Environmental Engineering Reference
in renewable energy plants, of which hydropower (19%) is the overwhelming contributor. 6 The
generation and distribution of electric power to numerous industrial, commercial, and residential
consumers is considered today to be a requirement for both advanced and developing economies.
The electric energy produced in power plants is very quickly transmitted to the customer, where
it is instantaneously consumed for a multitude of purposes: providing light, generating mechanical
power in electric motors, heating space and materials, powering communication equipment, and
so on. There is practically no accumulation of energy in this system, in contrast with the storage of
fuels (or water in hydrosystems) at power plants, so that electric energy is produced and consumed
nearly simultaneously. 7 Electric power plants must be operated so as to maintain the flow of electric
power in response to the instantaneous aggregate demand of consumers. This is accomplished by
networking together the electric power produced by many plants so that a sudden interruption in
the output of one plant can be replaced by the others.
Transportation of goods and people among homes, factories, offices, and stores is a staple ingredient
of industrialized economies. Ground, air, and marine vehicles powered by fossil-fueled combustion
engines are the principal means for providing this transportation function. 8 Transportation systems
require both vehicle and infrastructure: car, truck, and highway; train and railway; airplane and
airport; ship and marine terminal. Ownership, financing, and construction of the infrastructure is
often distinct from that of the vehicle, with public ownership of the infrastructure and private
ownership of the vehicle being most common.
In economic terms, the largest transportation sector is that of highways and highway vehicles.
Worldwide, highway vehicles now number about 600 million, 200 million of them in the United
States. In the United States, 96% of the road vehicles are passenger automobiles and light-duty
trucks. The world and U.S. vehicle populations are growing at annual rates of 2.2% and 1.7%,
respectively. On average, U.S. vehicles are replaced every 13 years or so, providing an opportunity
to implement relatively quickly improvements in vehicle technology. 9
Transportation fuels are nearly all petroleum-derived. In the United States, transportation
consumes 70% of the petroleum supply, or 32% of all fossil fuel energy. Highway vehicles account
for 46% of petroleum consumption, or 21% of all fossil fuel energy. Transportation systems are
especially vulnerable to interruptions in the supply of imported oil, which now exceeds the supply
from domestic production. Unlike some stationary users of oil, transportation vehicles cannot
substitute coal or gas for oil in times of scarcity.
While substantial reduction in highway vehicle air pollutant emissions has been achieved in the
United States since 1970, and more reductions are scheduled for the first decade of the twenty-first
century, the focus of vehicle technology has shifted to improving vehicle fuel economy. Doubling
current fuel efficiencies without penalizing vehicle performance is technically possible, at a vehicle
6 When comparing the amount of hydropower energy with that of fossil and nuclear, the former is evaluated
on the basis of fuel energy needed to generate the hydroelectric power output of these plants.
7 In some renewable energy electric power systems, such as wind and photovoltaic power systems, there is
usually no energy storage; these systems can comprise only a part of a reliable electric energy system.
8 In developing countries, human-powered bicycles may be important components of ground transportation.
9 In contrast, fossil and nuclear power plants have a useful life of 40 years or more.