Experimental study on the heat transfer characteristics of a low melting point salt in a parabolic trough solar collector system

• A low melting point molten salt was applied in CSP systems.
• Experiment indicates a low risk of freezing and plugging.
• The results show the proportion of the thermal loss at the joints.
• Total heat transfer coefficient of the water-to-salt heat exchanger was obtained.

An experimental system of parabolic trough solar collector and heat transfer was set up with a new molten salt employed as the heat transfer medium (with a melting point of 86 °C and a working temperature upper limit of 550 °C). The circulation of molten salts in the system took place over 1000 h. Experiments were conducted to obtain the heat loss of the Heat Collector Element (HCE), the total heat transfer coefficient of the water-to-salt heat exchanger, and the convective heat transfer coefficients for the low melting point molten salt in a circular tube. The results show that the thermal loss of the tested HCE is higher than that of the PTR70, and the thermal loss at the joints of the collector tube represents about 5% of the total loss in the entire tube. The total heat transfer coefficient of the water-to-salt heat exchanger was between 600 and 1200 W/(m2·k) in the ranges of 10,000 < Re < 21,000 and 9.5 < Pr < 12.2. The experimental data show good agreement with existing well-known correlations presented by the Sieder-Tate equation and the Gnielinski equation. This experimental study on heat loss from molten salt flow in a receiver tube will hopefully serve as a helpful reference for applications in parabolic trough systems.