Turbines, gas efficiency

The gas turbine efficiency drops off quickly at part load as would be expected, as the gas turbine is very dependent on turbine firing temperature and mass flow of the incoming air. The gas turbine heat rate increases rapidly at part load conditions. 

The increase in unit size and complexity together with the higher turbine inlet temperature, and higher pressure ratio has lead to an increase in overall gas turbine efficiency. The increase in efficiency of 7-10% has in many cases lead to an availability decrease of the same amount or even more as seen in Figure 21-5. A 1% reduction in plant availability could cost 500,000/yr in income on a 100 MW plant, thus in many cases offsetting gains in efficiency. 

Fig. 5.2 shows that for the single-step cooled CBT plant at a given combustion temperature, the overall efficiency of the cooled gas turbine efficiency increases with pressure ratio initially but, compared with an uncooled cycle, reaches a maximum at a lower optimum pressure ratio. Fig. 5.3 shows that for a given pressure ratio the efficiency generally increases with the combustion temperature even though the required cooling fraction increases. 

Figure 15.7 Approximate relationship showing variation of gas turbine efficiency for changes in load...

Part load As the gas turbine operates with a fixed inlet air volume, its efficiency at part load deteriorates significantly. Figure 15.7 shows a typical relationship for variation in gas turbine efficiency for changes of load. 

Aero-derivative gas turbines are typically used for offshore applications where weight and efficiency are a premium, to drive compressors for natural gas pipelines, and stand-alone power generation applications for peak periods of high power demand. For stand-alone applications, gas turbine efficiency becomes a critical issue. However, if heat is to be recovered from the gas turbine exhaust, the efficiency becomes less important as the waste heat is utilized. 

Wmax = maximum turbine power hGT.max = gas turbine efficiency at maximum load a,b = correlating parameters... 

Improvement to gas turbine efficiencies is a general goal of manufacturers. It further enhances the attractiveness of... 

C. W. Janes, "Increasing Gas Turbine Efficiency Through the Use of a Waste Heat Methanol Reactor", IECEC 799423 (1979). 

Often the maximum temperatures in the combustion chamber exceed the TIT. Dilution air is mixed with the hot combustion gases downstream of the combustion chamber to obtain a suitable temperature. Therefore, the gas turbine efficiency is not necessarily related to the maximum temperature in the combustor. This fact is important for catalytic combustors, operating at relatively low temperatures, as will be discussed in the next section. 

Gas turbines efficiency is higher with high temperature gases... 

Compressors may be driven by electric motors, steam turbines, or gas turbines, but the former is the most common driver and the gas turbine is the least common. All drivers are available up to at least 20,000 Hp. For applications below about 200 Hp, electric motors are used almost exclusively. Most efficient is the electric motor least efficient is the gas turbine. Efficiencies of all three drivers increase with horsepower. At 1,000 Hp, typical efficiencies are... 

Power plants under about 350 MWe cannot use the latest high-efficiency combined cycle technologies. Those below about 250 MWe cannot use particularly high-efficiency steam turbines because of friction losses and leaks in small dimension gas paths. Those below about 100 MWe cannot economically use reheat steam cycles, giving a further efficiency drop. Moving further down in size gives a steady reduction in efficiency of the gas turbine, whichever manufacturer is selected. The scale effect of gas turbine efficiencies is due to flow paths and pressure drops and can only be partly compensated for with additional components such as intercoolers or reheaters. 

Gas turbine efficiency can be enhanced by the use of recuperator (see Figure 23.25), which pre-heats the gas prior to the heat addition process. 

W. H. Hadley, Effect of gas turbine efficiency and fuel cost on the cost of producing electric power. Contract 68-02-1320. U.S. Environ. Protection Agency to Monstanto Res. Corp., Dayton Lab., Dayton Ohio, 1972. 

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