Decarbonizing passenger cars using different powertrain technologies: Optimal fleet composition under evolving electricity supply

Abstract

Alternative powertrain technologies are seen as key elements to meet decarbonization targets for road-based motorized passenger transportation. But which technology can result in the largest avoidance in CO2, if the electricity sector is evolving in parallel? In this paper we investigate the maximum CO2 reduction potential supply side interventions can have on a national fleet under the best imaginable circumstances, considering the emissions from supply of fuels and electricity. The optimal fleet may contain battery-electric, fuel-cell electric and hybrid-electric propulsion systems, as well as their plug-in variants. Contrary to typical well-to-wheel analyses, we use disaggregated mobility data from a representative national household travel survey to represent individual vehicle usage behavior – some technologies may not be able to cover the demands of certain individuals. Also, we describe the energy conversion efficiency of all powertrain technologies using empirical data from a chassis dynamometer measurement campaign on actual, current vehicles. Using this information, we compute the CO2 optimal fleet compositions, i.e the fleet with largest CO2 avoidance under different CO2 intensities of the electricity. The latter serves as a proxy for the evolution of the electricity supply sector. The study is a potential analysis based on Swiss data.We identified three domains: (1) above 600gCO2/kWh the optimal passenger car fleet only operates on fossil fuels in the from of compressed natural gas, but could avoid about 35% in CO2 emissions, (2) between 600 and 235gCO2/kWh electricity is used as energy carrier, but the long daily distances still utilize natural gas, (3) below 235gCO2/kWh the entire fleet is powered through electricity, directly or by hydrogen electrolysis.Plug-in vehicles have the highest fleet share of around 80%. They are mainly operated as small capacity battery electric vehicles, since the majority of daily trips are rather short. A significant decarbonization requires a large amount of additional electricity, in the optimal case about 34% of the current chemical fuel energy.The potential CO2 avoidance is large even with todays technologies. Hydrogen is not crucial to substantially decarbonize passenger cars.