5. Gas Turbine Cycles - Engsoft Cycle Power Plants 5. Gas Turbine Cycles 3 /64 HIoPE Simple Cycle...

Combined Cycle Power Plants 5. Gas Turbine Cycles 1 /64

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5. Gas Turbine Cycles


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Regenerative Cycle 21 2

Combined Cycle 52 5

Closed Cycle 61 6

Simple Cycle 2 1

Intercooled Cycle 43 4

Reheat Cycle 36 3


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Simple Cycle

Cycle Arrangement

Compressor

Fuel Combustor

Turbine

Air

Power

Exhaust gas 1

2 4 3

p

2

1

T

(h)

s

qin

3

4 1

2

3

4

qout

win wout

win

wout

qin

qout


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Simple Cycle

The term simple cycle is used to distinguish this configuration from the complex cycles, which utilizes

additional components, such as heat exchanger for regeneration, intercooler, reheating system, or steam

boilers.

This cycle is suitable for a fixed speed and fixed load operation, such as power generation.

In order to analyze gas turbine system in a convenient form, the assumptions listed below are frequently

used.

1) The working fluid is treated as the air. The air is an ideal gas and has a constant specific heat. (In

practice, there is a change in the composition of the working fluid because of the combustion process)

2) The combustion process is replaced by a heat transfer process from an external source. In other words,

the mass flow rate remains constant throughout the system.

3) The inlet and exhaust processes are replaced by a constant pressure process that will in turn complete

the gas turbine cycle.

4) All processes are internally reversible.

The combination of these assumption is called the air-standard cycle approach.

General Notes


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Simple Cycle Analysis [1/13]

Process Component Heat Work Process

12 Compressor q12 = qC = 0 w12 = wC = (h2h1) Power in (adiabatic compression)

23 Combustor q23 = qB = h3h2 w23 = wB = 0 Heat addition at constant pressure

34 Turbine q34 = qT = 0 w34 = wT = h3h4 Power out (adiabatic expansion)

41 Exhaust q41 = qE = (h4h1) w41 = wE = 0 Heat release at constant pressure

121212 whhq

Cycle Analysis

p

2

1

T

(h)

s

qin

3

4 1

2

3

4

qout

win wout

win

wout

qin

qout


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0 5 10 15 20Pressure Ratio [r]

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Th

erm

alE

ffic

ien

cy

Thermal efficiency in a simple cycle gas

turbine increases with pressure ratio

and specific heat ratio.

The increasing rate of the thermal

efficiency is getting smaller as the

pressure ratio increases.

4

3

1

2

p

p

p

pr

crth

111

/1

1

rc

2

1

23

14

23

1423

23

4123

23

3412 11T

T

TT

TT

hh

hhhh

q

qq

q

ww

q

w

inputheat

ouputworknet

in

sys

th

Thermal Efficiency



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1

3

T

Tt

Specific work output:

124341342312 TTcTTcwwwww ppsys

1

111

11 /1

/1

1

c

ctr

rt

Tc

w

p

sys

The specific work output, which is the output per unit mass flow rate of working fluid, is a function of

pressure ratio and maximum cycle temperature.

The specific work output increases with the pressure ratio when the maximum cycle temperature is greater

than a certain value.

There is a pressure ratio having a maximum specific work out in a constant t-curve.

Specific Work Output

4

3

1

2

p

p

p

pr

1

rc



0.0

0.5

1.0

1.5

2.0

Sp

ecific

Wo

rkO

utp

ut[w

/CT

]sys

1p

t = 2

t = 3

t = 4

t = 5


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Optimum Pressure Ratio for a Given TIT [1/3]

There are different optimum pressure ratios in

terms of thermal efficiency and specific work

output for a given maximum cycle temperature.

The thermal efficiency increases with the

pressure ratio, and it has a maximum value

when the air temperature at the compressor

outlet is equal to TIT.

