Cascade hydropower retrofitting for pumped storage in high-penetration renewable energy systems: Techno-economic perspectives

Retrofitting hydropower plants with pumping stations (HPSH-P), reversible units (HPSH-PT), or new reservoirs (PSH) is critical for boosting grid flexibility and variable renewable energy (VRE) integration. However, comparative multi-scale operational characteristics and techno-economic trade-offs among these pathways remain underexplored. This study develops a scheduling model that captures multi-scale regulation for these retrofitting approaches, driven by a net-load-based time-of-use price curve. The performance of different pathways is further evaluated through techno-economic and sensitivity analyses, considering hydrological inflows, economic parameters, and round-trip efficiency. A Southwest China case study reveals that HPSH-P provides a cost-effective near-term solution for capital-constrained or low-capacity-factor plants. In contrast, HPSH-PT achieves a peak net present value of 7.0 billion CNY by effectively leveraging existing storage and expanded capacity at moderate investment. While PSH delivers superior technical performance, with an overall energy conversion efficiency of 79.8%, a VRE curtailment absorption rate of 9.9%, and an annual greenhouse gas reduction of 10.5 Mt CO2-eq, substantial capital costs for new reservoirs raise its break-even threshold. Notably, all schemes at their optimal capacities exhibit competitive levelized cost of energy (LCOE) below 200 CNY/MWh. These findings provide a systematic framework for transforming conventional hydropower into flexible, low-carbon energy storage assets.