gas-vapor power cycles

Gas Turbine Power Plants

Chapter 9.5

Timeline

Week Book Sections Covered1 9.5 – 9.72 9.10, 9.93 9.8 , 9.94 9.1 – 9.4, 9.6.25 9.11 – 9.14

Homework Discussion

• Homework 2 hint: read Section 8.2.3

Simple Gas Turbine

Open System Closed System

Compressors

Turbine Engine

Air-Standard Brayton Cycle

Air-Standard Brayton CycleHeat input

Qin/m + (h2 – h3) = 0

Heat output Qout/m + (h4 – h1) = 0

Note: The book uses different signs for these equation.

Air-Standard Brayton CycleTurbine work

(h3 – h4) - Wt/m = 0

Compressor work(h1 – h2) - Wc/m = 0

Note: The book uses different signs for these equation.

Air-Standard Brayton Cycle

Compressor Pressure Ratio P2/P1

Thermal efficiency η = (Wt/m + Wc/m)/(Qin/m) η = [(h3-h4) + (h1-h2)]/(h2 – h3)

Back Work Ratio (bwr) bwr = (Wc/m)/(Wt/m) (absolute value) bwr = (h1 – h2)/(h3 – h4) (absolute value)

Note: The book uses different signs for these equation.

Air-Standard Brayton CycleTurbine work

Wt/m = h3 – h4

Compressor workWc/m = h1 – h2

Heat input-Qin/m = h2 – h3

Heat output-Qout/m = h4 – h1

Thermal efficiency η = (Wt/m + Wc/m)/(Qin/m) η = [(h3-h4) + (h1-h2)]/(h2 – h3)

Back Work Ratio (bwr) bwr = (Wc/m)/(Wt/m) (absolute) bwr = (h1 – h2)/(h3 – h4) (absolute)

Note: The book uses different signs for these equation.

Irreversibilities and lossesFriction and heat loss cause process inefficiencies in• Compressors (heat loss)• Turbines (heat loss)• Heat exchanger pipes

(pressure drop)

Irreversibilities and lossesPressure drop in heat exchanger pipes <<< heat loss in compressors and turbines<<<Inefficiencies during combustion

Irreversibilities and lossesηt = (Wt/m)/(Wt/m)s

= (h3 – h4)/(h3 – h4s)

ηc = (Wt/m)/(Wt/m)s

= (h3 – h4)/(h3 – h4s)

EXAMPLE 9-6 WITH EES

Regenerative Gas Turbines

Regenerative Gas Turbine CycleThe regenerator preheats the turbine inlet stream using heat from the exhaust gas

This reduces Qin (everything else remains the same)

Note Tx can be higher than Ty

Regenerative Gas Turbine

Regenerator efficiencyηreg = (hx – h2)/(h4 – h2)

Gas Turbine Combined CycleUse rejected heat from the Top cycle as heat input to a Bottom cycle

Improves efficiency

η = (Wgas + Wvap)/(Qin)

Regenerator Energy Balancemv *(h6-h7) + mg*(h4-h5) = 0

Gas Turbines with Reheat

Turbines require temperature control to prevent material deterioration

One strategy is to provide cooling with excess air

Reheat takes advantage of the excess air to burn more fuel

What are some disadvantages?

Gas Turbines with Reheat

Compression with Intercooling

It is easier to compress a cooler gas (think of a hot air balloon)

But… a cooler gas requires additional heat input

Optimization can help select number and temperature of intercooler stages

Integrated Gasification Combined Cycles