In this limiting case, the cycle net work tends

toward zero, and the thermal efficiency

approaches the Carnot efficiency. In terms of

available work output (= turbine work –

compressor work), it increases with pressure

ratio and reaches a maximum at a certain

pressure ratio, then it becomes smaller, and

finally reaches zero when the air temperature at

the compressor outlet is equal to TIT.

Therefore, it is clear that there are different optimum pressure ratios in terms of thermal efficiency and

specific work output for a given maximum cycle temperature.

This means that the maximum net specific work and the maximum thermal efficiency do not occur at the

same pressure ratio. Therefore, in designing gas turbines, the design pressure ratio must be a compromise

between the maximum thermal efficiency and the maximum specific work.


T

(h)

s

1

2

3

4

4

3

2

2

4

3

Tmax


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This can be understand more easily with the

investigation of the diagram shown relationship among

thermal efficiency, specific output, and pressure ratio.

In a real (non-ideal) gas turbine, the specific work

produced by the turbine increases more rapidly than

the specific work that the compressor consumes, as

the pressure ratio increases (from 5 to 8).

As a result, both shaft specific work output and

efficiency increase with pressure ratio.

As the pressure ratio increases, the compressor outlet

temperature also increases. This in turn reduces the

combustor temperature rise necessary to achieve a

given TIT. As a result, the combustor heat input

decreases and cycle efficiency increases.



Specific output, MW/lb/s

Th

erm

al e

ffic

ien

cy

Maximum thermal efficiency

Maximum shaft output

Pressure ratio = 5

10 15

20

TIT = 1,800F (982C)


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At the point corresponding to the maximum shaft specific work output (turbine specific work produced

minus compressor specific work consumed), the turbine specific work produced and the compressor

specific work consumed increase at the same rate.

As pressure ratio increases further (beyond the maximum shaft output), the compressor specific work

consumed increases as a greater rate than the turbine specific work produced and, as a result, the shaft

work output actually decreases.

However, because the combustor heat input continues to decrease, the efficiency continue to increase as

pressure increases (from 8 to 16). The reason for this is that the shaft specific work output decreases

because compressor specific work consumed is increasing faster than turbine specific work output, as the

pressure ratio increases. Therefore, shaft specific work output decreases.

However, fuel consumption is reduced and overall thermal efficiency increases because of higher

compressor outlet temperatures due to higher pressure ratios.

As the pressure ratio increases further (greater than 16), the increase in compressor specific work

consumed offsets the advantage of higher compressor outlet temperature and overall efficiency begins to

decrease.

The actual pressure ratio at which this occurs depend on the specific gas turbine considered.




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The Performance Map of a Simple-Cycle Gas Turbine



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h1 = enthalpy at compressor inlet

h2s = enthalpy at constant entropy and

compressor discharge pressure

h2 = actual enthalpy at compressor discharge

pressure

h3 = enthalpy at turbine inlet

h4s = enthalpy at constant entropy and turbine

exit pressure

h4 = actual enthalpy at turbine exit pressure

p

2

1

T

(h)

s

Qin

3

4 1

2s

3

4

Qout

Win

Wout

2

4s

12

12

hh

hh sC

s

Thh

hh

43

43

Practical Brayton Cycle



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General Expression of the Simple Cycle Net Work and Efficiency [1/4]

1

1

1

21

12T

TTcTTcw s

C

p

sp

C

C

crp

p

T

T s

1

1

1

2

1

2

where, r is a pressure ratio. Therefore, compressor

work is

11 c

Tcw

C

p

C

Similarly, the turbine work is given by

cTcTTcw pTspTT

11343


T

(h)

s

1

2s

3

4

4s

2


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CTnet www

11

1

c

tc

Tcw TC

C

p

net

1

2

1

3123

T

T

T

TTcTTcq ppin

12

12

TT

TT sC

1

11

1

2 cT

T

C

where, t is a maximum temperature ratio. Therefore, heat input is expressed by

111

23 ctTc

TTcq C

C

p

pin

11

1

ctc

ctc

q

w

C

TC

in

netcy

Four major parameters affecting the cycle efficiency of a simple gas turbine are pressure ratio (c), TIT

(t), compressor efficiency, and turbine efficiency.

