Critical thresholds of daily average reservoir release for PV integration in coordinated pump-back pumped-storage and associated cascade hydropower system

The rapid expansion of wind and photovoltaic energy has significantly driven the growth of pumped-storage hydropower plants (PSHP). As a prominent PSHP configuration, pump-back PSHP (PSHPPB) utilizes cascade in-stream reservoirs from conventional hydropower plants (HP) by integrating reversible pump-turbines technology. However, this configuration imposes considerable operational pressure on cascade reservoirs, necessitating a delicate balance between fulfilling water supply demands, managing PSHPPB pumping and generation cycles, and maintaining conventional HP output. This study: (1) investigates the coordinated operation of PSHPPB-HP system to enhance photovoltaic (PV) power integration under water supply priorities; and (2) evaluates the optimal PV capacity that can be integrated into the PSHPPB-HP system, accounting for long-term basin-scale water supply variability. A case study of the PSHPPB located in the upper Yellow River Basin of China, utilizing the cascade reservoirs of Longyangxia (LYX) and Laxiwa (LXW) hydropower stations, is presented. Results indicate that: (1) an increase in the daily average release from the LYX reservoir shifts the PSHPPB operation from a pumping-generating mode to a generation-only mode; (2) energy consumption of the PSHPPB decreases to zero as the daily average reservoir release increases, while its power generation follows a distinct dip-and-climb pattern; (3) two critical daily average reservoir release thresholds, approximately 900 m3/s and 1500 m3/s, are identified for optimizing the PV integration in the PSHPPB-HP system; (4) the PSHPPB-HP system can support up to 4141.6 MW of PV capacity (62% of the total installed capacity of the PSHPPB-HP system) with PV curtailment limited to 5%, accounting for long-term water supply variability in the Yellow River Basin. These findings provide valuable insights for PSHPPB planning and operation, not only in the Yellow River Basin but also in other global regions.