The start-up energy consumption of parabolic trough concentrated solar power(PTCSP) systems is high due to the characteristics of their daily start-up and shut-down processes. To reduce this energy consumption

quasi-steady state models for a 50 MW PTCSP are established

and a daily optimal operating mode is proposed by replacing the start-up and shut-down processes with low-load operation. The critical shut-down time is derived theoretically to determine whether the optimal operating mode could reduce energy consumption. The results show that

the daily accumulated electricity production of a PTCSP can be improved by 16.3～26.3 MW·h with the optimal operating mode when the electricity production decreases from 25% to 10% of the rated production under low-load condition. When the initial energy storage duration in the thermal energy storage system increases from 0 to 3.0 h

the daily accumulated electricity production of the PTCSP system can be improved by 7.2～8.1 MW·h.Additionally

the critical shut-down time of the PTCSP system is 14.0 h when the load is 15% of the rated electricity production under low-load operating conditions. The start-up energy consumption of the PTCSP system with the optimal operating mode can be reduced when the practical shut-down time is shorter than the critical shut-down time. The annual electricity production of the PTCSP system can be increased by 0.9% with the optimal operating mode.