Page 172 - Handbook of Energy Engineering Calculations

simple-cycle plants employing steam injection. Further, combined-cycle
plants may also be considered more adaptable to cogeneration compared to
steam-injected gas turbines. The reason for this is that the maximum
achievable electrical output decreases significantly for steam-injected units in
the cogeneration mode because less steam is available for use in the gas

turbine. In contrast, the impact of cogeneration on electrical output is much
less for combined-cycle plants.
Repowering in the utility industry can use any of several plant-

revitalization schemes. One of the most common repowering options
employed or considered today by utilities consists of replacing an aging
steam generator with a gas-turbine/generator and HRSG, Fig. 22. It is
estimated that within the next few years, more than 3500 utility power plants
will have reached their 30th birthdays. A significant number of these facilities

—more than 20 GW of capacity by some estimates—are candidates for
repowering, an option that can cut emissions and boost plant efficiency,
reliability, output, and service life.

FIGURE 22 HRSG and gas turbine used in repowering. (Power.)

And repowering often proves to be more economical, per cost of kilowatt