Phase diagram of water

Combined Cycle Gas Turbine (CCGT) Stations
v) h 4 = (1−x)h f4 + xh g4
h 4 = (1−x)126 + 2566 x
x = 0.769
h 4 = 2002 kJ/kg
In recent years gas turbines and steam turbines have been combined to
increase the efficiency to around 50-60% (upper temperature ~1200 C)
Gas Turbine
air
34 isentropic so
W 34 = h 3 – h 4
= 3564 – 2002 = 1562 kJ/kg
vi) η = useful work/heat in
= (W 34 – W 12 )/Q 23 = (1562 – 17)/3421
= 0.452 = 45.2%
vii) cf Carnot Cycle
η c = (T 3 − T 4 )/T 3 = (873 − 303)/873
= 0.653 = 65.3%
T
W 12
Q
3
23
2
W 34
1
4
Q 41 S
T
T max
Compressor
Combustion
Compressor
Gaseous
fuel
p
compressed
air
Combustion
Chamber
p atmos
Turbine
Brayton Cycle
S
Turbine
Exhaust gas
a) Heat generated by internal combustion rather than
via a high temperature heat exchanger (boiler)
b) No cooler required since exhaust gases vented to
atmosphere
Plant much smaller. Work done by compressor is
significant, though this is compensated by very high
temperature ~ 1200 C (Turbine blades ceramic coated
and water cooled)
ω
CCGT Station
660 MW Power Plant
air
Compressor
Gaseous
fuel
compressed
air
Combustion
Chamber
Turbine
Exhaust gas
Water
Pump
Heat in
Boiler
Turbine
Exhaust gas
Condenser
ω
combustion
Cooling water
Heat out
T
Rankine
Cycle
compressor
S
turbine
boiler
Brayton
Cycle
Heat of exhaust gases used to
raise steam for steam turbine
Many CCGTs have been built in the
UK in the 90s due to availability of
cheap gas and relaxation of
governmental controls
Stator for a 660 MW
generator being assembled
Low pressure turbine, part of a
660 MW assembly
Power density ~ 1000 MW e per sq km