Using Natural Gas for Vehicles: Comparing Three Technologies
In the United States, natural gas is used primarily for electricity generation and residential, commercial, and industrial applications, but it is also used as a fuel for on-road vehicles, especially in medium- or heavy-duty vehicles in centrally- fueled fleets.
It has been proposed for greater use as a fuel for on-road vehicles, particularly in light-duty vehicles. This can mean burning natural gas in an internal combustion engine like those used in most natural gas, gasoline, diesel-powered vehicles on the road today. However, natural gas can also serve as the energy source for plug-in electric or hydrogen fuel cell electric vehicles. This fact sheet compares some efficiency and environmental metrics for three possible options for using natural gas in light-duty cars.
The analysis presented here compares these pathways. It is not intended to recommend for or against increased use of natural gas in light-duty vehicles. Related ongoing analysis considers use ofnatural gas with these three technology pathways in medium- or heavy-duty vehicles (for example, in centrally-fueled fleets that use large quantities of fuel), but will not be covered in this fact sheet.
This comparison of pathways in light-duty vehicles is based on a detailed analysis (Wang and Elgowainy 2015), which uses the GREET model (Greenhouse Gases, Regulated Emissions, and Energy use in Transportation). Scientists at Argonne National Laboratory developed GREET to estimate energy use and emissions across the entire fuel life cycle from extraction to end use in transportation.
Considering Life Cycle Efficiency
Each of the three pathways for using natural gas has strengths and weaknesses that determine where efficiency losses and emissions occur across the entire life cycle. Vehicle efficiency determines only part of the story.
Figure 2, which is based on GREET results, gives a look at the fuel life cycle for the three types of natural gas cars, showing how energy conversion efficiency at each major step leads to the ultimate system efficiency for each pathway. The percentages (light blue circles) show natural gas conversion efficiencies along steps of the life cycle. Below each pathway, the changes in the height of the light blue bar show how much of the energy in one mmBtu of natural gas is used during each step.
Natural gas internal combustion engine vehicles running on compressed natural gas (CNGV) have the greatest losses during combustion at the vehicle propulsion stage. Fuel cell electric vehicles (FCEVs) have the greatest losses during hydrogen production via methane reforming and during electricity generation from hydrogen in the vehicle. In addition, plug-in electric vehicles (PEVs) have the greatest losses during electricity generation.
US Dept of Energy
http://www.nrel.gov/docs/fy16osti/64267.pdf
