After processing the data of four million driving days of PHEVs, Ahmet Mandev can also say how charging should take place to maximise electric power, while minimising fuel consumption and emissions.
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, a doctoral student at the Department of Space, Earth and Environmental Science at Chalmers, aimed to answer in his doctoral studies, supervised by Associate Professor Frances Sprei.
“The reason why we want to take a closer look at PHEVs is that there are different views on their role in electrifying personal transport. It is vital to learn as much as we can about their electrical potential, so that we can determine which policy instruments – laws, regulations and subsidies – can be most effective for such vehicles,” says Ahmet Mandev.
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.
“It is easy to see the breakdown of the kilometres for which a PHEV uses the electric motor or the internal combustion engine. But the unique thing about this study is that we looked at the household level – we mapped all the vehicles in different multi-car households. Then 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 shows that households with a fully electric car and a conventional car, drive on average 45 percent of their total kilometres on electricity, while the households with a 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 electric car – in the case of this study, the Nissan Leaf – and just under half 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 for the longer journeys. So then at least some distance of those trips is driven using electricity. The figures also show that the PHEVs are more often used while another person in the household is using the conventional vehicle. The range for both BEVs and PHEVs have increased since the study, but the results are still relevant and show that plug-in hybrid vehicles have an important role to play when it comes to electrification of personal transport. As the next step it will be interesting to follow up the effect of these longer ranges on electrification rates”, says Frances Sprei.
Most important to charge overnight
Another issue that Ahmet Mandev investigated was how and when to charge a PHEV to get as many electrically driven kilometres as possible, with the lowest possible fuel consumption and emissions. In two other studies, he used about 4 million driving days of data, collected over a ten-year period from the plug-in hybrid model Chevrolet Volt. By processing the data, Ahmet Mandev calculated how often the vehicles were charged, and can thus empirically prove several points about the PHEVs.
The most positive effects result from charging your car once a day – perhaps not so surprising. But Ahmet Mandev made a further discovery which did stand out.
“If you decrease from charging your car every night, to 90 percent of 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 achieve a high share of 76 percent of kilometers driven on electricity, provided, that they are fully charged once a day. Ahmet Mandev and Frances Sprei point out that supplementary charging during the day also gives positive effects, but for maximum effect, a full charge overnight is the best option.
“In our studies, we focused on studying data and drawing conclusions about charging and electric operation based on that. But if one were to translate our results into policy suggestions, it would be to give more people the opportunity to recharge vehicles overnight. Currently, many people, for example those who live in apartment buildings, do not have that opportunity,” says Ahmet Mandev.
In his further doctoral studies, he plans to make international comparisons, to see how charging patterns and electric power differ between countries with different conditions, laws and guidelines regarding PHEVs. Based on this, it will then be possible to see which policy guidelines and recommendations would make the biggest difference.
Text and photos: Christian Löwhagen.
The research was financed by the Swedish Electromobility centre and carried out in collaboration with UC Davis in California and the Fraunhofer Institute for Systems and Innovations Research in Germany.