CMU: Plug-in Hybrids “More Sensible” Use of Coal Than Coal-to-Liquids
8 June 2007
|Comparing life cycle CO2 emissions from plug-in hybrids, coal-to-liquids gasoline, and conventional gasoline. Click to enlarge.|
A study from the Carnegie Mellon Electricity Industry Center (CEIC) concludes that while enacting policies to subsidize the production of coal-to-liquids transportation fuel would enhance national security by lowering oil imports, encouraging plug-in hybrids powered by coal-generated electricity is a less costly policy that also reduces oil imports and does more to lower greenhouse gas (GHG) emissions.
CEIC produced the paper in the context of the current work by the US House Committee on Energy and Commerce on transportation energy legislation, the current draft of which includes significant support for CTL. (Earlier post.) The CEIC paper compares GHG emissions of CTL gasoline to the emissions of plug-in hybrid vehicles powered with electricity generated from coal on a full life cycle basis.
Although CTL conventionally produces more diesel than gasoline, the process can be altered with catalysts to upgrade some of the diesel and waxes produced in the standard F-T process to gasoline, with an overall efficiency of around 52% (HHV).
The CEIC team used CTL inputs and outputs derived by Bechtel in 1993, and allocated the total emissions factor among the various CTL co-products using the method in the GREET model (by energy content of the co-products).
The allocated worst-case well-to-plant emission factor (no carbon capture and sequestration, current electricity generation mix) is 190 pounds CO2 equivalent per MMBtu of CTL gasoline, and 50 pounds CO2 equivalent per MMBtu of CTL diesel. With 80% CCS and zero-carbon electricity, the allocated factors drop to 50 pounds CO2 equivalent for gasoline and 15 pounds for diesel.
Adding in the other complete lifecycle factors (transportation for distribution, combustion in the engine) resulted in complete well-to-wheel CTL lifecycle emissions of 360 pounds CO2 equivalent per MMBtu of gasoline in the worst-case scenario and 220 pounds CO2 equivalent per MMBtu of gasoline in the best-case scenario.
CEIC then used a fuel consumption figure of 34 mpg and an annual driving distance of 12,000 miles to calculate the annual CTL gasoline emissions: 1.18 lbs/mile (536.7 g/mi) in the worst case; 0.72 lbs/mile (325 g/mi) in the best case.
For plug-ins, the CEIC researchers calculated the impact of both electricity and gasoline. For electricity generation, they used two scenarios: bituminous coal in a pulverized coal power plant and bituminous coal in an integrated gasification combined cycle power plant with carbon capture and sequestration (IGCC w/ CCS).
|Well-to-Wheel Greenhouse Gas Emissions|
|CTL w/ CCS gasoline||325.1|
|Gasoline base case||344.0|
|PHEV, coal generation||264.6|
|PHEV, coal IGCC w/CCS||105.8|
For a vehicle, they assumed a plug-in hybrid built on a Toyota Prius platform in a parallel configuration with an all-electric range of 60 miles. To determine the fraction of vehicle travel powered by electricity or gasoline, they used the percentages resulting from the cumulative distribution function of daily vehicle miles traveled constructed in another paper from CMU (Samaras and Meisterling, “Decarbonized Electricity Needed for Plug-in Hybrids” 2007). The CEIC distribution estimates electricity would power about 85% of average annual vehicle travel for a plug-in hybrid with a 60-mile electric range, assuming vehicles are charged once per day.
The results: total well-to-wheel emissions of 264.6 g/mi for the conventional coal-generated scenario; 105.8 g/mi for the scenario with advanced IGCC with CCS). The conventional gasoline baseline in the study was 344 g/mi.
It can be seen that gasoline derived from CTL plants with no CCS could increase GHG emissions from vehicles by almost 60%. If CCS is available, then a reduction of less than 6% could be obtained. It is important to note, once again, that in this best-case CTL scenario, not only is there CCS at the CTL plant, but also a low-carbon electricity source is used for CTL production. This might not be a very realistic assumption, but is presented here to show that at best we could only obtain a very small reduction in GHG emissions following a path of increased CTL production.
Plug-in hybrids look more promising as a pathway for reduction of GHG emissions. Even if coal electricity without CCS is used, plug-in hybrids could lead to a GHG emissions reduction of almost 25%. This demonstrates the worst case for plug-in hybrids, as GHGs would be further reduced with a low-carbon electricity portfolio. It is important to note however, that this analysis does not include the emissions from manufacturing the storage battery used in plug-in hybrids. If GHG emissions from lithium-ion batteries for plug-in hybrids are included, total annual GHGs from plug-ins would increase by about 800-1,500 pounds of CO2 equivalents, depending if a twelve or eight-year vehicle life is assumed (Samaras and Meisterling 2007). Battery technologies are difficult to predict, but even when emissions from current battery production are included, plug-in hybrids result in substantially lower emissions than CTL pathways.
The Carnegie Mellon Electricity Industry Center (CEIC) was established in August 2001 as one of 20 centers of excellence in different industries that the Alfred P. Sloan Foundation has established at 13 universities. CEIC’s core funding comes jointly from Sloan and from the Electric Power Research Institute (EPRI).
“For energy security and greenhouse gas reductions, plug-in hybrids a more sensible pathway than coal-to-liquids gasoline”; Paulina Jaramillo and Constantine Samaras; CEIC Working Paper CEIC 07-04 – June 2007
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