Working Group III: Mitigation

Other reports in this collection Use of Biofuels

Liquid and gaseous transport fuels derived from a range of biomass sources are technically feasible (see Section They include methanol, ethanol, di-methyl esters, pyrolytic oil, Fischer-Tropsch gasoline and distillate, and biodiesel from vegetable oil crops (Section Ethanol is commercially produced from sugar cane in Brazil and from maize in the USA where it has been sold neat or blended for more than a decade. Ethanol is blended with gasoline at concentrations of 5-15%, thereby replacing oxygenates more typically used in North America such as methyl-t-butylether (MTBE) and ethyl-t-butylether (ETBE) additives. ETBE production from bio-ethanol is also a promising market in Europe but the production costs by hydrolysis and fermentation from cereals or sweet sorghum crops remain high (Grassi, 1998).

In Brazil the production of ethanol-fuelled cars achieved 96% market share in 1985 but declined to 3.1% in 1995 and 0.1% in 1998. Since the government approved a higher blend level (26%) of ethanol in gasoline the production of ethanol has continued to increase achieving a peak of 15,307m3 in the 1997/98 harvesting season. This represented 42.73% of the total fuel consumption in all Otto cycle engines giving an annual net carbon emission abatement of 11% of the national total from the use of fossil fuels (IPCC, 2000).

National fuel standards are in place in Germany for biodiesel and many engine manufacturers such as Volkswagen now maintain warranties (Schindlbauer, 1995). However, energy yields (litres oil per hectare) are low and full fuel cycle emissions and production costs are high (see Section Aircraft Technology

Several major technologies offer the opportunity to improve the energy efficiency of commercial aircraft by 40% or more (Table 3.10). The Aeronautics and Space Engineering Board of the National Research Council (NRC, 1992, p. 49) concluded that it was feasible to reduce fuel consumption per seat mile for new commercial aircraft by 40% by about 2020. Of the 40%, 25% was expected to come from improved engine performance, and 15% from improved aerodynamics and weight. A reasonable preliminary goal for reductions in NOx emissions was estimated to be 20%–30%.

Table 3.10: Energy information administration aircraft technology estimates
Technology Year of
% gain in
seat-km per kg
Ultra-high bypass engine 1995 10
Propfan engine 2000 23
Hybrid laminar flow 2020 15
Advanced aerodynamics 2000 18
Material substitution 2000 15
Engine thermodynamics 2010 20

An assessment of breakthrough technologies by the US National Research Council (1998) estimated that the blended wing body concept alone could reduce fuel consumption by 27% compared to conventional aircraft, assuming equal engine efficiency. The NRC report also identified a number of breakthrough technologies in the areas of advanced propulsion systems, structures and materials, sensors and controls, and alternative fuels that could have major impacts on aircraft energy use and GHG emissions over the next 50 years.

Noting that the energy efficiency of new production aircraft has improved at an average rate of 1-2% per year since the dawn of the jet era, the IPCC Special Report on Aviation and the Global Atmosphere concluded that the fuel efficiency of new production aircraft could improve by 20% from 1997 to 2015 (Table 3.11), as a result of a combination of reductions in aerodynamic drag and airframe weight, greater use of high-bypass engines with improved nacelle designs, and advanced, “fly-by-light” fibre optic control systems (Penner et al., 1999, Ch. 7). Advanced future aircraft technologies including laminar flow concepts, lightweight materials, blended wing body designs, and subsystems improvements were judged to offer 30%-40% to 40%-50% efficiency improvements by 2050, with the lower range more likely if reducing NOx emissions is a high priority. The purpose of these scenarios was not to describe the technological or economic potential for efficiency improvement and emissions reductions, but rather to provide a “best judgement” scenario for use in assessing the impacts of aviation on the global atmosphere through 2050. A number of alternatives to kerosene jet fuel were considered. None were considered likely to be competitive with jet fuel without significant technological breakthroughs. On a fuel cycle basis, only liquid methane and hydrogen produced from nuclear or renewable energy sources were estimated to reduce greenhouse gas emissions relative to jet fuel derived from crude oil.

In operation, aircraft seat-km per kg is also influenced by aircraft size, and overall passenger-km per kg efficiencies depend on load factors as well. Industry analysts (Henderson, 1999) have forecasted an increase in global load factors to 73% by 2018, but foresee only a small potential for increasing aircraft size, however, since most additional capacity is expected to be supplied by increased flight frequencies. If average aircraft size could be increased, perhaps as a strategy for reducing airport congestion, further reductions in energy intensity could be achieved.

Table 3.11: Historical and future improvements in new production aircraft energy efficiency (%)
(Lewis and Niedzwiecki, 1999,
Table 7.1).
Time period Airframe Propulsion Total percent per year
1950 to 1997 30 40 70 1.13
1997 to 2015 10 10 20 1.02
1997 to 2050 25 20 45 0.70

Other reports in this collection

IPCC Homepage