1. Combined Cycle Theory
Dalton Plant
Ouachita Plant

2. What is a “Combined Cycle” Power Plant?
Uses 2 thermodynamic cycles to generate electricity… Brayton - Gas Turbine & Rankine - Steam Plant
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The Brayton cycle gets its heat from burning the fuel.
The Rankine cycle uses heat rejected from the Brayton cycle.

3. Combined Cycle Designation
S207FA
Steam and Gas (STAG)
The Number of CT’s (and HRSG’s) per Steam Turbine
Model Series
MS6001FA
MS7001FA
MS9001FA
MS7001FB
MS9001FB
MS9001H
MS7001H
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4. Brayton Cycle
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5. Ideal Brayton Cycle 3 4 1 2
Compressor
Turbine 1 2 3 4
Fresh Air
Exhaust
Combustion
Fuel 1 2 3 4
Entropy S
Temperature
q in
q out
Const. p
Generation
Heat Rejected 1 2 3 4
Pressure
Specific Volume v
q in
q out
Const. s
Work Out
A Brayton Cycle is an all gas cycle where combustion and exhaust take place at different, but constant pressures.
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6. Ideal Brayton Cycle Efficiency
The theoretical maximum Brayton Cycle thermal efficiency is a function of: 1 2 3 4
Entropy S
Temperature
q in
q out
Const. p
Generation
Heat Rejected
The Pressure ratio - Ratio of Combustion to Atmospheric Pressure
The maximum temperature achieved or the maximum temperature that the machine can withstand
Ambient temperature
Expressed as: th = rp (1-k)/k
Where:
rp = Pressure Ratio
k = specific heat ratio of the working fluid
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Typically 30% - 40%

7. Perryville Simple Cycle GT
Inlet
Transformer
Generator
Gas Turbine
Exhaust Stack

8. Rankine Cycle
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9. Rankine Cycle (Steam Side)
Entropy (Btu/#/degF)
Boiling
Condensation
Pump
FW Heating
LPT
Reheating
HPT
Absolute Temperature (degF)
Heat Rejected to Condenser
Electrical Generation
LPT Exhaust enters condenser
A Rankine Cycle is a condensing cycle. In our case, steam is condensed.
Heat is added independent of pressure.
Superheating
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10. Ideal Rankine Cycle Efficiency
Entropy (Btu/#/degF)
Boiling
Condensation
Pump
FW Heating
LPT
Reheating
HPT
Absolute Temperature (degF)
Heat Rejected to Condenser
Electrical Generation
LPT Exhaust enters condenser
Superheating
The theoretical maximum Rankine Cycle thermal efficiency is a function of:
Boiler pressure
The max. temperature achieved (the max. temperature the machine can withstand)
Ambient temperature
Expressed as: th = (Wout - Win) / Qin
Where: Wout = work done by the system,
Win = work done on the system, and
Qin = heat added
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Typically 30% - 40%

11. Combined Cycle
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12. Combined Cycle Brayton Rankine
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13. The Concept
Generator
Intake
Fuel
Combustion Turbine
Heat Recovery Steam Generator
Steam Turbine
Cooling Tower
A Combined Cycle Power Plant has
One or more Gas Turbines with Electrical Generators and
One or more Steam Turbines with Electrical Generators
A Combined Cycle Power Plant produces electric power from fossil fuel using:
Gas Turbine, supplying turbine exhaust heat to
Heat Recovery Steam Generator, supplying steam to
Steam Turbine
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14. Perryville Combined Cycle Plant

16. For any system……. Q – E = W Heat In – Energy Change = Work Out
Combined Cycle
For any system……. Q – E = W
Heat In – Energy Change = Work Out
WORK AND ENERGY
ARE INTERCHANGEABLE !
To a power plant this means:
Heat in from fuel BTU/Hr
Electrical Generation MW
Heat rejected MW - =
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17. So why use a Combined Cycle?
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18. Because it increases overall efficiency!
Fuel Energy In
GT Gen.
GT Waste Heat (HRSG Energy In)
ST Gen.
ST Waste Heat
Total Gen.
Out the Stack
CT Efficiency = ~ 35%
CT Gen.
Fuel Energy In
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ST Efficiency = ~ 35%
ST Gen.
HRSG Energy In
Overall Efficiency = ~ 58%
Total Gen.
Total energy In
OA = CT + ST - CT ST

19. Why use a Combined Cycle?
Let’s put in numbers…
907 MW (3 Billion BTU/Hr)
Fuel
330 MW
577 MW CT
190 MW
349 MW ST
520 MW
Total
38 MW
CT Efficiency = ~ 36%
330 MW
907 MW
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ST Efficiency = ~ 33%
190 MW
577 MW
Overall Efficiency = ~ 57%
520 MW
907 MW
OA = x .33 = .57 = 57%

20. Combined Cycle 349 MW 38 MW 190 MW 907 MW 330 MW 577 MW
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330 MW
577 MW

21. GT/ST Combined Cycle Advantages: Disadvantages:
Short Project Schedules ~ 24 Months
High Thermal Efficiency ~ almost 60%
Uses multiple fuels - Oil or Gas
Quick Startup - 1 to 5 hours
Low Environmental Emissions per kW
Operating Flexibility – Run CT’s alone
Easier to Operate and Maintain
High Availability ~ 90%
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Disadvantages:
Burns “expensive” fuel.

22. Questions?
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