We know that electric cars can reduce carbon emissions – they are far more efficient as using energy, for example, simply by not losing it through the vibrations and therefore heat typically produced by an internal combustion engine.
But how much can they actually reduce emissions, and how is this affected by the types and extent of renewables present in particular grids?
A new study published in Environmental Science and Technology by researchers from MIT Energy Initiative (MITEI) demonstrates a novel model for working out reductions in emissions, and shows that the time of day an electric vehicle is charged has a significant impact on savings.
While the study is US-based, it has some relevance for Australia, particularly as it shares common attributes with California which is also undergoing a substantial transition to solar power.
The researchers found that in solar-heavy California, if an electric vehicle is charged at night it produces 70% more emissions than if charged during the day (noting that during the day is still producing less emissions than driving a petrol or diesel car).
Obviously, in solar-rich grids this has huge implications for how effectively electric vehicles can contribute to the reduction of emissions.
The researchers say that this sort of data is important to understand to ensure the correct type of charging infrastucture is present in appropriate places.
For example, if you live and work in a solar-rich area, it might be more useful to have access to charging at work.
“Charging infrastructure is a…big determinant when it comes to facilitating charging at specific times — during the day especially,” said Emre Gençer, co-author and a research scientist at MITEI.
“If you need to charge your EV midday, then you need to have enough charging stations at your workplace.”
In New York which has a large percentage of nuclear or hydro power – the best time to charge an electric vehicle is actually the exact opposite, because by charging at night an extra 20% of emissions can be saved.
Ian Miller, the lead author of the study and a research associate at MITEI notes that the data created from their modelling can also be useful for informing law makers and regulators about when best to introduce time-of-use tariffs to encourage EV charging at particular times.
“If you facilitate charging at particular times, you can really boost the emissions reductions that result from growth in renewables and EVs,” says Miller
“So how do we do this? Time-of-use electricity rates are spreading, and can dramatically shift the time of day when EV drivers charge. If we inform policymakers of these large time-of-charging impacts, they can then design electricity rates to discount charging when our power grids are renewable-heavy.”
The researchers noted that in the past, not modelling by hourly variations in the grid or temperature-driven variations in fuel economy can mean that projected emissions reduction data can be off by 10% in almost a third of cases. In solar-rich grids like California, the error can be up to a 50% deviation.
“If you don’t model time of charging, and instead assume charging with annual average power, you can mis-estimate EV emissions,” says Maryam Arbabzadeh, a postdoc who co-authored the report.
“To be sure, it’s great to get more solar on the grid and more electric vehicles using that grid. No matter when you charge your EV in the U.S., its emissions will be lower than a similar gasoline-powered car; but if EV charging occurs mainly when the sun is down, you won’t get as much benefit when it comes to reducing emissions as you think when using an annual average.”
To lessen the margin of error, the researchers used hourly grid data from 2018 and 2019 — along with hourly charging, driving, and temperature data – and applied this to a novel model they refer to as the “average day” method.
“We found that you can ignore seasonality in grid emissions and fuel economy, and still accurately estimate yearly EV emissions and charging-time impacts,” says Miller.
“This was a pleasant surprise. In Kansas last year, daily grid emissions rose about 80 percent between seasons, while EV power demand rose about 50 percent due to temperature changes. Previous studies speculated that ignoring such seasonal swings would hurt accuracy in EV emissions estimates, but never actually quantified the error. We did — across diverse grid mixes and climates — and found the error to be negligible.”
The modelling was also used to project future emissions. In this case, modelling in the southeastern United States from 2018 to 2032 showed that by 2032, compared to a similar hybrid car, EV emissions per kilometre might be 30 percent lower if charged overnight, and 65 percent lower if charged midday.
“To mitigate climate change, we need to decarbonize both the transportation and electric power sectors,” says Gençer.
“We can electrify transportation, and it will significantly reduce emissions, but what this paper shows is how you can do it more effectively.”
Citation: Hourly Power Grid Variations, Electric Vehicle Charging Patterns, and Operating Emissions
Ian Miller, Maryam Arbabzadeh, and Emre Gençer
Environmental Science & Technology 2020 54 (24), 16071-16085
Bridie Schmidt is lead reporter for The Driven, sister site of Renew Economy. She specialises in writing about new technology and has been writing about electric vehicles for two years. She has a keen interest in the role that zero emissions transport has to play in sustainability and is co-organiser of the Northern Rivers Electric Vehicle Forum.