CMU analysis finds BEVs powered with natural gas-based electricity have about 40% of the lifecycle GHGs of a conventional gasoline vehicle
According to a new lifecycle analysis by a team at Carnegie Mellon University, a battery electric vehicle (BEV) powered with natural gas-based electricity achieves around an average 40% lifecycle greenhouse gas (GHG) emissions reduction when compared to a conventional gasoline vehicle. Plug-in hybrids (PHEVs), either with a 30- or 60-km range, when powered by natural gas electricity, have the second lowest average emissions. Both BEVs and PHEVs provide large (more than 20%) emissions reductions compared to conventional gasoline, but none of them is a dominant strategy when compared to gasoline hybrid electric vehicles (HEVs), the team found.
Gaseous hydrogen fuel cell electric vehicles (FCEVs) and compressed natural gas (CNG) vehicles have comparable life cycle emissions with conventional gasoline, offering limited reductions with 100-year global warming potential (GWP) yet leading to increases with 20-year GWP. Other liquid fuel pathways using natural gas—methanol, ethanol, and Fischer–Tropsch liquids—have larger GHG emissions than conventional gasoline even when carbon capture and storage technologies are available. The study is published in the ACS journal Energy & Fuels.
For the study, the team used a Monte Carlo analysis combined with a full lifecycle analysis framework to assess the greenhouse gas (GHG) implications of a transition to light-duty natural gas-powered vehicles. The researchers considered six different types of transportation fuels—CNG; natural gas-based electricity; natural gas-based hydrogen; natural gas-based Fischer−Tropsch liquids (gasoline and diesel); natural gas-based methanol; and ethane-based ethanol—in two representative light-duty vehicles: a passenger vehicle and a sport utility vehicle.
They evaluated six vehicle technologies: a spark ignition internal combustion engine vehicle (SI-ICEV); a flex fuel vehicle (FFV); a compression ignition internal combustion engine vehicle (CI-ICEV); a hybrid electric vehicle (HEV); a plug-in hybrid electric vehicle (PHEV); a battery electric vehicle (BEV); and a fuel cell electric vehicle (FCEV). For both passenger vehicles and SUVs, the functional unit was one vehicle kilometer traveled.
The study on light-duty vehicles is a companion piece to a study published earlier this year assessing the GHG implications of the same transition to natural-gas powered medium- and heavy-duty vehicles. (Earlier post.)
In addition to the findings outlined above, the researchers also determined that SUVs have larger life cycle GHG emissions than passenger cars for all natural gas pathways. For example, a hydrogen-powered fuel-cell SUV has life cycle GHG emissions that are at least 19% higher than those of a fuel-cell passenger vehicle, while a battery electric SUV has life cycle GHG emissions that are 41% higher than a battery-electric passenger vehicle.
We find that the benefits of natural gas pathways largely depend on two factors, vehicle fuel efficiency and carbon intensity of the fuel (cradle-to-gate GHG emissions per MJ of fuel delivered; see the Supporting Information for our estimates). Pathways that run on electric vehicles (except liquid hydrogen) have smaller emissions than pathways that run on ICEVs. Within each vehicle technology group, if vehicles’ fuel efficiencies are comparable, pathways with higher supply chain efficiency emit less than pathways with lower supply chain efficiency (e.g., CNG vs methanol or gaseous hydrogen vs liquid hydrogen). To assess the contribution of these two effects, we developed a break-even analysis that shows the trade-offs between vehicle fuel efficiency and methane leakage rate that influence the carbon intensity of a natural gas-based fuel.—Tong et al.
The break-even rate is the methane leakage rate at which life cycle GHG emissions from a natural gas pathway equals that of conventional gasoline. If the actual methane leakage rate from the natural gas systems is lower than the calculated break-even rate of a specific pathway, that pathway has lower life cycle emissions than conventional gasoline. The higher the break-even rates, the larger the emissions reduction coming from that pathway.
They performed the break-even analysis on the CNG, gaseous hydrogen fuel cell vehicle and battery-electric vehicle pathways because those emissions are highly dependent on fugitive methane emissions. Further, these pathways are the closest to commercial deployment.
They found that CNG vehicles offer emissions reductions if the lifecycle methane leakage rate is lower than 2.3% (using the 100-year GWP) or 0.9% (using the 20-year GWP). A shorter time horizon (such as 20 years), which considers a higher warming potential of methane, requires a lower break-even rate than a longer time horizon (such as 100 years). Current FCEVs offer emission reductions if the life cycle methane leakage rate is lower than 2.8% (100 years) or 1.2% (20 years). Of the three pathways considered, BEVs have largest break-even rates with current vehicle technologies, 10.8% (100 years) and 4.5% (20 years).
Their baseline estimate of methane leakage was 1.3%, with a pessimistic estimate of 2.0%.
Increasing vehicle fuel efficiency and reducing methane emissions from the natural gas system are promising strategies to reduce GHG emissions from using natural gas for road transportation. For the same mobility service, a higher vehicle fuel efficiency leads to lower life cycle emissions or translates into a higher allowable break-even methane leakage rate. Recent studies find evidence of “super emitters” in natural gas systems: a small number of emission sources that lead to a significant share of methane emissions. There are cost-effective technologies to reduce these emissions that would reduce the methane leakage rate and provide better opportunities for reducing GHG emissions from light-duty vehicles by using natural gas as a transportation fuel.—Tong et al.
Fan Tong, Paulina Jaramillo, and Inês M. L. Azevedo (2015) “Comparison of Life Cycle Greenhouse Gases from Natural Gas Pathways for Light-Duty Vehicles” Energy & Fuels doi: 10.1021/acs.energyfuels.5b01063