Direct Steam Generation Methodology in Horizontal Tubes for Parabolic Trough Solar Collectors Designing
Authors: Contreras Ernesto Enciso , Alejo Jesús de la Cruz , Saldaña Juan Gabriel Barbosa , Guillermo Irving Cardel Alcocer , Anastasio José Carlos Alejandro Aguilar 
Published: October 2024

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 user. At the evaporation section, the heat transfer coefficient depends on the flow patterns formation as well as the pressure drop, where a separated flow model is applied for this analysis.

Results and Conclusion: The optimization methodology for a computational tool, that is used for parabolic trough solar collector designing and assessment for direct steam generation system was analyzed. The absorber tube is divided into three segments, the first one for preheating, reaching the water from initial temperature to saturation temperature for the working pressure, the software will find the minimal length for reaching that condition and necessary mass flow is computed. The second step is evaporation, maintaining constant mass flow, evaporation begin when the saturation temperature is reached at the saturated liquid state and finishes at the saturated steam point, and the software find the minimal length for reaching this condition. The most important and complex variable for this section is the heat transfer coefficient, because of the formation of flow patterns, dominating two main phenomena, nucleate boiling and convective boiling, which are determined using the Froude number. Once saturated steam condition is covered, next section overheats the steam and the software finds the minimal length for the final temperature condition dictated by the user, also the quantity of steam produced maintaining constant mass flow condition. Finally, for the two phase flow phenomena carried out at the evaporation zone, the pressure drop is considered, analyzing some methods for frictional pressure drop in two phase flow, Friedel method is recommended due to the applicability and accuracy.

Implications of research: The comprehension of PTC designing with DSG system implies a good knowledge of heat transfer in forced two phase flow into circular tubes, as well as pressure drop using a separated flow model.

Originality/value: Using the methodology described in this work, the software´s applicability and versatility is enhanced, because of the implementation of another PTC collector configuration, generating steam directly, avoiding the use of great heat exchangers and synthetic oils as working fluids, which implies the reduction in the installation size and diminishing the total installation price.