E‐Mobility and the Energy Transition

Since the reduction of greenhouse gases is the top priority of the Energy Transition, primary electricity should be converted to material energy carriers. In this way electricity can be “stored” and made accessible for other applications. This Essay focuses on the integration of mobility in the Energy Transition and the development of sustainable alternatives to electricity-based transportation. The Energiewende (Energy Transition) is leading to continually larger quantities of renewable electricity (green electricity) in the electrical grid. Despite this, CO2 emissions are not decreasing as expected. One reason is the integration of energy supply systems has been neglected. The mobility sector, which makes up a substantial part of today's energy regime, is currently experiencing the largest upheaval in its history through the discussion of new types of power trains. If one also considers biomass in this scenario, a sustainable solution begins to emerge with a realistic chance of implementation. Individual mobility based on the automobile is a cornerstone of the global economy. Currently, there are 1.21×109 automobiles on the planet (6.1×107, or about 5 %, in Germany alone). This number is growing at a rate of 8.3×106 automobiles per year, or 2.5 per second, and is equal to the current growth rate of the human population. The global level of motorization is 160 cars per 1000 people, although the distribution shows a strong regional dependence: in Germany there are 672 cars for every 1000 people with similar numbers in the USA.1 Nearly all of these vehicles use power trains with a gasoline or diesel engine. The use of automobiles (to transport people and freight) in Germany in 2014 cost an equivalent of 2.43×1012 kJ of energy and produced 1.61×108 tons of CO2.2a In addition to these emissions, the conversion losses when starting from chemical energy sources must also be considered: for crude oil the conversion efficiency (from 2015) is 73 %. As a comparison the efficiency for the conversion of the primary German energy mix to electricity (from 2015) is 36.8 %.2b If the power for mobile power trains were generated purely electrically, as is the topic of current discussion, it would require 295 TWh of electricity in Germany alone. This assumes an average 40 % efficiency for the internal combustion engine and 90 % for an electric motor.3 Based on the current primary energy mix, the production of electricity for mobility would cause an increase of 1.51×108 tons of CO2 above the 3.26×108 tons already emitted from the normal production of electricity.4 These numbers show that e-mobility offers no appreciable contribution to environmental protection in the intermediate term. After nuclear energy is cut out of the energy mix, the inclusion of power production for mobility would actually mean an increase in emissions compared to the continued use of the internal combustion engine.5 A point of uncertainty is the potential of e-mobility for cargo transport for which as there are not enough data. In addition, considerable new investments in infrastructure for the production and distribution of electricity or hydrogen would be necessary. The future of the drive train has been discussed6 widely and emotionally since “Dieselgate.” The general mistrust in the automobile manufacturers about their data on fuel consumption and emissions is compounded by the certainty of technical and legal manipulation. The letter of the law may be followed but not the spirit: practical expectations of the reliability of a manufacturer's specifications in realistic driving situations are not fulfilled. Two objectives of this discussion have become conflated. On one hand, one would like to use changes in automobiles to contribute to environmental protection if the changes conform to the concept of a sustainable energy supply and implement renewable energy sources in the transport sector. In this context it is argued that battery-powered electric cars or the use of hydrogen in fuel cells could eliminate CO2 emissions from vehicle use. On the other hand, there is a discussion of local environmental protection in congested areas along with worries about the effect of the transport sector on air quality. In this case the focus is on regulated emissions such as atmospheric particulates or NOx as a part of a reaction chain leading to secondary particles and condensation nuclei. These emissions are becoming increasingly problematic in megacities in China in the form of smog.7 The result of this discussion is that the use of internal combustion engines—regardless of the technology, their future developmental potential, and the target to change propulsion technology—is disputable. If local conditions such as extensive access to green electricity, a suitable network of charging8 or hydrogen stations,9 and an appreciable development of battery10 and fuel cell11 technology are assumed, it is conceivable that the internal combustion engine will be abandoned for passenger vehicles. However, the necessary combination of all the abovementioned conditions will not be realized quickly: few regions of the world are expected to achieve this in the next 30 years. The internal combustion engine is an aggregate technology12 that has seen manifold developments. If one considers that fuel has changed in its composition but not in its specifications, then the potential of optimizing these system components simultaneously becomes apparent. The idea has already been put forward13 but was received with little fanfare in contrast to numerous proposals for developing “greener” fuel mixtures to improve the sustainability of combustion engines without a fundamental change in the technical details.14 A new “designer” fuel in a sustainable energy regime should, in the best case, have few regulated emissions and zero particle emissions. This would significantly simplify exhaust treatment. In order to reach this goal, the new fuel should consist of molecules that burn as homogeneously as possible and the actual burning15 should take place in a way that limits the generation of carbon radicals and their condensation products.16 In addition, the fuel should of course be manufactured in a sustainable way. Knowledge about the processes of burning small, oxygen-containing molecules in motor-relevant conditions17 shows that C1 units linked via oxygen are a good choice for such a fuel. There are no reaction pathways to molecules that can generate particulates. These kinds of molecules can also be easily obtained18 via hydrogenation of CO2 with green hydrogen whereby the intermediate product is methanol. One family of molecules with excellent fuel potential is the oxymethylene ethers (OMEs), CH3O(CH2O)nCH3 (n=1–7). OMEs burn in diesel motors without producing particulates and have low NOx emissions.20 The reduction of emissions can also be achieved in mixtures with diesel fuel21 and their synthesis can be facilitated22 by the use of low-moisture sources of methanol and formaldehyde. The main disadvantage of OMEs is their low energy density. Therefore, for a sensible application as fuel other chemical structures are needed with higher energy densities. One example are those available from biomass.19, 23 Research should continue along several paths and remain open to different technologies with the goal of developing the whole process chain