Analyzing the Possibility of Reducing Power Loss in a Cooled Power-Generating Gas Turbine Unit

Abstract

Evolutionary development of power-generating gas turbine units (GTUs) is characterized by an increase of their parameters and the compressor output and improvement of turbine machinery aerodynamics. With the initial gas temperature increased to 1600°С, the flowrate of compressor air taken for cooling the turbine increases, and its value in the most powerful single-shaft GTUs operating according to a simple thermodynamic cycle reaches 20% of the compressor output. Admixture of air to hot gases causes them to become cooler, which entails heat loss proportional to the total cooling air flowrate. The extent to which the cooling air flowrate is reduced has an essential effect on the improvement of the GTU and combined-cycle power plant (CCPP) efficiencies. The article discusses the possibilities of reducing the cooling air flowrate due to applying closed-loop cooling of the turbine initial stages and using the heat removed in the power generating cycle. In the case of using well-known steam cooling, the removed heat is used in the CCPP steam part, and it is used in the gas turbine cycle with water cooling. In the latter case, the removed heat can be returned into the cycle by using advanced, single-loop water cooling of nozzle vanes and rotor blades. The effectiveness of using closed-loop cooling is evaluated for two values of initial gas temperature of 1430 and 1600°С. With the initial gas temperature of 1430°С, compressor pressure ratio of 23, compressor output of 675 kg/s, and heat removal rate of 21.1 MJ/s, the efficiency of a GTU equipped with water cooling of only its first stage (41.3%) is higher than it is with the initial fully air cooling (39.5%) and existing steam cooling (40.5%) of two stages. The efficiency values of a CCPP with water-cooled and steam-cooled GTUs become essentially equal to each other (60%) and are by 2% higher than the efficiency of an air-cooled CCPP (58%). With the initial gas temperature of 1600°С and the first stage equipped with water cooling, the air flowrate extracted for cooling decreases down to 10%, the GTU efficiency increases to 44.7%, and the CCPP efficiency makes 65.2%, which is 2.1% (abs.) higher than the efficiency of a CCPP equipped with a modern fully air cooled GTU.