Thermodynamic optimization of an integrated gas turbine cycle, heat exchanger and organic Rankine cycle for co-generation of mechanical power and heating load

A thermodynamic evaluation is conducted on a combined heat and power system integrating a gas turbine (GT), a heat exchanger (HX1), and an organic Rankine cycle (ORC). Traditionally, ORC bottoming GT cycle is limited to mechanical power production. The novelty of this study is to recover wasted heat from the GT cycle in multistage, which is used for the simultaneous production of mechanical power and hot water supply. In the first stage, the HX1 recovers heat from the GT cycle compressed air to heat the water stream. In the second stage, the ORC cycle recovers thermal energy from the GT turbine exhaust stream to produce extra mechanical power with the remaining latent heat used to heat the water. Two models are proposed for comparison using ASPEN Plus software linked with the RAFPROP database. The modelled GT, in this study, is adopted from an actual machine. The steady-state results show that the combined system achieves 51.55% thermal efficiency compared with a standalone GT efficiency, which is only 21%. The thermal efficiency is divided into 24% mechanical power and 27.55% thermal load. The output hot water temperature is 65 oC. The outcomes of increasing the GT pressure ratio (12-25) are higher combined cycle net power output by up to 16% with a 9.5% reduction in the thermal energy rejected to the environment. Also, the GT efficiency increases from 20% to 22.5%; however, the final water temperature declines from 67 oC to 60 oC, which is still appropriate for various heating applications.