The challenge of energy-efficient transportation
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REVIEW The challenge of energy-efficient transportation
Jo Hermans, Huygens Laboratory, Leiden University, 2300RA, Leiden, The Netherlands Address all correspondence to Jo Hermans at [email protected] (Received 26 October 2016; accepted 11 January 2017)
ABSTRACT The efficiencies of present-day modes of transportation are reviewed. Future sustainable options are discussed. Transportation takes about 20% of the energy use worldwide [1], and this figure is likely to increase. The fact that transportation requires some form of mobile energy storage makes this topic especially challenging for the post-fossil-fuel era. When looking at alternatives, we should realize that nothing matches the energy density of liquid fuels like gasoline or diesel, if the system as a whole is considered. It is, therefore, important to consider the efficiency of various modes of transportation. To assess the possibilities of improvements in efficiency, a brief introduction into the physics of transportation is given first. Subsequently, the efficiencies of present-day modes of transportation— cars, buses, trains, air transport, and bicycles—are reviewed. Finally, new technologies relying on biofuel, electricity, solar power, and hydrogen are discussed. Keywords: environment; energy storage; efficiency; photovoltaic; transportation
DISCUSSION POINTS • S ustainable transportation by cars may be feasible only if society is willing to accept a lower degree of comfort. • A worldwide tax on kerosene may be something to consider. • F or future aviation, liquid hydrogen deserves serious consideration as a potential energy carrier.
Resistances The key for determining energy use in transportation is resistance. The reason is that resistance—the force which has to be overcome to move at constant speed—is energy per unit distance: 1 N = 1 J/m. In other words, the resistance gives us directly the energy use in terms of the number of J/m or, more conveniently, kJ/km. For cars, buses, trains, and bicycles, we deal with two types of resistance in transportation, viz., rolling resistance Fr, and air resistance or aerodynamic drag Fd. The rolling resistance can be written as Fr = C r × mg,(1)
with Cr the rolling resistance coefficient (sometimes referred to as RRC), m the mass, and g the acceleration of gravity. Since mg is the weight (in newton), it is seen that the rolling resistance is
a fraction Cr of the weight. The rolling resistance for cars and other vehicles using rubber tires is mainly caused by the fact that the forces related to compression and expansion of rubber are not equal: there is some hysteresis. Note that the value of Cr must be of the order of 0.01, since a slope of approximately 1% suffices to get a car rolling. Extensive measurements by the National Research Council of the National Academy of Sciences2 showed that values of Cr vary from 0.007 to 0.014. Most tire models today, when measured new, have RRCs below 0.009. The value of Cr is affected by load and inflation pressure. Higher deformation of the tire resul
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