animals that have died millions of years ago also have stored energy.
Today, we use this stored energy in the forms of coal, oil, and natural
gas. These are just a few examples. Energy comes from many sources.
These sources, as you learned in Chapter 2, have been divided into three
categories:
• Nonrenewable energy sources.
• Renewable energy sources.
• Inexhaustible energy sources.
We will study energy resources in this chapter by classifying them as
either renewable or inexhaustible. Nonrenewable energy sources were
explained in Chapter 3.
Renewable Energy Sources
Renewable energy resources are those energy sources that can be
replaced as needed on a relatively short-term basis. See Figure 5-1. This
energy comes from plants and animals. Management of renewable energy
sources is critical to their effective use. Once a tree is cut down, it is
important to replant one to sustain this form of
energy. The basic renewable energy sources
include animals, food, wood, and alcohol,
including methanol. See Figure 5-2. Energy can
also be created from waste products, in a
process called bioconversion.
Animals
Animals were used as a main source of
energy for work in this country into the 1900s.
They are still used to produce power for
farming in many third world nations and in
certain cultures within the United States.
Animals are capable of pulling heavy loads on
sleds or wheeled vehicles that humans are not
able to pull. See Figure 5-3. Animals served as
a major source of mechanical power until the
development of mechanical machines. In the
United States and other developed nations, the
development and refinement of the external
and internal combustion engines slowly led to
the replacement of using animals for work.
Animals are also used for food.
Food
Food is another source of energy. Physical
labor, exercise, and recreation all require
energy. Energy from food also keeps our bodies
warm. Our bodies use the energy from the food
to produce heat, which keeps our bodies at the
correct temperature. Food is a renewable
source of energy because it can be regrown.
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Figure 5-1. This forest is an example of a
renewable energy resource. Trees can be
harvested, and new trees can be planted for
future harvesting.