The Electric Power Research Institute (EPRI) in conjunction with Arizona public power utility Salt River Project has completed a study of Tesla drivers that aims to help provide utilities with a better understanding of the charging behavior and driving practices of the owners of electric vehicles.
The study collected data on nine Tesla electric vehicles in Salt River Project’s territory in the Phoenix metropolitan area in Arizona from July 2017 through October 2018. Data loggers in the vehicles collected information on vehicle voltage, current, and power during driving and charging events.
EPRI says the resulting report may help grid operators prepare for widespread electric vehicle adoption by providing baseline information on electric vehicle charging characteristics in order to understand utility revenue opportunities as well as potential grid load impacts.
Among other things, an analysis of the data found about 63% of Tesla charging occurred at home, while public stations accounted for about 20% of all charging, with 80% of the public charging occurring at fast charging direct current stations.
The home charging rate is much lower than shown in previous EPRI EV charging studies, which found that 75% to 80% of charging occurs at home. In the analysis, EPRI researchers said the gap might be the result of Tesla’s policy of offering free charging at its SuperChargers for vehicles purchased before 2018.
The report also found that time-of-use rates are “very effective” at shifting peak loads to times when utilities have excess grid capacity.
“Understanding EV adoption’s impact on load shape and how drivers respond to different rates for charging provides energy companies with actionable data to inform grid investment and development,” Jamie Dunckley, EPRI technical leader and a principal investigator for the charging study, said in a statement.
“This collaboration with EPRI has provided the critical information we need to focus on planning for the large increase in the number of electric vehicles set to arrive in the near future,” said SRP Principal Environmental Scientist Kathy Knoop. “It is crucial for SRP to understand how longer-range electric vehicles will impact our grid.”
Previous studies of electric vehicle behavior have often fallen short, the EPRI researchers said. Some have used data from advanced metering infrastructure that fails to collect data when charging occurs away from the base meter. Other studies that use survey methods to log electric vehicle behavior often miss the mark because people generally cannot answer questions accurately on how much they drive and charge, the researchers said. And many studies track conventional vehicles and are then modified to adapt to an electric vehicle perspective, which requires far too many assumptions, EPRI said.
EPRI says its new report collected minute-level, high-resolution data and tracking of vehicle movement from home to work as well as on road trips. The data is able to show not only summary level statistics but also time-series analysis of load data, which is usually only available with meter data, EPRI said.
The data collected in the study included voltage, current, GPS location, and state of charge. Summary data included total yearly kilowatt-hours and whether charging occurred at home, at work, or in a public charging station, as well as average kWh per charge, miles driven per kWh, and grid-related data.
The new report grew out of an earlier 18-month study that tracked 100 electric vehicles in Salt River Project’s service territory. That study found that the largest demand spikes took place when Tesla owners were charging at public SuperChargers. The new study focused more narrowly on Tesla vehicles.
Tesla cars can charge at a higher power level than other electric vehicles, so the new study’s data represents the current upper range of potential grid impacts related to electric vehicle charging, EPRI said.
EPRI is in the process of developing a larger study to track 40 Tesla vehicles over 60 days using the publicly available Tesla application programming interface, which eliminates the need for added hardware, lowering the cost of tracking the vehicles.
And in the fourth quarter, EPRI expects to launch a study tracking thousands of vehicles. EPRI says that study will help it refine its understanding of where electric vehicle customers charge, how much they charge, and how that varies over time and season.
Additional information about the report is available here.
Other public power utilities are also examining EV charging
Along with SRP, several other public power utilities are studying EV charging.
Colorado-based Platte River Power Authority earlier this year said it is seeking electric vehicle owners to participate in a distributed charging study to evaluate vehicle electric energy consumption patterns and test smart charging technology.
Nebraska’s Lincoln Electric System on May 14 said that it will receive funds from the Nebraska Environmental Trust for the public power utility’s electric vehicle data collection project.
LES began collecting data for its two-year study, focused on charging behaviors and the associated impact on the local grid, in November 2018 and will conclude in 2020.