Startup characteristics of long direct-steam-generation loop in parabolic trough solar plant

Concentrating solar energy direct-steam-generation (DSG) technology is promising in kinds of industry fields and power plant. Understanding the thermo-hydraulic behavior during startup is essential to stabilize the system. Therefore, a transient optical-thermo-hydraulic model for a 600-m-long parabolic trough collectors (PTC) DSG loop is established with the finite volume method (FVM). The response behavior of flow boiling in the loop is examined during startup stage. The results indicate that during startup, the flow patterns in the evaporation section transit from intermittent flow to annular flow. The initial pattern of the evaporation is intermittent flow, which subsequently evolves into annular flow. The study reveals critical response characters for key thermo-hydraulic parameters in the DSG loop. The outlet parameters (temperature, velocity, pressure) stabilize after 2363 s, and mass flow rate 1046 s, respectively. The velocity shows three distinct stages during startup, marked by local maxima and minima. The pressure variation closely follows the changes in outlet velocity. Furthermore, the transient dynamic distribution of the preheating, evaporation, and superheating regions was analyzed, with the evaporation zone occupying 66.76 % of the total loop length after stabilization. Following a thermal disturbance in the DSG loop, the redistribution of the HTF's flow patterns always stabilizes sequentially along the flow direction. Additionally, higher irradiance intensifies the speed of flow redistribution. Before and after the thermal disturbance, the change in the proportion of intermittent flow and annular flow within the evaporation section is less than 0.2 %, remaining roughly at 11 % and 89 %, respectively. This research provides valuable insights into the dynamic behavior of DSG systems, which is crucial for optimizing their operational efficiency and reliability.