Monitoring of the thermoeconomic performance in an actual combined cycle power plant bottoming cycle
Introduction
Combined cycle power plant performance analysis is very important because of the large diffusion all over the world and the leading role in electric power production of this type of plant. A complete performance analysis is essential both for the power plant constructor and for the user.
In a plant monitoring system which can evaluate performance and efficiencies of each component, a thermoeconomic monitoring software is suggested as it could become a fundamental decision-support tool in the plant management.
The thermoeconomic approach to plant performance monitoring is a philosophy that started in the early 1980s. Many authors have faced the problem of thermoeconomic analysis of power plants, such as Frangopoulos [1], Von Spakovsky [2], Valero et al. [3], [4], Verda et al. [5].
As reported in Ref. [6], the monitoring activity consists in continuously evaluating the productivity capacity and the efficiency of the plant, using the data stream from plant instrumentation. The performance evaluation is repeated at regular intervals with the object of calculating the degradation of different elements of the plant. Degradation is defined as a worsening of performance and can be caused, depending on the plant component, by various phenomena: from compressor fouling to turbine blade oxidation or corrosion, heat exchanger fouling, and so on [7]. A detailed diagnostic thermoeconomic analysis can bring about important improvements in power plant management, such as:
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instant information about the operation of the plant, through the calculation of efficiency and degradation indexes;
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troubleshooting: fast identification of the malfunctioning component and the possibility of reducing maintenance shutdowns of the plant, using targeted interventions;
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performance improvement;
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better management of maintenance (predictive maintenance);
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optimization of plant operation.
Section snippets
Reference combined cycle power plant
The type of power plant, whose BC is the subject of this study, is a combined cycle producing around 400 MW, consisting of a gas turbine, a three level HRSG (constituted by 13 heat exchangers, including the reheater), a steam turbine with three pressure expansions (high, intermediate, low pressure), and an air or water condenser. Fig. 1 shows a schematic plant layout, including calculation sections and component names.
The topping cycle consists of a heavy duty, single shaft, axial gas turbine,
Summary of thermoeconomic theory
The thermoeconomic approach was developed starting from open literature publications [1], [8], [9], [10], adapted to the modular representation of power plants [11]. Exergoeconomic indexes were derived from Refs. [12], [13].
Actual values
The thermodynamic data (temperatures, pressures, mass flow rates) of all the sections of the bottoming cycle are all known: they can be either measured values (from plant sensors) or calculated values. The unknown values are derived through mass and heat balances [14].
HRSG unknowns: temperatures of the flue gas in all the sections and a few steam temperatures (desuperheating).
Steam turbine and condenser performance could be completely characterized using measurements only.
The complete set of
Input of the system
The inputs of the original Matlab software were taken directly from the monitoring system, selecting instantaneous operating conditions of the whole power plant at steady state. The criteria used for the choice of the exact moment for collecting the data were: stability of gas turbine and steam turbine power, stability of steam flow extracted, stability of steam pressures.
Overall, around 30 data points were collected, about two measurement points every month, over a time period of about 2
Conclusions
A complete thermoeconomic analysis of the described large-sized combined cycle power plant is examined in this paper. A description of the calculation procedure (developed in Matlab environment) is given and the results obtained are presented. This kind of analysis enables the calculation of all the characteristic thermoeconomic parameters, such as: exergy flows (y), their related costs (c), exergetic costs (y*) and the unit exergetic cost (k*). Then, the characteristic exergonomic indexes (Δc,
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