up to a pilot plant. Only after testing is completed can it be decided with any certitude which advantages are offered by individual fuels and how biomass compares to a direct conversion of CO2 from unavoidable point sources. Efficiency should always be considered in the context of an entire system and not only with regards to the single element of mobility. There are, after all, when seen globally, differing regional basic resources and patterns of consumption which require a portfolio of fuels to sustain them. The gigantic dimension of this synthetic fuel production becomes more understandable when we consider GTL (gas-to-liquid) sites. For this reason, the incorporation of these processes into a particular country's sustainable energy supply will be systemically challenging because green electricity would be necessary in quantities similar to those now used for all other industrial applications. If we observe the current advances in the electric power train, a multifunctional power platform could be developed alongside the conventional use of today's automobiles based on designer fuels. And it could be conceived to deal both temporally and regionally with all forms of renewable energy without necessitating further infrastructure. Part of this adaption would be regulated by the integration of intelligent control systems. The basis of this is an electrical power train system8 with a stable, cost-efficient, and safe battery which would suffice for intermediate-range trips of approximately 100 km. The system would incorporate the recuperation of braking energy and the use of green electricity via simple, convenient charging stations. The basic version could be augmented with a host of possible add-on improvements such as battery modules, a fuel cell, or a specially developed internal combustion engine that runs on a designer fuel such as OME. This type of chemical-to-electric energy converter would be driven at constant load conditions and would be of simple mechanical construction: it would not be based on a single mechanical power transmission, but would rather run on several electrical generators. Some of these electric automobiles could be designed with the capacity for both a motor-generator and a battery. This would provide not only an energy source for driving, but also a local standard load for the electric grid when the cars are parked24 which makes use of solar primary generators. A concept for the implementation of a closed carbon cycle is shown in Figure 1. Mobility does not need to be “de-carbonized,” but rather gradually “de-fossilized.” This can be achieved through the combined local and central use of green electricity. In addition to mobility, automobiles will also take over a regulatory or stabilizing role in the electric grid: the batteries and the conversion of chemical energy from designer fuels will act as a sink for energy spikes. Also, they will act as forms of delocalized electricity generators. The production of green hydrogen represents a considerable standard load for the electrical grid and will necessitate “demand-side management” from the very beginning as a new, large consumer of electricity. Manufacturing these fuels would be a form of chemical energy conversion and solar energy storage. Mobility in a closed cycle of carbon (red lines). The system-wide integration of mobility, electricity supply, and carbon-containing materials is represented schematically. The integration of heat was not included for reasons of clarity. The development of designer fuels (yellow) plays an integral role. The important role played by biomass as a collector of locally emitted CO2 as represented in Figure 1 could be extended by a function to represent a carbon sink (gray in Figure 1). If biomass were to be passivated through the use of its own energy content and converted into a solid form of carbon (for example via hydrothermal condensation25), it would no longer be easily biodegradable and CO2 could be sequestered without the difficulties of handling a gas. This idea is not new26 but substantial chemical and ecological research is required in order to permanently fix carbon without consuming additional energy. For true sustainability we will certainly need processes that return the nutrients and minerals contained in biomass back to the soil. In a sustainable system only a part of the yearly growth in biomass (approx. 5×1010 tons C/a27) is available for these applications. For reasons of eco-stability a significant portion must remain in the system—we must all be able to feed ourselves—and the rest could be used for energy or stocks for “sub-zero emissions” programs. The earth is covered by approximately 4.1×107 km2 28 of forest and we may assume that we can use 25 % of this surface area to harvest the annual growth of biomass (this would mainly be forests in the northern latitudes of the USA, Canada, and Russia). In these regions the growth of biomass is about 4×102 tons C/a km2 27 so that 4×109 tons C/a could be harvested. If unavoidable emissions of approximately 10 % are assumed to be released during the conversion of biomass to biostable forms of carbon, the biomass approach could still remove CO2 emissions from the atmosphere about equal to the amount generated by the mobility sector. In addition, this process, referred to as “CCSI” (carbon capture and immobilization), enables the use of concentrated ash-free biomass along with fresh biomass and fossil fuels during gasification. We would like to underscore that the current insecurity in the development of the mobility sector, with all of its seemingly rival solutions, can be channeled into a powerful force for the Energy Transition. We now have the chance to integrate different sectors and many of the partial solutions that have been found will be able to find a suitable purpose in this context. The ability to place emphasis on different aspects of the energy system will allow the emergence of a portfolio of possibilities which can adapt to local geographical conditions. A continuous and gentle transition can indeed be realized because no single energy form is excluded. This both aids the rapid reduction of CO2 and makes sure that the appropriate economic and social components are incorporated in the energy revolution. Without wanting to understate the many challenges hidden behind these cursory conceptions, any such idea would at least be a strong starting point for a discussion about the priorities of the Energy Transition. For researchers, these priorities include, in addition to the verification of the assumptions made here, the development of scalable chemical processes for the production of green hydrogen and the conversion of CO2 to designer fuels as well as to carbon-containing functional materials. In fact, these represent indispensable components of any future energy regime. The authors declare no conflict of interest. Robert Schlögl studied chemistry in Munich and completed his Habilitation with Prof. Ertl in Berlin. He has been director at the Fritz-Haber-Institut of the Max-Planck Society (Berlin) since 1994, and founding director of the Max-Planck Institute for Energy Conversion (Mülheim a.d. Ruhr) since 2011. His research interests include inorganic chemistry, heterogeneous catalysis, nanostructures, materials research for chemical energy conversion, and concepts for sustainable energy generation and storage.