‘Energy’ indicates (in general terms) a given amount of energy, with no reference to time, and it is measured in joules. In energy analysis it can refer to a given amount of a primary energy source, or to a given amount of an energy carrier. ‘Power’, on the other hand, indicates the given pace of an energy conversion in time the rate at which useful work is performed and its unit of measure is watts (joules per second). Power is intrinsically linked to the characteristics of the energy converter (generating power) and the useful work performed with such a power.

Unfortunately, in many applications of energy analysis the distinction between energy and power often becomes blurred because of the way data on energy flows are presented. In fact, when dealing with the analysis of the metabolism of human beings (endosomatic metabolism) or socio-economic systems (exosomatic metabolism), one gets easily confused because data on energy inputs are usually expressed on a time basis.
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The rapidly growing literature on biofuels and many initiatives looking for alternative energy sources shows an amazing variety of options proposed by experts and visionaries. All these options are being proposed as feasible and/or highly desirable solutions to replace our use of oil.

In order to ascertain whether or not a certain source of energy would be an appropriate input for a system, one must first carefully observe the characteristics of the energy system. We cannot feed gasoline to humans, or power a refrigerator with pizzas. When refuelling a vehicle in a modern gas station, the driver must first select the appropriate type of fuel: gas or diesel, regular or premium octane rating.
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These technologies affect engine performance either directly or indirectly in a manner that reduces fuel consumption. For example, a significant portion of this chapter discusses the state of readiness, cost, and impact of reducing vehicle mass. Reducing mass reduces the energy necessary to move a vehicle, and thus reduces fuel consumption.

The complexity of substituting advanced, lightweight materials affects the redesign of a part or a subsystem, component manufacturing (including tooling and production costs), and joining, and raises interface issues that mixing different materials can pose. The term material substitution oversimplifies the complexity of introducing advanced materials, because seldom does one part change without changing others around it.

Advanced lightweight materials show great promise for reducing mass throughout a vehicle’s body structure and interior. Low-rolling-resistance tires and reduction of aerodynamic drag are also discussed as technologies that can lower tractive force and result in reduced fuel consumption.

Improvements in energy-drawing devices such as air conditioner compressors and power steering can reduce fuel consumption either by electrification or by improving their efficiency. New transmissions with more gears or that are continuously variable improve power train efficiency. All these options either reduce the demand for power from the engine or enable operating the engine at a more efficient point to reduce fuel consumption.

The committee considers car body design (aerodynamics and mass), vehicle interior materials (mass), tires, vehicle accessories (power steering and heating, ventilation, and air conditioning [HVAC] systems), and transmissions as areas of significant opportunity for achieving near-term, cost-effective reductions in fuel consumption.

Fuel cell vehicles have the potential to significantly reduce greenhouse gas emissions (depending on how hydrogen is produced) as well as U.S. dependence on imported oil over the long term. However, fuel cell vehicle technologies have technical challenges that are severe enough to convince the committee that it is unlikely such vehicles will be deployed in significant numbers within the time horizon of this study.

A recent report states that under the following set of very optimistic assumptions, 2 million fuel cell vehicles could be part of the U.S. fleet in 2020:
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In spite of the significant progress that battery technology has experienced in the last 20 years, the battery is still the most challenging technology in the design of hybrid vehicles.

All production hybrid vehicles used batteries employing nickel-metal-hydride (NiMH) chemistry. It is anticipated that the NiMH battery will be replaced by Li-ion batteries in the near future. The acceptability of today’s hybrid vehicles has been shown to be strongly dependent on the price of gasoline, as evidenced by the rapid growth of hybrid sales in 2008, when gasoline prices were high, and the fact that hybrid sales dropped dramatically in early 2009 when prices returned to lower values.
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