A Rankine–Kalina combined cycle hybrid system integrated with low-temperature geothermal and solar energy for sustainable hydrogen production: Comprehensive 9E analysis

A new hybrid system model is proposed in which solar energy is integrated into low-temperature geothermal resources, which possess significant global potential but face challenges when directly utilized for energy production. In the proposed system, the low-temperature geothermal resource is used to preheat the thermal oil circulating in the parabolic trough solar collector. This approach allows the geothermal source to contribute to energy production and ensures system continuity during periods of insufficient solar radiation. The proposed hybrid system consists of a combined thermodynamic cycle in which the Rankine and Kalina cycles are used together. The Rankine cycle utilizes the system's primary heat source, whereas the Kalina cycle enhances overall efficiency by recovering the waste heat generated in the Rankine process. The electrical energy generated by the system is utilized for water electrolysis, thereby demonstrating the system's potential for hydrogen production. The entire system is comprehensively evaluated using energy, exergy, environment, economy, exergoeconomic, exergoenvironmental, enviroeconomic, energoeconomic, and energoenvironmental analyses (9E analysis method). The proposed hybrid system demonstrates an energy efficiency of 20.52 % and an exergy efficiency of 22.53 %. The hydrogen production mass flow rate is calculated as 0.05386 kg/s. The levelized cost of energy (LCOE) for the hybrid system was calculated as 0.1133 $/kWh, while the levelized cost of hydrogen (LCOH) was obtained as 7.7 $/kg H2. Based on energy-economic and exergy-economic evaluations, the specific costs of energy and exergy losses in the system are determined as 0.2099 Wh/$ and 0.2263 Wh/$, respectively. The energoeconomic environmental cost and the exergyeconomic environmental cost are found to be 68.79 $/year and 19.79 $/year, respectively. According to the environmental analysis, the system enables 22,283 tCO2/year of avoided emissions and yields a sustainability index of 1.291.