12- 1 Overview
This chapter focuses on technological options for sustainably delivering energy to motor vehicles and other mechanized transportation equipment. Each specific propulsion technology can be categorized among a limited number of generic “endpoint technologies,” and the strengths and weaknesses of each are considered. While the goal of sustainability may drive society toward adoption of endpoint technologies that will meet future expectations for transportation energy, the basic design of motor vehicles for the consumer market of today is driven by the need to trade off energy consumption against performance, size, and other consumer amenities. Therefore, a number of design factors are introduced and analyzed as a foundation for discussion of all technology alternatives. Thereafter we take a closer look at four major alternative propulsion platforms and fuels, namely, battery-electric, hybrid-electric, biofuel, and hydrogen vehicles. The chapter concludes with a discussion of “well-to-wheel” analysis of energy efficiency in transportation vehicles. Material in this chapter provides a basis for consideration of systems issues related to transportation energy in Chap. 14.
12- 2 Introduction
In the modern world of motorized cities, long-distance travel by jet or limited- access highway, and global trading of manufactured goods and commodities, it comes as no surprise that the transportation sector has become an enormous end user of energy. This sector is the single largest consumer of petroleum resources in the world today, and the second largest consumer of nonrenewable fossil fuels next to electric power conversion. Worldwide, the transportation sector accounted for approximately one-fourth of the total end-use energy consumption value of 426 EJ (403 quads) in 2003. In the United States alone, in that year the transportation sector accounted for 28.5 EJ (27.0 quads), which constitutes 27.5% of the total U. S. energy end-use budget of 103.7 EJ (98.3 quads), or 6.5% of the world total, as shown in Fig. 13-1. To put these values in context, the U. S. transportation energy consumption rate is equivalent to 9 billion 100-W lightbulbs burning continuously 24 h/day, 7 days a week, all year long, or 30 lightbulbs for every one of the United States’ approximate
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Figure 13-1 Comparison for 2003 of U. S. transportation energy consumption to other U. S. energy consumption (“U. S. other”), total energy consumption of other industrial countries (“Other industrial”), and emerging countries.
Total value 426 EJ (403 quads). Note that total energy consumption for all industrial countries is U. S. transport + U. S. other + other industrial. (Source for data: U. S. Energy Information Agency.)
population of 300 million people.[55] The world transportation energy consumption figure is equivalent to 36 billion lightbulbs, or 6 lightbulbs per person.
A unique challenge facing the ongoing use of various energy sources for transportation is the need to store energy in a concentrated form onboard the vehicle. One objective of vehicle design is to store a large quantity of energy per unit of weight and volume displaced on the vehicle, lest the weight or volume of the fuel should limit the capacity to carry passengers or cargo. Some vehicles bypass this problem by using the electric grid as a source of energy, through the use of electric catenary (e. g., electric trains with overhead wires, subways with third rails, trolleybuses, and so on), while others are nonmotorized (e. g., bicycles, cycleshaws, and so on). However, of the vehicle-miles traveled each year, only a small minority fall into these two categories. The great majority of vehicle-kilometers are generated by vehicles that are (a) mechanized and therefore not relying on human power and (b) carry their own power source (sometimes referred to as “free-ranging”).
Most of the energy used by free-ranging vehicles comes from petroleum in either a gasoline or diesel form. Gasoline or diesel fuels have a number of characteristics that make them well-suited for use in motor vehicles: they do not require pressurization, and they are liquids, so that they are relatively easily dispensed into the vehicle’s fuel storage tank and combusted in the internal combustion engine. They also provide high specific weight and volume, that is, for the amount of space and payload taken in the
vehicle, they provide a large amount of energy storage. Ideally, any alternative fuel must match these characteristics. Otherwise, society will need to radically rethink the way motorized transportation systems are used so that fuels that are less convenient or provide shorter range become the norm.
As discussed in Chaps. 4 and 5, the twin arguments of peaking of conventional petroleum production and CO2-induced climate change make a compelling case for exploring alternative fuels and alternative technologies. Many such alternative technologies exist on a small scale or in concept, including natural gas vehicles, fuels derived from coal, non-conventional crude oil sources such as oil shale, conversion of renewable energy sources into a form transferable to vehicles, and the like. However, in order to succeed, such technologies must be technically robust and financially viable. Under pressure from declining petroleum supplies, society may be willing to pay more for an alternative fuel source than they currently pay for gasoline, but people will refuse to support such a fuel if its price is exorbitant, or they may simply be unable to afford it. Furthermore, the energy source must be developed in parallel with the vehicles that use it and the infrastructure to distribute it, and all these things must fall into place in a short amount of time, or at least in a way that keeps pace with the decline in availability of gasoline and diesel from the market. Mature technologies that use petroleum more efficiently, such as hybrid drivetrains, are already expanding in the marketplace, but they are not a permanent solution unless the energy source is changed from petroleum. Cost of any new distribution infrastructure system is also a concern, although given the high value of the transportation fuels market—at an average cost of $3.00/gal, including taxes, the approximately 200 billion gallons of gasoline, diesel, and jet fuel purchased in the United States in 2006 would have a retail value of some $600 billion—it is clear that companies that deliver transportation energy products should be able to recoup a significant investment in new infrastructure through continued sales.
Compared to the challenge of finding a clean and abundant source of energy from which to make a future energy source for transportation, the ability to transfer that energy source onto the vehicle poses the more daunting of the two challenges. For example, nuclear energy and large-scale wind turbines are two proven technologies for generating electricity without CO2 emissions that have a similar cost per kWh to fossil fuel energy generation. However, we do not yet have an infrastructure to manufacture fuel cell or battery technologies on a scale to take nuclear or wind energy onboard the vehicle as a substitute for gasoline (though in the case of electricity, home recharging might provide a partial solution). We also do not have a network of refueling stations available to the public to distribute and dispense electricity or hydrogen. These obstacles may favor instead the development of a petroleum substitute, such as a biofuel, that behaves like petroleum so that we can use our existing distribution and dispensing infrastructure, but that does not depend on nonrenewable resources and that does not contribute to climate change.
To summarize, the development of a clean, abundant, and economical substitute for the petroleum-based transportation energy system is one of the major challenges facing the nations of the world today. Society would likely experience significant disruption from passing the peak oil point and not having a carefully prepared alternative waiting in the wings (see Chap. 5). Also, if the alternative were to be nonconventional petroleum sources, we might prolong for a time the worldwide transportation system based on liquefied fossil fuels, but greatly aggravate the climate change problem if no suitable system for mitigating CO2 emissions is in place. This challenge is arguably one of the most difficult technological and systems problems that we face in the pursuit of sustainable energy.
