Efficiency of gas turbines

The efficiency of gas turbines is limited by the maximum allowable turbine inlet temperature (TIT). The TIT may be increased by cooling of the blades and vanes of the high pressure turbine. Cooling channels can be casted into the components or may be drilled afterwards. Non-conventional processes like EDM, ECD or Laser are used for drilling. Radiographic examination of the drilled components is part of the inspection procedure. Traditional X-Ray film technique has been used. The consumable costs, the waste disposal and the limited capacity of the two film units lead to the decision to investigate the alternative of Real-Time X-Ray. 

Kaganovich, B. M., Keiko, A. V., Shamansky, V. A. and Shirkalin, I. A. "Application of Equilibrium Feasibility Models in the Analysis of Efficiency of Gas Turbine Technologies in Small-Scale Energy". ISEM SO RAN, Irkutsk, (2005). Preprint At, 5. 59 p. (in Russian)... 

Thermal-barrier coatings (TBCs) are ceramic coatings which are applied to components for the purpose of insulation rather than oxidation protection. The use of an insulating coating coupled with internal air cooling of the component, lowers the surface temperature of the component with a corresponding decrease in the creep and oxidation rates of the component. The use of TBCs has resulted in a significant improvement in the efficiency of gas turbines. ... 

Development of super alloys and ceramic materials to increase the efficiency of gas turbines (GTs). 

Gurappa L, Sambasiva Rao A. 2006. Thermal barrier coatings for enhanced efficiency of gas turbine engines. Surface Coatings Technology 201, 3016-3029. 

The fact that thermodynamically, some of the heat generated in the electrochemical reaction can be reinvested into the chemical fuel conversion explains that SOFC have the potential to reach a net electrical system efficiency above 60% with natural gas fuel [14]. In a combined cycle system, where the exhaust gas from the high-temperature fuel cell is used to drive a gas turbine, the overall system electrical efficiency may even reach over 70% [15]. The efficiencies of gas turbines and diesel engines decrease at part load, whereas the efficiency of the fuel-cell-based systems will be almost independent of part load up to very high turndown ratios [14]. 

The next generation of gas turbine-based, combined-cycle power plants, under constmction in many parts of the world, is to feature net plant efficiencies in the 60% range based on LHV of fuel input. These faciUties, scheduled for start-up in the latter 1990s, are anchored by large gas turbines capable of simple-cycle efficiencies >40% LHV in some cases. To develop these machines, manufacturers have scaled up and improved upon designs that have already proved to be highly rehable. 

These gas turbines have in many cases regenerators or recuperators to enhance the efficiency of these turbines. Figure 1-12 shows such a new recuperated gas turbine design, which has an efficiency of 38%. 

The work required to drive the turbine eompressor is reduced by lowering the compressor inlet temperature thus increasing the output work of the turbine. Figure 2-35 is a schematic of the evaporative gas turbine and its effect on the Brayton cycle. The volumetric flow of most turbines is constant and therefore by increasing the mass flow, power increases in an inverse proportion to the temperature of the inlet air. The psychometric chart shown shows that the cooling is limited especially in high humid conditions. It is a very low cost option and can be installed very easily. This technique does not however increase the efficiency of the turbine. The turbine inlet temperature is lowered by about 18 °F (10 °C), if the outside temperature is around 90 °F (32 °C). The cost of an evaporative cooling system runs around 50/kw. 

Figure 3-19. Combined cycie and simpie cycie efficiency as a function of gas turbine ioad.

The use of gas turbines in the petrochemical, power generation, and offshore industries has mushroomed in the past few years. It is to these users and manufacturers of gas turbines that this book is directed. The book will give the manufacturer a glimpse of some of the problems associated with his equipment in the field and help the user to achieve maximum performance efficiency and high availability of his gas turbines. 

Another reason for the increased use of gas turbines as prime movers in the process industry is the high thermodynamic cycle efficiencies and subsequent low operating cost. 

Many preliminary analyses of gas turbines are based on the assumption of a closed air standard cyclic plant, and for such analyses the use of tj as a thermal efficiency is entirely correct (as discussed in the early part of Chapter 3 of this book). But most practical gas turbines are of the open type and the rational efficiency should strictly be used, or at least its approximate form, the arbitrary overall efficiency tjq. We have followed this practice in the latter part of Chapter 3 and subsequent chapters even though some engineers consider this differentiation to be a somewhat pedantic point and many authors refer to tjo as a thermal efficiency (or sometimes the lower heating value thermal efficiency ). 

Modifications of gas turbine cycles to achieve higher thermal efficiency... 

Subsequently, in Chapter 4, we deal with cycles in which the turbines are cooled. The basic thermodynamics of turbine cooling, and its effect on plant efficiency, are considered. In Chapter 5, some detailed calculations of the performance of gas turbines with cooling are presented. 

A set of calculations using real gas tables illustrates the performance of the several types of gas turbine plants discussed previously, the [CBT]ig, [CBTX]ig, [CBTBTX]ig, [CICBTXIig and [CICBTBTX]ig plants. Fig. 3.15 shows the overall efficiency of the five plants, plotted against the overall pressure ratio (r) for = 1200°C. These calculations have been made with assumptions similar to those made for Figs. 3.13 and 3.14. In addition (where applicable), equal pressure ratios are assumed in the LP and HP turbomachinery, reheating is set to the maximum temperature and the heat exchanger effectiveness is 0.75. 

Therefore, merely reducing coal use will not he sufficient to satisfy the Protocol. Any plan to comply with the Protocol needs to assume substitution, first by non-combustion energy sources—that is by renewables or nuclear energy—and second by natural gas. This would have to be accompanied by achievement of far greater efficiencies in energy production (for example by introduction of far more fuel-efficient steam gas turbines, driven by natural gas) and by more efficient use of energy. 

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