1

3

T

Tt



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Pressure ratio

t = 3.25

t = 4.73

t = 5.69

Reversible

0.7

0.1

Therm

al effic

iency

2 4 6 8 0 10 12 14 16 18 20 22 24 26 28 30

0.6

0.5

0.4

0.3

0.2

0

1

3

T

Tt T = 0.90

C = 0.85




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In general, the cycle efficiency is relatively low, because of the high EGT, and because a significant portion

of the turbine output is used for compressor operation.

For a given turbine and compressor efficiency, the cycle performance is determined by the TIT and pressure

ratio.

The TIT is usually fixed by the metallurgical temperature limit of the first row of turbine blade.

As the TIT increases, the cycle efficiency is greatly improved.

The impact of the pressure ratio on the cycle efficiency is quite different.

There is an optimum pressure ratio that produces the maximum cycle efficiency.

The optimum pressure ratio increases with the TIT.




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Shaft power application

Simple Cycle Efficiency



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[Exercise 7.1]

Calculate the cycle efficiency and net work per pound of air. A gas turbine is operated under the

following conditions.

• Compressor inlet pressure and temperature 14.7 psia, 60F

• Pressure ratio 12

• Compressor efficiency 0.88

• Turbine inlet temperature 2000F

• Turbine efficiency 0.90

• Average constant pressure specific heat 0.25 Btu/lb-R

• Specific heat ratio 1.4

The pressure drops in the combustor, compressor inlet, and turbine outlet are assumed to be negligible.

Exercise for a Simple Cycle [1/3]

T

(h)

s

1

2s

3

4

4s

2


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[Solution]

Process 12s is an isentropic process. Thus,

T2s = T1 2.03394 = 1057.6 R

T2 = 1130.9 R

Similarly, process 34s is an isentropic process. Thus,

T4s = 1209.5 R

T4 = 1334.6 R

03394.212 4.1

4.01

1

2

1

2

p

p

T

T s

12

12

TT

TT sC

1

4

3

4

3

ss p

p

T

T

s

TTT

TT

43

43



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[Solution]

Heat input in the combustor is

Compressor work and turbine work are

Cycle efficiency is

cy = 0.387 or 38.7%

Cycle network is

128.6 Btu/lb

23 TTcq pin

12 TTcw pC

43 TTcw pT

23

141TT

TT

q

ww

in

CTcy

CTnet www

11

1

ctc

ctc

q

w

C

TC

in

netcy



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Regenerative Cycle 2

Combined Cycle 5

Simple Cycle 1

Intercooled Cycle 4

Reheat Cycle 3

Closed Cycle 6


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Regenerative Cycle

Cycle Arrangement

Compressor

Fuel

Combustor

Turbine

Air

Power

Exhaust gas

Heat exchanger

1

2

5

3 4

6

p

2

1

T

s

3

4 1

2

3

4 6

5

5

6

A B C D


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In order to increase the gas turbine efficiency, the gas turbine exhaust gas can be used to heat the air

leaving the compressor, thus reducing the amount of fuel required to reach the firing temperature.

This is achieved by the use of regenerators or recuperators, which heat the compressor exit air by the

exhaust gases from the turbine exit.

Regenerators or recuperators are usually used in small- to intermediate-sized gas turbines having output

less than 10 MW.

Regenerative cycle is also called as heat exchange cycle.

General Notes

Regenerative Cycle


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A Typical Regenerative Cycle Gas Turbine

Air flow path – Mercury 50 gas turbine (Solar)

Power = 4.6 MW

PR = 9.9:1

TIT = 2200F (1204C)

th = 38.5%

Regenerative Cycle


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25 Heat Ex. q52 = qin = h5h2 w52 = wHE = 0 Heat addition through heat exchanger

53 Combustor q53 = qB = h5h3 w53 = wB = 0 Heat addition in a burner


46 Heat Ex. q46 = qout = (h6h4) w46 = wHE = 0 Heat transfer to compressor discharged air

(h4h6 = h5h2)

61 Exhaust q61 = qE = (h6h1) w61 = wE = 0 Heat release to atmosphere

121212 whhq

Cycle Analysis

Regenerative Cycle Analysis [1/6]

p

2

1

T

s

3

4 1

2

3

4 6

5

5

6

A B C D


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압력비 vs 비출력


0.0

0.5

1.0

1.5

2.0S

pecific

Wo

rkO

utp

ut[w

/CT

]sys

1p

t = 2

t = 3

t = 4

t = 5

The regenerative cycle has

the exactly same specific

work out with a simple cycle.

