Environmental Engineering Reference
In-Depth Information
through a combination of taxes and phasing out of en-
vironmentally harmful subsidies?
ing the amount of energy
used
(the first law of thermo-
dynamics),
automatically wasted
(the second law of
thermodynamics), and
unnecessarily wasted
in finding,
processing, concentrating, and transporting the useful
energy to users.
Net energy is analogous to your net spendable in-
come—your wages minus taxes and job-related ex-
penses. Suppose for every 10 units of energy in oil in
the ground, we use and waste 8 units of energy to find,
extract, process, and transport that oil to users. Then
we have only 2 units of
useful energy
available from
every 10 units of energy in the oil.
We can express net energy as the ratio of useful en-
ergy produced to the energy used to produce it. In the
preceding example, the
net energy ratio
would be 10/8,
or approximately 1.25. The higher the ratio, the greater
the net energy. When the ratio is less than 1, there is a
net energy loss. Figure 13-5 shows estimated net energy
ratios for various types of space heating, high-tempera-
ture heat for industrial processes, and transportation.
Currently, oil has a high net energy ratio because
much of it comes from large, accessible, and cheap-to-
extract deposits such as those in the Middle East. As
those sources are depleted, the net energy ratio of oil
will decline and its price will rise sharply.
Science: Net Energy
Net energy is the amount of high-quality usable
energy available from a resource after subtracting the
energy needed to make it available for use.
It takes energy to get energy. For example, before oil
becomes useful to us, it must be found, pumped up
from beneath the ground or ocean floor, transferred to
a refinery and converted to useful fuels (such as gaso-
line, diesel fuel, and heating oil), transported to users,
and burned in furnaces and cars. Each of these steps
uses high-quality energy. The second law of thermo-
dynamics tells us that some of the high-quality energy
used in each step is wasted and degraded to lower-
quality energy.
The usable amount of
high-quality energy
available
from a given quantity of an energy resource is its
net
energy.
It is the total amount of energy available from
an energy resource minus the energy needed to find,
extract, process, and get that energy to consumers. It is
calculated by estimating the total energy available
from the resource over its lifetime and then subtract-
Space Heating
Passive solar
5.8
Natural gas
4.9
4.5
Oil
Active solar
Coal gasification
Electric resistance heating (coal-fired plant)
Electric resistance heating
(natural-gas-fired plant)
Electric resistance heating (nuclear plant)
1.9
1.5
0.4
0.4
0.3
High-Temperature Industrial Heat
28.2
Surface-mined coal
Underground-mined coal
Natural gas
Oil
Coal gasification
Direct solar (highly concentrated by mirrors,
heliostats, or other devices)
25.8
4.9
4.7
1.5
0.9
Transportation
Natural gas
4.9
Gasoline (refined crude oil)
Biofuel (ethyl alcohol)
Coal liquefaction
Oil shale
4.1
1.9
1.4
1.2
Figure 13-5
Science:
net energy ratios
for various energy systems over their estimated lifetimes. The higher
the net energy ratio, the greater the net energy available.
Critical thinking: how does the current use of energy
resources (Figure 13-3) compare with the usage based on net energy?
(Data from U.S. Department of Energy
and Colorado Energy Research Institute,
Net Energy Analysis,
1976; and Howard T. Odum and Elisabeth C.
Odum,
Energy Basis for Man and Nature,
3rd ed., New York: McGraw-Hill, 1981)
