Plug-in Hybrid Vehicle Simulation: How Battery Weight and Charging Patterns Impact Cost, Fuel Consumption, and Co2 Emissions

Plug-in hybrid electric vehicle (PHEV) technology is receiving attention as an approach to reducing U.S. dependency on foreign oil and emissions of greenhouse gases (GHG) from the transportation sector. Because plug-in vehicles require large batteries for energy storage, battery weight can have a significant impact on vehicle performance: Additional storage capacity increases the range that a PHEV can travel on electricity from the grid; however, the associated increased weight causes reduced efficiency in transforming electricity and gasoline into miles driven. We examine vehicle simulation models for PHEVs and identify trends in fuel consumption, operating costs, and GHG emissions as battery capacity is increased. We find that PHEVs with large battery capacity consume less gasoline than small capacity PHEVs when charged every 200 miles or less. When charged frequently, small capacity PHEVs are less expensive to operate and release fewer GHGs, but medium and large capacity PHEVs are more efficient for drivers that charge every 25-100 miles. While statistics on average commute length suggest that frequent charges are possible, answering the question of which PHEV designs will best help to achieve national goals will require a realistic understanding of likely consumer driving and charging behavior as well as future trends in electricity generation. INTRODUCTION Increasing concerns regarding high oil prices, oil dependency, and climate change have resulted in policymakers and the automobile industry evaluating alternative strategies for passenger transportation. PHEV technology offers a possible approach to reducing U.S. dependency on foreign oil and GHG emissions via the use of large rechargeable storage batteries to enable electricity to provide a portion of the propulsion requirements of a passenger vehicle. Since approximately 60% of U.S. passenger vehicle miles are traveled by vehicles driving less than 30 miles per day (USDOT 2004), PHEVs may be able to displace a large portion of gasoline consumption with electricity. Additionally, the price differential between retail electricity and gasoline could make electric-powered travel more cost effective than gasoline, depending on the additional vehicle capital costs (Scott et al. 2006). However, the reduced fuel use, economic costs, and GHG emissions of PHEVs depend on the vehicle and battery characteristics, as well as source of electricity used for recharging. For example, the full life cycle GHG emissions associated with manufacturing and operating a PHEV are comparable to traditional hybrids under the current U.S. mix of electricity generation (Samaras and Meisterling 2008). Trends in electricity generation, battery manufacturing, and the use of biofuels have critical implications on the relative advantages of PHEVs. There are several variants of PHEV design, and Bradley and Frank (2007) provide a review of the potential PHEV vehicle architectures. All PHEVs have a drivetrain that incorporates an electric motor and an internal combustion engine (ICE), both of which provide torque for vehicle propulsion (Bradley and Frank 2007). The storage battery of a PHEV, which can be recharged using conventional electrical outlets, would allow the vehicle to drive for a limited range solely (or primarily, depending on the configuration) via energy from the electricity grid. PHEV drivetrains could be arranged in parallel or in series. In a parallel configuration, generally both the electric motor and ICE provide power to the drivetrain after the storage battery has been depleted to approximately 20% of its initial state-of-charge (SOC). A parallel PHEV with a 20% SOC would perform similarly to a traditional Toyota Prius hybrid (HEV). In a series configuration, the electric motor provides power to the drivetrain, with the ICE responsible for battery energy management. Series designs have been estimated to have lower fuel economy, lower efficiency, and higher component costs (Bradley and Frank 2007) and hence, we focus on parallel PHEVs. Since PHEVs rely on large storage batteries for any economic or environmental benefits relative to traditional