Lecture 5 - The Gas-Turbine Cycles.pptx

The Gas-Turbine Cycles

1. The Gas-Turbine Cycles

2. Today’s Lecture The Gas-Turbine Cycle The Ideal Brayton Cycle Performance Parameters Back-Work Ratio The Non-Ideal Bryton Cycle Solved Problems Example 8.1 – Ideal Brayton Cycle – Optimal Pressure Ratio (El Wakeel) Example 9.4 – Ideal Brayton Cycle (Moran and Shapiro) 2

3. I am Dr Muhammad Alam Zaib Khan You can find me at @ [email protected] Hello! 3

4. Brayton Power Cycles Power Generation – Gas-Turbine Power Systems 1 4

5. The Gas-Turbine Cycle serves as a critical technology for global electricity generation, both in standalone applications and as the key component of highly efficient combined cycle plants. It forms the foundation for approximately 20-25% of the world's electricity. This translates to an estimated 30,000 to 35,000 Gas-Turbine power units of significant size worldwide, a figure that includes both simple-cycle peaking plants and the gas turbine units within combined-cycle facilities. 5

6. Gas-Turbine Power Plant 6 Air Intake and Compression System (The "Air Supplier") Combustion System (The "Heat Source") Expansion and Power Generation System (The "Engine") Exhaust and Heat Recovery System (The "Waste Heat Manager") The main advantages : Small in size, mass, and initial cost per unit output. Quick to install, quick-starting, and relatively smooth running. Their main disadvantages : Low cycle efficiency, and Incompatibility with the solid fuel. Gas-Turbine Power Plant Aerial-View of Gas -Turbine Power Plant

7. Ideal Brayton Cycle 7 Process 1 – 2: Isentropic compression in the compressor to state 2, where f resh air at ambient is drawn raising temperate and pressure . Process 2 – 3: The high-pressure air proceeds into the combustion chamber, where fuel is burned (heat added) at constant pressure . Process 3 – 4: Isentropic expansion of the high-temperature gases in the turbine, where they expand to the atmospheric pressure while producing power . Process 4 – 1: Constant-pressure heat rejection from the exhaust gases as it thrown out back to ambient (open cycle) Schematic of (a) An Open-Cycle Gas-Turbine Cycle (b) A Closed-Cycle Gas-Turbine Cycle

8. Performance Parameters 8 All four processes of the Brayton cycle are executed in steady-flow devices; thus, they should be analyzed as steady-flow processes. When the changes in kinetic and potential energies are neglected, the energy balance for a steady-flow process can be expressed, on a unit–mass basis, Processes 1-2 and 3-4 are isentropic, and P 2 = P 3 and P 4 = P 1 . Thus, Substituting ,   Thermal efficiency: Using the enthalpies, heat and work quantities and expressions, the thermal efficiency of the power cycle is;   Therefore, heat transfers to and from the working fluid are   Pressure Ratio

9. Performance Parameters 9 Under the cold-air-standard assumptions, the thermal efficiency of an ideal Brayton cycle depends on the pressure ratio of the gas turbine and, the specific heat ratio of the working fluid. The thermal efficiency increases with both parameters. The highest temperature in the cycle occurs at the end of the combustion process (state 3), and it is limited by the maximum temperature that the turbine blades can withstand. This also limits the pressure ratios that can be used in the cycle. For a fixed turbine inlet temperature T 3 , the net work output per cycle increases with the pressure ratio, reaches a maximum, and then starts to decrease. There is a compromise between the pressure ratio (thus the thermal efficiency) and the net work output. Thermal efficiency of the ideal Brayton cycle as a function of r p Net work of the cycle first increases with the r p , then reaches a maximum, and finally decreases The pressure ratio of gas turbines ranges from about 11 to 16.

10. Back-Work Ratio 10 The air in gas turbines performs two important functions: It supplies the necessary oxidant for the combustion of the fuel, and it serves as a coolant to keep the temperature of various component within safe limits. So, air–fuel mass ratio of 50 or above is not uncommon. Therefore, in a cycle analysis, treating the combustion gases as air does not cause any appreciable error. Also, the mass flow rate through the turbine is greater than that through the compressor, the difference being equal to the mass flow rate of the fuel. Thus, assuming a constant mass flow rate throughout the cycle yields conservative results for open-loop gas-turbine engines. The fraction of the turbine work used to drive the compressor is called the back work In gas-turbine power plants, the ratio of the compressor work to the turbine work, called the back work ratio , is very high. Usually more than one-half. The situation is even worse when the isentropic efficiencies of the compressor and the turbine are low. A power plant with a high back work ratio requires a larger turbine to provide the additional power requirements of the compressor. Therefore, the turbines used in gas-turbine power plants are larger than those used in steam power plants of the same net power output.

11. The Non-Ideal Brayton Cycle 11 The actual gas-turbine cycle differs from the ideal Brayton cycle on several accounts. For one thing, some pressure drop during the heat-addition and heat-rejection processes is inevitable. More importantly, the actual work input to the compressor is more, and the actual work output from the turbine is less because of irreversibilities. The deviation of actual compressor and turbine behaviour from the idealized isentropic behaviour can be accurately accounted for by utilizing the isentropic efficiencies of the turbine and compressor   The deviation of an actual gas-turbine cycle from the ideal Brayton cycle

12. Solved Problems Power Generation – Gas-Turbine Power Systems 2 12

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14. 14 Schematic & T – s Diagram of an Ideal Bryton Cycle

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16. Any questions ? You can find me at [email protected] Thanks! 16