Comparative thermo-hydraulic, energy, and exergy performance analysis of aromatic and silicone-based heat transfer fluids in a parabolic trough solar collector

In this study, the thermo-hydraulic performance of Therminol VP-1, Therminol 66, Dowtherm G, Dowtherm RP, and Syltherm 800 heat transfer fluids commonly used in parabolic trough solar collector (PTC) systems was comparatively investigated. A numerical model developed in Engineering Equation Solver (EES) was used to examine how fluid thermophysical properties and operating conditions affect flow, heat transfer, and system performance. The effects of mass flow rate, inlet temperature, solar beam radiation, and receiver length were evaluated. Increasing mass flow rate enhanced convective heat transfer but reduced outlet temperature by shortening fluid residence time. The mass flow rates corresponding to maximum exergy efficiency were 0.40, 0.65, 1.05, 1.25, and 1.75 kg/s for Therminol VP-1, Dowtherm G, Dowtherm RP, Therminol 66, and Syltherm 800, respectively, with Therminol VP-1 reaching 26.39%. Increasing inlet temperature reduced heat gain and thermal efficiency while increasing exergy efficiency. Higher solar beam radiation increased heat gain and exergy efficiency, whereas thermal efficiency decreased beyond a certain level. Increasing receiver length enhanced heat transfer and heat gain, but thermal losses became increasingly significant. The maximum exergy efficiency was achieved at a receiver length of 50 m for Therminol VP-1 and 60 m for Syltherm 800. For the other fluids, the maximum exergy efficiency was attained at the receiver length at which the fluid outlet temperature reached its maximum bulk temperature. The highest exergy efficiency of 27.25% was achieved using Therminol VP-1. Overall, enhanced convective heat transfer did not necessarily result in higher system efficiency because increased thermal losses at elevated temperatures limited performance gains. Therminol VP-1 exhibited the highest exergy efficiency across the investigated operating conditions, although the relative performance of the fluids varied with operating conditions. These findings demonstrate the importance of considering both fluid thermophysical properties and operating conditions when selecting HTFs for PTC systems.