modes of transportation for these commodities in the specific regions in which the water routes are available. The energy efficiency of domestic marine shipping is comparable to that of railroads on a ton-mile basis (Davis et al., 2008).

Various opportunities exist in the near term to improve the energy efficiency of waterborne transportation. On the technology side, they include better shore power management and electrification, high-efficiency propulsion technology, improved hull design, and the use of alternative fuels. The potential for technical measures to reduce CO2 emissions from diesel fuel has been estimated at 5–30 percent in new vessels and 4–20 percent in older ones. On the operations side, near-term opportunities include improved terminal operations to reduce idling, queues, and delays; improved vessel-loading and -unloading operations; better hull maintenance; and speed reduction or optimization. Increasing vessel size will also reduce energy use per ton of freight shipped, especially for container ships. Marintek (2000) estimated that these operational measures could provide up to a 40 percent increase in energy efficiency.

In analyzing measures that can be taken to improve energy efficiency, Kromer and Heywood (2008) estimated the potential gains in energy efficiency in marine shipping to be 20–30 percent by 2020. Speed reduction was found to offer the greatest potential, followed by implementation of new and improved technology. Speed reduction, however, would require strong incentives to achieve, in view of the incentives to move shipments rapidly. Moreover, because of continued growth in commerce and waterborne traffic, it is likely that total energy use will continue to rise (Marintek, 2000).

3.5
FUELS OLD AND NEW

Current U.S. transportation systems—land, water, and air—overwhelmingly use petroleum-based hydrocarbon fuels. These fuels dominate because they are liquid at ambient temperature, have very high energy density, and fit well with today’s engine technologies: spark-ignition engines, diesels, and gas turbines. As an illustration of their attractiveness, when refueling a car today, the fuel’s chemical energy flows through the nozzle in one’s hand at the rate of 570,000 Btu per minute, providing another 400 miles of driving with a 5-minute refueling time.

Current U.S. fuels and engine technologies have evolved together over many decades. Thus, U.S. petroleum extraction, delivery, refining, and distribution systems are cost-effective, and these fuels are well matched to what end-users—



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