In general, two approaches have been used in the gas turbine industry to improve Brayton cycle performance. One approach includes increasing Turbine Inlet Temperature (TIT) and cycle pressure ratio (β), but it is quite capital intensive requiring extensive research and development work, advancements in cooling (of turbine blades and hot gas path components) technologies, high temperature materials and NOx reducing methods. The second approach involves modifying the Brayton cycle. However, this approach did not become very popular because of the development of high efficiency gas turbine (GT) based combined cycle systems in spite of their high initial cost. This paper discusses another approach that has gained lot of momentum in recent years in which modified Brayton cycles are used with humidification or water/steam injection, termed “wet Cycles”, resulting in lower cost/kW power system, or with fuel cells, obtaining “hybrid Cycles”; the cycle efficiency can be comparable with a corresponding combined cycle system including better part-load operational characteristics. Such systems, that include advanced Steam Injected cycle and its variants (STIG, ISTIG, etc.), Recuperated Water Injection cycle (RWI), humidified air turbine cycle (HAT) and Cascaded Humidified Advanced Turbine (CHAT) cycle, Brayton cycle with high temperature fuel cell, Molten Carbonate Fuel Cell (MSFC) or Solid Oxide Fuel Cells (SOFC) and combinations of these with the modified Brayton cycles, have not yet become commercially available as more development work is required. The main objective of this paper is to provide a detailed parametric thermodynamic cycle analysis of the above mentioned cycles and discussion of their comparative performance including advantages and limitations.

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