ENTROPY GENERATION MINIMIZATION TO OPTIMIZE HEAT TRANSFER IN CSP TECHNOLOGIES USING MOLTEN SALT SYSTEM NaCl/KCl/MgCl2 AS HEAT TRANSFER FLUIDS

In order to optimize the flow and heat transfer of solar collectors and heat exchangers that use molten salts NaCl-KClMgCl2 or KCl-MgCl2, entropy generation minimization principle is used as the criterion. Gnielinski correlation and Dittus-Bolter correlation for Nusselt number versus Reynolds number as well as Moody friction factor given by Petukhov were used for the calculation of heat transfer coefficient and pressure loss due to friction. The objective function, the entropy production of the heat transfer system, was expressed as the function of Reynolds number, Prandtl number, heating flux, pipe diameter, etc. The minimum entropy production could be found from the first order deviation of the entropy production versus Reynolds number. As a result of the analysis, the optimum Reynolds number was determined and thereby determined the optimum Nusselt number, convective heat transfer coefficient, friction factor, and tube diameter, which helps the selection of optimum flow velocities and diameter of fluid pipes. The analysis was conducted in the fluid temperature range of 550 degrees C to 700 degrees C which covers the operation temperature for supercritical CO2 cycles. Results from using different heat transfer correlations (Gnielinski correlation and Dittus-Bolter correlation) have been compared and some obvious deviations were identified. It indicates that the selection of heat transfer correlations can cause some deviation of the optimal parameters of Reynolds number and pipe diameter. From the researcher's experience and experimental studies to molten chloride salts heat transfer, Gnielinski correlation for heat transfer is favorably recommended.