| Summary: | 碩士 === 國立成功大學 === 機械工程學系 === 87 === Since the oil embargo of the early 1970s, the increased awareness that the world''s energy resources are limited has caused some governments to reexamine their energy policies and take drastic measures in eliminating waste. The first law of thermodynamics deals with the quantity of energy and asserts that energy can not be created or destroyed. The second law of thermodynamics, however, deals with the quality of energy. It is concerned with the degradation of energy during a process, the exergy loss or the exergy destruction.
At first, in this paper, the thermodynamics of simple-cycle steam-injected gas turbine, STIG, are studied by means of exergy analysis. Calculations are performed for typical gas turbine operating parameters using an exergy analysis computer code. Under varied steam injection ratio, the different types of losses are considered and their relevance to plant performance is determined and compared with the Topping Cycle. The results indicate the possibility of achieving high efficiencies and large output improvements.
However, exergy analysis can only indicate the potential or possibilities of improving process performance, but cannot state whether or not the possible improvement is practicable and economic. This is because exergy analysis compares real performance to the ideal one. But any practical process needs a certain driving force for the process to take place, so that some exergy losses are inevitable. If the inevitable exergy losses can be determined, the attention can only be paid to the avoidable exergy losses, which can be converted into useful work.
Then, in this paper, a new method, a practical exergy analysis, will be used for better understanding of process performance and improvement of STIG. This is achieved by dividing exergy loss into avoidable and inevitable exergy losses. According to this improved understanding, a practical exergy efficiency is defined and a visualized exergy-loss-distribution graphic is proposed. Furthermore, an improvement number is defined for this new graphic representation.
From the results of the new method, we can see that although the most exergy losses occurs in the combustion chamber, most of it is inevitable; and most of exergy losses, occurring in the turbine, is avoidable. By increasing steam injection ratio and compression ratio, we can improve both the practical exergy efficiencies of the cycle and the turbine.
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