Beyond the Tailpipe: Why Your EV Might Be Dirtier Than a Hummer
- macncheesetango
- Feb 10
- 8 min read
Electric Vehicles (EVs) are gaining popularity as a sustainability choice to meet the Paris Agreement's goal of limiting global warming to 1.5 degrees Celsius. This is particularly significant given that transportation accounts for 29% of emissions in the US. However, the sustainability of EVs has become an increasingly complex and sometimes controversial topic. As with many environmental choices (and choices in life), the decision between an EV and a gasoline-powered vehicle isn't a black and white issue of good versus bad. In this blog, we'll explore the nuances of EV sustainability based on factors such as location, driving habits, and climate. We'll focus on the CO2 impact of vehicle use, leaving battery-related issues like end-of-life disposal and production emissions for a future post.
To set the stage, let's first understand the key differences between three main types of passenger vehicles: Electric Vehicles (EVs), Plug-in Hybrid Electric Vehicles (PHEVs), and conventional gasoline vehicles. We'll examine their power sources, charging or fueling methods, and basic operational characteristics to provide a clear picture of how these vehicle types differ in energy usage and environmental impact.
Electric Vehicles (EVs)
Electric Vehicles, also known as Battery Electric Vehicles (BEVs), are powered exclusively by electricity stored in rechargeable batteries. Key features include:
Power Source: Rely solely on electric motors powered by rechargeable batteries
Charging: Connect to external power sources through charging ports
Charging Locations:
Home charging stations (Level 1 or Level 2)
Public charging networks (including fast-charging stations)
Range: Typically 150-400 miles on a single charge, varying by model and battery capacity
Plug-in Hybrid Electric Vehicles (PHEVs)
PHEVs combine elements of both electric and conventional vehicles:
Dual Power Source: Equipped with both an electric motor and an internal combustion engine
Battery: Larger battery than traditional hybrids, allowing for extended electric-only operation
Charging: Can be plugged in to recharge the battery, similar to EVs
Fuel: Also have a fuel tank for gasoline, used when the battery is depleted or for longer trips
Operation Modes:
All-electric mode for short distances
Hybrid mode combining electric and gasoline power
Gasoline-only mode for extended range
Conventional Gasoline Vehicles
Traditional internal combustion engine vehicles powered by gasoline:
Power Source: Internal combustion engine burning gasoline
Fuel: Rely entirely on gasoline (or diesel in some cases)
Refueling: Exclusively at gas stations
Range: Typically 300-400 miles on a full tank, varying by model and fuel efficiency
The CO2 Impact
While EVs produce no tailpipe emissions, their overall carbon footprint depends heavily on the source of electricity used for charging. The electricity grid mix varies significantly across different states in the US, leading to substantial differences in EV emissions. The map below shows the average grid electricity mix in each state, which varies wildly. For instance, Vermont's grid is 100% renewable, while West Virginia's is predominantly coal-based (80%), with small contributions from biomass, natural gas, and other sources.
To put this into perspective, an EV traveling one mile in Vermont produces about 0.09 lbs of CO2, compared to 0.6 lbs in West Virginia - a sixfold difference. Over a year, assuming average driving of 13,500 miles, an EV in West Virginia would produce 6,950 lbs more CO2 than one in Vermont. This difference is equivalent to the emissions from 2.5 round-trip flights between San Francisco and New York City, or the amount of carbon sequestered by 143 trees annually. This shows that even within EVs, your EV can be considered more sustainable in one part of the US versus another part of the US.
However, it's crucial to understand that these figures are based on average grid mix, which doesn't always reflect the real-time impact of charging an EV. This brings us to an important distinction: the difference between marginal and average grid mix.
The average grid mix represents the overall composition of electricity sources over time (as shown in the figure above). In contrast, marginal emissions reflect the impact of additional electricity demand on the grid. When you charge your EV, the grid responds by increasing production from the most readily available (and often cheapest) source, which may not align with the average mix.
When comparing marginal and average emissions, this can create a huge discrepancy. To illustrate this, imagine we have a simplified grid with just a solar panel and a gas turbine shown in the figure below. During sunny hours, solar power might be prioritized due to its lower cost. However, at night or during peak demand, the gas turbine would likely meet any additional power needs (one can’t demand that the solar panel produces more, as one can’t ask the sun to shine brighter). This means that the marginal emissions caused by your specific charging action is higher than what the average grid mix suggests. On this simplified grid, the average signal is lower during sunny hours so the average emissions are lower. However, the marginal signal is constant, as the marginal power plant is always the gas turbine.
This discrepancy between marginal and average emissions creates a dilemma for EV owners trying to minimize their carbon footprint. Often making choices based solely on average emissions will often give biased results that display higher carbon emissions saving than we see in real world data.
