Parametric analysis of combined power and freshwater producing system for natural gas engine heat recovery

Abstract

The present paper focuses on exergoeconomic optimization of a proposed system with the goal of cogenerating freshwater and power, simultaneously. The cogeneration system includes Internal Combustion Engine, Organic Rankine Cycle, and Reverse Osmosis desalination unit. The thermodynamic model of the cogeneration system is investigated from energy, exergy, and exergoeconomic viewpoint. In addition, the variation of some key parameters including the engine speed, turbine inlet pressure, the number of the active vessels, freshwater mass flow rate, and the unit cost of freshwater to obtain optimum working condition is analyzed. The results show that the optimum value of the unit cost of power generation and freshwater occurs when the engine speed is 4000 rpm. Based on the optimization, the optimal condition does not occurred at the speed that exergy destruction becomes minimum. Furthermore the optimum value of the turbine inlet pressure was obtained respect to the maximum exergy efficiency and minimum unit cost of fresh water. The result showed that the optimum value of the turbine inlet pressure is 2690 kPa. There is an optimum value for the number of the active vessels that is achieved when 3 pressure vessels exist in the stage. In addition, the multi-objective optimization of the system based on the Pareto method revealed that at the optimum working condition, the exergy efficiency and freshwater mass flow rate are 30.7% and 2.926 kg/s, respectively. As a final result, it can be stated that the optimal values are not achieved in the minimum exergy destruction.