Gas turbines and combined cycles

1
Gas turbines and combined cycles

• CB 8-8, 9-9

2
One pass gas engines

• Characteristics
• Burn a fuel
• Compress air
• Expand combustion gases
• Examples
• Internal combustion engine
• Diesel engine
• Gas turbine
• CB Chapter 8

3
One pass gas engines

• Ideal model for the engine
• Closed cycle
• Ideal gas working fluid similar to air
• Examples of ideal cycles
• I.C.E. Otto cycle
• Diesel diesel cycle
• Gas turbine Brayton cycle

4
Gas turbine cycle

• Working fluid ideal gas
• 4 steps
• Adiabatic, reversible compression AB
• Constant pressure heat transfer (combustion) BC
• Adiabatic, reversible expansion CD
• Constant pressure heat transfer (exhaust gas) DA

5
Gas turbine efficiency
6
Gas turbine efficiency

• Best rp from T limits
• Tmax determined by
• metallurgy of turbine
• Large gas turbines
• efficiencies 0.4

7
Gas turbine cycle

• Natural gas as a fuel
• Typical values
• pb8 atm
• TC1500 C
• It follows that

• Issues
• Large fraction of output work to run compressor
• High fuel cost
• Easy to start up
• Good greenhouse credentials
• Low pollution

8
A new approach to power generation?

• Standard Rankine

• Standard gas turbine

Can you come up with an improved approach to give
a cycle with much higher efficiency?
9
CURRENT TECHNOLOGYNo. 2 boiler in Pleystowe
Sugar Mill
10
Fluid bed combustor
11
Combined Cycles

• Use gas turbine cycle to deliver high temperature
  energy to a Rankine cycle
• High temperature exhaust gas to generate steam
  for power plant turbine
• Higher real efficiencies 0.6, compared with
  separate cycles (0.4)

12
Combined cycle
13
Gas-steam combined power plant