CHAPTER 4
ANALYSIS RESULTS
In this chapter, implementation of an array of combined cycle power plant options will be
considered as options for the replacement of the simple gas turbine standard used to power
conventional navy ships. Doerry’s Economical Transit operational mode, which was introduced
in Table 7, will be deployed for analysis. Recall that the maximum power required for
Economical Transit mode is 22 MW. This reference will serve as the design target for the power
output of all combined cycle power plant configurations discussed below. Using the tool
developed in chapter 2, for evaluation of combined cycle power plants, and the weight analysis
techniques presented in chapter 3, combined cycle power plant alternatives will be assessed for a
variety of design scenarios. The nominal quality and pinch point values used in each of the
section of this chapter are 95% and 20 °C, respectively, and the efficiency and exhaust gas
temperature of the nominal gas turbine are 30% and 500 °C, respectively.
In section 4.1 the gross effects of recovering power from the gas turbine exhaust will be
discussed. In section 4.2, the focus shifts to the impact that changing the quality of steam exiting
the turbine at state 8 has on combined cycle power plants. Section 4.3 concentrates on the effects
of varying the pinch point in the heat recovery steam generator (HRSG). Finally, section 4.4
utilizes the information of the previous three sections to assess the effects that altering gas
turbine operating parameters (efficiency and exhaust gas temperature) have on the system. This
analysis will help determine what type of gas turbine is best suited for retrofit with a combined
cycle power plant. The chapter concludes with a brief summary of the findings in section 4.5.
Throughout the chapter, the primary assessment parameter employed in the evaluation of other
system parameters is the power recovered from the gas turbine’s exhaust gases. This technique is
intended to demonstrate that the power recovered from the exhaust gas stream is the driving
factor that affects every parameter of the bottoming cycle power plant. However, the power
recovered in the HRSG will not be displayed explicitly on the abscissa; rather, the temperature of
the gas turbine exhaust exiting the HRSG (T5) will be displayed. This choice leverages the
presumed proportional relationship between HRSG power and the temperature drop of the gas
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turbine’s exhaust through that device. Recall from Eq. (8) that under steady state operation, if the
mass flow rate and the specific heat of the gas turbine exhaust remain constant, then the power
recovered in the HRSG is directly proportional to the temperature drop across the heat
exchanger. Further, it is reasonable to assume that the gas turbine’s exhaust temperature at state
4 remains constant at 500 °C. Under these circumstances, the change in temperature at state 5 can
be used as a qualitative means for analyzing the power recovered by the HRSG. As T5 falls
below 500 to 100 °C, the power recovered from the gas turbine exhaust increases from the lowest
to the highest level considered in this work. Figure 16 highlights the measurement location of T5
in the combined cycle power plant.
Figure 16 –HRSG exhaust gas temperature is a qualitative measure
recovered power
The tool, as described in the previous chapters, is limited to analysis for gas turbines, whose
exhaust stream is directed through the HRSG of a simple, Rankine cycle steam turbine plant.
Although the configuration analysis could be used to design a steam turbine cycle who’s HRSG
absorbs power from the waste heat from any prime mover, the weight analysis provided in the
previous chapter constricts the tool’s use to gas turbines. Moreover, only simple, Rankine cycle
steam turbine power plants may be considered using this tool. Higher efficiency steam turbine
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cycles that incorporate regeneration, re-heating, and other efficiency optimization methods are
beyond the capability of this first approximation tool.
4.1 Case I – The Effects of Varying HRSG Exhaust Gas Temperature (T5)
The temperature drop of the gas turbine exhaust as it passes through the HRSG provides a good
reference to qualitatively understand how the power recovered from the gas turbine exhaust
affects various parameters of the combined cycle. The evolution of the combined cycle
efficiency as a result of this power recovery is displayed in Figure 17. This graphic provides the
framework for most of the figures that follow. The left axis is reserved for the parameter(s)
considered later in the chapter, while the right axis is dedicated to tracking the combined cycle
efficiency. The line representing the efficiency versus HRSG exhaust gas temperature is
ubiquitous in the figures of this section.
