Pittsburgh, Pennsylvania – A new study by researchers at Carnegie Mellon University reports that while plug-in hybrid electric vehicles (PHEVs) can reduce greenhouse gas (GHG) emissions by 32 per cent over their life cycle when compared to conventional vehicles, their benefit over current hybrid vehicles depends largely on the source of the electricity used to charge them.

The study, published in Environmental Science and Technology, looked at baseline, carbon-intensive and low-carbon energy scenarios to see what the electricity system should be if major GHG reductions are to be realized from PHEVs. The paper examined a conventional sedan, a traditional self-charging hybrid and PHEVs with 30-, 60- and 90-km electric range, and estimated the life cycle energy use and GHGs from vehicle production, storage battery production, combustion and production of liquid fuel, and combustion and production of electricity to power the vehicle.

Not surprisingly, the carbon intensity of the electricity used to power the PHEVs greatly affected the GHG over the vehicle’s life cycle. In areas where coal is or could be the dominant fuel for charging, PHEVs would still edge out conventional vehicles, but would be nine to 18 per cent higher in GHGs than conventional hybrids; with a low-carbon electricity source, PHEVs improve over conventional vehicles by 51 to 63 per cent, and over conventional hybrids by 31 to 47 per cent.

“If PHEVs are going to be predominately charged overnight, we need to think about how renewable portfolio standards could be updated to help reduce the carbon intensity of cheap off-peak power, which in many regions is coal,” the authors stated. “We should also think about how smart chargers could not only reduce grid impacts of PHEV charging (as is being done in ongoing work in various places), but potentially the CO2 impacts as well, either by prices or credits for the user.

“Power plants that get constructed stick around for a long time. Vehicles have a much shorter time horizon. So if you want to buy a PHEV two or three vehicles down the line, the types of power plants we build in the next few years will be part of the mix that charges those future vehicles.”