The most effective cycle is the Brayton-Rankine cycle. This cycle has tremendous potential in power plants and in the process industries where steam turbines are in use in many areas. The initial cost of this system ishigh; however， in most cases where steam turbines are being used this initial cost can be greatly reduced.
Regenerative cycles are popular because of the high cost of fuel. Care should be observed not to indiscriminately attach regenerators to existing units. The regenerator is most efficient at low-pressure ratios. Cleansingturbines with abrasive agents may prove a problem in regenerativeunits， since the cleansers can get lodged in the regenerator and cause hot spots.
Water injection， or steam injection systems， are being used extensively to augment power. Corrosion problems in the compressor diffuser and com-bustor have not been found to be major problems. The increase in work and efficiency with a reduction in NOx makes the process very attractive. Split-shaft cycles are attractive for use in variable-speed mechanical drives. The off-design characteristics of such an engine are high efficiency and high torque at low speeds.
A General Overvie. of Combined Cycle .lants
There are many concepts of the combinedcycle， these cycles range from the simple single pressurecycle， in which the steam for the turbine is generated at only onepressure， to the triple pressure cycles where the steam generated for the steam turbine is at three different levels. The energy flow diagram Figure 2-30 shows the distribution of the entering energy into its useful component and the energy losses which are associated with the condenser and the stack losses. This distribution will vary some with differ-ent cycles as the stack losses are decreased with more efficient multilevel pressure Heat Recovery Steam Generating units (HRSGs). The distribution in the energy produced by the power generation sections as a function of the total energy produced is shown in Figure 2-31. This diagram shows that the load characteristics of each of the major prime-movers changes drastically
Figure 2-.0. Energy distribution in a combined cycle power plant.
70
60
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40
Gas Turbine Steam Turbine
30
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0
0 20 40 60 80 100 120
Percent Overall Load
Figure 2-.1. Load sharing between prime movers over the entire operating range of a combine cycle power plant.
with off-design operation. The gas turbine at design conditions supplies 60% of the total energy delivered and the steam turbine delivers 40% of the energy while at off-design conditions (below 50% of the design energy) the gas turbine delivers 40% of the energy while the steam turbine delivers 40% of the energy.
To fully understand the various cycles， it is important to define a few major parameters of the combined cycle. ln most combined cycle applica-tions the gas turbine is the topping cycle and the steam turbine is the bottoming cycle. The major components that make up a combined cycleare the gas turbine， the HRSGand the steam turbine as shown in Figure 2-32 a typical combined cycle power plant with a single pressure HRSG. Thermal efficiencies of the combined cycles can reach as high as 60%. ln the typical combination the gas turbine produces about 60% of the power and the steam turbine about 40%. lndividual unit thermal efficiencies of the gas turbine and the steam turbine are between 30-40%. The steam turbine utili.es the energy in the exhaust gas of the gas turbine as its input energy. The energy transferred to the Heat Recovery Steam Generator (HRSG) by
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