Thermo-Economic and Exergetic Optimization of a Sub-critical Organic Rankine Cycle for Micro Gas Turbine Exhaust Heat Recovery

Abstract

Micro gas turbines (mGTs) are used in distributed generation and combined heat and power systems, but their post-recuperator exhaust often leaves at 200–300 °C and is released without further use. This remaining heat can be recovered with a subcritical Organic Rankine Cycle (ORC) to produce additional electricity. However, the design is not straightforward because net power output, exergy destruction, and specific investment cost cannot all be improved at the same time. This thesis aims to identify the best-compromise ORC design for a 240 °C mGT ex-haust stream using a three-objective NSGA-II optimisation framework. The ORC model was built for steady-state operation. It calculates the cycle performance and represents the heat recovery unit with separate preheater and evaporator zones. It also includes gas-side pressure-drop losses, exergy destruction, and purchased equipment cost. For each feasible design, it gives Pnet, TED, and SIC. NSGA-II is used to optimise these objectives through seven decision variables, including operating pressures, working-fluid mass flow rate, and heat-exchanger geometry. Four hydrocarbon fluids are analysed: n-pentane, cyclopentane, n-hexane, and n-heptane. The final Pareto solutions are then ranked using TOPSIS. n-Pentane is selected as the best-compromise fluid. It gives a Pnet of 15.96 kW, an SIC of €7,169/kW, and an estimated simple payback of about 2.7 years at 8,000 h/yr under Belgian household electricity tariffs. Cyclopentane is the preferred alternative when minimising TED is more important, giving 22 % lower TED at 87 % of n-pentane’s Pnet. Overall, n-pentane is the stronger choice when cost and power are the main priorities, while cyclopentane is better when thermodynamic quality is prioritised.