Thermodynamic optimisation of ORC systems using hybrid renewable energy sources and nanofluid-zeotropic mixtures

Abstract

This study examines the thermodynamic performance of a hybrid solar–biomass Organic Rankine Cycle system, focusing on the selection of the working fluid and the integration of nanoparticle-enhanced zeotropic mixtures. The work is motivated by the need for decentralised renewable energy systems in regions such as sub-Saharan Africa, where solar potential is high but inconsistent, and biomass provides a reliable complementary resource. A simulation framework was developed using ASPEN Plus V14, with the Peng–Robinson Equation of state used to estimate thermophysical properties. The hybrid heat source configuration combines evacuated-tube solar collectors with a biomass-fired boiler, stabilised by a buffer tank to ensure a consistent thermal input to the ORC evaporator. System performance was assessed under solar-dominant, balanced, and biomass-dominant scenarios, with operating temperatures ranging from 120 to 200 °C. Two sets of zeotropic mixtures were evaluated; transitional refrigerants (R245fa/R123, R245fa/R1233zd(E), and R1233zd(E)/R123) and newer low-GWP alternatives (R1224yd(Z)/R1233zd(E), R1224yd(Z)/R1336mzz(Z), and R1233zd(E)/R1336mzz(Z)). Performance metrics included thermal efficiency, net power output, and heat absorption, supported by system-level indicators such as condenser load and specific work. Nanoparticles of Ag, Al₂O₃, CuO, TiO₂, and ZnO were introduced at varying concentrations to examine their effect on thermal conductivity and cycle stability. Results show that zeotropic mixtures significantly improve thermal matching in heat exchangers through temperature glide, thereby reducing irreversibility and enhancing cycle efficiency. Among the tested fluids, R1233zd(E)/R123 and R1224yd(Z)/R1233zd(E) demonstrated the best performance, delivering higher net power and efficiency across a range of conditions. Ag nanoparticles provided the most substantial enhancement, achieving up to a 16% efficiency improvement at optimal concentrations; however, higher mass fractions introduced viscosity penalties and stability concerns. Oxide nanoparticles yielded modest but more stable performance. The findings demonstrate the technical feasibility of hybrid solar–biomass ORC systems using advanced fluid formulations. While the study is limited to thermodynamic modelling, it provides a reproducible foundation for future experimental validation and techno-economic assessments. The work contributes to ongoing efforts to develop sustainable, decentralised energy systems suitable for regions with variable renewable resources.