Charging cars at home at night is not the way to go, finds a Stanford study

September 22, 2022

The shift to electric vehicles will entail huge costs for generating, transmitting and storing more energy. Moving current EV charging from home to work and night to day could cut costs and help the network, according to a new Stanford study.

By Marco Dorato

The vast majority of electric vehicle owners charge their car at home in the evening or overnight. We’re wrong, according to a new Stanford study.

According to a new study from Stanford University, if the common charging of electric vehicles at home in the evenings or night shifts at work shifts during the day to work as more cars become electric, that would limit the additional costs for electrical systems. (Image credit: Amy Adams)

In March, the research team released a paper on a model created to charge demand that can be applied to a range of populations and other factors. In the new study, published on 22 September a Energy of nature, applied their model across the western United States and examined the stress the region’s power grid will experience by 2035 from growing ownership of electric vehicles. In just over a decade, they found that the rapid growth of electric vehicles alone could increase peak electricity demand by up to 25%, assuming a continued dominance of overnight charging.

To limit the high costs of all that new electricity generation and storage capacity, the researchers say, drivers would need to switch to daytime charging at work or public charging stations, which would also reduce greenhouse gas emissions. This discovery has political and investment implications for the region and its utilities, especially since California moved in late August to ban the sale of gasoline cars and light trucks starting in 2035.

“We encourage policy makers to consider utility rates that encourage daily charging and incentivize investment in charging infrastructure to move drivers from home to work for charging,” said the study’s senior co-author. Ram Rajagopal, associate professor of civil and environmental engineering at Stanford.

In February, cumulative sales of electric vehicles in California hit one million, or about 6% of cars and light trucks. The state has targeted five million electric vehicles on the road by 2030. When penetration reaches 30% to 40% of cars on the road, the network will experience significant stress without major investments and changes in charging habits, he said. Rajagopal. Building such infrastructure requires significant lead times and cannot be done overnight.

“We looked at the entire western US region, because California is heavily dependent on electricity imports from other western states. Electric vehicle charging and all other uses of electricity have consequences for the entire western region given the interconnected nature of our power grid, “said Siobhan Powell, lead author of the March and new study.

“We were able to show that with fewer home and more daytime charges, the western United States would need less generation and storage capacity and wouldn’t waste as much solar and wind energy,” said Powell, PhD in mechanical engineering ’22. .

“And it’s not just California and the western states. All states may need to rethink electricity pricing structures as their electric vehicle charging needs increase and their network changes, ”added Powell, who recently took a postdoctoral research position at the ETH of Zurich.

Once 50% of the cars on the road are powered by electricity in the western United States – of which roughly half the population lives in California – more than 5.4 gigawatts of energy storage would be required if charging habits took the course. current. This is the equivalent capacity of 5 large nuclear reactors. A big shift to charging at work instead of home would reduce the storage space needed for electric vehicles to 4.2 gigawatts.

Change the incentives

Current hourly rates encourage consumers to switch electricity use to night hours whenever possible, such as turning on the dishwasher and charging electric vehicles. This tariff structure reflects the period prior to significant solar and wind energy supplies, when demand threatened to outstrip supply during the day, particularly in the late afternoon in the summer.

Today, California has an excess of electricity during the late morning and early afternoon, thanks largely to its solar capacity. If most electric vehicles were to charge during these times, cheap energy would be used rather than wasted. Alternatively, if most electric vehicles continue to charge at night, the state will have to build more generators – probably powered by natural gas – or expensive large-scale energy storage systems. Electricity that goes first to a huge battery and then to an EV battery loses power from the extra stop.

Locally, if a third of homes in a neighborhood have electric vehicles and most homeowners keep setting charging to start at 11pm or whenever electricity rates drop, the local grid could become unstable.

“The findings of this paper have two profound implications: the first is that the price signals are not aligned with what would be best for the network and for taxpayers. The second is that it requires you to consider investing in charging infrastructure for where people work, “said Ines Azevedo, the other co-senior author of the new paper and associate professor of energy sciences and engineering at the Stanford Doerr School of Sustainability, which opened on 1 September.

“We need to move quickly towards decarbonising the transportation sector, which accounts for the majority of emissions in California,” continued Azevedo. “This work provides information on how to get there. Let’s make sure we pursue investment policies and strategies that allow us to do so in a sustainable way “.

Another problem with electricity pricing design is charging high tariffs to commercial and industrial customers based on their peak electricity consumption. This can discourage employers from installing chargers, especially when half or more of their employees have electric vehicles. The research team compared different charging infrastructure availability scenarios, along with different rates for residential use time and commercial demand rates. Some tariff changes have worsened the situation at the network level, while others have improved it. However, a scenario of having a charging infrastructure that encourages more daytime charging and fewer home charging provided the greatest benefits, the study found.

Rajagopal and Azevedo are also co-directors of the Bit & Watt initiative at Stanford’s Precourt Institute for Energy. Other co-authors of this study are Gustavo Cezar, PhD student and personnel engineer at Stanford’s SLAC National Accelerator Laboratory; And Liang MinCEO of the Bits & Watts Initiative.

This work was funded by the California Energy Commission, the National Science Foundation and the Bits & Watts Initiative with support from Volkswagen.

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