Page 122 - Handbook of Energy Engineering Calculations

increasing the plant’s GHG output by much. At their introduction, gas
turbines were planned to handle 1000 to 2000 peaking hours per year.
Since then, their successful performance has far exceeded these plans.
Today, gas turbines are widely used in base-load service in combined-cycle
plants. And the capacity of such combined-cycle plants can exceed 500

MW, a far cry from the 75 MW capacity when first introduced.

• Personnel requirements for operating the new gas turbines are minimal;
most plants did not need to add operators to their existing staff.

• Manufacturers, worldwide, have added gas turbines to their list of products.

And aero-gas-turbine manufacturers quickly increased the size and
capacity of their product line to meet the needs of electric utilities for
bigger gas turbines.

• Heat rates of 9600 to 10,400 kJ/kWh are reported for free-turbine-drive gas
turbines operating at variable speed. Overall combined-cycle plant

efficiency exceeding 60 percent has been reported by some plants.

• When gas turbines were first introduced in combined-cycle electric-
generating plants, the ratio of the output of the steam turbine to the gas
turbine was 8:1. Today, gas turbines are filling a much larger output role in

combined-cycle plants worldwide.
The result of all this activity is that high-efficiency combined-cycle plants
are now the norm for most electric utilities, and the gas turbine has become as
popular as the steam turbine once was. Today’s gas turbines show overall
thermal efficiencies exceeding 30 percent. Such performance is highly

desirable in times of increasing fuel costs and uncertainties in the
dependability of certain fuel supplies.

GAS TURBINE-STEAM TURBINE CYCLE ANALYSIS

Sketch the cycle layout, T-S diagram, and energy-flow chart for a combined
steam turbine–gas turbine cycle having one stage of regenerative feedwater
heating and one stage of economizer feedwater heating. Compute the thermal