Optimising Car Life for Minimum CO$_2$ Emission

In this paper, the historical trends and future projections of whole of life CO₂ emissions is followed and includes the changing effects on embedded production energy as vehicles have been made lighter. Even so, the rapid reduction in fuel consumption of conventional vehicles leads to the ratio of embedded to in-use CO₂-e to have doubled in the last 30 years. This embedded energy sourced CO₂ recurs each time a new car is made, so the front end energy has to be amortised over the life of the vehicle. It is shown that the ratio is several times higher for battery electric vehicles, while hybrids fall between electric and conventional. The importance of vehicle useful life is emphasized. In the past, the optimum life to amortise the embedded energy was about 17 years but this depends on the prevailing rate of improvement in in-use energy of the marketed fleet. The paper concludes on the basis of the evidence presented that the optimum life for present conventional vehicles is between 10 and 12 years and for battery electric vehicles approaching 20 years with hybrids falling between. As the rate of annual fuel consumption improvement reduces from the present level of 5%/y, the desirable life-times of vehicles will increase. It is recommended that some form of government policy be implemented to achieve the changes in optimum vehicle life-time, over the next few decades, through support for ‘Cash for clunkers’ or equivalent mechanisms. This will enable the most rapid achievement of greenhouse gas emissions reduction. Incentives or other mechanisms need to be found to encourage hybrids rather than all electric vehicles to achieve best possible vehicle fleet CO₂ reduction.