Summary Hybrid nanofluids exhibit enhanced thermal properties compared to their base fluid which makes them more suitable as heat transfer fluids for various applications including solar thermal energy conversion. In the present study, ethylene glycol‐based ternary nanofluid composed of Al2O3, ZnO and multiwall carbon nanotube nanoparticles is synthesized and analyzed for its thermophysical properties such as viscosity and thermal conductivity for different volume concentrations. Thermal conductivity was determined for a temperature range of 25 to 40°C and viscosity was measured for a temperature range of 40 to 70°C. When the volume concentration of nanoparticles for ternary nanofluid was increased from 0.01% to 0.05%, an enhancement in thermal conductivity along with a reduction in viscosity was observed. For a maximum volume concentration of 0.05%, the thermal conductivity was enhanced by 26.6% and 11.2% at a temperature of 25 and 40°C, respectively. Percentage reduction in viscosity was observed at 19.46% and 13.86% for 40 and 70°C, respectively for the volume concentration. Thus, the study suggests that the ternary nanofluid can be used as heat transfer fluid for medium‐temperature heating and cooling applications.

Summary The performance of renewable energy systems can be significantly improved if power is produced from low‐temperature heat sources. The organic Rankine cycle (ORC) is suitable for producing power from low‐temperature heat sources. ORC finds applications alongside solar power, geothermal power, waste heat and biomass. ORC‐enabled systems can contribute significantly to raising the effectiveness of renewable power sources. Solar power, for example, can deliver affordable electricity to remote locations and small industries. By employing ORC, power generation is possible at lower temperatures on cloudy days. Its main difference from the steam Rankine cycle is its working fluid. The ORC makes use of an organic, high molecular mass fluid as the working fluid. Compared to steam Rankine cycles, ORC works at a lower temperature (<250°C) and pressure. The only working fluid in the steam Rankine cycle is water, but the ORC can operate on hundreds of different working fluids. The design and performance of ORC systems are entirely dependent upon the suitable working fluid and, hence identification of the working fluid for ORCs is of utmost importance for diverse applications. Properties of working fluid have a considerable impact on the efficiency of the system. The main objective of this study is to identify the most suitable organic fluids having characteristics necessary for use in solar ORCs. In addition, a detailed review of the various types of heat transfer fluids now available in the market that are often used in ORC systems was conducted. This research assists in selecting the most appropriate organic fluids for various solar ORC applications in terms of operating circumstances.