How green are electric vehicles really?
Aug 07, 2012
Electric vehicles are not green just because they use electricity rather than fuel, but their green credentials critically depend on the materials used, the driving conditions and, most importantly, the carbon intensity of their electricity sources. Indeed, in some conditions, electric vehicles (EV) emit more CO2 than an internal combustion engine vehicle (ICE).
Why are EVs a better option in environmental terms than ICEs? While the obvious answer would be because of the avoidance of the combustion of fossil fuels, a proper accounting must justify the claim. Lifecycle Analysis (LCA) is an approach to assess the environmental impact of a product, including the accounting of all CO2 emissions produced in the whole life cycle of the product.
Breaking up this accounting of CO2 emissions, in the case of electric vehicles, we could represent it in mainly two parts: first, the CO2 emissions involved in the production and disposal of the vehicle, starting from the mining of their constitutive raw materials all the way through to the final disposal of the vehicle; and second, the emissions during the vehicle's lifecycle, which depend on its energy sources and efficiency.
LCA studies1,2 suggest that more CO2 emissions are generated during the production of electric vehicles than ICEs. This can however be more than compensated during the operational life of a car, provided that electric vehicles are powered using green energy sources. This suggests that the ultimate green credentials of EVs will vary from country to country, depending on the specifics of their electricity mix.
The figure below shows the fraction of fossil fuels as a percentage of total energy sources used for power generation and the carbon intensity of the electricity mix (CO2 kg/kWh) for different countries. Countries with higher use of fossil fuels will have higher CO2 per kWh. It can be readily deduced that EVs make a more positive environmental impact in countries such as Norway and France than Poland.
Figure 1 Carbon Intensity CO2 kg/kWh and % Fossil fuel in power sectora
The ultimate green credentials are also controlled by the efficiency of EVs, which is affected by the driving conditions and the make of the car. EVs demand much less energy than ICEs in urban use; this is mainly due to the low combustion engine efficiency in partial load. For EVs, the highest energy consumption occurs on a motorway.
Considering all these variables, pure EVs (exclusive use of electric) will have lower carbon footprints than ICE gasoline, but not necessarily than ICE diesel, provided that the country has a low carbon intensity of the electricity mix. In contrast, pure EVs will have higher carbon footprints than ICE gasoline, diesel and even hybrid EV when high carbon electricity is being used (e.g., as in Poland with 1 CO2 kg/kWh)
Figure 2 LCA greenhouse emissions of a compact car with different drive trains (120,000km, 70% urban driving, 0.62 kg/kWh)3 At increased levels of use of renewable electricity in the energy mix, major CO2 reductions can take place by the use of EVs compared with ICEs (see wind power case in Figure 2).
For the sake of simplicity the electric vehicle industry reports CO2 emissions per km, based on a fixed carbon intensity of the electricity mix and standard driving conditions to compare their performance against combustion engine vehicles. We can find interesting comparisons between specific electric car models, as the one following.
Figure 3 Carbon emissions, in grams CO2 per km, of a selection of cars for sale in the UK. The horizontal axis shows the emission rate, and the height of the blue histogram indicates the number of models on sale with those emissions in 2006.4
In conclusion, we can safely assume that we are reducing CO2 emissions while using an EV, provided it is used more in urban driving than motorway conditions and is used in a country which has an intermediate carbon intensity of electricity mix (e.g., UK or Germany with around 0.5-0.6 CO2 kg/kWh). Furthermore, if the country has increasing targets of renewable energy use in the electricity mix, major additional reductions in CO2 emissions will be achieved in the transport sector, particularly as the electricity mix becomes greener.
Well you know now, why and under what conditions, electric vehicles are greener than internal combustion engine vehicles.
1Notter D. et al, Contribution of Li-Ion Batteries to the Environmental Impact of Electric Vehicles, Environ. Sci. Technol. 2010, 44, 6550-6556
2Samaras C. et al, Life Cycle Assessment of Greenhouse Gas Emissions from Plug-in Hybrid Vehicles: Implications for Policy, Environ. Sci. Technol. 2008, 42, 3170-3176
3H. Helms, M. Pehnt, U. Lambrecht and A. Liebich 2010 Electric vehicle and plug-in hybrid energy efficiency and life cycle emissions, 18th International Symposium Transport and Air Pollution, Dubendorf Switzerland
4MacKay D. 2009, p.122 Sustainable Energy without the hot air, UIT Cambridge, UK
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