In a combined cycle power plant (CCPP), the overall performance is usually parametrized in terms of the efficiency (heat rate), and power output. These values are usually easily calculated when there is access to the necessary data. Still, the calculated efficiency and output of a CCPP will hardly ever be the same two times. This is because there are many ever-changing conditions that affect the efficiency and output of the plant.
In the short term case, the performance is dominantly affected by changes in atmospheric conditions. In general, performance tends to be better during night and morning periods when ambient air temperature tends to be lower, and worse on midday and afternoon when ambient air temperatures are higher. In the long term case, performance tends to decrease due to the effects of component degradation. This could be caused by gas turbine compressor fouling, gas turbine backpressure increase due to HRSG flue gas path fouling, dirty air filters, mechanical degradation of equipment such as pumps and generators, increased tolerances due to friction and erosion, etcetera.
One can see that there are very many factors that affect the performance of a plant, and a question that comes out of this is: Which are the main factors that affect performance, and by how much do they affect the performance? Not surprisingly, this is a very difficult question to answer since the effects of all these factors aggregate and their individual contributions to the efficiency and output of the plant cannot be singled out.
TGPS makes use of power plant modelling software for detailed and precise analysis of CCPP performance. Within the capacities of this approach is the prediction of performance change due to variations in any of thousands of parameters. For example, the performance of a plant can be accurately predicted under different ambient conditions, or on operation with fouled components.
A case study for a modern combined cycle power plant with a 2×1 configuration, triple pressure reheat, is presented here for demonstration. The plant has been modeled with the main design criteria shown in Table 1 below.