Numerical and Experimental Investigation of Transient Two-phase Flow Phenomena in Concentrated Solar Power Plants with Direct Steam Generation

Abstract

The direct steam generation process in parabolic trough collectors of CSP plants heats and evaporates water to obtain steam that is subsequently superheated to feed a steam turbine. Recently, this process was proven in economic and operational perspectives in the once-through mode to be a promising option for large-scale commercial plants. Nevertheless, knowledge gaps about the two-phase flow could deter the deployment of this type of power plant. The once-through mode implies temperature oscillations at the transition region from two-phase flow to superheated steam that induces thermal stresses in the receiver and could reduce the receiver's lifetime. The uncertainties complicate a reliable economic analysis. The thesis aims to enrich the existing pool of knowledge about the two-phase flow under specific conditions in state-of-the-art parabolic trough facilities with direct steam generation. The objective is to state the relevance of intermittent flows and wavy flows, severe slugging, and flow instabilities regarding the implications at the end of the two-phase flow region by means of advanced numerical and experimental tools. The numerical simulations use the capabilities of a one-dimensional two-fluid model that gives a deeper insight into the two-phase flow compared to the previously used homogeneous equilibrium models. The unique experiments are performed with a single wire-mesh sensor, which measures the local instantaneous void fraction and has been installed in the low-quality region of the DISS test facility at the Plataforma Solar de Almería in Spain for the first time. It is analytically shown that the two-fluid model can identify unstable liquid-gas interfaces in certain conditions that would initiate the transition to a non-stratified flow. The numerical results of intermittent and wavy flow are validated by wire-mesh sensor experiments, suggesting that thermal oscillations at the end of the two-phase flow are linked to the transient two-phase flow pattern in the evaporator. The wire-mesh sensor measures slug flow, roll waves, wavy flow, and stratified-wavy flow in the horizontal receiver pipes within pressures of 30 to 80 bar.In the simulation, severe slugging could not be observed in the U-shaped connection pipes under typical operating conditions - this can mainly be ascribed to high-pressure conditions. The obtained findings enhance the understanding of the evaporation process in the receiver pipes and improve the confidence in this technology. Subsequent research should increase the number of experiments in the low-quality region and extend the measurements towards the high-quality region. These efforts will yield a full collection of reliable and reproducible data for the investigated system that is affected by many and various influencing parameters