Coupling of 3D thermal with 1D thermohydraulic model for single-phase flows of direct steam generation in parabolic-trough solar collectors

Abstract

Estimating 3D temperature distribution around the heat collector element (HCE) is an essential matter for the safety and efficiency of parabolic-trough solar collectors (PTC) due to non-uniform heat flux distribution (NUHFD). Particularly in 2nd and 3rd generation PTC systems, such as direct steam generation (DSG), because of the higher thermal stress around the absorber. 3D HCE thermal models linked to a 1D approach of fluid flow appear as the fastest alternative to study this phenomenon. However, there are still challenges such as the thermal coupling between HCE and heat transfer fluid (HTF) for accurately estimating the receiver thermal field. In this work, the coupling is studied in depth for single-phase flows of DSG in PTC, with a fast and realistic 3D HCE − 1D HTF model, which is validated with experimental data of the Direct Steam Solar (DISS) facility for heat transfer and absorber thermal field variables. Firstly, the inability of standard heat transfer coefficient (HTC) correlations to predict absorber thermal field is constated, being deviations greater than −36.1% for absorber cross-sections (CS) maximum thermal gradients. Assuming underestimated and constant thermophysical properties of the absorber, as former models did, underestimations of more than −29.6% are reported. Besides, the heat transfer variables are overestimated by around 14.3–15.9% for temperatures above 600 K. Secondly, a new correction factor (CF) for standard HTC is proposed involving the NUHFD. Considering this CF and the thermal dependence of the absorber thermophysical properties, deviations fall below 7.5% for heat transfer variables, whilst the absorber CS maximum thermal gradients are under 6.0%. This approach also improves the results reported by former models around 4.3–7.3% for heat transfer variables, without underestimating absorber CS maximum thermal gradients. These deviations are between 4.9–19.1% guaranteeing the safety of the absorber when the model is used during the design stage.