Direct Steam Generation Methodology in Horizontal Tubes for Parabolic Trough Solar Collectors Designing

Purpose: The present work has the objective to describe the methodology that is been carried out for the optimization of a computational tool for Parabolic Trough Solar Collector (PTC) designing and assessment, the optimization is centered in a new section where de solar collector produces steam directly, named Direct Steam Generation (DSG). Theoretical reference: A PTC collector consist of a sheet with parabolic cross section with a high reflectance surface, at the focus of the parabola is placed a tube whose material must have high thermal conductivity, this tube must be recovered by a selective coating for high solar radiation absorptivity and less emissivity. Surrounding the absorber, a coating transparent tube is placed concentric with high solar radiation transmissivity value. Into the absorber tube, the heat transfer fluid gains energy due to the concentration of the solar radiation giving as a result, a temperature increase at the outlet of the solar collector. Inside the absorber tube, the working fluid considered in this work is water, due to the objective is to produce steam directly. At the first and third section of the tube, preheating and overheating respectively, a full developed single phase flow is considered and the heat transfer depend on the Reynolds number, Prandtl number, the geometry and conductivity of the tube, related on Dittus-Boelter equation. The mayor analysis is located in the evaporation section, because of the flow patterns formation, depending directly on the mass flow and the steam quality, the Froude number indicates the flow pattern formed during the evaporation process, being the stratified and annular flow patterns the most significant. Method: The first version of the computational tool has some limitations: heat transfer fluid doesn´t have phase change, it means, liquid state is maintained, the parabola rim angle is 90° and only one concentric coating tube is considered. The software’s optimization is centered on the PTC collector designing and assessment for direct steam generation system (DSG). For this purpose, the absorber tube is divided into three sections: first for preheating, where water enters to ambient temperature and at the end reaches saturation temperature, at second section, the evaporation process begins with the water in saturated liquid state and ends to saturated steam state and at the third section, steam is superheated reaching the temperature´s condition imposed by the use