Post on 14-Dec-2014
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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