4E performance analysis and multi-objective optimization of a power-heat-freshwater trigeneration system based on ammonia-fed protonic ceramic fuel cell, micro gas turbine, and air gap membrane distillation

To address the increasing global demand for clean energy and freshwater, particularly in islands and coastal regions, an innovative power-heat-freshwater trigeneration system integrating an ammonia-fed protonic ceramic fuel cell, a gas turbine, and air gap membrane distillation is proposed. A comprehensive 4E (energy, exergy, economic, and environmental) mathematical analysis model is established based on thermodynamics, electrochemistry, economics, and environmental theories, deriving various performance evaluation indicators and quantifying irreversible losses in components. The system’s reliability, feasibility, and competitiveness are assessed, and the performance of different subsystems, components, and operating modes is compared. Local sensitivity analysis parameters, global input–output correlation evaluation, Sobol-based global sensitivity analysis, and multi-objective optimization using a genetic algorithm are performed on seven key parameters to identify potential performance enhancement pathways. Results indicate that the system’s power output, exergy efficiency, overall energy efficiency, and levelized costs of electricity and freshwater are determined as 122.53 kW, 45.78 %, 94.69 %, 0.082$ kWh⁻¹, and 18.41$ m⁻³. Compared with the NH₃-PCFC/GT subsystem, power output and overall energy efficiency increased by 4.39 % and 49.84 %. Over the system lifecycle, 6461.13 tons of seawater are desalinated, and 3690.90 tons of CO₂ emissions are reduced. Regulating current density or air utilization is more sensitive for improving 4E performance, while adjusting stack inlet temperature, feed temperature or heat exchanger #3 cold end difference is a less sensitive option. Optimization results determined by the decision-making method yield a power output of 148.26 kW and a cost of 12.81$ m⁻³, with corresponding parameter design schemes provided.