Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

to engineers, and the second law provides the necessary means to determine
the quality as well as the degree of degradation of energy during a process.
As discussed later in this chapter, more of high-temperature energy can be
converted to work, and thus it has a higher quality than the same amount of
energy at a lower temperature.
The second law of thermodynamics is also used in determining the
theoretical limitsfor the performance of commonly used engineering sys-
tems, such as heat engines and refrigerators, as well as predicting the degree
of completionof chemical reactions.

6–2 ■ THERMAL ENERGY RESERVOIRS

In the development of the second law of thermodynamics, it is very conve-
nient to have a hypothetical body with a relatively large thermal energy
capacity(mass
specific heat) that can supply or absorb finite amounts of
heat without undergoing any change in temperature. Such a body is called a
thermal energy reservoir,or just a reservoir. In practice, large bodies of
water such as oceans, lakes, and rivers as well as the atmospheric air can be
modeled accurately as thermal energy reservoirs because of their large ther-
mal energy storage capabilities or thermal masses (Fig. 6–6). The atmo-
sphere, for example, does not warm up as a result of heat losses from
residential buildings in winter. Likewise, megajoules of waste energy
dumped in large rivers by power plants do not cause any significant change
in water temperature.
A two-phase systemcan be modeled as a reservoir also since it can absorb
and release large quantities of heat while remaining at constant temperature.
Another familiar example of a thermal energy reservoir is the industrial fur-
nace. The temperatures of most furnaces are carefully controlled, and they
are capable of supplying large quantities of thermal energy as heat in an
essentially isothermal manner. Therefore, they can be modeled as reservoirs.
A body does not actually have to be very large to be considered a reser-
voir. Any physical body whose thermal energy capacity is large relative to
the amount of energy it supplies or absorbs can be modeled as one. The air
in a room, for example, can be treated as a reservoir in the analysis of the
heat dissipation from a TV set in the room, since the amount of heat transfer
from the TV set to the room air is not large enough to have a noticeable
effect on the room air temperature.
A reservoir that supplies energy in the form of heat is called a source,and
one that absorbs energy in the form of heat is called a sink(Fig. 6–7). Ther-
mal energy reservoirs are often referred to as heat reservoirssince they
supply or absorb energy in the form of heat.
Heat transfer from industrial sources to the environment is of major con-
cern to environmentalists as well as to engineers. Irresponsible manage-
ment of waste energy can significantly increase the temperature of portions
of the environment, causing what is called thermal pollution. If it is not
carefully controlled, thermal pollution can seriously disrupt marine life in
lakes and rivers. However, by careful design and management, the waste
energy dumped into large bodies of water can be used to improve the qual-
ity of marine life by keeping the local temperature increases within safe
and desirable levels.

Chapter 6 | 281

ONE WAY

FIGURE 6–4

Processes occur in a certain direction,

and not in the reverse direction.

PROCESS 1st law 2nd law

FIGURE 6–5

A process must satisfy both the first

and second laws of thermodynamics to

proceed.

ATMOSPHERE

LAKE

RIVER

OCEAN

FIGURE 6–6

Bodies with relatively large thermal

masses can be modeled as thermal

energy reservoirs.

Thermal energy

SINK

Thermal energy

SOURCE

HEAT

HEAT

FIGURE 6–7

A source supplies energy in the form

of heat, and a sink absorbs it.

SEE TUTORIAL CH. 6, SEC. 2 ON THE DVD.

INTERACTIVE

TUTORIAL