Quality Key Numbers of Gas Turbine Combined Cycles

It is an attractive task to evaluate the most relevant quality key numbers for the largest and most efficient Gas Turbine Combined Cycles (GTCC). Such numbers are here evaluated with educated guesses based on published data of the latest announcements of the “big four OEM’s”. Such data are of interest for potential customers but also for nailing down the current state of the art for all kind of further cycle studies using turbomachinery components.

Making educated guesses means thermodynamic 1D simulation based on additional assumptions for pressure losses and other cycle data, which have a limited influence on the target quality numbers, such as:

·        Combustor Exit Temperature CET. This is a key value describing the technology level. It is important for the design of the cooled combustor walls and for the first turbine vane row.

·        Polytropic efficiency of the compressor blading. This number describes the aerodynamic quality of the compressor blading.

·        Polytropic efficiency of the turbine blading. It describes the quality level of both the aerodynamic and of the open air cooling design.

·        Balance of the exergy losses within the GT and in the bottoming cycle. The exergy loss distribution describes the remaining opportunities for further improvements in the thermodynamic cycle design. But it also indicates its limits.

However already the determination of the CET is tricky. It depends strongly on the analysis method and on the fluid data applied. The polytropic efficiency of the turbine blading and the exergy losses will depend both on the used methods and on the CET found. Achieving a trustable result therefore requires a transparent and reproducible method. In case of application of the found results for performance prediction of similar cycles the same method has to be applied in order to avoid mistakes.

In this paper real gas data with consideration of dissociation in equilibrium are used, while the polytropic efficiencies are determined with an incremental method based directly on the classic definitions of Stodola and Dzung. Therefore the still most used method using semi-perfect gas properties and corresponding formulas is bypassed.

In order to keep it as simple as possible the evaluation is limited to base load at ISO ambient condition (15°C, 60% relative humidity, sea level). The fuel is limited to pure methane according to the practice in current catalogue data.

The main focus is on the gas turbine with its components. The steam bottoming cycle is captured with its effect on the overall exergy and energy balance of the GTCC, which includes exhaust and condensation losses.

The accuracy and the unavoidable uncertainties of the results will be discussed too.