Thermal and thermodynamic benchmarking of liquid heat transfer fluids in a high concentration ratio parabolic trough solar collector system

Abstract

The thermal oil-based heat transfer fluids (HTFs) used in parabolic trough solar collector (PTSC) systems suffer from degradation at temperatures above 400 °C, limiting the thermal efficiencies of these systems. As such, several researchers have investigated various HTFs for high-temperature applications of PTSCs. In this study, the thermal and thermodynamic performance of a PTSC system with a geometrical concentration ratio of 113 is numerically investigated. The developed and thorough validated numerical model combines Monte-Carlo ray tracing and computational fluid dynamics for optical analysis, and thermal and thermodynamic studies, respectively. Ten HTFs with temperature-dependent thermal physical properties are considered. They are –Liquid metals: liquid sodium, Lead-Bismuth Eutectic (LBE), –Molten salts: Solar Salt, Hitec, Hitec XL, a ternary salt mixture: LiNO3 + NaNO3 + KNO3 (18 wt%, 52 wt%, and 30 wt%), a quaternary salt mixture: NaNO3 + KNO3 + LiNO3 + Ca(NO3)2 (9 wt%, 54 wt%, 18 wt%, and 18 wt%), a new salt mixture: NaCl+KCl + ZnCl2 (7.5 wt%, 23.9 wt%, and 68.6 wt%), and –Thermal oils: Therminol VP-1 and Dowtherm A. Results show that liquid sodium gives the best thermal, hydraulic, and thermodynamic performance of the considered HTFs at all flow rates and inlet temperatures. LBE gives the second highest heat transfer performance, however, its thermal and thermodynamic performance degrade as flow rates increase above 32.75 m3/h owing to the high pumping power and fluid flow irreversibilities. As liquid sodium and LBE are expensive, LiNO3 + NaNO3 + KNO3 (18 wt%, 52 wt%, and 30 wt%) shows better overall performance as compared with other molten salts. Moreover, it possesses a low melting point and high thermal stability temperature. In addition, the thermal efficiency is within ±0.4% for molten salts at flow rates between 16 and 36 m3/h that give optimal performance.