Conflicting interests in defining an ‘optimal’ battery size when introducing the PHEV?

The PHEV is an interesting option for reducing greenhouse gas emissions from transport or to increase energy security without losing performance in car operation. However finding an optimal battery size is of great importance for the overall economic and environmental performance of the PHEVs. This study investigates the resulting vehicle design, and fleet composition and performance when optimising the PHEV battery with respect to different objective functions possibly reflecting different actors´ interest: number of PHEVs, cumulative cost savings and share of electric driving respectively under various assumptions concerning policies and costs. A recently available data set of car movements, containing 445 privately driven Swedish cars that have been measured with GPS-equipment for 1-2 months each is utilized to get a representative car fleet. We find that the battery size and fleet performance are heavily influenced by not only the choice of objective function for the optimization but also by its interaction with the cost structure and performance requirement in the transition from an energy efficient fuel-driven car to a PHEV. The effect of different policies may also vary depending on these conditions and may favour various actors´ interests differently. We conclude that these aspects are important to consider both when designing vehicles and when formulating policies for the introduction of PHEVs.