Study & Analysis of Vapor and Gas Power Cycles

*The break up of other renewable energy sources (RES) is: wind power (34,293.48 MW), Solar power(23,022.83 MW), Biomass power & gasification (8,700.80 MW), Small hydro (4,493.20 MW), Waste-to-energy(138.30 MW).

#Facts• Thermal power is the "largest" source of power in India. There are

different types of thermal power plants based on the fuel used togenerate the steam such as coal, gas, and Diesel, natural gas. About71% of electricity consumed in India is generated by thermal powerplants.

• More than 62% of India's electricity demand is met through thecountry's vast coal reserves.

Classification of Thermodynamics Cycles

Heat Energy Mechanical EnergyPower Cycle (+)

Power Cycle (-)

Working Fluid

Gas Cycle: no phase-change of working fluid during cycle

Vapor Cycle: phase-change of working fluid during cycle

Actual Computer window of 210 MW Coal base Thermal Power Plant

Carnot Vapor Cycle

Analysis of Carnot Cycle

Several Impracticalities are associated with this cycle:

(Limitation of Carnot Cycle)

1. It is impractical to design a compressor that will handle two phases for isentropic compression

process(4-1).

2. The quality of steam decrease during isentropic expansion process(2-3) which do harm to

turbine blades.

3. The specific volume of steam is much higher than that of water which needs big equipments

and large amount of work input.

Rankine Vapor Cycle

4-6 Constant pressure heat addition in a boiler6-1 to Superheat Vapor1-2 Isentropic expansion in a turbine2-3 Constant pressure heat rejection in a condenser3-4 Isentropic compression in a pump

S

4

6

1

2

3

Simple Steam Power Plant

S

4

6

1

2

3

T

s

1

6

5

4

3 2

p

v

1654

32

p1

p2

Analysis of Rankine Vapor Cycle

4-5-6-1 Constant pressure heat addition in a boiler

1 1 4q h h

1-2 Isentropic expansion in a turbine

tT 1 2w h h

2-3 Constant pressure heat rejection in a condenser

2 2 3q h h

3-4 Isentropic compression in a pump

tP 4 3w h h

Efficiency

Because of uncompressibility of water

( )tP 4 3 tTw v p p w

4 3h h

o tT tP 1 2 1 2 s

o 1 2t

1 1 3

w w w q q h h w

w h h

q h h

0, 0k pE E

1 21 2

1 2

2t

1

Q QT T

S S

T1

T

Steam Rate (Steam Consumption)

s

o 1 2

3600 3600m

w h h

ms— the steam required to generate work of 1 kWh (kg/kWh)

, equipment size , investmenttd

o 1 2t

1 1 3

w h h

q h h

Entralpy of steam, turbine inlet

Entralpy of exhaust air , turbine outlet

Entralpy of condensed water

1

2

3

h

h

h

,1 1p t

2p

Influencing factors

1. － Pressure of Steam, Turbine Inlet1p

3

4

5

5’

1’ 1

22’

,1 2t p －Unchange

1p '1p

Two Cycles:

① 3-4-5-1-2-3

② 3-4-5’-1’-2’-3

Method of improving the efficiency of Rankine Vapor Cycle

3

4

5

5’

1’ 1

22’

1 tp

Disadvantages:

1p 1. x

decrease the turbine efficiency anderodes the turbine blades.

2. 1p Increase of requirements on pressurevessels and equipment investment.

2. － Temperature of Steam, Turbine Inlet1t

3

4

5

1

1’

2’2

6

,1 2p p －Unchange

1t '1t

Two Cycles:

① 3-4-5-6-1-2-3

② 3-4-5-6-1’-2’-3

3

4

51

1’

2’2

6

Advantages:

' 1 1 tT T i

ii it decreases the moisture contentof the steam at the turbine exit.

Disadvantages:

Superheating temperature is limitedby metallurgical considerations.

6001t ℃

3. － Condenser Pressure, Turbine Exit2p

,1 1t p －Unchange

2p '2p

Two Cycles:

① 1-2-3-4-5-6-1

② 1-2’-3’-4’-5-6-13

4

5

1

3’ 2’

2

6

4’

3

4

5

1

3’ 2’

2

6

4’

' 2 2 tT T i

iiDisadvantages:

i Condense pressure is limited

by the sink temperature.

ii It increases the moisture

content which is highly

undesirable.

Example

Consider a steam power plant operating on the ideal Rankine

cycle. The steam enters the turbine at 2.5MPa and 350℃ and

is condensed in the condenser at pressure of 70kPa. Determine

(a)The thermal efficiency of this power plant

(b)The thermal efficiency if steam is condensed at 10kPa

(c)The thermal efficiency if steam is superheated to 600 ℃

(d)The thermal efficiency if the boiler pressure is raised to 15MPa while

the turbine inlet temperature is maintain at 600 ℃

State 1:

State 2:

1 1

1

1

2.5MPa, 350

3128.2 kJ/kg

6.8442 kJ/kg K

℃p t

h

s

Ideal Rankine Cycle

2 2 1

2 2

2 2

70kPa,

' 1.1921kJ/(kg K), '' 7.4804kJ/(kg K)

' 376.77kJ/kg, " 2660.1kJ/kg

'

'' '

6.8442 1.1921 0.8988

7.4804 1.1921

x

p s s

s s

h h

s sx

s s

h2 ' ''

376.77 0.8988 2660.1 2767.7kJ/kg

h xh

State 4:

1 1 4

2 2 3

3128.2 381.83 2746.37

2767.7 376.77 2390.93

q h h

q h h

2

1

1 12.9% t

q

q

4 4 3

3 4 3

4 3

2.5MPa,

( ) 2.53kJ/kg

376.77 2.53

=381.83kJ/kg

tp

tp

p s s

w v p p

h h w

3

3

3

3

70kPa, Saturate Liquid

376.77kJ/kg

0.00104m /kg

p

h

v

State 3:

Irreversibility

• Flow friction

• Heat transfer under temperature

difference

• Heat loss to the surroundings

Actual Rankine Vapor Cycle

Actual cycle

2’2

1

56

1 2' 'tT

w h h

Turbine Efficiency

1 2

1 2

' '0.92tT

i

tT

w h h

w h h

Ideal Cycle

1 20

3600

h hN

Actual Cycle

1 20

'

3600

i i

h hN N

Actual Rankine Vapor Cycle

Analysis of Actual Rankine Vapor Cycle

Ideal Regenerative Cycle

Boiler Turbine

Regenerator

Condenser

Mixing Chamber

Pump II Pump I

1

27

34

56

ExtractingRegeneration

1

Analysis Ideal Regenerative Cycle

3(4) 2

7

1

6

5

1kg

akg

(1-a)kg

T

s

( ) ( )( )

( ) ( )( )

7 5 5 4

5 4

7 5

0 1 7 7 2 tp

1 1 5

0t

1

a h h 1 a h h

h ha

h h

w h h 1 a h h w

q h h

w

q

( ) ( )

2 3t Rankine

1 3 1 7

h h1

ah h h h

1 a

>0

Ideal Regenerative Cycle

Boiler Turbine

Regenerator

Cond-enser

Mixing Chamber

Pump II Pump I

1

27

34

56

8

93 2

7

1

6

5

T

s

4

89

1

1

Ideal Reheat Cycle

3 c 2

a1

5

4

6 b

Analysis Ideal Reheat Cycle

bp intermediate pressure

( ) ( )

( ) ( )

1 b a 2t

1 3 a b

h h h h

h h h h

Regenerative-Reheat Cycle