Power plant : Gas turbine Cycle

By Akhil Gupta|Updated : February 10th, 2022

GAS TURBINE CYCLE:

A gas turbine cycle works on Brayton cycle. Sometimes, these words are used interchangeably. A gas turbine cycle could be of the open gas turbine cycle and closed gas turbine cycle.                                     

 

OPEN GAS TURBINE CYCLE

Fresh air enters the compressor at ambient temperature where its pressure and temperature are increased.

The high-pressure air enters the combustion chamber where the fuel is burned at constant pressure.

The high temperature and high-pressure gas enters the turbine where it expands to ambient pressure and produces work.

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 Schematic for an open gas-turbine cycle.

Features:

  • Gas-turbine is used in aircraft propulsion and electric power
  • High thermal efficiencies up to 44%.
  • Suitable for combined cycles (with steam power plant)
  • High power to weight ratio, high reliability, long
  • High back work ratio (ratio of compressor work to the turbine work) up to 50%, compared to few percent in steam power


BRAYTON CYCLE

The ideal Brayton cycle is made up of four internally reversible processes:

 

1-2

isentropic compression (in compressor)

2-3

constant pressure heat-addition (in combustion chamber)

3-4

isentropic expansion (in turbine)

4-1

constant pressure heat rejection (exhaust)

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Fig.10: Brayton cycle on P-V and T-S diagram.

Heat added = h3 – h2

Heat rejected = h4 – h1

Turbine work (WT) = h3 – h4

Compressor work (WC) = h2 – h1

Net-work = WT – WC = (h3 – h4) – (h2 – h1)

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                                      Variation of Net work with pressure ratio

ACTUAL BRAYTON CYCLE

Irreversibilities exist in actual cycle. Most important differences are deviations of actual compressor and turbine from idealized isentropic compression/expansion, and pressure drop in combustion chamber.

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BRAYTON CYCLE WITH REHEAT

(i) In this cycle, high pressure and high temperature air (state 3) expands in the high- pressure turbine (turbine 1).

(ii)The air coming out of the high-pressure turbine (at state 4) is then reheated at constant pressure in a reheater to increase its temperature (state 5)

(iii) Then this reheated air is allowed to pass through the low-pressure turbine (turbine 2).

(iv)The thermal efficiency with reheating always decreases. This is because by reheating the mean temperature of heat rejection increases.

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BRAYTON CYCLE WITH INTERCOOLING

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THE BRAYTON CYCLE WITH REGENERATION

The high-pressure air leaving the compressor can be heated by transferring heat from exhaust gases in a counter-flow heat exchanger which is called a regenerator. Hence the amount of heat to be supplied by the combustion chamber reduces and hence efficiency of the cycle increases.

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