CMU study finds controlled EV charging can reduce generation cost, but at greater health and environmental costs depending upon the generation mix
In a study focused on the PJM portion of the US electricity grid, researchers at Carnegie Mellon University (CMU) found that although charging electric vehicles at night (when electricity is cheap and wind power is typically more plentiful) could lower electricity costs, doing so also creates more air emissions, and that the health and environmental costs from these emissions outweigh the electricity cost savings. A paper describing the work is published in the ACS journal Environmental Science & Technology.
Results from the study also suggest that with sufficient coal plant retirement and sufficient wind power, controlled charging could result in positive net benefits instead of negative. The result of the analysis depends on the details of the region, notes CMU Professor Jeremy Michalek, corresponding author—i.e., other parts of the US and the world could be different. The question of electricity costs vs. health and environmental cost is important to ask everywhere, Michalek said.
Although electric vehicles have lower tailpipe emissions than gasoline powered vehicles, the changes in emissions associated with vehicle electrification on a life cycle basis will depend on the emissions associated with the operations of the power plants used to charge the battery. Power plants currently produce 71% of national SO2 emissions, 1% of primary particulate matter emissions, and 14% of NOx emissions, which cause their own set of health and environmental problems. SO2 from power plants is a particular concern, as SO2 is a precursor of particulate matter. Power generation also accounts for over 40% of GHG emissions. Electric vehicle charging may affect these trends. Finally, the additional electricity demand from charging vehicles will affect the operations of the power system and potentially affect the costs of electricity.—Weis et al.
In the study, the CMU team evaluated the economic, environmental, and health costs and benefits of controlled electric vehicle charging in the PJM interconnection. The PJM Interconnection is a regional transmission organization (RTO) that coordinates the movement of wholesale electricity in all or parts of Delaware, Illinois, Indiana, Kentucky, Maryland, Michigan, New Jersey, North Carolina, Ohio, Pennsylvania, Tennessee, Virginia, West Virginia and the District of Columbia.
PJM the team noted, is an “interesting” power system to examine, they said, as it is the largest independent system operator in the United States by population and has a large installed coal capacity.
The team developed a unit commitment and economic dispatch model to estimate the operation costs and the air emissions externality costs attributable to new electric vehicle electricity demand under controlled vs. uncontrolled charging schemes.
Unlike earlier studies, the CMU team combined detailed modeling of the operating constraints of the electric grid with an estimate of the environmental and health damages from the additional emission due to vehicle charging, in addition to evaluating the change in operating costs.
The researchers used five different scenarios to investigate how different factors will affect emissions and the costs of charging:
Base Case: In this scenario, they assumed an electric vehicle fleet based on the PHEV35 model in GREET10 (similar to the Chevy Volt) and a fleet of power plants representing the PJM system in 2010.
Small Battery: For this scenario, they modified the base case so that the vehicle fleet is based on the Toyota Plug-in Prius.
Large Battery: For this scenario, they modified the base case so that the vehicle fleet is based on the Tesla Model S.
Future: For this scenario, they modified the base case to model a power plant fleet in 2018 by accounting for planned new power plant construction, plant retirement, and updated emissions rates and marginal generation costs.
High Wind Future: In this scenario, they modified the future case to add wind plants sufficient to produce 20% of generation.
For each scenario, they evaluated uncontrolled electric vehicle charging, in which drivers plug in their vehicles immediately after the last trip of the day, and controlled charging, in which vehicle charging is optimized to minimize the cost of generating electricity.
They found that controlled electric vehicle charging can reduce associated generation costs by 23%−34% in part by shifting loads to lower-cost, higher-emitting coal plants. However, this shift results in increased externality costs of health and environmental damages from increased air pollution.
The net implication is that controlled electric vehicle charging creates negative net social benefits in the recent grid scenarios but might produce positive net social benefits in a future grid with sufficient coal retirement and wind penetration. This finding is robust to uncertainty in vehicle adoption patterns, transmission constraints, reserve requirements, fuel prices, and air emissions implications.
In general, controlled charging has potential for reducing generation costs, but its net implications depend on the characteristics of the power plant fleet. In other regions with tighter environmental regulations, more renewable generation, less coal power, and/or inexpensive natural gas plants, controlled charging could lead to lower environmental and health damages. Our results also suggest that the externality costs missing from the current power system operations based on generation cost minimization are substantial and should be considered when making policy decisions to avoid large increases in human health and environmental costs.—Weis et al.
Allison Weis, Jeremy J. Michalek, Paulina Jaramillo, and Roger Lueken (2015) “Emissions and Cost Implications of Controlled Electric Vehicle Charging in the U.S. PJM Interconnection” Environmental Science & Technology doi: 10.1021/es505822f