Computer-Aided Exergy Study of a Gas Microturbine Cogeneration System

In the following article, an exergetic analysis of a microturbine operating with a regenerative Brayton cycle was carried out in order to identify the variation in exergy and exergy destruction behaviour generated in each component of the system by comparing these results to different microturbine loads. The study was carried out on a Brayton cycle with cogeneration which is composed of a compressor, combustion chamber, gas turbine, HRSG and an air preheater. In which the output power of the turbine was varied for the five case studies starting at 25kW to 45kW. As the study is carried out, at 45kW the greatest exergy is consumed and in the combustion chamber it is the one that contributes most to the destruction of exergy, adding up to an average of 36.5% of the total destroyed. With this it was shown that the increase of the power output of the turbine increases the needs of each component of the system and also increases the exertions of the system.

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The fund the litera cycle.The obtaining In additio equation Isotropic isentropic compress owing is a det case studies d for the cacu ents. cess descripti owing is a det case studies d for the cacu ents Figure 1. system the ou valuating the b ndamental Eq damental therm ature [20],whic e first law of g equation (1) on, the second (2 For ideal gases, the analysis of entropy between two states is governed by equation (5).
According to the third law of thermodynamics entropy at the same temperature and any pressure for ideal gases is determined according to equation (6).
Exergy can be divided into physical exergy ( , potential exergy , kinetic exergy and chemical exergy and omitting nuclear, magnetic, electrical and surface tension effects its equation (7) The total exergy specified in units of mass is.
. (7) The physical exergy of a closed system in a specific state is given by equation (8), where h and s denote, respectively, the enthalpy and entropy of the system in the specific state, and are the values of the same properties when the system is in a restricted dead state.
. (8) The chemical exergy of a mixture of gases, which are present in the gaseous phase of the environment, can be obtained by means of equation (9).
∑ . (9) III. RESULTS AND ANALYSIS One of the parameters analyzed was the physical exergy which was studied in each of the states of the plant. Figure 1 shows a comparison of physical exergy in each state. In addition, the physical exergy is shown for each power ranging from 25kW to 45kW. It was possible to observe that in the first state there is no exergy due to the fact that it is air at room temperature and therefore does not present an energetic potential. On the other hand, in state four a peak of exergy is observed since in this state the gases have just left the combustion chamber having here if higher temperature, this thermal condition is reflected in a great available energy which is used in the process of the plant. The increase of this temperature is a determining point for the design of these cycles, this is due to the fact that as the temperatures handled by the plant increase, the demands on the materials with which they are built increase and the costs of assembling the plant also increase.On the other hand, chemical exergy was also a parameter that was reviewed in the study. In states 4 to 7 the chemical exergy maintains the value reached after leaving the combustion chamber as shown in Figure 3. In the same way, the values of total exergy in each of the states were reviewed, this value is given by the sum of chemical and physical exergy. Figure 4 shows two high peaks due to the influence of high chemical exergy in state 10 due to the energy potential of methanol and physics in state 4 due to the increase in temperature generated by the passage of methanol through the combustion chamber.  The component that most contributes to the destruction of exergy is the combustion chamber. This destruction is presented in this component due to the transformation of the energy potential into methanol which is used to generate an increase in the temperature in the chamber. Adding up an average of 36.5% of the total destroyed exergy, its contribution to the destruction is very significant. This component shows its highest peak at 45kW. Finally, Table 2 showed how fuel flow needs changed in each of the case studies. An increase proportional to the change in output power was observed, and as always the air flow was on average 1.72% lower than that of the products. IV. CONCLUSIONS In conclusion, this study showed how the increase in turbine output power increases the needs of each component of the system. Parameters such as fuel mass flows increase and the temperature handled by the system components increases, which is critical for the design considerations of the system. In addition, it is observed that as the power generated increases, both the chemical exergy in the fuel and the physical exergy after combustion increase proportionally, due to the increase in the mass flows previously mentioned. It is hoped that the results of this study will serve as a basis for further research focused on the Brayton cycle with microturbines.