Exergy analysis of a gas turbine trigeneration system using the Brayton refrigeration cycle for inlet air cooling

Abstract

Trigeneration with gas turbines as prime mover can improve energy utilization efficiency significantly because of their potential of high economic and energy saving characteristic. In this article, a novel method for compressor inlet air cooling in gas turbine trigeneration is achieved by the Brayton refrigeration cycle driven by the gas turbine and air is used as a working fluid. Introducing the air refrigeration cycle for inlet air cooling provides the advantage of quite low temperatures close to 0 °C and even lower, and enhances the performance significantly. The performance was evaluated by both energy and exergy efficiencies, with the latter providing good guidance for system improvement. The influences of key parameters, which include extraction pressure ratio, extraction mass rate, turbine inlet temperature (TIT), process heat pressure, and ambient relative humidity on the cycle performance parameters, have been investigated. It is found that the inlet air-cooled gas turbine trigeneration cycle has a good thermal performance, with first and second law efficiencies of 91 per cent and 55 per cent, respectively, for the base case studied (having a maximum cycle temperature of 1327 °C). For a minimum process heat pressure of 4 bar, the energy and exergy efficiencies were obtained as 89 per cent and 51 per cent, respectively. Gas turbine trigeneration plants with and without inlet air cooled by the Brayton refrigeration cycle are compared with each other, and the effect of inlet air cooling on energy and exergy efficiencies is clarified. Inlet air cooling increases the energy and exergy efficiencies and decreases the electrical to thermal energy ratio significantly. Exergy analysis indicates that maximum exergy is destroyed during combustion and steam generation, which represents around 65 per cent of the total exergy destruction in the overall system. The exergy destruction in major components of trigeneration varies significantly with TIT and process heat pressure. The present analysis provides further information on the role of inlet air cooling through extracted mass rate influence on the performance of a gas turbine-based trigeneration system.