The Basic Performance of Combined Cycles and Their Optimum Gas Turbine Pressure Ratios

Based on the idea of the Comparison Method proposed by the author in 80's,the expressions for efficiencies and power ratios between gas turbine and steam turbine of various typical combined cycles such as pressurized fluidized bed type,fully fired type and pure heat recovery types are derived. These expressions are very simple,physically clear and accurate enough. In addition,the values of optimum gas turbine pressure ratios for efficiency of these combined cycles are derived or discussed also. It is found that the optimum gas turbine pressure ratio for efficiency of pressurized fluidized bed type combined cycle can be expressed as the same as the optimum pressure ratio for specific output of the simple cycle gas turbine,and the optimum pressure ratios for efficiency of other combined cycles are only a little bit higher than the optimum pressure ratio for specific output of the simple cycle gas turbine.