Alternatives for the individual mobility of tomorrow

Nuremberg, January 2021. In September 2016, the German Bundestag unanimously approved accession to the United Nations Framework Convention on Climate Change, thereby setting the goal of climate neutrality in around 30 years; according to this, in the future we may only emit a maximum of as much CO ₂ in the overall system as we simultaneously absorb. The call for alternative drive systems is growing louder and louder, presenting industry and society with major challenges. But what alternatives are there and which are the right ones? A critical commentary by Dr. Guido Weißmann (Head of the Electromobility Competence Center) and Prof. Dr.-Ing. habil. Oliver Mayer (Head of Specialization Field Energy) on the drive alternatives of the future and their respective strengths and weaknesses.

Totally, we now meet slightly less than 20 percent of our total energy needs through renewable sources (heat and electricity). The remaining 80 percent is supplied by oil, coal , gas and nuclear power that we will have to replace in the future. If we look only at the electricity sector, the proportion of green power in the balance sheet is already at 40 to 50 percent, but overall it is still far too low.

If we now withdraw green power from the power grid for new applications, it will be missing from the grid. But other consumers run anyway and consequently get their energy from fossil power. Even with newly built PV or wind power plants , you can offset the fact that you could also feed in this additional green power as a coal substitute, at least on balance.

For a simplified overall view, it is therefore legitimate to always apply the CO ₂ emission in the German electricity mix of 400 g per kWh today for every kilowatt hour consumed (by the way: humans generate 1450 g CO ₂ per kWh). These are the framework conditions to which our actions as well as our individual desires and interests must increasingly be oriented.

Optimizing visions of the internal combustion engine

In discussions, the optimization potential of internal combustion technology is frequently mentioned. Yet the search for an internal combustion engine with less CO ₂ emission is actually a fallacy. Strictly speaking, you want to produce as much CO ₂ as possible, because - to put it simply - that's the chemical process to get the energy out of the fuel (ignoring side processes that produce other pollutants, e.g. NOx, SOx, etc.): "Hydrocarbon burns with oxygen to form CO ₂ , releasing energy in the process."

Any C atom that has not been converted to CO ₂ does not fully contribute to vehicle propulsion and ultimately ends up unused as soot in the exhaust. Ideal combustion thus necessarily produces 2.6 kg / 2.3 kg of CO ₂ per liter of diesel / gasoline burned. Modern internal combustion engines come closer to this limit and produce more CO ₂ per liter of fuel than older units, because they burn more efficiently.

According to a recent study by the TU Eindhoven, there are still over 30 percent emissions from

• fuel production,
• fuel transport,
• fuel storage and
• fuel supply in addition.

The often conjured up combustion potentials, if after about 150 years of engine developments here at all still significant developments are to be expected, therefore refer exclusively to how efficiently in the figurative sense the generated CO ₂ amount is converted into engine or drive power.

From an environmental point of view, however, only the generated, about 3 kg CO ₂ per burned liter of fuel are decisive. Notwithstanding various glossy vehicle brochures, actual fuel consumption in Germany averages just over 7 liters per 100 km. In contrast, the legal situation in terms of fleet emissions practically demands a 2.5 liter car by 2030. With a view to the physical-chemical bases as well as the trend to the achievement-stakten SUV it seems therefore already almost absurd to attribute still environmental optimization potentials to the internal combustion engine.

Taking electricity to the road

If you look at mobility in isolation, e-cars are particularly environmentally friendly when powered by renewable energy. In reality, therefore, virtually all notable charging infrastructure operators offer only green power at their charging stations. From the perspective of the overall system, however, it is irrelevant whether the limited green power is used for an electric car, a PC or a cook stove. It can only be consumed once.

