Thermo-hydraulic performance modeling of thermal energy systems using parabolic trough solar collectors
Tagle Salazar, Pablo Daniel
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Solar energy is one of the most important emerging renewable energy resources. Parabolic trough solar collectors are one of the most used technologies for solar concentrating applications. The main purpose of this research is to develop a mathematical model for predicting thermodynamics and hydraulics of solar-to-heat conversion of thermal systems using parabolic trough collectors. Thermal model is based on energy balance of a one-dimensional steady-state heat transfer thermal resistance circuit. The receiver and its surroundings are considered as the control volume of the analysis. Heat transfer coefficients are obtained using experimental correlations found in the literature. The model considers single-phase and two-phase flow with phase-change effects, where pressure drop is solved simultaneously with thermal energy balance. The input data corresponds to optics properties, design of collector, weather data, and basic hydraulic parameters (for series or parallel configurations). Parabolic trough collectors with Al2O3/water nanofluid is also considered as a case of study. Computational simulations are carried out using Engineering Equation Solver (EES), a software developed to solve complex systems of non-linear equations. This software was selected due to its simplicity in programing systems of non-linear equations and the available database of thermophysical properties of number of substances, including water and steam. Experimental data is used to validate the model, comparing with simulation results. Simulations are realized using same ambient and inlet operational conditions as described in test results. Sources of experimental data are test results of four collectors (for efficiency curves case), a M.Sc. Thesis previously presented (for nanofluid case), and from data provided by Plataforma Solar de Almeria (for direct steam generation case). Results show a good agreement between simulations and experiments. Thermal parameters (such as thermal efficiency and temperatures) are predicted with high accuracy. There was obtained a global absolute error of around 1.5 °C for temperatures and 2% in thermal efficiency. Comparison of temperature and pressure profiles using simulation results and experimental data of a direct steam generation system in once-trough mode show that the model can predict phase-change phenomena with high accuracy. Although, the model fails in predicting pressure drop with high steam quality two-phase flow.