|A summary of liquid and gaseous fuels, not including hydrogen. Alternative-fuel pathways show WTW GHG improvements over gasoline and diesel, but not WTW energy-efficiency improvements. Click to enlarge.|
The European Council for Automotive Research and Development (EUCAR); Oil Companies’s European Organization for Environment, Health and Safety (CONCAWE); and the Joint Research Center of the European Commission (JRC) have completed a joint evaluation of the Well-to-Wheels (WTW) energy use and greenhouse gas (GHG) emissions for a wide range of potential future fuels and powertrains options.
The trio published an earlier version in December 2003. The second version, released in May 2006, includes a number of new and updated pathways as well as revised cost calculations and availability estimates.
Among the changes or new factors in the study are: a reduced fuel penalty for diesel particulate filters (2.5% down from 4%); optimized hybrid performance; revised and new pathways for ethanol production; Coal-to-Liquids (CTL) and carbon capture and sequestration (CCS).
Among the findings or conclusions are:
Engines. Continued developments in engine and vehicle technologies will reduce energy use and GHG emissions. Spark ignition engines have more potential for improvement than diesel, and hybridization can provide further GHG and energy use benefits.
Compressed Natural Gas (CNG) engines are currently slightly less efficient than gasoline engines, but in the future the improvements on spark ignition engines will bring CNG close to diesel efficiency. Hybridization is particularly favorable for CNG. Today the WTW GHG emissions for CNG lie between gasoline and diesel, approaching diesel in the best case.
The origin of the natural gas and the length of the supply pathway are critical to the overall WTW energy use and GHG emissions.
Biofuels. Conventional production of ethanol as practiced in Europe gives modest fossil energy/GHG savings compared with gasoline. Existing European pathways can be improved by use of co-generation and/or use of byproducts for heat. Advanced processes (from wood or straw) can give much higher savings. Using straw as fuel would yields the best GHG balance. Cellulosic ethanol is very promising (although sugar cane as a feedstock uses very little fossil energy).
Biodiesel saves fossil energy and GHG compared to conventional diesel. Sunflower is more favorable than rape as a feedstock.
Synthetics. Fischer-Tropsch (FT) diesel synthesis requires more energy than conventional diesel refining from crude oil. Gas-to-liquids (GTL) is nearly GHG neutral compared to conventional diesel; Coal-to-Liquids (CTL) produces considerably more GHG. The use of biomass-to-liquids (BTL processes) involves very little fossil energy and therefore produces few GHG emissions because the synthesis processes are fuelled by the biomass itself.
DME can be produced from natural gas or biomass at lower energy use and GHG emissions than FT diesel.
Hydrogen. If hydrogen is produced from natural gas, GHG emissions savings are only achieved with fuel cell vehicles—not when burning hydrogen in an engine. Direct use of natural gas as a fuel is more energy/GHG efficient in engines than using hydrogen produced from natural gas.
Liquefied hydrogen is less energy-efficient than compressed hydrogen.CCS. Carbon capture and storage requires some additional energy (mainly for CO2 compression), and is most attractive for processes that use large amounts of high-carbon energy such as CTL or processes that decarbonize fuels to produce hydrogen.
The researchers concluded that a shift to renewable/low fossil carbon routes may offer a significant GHG reduction potential but generally requires more energy. They also note that applying renewable energies directly to transport may not maximize the GHG reduction potential of those renewable energies—more might be gained by direct application to electricity rather than to road fuels.