advanced thermodyamic cycles for power plants.pps

7/27/2019 Advanced thermodyamic cycles for Power Plants.pps

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All conventional and most non-conventional power plants alwayshave a generator which is drivenby a prime mover.

The prime mover may be adevice which traps any form ofenergy found in nature and

converts into work or it may be aheat engine.

There can again be several kindsof heat engines in a power plant

viz. steam turbine or gas turbine.However, here we consider themost common type: a steamturbine power plant.


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Rankine cycle Coal fired thermal power plants

Oil fired boiler power plants Nuclear power plants

Brayton cycle Gas turbine power plant

Stirling cycle Solar power generation units.

(where the temperature difference is

low)


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540

Turbine

Condensor


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540

I.PTurbine

Condensor

L.P

Turbine

H.P

Turb

ine


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Reduces maximum temperatureto safe limits.

Decreases heat consumption ofthe boiler.

The same mount of heating at highertemperature would be less efficient,

because the higher heat dissipation.Yet reheating is a byproduct ofheating.

Moves closer to the Carnot cyclethus increases efficiency.


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540

I.PTurbine

Condensor

L.P

Turbine

H.P

Turb

ine


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ARE SO MANY REHEATS USEFUL?

Putting many turbines with less

work output (through low

pressure drops) will not beuseful.

It may give rise to more entropy

generation only

It is desired that we go to

regeneration after we reach

saturated pressure.

That will help in regeneration


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0

100

200

300

400

500

600

0 2 4 6 8 10s

T

Region boundary

Main steam/water cycle

540

LP REGENERATION

CONDENSATION

HP

TURBINE

IP

TURBINE

LP

TURB

INE

BOILER DRUM

CEP

BFP


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Main advantages ofregeneration

Less heat required by boiler.

Less size of turbine (since some steam isbled).


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Where and when can we useregeneration?

It is better to use regeneration withsaturated steam as hot fluid ratherthan superheated steam.

So we use regeneration as soon as the steambecomes saturated.

Sat

urationpoint

Distance

Temperature

Distance

Temperature

Regeneration withsuperheated steam

Regeneration withsaturated steam


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0

100

200

300

400

500

600

0 2 4 6 8 10s

T

Region boundary

Main steam/water cycle

Regeneration

540

HP REGENERATION

LP REGENERATION

CONDENSATION

HP

TURBINE

IP

TURBINE

LP

TURB

INE

BOILER DRUM

CEP

BFP


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Supercritical

Rankine CycleFrom the principle of the Carnot cycle we see by raising the

boiler temperature, the efficiency can be increased. In the

Rankine cycle, which only is a modification of the Carnot

cycle, higher boiler temperature means higher boiler

pressure. In the T-S diagram of water density difference of

water and steam decreases at higher pressure and finally

becomes zero at critical pressure. Most older boilers

required this density difference to produce flow in the boiler.

But modern forced convection boilers can handle water at

supercritical pressure.

Turbine

Condensor


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Cogeneration PlantsThe temperature which is usually the boilertemperature (heat addition temperature) of a Rankine

cycle using steam is generally same or below theexhaust (heat rejection temperature) of an ordinarygas turbine. So, the flue gas from the gas turbine isused to heat the water (in the boiler) of the Steamturbine.

Thermodynamic cycles for cogent plants naturallyconsist of two separate cycles having different workingfluids. Only the exhaust temperature of the gas turbineis a little higher than the inlet temperature of thesteam turbine and the inlet temperature of the gas

should be a little higher the exit temperature of theBFP of the steam turbine.


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0

100

200

300

400

500

600

0 2 4 6 8 10

T

sBoiler Feed Pump

Turbine

Condensor

Schematic block diagram of combined cycle power plant

Thecombinedcycle


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Acknowledgements


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advanced thermodyamic cycles for power plants.pps

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