Study Concludes That Smaller Capacity PHEVs Would Be a “Robust” Option for Minimizing Fuel Consumption, Cost and GHG Emissions
26 February 2009
|Best vehicle choice for minimum fuel consumption, cost, or greenhouse gas emissions as a function of distance driven between charges across sensitivity scenarios. Shiau et al. (2009) Click to enlarge.|
A team of researchers from Carnegie Mellon University has analyzed the impact of plug-in hybrid electric vehicle (PHEV) battery pack size on fuel consumption, cost and greenhouse gas emissions over a range of charging frequencies (distance traveled between charges). The study will appear in an upcoming issue of the journal Energy Policy.
When charged frequently (every 20 miles or less), using average US electricity, small capacity (i.e., lower all-electric range) plug-in hybrid electric vehicles (PHEVs) are less expensive operationally and release fewer greenhouse gases (GHGs) than hybrid-electric (HEVs) or conventional vehicles, according to the study’s findings.
|“Larger battery packs allow drivers to go longer distances on electric power. But batteries are heavy and expensive. Over a range of scenarios—including fluctuating gas prices, new battery technologies or high taxes on carbon dioxide emissions—plug-ins with small battery packs are economically competitive with ordinary hybrid and conventional vehicles for drivers who charge frequently.”|
—Prof. Jeremy Michalek
For moderate charging intervals of 20-100 miles, PHEVs release fewer GHGs, but HEVs are more cost-effective, the study found. Large-capacity PHEVs—sized for 40 or more miles of electric-only travel—are not cost-effective in any scenario, according to the findings, although they could minimize GHG emissions for some drivers and provide potential to shift air pollutant emissions away from population centers.
The study, led by Assistant Professor Jeremy Michalek, indicated that the impacts of increased battery weight from larger packs on charge-depleting (CD) mode electrical efficiency and charge-sustaining (CS) mode fuel economy are measurable, with about a 10% increase in Wh/kg and an 8% increase in gallons per mile when moving from a PHEV7 to a PHEV60. This implies, the researchers said, that the additional weight of a PHEV60 results in a 10% increase in operation-related costs and greenhouse gas emissions per mile relative to a PHEV7 for drivers who charge frequently (every 7 miles or less).
The best choice of PHEV battery capacity depends critically on the distance that the vehicle will be driven between charges. Our results suggest that for urban driving conditions and frequent charges every 10 miles or less, a low-capacity PHEV sized with an AER of about 7 miles would be a robust choice for minimizing gasoline consumption, cost, and greenhouse gas emissions.
An increase in gas price, a decrease in the cost of usable battery capacity, or a carbon tax combined with low carbon electricity generation would increase PHEV cost effectiveness for a wide range of drivers. In contrast, a battery technology that increases specific energy would not affect net cost and GHG emissions significantly, and a $100 per ton carbon tax without a corresponding drop in carbon intensity of electricity generation would not make PHEVs significantly more competitive.
These results suggest that research on PHEV battery technology improvements would be better targeted toward cost reduction than improvement of specific energy, and the effect of carbon taxes on the PHEV market will depend on their effect on the electricity generation mix, such as encouraging renewables, carbon capture and sequestration, and nuclear.
...The dominance of the small-capacity PHEV over larger-capacity PHEVs across the wide range of scenarios examined in this study suggests that government incentives designed to increase adoption of PHEVs may be best targeted toward adoption of small capacity PHEVs by urban drivers who are able to charge frequently.—Shiau et al. (2009)
The study, which was funded by the National Science Foundation and the Teresa Heinz Scholars for Environmental Research Program, points out that targeting drivers with the potential to charge frequently would not limit plug-ins to a boutique market: nearly 50% of US passenger vehicle miles are traveled by vehicles driving less than 20 miles per day.
The researchers used the split drivetrain configuration of a 2004 Prius as the baseline HEV, and examined PHEV versions of it sized for 7, 20, 40, and 60 miles (11, 32, 64 and 96 km) of all-electric range (AER) with comparable performance characteristics.
For simplicity, they assumed an operating strategy in which the PHEVs run entirely on electric power in the charge-depleting (CD) range (i.e., not blended mode), and then switch to operate like an HEV in the charge-sustaining (CS) range. The battery packs used Saft Li-ion cells with 6 Ah capacity and nominal output voltage of 3.6V. The researchers used Argonne National Laboratory’s Powertrain System Analysis Toolkit (PSAT) to model and examine design tradeoffs between battery capacity and PHEV benefits.
The researchers modified the control strategy so that the PHEVs operated in electric-only charge-depleting mode until the battery reaches 35% SOC, after which time the vehicle switches to CS-mode and operates like a Toyota Prius, using the split control strategy with a target SOC of 35% and SOC operating range of 30-40%.
Ching-Shin Norman Shiau, Constantine Samaras, Richard Hauffe, Jeremy J. Michalek. Impact of battery weight and charging patterns on the economic and environmental benefits of plug-in hybrid vehicles. Energy Policy
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