Households with plug-in hybrid vehicles, or PHVs, and smart meters actively managed how, when and where they charged their cars based on electricity rates but rarely took advantage of online feedback, a University of Colorado Boulder study sponsored by Toyota Motor Sales USA has found.
CU-Boulder’s Renewable and Sustainable Energy Institute, or RASEI, recently presented findings from the two-year study combining both household and vehicle data in a smart-grid context.
“Although households had access to online feedback on electricity use, we were surprised that most were not interested in using it to control their vehicle charging,” said Barbara Farhar, principal investigator and senior research associate at RASEI. “However, households still actively managed their charging in other ways.”
Toyota Motor Sales sponsored the study in partnership with Xcel Energy, which installed smart plugs in the garages of the 142-strong sample’s households, randomly selected to participate from among early volunteers for Boulder’s SmartGridCity project. Toyota lent 28 Prius Plug-in Hybrid demonstration programme vehicles to the study. Each household used the car for one nine-week period. Participating households had access to two websites. One served as a nearly instantaneous meter of vehicle electricity consumption when the car was plugged in. The other gave delayed feedback on overall household electricity use. Approximately 90% of the households looked at the websites only a few times or less. Some never looked at them.
Households were found to create distinct methods of managing their vehicle charging based on personal preferences, pricing and convenience. Initially, about half the households were randomly assigned to an ‘unmanaged’ scenario, allowing PHV charging through their in-home smart plug at any time of day. The other half were randomly assigned to a ‘managed’ scenario, which meant their smart plugs were initially programmed to charge only from 10pm to 6am daily. Households were shown how to change their charging scenarios from ‘managed’ to ‘unmanaged’ or vice versa and were free to alter the scenarios in any way they wanted. About half of the households had standard and the other half had time-of-use electricity rates.
Most of those with standard rates preferred the ‘unmanaged’ scenario, and most of those with time-of-use rates preferred the ‘managed’ scenario, many using a ‘set it and forget it’ approach. Quite a few found the time constraints of the ‘managed’ scenario inconvenient.
“Electricity pricing appeared to drive charging behaviour and time cost or convenience was also very important,” said Farhar. “People loved not having to go to the gas station.”
Other findings of the study included a high level of satisfaction among households with the Prius Plug-in Hybrid, but a low level of satisfaction with its electric-only range, about 14 miles of cruising from a full charge, which took three hours using a regular 110V outlet. The Prius PHVs averaged 68mpg on gasoline and were used for an average of 3.2 trips per day. Altogether, the cars used 27MW/h of electricity. As might have been expected, it proved less expensive to drive on electricity as a fuel than gasoline, even when paying higher on-peak electricity rates, Xcel Energy’s analysis confirmed.
Some households charged at locations other than home. Using data from the vehicles, study investigators are continuing to look into where and when away-from-home charging took place.
The two-year study also allowed Toyota to test the PHEVs in the Colorado environment including high altitudes, temperature extremes and mountainous terrain. “The RASEI study demonstrates the importance of testing new technologies with real customers in everyday circumstances,” said Bill Reinert, Toyota advanced technology vehicle National Manager. “The results are often unexpected but help us understand the needs of potential customers and how to successfully introduce advanced technologies to the market.”
Dragan Maksimovic, CU-Boulder Professor of electrical, computer and energy engineering, was the study’s co-principal investigator. Alison Peters, Managing Director of the Deming Center for Entrepreneurship at CU-Boulder’s Leeds School of Business, was its senior manager.
RASEI is a joint venture with the US Department of Energy’s National Renewable Energy Laboratory (NREL).
