MATHEMATICAL MODELING AND STATISTICAL EVALUATION OF COGENERATION PLANT IN TROPICAL REGION

The widespread use of gas turbines and cogeneration plants as a means of
independent power generation have provided a considerable momentum for further
study of cogeneration plant. Furthermore, in the design of new systems and an
existing system improving their performance is a challenging task. This is largely
achieved by studying the system performance as a whole or as an individual
component. In order to do that, greater understanding of the behaviour of the plant
during off-design operation and identifying the potential components that have wide
margin of improvement are important.
Thus, this thesis is concerned with a detailed investigation of how off-design
conditions affect the cogeneration performance and the associated exergy destruction
or loss. To carry out the investigation a new modeling procedure based on component
matching is developed. The model is used to predict the design performance, offdesign
performance, and the exergy destruction of the cogeneration plant. The
cogeneration plant consists of gas turbine and heat recovery steam generator. The gas
turbine compressor has variable stator vanes whose position may be set to control the
inlet air flow to the compressor. During off-design the variable vanes are re-staggered
to improve the overall cogeneration performance. Two modes of gas turbine
operation are identified. The first mode is for part load less than 50% running to meet
the part load demand. This is achieved by controlling the fuel flow and air bleeding at
the downstream of the compressor to avoid surge formation. The second mode of
operation is for part load greater than 50% and running to meet both the part load
demand and the exhaust gas temperature set value by simultaneously regulating the
fuel feeding and the variable vanes opening. To accommodate change of compressor
parameters during variable vanes re-stagger correction coefficients are introduced.
The unavailable information such as the compressor and turbine design point data
are obtained using energy and mass conservation, and thermodynamic properties ratio
relationships. The compressor and turbine maps are developed using scaling method from similar configuration known component maps. Both energy and exergy models
of the components are developed. First, an energy based components model and their
interactions using modified component matching concept are developed. To support
the calculations required for off-design analysis, a computer program is developed in
MATLAB software. The effect of variation of load on the cogeneration parameters
such as fuel consumption, temperatures, pressure ratios, variable vanes opening,
efficiencies, specific fuel consumption, and steam production rate are examined. The
simulated results are compared with available actual data. Furthermore, statistical
errors evaluation using Minitab program indicated that the error mean and standard
deviations values were small and hence the developed model represents the real
process.
Once the model has been validated, based on the inlet and outlet properties of
each component the exergy analysis is performed to find out the exergy destruction or
loss in each component. Sensitivity analysis of the effect of ambient temperature on
the cogeneration performance is carried out. It is found that the smaller the ambient
temperature, the better is the gas turbine performance in the first mode of operation.
In the second mode of operation the VV s is modulated to maintain the turbine exhaust
gas temperature. Consequently, effect of ambient temperature on the thermal
efficiency is not significant; but the higher the ambient temperature, the higher is the
overall performance of the cogeneration plant at a given load.
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