Thermodynamic design and multi-objective optimization of supercritical CO 2 recuperated cycle for gas turbine waste heat recovery

Abstract

Enhancing power plant energy efficiency through waste heat recovery (WHR) bottoming cycles appears to offer great potential in approaching the net-zero targets. This research aims to recover power from flue gases of a 25 MW gas turbine through utilization of a recuperated WHR cycle with supercritical carbon dioxide (sCO₂) as its working fluid. Multi-objective optimization, based on the non-dominant sorting genetic algorithm II technique, was used to maximize the sCO₂ WHR cycle thermal efficiency and output power, simultaneously. Optimization results suggested a recuperated sCO₂ cycle capable of producing 1.3 MW electric power with a thermal efficiency of 16.5% at design condition. Results indicated possibility of generating even 3.2 MW of electricity with a thermal efficiency of 18.1% at the expense of slightly higher stream flow rates at design condition. To evaluate performance of the designed WHR cycle under different operating conditions, sensitivity analyses were conducted, including variations in compressor inlet pressure, temperature and pressure ratio. Topping cycle gas turbine off-design conditions was also investigated. Operating the gas turbine at 90% load resulted in combined cycle efficiency of about 35%, which is accompanied by 9% fuel saving and power plant emissions reductions. Moreover, the prospect of combined cycle thermal efficiency of 33.5% at 70% topping gas turbine load and 25% fuel saving appeared achievable. The estimated reduction in the power plant fuel consumption results in approximately 135 kgCO₂/h emissions reduction from the topping gas turbine flue gases.