11

11

c

ct

Tc

w

p

sys

1

3

T

Tt

1

rc

1

2

p

pr



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압력비 vs 열효율


0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Th

erm

alE

ffic

ien

cy

Simple cycle efficiency

t = 2

t = 3

t = 4

t = 5t = 6

t

cth 1

1

3

T

Tt

1

rc1

2

p

pr

4

1

3

2 11T

T

T

Tth

t

cth 1

사이클 최고온도가 일정한 상태에서

압력비가 작아질수록 재생사이클 가스터빈 열효율 향상.

압력비가 일정한 상태에서 최고온도가 증가할수록 열효율이 향상된다.

4

1

3

2 11T

T

T

Tth

재생사이클 가스터빈은 TIT가 증가할수록, 그리고 EGT가 증가할수록 열효율 향상.



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The cycle efficiency decreases as the pressure ratio increases, which is opposite to that of a simple cycle.

This is due to the fact that, as the pressure ratio increases the air temperature exiting the compressor

increases and ultimately will exceed that of the turbine exhaust gas temperature. Then heat in the heat

exchanger (regenerator) will be lost from the air to the exhaust gases instead of desired gain.

The efficiency with lower TITs, say at t=2, is seen to become negative soon after the pressure ratio 11.3 is

exceeded. The reason is that the temperature at compressor outlet actually exceeds the assumed

combustion temperature in this case.

In many cases, regeneration is not desirable. This is because the efficiency increases with pressure ratio, if

regenerative cycle is not employed.

Efficiency, with regenerative cycle rises very rapidly with increase in maximum temperature of the cycle.

Lower pressure ratios and high cycle temperatures are favorable for the regenerative cycle, since a large

heat recovery is then possible.

After the efficiency becomes equal to that of simple cycle, any further increase of pressure ratio will yield an

efficiency which is lower than that of simple cycle and that is of no interest.

Power output may be reduced by 10% for a given size of plant because of the pressure losses occurred in

the heat exchanger.

Normally, micro gas turbines having lower thermal efficiency adopts this kind of arrangement to improve

thermal efficiency.



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24

25

24

25

TT

TT

hh

hhreg

53

1243

TT

TTTT

q

ww

in

CTcy

2423

1243

TTTT

TTTT

reg

cy

1

2

1

3

3

4

1

2

1

3

1

2

1

3

3

4

1

3 1

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

T

reg

cy

11

1

2

C

c

T

T

cT

TT

111

3

4

It can be seen that the irreversibility of the system significantly lower the cycle efficiency.

Compared with the simple gas turbine system, the optimum pressure ratio is smaller for the regenerative

cycle.

The small pressure ratio means a small cycle net output.

Therefore, the cost associated with this output reduction must be weighted against the saving that can be

affected by the cycle efficiency improvement.

Cycle Efficiency of Practical Regenerative Cycles


Pressure ratio

t = 4.73

t = 5.69

Reversible 0.7

0.1

Therm

al e

ffic

iency

2 4 6 8 0 10 12 14 16 18 20 22 24 26 28 30

0.6

0.5

0.4

0.3

0.2

0

1

3

T

Tt

T = 0.90

C = 0.85

reg = 0.80

0.8

0.9

1.0

Practical t = 5.69

t = 4.73


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An Alternative Regenerative Cycle

This modified form is more suitable for fuels those combustion products contain constituents which may

corrode or erode the turbine blades.

It is much less efficient than the simple cycle power plant because of the efficiency of the heat exchanger.

