Numerical study of parabolic-trough direct steam generation loop in recirculation mode: Characteristics, performance and general operation strategy

• Exergy efficiency profile shapes diversely under various weather conditions.
• Optimal working conditions are with high separation steam quality and high DNI.
• A novel indicator is proposed to solely quantify the flow pattern along the loop.
• A general operation strategy regarding system performance and safety is proposed.
• The general operation strategy is verified by a case study in North China.

The characteristics, performance and general operation strategy of parabolic-trough direct steam generation (DSG) loop in recirculation mode have been numerically studied. The controllable separation steam quality (xsep=0.2–0.8)(xsep=0.2–0.8), which characterizes the two-phase flow, and the corrected direct normal irradiance (DNIcorr=200–1000DNIcorr=200–1000 W m−2), the major weather impact, were chosen as two independent parameters. The results show that for a given DNIcorrDNIcorr, the onset of evaporation is advanced with increasing xsepxsep. The convective heat loss is less or equal to the radiative part at the inlet, then surpasses and finally could reach up to 1.6 times of the radiative part at outlet. The energy efficiency decreases along the loop regardless of DNIcorrDNIcorr or xsepxsep while the exergy efficiency varies diversely according to DNIcorrDNIcorr in the superheating stage. This diverse variation is due to the opposite trends of energy level and heat gain. When DNIcorr<400DNIcorr<400 W m−2, the stratified flow shows an apparently increasing probability to emerge over the loop with decreasing xsepxsep, where the thermal-stress risk may exist. To quantify the probability, a novel flow pattern indicator is proposed. In addition, a general DNIcorr-xsepDNIcorr-xsep-based operation strategy of DSG loop concerning both performance and safety is given, of which the merit is verified through a case study in typical summer and winter days in North China.