Numerous studies have tackled the future environmental impact of battery-electric vehicles; the resulting assessments are highly uncertain. Now, a team of researchers at the Paul Scherrer Institut in Switzerland, with colleagues at Leiden University (Netherlands) and PBL Netherlands Environmental Assessment Agency have used Monte Carlo and global sensitivity analysis to quantify parametric uncertainty of the impacts.

They also included two additional factors that have not yet been addressed in the field, they said. The first is changes to driving patterns due to the introduction of autonomous and connected vehicles. The second is the impacts of changes to the electricity sector on the environmental burdens of producing and recharging future EVs.

Life cycle climate change contribution of current (production year 2017) and future (production year 2040) electric vehicles per kilometer driven. The boxes contain 50% of the results, while the whiskers show the 5th and 95th percentiles and the red line shows the median result. Y axis labels show the background database that was used for the calculation: ecoinvent uses the current electricity scenario while Baseline and ClimPol show middle-of-the-road and aggressive climate policy scenarios for the future electricity sector respectively. Credit: ACS, Cox et al. Click to enlarge.

We find that the electricity used for charging is the largest source of variability in results, though vehicle size, lifetime, driving patterns and battery size also contribute to variability in results. We also develop a method of including the energy savings potential of future autonomous and connected vehicles in our model and find that this new technology is most likely to result in roughly 10% energy savings, though energy savings could be up to 30% with ideal technology performance.

Future electric vehicles are likely to have roughly 20% lower environmental impacts compared to current vehicles based on technological improvements alone. However, when considering also the future improvement of the electricity sector, these improvements are in the range of 30-70% for baseline and climate policy scenarios. This is an important conclusion that must be considered when making policy in the mobility sector as use of current results as a proxy for future technologies would be highly misleading.

It is common practice when performing LCA of future technologies such as electric vehicles to assume that the background database used for infrastructure production can be considered constant and that the future electricity mix needs only be considered for direct consumption. We find that neglecting the improvement of the future electricity system when calculating the upstream impacts of EV production changes the results by 23-52 g CO₂ eq / km. This is an error of over two standard deviations for the ClimPol scenario! Thus, especially in future scenarios where renewable electricity is used to charge the EVs, it is extremely important to consider also the development of the electricity sector, as the upstream impacts due to electricity production can significantly impact results.

—Cox et al.

In their study, the team used a Life Cycle Assessment (LCA) modeling framework. They quantified the impact of uncertain input parameters and key life cycle inventories on the environmental performance of current (production year 2017) and future (production year 2040) EVs. They integrated Monte Carlo analysis with driving cycle simulation to calculate vehicle energy consumption. They also included the impacts that connected and autonomous vehicles may have on energy demand, such as traffic smoothing, eco-driving, and platooning.

They also factored in changes in the background energy systems so that the entire EV production chain was consistent with the particular electricity generation scenario used.

The model considers vehicle sizes ranging from mini to luxury car, with vehicle mass as an uncertain parameter. The most likely vehicle size in the model is comparable to the Renault Zoe and the Nissan LEAF.

Resources

• Brian Cox, Christopher L Mutel, Christian Bauer, Angelica Mendoza Beltran, and Detlef P. van Vuuren (2018) “Uncertain Environmental Footprint of Current and Future Battery Electric Vehicles” Environmental Science & Technology doi: 10.1021/acs.est.8b00261