Superstructure Optimization of Absorption Chillers integrated with a Large Internal Combustion Engine for Intake Air Conditioning

Abstract

This work aims to propose a thermal design for a chilled water system that provides cooling effects and dehumidification process in the intake air, therefore, allowing engine operation under reduced knocking restrictions which implicates in performance enhancement. The reduction of electrical energy demand at the radiator is also investigated as second benefit. The main contribution is to apply superstructure optimization methodology by considering several options of absorption chiller systems integrated to one Wärtsilä 20V34SG engine of 8.7 MW. The superstructure modelling and optimization are carried out in the Engineering Equation Solver (EES) software and two distinct Objective Functions are explored separately. Firstly, it is minimized the chilled water specific cost, and posteriorly, the gross profit is maximized. Furthermore, a detailed exergy analysis is developed for each thermal equipment that composes the optimized thermal system. Finally, an economic feasibility study is also developed. Regarding the main results, the optimal solution in order to reach the maximum profit of 30.7 US$ h −1 is a single-effect configuration with high solution heat exchanger in its structure and recovering totally the engine’s cooling water while preheating it at the exhaust gases heat exchanger. For the minimum chilled water specific cost of 311.7 US$ ton−1 , the best configuration is a half-effect chiller with no solution heat exchangers. The exergy analysis allows to comment that the single-effect chiller concentrates more total exergy losses in the dissipative heat exchangers (29.56%) than the half-effect chiller (22.08%). Notwithstanding, there is more concentration of irreversibility in the half-effect chiller (67.09%) than in the single-effect chiller (48.24%). The exergy efficiency is higher in single-effect chiller, presenting a value of 0.222 against 0.108 of exergy efficiency in half-effect chiller. The engine under a repowered shaft power output of 10.2 MW presents calculated energy efficiency of 45.26% against 44.87% for the rated condition of 8.7 MW. Moreover, the engine exergy efficiency is increased from 43.27% to 43.64%. This repowering application gives a LCOE of 19.2 US$ MWh−1 whereas a commercial ORC system under similar conditions presents a LCOE of 57.15 US$ MWh−1 . The economic indicators present promising profitable scenario for the studied engine, demonstrating a maximum payback period of 3 years.