A novel two-phase numerical model for evaluating thermal crisis in the absorber tube of a parabolic trough collector for direct steam generation

In line with the need for further decarbonization of electricity production and to promote the use of renewable energy sources, this article proposes a steady-state quasi 2-D homogeneous equilibrium model for a parabolic trough collector (PTC) intended for steam generation within a Rankine cycle. The thermal behaviour of the collector has been modelled considering that the heat transfer fluid (HTF) enters the absorber as subcooled liquid, reaches saturation temperature, and then undergoes phase change. No models are currently available that can predict the onset of the film dryout phenomenon and accurately simulate heat transfer under this flow regime. In this work the two-phase region has been modelled by assuming an annular flow regime, followed by a possible dispersed droplet flow pattern, to describe the thermal behaviour of the collector even under thermal crisis conditions. Furthermore, the model allows for the determination of the internal characteristics of the absorber to identify the static stability conditions of the collector. The thermal model was validated using experimental data from the Direct Solar Steam Test Facility at the Almeria platform (Spain). A case study of a PTC operating in a direct steam generation (DSG) system has been analyzed, and a parametric analysis was conducted by varying the main operating parameters (Direct Normal Irradiance, HTF mass flow rate, and absorber length) to assess the effect of dryout on the energy performance of the collector. The results show that the efficiency ranges between 72.9 % and 66.5 %, and furthermore, highlight that, within the range of thermal fluxes typical of solar applications, the onset of a thermal crisis does not lead to destructive phenomena. In fact, although the wall temperature increases suddenly, the values reached are contained, allowing the absorber to work safely.