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

The net work output of this power plant is simply the difference between
the total work output of the plant and the total work input (Fig. 6–11):

(6–1)

The net work can also be determined from the heat transfer data alone. The
four components of the steam power plant involve mass flow in and out, and
therefore they should be treated as open systems. These components, together
with the connecting pipes, however, always contain the same fluid (not count-
ing the steam that may leak out, of course). No mass enters or leaves this com-
bination system, which is indicated by the shaded area on Fig. 6–10; thus, it
can be analyzed as a closed system. Recall that for a closed system undergoing
a cycle, the change in internal energy Uis zero, and therefore the net work
output of the system is also equal to the net heat transfer to the system:

(6–2)

Thermal Efficiency

In Eq. 6–2,Qoutrepresents the magnitude of the energy wasted in order to
complete the cycle. But Qoutis never zero; thus, the net work output of a heat
engine is always less than the amount of heat input. That is, only part of the
heat transferred to the heat engine is converted to work. The fraction of the
heat input that is converted to net work output is a measure of the perfor-
mance of a heat engine and is called the thermal efficiencyhth(Fig. 6–12).
For heat engines, the desired output is the net work output, and the
required input is the amount of heat supplied to the working fluid. Then the
thermal efficiency of a heat engine can be expressed as

Thermal efficiency (6–3)

Net work output

Total heat input

Wnet,outQinQout¬¬ 1 kJ 2

Wnet,outWoutWin¬¬ 1 kJ 2

Chapter 6 | 283

System boundary

Boiler

Pump Turbine

Qout

Win

Wout

Qin

Energy source

(such as a furnace)

Energy sink

(such as the atmosphere)

Condenser

FIGURE 6–10

Schematic of a steam power plant.

HEAT

ENGINE

Wout Wnet,out

Win

FIGURE 6–11

A portion of the work output of a heat

engine is consumed internally to

maintain continuous operation.

Heat input

100 kJ 100 kJ

1

Net

work

output

20 kJ

(^2) Net
work
output
30 kJ
Waste heat SINK
80 kJ
Waste heat
70 kJ
ηth,1= 20% ηth,2= 30%
SOURCE
FIGURE 6–12
Some heat engines perform better than
others (convert more of the heat they
receive to work).