Environmental Engineering Reference
In-Depth Information
The actual production of steam for powering turbines for energy conversion is
accomplished by machines called
heat engines
. A few heat engine examples include
an automobile engine that converts the chemical energy of gasoline into the mechani-
cal energy of a piston and camshaft or the turbine in an electrical generating plant that
converts heat into shaft work to run a generator which, in turn, produces electrical
power. The heat engines of interest to us in this text are the Rankine cycle (or vapor
cycle) and the Stirling cycle (or gas cycle) heat engines (Hinrichs and Kleinbach, 2006).
HEAT ENGINES
A heat engine is a device that
converts thermal energy to mechanical output. The
thermal energy input is called
heat
, and the mechanical output is called
work
.
Typically, heat engines run on a specific thermodynamic cycle. Heat engines can
be open to the atmospheric air (open cycle) or sealed and closed off to the outside
(closed cycle). The driving agent of a heat engine is a temperature differential. That
is, heat engines convert heat energy to mechanical work by exploiting the tempera-
ture gradient between a hot “source” and a cold “sink” (see Figure 1.1). Heat is trans-
ferred from the source, through the “working body” of the engine, to the sink, and in
this process some of the heat is converted into work by exploiting the properties of a
gas or liquid (the working substance). The lower the sink temperature or the higher
the source temperature, the more work is available from the heat engine.
Hot source (
T
H
)
Q
H
W
Q
L
Cold sink (
T
L
)
where
Q
H
= Heat energy taken from the high-temperature system.
Q
L
= Heat energy delivered to the low-temperature system.
W
= Work.
T
H
= Absolute temperature of heat source.
T
L
= Absolute temperature of cold sink.
FIGURE 1.1
Heat engine diagram.
