1 /GE /

June 2005

Combined Cycle Theory

2 /GE /

June 2005

Combined Cycle Theory Objectives•Explain the Brayton Cycle•Explain the Rankine Cycle•Explain how different thermodynamic cycles make up a combined cycle power plant•Describe the effects of combining the different cycles (efficiency)

3 /GE /

June 2005

Simple Gas Turbine CycleGas TurbineBrayton Thermodynamic Cycle

4 /GE /

June 2005

1 to 2: Isentropic compression 2 to 3: Isobaric heat supply 3 to 4: Isentropic expansion 4 to 1: Isobaric heat rejection

In Brayton Cycle (Constant Pressure Cycle), combustion and exhaust take place at different, but constant pressures.

Brayton Thermodynamic Cycle

5 /GE /

June 2005

Brayton Cycle Efficiency

• Brayton Cycle thermal efficiency measured using compressor ratio.

• Compressor ratio = Ratio of maximum and minimum pressures in cycle (pressure at combustor outlet vs. hot gas pressure at turbine exhaust).

6 /GE /

June 2005

7 /GE /

June 2005

Best expressed as: th = 1 - rp (1-k)/k

Where: rp = Compressor Ratio

and k = ratio of specific heats of working fluid.

• Average simple gas turbine cycle efficiency is 38% - 45%

Brayton Cycle Efficiency

8 /GE /

June 2005

Simple Steam Turbine Cycle

Steam TurbineRankine Thermodynamic Cycle

9 /GE /

June 2005

Rankine Thermodynamic Cycle

Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy.

Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become a dry saturated vapor.

Process 3-4: The dry saturated vapor expands through a turbine, generating power. This decreases the temperature and pressure of the vapor, and some condensation may occur.

Process 4-1: The wet vapor then enters a condenser where it is cooled at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase change.

There are four processes in the Rankine cycle, each changing the state of the working fluid.

10 /GE /

June 2005

• Average normal Rankine Cycle efficiency = 27% - 35%.

• Best expressed as: th = (wout - win) / qin

• Where: wout = work done by the system, win = work done on the system, and qin = specific heat added

Rankine Cycle stated simply, is E = Q – W

•Total change in energy entering or leaving system equals heat added to system MINUS work performed by system

Rankine Cycle Efficiency

11 /GE /

June 2005

Rankine Cycle

SubcooledLiquid

SaturatedLiquid-Vapor Mix

SuperheatedVapor

Boiling →

← Condensation

Cond / FW Pp →

FW Heating →

Superheating →

← LP Turbine

← Reheating

← HP Turbine

12 /GE /

June 2005

What is a “Combined Cycle” Power Plant?

A combined cycle system uses same heat energy to generate power using two different thermodynamic cycles.

Plant uses two distinctly different thermodynamic cycles to perform useful work (generate electricity).

•Brayton (Gas Turbine) Cycle

•Rankine (Steam Plant) Cycle

Advantage -- overall plant efficiency (electrical energy out compared to heat energy in) greatly improved over either simple gas turbine cycle alone or simple steam turbine cycle alone.

13 /GE /

June 2005

To Steam Turbine

2 different cycles

14 /GE /

June 2005

Combined Cycle Efficiency

• When two power cycles combined into single power unit, total cycle efficiency is function of individual cycle performance.

•Brayton Cycle and Rankine Cycle in series.

•Any changes in Brayton Cycle efficiency directly effects efficiency of Rankine Cycle and,

ultimately, overall plant efficiency.

15 /GE /

June 2005

• Combined Cycle thermal efficiency can be calculated as;

th = A + B - A B

•With Cycle A representing Brayton Cycle and Cycle B representing Rankine Cycle.

•This gain in efficiency is basis for preferring Combined Cycle power plants over standard fossil power generating stations.

Combined Cycle Efficiency

16 /GE /

June 2005

Combined Cycle Power Plant

17 /GE /

June 2005

Combined Cycle Advantages

• Short Project Schedules ~ 24 Months•High Thermal Efficiency ~ 50%•Low Environmental Emissions• High Operating Flexibility• High Availability ~ 90%

18 /GE /

June 2005

Review

•Explain the Brayton Cycle•Explain the Rankine Cycle•Explain how different thermodynamic cycles make up a combined cycle power plant•Describe the effects of combining the different cycles (efficiency)