Waste heat recovery of a combined internal combustion engine and inverse brayton cycle for hydrogen and freshwater outputs: 4E optimization and comparison

Abstract

In power plants, internal combustion engines (ICEs) continue to serve as a prevalent source of power generation. Despite advancements in ICE performance over the past decade, significant amounts of energy are still wasted through exhaust gases and jacket cooling water. In this study, advanced waste heat recovery technologies are explored to enhance the overall efficiency of ICE power plants. Initially, an inverse Brayton cycle (IBC) is employed for exhaust gas energy recovery in a 500 kW ICE. Subsequently, waste energy in the heat rejection stage of the IBC and the exhausted gas from the compressor is recovered using a thermoelectric generator (TEG) and an absorption chiller. Additionally, the extra electricity generated by the TEG is directed to a proton exchange membrane electrolyzer and a reverse osmosis desalination unit, producing hydrogen and potable water as additional outputs. Furthermore, energy from the jacket cooling water is recovered in a hot water unit. The 4E (energy, exergy, exergy-economic, and environmental) performance of the two configurations is compared by sensitivity analysis and design optimization. Exergy efficiency and unit cost of product (UCOP) are obtained as 36.09% and 58.303 $/GJ for the stand-alone engine. They are calculated at 39.62% and 64.553 $/GJ and 43.05% and 62.06 $/GJ for Configurations 1 and 2, respectively, in the optimum mode. In this case, Configuration 2 leads to the highest efficiency, and while the UCOP of Configuration 2 is better than that of Configuration 1, the stand-alone engine has the best value of UCOP. This suggests that converting a stand-alone engine to an integrated system with multiple useful outputs is a feasible and beneficial strategy for improving power plant efficiency and sustainability. Furthermore, in the optimum mode and for Configuration 2, improvements of 1.36% and 1.49% are reported for exergy efficiency and UCOP in comparison with the base case mode.