Figure 17 – Efficiency versus T5 in the format used to evaluate other
parameters
In most of the graphs in this section, a representation of the power recovered from the gas turbine
exhaust will serve as the abscissa, the parameter under consideration will be the primary
ordinate, and the efficiency of combined cycle power plant will frequently be represented by a
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secondary ordinate located on the right side of the graph. This technique is used to underscore
the importance of efficiency in the evaluation of combined cycle operation.
The variable parameters manipulated in this chapter are the steam quality of the bottoming cycle
turbine exit, the pinch point in the heat recovery steam generator (HRSG), and the primary
operational parameters of the gas turbine, exhaust gas temperature and operating efficiency. In
this section, the values for steam turbine quality and HRSG pinch point are 95% and 20 °C,
respectively, while the gas turbine modeled has an exhaust gas temperature of 500 °C and an
efficiency of 30%. Doerry’s Economical Transit mode of operation provides the basis for
evaluation.
The energy recovered from the gas turbine exhaust is used to drive the Rankine Cycle which
produces work with the steam turbine. Figure 18 illustrates the effect that recovering varying
amounts of power in the HRSG has on the power output of the steam turbine. The increased
power output per unit of fuel consumed results in increased combined cycle efficiency as
anticipated by Eq. (19). By augmenting the prime mover with a bottoming cycle, the efficiency
of the combined cycle power plant can be elevated to as high as 42.4%. Initially, increasing
power recovery in the HRSG corresponds with gains in both steam turbine output power and
power plant efficiency. This trend continues until the HRSG exhaust gas exit temperature
reaches 186 °C, when both the power output of the steam turbine and the efficiency of the
combined cycle power plant drop precipitously. The power output of the steam turbine regresses
from a maximum of 6.44 MW at 186 °C to 5.36 MW at 100 °C. Simultaneously, the combined
cycle efficiency falls from 42.4% to just below 40%. If the earlier trends had continued, the 100
°C mark would have corresponded to the maximum steam turbine power output in excess of 7
MW, and combined cycle power plant efficiency greater than 45%.
The change in behavior in the region between 186 °C and 100 °C can be explained by the fact
that the pinch point changed location. Recall the inequalities of Eqs. (13) and (14), as well as the
discussions in sections 2.3.1 and 2.3.2 regarding the effects of moving the pinch point. In the
region in which the steam turbine power and efficiency increase from 0 to 6.44 MW and from
30-42.4%, respectively, the pinch point is located at the interface between states 4 and 7, where
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live steam exits the super heater. In this regime, power and efficiency increase with decreasing
gas turbine exhaust temperature from the HRSG, as expected. However, as the exhaust gas
HRSG exit temperature (T5) drops, it brings T4b with it, eventually compelling the pinch point to
move to the interface between states 4b and 6a where saturated liquid water enters the boiler,
after being heated in the economizer. Figure 19 illustrates the change in pinch point location that
is required to comply with the inequalities presented in Eqs. (13) and (14).
Figure 18 – The effects of changing T5 on the steam turbine power
output
The change in pinch point position is followed by dramatic drops for both live steam (T7) and
evaporator (T6a) temperature. Since any reduction in evaporator temperature results in a drop in
HRSG pressure, the presumption of constant condenser pressure results in corresponding
reductions in steam turbine pressure ratios in the regime between 186 °C and 100 °C. These
drops in pressure ratio are manifested by the reduced efficiencies. The green line representing the
live steam temperature (T7) in Figure 19 also indicates that although heat recovery from the
exhaust gas stream continues to increase (T5 continues to decrease), when the pinch point moves
to the interface between states 4b and 6a, less of that power is being used to elevate the
temperature of the superheated steam.
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In order to achieve the efficiency gains that are apparent in either pinch point regime, additional
mechanical components must be added to the system. Using the analysis methods presented in
chapter 3, the weights of the array of combined cycle configurations have been calculated.
However, it is also apparent from the fuel weight analysis of chapter 3 that net fuel savings will
coincide with the efficiency gains. The blue line in Figure 20 indicates the effect that varying the
HRSG exhaust gas temperature has on the weight of the combined cycle power plant. The green
line shows the response of the fuel to the various amounts of heat recovery in the HRSG, and the
red line represents the effect on the mechanical components. Analysis of this figure shows that
although the addition of mechanical components could result in a maximum weight increase of
128 tonne, the maximum fuel weight reduction achieved by elevating