Transport-related emissions are increasing globally. As per the recent report from MIT, currently, light-duty vehicles – that is, passenger cars, such as sedans, SUVs, or minivans – contribute about 20 percent of the net greenhouse gas emissions in the United States. But studies have shown that swapping a conventional gasoline-heavy car for one that runs on electricity can make a significant dent in reducing these emissions.
A recent study published in Environmental Science and Technology takes this a step further by examining how to reduce the emissions associated with the source of electricity used to charge an electric vehicle (EV). Taking into account regional charging patterns and the effect of ambient temperature on the fuel economy of automobiles, researchers from the MIT Energy Initiative (MITEI) find that the time of day an electric vehicle is charged impacts significantly on vehicle emissions.
“By making charging easier at particular times, you can really increase the emission reductions that result from the growth of renewables and electric vehicles,” says Ian Miller, lead author of the study and a research associate at MITEI. “So how do we do this? Time-of-use electricity rates are spreading and can dramatically change the time of day that EV drivers charge. If we inform lawmakers about these big impacts on charging time, then they can design electricity rates to discount the load when our power grids are renewable. In regions with a lot of solar energy, that’s noon. In windy regions, like the Midwest, it’s overnight. “
According to his research, in California, with lots of solar energy, charging an electric vehicle overnight produces 70 percent more emissions than charging it at noon (when more solar energy powers the grid). Meanwhile, in New York, where nuclear and hydro power make up a larger part of the electricity mix at night, the best charging time is the opposite. In this region, charging a vehicle overnight reduces emissions by 20 percent relative to daytime charging.
“The charging infrastructure is another big determinant when it comes to facilitating charging at specific times, especially during the day,” adds Emre Gençer, co-author and scientific researcher at MITEI. Today, most people charge their vehicles in their garages at night, resulting in higher emissions where it is best to charge during the day. “
“If you don’t model charging time and instead assume charging is done with annual average power, you can mis-estimate emissions from electric vehicles,” says Arbabzadeh.
To reduce this margin of error, the researchers use hourly network data from 2018 and 2019, along with hourly charge, drive and temperature data, to estimate emissions from electric vehicle use in 60 cases across the U.S. They then introduce and validate a novel method (with less than 1 percent margin of error) for accurately estimating emissions from electric vehicles. They call it the “average day” method.
This finding has useful implications for modeling future electric vehicle emissions scenarios. “You can get precision without computational complexity,” says Arbabzadeh. “With the average day method, you can accurately estimate electric vehicle emissions and load impacts in a future year without the need to simulate 8,760 grid emissions values for each hour of the year. All you need is an average day profile, which means only 24-hour values, for network emissions and other key variables. It is not necessary to know the seasonal variation of these average day profiles. “
The model used in this study is a module of a broader modeling program called Sustainable Energy Systems Analysis Modeling Environment (SESAME). This tool, developed at MITEI, takes a systems-level approach to assessing the full carbon footprint of the currently evolving global energy system.
In ongoing and future research, the team is expanding its individual vehicle load analysis to entire fleets of passenger vehicles in order to develop fleet-level decarbonization strategies.
“To mitigate climate change, we need to decarbonize both the transport and electrical energy sectors,” says Gençer. “We can electrify transportation and it will significantly reduce emissions, but what this document shows is how you can do it the right way.
