2 - The combined cycle and variations that use HRSGs

Abstract

The Brayton cycle (gas turbine) and the Rankine cycle (steam turbine) are two venerable cycles that have served mankind well. However, the combined cycle, which combines the Brayton and Rankine cycles, has resulted in cycle efficiencies exceeding 60% on a lower heating value basis. This is a much higher efficiency than can be achieved by either the Brayton or Rankine cycle alone. To combine these two cycles, a means to recover the waste heat from the gas turbine exhaust must be provided. The modern day heat recovery steam generator (HRSG) has met this need and is the bridge between the two cycles. The versatility of the modern day HRSG has allowed great flexibility in combined cycle design: single pressure, double pressure, triple pressure steam levels; nonreheat, reheat; and supplementary firing. In addition, the adaptability in HRSG design has provided the prerequisite heat recovery for variants of the combined cycle such as cogeneration, steam power augmentation, integrated gasification combined cycle, and solar hybrid cycles. Without the HRSG, the combined cycle and its variants would not be technically feasible.