Contribution of country-specific electricity mix and charging time to environmental impact of battery electric vehicles: A case study of electric buses in Germany

Abstract

In many countries, the replacement of internal combustion engine vehicles (ICEVs) with battery electric vehicles (BEVs) offers an opportunity to reduce CO2 equivalent (CO2eq) emissions in traffic. BEVs have significantly greater energy conversion efficiency from energy storage to wheel (tank-to-wheel [TTW]) and zero tailpipe emissions; these features are in stark contrast to the environmental burdens posed by vehicle production and electricity generation (well-to-tank [WTT]). Since CO2eq emissions from vehicle use of BEVs account for the majority of the entire life cycle emissions, it is important to closely examine the CO2eq intensity of electricity generation, which varies based on the sources of energy in the electricity mix of each country and time of day.This paper compares the environmental impact of diesel and electric buses in the city of Aachen throughout their life cycles and analyzes the effects of electricity generation with reference to variation in charging time. Based on recorded driving profiles, the vehicle dynamics of the longitudinal motion are simulated, and the energy demand as a function of the altitude profile and ambient temperature is calculated. The calculation of the quarter-hourly CO2eq intensity is combined with the vehicle life cycle model in order to examine the effects of electricity generation on BEV life cycle emissions. To analyze the correlation between charging time and BEV environmental impact, different charging scenarios are defined, and CO2eq savings from replacing ICEVs with BEVs are estimated. Scenario analyses are conducted over both short- and long-term time scales to predict the future environmental and economic benefits and costs of BEVs compared to ICEVs.The results demonstrate a significant dependency of BEV environmental impact on charging time, which varies by time of day and season. At night, the contribution of renewable energies to the electricity mix is low, and the variation in CO2eq intensity is limited; therefore, the environmental impact of charging time is lower compared to charging at noon. The larger CO2eq emissions of BEV production are compensated for all scenarios within the first year of operation. However, if BEVs are charged with electricity generated exclusively by lignite-fired power plants, more CO2eq emissions are released over their lifetimes compared to ICEVs; this scenario also holds when ICEVs are fueled with biodiesel. Overall, the positive environmental balance of BEVs is not based on a low CO2eq intensity, but on the greater energy conversion efficiency and the possibility of recuperating braking energy. These properties are particularly advantageous in urban traffic.