Such a cycle may be considered only if low grade fuels are to be used.

Gas turbines of a more complicated design (i.e., with intermediate cooling in the compressor or

recuperator) are less suitable for combined cycle plants. They normally have a high simple cycle

efficiency combined with a low exhaust gas temperature, so that the efficiency of the water/steam cycle

is accordingly lower.

Compressor

Fuel Combustor

Turbine

Air

Power

Exhaust gas Heat exchanger

1

2 3 4


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1 초소형 발전 정의: Micro Gas Turbine (and/or Fuel Cell)을 이용한 전력생산

기술적 특성:

구(old) 기술 (시장출현 후 거의 1세기 경과 과거 가격 경쟁력 열위)

초소형 발전시스템은 현재 기존설비(디젤발전기)에 대한 가격경쟁력 확보

2 초소형 발전 확산 배경

전력시장 자유화 (독점체제 경쟁체제)

환경규제 강화

신기술 개발 공기베어링(air bearing), 변환기(inverter), 재생사이클 채택

전력 안정성 확보

3 초소형 발전 확산 장애물

세금

표준화

규제정책

4 초소형 발전 잠재력

기술개발을 통한 경제성 확보

개발도상국 (고압 송전시설 불필요)

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중소업체 상용제품 개발 완료

GE, ABB 같은 다국적 전력업체 경쟁적 개발 참여

신규 자본투자 급상승 향후 10년 이내 600억불 시장 예측

지역분산형(초소형)발전 전기 혁명


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ITEM MGT (Micro Gas Turbine) 디젤발전기

연료 천연가스, 프로판가스, 경유, 등유, 메탄올 등 등유

크기 소형 냉장고 2대 정도 variable

무게 1 5

축 회전속도 90,000 ~ 116,000 1,000

열효율 25 ~ 30 % 35 %

NOx 9 ppm

Noise level 65 dB

설비 가격 $400~550/kW (석탄화력: $1,000/kW)

Cycle Regenerative cycle Diesel

발전기 • 회전계자방식 영구자석 2,4극 • AC 1,500~4,000 Hz 발생시켜 직류로 전환 • 변환기를 이용 400~480 V, 50/60 Hz, 3상 변환

운전비용 1 3

MGT와 디젤발전 비교


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발전분야: 전력시장 자유화, 환경운동, 기술진보에 의해 통신분야와 같은 변화 시작 분산형 발전 확대

국가별 현황

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개발도상국: 민영화 적극 추진 (인도, 아르헨티나 등)

2 환경규제 강화

미국: 석탄화력 규제치 만족 불가 (2000년 현재 절반 이상의 전력 석탄화력 이용 생산)

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3 신기술 개발

공기베어링 (미국 Capstone 社)

변환기(Inverter) 감속기어 배제

재생사이클 열효율 향상

4 경쟁력 확보

막대한 송전손실 배제

폐열 이용 가능 Co-generation System

5 안정적 전력 공급 희망

선진국을 중심으로 정전문제 중요 사안으로 부각

지역적으로 안정성 문제 부각 California(USA), Kobe(Japan)

미개발국 대규모 자본투자가 요구되는 중앙집중화 발전방식 배제

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피해장소; 신주 과학단지(대만 실리콘밸리), 피해업체 수; 6

The efficiency of a small gas turbine is usually much lower

than a large unit because of the limitation of the TIT and the

lower component efficiencies.

