Current and theoretical maximum well-to-wheels exergy efficiency of options to power vehicles with natural gas

Lower prices and increased supply of natural gas from hydraulic fracturing could lead to widespread use of natural gas in transportation. There are three primary ways that natural gas could be used in personal vehicles: compressed natural gas (CNG) in a combustion engine, as a source of hydrogen for a fuel cell electric vehicle (FCEV), and to generate electricity for a battery electric vehicle (BEV). In this work, we compare these three paths by analyzing their current and theoretical maximum well-to-wheels (WTW) exergy efficiencies. Each pathway begins with the extraction of natural gas and ends with delivery of work to the vehicle’s wheels. The best current and theoretical maximum well-to-wheels exergy efficiencies for CNG, FCEV, and BEV pathways are found to be 31%/63%, 25%/87% and 44%/84% respectively. The largest exergy destruction for the CNG pathway occurs within the vehicle’s internal combustion engine (ICE) plant, which has a best current efficiency of 35%. For the FCEV pathway the main current sources of exergy destruction are the reforming stage and within the fuel cell engine plant, with best current efficiencies of 69% and 50% respectively. For the BEV pathway, the largest exergetic loss occurs during the conversion from natural gas to electricity at a combined cycle power plant, with a best current efficiency of 59%. While the theoretical maximum succeeds in identifying process steps that limit efficiency, it does not inform how much progress could be made to improve efficiency with what effort.