|Modeled GHG emissions from conventional vehicles, hybrids, and plug-in hybrids using current US average GHG intensity of electricity. Click to enlarge.|
Plug-in hybrids (PHEVs) can displace a large fraction of gasoline use and the greenhouse gas emissions (GHG) associated with its combustion. However, total lifecycle GHG reductions from PHEVs depend heavily on the carbon intensity of the electricity used to grid-charge the PHEV battery pack.
A new study by researchers at Carnegie Mellon University concludes that—given US average GHG intensity of electricity (670 g CO2-eq/kWh)—PHEVs can reduce total lifecycle GHG emissions by 32% compared to conventional vehicles (CVs), but offer only a small reduction compared to conventional charge-sustaining hybrids (HEVs). A paper on their work appears in the 7 April edition of the journal Environmental Science & Technology.
Under a carbon-intensive electricity scenario (950 g CO2-eq/kWh), lifecycle PHEV GHG impacts are 9–18% higher than those of HEVs, Constantine Samaras and Kyle Meisterling conclude. Under a low-carbon scenario (200 g CO2-eq/kWh), however, PHEVs can deliver large lifecycle GHG reductions: 51–63% and 30–47% compared to CVs and HEVs, respectively.
The potential for PHEVs to achieve large-scale GHG emission reductions is highly dependent on the energy sources of electricity production...If large life cycle GHG reductions are desired from PHEVs, a strategy to match charging with low-carbon electricity is necessary.
...For large GHG reductions with plug-in hybrids, public policies that complement PHEV adoption should focus on encouraging charging with low-carbon electricity.—Samaras and Meisterling
A 20% improvement in liquid fuel economy across the vehicle technologies results in HEVs having 4-13% lower life cycle GHGs than plug-in hybrids. However, if kWh/km (electric energy) requirements for PHEVs improve by 20% while holding liquid fuel economy constant for all vehicles, lifecycle GHGs from PHEVs are 10-13% lower than HEVs.
|Lifecycle greenhouse gas emissions for the different vehicles under different scenarios of carbon intensity of electricity;fuel (liquid and electricity) consumption; and E85 use. Click to enlarge.|
Their analysis includes GHG emissions associated with vehicle and storage battery production along with energy use and fuel production under three electricity generation scenarios: baseline average, carbon-intensive, and low-carbon. They also consider cellulosic ethanol use.
Vehicles considered in the study are: a conventional internal-combustion (IC) sedan such as the Toyota Corolla (CV); a hybrid electric sedan (HEV), such as the Toyota Prius; and three PHEVs, powered with liquid fuel and electricity from the grid. The PHEVs considered have electric ranges of 30 km (PHEV30), 60 km (PHEV60), and 90 km (PHEV90).
When charging PHEVs with electricity that has a GHG intensity equal to or greater than our current system, our results indicate that PHEVs would considerably reduce gasoline consumption but only marginally reduce life cycle GHGs, when compared to gasoline–electric hybrids or other fuel-efficient engine technologies. With a low-carbon electricity system, however, plug-in hybrids could substantially reduce GHGs as well as oil dependence.
...With the slow rate of capital turnover in the electricity sector, a low-carbon system may require many years to materialize. Considerable reductions in greenhouse gas emissions using plug-in hybrids in the coming decades will likely require decisions within the next ten years to develop a robust low-carbon electricity supply.
Samaras, C., Meisterling K., 2008. Life cycle assessment of greenhouse gas emissions from plug-in hybrid vehicles: Implications for policy. Environ. Sci. Technol. Web release 5 April 2008. DOI: 10.1021/es702178s