지역분산형 발전 확산 배경


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AIR BEARING

Air is used as a lubricant

• Lube oil system is not required

• System becomes much simpler

• No maintenance, such as oil replacement and oil supplement, is required

INVERTER

High frequency alternating current is produced at a generator

Then, it converted into direct current

And then, it converted into alternating current of 50 or 60 Hz by the inverter

• Reduction gear used for high rpm machines is not required

• Structure of the machine becomes simpler

REGENERATIVE CYCLE OF GAS TURBINE ENGINE

The thermal efficiency is increased greatly by the addition of a device for transferring energy from the hot

turbine exhaust gas to the air leaving the compressor

The typical thermal efficiency of previous microturbines, rating of lower than 100 kW, was 15%. But, it is

improved up to 25-30% by the adoption of regenerative cycle

New Technologies for MGT


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1 세금

다수의 대형 석탄발전소에 대한 탄소세 면제 및 기금 지원

미국: 기존발전소 환경규제로부터 면제해 주는 포기각서를 통해 간접지원

EU: 지원기금 납세자들로부터 직접 염출

초소형 발전 경쟁력확보를 위하여 왜곡된 세금제도 개선 필요

2 표준화 (기술기준)

초소형 발전기 소유자가 전력을 사용하지 않을 때 남는 전기 송전선에 보내기 위한 지능형 전자통제장비 필요

현재 이에 대한 국가기준을 마련한 나라 매우 적음

초소형 발전기 제조업체와 소유자는 전력을 사고 팔 수 있는 권한을 획득하기 위하여 무수한 문제 직면

기존 전력업체의 새로운 경쟁자에 대한 훼방

사이비 안전문제 제기

기나긴 허가절차 부과

부당한 요금 요구

각국 정부 적절한 기준 제정 필요

3 규제정책

각종 규제 난립

주정부간 전력매매에 대한 조정작업 미실시

단일 규제기관 부재 - 시장 신규진입 방해

첨두부하용 비축용량 보유 요구사항

규제철폐 및 제도개선 초소형 발전 기존시장 진입 적극 권장

지역분산형 발전 장애물


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Combined Cycle 5

Simple Cycle 1

Intercooled Cycle 4

Reheat Cycle 3

Closed Cycle 6


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Reheat Cycle

Fuel Reheat combustor

LP turbine

Power

Exhaust gas 5

4 6

Compressor

Fuel

Combustor

Air

1

2

3

HP turbine

Cycle Arrangement

p

2

1

T

s

3

6 1

2

5

6

4 5 4

3

4

4


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A Typical Reheat Cycle Gas Turbine for Power Generation - GT26 Gas Turbine

Reheat Cycle


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F-14

Reheat Cycle Gas Turbine for Military Aviation

F100-PW-229 (F-15, F-16)

Reheat Cycle

Reheat Cycle Gas Turbine for Civil Aviation

Olympus 593


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Cycle Analysis

Reheat Cycle Analysis [1/4]




34 Turbine q34 = qHPT = 0 w34 = wHPT = h3h4 Power out (adiabatic expansion)

45 Reheat q45 = qin,R = h5h4 w45 = wR = 0 Heat addition in a reheat combustor


121212 whhq

p

2

1

T

s

3

6 1

2

5

6

4 5 4

3

4

4


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11

121

max

c

ct

TC

w

p 1

3

T

Tt

1

rc1

2

p

pr


0.0

0.5

1.0

1.5

2.0

2.5

Sp

ecific

Wo

rkO

utp

ut[w

/CT

]

t = 2

t = 3

t = 4

t = 5

Reheat cycle

Simple cycle

sys

p1



0.0

0.1

0.2

0.3

0.4

0.5

0.6

Th

erm

alE

ffic

ien

cy

Simple cycleefficiency

t = 2

t = 3

t = 4

t = 5

ctct

cctth

/2

1/112


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An increase of specific work output can be obtained by splitting the expansion and reheating the gas

between the high pressure and low pressure turbines.

The increase of specific work output can be seen in p- diagram.

The turbine work increase is obvious from the fact that the vertical distance between any pair of constant

pressure lines increases as the entropy increases. Thus,

(T3T4) + (T5 T6) (T3 T4’)

The shaft length becomes longer and the control of shaft vibration becomes difficult.

The maximum temperature in low pressure turbine is the same as in high pressure turbine.

Thermal efficiency of the reheat cycle is lower than that of the simple cycle. This is because the reheat

cycle is made by the combination of a simple cycle and a less efficient cycle which is operated over a lower

temperature range.



