Alternative fuels
Today, 10% so-called bioethanol is usually added to the gasoline in order to reduce the CO2 emissions of the German motor vehicle fleet. Would an expansion be a sensible option? The consideration of the efficiencies gives a somewhat better insight into the situation here: plants that produce biomass through photosynthesis do so with an efficiency between one and two percent of the incident solar energy[1]. In addition, fertilizer is still needed for the plants and diesel for the agricultural vehicles. The conversion of biomass into bioethanol further reduces the efficiency. Ultimately, the use of bioethanol in an internal combustion engine with an efficiency of also below 30% is very low. Since we have to multiply efficiencies, we come to an overall efficiency of less than 0.5%[2]. If we compare this with the PV and a battery-powered electric car, we need for each kilometer we want to drive, more than 30 times the area for energy production. This area is not available in Germany! And here, there is a significant ethical conflict between food production and the production of energy crops.
Another technical option is the conversion of electricity into a synthetic fuel, with which conventional internal combustion engines can be operated. On the vehicles one would not even have to make any major changes for these "e-Fuels"[3]; they could basically be driven directly with such fuels. Here is the problem especially the low efficiency. If we completely shifted our mobility to e-fuels, we would need around 5 times more renewable electricity than battery-powered vehicles for the transport sector alone.
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[1]Barber, J. (2009): Photosynthetic energy conversion: natural and artificial. In: Chem Soc Rev. 38(1); 185–196