Thermoeconomic and multi-objective optimization of a solar-powered MED-TVC desalination system considering ejector positioning and seasonal energy input

The global scarcity of freshwater resources, exacerbated by climate change and increasing consumption, necessitates the development of efficient and sustainable desalination technologies. This study addresses this challenge by proposing a solar-powered thermal desalination system based on multi-effect distillation integrated with a thermal vapor compression unit (MED-TVC). The required thermal energy is supplied by solar parabolic trough collectors (PTC), which are modeled to reflect seasonal variations in solar radiation. A comprehensive multi-objective optimization framework is developed to evaluate and improve the thermoeconomic performance of the system. Key design variables including the number of effects, top and bottom brine temperatures (TBT and BBT), pinch temperature differences in the heat exchanger and condenser, and the working fluid flow rate in the solar loop are considered to assess system performance across the four seasons. The optimization approach follows a two-stage strategy: first, determining the optimal decision variables for each season, and second, analyzing the influence of ejector positioning within the MED effects during summer conditions. A multi-criteria decision-making method using the TOPSIS technique is applied to resolve trade-offs among three conflicting objectives: gain output ratio (GOR), exergy efficiency, and total system cost rate. Placing the ejector after the fourth effect was determined to provide the best configuration. According to the optimization results, the system achieved a total cost rate of 33.53 $/h, a GOR of 7.691, and an exergy efficiency of 3.268%. The highest exergy destruction in the PTC system, amounting to 75.07% of the total exergy destruction, is followed by the TVC exergy destruction of 13.54%, which represents the highest exergy destruction percentage in the MED-TVC system configuration.