Thermoeconomic Evaluation of a High-Performance Solar Biogas Polygeneration System

Abstract

Because of the higher efficiencies achieved by polygeneration systems compared with conventional generation systems, they have been increasingly adopted to reduce the consumption of resources and consequent environmental damage. Heat dissipated by equipment can be harnessed and reused in a cascade manner. This study applies the Theory of Exergetic Cost (TEC), a thermoeconomic approach, to a high-performance polygeneration system. The system includes a biogas-fueled internal combustion engine, a water–ammonia absorption refrigeration system driven by the engine’s exhaust gases, and a set of photovoltaic panels with a cooling system coupled to solar panels and a hot water storage tank. The pieces of equipment are dimensioned and selected according to the energy demands of a hotel. Then, the temperature, pressure, and energy flows are established for each point of the system. Mass, energy, and exergy balances are developed to determine exergy flows and efficiencies. The main component in terms of exergy and operation costs is the engine, which consumes 0.0613 kg/s of biogas, produces 376.80 kW of electricity, and provides thermal energy for the refrigeration system (101.57 kW) and the hot water tank (232.55 kW), considering the average operating regime throughout the day. The levelized costs are 2.69 USD/h for electricity, 1.70 USD/h for hot water (thermal energy tank), and 1.73 USD/h for chilled water (absorption chiller). The thermoeconomic diagnosis indicated that the hot water tank and the engine are the most sensitive to changes in the maintenance factor. Reducing operating expenses by 20% for the tank and engine lowers energy costs by 10.75% for the tank and 9.81% for the engine.