Chapter 13 Gas Turbine Power Plants

13.8 Combined Brayton and Rankine Cycles
Figure 13.11 shows a schematic arrangement of a combined gas
turbine and steam power plant. The hot exhaust gases from the
gas turbine are used in process 04' -05 to boil water in process 06-
09' in the boiler of a Rankine-cycle plant. Steam expands in
process 06-07' in the steam turbine, and gas expands in process
03-04' in the gas turbine. The net power for the combined plant is
the sum of the powers from the two turbines less the power to the
compressor and to the pump. The net power output of the steam
and water cycle can be written in terms of total enthalpies as
P~* =rn s [(h 06 -h or )-(h 09 , -/z 08 )] (13.26)
where m s is the mass flow rate of the steam. For the gas cycle the
net power output is
P as = m a ((h 03 -h w )-(h m , -h ol )] (13.27)
where m a is the mass flow rate of air entering the compressor.
The thermal efficiency of the combined system is the net power
divided by the rate at which chemical energy is supplied in the
combustor; thus,
P + P
£=——f2!_ (13.28)
13.9 Future Gas Turbines
Gas turbines with outputs of hundreds of megawatts are currently
used in power plants around the world. Their use in central stations
for topping and in combination with steam cycles will continue.
In September 1998 IPG International reported that the