Modern Engineering Thermodynamics

482 CHAPTER 13: Vapor and Gas Power Cycles
Thermal efficiencies %
55
50
45
40
35
30
Low-speed
diesel engine
Medium-speed
diesel engine
Combined cycle
gas turbine
25
Gas turbine
Steam turbine
FIGURE 13.30
Thermal efficiency ranges of various power-producing technologies.
20 1 10 50
Capacity (MW)
2. A similar performance measure, the heat rate of the power plant, which is defined to be the inverse of the
thermal efficiency but in mixed units (e.g., Btu/(kW ·h)). These two measures are related by
Heat rate in Btu/ðkW . hÞ = 3412 Btu/ðkW . hÞ
ð
η T Þ actual
where the decimal form (not percent) of (η T ) actual is used.
3. The actual steam flow rate divided by the actual plant electrical output power, _m steam / _W elect , in mixed units
of (lbm steam)/(kW ·h).
4. The ratio of the actual thermal efficiency of the power plant to the isentropic Rankine cycle thermal
efficiency, (η T ) actual /(η T ) isentropic . This ratio is often expressed as a percentage and is commonly called by the
misleading term engine efficiency. More accurately, it is an overall heat engine thermal efficiency ratio.
By the 1930s, it had been realized that water was not necessarily the best working fluid for a vapor cycle heat
engine. Since the deviations between the Carnot cycle and the isentropic Rankine cycle are due to the characteristics
of the working fluid, clearly, the ideal working fluid for a heat engine should make the Rankine cycle as
close to the Carnot cycle as possible. More specifically, the ideal working fluid should have the following characteristics
(see Figure 13.31):
1. It should have a critical temperature well above the metallurgical limit of the boiler and turbine, so that
efficient isothermal high-temperature heat transfer can occur in the boiler.
2. It should have a relatively low saturation vapor pressure at high temperatures, so that high mechanical
stresses are not produced in the boiler or turbine.
3. It should have an ambient temperature saturation pressure slightly above atmospheric pressure, so that the
condenser does not have to be operated at a vacuum.
4. It should have large phase change enthalpies (h fg )
and low liquid specific heats, so that the heat
required to bring the liquid condensate up to the
vaporization temperature is a small percentage of the
vaporization heat (this reduces boiler heat transfer
irreversibilities and ensures that regeneration devices
are effective).
5. It should have the slope of its saturated vapor and
liquid lines as nearly vertical as possible on a T–s
diagram.
6. It should have a triple-point temperature well below
the ambient temperature to prevent the formation of
solids (i.e., freezing) within the system.
7. It should be chemically stable (i.e., not dissociate at
high temperatures), nontoxic, noncorrosive, and
inexpensive.
T H
T
T L
4
3 2
s
Vapor dome of an
ideal working fluid
1
Metallurgical limit
Rankine and Carnot
cycles
Ambient
FIGURE 13.31
The Rankine cycle with the ideal working fluid becomes a
Carnot cycle.