Assessing the CO2 impacts in this way is known as a “well-to-wheels analysis”. A well-to-wheels analysis can be taken further still by asking about the relative emissions associated with manufacturing a traditional combustion engine car versus an electric car and its batteries. We won’t answer that very complex and detailed question with figures here, but suffice to say that based on our experience, the embodied emissions of the materials used in manufacture of a vehicle tend to be 15% to 20% of a vehicle’s “lifetime” emissions, when all the emissions associated with fuelling the car throughout its life are taken into consideration. The key point is that, at least on a CO2 basis, the main impact a vehicle has in its lifetime is in the consumption of energy when it’s being driven.
Is an electric car more efficient than a petrol or diesel car?
This is actually a more complicated question to answer than it first appears. If we looked at the efficiency [2] of the vehicle itself, we can see that an electric vehicle is seven times more efficient than a petrol car and four times more efficient than a diesel car. This is because in an electric car, there are far fewer energy losses, principally because there are fewer moving parts; an electric car has about four main moving parts compared to an internal combustion engine car with over 300.
However, this is a simplistic view as, in essence, an electric car does have a combustion engine – at the power station. So if we include the thermal efficiency of the power station, which varies greatly from power station to power station, and the transmission and distribution losses associated with getting the electricity to the car, we can see that, in the U.K., an electric car is slightly less efficient than the best diesel cars but more efficient than the best petrol cars. It must be noted that these results are very sensitive to the assumptions made about the efficiencies of the power stations and the assumed efficiency of the comparative petrol and diesel car. One could easily obtain quite different results using different assumptions.
What we’ve presented here is effectively the worst case scenario for electric cars as we have pitched them against some of the most efficient petrol and diesel models currently available, and we have assumed that the manufacturers’ claimed MPG figures would be achieved in the real world (For calculations, check Appendix I).
So will electric vehicles reduce emissions in the U.K.?
Yes. Even including the emissions at the power station, electric cars have lower emissions than an average petrol or diesel car.
To give you an idea of the scale of the potential emissions reduction, we’ve estimated what the total reductions would be if the U.K. government’s plug in car grant was fully subscribed by 2012. This could be used to subsidize 8,600 electric vehicles in the U.K., and assuming that these electric cars displace a mix of the most efficient diesel and petrol cars on sale, this would reduce CO2 emissions by 7,483 tonnes of CO2 each year, equivalent to taking over 3,600 cars off the road.
Notes:
[1] Some analysts have suggested using the carbon intensity of the marginal generating capacity, which often differs from the average grid intensity. However, given the gradual introduction of electric vehicles and the fact that charging is quite likely to be done at night rather than peak time, we considered the average intensity to be a reasonable basis for estimation.
[2] Efficiency is the amount of energy used per unit of distance traveled. Energy per unit of distance is different from emissions per unit of distance as different energy sources create different amounts of CO2.
About the Author:
Gary Davis is the operations director at Ecometrica and an experienced consultant specializing in the quantification of greenhouse gas emissions. He has a first class B.Sc. (Hons) in Physical Geography from the University of Edinburgh.
