Modern Engineering Thermodynamics

13.10 Modern Steam Power Plants 483
4
Mercury boiler
Mercury condenser
8
3
Water boiler
5
1
2
W Hg
T
4
3 2
1
Mercury vapor dome
Mercury cycle
5
Steam cycle
7
Water condenser
(a) Equipment schematic
6
W H2 O
8
7 6
s
(b) Thermodynamic T–s diagram
Steam vapor dome
FIGURE 13.32
A mercury-water binary power plant.
No known fluid meets all seven of these conditions quite as well
as water. But other fluids meet some of these conditions significantly
better than water. For example, the critical state of mercury
is at 1649°F and 2646 psia, which meets item 1 much
better than water, whose critical state is 705°F and 3204 psia.
However, the saturation pressure of mercury at 100°F isavery
high vacuum (thus violating item 3). At 1000°F, the saturation
pressure of mercury is about 180 psia, which makes it attractive
for use in a dual working fluid or binary cycle system, as shown
in Figure 13.32. Here, the mercury condenser also serves as the
steam generator, and the combined binary cycle thermal efficiency
is much higher (in the range of 50–60% for isentropic
systems) than either one operating alone (in the range of 30–
40% for isentropic systems).
Between the 1930s and 1960s, several mercury-water binary
cycle power plants were put into commercial operation. But,
despite their superior thermal efficiencies, the problems of high
initial cost, mercury toxicity, and numerous operating and maintenance problems prevented such plants from
being commercially successful. However, the use of two or more working fluids within the same power plant
still holds promise for significantly improving overall thermal performance in the future.
Supercritical power plants were developed after 1950 with boiler pressures as high as 5000 psia at 1200°F.
Figure 13.33 illustrates a supercritical Rankine cycle with two stages of reheat. However, the high operating
and maintenance costs of supercritical plants often offset the cost benefits due to their increased thermal
efficiencies.
T
Supercritical boiler
s
First reheat
Second reheat
FIGURE 13.33
A supercritical Rankine cycle with two stages of
reheat.
EXAMPLE 13.8
The Philadelphia Electric Power Company Eddystone Power Plant has the highest operating conditions of any electrical
generating facility in the world. The boiler has a supercritical outlet state of 5000. psia at 1200.°F. After expansion in the
first and second stages of the turbine, the steam is reheated to 1000.°F at 1000. psia and to 1000.°F at 300. psia, respectively.
The condenser pressure is 0.400 psia, and the power plant has eight regenerators. The steam mass flow rate is
1.50 × 10 6 lbm/h and the power plant produces 325 MW of electrical power. Neglecting the eight regenerators in this
plant, determine
a. The isentropic thermal efficiency of this power plant.
b. The isentropic efficiency of the turbine-generator power unit.
Solution
First, draw a sketch of the system (Figure 13.34).
(Continued )