Included in the analysis are fuel cell, electric, hybrid-electric and conventional ICE drivetrains; energy carriers include gasoline, diesel, CNG, biogas, hydrogen and electricity.

The reference vehicle has a mass of 1,350 kg (2,976 lbs), with 70 kW (94 hp) equivalent ICE power. Battery-electric range for plug-in hybrids is 60 km, while electric range for full battery-electric and fuel cell vehicles is 180 km. Li-ion battery specific energy density is presumed to be 150 Wh kg^-1, and fuel cell specific weight is 3 kg kW-1. Total lifetime driven distance is 150,000 km (93,206 miles).

Primary energy demand and GHG emissions for H2 production processes; abbreviations indiciate energy source and/or process technology. Click to enlarge.

Hydrogen production pathways include low- and high-temperature electrolysis; solar thermochemical disassociation; photobiological splitting (four pathways: direct and indirect biophotolysis using hydrogenase and nitrogenase); gasification (coal, biomass); steam reforming (methane, ethanol); and partial oxidation.

Electricity sources include nuclear; combined cycle plant using natural gas; coal; oil; waste incineration; combined heat and power (wood, diesel, natural gas, biogas); wind; solar; hydro; and pumped storage. They considered three mixes: Swiss production, Swiss consumer, and the Union for the Coordination of the Transmission of Electricity (UCTE) mix.

Among the broader findings of the study are:

• Full battery-electric vehicles demonstrate some of the lowest WTW energy demand and GHG emissions of all drivetrains due to relatively high energy conversion chain efficiences. However, WTW performance strongly depends on the electricity generation method or mix.

• Renewable energy-based electricity generation results in low WTW GHG emissions, while fossil fuel-based electricity generation can result in higher emissions than a gasoline-fueled vehicle.

• The lowest WTW energy demand and GHG emissions were achieved using electricity from waste incineration, biogas CHP, hydropower, wind power, and photovoltaic power.

• Plug-in hybrid electric vehicles yielded lower WTW energy demand and GHG emissions than the gasoline vehicle for all electricity sources evaluated. However, the result is a function of the electric share and the mix. A critical electricity mix can be identified which divides optimal drivetrain performance between the BEV, PHEV and ICE vehicle.

• Fuel cell drivetrains using hydrogen produced via electrolysis are more sensitive to variations in the electricity mix compared to plug-in vehicles due to a lower overal energy conversion efficiency chain. Direct chemical conversion processes for hydrogen offer efficiency gains which can reduce or eliminate the gap between EV and FCV operational WTW performance.

• Both fuel cell and PEV drivetrains yielded remarkably lower WTW GHG emissions than the gasoline ICE where biomass gasification was used for hydrogen production and electricity was generated via a wood CHP plant.

• Natural gas pathways showed that both WW GHG emissions and WTW energy demand were significantly reduced using steam methane reforming-based fuel cell drivetrains and natural gas CCP-based plug-in drivetrains.

• EV electrification using photovoltaic power resulted in the lowest solar energy pathway WTW energy demand and GHG emissions. Fuel cell drivetrains were able to achieve GHG emissions in a similar range, but the corresponding WTW energy demand exceeded that of the ICE vehicle.

• Direct photobiological splitting using hydrogenase performed well compared to other direct solar energy conversion pathways for H₂ production.

• Biogas use in the ICE vehicle and hybrid electric vehicle reduced WTW energy demand and GHG emissions more than 50% compared to the gasoline-fueled vehicle. The natural gas hybrid also resulted in the lowest WTW energy demand and GHG emissions of all natural-gas based pathways evaluated.

Hence, alternative fuel sources, and not only alternative drivetrain technologies, play a key role in improving WTW energy demand and GHG emissions. This is a positive outcome in light of the implementation and infrastructure challenges associated with alternative drivetrain technologies.

—Yazdanie et al.

Resources

• Mashael Yazdanie, Fabrizio Noembrini, Lionel Dossetto, Konstantinos Boulouchos (2014) “A comparative analysis of well-to-wheel primary energy demand and greenhouse gas emissions for the operation of alternative and conventional vehicles in Switzerland, considering various energy carrier production pathways,” Journal of Power Sources, Volume 249, Pages 333-348 doi: 10.1016/j.jpowsour.2013.10.043