Exergy Balance Applied to a Gas Turbine in a Modified Brayton Cycle to Assess the Power Output Recovery

Abstract

Introduction— Gas turbines play a key role in the power generation industry. The UPME (Unidad de Planeación Minero-Energética), in its monthly report of the generation variables and the Colombian electricity market, quoted data from December 2016, highlights that the net effective capacity in megawatts is 2,093 and represents about 12.61% of the total energy capacity through gas thermal plants. During hot seasons, the performance of the turbines is affected by the elevated air temperature flowing into the cycle, because the power output depends on the air mass flow through the compressor, the mass flow decreases as the temperature increases, resulting in a decline in efficiency and power generation.

Objetivo— A gas turbine in open cycle was analyzed based on the first and second law of thermodynamics, irreversibilities were considered using exergy as the criterion to establish the overall performance of the system.

Methodology— An alternative analysis was proposed modifying the ideal Brayton cycle, humidity of the air, the height above sea level, and the detailed molar composition by component were considered. Furthermore, the heat adding process was replaced by an adiabatic and isobaric combustion progression with a subsequent cooling by means of an adiabatic mixture of ideal gases with the compressor discharged air to condition the incoming flow mixture of gases to the turbine aimed to maintain the maximum allowed temperature controlled.

Results— The simulation was developed in the software EES (Engineering Equation Solver) and the free software CEA (Chemical Equilibrium with Applications) from NASA was used to validate results for the combustion process, under the criterion of chemical equilibrium. Few investigations about inlet cooling methods for gas turbines have supported the implementation of such kind of technologies by means of exergy balance, which is the main goal of this research.