Advanced solar–geothermal polygeneration system for CO2-based power, hydrogen, and freshwater recovery via transcritical CO2 rankine cycle

The use of carbon dioxide as a high-performance working-fluid in advanced thermodynamic cycles provides a compelling route for developing low-carbon, multi-output renewable-energy systems. The study develops and assesses an advanced hybrid solar–geothermal polygeneration facility designed to produce electricity, hydrogen, and freshwater under the real resource conditions of the Harrat Rahat geothermal zone in Saudi Arabia. The configuration combines a double-flash geothermal cycle with a Transcritical CO2 Rankine cycle, a Kalina cycle, an alkaline electrolyser, and a reverse-osmosis desalination unit, supported by parabolic trough solar thermal augmentation. A full 3E+S evaluation—covering energy, exergy, economic, and sustainability metrics—is carried out alongside multi-objective optimization using the Secretary Bird metaheuristic algorithm. Under the real resource inputs of the Harrat Rahat site—geothermal reservoir temperatures exceeding 220 °C and mean solar irradiance of ∼6.6 kWh m−2 day−1, the results show the system could deliver 3.65 MW of net electricity, 9.35 kg.h−1 of hydrogen, and 10.23 m3.h−1 of freshwater, with overall energy and exergy efficiencies of 42.7 % and 38.18 %. Optimization enhances exergy efficiency by about 1.54 % and lowers the levelized cost of energy by roughly 2.2 %, yielding an LCOE of 0.04039 USD/MJ and a sustainability index of 0.238. Exergy-destruction profiling shows that condensers (≈47 %) and the solar thermal subsystem (≈16 %) are the main contributors to irreversibility. Overall, the results indicate that integrating high-enthalpy geothermal resources with concentrated solar power and advanced thermodynamic cycles can deliver a robust, efficient, and economically competitive polygeneration pathway suited to arid regions with strong energy and water needs.