Comprehensive analysis of energy, exergy and exergo-economic of cogeneration of heat and power in a combined gas turbine and organic Rankine cycle

Abstract

Organic Rankine cycles (ORCs) are appropriate technology for the conversion of low quality thermal energy to electrical power. Meanwhile the waste heat from the exhaust gases of the top cycle of the energy conversion systems to the environment is prevented. The aim of this work is thermodynamic, exergo-economic and environmental assessment of a cogeneration of heat and power cycle (CHP), considering the three objective functions of first and second law efficiencies and the total cost rates of the system. The proposed combined heat and power cycle combines a gas turbine (GT) and an ORC through a single-pressure heat recovery steam generator (HRSG). In this regard, after the thermodynamic simulation of the cycle, validations of the results of top and bottom cycles are evaluated using the works available in the literature. The results show that the most exergy destruction rate takes place in the combustion chamber, and after that in heat recovery steam generator and gas turbine, respectively. The exergo-economic factor for the whole cycle is 10.59% indicating that the exergy destruction cost rate is more than capital investment cost rate. In order to assess the effects of design parameters on the objective functions, a parametric study is conducted. The results reveal that the increase in pressure ratio and isentropic efficiency of air compressor and gas turbine efficiency improves thermodynamic performance of the system, however, the more increase of these parameters deteriorates the total cost rates. Furthermore, the increase in air preheater exiting temperature will be useful for the system both in terms of thermodynamic and exergo-economic. The increase in the condensation temperature and pinch point temperature difference of evaporator of ORC and increase in pinch point temperature difference of heat recovery steam generator cause the first and second law efficiency to fall down and also to increase the total cost rates of the system.