The Second Law of Thermodynamics:
Kelvin–Planck Statement
We have demonstrated earlier with reference to the heat engine shown in
Fig. 6–15 that, even under ideal conditions, a heat engine must reject some
heat to a low-temperature reservoir in order to complete the cycle. That is,
no heat engine can convert all the heat it receives to useful work. This limi-
tation on the thermal efficiency of heat engines forms the basis for the
Kelvin–Planck statement of the second law of thermodynamics, which is
expressed as follows:
It is impossible for any device that operates on a cycle to receive heat from a
single reservoir and produce a net amount of work.That is, a heat engine must exchange heat with a low-temperature sink as well
as a high-temperature source to keep operating. The Kelvin–Planck statement
can also be expressed as no heat engine can have a thermal efficiency of
100 percent(Fig. 6–18), or as for a power plant to operate, the working fluid
must exchange heat with the environment as well as the furnace.
Note that the impossibility of having a 100 percent efficient heat engine is
not due to friction or other dissipative effects. It is a limitation that applies
to both the idealized and the actual heat engines. Later in this chapter, we
develop a relation for the maximum thermal efficiency of a heat engine. We
also demonstrate that this maximum value depends on the reservoir temper-
atures only.
6–4 ■ REFRIGERATORS AND HEAT PUMPS
We all know from experience that heat is transferred in the direction of
decreasing temperature, that is, from high-temperature mediums to low-
temperature ones. This heat transfer process occurs in nature without requir-
ing any devices. The reverse process, however, cannot occur by itself. The
transfer of heat from a low-temperature medium to a high-temperature one
requires special devices called refrigerators.
Refrigerators, like heat engines, are cyclic devices. The working fluid
used in the refrigeration cycle is called a refrigerant.The most frequently
used refrigeration cycle is the vapor-compression refrigeration cycle, which
involves four main components: a compressor, a condenser, an expansion
valve, and an evaporator, as shown in Fig. 6–19.
Chapter 6 | 287To supply energy at this rate, the engine must burn fuel at a rate ofsince 19,000 Btu of thermal energy is released for each lbm of fuel burned.
Discussion Note that if the thermal efficiency of the car could be doubled,
the rate of fuel consumption would be reduced by half.m# 689,270 Btu>h
19,000 Btu>lbm36.3 lbm/hHEAT
ENGINEWnet,out = 100 kWQH= 100 kWQL = 0Thermal energy reservoir···FIGURE 6–18
A heat engine that violates the
Kelvin–Planck statement of the
second law.SEE TUTORIAL CH. 6, SEC. 4 ON THE DVD.INTERACTIVE
TUTORIAL