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Combined Cycle 5

Simple Cycle 1

Intercooled Cycle 4

Reheat Cycle 3

Closed Cycle 6


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Intercooled Cycle

HP compressor

Fuel

Combustor

Power turbine

Air

Power

Intercooler

Coolant in Coolant out

HPT

LP compressor

LPT

1 2 3 4 5

6

Cycle Arrangement

p

2

1

T

s

3

6 1

2

5

6

4 5

4

3

2

2


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Typical Intercooled Cycle Gas Turbines

Air-to-Water Heat Exchanger Air-to-Air Heat Exchanger

Intercooler

LMS 100 (GE)


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The specific work output of a gas turbine may be improved substantially by reducing the work of

compression.

If, therefore, the compression process is carried out with intercooling, the work of compression will be

reduced, as can be seen in p- diagram.

The compressor work decrease is obvious from the fact that the vertical distance between any pair of

constant pressure lines decreases as the entropy decreases. Thus,

(T2T1) + (T4 T3) < (T2′ T1)

Heat is extracted by an intercooler between the first and second compressors.

Rejecting heat worsens SFC, since more fuel should be burnt to raise cooler compressor delivery air to any

given TIT.

Therefore, the thermal efficiency of the intercooled cycle will be less than that for a simple cycle.

Intercooling is useful when the pressure ratios are high and the efficiency of the compressor is low.

Intercooled Cycle


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Intercooled Cycle Analysis [1/5]

Cycle Analysis


12 LP compressor q12 = qC = 0 w12 = wLPC = (h2h1) Power supply in a LP compressor

23 Intercooler q23 = qIC = (h3h2) w23 = wIC = 0 Heat rejection in an intercooler

34 HP compressor q34 = qC = 0 w34 = wHPC = (h4h3) Power supply in a HP compressor




121212 whhq

p

2

1

T

s

3

6 1

2

5

6

4 5

4

3

2

2


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압력비 vs 비출력

Simple Cycle Intercooled Cycle


0.0

0.5

1.0

1.5

2.0

Sp

ecific

Wo

rkO

utp

ut[w

/CT

]

t = 2

t = 3

t = 4

t = 5

max

p1


0.0

0.5

1.0

1.5

2.0

Sp

ecific

Wo

rkO

utp

ut[w

/CT

]sys

1p

t = 2

t = 3

t = 4

t = 5

121

max cc

tt

Tc

w

p 1

3

T

Tt

1

rc



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압력비 vs 효율


0.0

0.1

0.2

0.3

0.4

0.5

0.6

Th

erm

alE

ffic

ien

cy

Simple cycleefficiency

t = 2

t = 3

t = 4

t = 5


0.0

0.1

0.2

0.3

0.4

0.5

0.6

Th

erm

alE

ffic

ien

cy

t = 2

t = 3

t = 4

t = 5

Intercooled Cycle Reheat Cycle

1

3

T

Tt

1

rcct

cctth

2/1



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A Typical Spray Intercooling Gas Turbine

LM6000-SPRINT Gas Turbine

LM6000 (GE)



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Spray Intercooling

Intercooling can also be accomplished by fog spraying atomized water between the HP and LP

compressors.

GE LM6000-SPRINT is one example of such a system.

Water in injected through 24 spray nozzles.

Water is atomized to a droplet diameter of less than 20 microns using high-pressure air taken from the

eighth-stage of HP compressor.

Injecting water significantly reduces the compressor outlet temperature.

The result is higher output and better efficiency.

Output increases of more than 20% and efficiency increases of 3.9% are possible on 90F(32C) day.



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Combined Cycle 5

Simple Cycle 1

Intercooled Cycle 4

Reheat Cycle 3

Closed Cycle 6


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[ A simple schematic diagram of a combined cycle system ]

Compressor

Fuel Combustor

Turbine

Air

Steam turbine

Exhaust gas 1

2

4

3

G G

HR

SG

5

6

7

8

9 Condenser

Pump

Combined Cycle

Cycle Arrangement


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Combined cycle power plants have a higher thermal

efficiency because of the application of two complementary

thermodynamic cycles

Combined Cycle

T

s

Topping cycle

(Brayton cycle)

Bottoming cycle

(Rankine cycle)


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Simple cycle

Combined cycle

Thermodynamics of a Combined Cycle [1/2]


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In simple cycle application, thermal efficiency increases with the pressure ratio at a given TIT.

