Effect of transient concentrated solar flux profile on the absorber surface for direct steam generation in the parabolic trough solar collector

Abstract

The parabolic trough solar collector with Direct Steam Generation (DSG) has the potential to bring down the electricity cost. The viability of DSG technology is proved using various experimental demonstrations (e.g., DISS test facility) and running commercial-scale power plants (Thai Solar One). However, the DSG technology is in the nascent stage and needs improvement in the solar collector's components and control mechanism. The thermo-hydrodynamic instability induced in the absorber tube due to the non-uniform concentrated solar flux and flow boiling. This study investigates the effects of transient solar flux profiles on the thermal-hydraulics of flow boiling in the DSG process using CFD software ANSYS Fluent. The Eulerian two-fluid model is developed using conservation equations, turbulence model, and phase change model. The concentrated solar flux profiles are considered corresponding to various times of the day from 12:00 h (solar noon) to 17:00 h at an interval of 1 h. The three dimensional (3-D) numerical investigations are performed for DNI of 750 W/m2 and 1000 W/m2; operating pressures 60 bar and 100 bar; mass flow rates 0.4 kg/s and 0.6 kg/s. The flow regimes under the considered boundary conditions are observed as stratified flow. The vapor volume fractions at the absorber outlet exist between 0.30 and 0.45 for DNI of 750 W/m2 and 0.35 to 0.50 for DNI of 1000 W/m2. The absorber surface temperature has been calculated for operation at various times of the day, and it stands between 550 K and 656 K. Further, the circumferential temperature distributions and temperature contours have been plotted for various operating conditions. The adopted multiphase model shows very good performance and is useful for designing and optimizing solar collectors for DSG.