirement in the form of power-to-X (PtX) by 2050.
Using CO2-neutral sources of energy is the most efficient way to reduce its carbon footprint. As drop-in fuels, they can also reduce the carbon footprint of existing vehicle fleets. Due to their molecular structures, many prospective PtX fuels have different chemical and physical properties. The candidates that best meet the key criteria (energy density, fuel availability and established production paths, compatibility with the existing fleet) are Fischer-Tropsch fuels and longer-chain alcohols for diesel engines, as well as methanol-to-gasoline (MTG) and methanol for gasoline engines. It is possible to use hydroformylated Fischer-Tropsch fuels containing long and medium-chain alcohols to produce an e-fuel for diesel engines that is compatible with the current EN 590 standard and can thus be mixed into the existing fleet at any ratio. As shown in Figure 4, these fuels also allow for a significant reduction of soot and/or NOx.
Ethanol could be a highly promising candidate for gasoline engines. In 2018, some
110 million tons were synthesized and traded, primarily for the chemical industry. Due its very high knock resistance and good lean-burn characteristics, methanol can be used to significantly improve the efficiency of gasoline engines. As a result, e-fuels can be used to achieve similar tank-to-wheel efficiencies as in fuel cell vehicles. Figure 5 shows the increase in efficiency for engine measures implemented thus far and how the efficiency target of 50 percent could be achieved in the future.
Some countries, such as China, are massively promoting the use of methanol. In Europe, the methanol content in gasoline is currently limited to 3 percent v/v in EN 228, even though most fuel system materials are already certified up to 15 percent v/v. In addition to its direct use as fuel, methanol is also very well suited as a starting material for other fuels. For instance, the methanol-to-gasoline (MTG) process can be used to produce a gasoline-equivalent synthetic fuel that can also be mixed with conventional fuel in large quantities.
Areas of application for e-fuels
In the future, Germany will need to depend heavily on PtX imports. Cost will be the primary factor for the expansion of various technologies, however. Since local circumstances have a critical impact on the availability of renewable energies, the costs of synthesis also vary quite dramatically around the world. Figure 6 shows how greatly fuel production costs depend on the costs of power.
In many countries, e.g. in the Middle East and North Africa (MENA), the costs of synthesis will drop to below 1 euro/l diesel equivalent by 2030 due to low electricity rates. Although the potential for e-fuels has also been discussed outside of Europe, no large-scale PtX plan is currently in the works since legislation still does not acknowledge any CO2 reduction through e-fuels. As a result, market players still do not see a substantial enough business case to invest in e-fuels. A quick market introduction would be possible if one stakeholder were to profit from producing, marketing, or using e-fuels. A certificate trading system could also be introduced to allow carmakers to purchase CO2-neutral fuel and the corresponding certificates. By mixing it into the existing infrastructure, the fuel would be used by all customers.
The CO2 savings resulting from the use of e-fuels would then be counted towards the CO2 emissions of the manufacturer’s vehicle fleet. Another option would be to redesign the energy tax by lowering it on renewable sources of energy and gradually increasing the costs for CO2 emissions from fossil fuel combustion. The stakeholders addressed could be the petroleum industry or carmakers. This would yield a sustainable business model with the urgently needed investment security.
All technically feasible options will need to be used to achieve a quick reduction in CO2 emissions. However, the powertrain systems for heavy-load and long-distance vehicles cannot be electrified to the same extent as those for light commercial vehicles and cars. Goods transport will still need to rely on liquid or gas chemical sources of energy. As such, Europe will also rely on substantial imports of chemical sources of energy. The tank-to-wheel accounting currently gives a strong preference to electromobility – which will contribute enormously to lowering fleet CO2 emissions – over alternative technologies.
E-fuels have not been counted toward fleet emissions yet. Legislation thus requires urgent revision. There are various ways to make synthetic fuels more attractive, e.g. a tax on CO2 or carbon from fossil sources. Another option is to count e-fuels toward fleet emissions using a certificate trading system. Regardless of the political instrument used, synthetic fuel absolutely must be compatible with the existing fleet in order to quickly achieve market penetration.