For a given pressure ratio, thermal efficiency decrease as the TIT increases. This is because the lager

amount of cooling air for turbine blade is required as the TIT increases.

The pressure ratio resulting in maximum output and maximum efficiency change with TIT.

The higher the pressure ratio, the greater benefits for a given TIT.

The power increases with the TIT at a given pressure ratio. However, efficiency decrease because the

flow of the cooling air extracted increase with the TIT.

Simple cycle

Combined cycle

In combined cycle applications, pressure ratio have a less pronounced effect on efficiency.

As pressure ratio increases, specific power decreases.

Thermal efficiency increases with firing temperature.

The optimum cycle parameters for combined cycle are different from simple cycle.

Simple cycle efficiency is achieved with high pressure ratios. However, combined cycle efficiency is

obtained with more modest pressure ratios and higher TITs.

Thermodynamics of a Combined Cycle [2/2]


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Efficiency of Combined Cycle [1/4]


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SFGT

STGTCC

qq

ww

CC: gross efficiency of the combined cycle

wGT: output of gas turbine

wST: output of steam turbine

qGT: heat input in the gas turbine

qSF: heat input through supplementary firing in the HRSG

SFGT

AuxSTGTnetCC

qq

www

,

CC,net: net efficiency of the combined cycle

wAux: auxiliary consumption (=station service power consumption +

electrical losses)



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The efficiency of the simple cycle gas turbine,

GT

GTGT

q

w

The efficiency of the simple cycle steam turbine,

SFExhGT

STST

qq

w

,

GTGTExhGT qq 1,

SFGTGT

STST

qq

w

1

The efficiency of the combined cycle without supplementary firing,

GT

GTGTSTGTGTCC

q

qq

1

GTSTGTCC 1


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[Exercise 7.2]

Calculate the combined cycle efficiency. The efficiencies are 26 and 33% for a gas turbine cycle and a steam

turbine cycle, respectively.

[Solution]

The efficiency of the combined cycle is

[Note]

This calculation is based on the assumption of no pressure drops at the various cycle locations. Therefore,

this cycle efficiency represents an optimistic estimate. When the pressure drops are taken into account, the

efficiency of the combined cycle is greatly reduced. When detailed design of a combined cycle plant is made,

it usually shows the plant efficiency in the range of 38 to 42%.

GTSTGTCC 1

504.0CC



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Combined Cycle 5

Closed Cycle 6

Simple Cycle 1

Intercooled Cycle 4

Reheat Cycle 3


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Closed Cycle


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In a closed cycle, the working fluid is continuously recirculated. It may be air or another gas such as helium.

Usually, the gas turbine is of intercooled recuperated configuration.

However, the combustor is replaced by a heat exchanger as fuel can not be burnt directly.

The heat source for the cycle may be a separate combustor burning normally unsuitable fuels, such as coal,

a nuclear energy, etc.

On leaving the recuperator, the working fluid must pass through a pre-cooler where heat is rejected to an

external medium, such as sea water, to return it to the fixed inlet temperature, usually between 15C and

30C.

The pressure at inlet to the gas turbine is maintained against leakage from the system by an auxiliary

compressor supplying a large storage tank called an accumulator.

The high density of the working fluid at engine inlet enables very high power output, which is the main

benefit of the closed cycle.

Closed Cycle


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질의 및 응답

작성자: 이 병 은 (공학박사) 작성일: 2015.02.11 (Ver.5) 연락처: [email protected]

Mobile: 010-3122-2262 저서: 실무 발전설비 열역학/증기터빈 열유체기술

5. Gas Turbine Cycles - Engsoft Cycle Power Plants 5. Gas Turbine Cycles 3 /64 HIoPE Simple Cycle...

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