Novel solar-based cogeneration system: Parabolic trough integrating supercritical Brayton and organic Rankine cycles with membrane distillation

This research addresses advancements towards third-generation concentrated solar power (CSP) systems, highlighting the critical need for improved system efficiency through advanced power cycles and optimized waste heat utilisation. By integrating a direct parabolic trough solar collector (PTSC) with a supercritical CO₂ (sCO₂) Brayton cycle and direct contact membrane distillation (DCMD), coupled with a bottoming organic Rankine cycle (ORC), this study proposed a novel solution for the co-generation of power and clean water. Innovative design and optimisation methodologies for the integrated system were introduced, and a comprehensive mathematical model was developed and validated, facilitating a comparative analysis of various ORC fluids' techno-economic performance. The investigation revealed that under baseline conditions, neopentane and isobutane demonstrated slightly better cycle's net-work, whereas toluene showed significantly higher water production due to the higher mass flow rate of seawater needed, resulting from the lower condenser inlet temperature. Furthermore, parametric analysis revealed that varying ambient temperatures resulted in different optimal fluids, with cyclohexane, n-octane, n-nonane, and n-heptane achieving the highest thermal efficiency of around 36.52% at 10°C. Additionally, this study highlighted substantial exergy destruction in the DCMD desalination process, accounting for 63.12% of the exergy destruction in the bottoming cycle, predominantly at the membrane where approximately 67% occurred. Moreover, the second compression and expansion stages, especially at higher Direct Normal Irradiance (DNI) levels, contributed the most to exergetic destruction in the topping cycle. Significantly, the double cycle showed exergetic efficiency improvements between 0.12% and 0.35% across different DNI levels while in contrast, single cycles demonstrated marginally superior water production capacity. Economic analyses using bare module costing to assess the levelized cost of electricity and water, along with net present value calculations, revealed that varying ambient temperatures resulted in different optimised fluids. Notably, n-octane achieved the highest net present value, approximately 4.53% above baseline conditions, at an ambient temperature of 10°C. Finally, recommendations for organic fluids for each ambient temperature based on techno-economic optimisation were detailed.