DUAL CYCLE PROCESS Process 1 2 Isentropic compression (Reversible Adiabatic) Process 2 2.5 Constant volume heat addition Process 2.5 3 Constant pressure heat addition Process 3 4 Isentropic expansion (Reversible Adiabatic) Process 4 1 Constant volume heat rejection (Cooling) 2.5
Qin
3
3
2
Qin
4
2.5
4
2
1 1
Qout
BRAYTON CYCLE
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Open cycle gas turbine power plant
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Closed cycle gas turbine power plant
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Advantages of gas turbine power plant
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Storage of fuel requires less area and handling is easy. The cost of maintenance is less. It is simple in construction. There is no need for boiler, condenser and other accessories as in the case of steam power plants. Cheaper fuel such as kerosene , paraffin, benzene and powdered coal can be used which are cheaper than petrol and diesel. Gas turbine plants can be used in water scarcity areas. Less pollution and less water is required.
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Disadvantages of gas turbine power plant 66% of the power developed is used to drive the compressor. Therefore the gas turbine unit has a low thermal efficiency. The running speed of gas turbine is in the range of (40,000 to 100,000 rpm) and the operating temperature is as high as 1100 – 12600C. For this reason special metals and alloys have to be used for the various parts of the turbine. High frequency noise from the compressor is objectionable. 10 September 2016
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COMBINED GAS–VAPOR POWER CYCLES • The continued quest for higher thermal efficiencies has resulted in rather innovative modifications to conventional power plants. • A popular modification involves a gas power cycle topping a vapor power cycle, which is called the combined gas–vapor cycle, or just the combined cycle. • The combined cycle of greatest interest is the gas-turbine (Brayton) cycle topping a steam-turbine (Rankine) cycle, which has a higher thermal efficiency than either of the cycles executed individually. • It makes engineering sense to take advantage of the very desirable characteristics of the gas-turbine cycle at high temperatures and to use the high-temperature exhaust gases as the energy source for the bottoming cycle such as a steam power cycle. The result is a combined gas–steam cycle. • Recent developments in gas-turbine technology have made the combined gas–steam cycle economically very attractive.
• The combined cycle increases the efficiency without increasing the initial cost greatly. Consequently, many new power plants operate on combined cycles, and many more existing steam- or gas-turbine plants are being converted to combined-cycle power plants. • Thermal efficiencies over 50% are reported. 12
Combined gas–steam power plant.
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Advantages of Combined Cycle Power Plant • The efficiency of the combined cycle plant is better or higher than the turbine cycle or steam cycle plant. The efficiency of combined cycle power plant will be of the order of about 45 to 50%. • fewer moving parts and less vibration than a reciprocating engine • very low toxic emissions • runs on a wide variety of fuels • high operating speeds
Disadvantages of Combined Cycle Power Plant • • • •
Higher cost longer start-up less responsive to power demands shrill whining noise.
