Science, Matter, and Energy - Environmental Science - page 31

Source of Energy Relative

Energy Quality
(usefulness)

Energy Tasks

Electricity
Very high temperature heat
(greater than 2,500 ° C)
Nuclear fission (uranium)
Nuclear fusion (deuterium)
Concentrated sunlight
High-velocity wind

Very high-temperature heat
(greater than 2,500 ° C)
for industrial processes
and producing electricity to
run electrical devices
(lights, motors)

Very high

High-temperature heat
(1,000-2,500

Mechanical motion (to move
vehicles and other things)
High-temperature heat
(1,000-2,500

° C)

Hydrogen gas
Natural gas
Gasoline
Coal
Food

C) for
industrial processes and
producing electricity

°

High

Normal sunlight
Moderate-velocity wind
High-velocity water flow
Concentrated geothermal energy
Moderate-temperature heat
(100-1,000

Moderate-temperature heat
(100-1,000

C) for industrial
processes, cooking,
producing steam,
electricity, and hot water

°

Moderate

C)
Wood and crop wastes

°

Dispersed geothermal energy
Low-temperature heat
(100 ° C or lower)

Low-temperature heat
(100

C or less) for
space heating

°

Low

Figure 2-10 Natural capital: categories of the quality of different sources of energy. High-quality energy is
concentrated and has great ability to perform useful work. Low-quality energy is dispersed and has little ability
to do useful work. To avoid unnecessary energy waste, you should match the quality of an energy source with
the quality of energy needed to perform a task.

another. No one has ever found a violation of this fun-
damental scientific law. It is another one of Mother
Nature's basic rules.
Consider three examples of the second law of ther-
modynamics in action. First, when you drive a car,
only 20-25% of the high-quality chemical energy avail-
able in its gasoline fuel is converted into mechanical
energy (to propel the vehicle) and electrical energy (to
run its electrical systems). The remaining 75-80% is de-
graded to low-quality heat that is released into the en-
vironment and eventually lost into space. Thus, most
of the money you spend for gasoline is not used to
transport you anywhere.
Second, when electrical energy in the form of mov-
ing electrons flows through filament wires in an incan-
descent light bulb, it changes into about 5% useful
light and 95% low-quality heat that flows into the en-
vironment. In other words, the light bulb is really a heat
bulb. Good news. Scientists have developed compact
fluorescent bulbs that are four times more efficient

than incandescent bulbs, and even more efficient ver-
sions are on the way. Do you use compact fluorescent
bulbs?
Third, in living systems, solar energy is converted
into chemical energy (food molecules) and then into
mechanical energy (moving, thinking, and living). Dur-
ing each conversion, high-quality energy is degraded
and flows into the environment as low-quality heat.
Trace the flows and energy conversions in Figure 2-11
(p. 32) to see how.
The second law of thermodynamics also means we
can never recycle or reuse high-quality energy to perform
useful work. Once the concentrated energy in a serving
of food, a liter of gasoline, a lump of coal, or a chunk of
uranium is released, it is degraded to low-quality heat
that is dispersed into the environment.
Energy efficiency, or energy productivity, is a
measure of how much useful work is accomplished
by a particular input of energy into a system. Good
news. There is plenty of room for improving energy