A novel modified biogas-driven electricity/cooling cogeneration system using open-and-closed Brayton cycle concepts: Environmental Analysis and Optimization

Abstract

This study investigates and describes a novel method for using biogas to run a cogeneration system that generates electricity/cooling and cools with recovered heat from liquefied natural gas (LNG). The investigated setup comprises an open-loop Brayton cycle fed by biogas, a close-loop Brayton cycle (CLBC), an LNG open power generation cycle, and a dual-stage combined cooling and power (CCP) unit composed of an organic Rankine cycle (ORC) integrated with an ejector refrigeration cycle (ERC). The performance of the system is scrutinized from many perspectives, such as the ones of energy, exergy, exergoeconomic, CO2 emmision rate, payback, and multi-objective optimization. It is deduced that TGT1 is the most influential decision variable from exergy and cost outlooks, while energy efficiency undergoes considerable changes with variation in Teva. Based on optimization, the developed unit generated 424.1 kW of cooling and 1,864 kW of net electricity, corresponding to 80.4% energy efficiency, 41.24% exergy efficiency, and 10.07 $/GJ for the unit’s overall product cost. In base and optimum design modes, the combustion chamber (1058 kW) and condenser (1023 kW) are responsible for the high levels of total exergy destruction. At a selling price of 0.27 $/kWh for cooling and 0.06 $/kWh for electricity, payback period equals 12.13 years. Furthermore, it is also expected that over the course of 20 years, the system can earn 3.5 million dollars in profits. Based on environmental aspect, the cogeneration system emits 6091 kg/ MWh of CO2, which decreased to 3913 kg/MWh after optimization.

Keywords

• Biogas
• Thermodynamic analysis
• Economic analysis
• CO2 emission
• Genetic algorithm
• Sensitivity index analysis