Investigation of Heat Transfer and Entropy Production of High Temperature Molten Chloride Salts Circulation in Concentrating Solar Power Systems

Abstract

The global warming and worsening environment on the earth has been a great concern to human society in the last two decades. The major solution to the problem is to use clean and renewable energies for electricity generation, thus reducing the use of fossil fuels and cutting emission of CO2. One of the most feasible approaches for accomplishing the goal is to significantly increase the harvest and utilization of solar energy, using Concentrating Solar Power (CSP) technology combined with Thermal Energy Storage (TES) systems.Heat transfer fluids (HTFs) are used in CSP plants to receive heat from solar concentrator and then transfer it to heat exchanger, power turbine, or thermal storage system. To achieve higher energy efficiency from the CSP systems, the development of a new generation HTF to approach a higher temperature limit by using eutectic high temperature molten chloride salts is sponsored by the U.S. Department of Energy. The first part of this dissertation is the experimental study of the convective heat transfer of the molten salt at high temperatures. For the first time, a circulation system and instrumentation of flow and heat transfer was designed and constructed to measure the heat transfer coefficient of the . Experimental measurement of the convective heat transfer coefficients of NaCl-KCl-ZnCl2 (molar fraction: 13.8%-41.9%-44.3%) inside tubes has been accomplished to find the most suitable heat transfer correlations. This provides valuable information for the design of heat transfer devices in CSP plants that use molten chloride salts as heat transfer fluid and thermal energy storage material. The second part of this dissertation mainly focuses on the analysis to the transient heat transfer phenomenon between the hot fluid and the cold pipe. Currently, most of the modern concentrated solar thermal power plants employ molten salts as the heat transfer fluid to carry the thermal energy from solar concentrators and deliver to thermal storage systems or thermal power plants for the need of power generation. For the startup operation of solar concentrators, molten salts need to be pumped to flow into the pipes which may have lower temperature than the molten salt due to cold ambient overnight or over the suspend period of operation. As the freezing point of various molten salts ranges from 220 oC to 430 oC, preventing the freezing of molten salt flowing in cold pipe is a very important requirement for the safe operation of a concentrated solar thermal power plant. A basic heat transfer analysis of transient heat exchange between molten salts and the flow pipe is conducted to find a criterion or the critical condition of preventing molten salt from freezing. The effects of molten salt flow velocity, heat capacities of molten salt and pipe, dimensions of pipes, and the initial temperatures of salts and cold pipes are all correlated theoretically in the analysis through modeling of transient heat transfer between a pipe and the fluid. The results are very helpful to the understanding and management of a safe startup of hot molten salt flowing in cold pipes on cyclic operations. The third part of this dissertation introduces details about the modeling that provides a fundamental approach for the comparison of various heat transport systems which may have different designs and using different heat transfer fluids/media (gas, liquid, or solid particles) in CSP systems. For various high temperature heat transfer fluids, such as, synthetic oils, various molten salts, and liquid metals, a general criterion is proposed in this work to evaluate the merit of fluids regarding their transport properties. For the goal of transferring a desired amount of heat, a fluid that causes less entropy production is believed to have better figure of merit (FOM). This is due to the fact that entropy production is associated with the destruction of exergy or useful energy. The entropy production in a heat transfer system in a solar thermal power plant includes the part due to the processes of heat addition and removal and the other part due to pressure losses in the flow in heat exchangers and pipes. Theoretical analysis and relevant equations for total entropy production are derived. As an example, the FOM for several heat transfer fluids used in CSP industry are compared for the goal of heat transport in the range of 50 MWth to 600 MWth. This work offers one very important approach leading to the development and optimization of a heat transport system for CSP plant with all factors considered. The investigations included in this dissertation for the heat transfer and system analysis in concentrating solar power technology are of particular interest to the renewable energy engineering community. It is expected that the proposed methods can provide useful information for engineers and researchers.