Balance-wise, each kilowatt-hour in the German electricity mix always adds about 400 g of CO ₂ per kWh. A medium-sized e-car consumes an average of 16 kWh per 100 km. Added to this are a total of around 20 percent efficiency losses in the power grid and during the battery charging process. This raises the actual energy requirement to 20 kWh, which, when considered as a whole, results in emissions of 80 g CO ₂ per km. Since one avoids the corresponding gasoline or diesel consumption with each kilometer driven electrically, the e-car reduces the CO ₂ emission by about half. In addition, the emission is continuously reduced further with each new PV or wind power plant.

In the last five years alone, the CO ₂ emission in the energy mix has fallen by around 25 percent, whereas no further significant improvements can be expected for the internal combustion engine for physico-chemical reasons. Strictly speaking, it would be even better from an environmental point of view to convert the fuel into electricity in a modern CHP plant (combined heat and power) and use it to power e-cars. Moreover, this could also make use of the additional heat that would otherwise have to be generated separately with fossil fuels.

Or hydrogen after all?

In public discussions, the switch to hydrogen (H2) is often called for. Fuel cells generate electricity from it in the vehicle for the electric motor. Therefore, H2 vehicles actually also count as electromobility. Only the energy storage system is not a battery, but a fuel cell system. Water is produced as a waste product. That is environmental protection par excellence.

In the vehicle ("tank-to-wheel") this is also quite correct. Annoyingly, however, the problem lies before the H2 vehicle. Because pure hydrogen practically does not occur in nature. Rather, it must be extracted from the fairly stable water molecule at great energy expense. This one step alone is about as efficient as the entire battery electromobility power chain from wind turbine to tire (well-to-wheel). In addition, H2 involves significant losses due to transport, storage, provision and conversion back into electricity.

If you believe the relevant studies, only around 20 to 30 percent of the energy used ends up at the tire. If one saves the last conversion step and burns H2 directly in the cylinder, then even less usable energy remains at the end (approx. 18 percent). In comparison, it's the other way around with battery electromobility, 20 percent loss and 80 percent energy utilization. If one also sets an energy requirement of 16 kWh per 100 km for the H2 vehicle, one ultimately obtains a disastrous emission value of over 300 g CO ₂ per km in the overall balance.

However, hydrogen is a good storage medium and offers a weight advantage, especially for large amounts of energy, when batteries become too heavy due to their size. However, H2 can actually only bring this advantage to bear above the weight of a passenger car. In fact, a Toyota Mirai weighs about the same as a Tesla Model 3 for a comparable range, but it becomes interesting for trucks or coaches, shipping, or air travel. Nevertheless, it is also crucial here that H2 is not simply generated with green electricity from the grid, but with renewable surplus electricity.

This refers to electricity that cannot be fed into the grid due to local grid bottlenecks and would therefore be lost. However, the potential for this is currently relatively manageable at 5 to 6 terrawatt hours per year and would only suffice to power around 500,000 H2 cars, correspondingly less for H2 trucks. On the other hand, this truly green hydrogen is also urgently needed for many other applications (industrial processes, e.g. steel production, grid stabilization, etc.) and is thus actually too good to be used on the road. Without massive import of green hydrogen and the associated geopolitical, economic, large-scale industrial and societal impacts, it will not work. In this respect, climate-friendly H2 mobility in the passenger car sector is actually not yet seriously feasible in the foreseeable future.

Why not more biofuels?

Biofuels are hydrocarbons produced from plants, such as vegetable oil, bioethanol, biomethane and biodiesel. In principle, biofuels are renewable and CO ₂ neutral, since as much CO ₂ was absorbed via the plant during their production as is released during combustion. However, their carbon footprint deteriorates when they are produced on an agro-industrial scale using large amounts of fossil fuel, nitrogen fertilizer, or electricity. Biofuels from waste or residual biomass are less of a problem here, since the plants had already fulfilled their actual purpose and would have been thermally recycled or composted anyway. However, the potential for fuel production is manageable.

