Close Supercritical Versus Inverse Brayton Cycles for Power Supply, Using Waste of a Biogas-Driven Open Brayton Cycle

Power generation via a biogas-driven Brayton cycle (BC) can be regarded as the best scenario for electricity supply of decentralized complexes or small communities. However, the central problem associated with such technology is the high temperature of its exhaust gases, which can be recovered via appropriate waste heat elimination schemes. Although various studies have previously discussed optimal operating conditions of the enhanced biogas-driven BC in terms of thermodynamics and economic, no comprehensive investigation in terms of selecting the best bottoming cycle for the biogas-driven BC has been carried out up to yet. This spurs the current investigation to recommend the it best bottoming cycle between a close supercritical BC (CSBC) and an inverse BC (IBC) for waste heat recovering of a biogas-driven BC around the optimum point. Another novelty of the present study is the inclusion of the environment index (EI) along with energy, exergy, and economic metrics in the performed multi-objective optimization scheme, resulting in the design of a highly sustainable energy system. The results indicated that no single optimal solution exists in selecting the best bottoming cycle by accounting energy, exergy, exergoeconomic, and exergoenvironmental metrics, simultaneously. Hence, a trade-off should be deliberated in selecting the best case in the design process. Accordingly, the integrated BC/CSBC system is superior to the BC/IBC system in terms of thermodynamics (i.e., both energy and exergy metrics) around both base and optimal design points; however, it is not commendable in terms of economic and exergoenvironmental viewpoints. Quantitatively speaking, selecting the BC/CSBC system can lead to thermal and exergetic performance enhancement of around 3.3%, while degrading economic and exergoenvironmental metrics around 7.2% and 8.3%, respectively.