There are many types of electric vehicles depending on how they are powered. These include hybrid electric vehicles (HEV) like the Toyota Prius, which has been on the market for several years now. These vehicles have a gasoline engine that can drive the car and charge the on-board battery in addition to an electric motor driven by the on-board battery-bank, which can drive the car by itself or in conjunction with the gasoline engine. There are no provisions to charge this battery pack with electricity from a wall socket. HEV’s are now being followed by plug-in hybrid electric vehicles (PHEV), which have larger battery packs that can be charged from a charging station. A plug-in HEV can run on battery alone – typically about 50 miles (80 kilometers). Some of the commercially available models include the Ford Fusion, and the Toyota Prius, which will have a plug-in version available in 2012
Then there are pure electric vehicles that run on electricity only. These are classified as battery electric vehicles (BEV) and fuel cell electric vehicles (FCEV). One of the commercially available BEV’s is the Nissan Leaf, which has a range of 100 miles (160 km). The other BEV on the market today is the Tesla Roadster, which has a range of 245 miles (394 km) as claimed by the manufacturer. Another BEV is the Chevy Volt, which is unique in the sense that it has an onboard gasoline engine that can be used to recharge the battery, thus providing an almost limitless range as long as gasoline is available. This car can go up to 40 miles (64 km) using the on-board battery alone (which can be recharged from a charging station).
There have been several prototypes of fuel cell electric vehicles where the electricity from the on-board fuel cell drives the electric motors that run the car. But none of these are available commercially.
Now the question is how will these electric vehicles be charged and what will be their impact on the electric power grid and the emissions coming out of the power plants providing the additional electricity? Let us assume we will have one million PHEV’s and EV’s on the road in the United States in the next five years—a target of the U.S. Department of Energy. In order to estimate the impact of these cars at a regional level, let us look a the Pennsylvania Jersey Maryland (PJM) interconnection area that serves about 54 million people in 13 states on the East Coast and the District of Columbia. This represents a sixth of the U.S population. Assuming PJM’s area represents the demographic mix of the U.S. population, one can expect one-sixth of the EV’s or 160,000 such vehicles on the street in the PJM area in the next five years. Assuming an average charging capacity of 5 kilowatts per car this represents a total electric demand of 5 kW x 160,000 or 800 megawatts. Given that all cars will not be charged at the same time, and assuming a diversity factor of 50 percent, we can expect an additional peak demand of 400 MW from these 160,000 EV’s. Given that the PJM peak load in the summer of 2011 was about 160,000 MW, this additional demand from EV’s amounts to a 0.25 percent increase in overall demand, which the system can easily absorb without any new power plants. Assuming an average of six hours of charging for these 160,000 EV’s, the daily electric energy requirement will be 160,000 x 5 kW x 6 = 4800 megawatt hours. Given that the PJM interconnection uses 40 percent coal and 19 percent nuclear generation, and has a load factor of 60 percent, if the EV’s are charged primarily during off-peak hours, these base-load power plants will provide the bulk of the electricity with small incremental CO2 emissions.
While the system-wide impact of EV’s will be small in the near future, the story is different at the distribution level. Distribution transformers serving residential customers can easily be overloaded if only two EV’s are plugged in the homes connected to the same transformer during peak load hours. Since upgrading of transformers en masse is an expensive option, this issue needs close attention as the EV penetration increases.
Editor's Note: Saifur Rahman is an IEEE Fellow and Editor-in-Chief of IEEE Transactions on Sustainable Energy. He also is the Joseph R. Loring Professor and director at Virginia Tech, Arlington, Va. He was interviewed for Going Electric: Things to Consider (September 2011).