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Opportunities for reducing energy use and GHG emissions from waterborne transport were not covered in the SAR. The predominant propulsion system for waterborne transport is the diesel engine. Worldwide, 98% of freighters are powered by diesels. Although the 2% powered by steam electric drive tend to be the largest ships and account for 17% of gross tonnage, most are likely to be replaced by diesels within the next 10 years (Michaelis, 1997). Still, diesel fuel accounted for only 21% of international marine bunker fuel consumed in 1995 (Olivier and Peters, 1999). Modern marine diesel engines are capable of average operating efficiencies of 42% from fuel to propeller, making them already one of the most efficient propulsion systems. The best modern low-speed diesels can realize efficiencies exceeding 50% (Farrell et al., 2000).
Fuel cells might be even more efficient, however, and might possibly be operated on fuels containing less carbon (Interlaboratory Working Group, Appendix C, 1999). Design studies suggest that molten carbonate fuel cell systems might achieve energy conversion efficiencies of 54%, and possibly 64% by adding a steam turbine bottoming cycle. These studies do not consider full fuel cycle emissions, however. Farrell et al. (2000) estimated the cost of eliminating carbon emissions from marine freight by producing hydrogen from fossil fuel, sequestering the carbon, and powering ships by solid oxide or molten carbonate fuel cells at US$218/tC, though there is much uncertainty about costs at this time.
A number of improvements can be made to conventional diesel vessels in, (1) the thermal efficiency of marine propulsion (5%–10%); (2) propeller design and maintenance (2%–8%); (3) hydraulic drag reduction (10%); (4) ship size; (5) speed (energy use increases to the third power of speed); (6) increased load factors; and (7) new propulsion systems, such as underwater foils or wings to harness wave energy (12%–64%) (CAE, 1996). More intelligent weather routing and adaptive autopilot control systems might save another 4%–7% (Interlaboratory Working Group, Appendix C, 1999).
Modern heavy trucks are equipped with turbo-charged direct-injection diesel engines. The best of these engines achieve 45% thermal efficiency, versus 24% for spark-ignited gasoline engines (Interlaboratory Working Group, 1997). Still, there are opportunities for energy efficiency improvements and also for lower carbon alternative fuels, such as compressed or liquified natural gas in certain applications. By a combination of strategies, increased peak pressure, insulation of combustion chambers, recovery of waste heat, and friction reduction, thermal efficiencies of 55% might be achievable, though there are unresolved questions about nitrogen oxide emissions (US DOE/OHT, 1996). For medium-heavy trucks used in short distance operations, hybridization may be an attractive option. Fuel economy improvements of 60%-75% have been estimated for smaller trucks with 5-7 litre engines (An et al., 1999). With drag coefficients of 0.6 to 0.9, heavy trucks are much less aerodynamic than light-duty vehicles with typical drag coefficients of 0.2 to 0.4. Other potential sources of fuel economy improvement include lower rolling resistance tyres and reduced tare weight. The sum total of all such improvements has been estimated to have the potential to improve heavy truck fuel economy by 60% over current levels (Interlaboratory Working Group, 2000).
Recognizing the growing levels of external costs produced by the continuing growth of motorized transport, cities and nations around the world have begun to develop plans for achieving sustainable transport. A recent report by the ECMT (1995) presents three policy “strands”, describing a progression of scenarios intended to lead from the status quo to sustainability. The first strand represents “best practice” in urban transport policy, combining land-use management strategies (such as zoning restrictions on low-density development and parking area controls) with advanced road traffic management strategies, environmental protection strategies (such as tighter pollutant emissions regulations and fuel economy standards), and pricing mechanisms (such as motor fuel taxes, parking charges, and road tolls). Even with these practices, transport-related CO2 emissions were projected to increase by about one-third in OECD countries over the next 20 years and by twice that amount over the next 30 to 40 years. A second strand added significant investment in transit, pedestrian, and bicycle infrastructure to shape land use along with stricter controls on development, limits on road construction plus city-wide traffic calming, promotion of clean fuels and the setting of air quality goals for cities, as well as congestion pricing for roads and user subsidies for transit. The addition of this strand was projected to reduce the growth in CO2 emissions from transport to a 20% increase over the next 20 years. The third strand added steep year-by-year increases in the price of fuel, full-cost externality pricing for motor vehicles (estimated at 5% of GDP in OECD countries), and ensuring the use of high-efficiency, low-weight, low-polluting cars, vans, lorries, and buses in cities. Addition of the third strand was projected to reduce fuel use by 40% from 1995 to 2015.
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