CHALMERS UNIVERSITY OF TECHNOLOGY - Plug-in hybrid electric vehicles (PHEVs) are those which can run on both electricity and fossil fuels. But how environmentally friendly are they? And how well can they help prepare for an eventual transition to a fully fossil-free vehicle sector? A unique study from Chalmers University of Technology, Sweden, now shows that PHEVs with, in this case, a range of about 56 km, have an important role to play and can result in the same amount of kilometres driven electrically as short range fully electric cars.
“In
comparing a number of multi-car households, we can see that households which
own a fossil-fuel vehicle and a PHEV can drive as many purely electric
kilometres as a household owning a fossil fuel vehicle and a fully electric
one,” says Ahmet Mandev, doctoral student at the Department of Space, Earth and
Environmental Science at Chalmers, whose doctoral studies were supervised by
Associate Professor Frances Sprei.
Despite
the fact that over 20 years have passed since the first mass-produced PHEV car
appeared on the market, many questions remain regarding optimal usage of such
vehicles. These are questions that Ahmet Mandev aimed to answer. “There are
different views on PHEVs’ role in electrifying personal transport. It is vital
to learn as much as we can about their electrical potential, to determine which
policy instruments – laws, regulations and subsidies – can be most effective
for such vehicles,” he explains.
In
the first of the studies included in his licentiate thesis The Role of Plug-in Hybrid
Electric Vehicles in Electrifying Personal Transport – Analysis of Empirical
Data from North America, he processed and analysed one year of
driving data for 71 households in California using either the Nissan Leaf, a
fully electric vehicle, or different PHEV models with various ranges.
“It is easy enough to see the
breakdown of the kilometres between electric and fossil fuel operation. But the
unique thing about this study is that we looked at the household level – mapping
all the vehicles in different multi-car households. We saw how many kilometres
a household travelled using electric power and compared that between households
which own a fully electric car, or a PHEV, alongside a conventional vehicle,”
he explains.
As
usual with all types of electric vehicles, range is an important factor. The
study showed that households with a fully electric car and a conventional car
drove on average 45 percent of their total kilometres on electricity, while the
households owning a long-range PHEV and a conventional car, reached 46 percent
electric operation on average. This is despite the fact that the range for the
vehicles at full electric operation was 130 km for the fully electric car and
just under half that for the plug-in hybrid – 56 kilometres.
The reason that the PHEV
performs better, despite the considerably shorter range, is that it is taken
more often on longer journeys. “That means that at least some of the distance
of those trips is driven using electricity. The figures also reveal that the
PHEVs are more often used at the same time as the fossil vehicle, by another
member of the household.
"All
in all, this shows that plug-in hybrid vehicles have an important role to play
when it comes to electrification of personal transport,” says Associate
Professor Frances Sprei.
Important conclusions
relating to charging
In
addition to these findings, Ahmet Mandev can also say, after processing the
data of four million driving days of the plug-in hybrid model Chevrolet Volt
collected over a ten-year period, how charging should take place to maximise
the share of electric driving, while minimising fuel consumption and emissions.
The
most positive effects result from fully charging your car every night – perhaps
not so surprising. But he made a further discovery which did stand out.
“If
you decrease from fully charging your car every night, to 9 out of 10 nights,
emissions triple – from 1.7 kg of carbon dioxide to 5.7 kg for 100 kilometres
of driving. Fuel consumption increases in a similar way, from 0.7 liters for
100 kilometers to 2.5 liters. These are still low emissions and low levels of
fuel consumption, but it is a big difference for such a small change in
behaviour,” he explains.
The
PHEVs in the study achieved a high of 76 percent electric power usage, provided
that they were fully charged once every 24 hours. Ahmet Mandev and Frances
Sprei point out that supplementary charging during the day also gives positive
effects, but for maximum effect, full charging every night is the best option.