This is why biofuel is mainly obtained from energy crops grown explicitly for this purpose, e.g. rapeseed in Germany or sugar cane in Brazil. Consequences of monocultures and one-sided soil pollution due to a lack of crop rotation are often unavoidable. It becomes even more problematic when rainforests, moors or other biotopes are destroyed in order to develop them as cultivation areas. Relevant studies claim that biodiesel from cleared rainforest areas is even more harmful to the climate than classic diesel from petroleum. Regardless of this, one must also face the "plate-tank discussion", especially when agricultural land in poorer regions is used for the mobility of industrialized nations.

Overall, one can assume that liquid or gaseous biofuels can certainly be a suitable option as an admixture in classic fuels (e.g. E10 gasoline), niche applications or manageable mobility systems (urban mass transit, etc.) but rather not a viable solution for our entire mass mobility.

And then there are the synthetics

In the discussion about the powertrain of tomorrow, synthetic fuels keep coming into play. What's the deal with them? Synthetic fuels are also regenerative, but they are not based on biomass. In simple terms, H2 is first produced by electrolysis, which is then carbonized to hydrocarbons, i.e. C-H chains, with the help of CO ₂ . The resulting artificial fuel is almost identical to conventional gasoline. In the process, CO ₂ is absorbed but re-emitted during subsequent combustion in the engine.

Due to its very low concentration (0.04 percent), however, the CO ₂ cannot be extracted directly from the air. Synthetic fuel production plants are therefore ideally located where industrial processes automatically generate high CO ₂ concentrations. This is not trivial from a technical point of view, but it is certainly possible to get to grips with it. More problematic, however, is the energy balance, because carbonization of H2 reduces the already poor efficiency of the hydrogen chain by another 5 to 10 percent. Therefore, if we are already dependent on imports for green hydrogen, this will become even more pressing for synthetics.

However, synthetic fuel offers two important advantages:

• On the one hand, it can be transported more efficiently or economically over longer distances and, unlike pure H2, does not require energy-intensive cooling or compression.
• On the other hand, it can be used immediately and without adaptation of engine technology or infrastructure as a complete fuel substitute for our current burners.

Even if the required amounts of surplus renewable energy are not available and the necessary production facilities are not even planned, synthetic fuels could nevertheless represent a sensible option in the medium term to make the gigantic vehicle population of over 1.3 billion burners worldwide climate neutral. Today's cars (at least in Germany) have a statistical life expectancy of 12 years. Many nations do not want to register any new combustion vehicles in 10 to 20 years. The enormous added value of synthetics is therefore probably limited to the next 30 to 40 years. This makes corresponding investment decisions much more difficult. Moreover, this alternative would contradict the politically favored H2 energy economy, because green hydrogen would then not be directly available for a long time in favor of fuel synthesis.

Are there any other alternatives?

Apart from battery electromobility, the efficiencies of the drive alternatives do not look exhilarating. Or to put it in the words of VW CEO Herbert Diess: "We have not stopped research. On the contrary. It is precisely because we have looked intensively at all alternatives to the internal combustion engine that we are so clearly committed to E-mobility ."

However, we have not yet addressed other aspects of our current mobility at all, with a view to clarity and complexity of this text, e.g. the enormous use of raw materials, the relatively high space requirement of a car or the immense costs of a system conversion. Therefore, when thinking about mobility, one inevitably also comes to the core question: "How much mobility do we need at all and how much can we still afford in the future?"

Initially, this is difficult to answer, since we have always been so used to our current mobility. How could it ever be any different? But this is precisely where an approach to rethink mobility presents itself. And this is not about restricting personal freedoms. Today, for example, we transport people back and forth between home and work. Is that always necessary? In times of Covid-19 , by home office arrangements other options are visible that may well work for many activities and professions. Do goods and food always have to be shipped halfway around the world? Already today, supermarkets are offering the first shelves of domestic products, still in moderation, but definitely a start. In the future, processes are also conceivable in which a company develops a product but only sends the production data and, for example, sends it via 3D printing manufactures on site.

Many such concepts are still theory and, on closer inspection, often bring other disadvantages. What is important, however, is that we start thinking about mobility holistically, i.e., especially beyond the car, and develop concepts that are suitable for everyday use.