Even if enough renewable energy is available, simply switching from gas-powered cars to electric vehicles (EVs) won't be enough to fight climate change - unless the U.S. also upgrades its transmission grid, a new Northwestern University study finds.
If every gas-powered vehicle in the U.S. were replaced by an EV, transmission constraints would prevent the cleanest available electricity from reaching many charging locations. This "grid congestion" would force greater reliance on nearby fossil fuel power plants, undercutting the emissions benefits of electrification.
After identifying the issue, the study also recommends a modest set of targeted transmission grid upgrades to alleviate congestion and unlock the full emissions-reduction potential of EV adoption.
The study was published today (Aug. 6) in the journal Nature Communications.
"Even if the U.S. fully adopts EVs and generates enough renewable electricity to charge them, it still won't be enough," said Northwestern's Adilson Motter, who led the study. "We found the limiting factor for cars to be powered by clean energy has less to do with the availability of renewable energy and more to do with the ability to transmit that energy from generation sites to where it's needed. The power lines are congested, and that leads to congestion-induced CO2 emissions."
An expert on complex systems, Motter is the Charles E. and Emma H. Morrison Professor of Physics at Northwestern's Weinberg College of Arts and Sciences and the director of the Center for Network Dynamics. Motter conducted the research with Chao Duan, a former Research Assistant Professor at Northwestern.
The grid gets in the way
Like a highway system, the power grid is a vast infrastructure that transmits electricity across the U.S. After being generated at plants, electricity travels long distances through high-voltage transmission lines that span entire states and regions. It then reaches substations, where its voltage is reduced. Power then flows through distribution lines to homes, businesses and EV charging stations.
To analyze electricity's journey through power lines, the researchers combined data on vehicle usage and power grid infrastructure. Using advanced computer models, the team simulated the flow of electricity across the U.S. under varying levels of vehicle electrification and renewable energy generation
In every scenario with high EV adoption, grid congestion emerged as a critical bottleneck.
As EV adoption increases, so does electricity demand, especially in urban areas. But renewable energy sources like wind and solar are typically located far from cities, such as on rural wind farms or solar arrays in the desert. While clean energy is available, transmission capacity is often too limited to deliver it where it is needed, including EV charging stations. As a result, the grid draws electricity from closer - but more polluting - power plants that generate electricity by burning coal, oil and gas.
In the study's most ambitious simulation, scientists converted the entire U.S. vehicle fleet to electric. If the grid had adequate transmission capacity, this shift could eliminate nearly all vehicle-related CO2 emissions once renewable energy matches nonrenewable energy generation. But, with current grid constraints, one-third of those potential emissions savings would be lost.
"The charging schedule of EVs can be optimized to align with intermittent renewable generation," Motter said. "But even with smart charging, efficient use of clean energy still depends on having enough transmission capacity to deliver it where it's needed."
Smart, targeted upgrades
To address this bottleneck, the researchers calculated how much additional transmission capacity would be needed. They found that increasing the existing grid's transmission capacity by as little as 3 to 13% would significantly reduce congestion. This could involve building new high-voltage lines or expanding existing ones - enabling more clean power from remote wind and solar farms to reach the cities and suburbs where EV charging demand is highest.
Motter stresses that the entire grid does not need to be rebuilt. Instead, he recommends targeted upgrades in areas where congestions are most likely to occur. The U.S. grid is divided into three largely independent regions - Eastern, Western and Texas - with limited ability to transfer power among them. Improved connections and coordination among regions would help clean energy reach the areas that need it most.
"Power grids began as local networks, where consumption was close to generation," Motter said. "Over time, they evolved into nationwide - even continent-wide - systems. It was a gradual growth process built on existing infrastructure. No one wants to redesign it from scratch, but we do need targeted upgrades that reflect the large-scale reach of today's grid."
The study, "Grid congestion stymies climate benefit from U.S. vehicle electrification," was supported by Leslie and Mac McQuown through the Center for Engineering Sustainability and Resilience, a Resnick Award from the Paula M. Trienens Institute for Sustainability and Energy and the National Natural Science Foundation of China.
