Gas Turbine Performance

Gas turbine thermal efficiencies (shaft power) range from 15% to more than 40% (LHV). A simplified heat balance diagram, showing average annual performance for a 4.7 MW gas turbine cogeneration system, is shown in Figure 10-43.

As demonstrated above in the discussion of the Brayton

Showing Symmetry Engineering Prints
Fig. 10-43 Simplified Heat Balance Diagram Showing Average Annual Performance for 4.7 MW Gas Turbine Cogeneration System. Source: Cogen Designs, Inc.
Fig. 10-44 Simple-Cycle and Total Efficiency as a Function of Gas Turbine Cogeneration System Capacity. Source: Cogen Designs, Inc.

Fig 10-45 Heat Rate vs. Capacity for Industrial, Aeroderivative and Steam Injection Models. Source: Cogen Designs, Inc.

airflow, the smaller the gas turbine required for the same output power. Note the importance of cycle-pressure ratio on output and thermal efficiency, and that at each firing temperature, maximum efficiency occurs at a pressure ratio other than that of maximum output. Also note that the pressure ratio resulting in maximum output and maximum efficiency changes with firing temperature, and the higher the pressure ratio, the greater the beneficial impact of increased firing temperature.

Gas turbine thermal efficiency is reduced under part-load operation. Figure 10-47 provides a representative sample of fuel rate versus load for applied turbine designs featuring single-shaft, two-shaft, and single-shaft with IGVs. The IGVs restrict mass flow under part-load operation, thereby

Fig. 10-47 Gas Turbine Fuel Rate vs. Load. Source: Cogen Designs, Inc.

Fig 10-45 Heat Rate vs. Capacity for Industrial, Aeroderivative and Steam Injection Models. Source: Cogen Designs, Inc.

more than 70 gas turbine cogeneration systems. Figure 1045 depicts gas turbine heat rate as a function of capacity (in kW) for industrial, aeroderivative, and steam injection (STIG) cycle models.

Figure 10-46 shows a plot of output and thermal efficiency for different gas turbine firing temperatures and various pressure ratios. The higher the output per unit mass of

Fig. 10-47 Gas Turbine Fuel Rate vs. Load. Source: Cogen Designs, Inc.

improving thermal efficiency relative to the standard singleshaft design. Notice that two-shaft design offers the highest part-load thermal efficiency, increasingly so as loads are reduced.

While in the ideal Brayton cycle, efficiency improves with increased pressure ratio, in actual practice, the effect of increased pressure ratios is linked to turbine inlet temperatures.

Fig. 10-46 Simple Cycle Gas Turbine Thermodynamics. Source: General Electric Company

For each inlet temperature, there is a certain pressure ratio that results in maximum thermal efficiency. Beyond this point, efficiency begins to decrease with increasing pressure ratios. Maximum pressure ratios and inlet temperatures are limited by current technological constraints of material temperature tolerance and cooling system effectiveness.

Turbine efficiency is also a function of turbine airflow design and turbine inlet temperature. Most of the efficiency losses in the turbine section are caused by flow conditions in the blade-to-blade channel of the rotating blades. A critical efficiency issue is the need to balance the impact on efficiency of higher-pressure ratios and firing temperatures with the need for increased cooling airflow.

As firing temperatures increase, some or all of the stationary nozzles and rotating blades must be cooled. As pressure ratios increase, the temperature of the cooling air typically bled from the compressor also increases. For modern industrial turbines, cooling airflow is about 12 to 15% of the total compressor flow. Since allowable component metal temperatures are limited to a maximum of about 1,750°F (954°C), increased cycle efficiency requires some form of turbine part cooling. Compressor air, diverted for cooling turbine parts, is expensive in terms of the energy required to deliver it. The impact of cooling is doubly felt because this air makes no contribution to the combustion or dilution processes in the cycle.

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