So then, how do we make a decision with these conflicting results? While the average grid mix might suggest charging during peak solar hours to be on the cleaner grid and reduce emissions, the marginal emissions might be similar regardless of charging time if additional demand is always met by fossil fuel plants. This decision is often complicated by the fact that most people drive their EV to work and then charge their cars at night when they come home and don’t have access to charging stations around their community or at the work location.
While there may seem like no good solution to this problem, like usual in sustainability questions - The solution to this lies in long-term planning and community engagement. If there is persistent demand during the daytime sunny hours, this would create incentives to install new solar panels, as they are probably the cheapest (and cleanest) option to expand the grid to deliver electricity at these times. If the additional demand wouldn’t have happened during sunny hours, chances are that the grid would need to be expanded with new fossil fuels, or with storage systems.
The UK provides an encouraging example of this principle in action. From 2010 to 2019, the country significantly reduced its reliance on fossil fuels for electricity generation while simultaneously expanding its EV fleet from 1,000 to 74,000 vehicles, the third largest in Europe. The figure below shows this effect in action and how expanding the clean energy grid can reduce emissions over time. From 2010 to 2019, 75% of energy came from fossil fuels whereas in 2019 50% of the energy came from fossil fuels. This cleaning up in the energy grid decreased carbon emissions from the same EV by about 20%.
While the electricity source is a major factor in EV sustainability, other regional aspects also play a role. Climate, for instance, can significantly impact emissions. Colder states generally show higher emissions for all vehicle types, with EVs being particularly affected due to battery inefficiency and increased energy demand for cabin heating in low temperatures.
Driving patterns also influence the relative efficiency of EVs versus conventional vehicles. Urban environments with frequent stop-and-go traffic tend to favor EVs, as their regenerative braking systems are more efficient in these conditions. In contrast, rural areas with more highway driving might see conventional vehicles performing comparably or even better in terms of emissions.
How do all these factors compare in the real world?
A comprehensive 2023 study examined these factors along with vehicle-specific characteristics to compare emissions across different car models and US counties. The study looked at two most efficient gasoline hybrids on the market (Toyota Prius and Honda Accord Hybrid) and three electric vehicles (Tesla Model S, Chevrolet Bolt, and Nissan Leaf) with varying battery capacities.
The figure below shows their results and illustrates the differences in lifecycle emissions between EVs and hybrid vehicles across various U.S. counties. Counties shaded in blue indicate areas where the lifecycle emissions of the EV are lower than those of the hybrid vehicle, making the EV the more sustainable choice. Conversely, counties shaded in red represent areas where the EV produces higher lifecycle emissions compared to the hybrid vehicle, highlighting regional disparities in sustainability based on energy sources and driving conditions.
The results showed that the Bolt and Leaf had lower emissions than the gasoline hybrids in most counties in the West, Texas, Florida, and New England. However, these EVs had higher emissions in rural counties of the Midwest and South. Interestingly, the Tesla Model S, despite its popularity, had higher emissions than the gasoline hybrids in all counties, likely due to its larger battery and higher energy consumption.
These findings highlight the regional variations in EV sustainability and underscore the need for grid improvements. For EVs like the Leaf or Bolt to achieve emissions parity with gasoline hybrids in the Midwest and parts of Appalachia, the marginal emissions factor would need to be reduced by 207 gCO2/kWh. This would require a significant shift in the energy mix, such as Ohio moving from 60% coal, 25% gas, and 15% renewables to a cleaner mix of 20% coal, 40% gas, and 40% renewables.
The challenge is even greater for larger EVs like the Tesla Model S. For this vehicle to have lower lifecycle emissions than gasoline hybrids, most US regions would need to reduce their marginal emission factors by up to 342 gCO2/kWh. This translates to an average state needing to transition from 75% non-renewable electricity sources to just 25% - a monumental shift requiring substantial time and investment.
It's worth noting that these comparisons are based on 2018 data, and the landscape of EVs is rapidly evolving. As of 2024, the best-selling EV in the US is the Tesla Model Y, however these vehicles have similar emissions characteristics with the Model S used in the study.
In conclusion, the sustainability of EVs is a complex issue influenced by numerous factors including vehicle type, location, charging habits, electricity sources, climate, and driving patterns. While there's no one-size-fits-all answer, understanding these variables can help individuals make informed choices suited to their specific circumstances. Ultimately, for EVs to become more universally sustainable in the US, a concerted effort to increase renewable energy in the electricity grid is crucial. This transition would not only make EVs more competitive with gasoline cars in terms of emissions but could also lead to lower electricity bills for consumers.
Sources:
コメント