But researchers in Texas, US, said that historical claims about vehicle-to-grid (V2G) back-up power might have been too optimistic.

"Vehicle availability is complementary to net load during cooler months in hot climates, when electricity demand is bimodal and brackets the hours of highest vehicle use," Michael Webber of the University of Texas at Austin told environmentalresearchweb. "In hotter months, vehicle use and net load are closely aligned, thus the availability of vehicles to provide load shifting, peak shaving, or valuable ancillary services [backup generation] is more limited."

Together with colleague Chioke Harris, Webber examined data from GPS vehicle-tracking devices on 429 vehicles in the Puget Sound region, along with wind-generation and electricity-demand data from the Electric Reliability Council of Texas (ERCOT). In the week, vehicle use rose sharply in the morning before peaking around midday. There was a second peak around 6.00 p.m., during the evening commuter period. At weekends, use increased later in the morning and there was a single peak around noon, which tailed off gradually throughout the afternoon and evening.

"Vehicle-use patterns are more complex than previously thought," said Webber. "While driving patterns do not show significant variation across seasons, they do vary significantly between weekdays and weekends. Averaging weekday and weekend driving patterns together yields a use curve that reflects the characteristic patterns of neither, indicating that researchers should avoid using yearly or monthly averaged data – doing so could result in under-prediction of vehicle use."

In cooler months, vehicle availability and electricity demand were aligned best between 6.00 and 8.00 a.m. and in the evening after 9.00 p.m., that is, before and after people had used their automobiles. Wind generation makes up a greater proportion of electricity in Texas in cooler months. Demand tends to peak in the morning and evening when people are at home and there is limited daylight. In the summer, however, electricity demand peaked during the afternoon and evening because of air-conditioning needs.

Batteries had the most capacity for regulation up of the grid when most vehicles were stationary and fully charged in the early morning. Regulation down capacity (for when the grid needs an increase in load – or reduction in generation – to correct its frequency) was highest in the evenings, after vehicles had been used for the day and their batteries were partially depleted.

The researchers reckon that, while this result is dependent on data specific to Texas, it might apply to other places. "That being said, the significant differences in electricity markets and electric load between regions, both as a function of time of day and time of year, mean that any results from studies comparing electricity markets and vehicle use are likely regional," said Webber. "It is important that researchers be attentive to the generality of their results when looking at generation data from specific regions. Further, it is possible that vehicle-use patterns vary between regions in the United States, thus consideration of region-specific vehicle-use patterns should also be undertaken."

According to Webber, even before major automakers' recent commitments to series production of electric vehicles, the potential benefits of V2G for vehicle owners, utilities, and grid operators had been identified, but not well quantified. "While prior research typically assumed either that vehicle use is constant throughout the day or that it varies according to a single pattern year-round, our research advances the state of scientific understanding by comparing weekday and weekend driving patterns to see if vehicles are actually available to the grid when they are needed most," he said.

Now the team will continue to address critical assumptions in the V2G literature. "In particular, we intend to study potential changes in ancillary service procurements as a consequence of battery electric-vehicle charging needs, the need for scheduled charging and how that varies throughout the year, and the potential revenue opportunities for vehicle owners and aggregators offering ancillary services through a V2G system," said Webber.

Webber and Harris reported their study in Environmental Research Letters.