Session: 11-06-01 Heat Transfer in Solar and Renewable Energy Systems - Concentrated Solar Power and Thermal Storage

Paper Number: 67195

Start Time: Monday, 11:25 AM

67195 - Entropy Generation Minimization Optimize Heat Transfer in CSP Technologies Using Molten Salt System NaCl/KCl/MgCl2 as Heat Transfer Fluids 

Concentrated solar power technology has been experiencing development of improving the operation temperature of concentrated solar energy in order to have higher energy conversion efficiency from solar to electrical energy. In the recent years, using eutectic molten salts mixed up by the system of NaCl, KCl, and MgCl2 for high temperature heat transfer fluid have been recognized in the community of concentrated solar power technologies. The molten salts by eutectic NaCl-KCl-MgCl2 or KCl-MgCl2 have the advantages of stability at high temperatures up to 800 ^oC, reasonable transport properties for heat transfer performance and thermal energy storage, as well as compatibility with high temperature alloys for acceptable low corrosion to the pipes, heat exchangers, pumps, and thermal storage tanks. For the heat transfer design, selection of proper flow velocities and pipe diameters is important which should be based on a proper principle and is discussed in this paper. The analysis starts with the objective of minimal entropy production in the heat transfer system caused by heat transfer and pressure loss due to friction between fluids and pipes.  In order to optimize the heat transfer of solar collectors and heat exchangers that use molten salts NaCl-KCl-MgCl2 or KCl-MgCl2, entropy generation minimization principle is used as the criterion. Gnielinski correlation and Dittus-Bölter correlation for Nusselt number versus Reynolds number as well as Moody friction factor developed by Petukhov were used for the calculation of heat transfer 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, pipe diameter. The minimum entropy production could be found due to the first order deviation of the entropy production against Reynolds number, pipe diameter etc., which determines the best Reynolds number and pipe diameter. As the results of the optimization analysis, optimum operational Reynolds number and thereby determined Nusselt number, convective heat transfer coefficient, friction factor, and Stanton number were determined, which helps the optimum selection of flow velocities, diameter of fluid pipes. Optimization analysis was conducted for the temperature range of 500 ^oC to 700 ^oC which covers the operation temperature for super critical CO₂ cycles. Different optimization results from using different heat transfer correlations (Gnielinski correlation and Dittus-Bölter correlation) have been compared and some obvious deviations were identified. This 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 about molten chloride salts heat transfer, Gnielinski correlation for heat transfer is favorably recommended.    

Presenting Author: Fouad Hadad University of Arizona

Authors:

Fouad Hadad University of Arizona
Peiwen Li University of Arizona