Beyond fixed-speed pumped storage: A comprehensive evaluation of different flexible pumped storage technologies in energy systems

Traditional fixed-speed pumped storage (PS) has been a reliable measure to provide power system flexibility. However, the increasing need for flexibility of power systems due to adverse environmental events and amplified utilization of renewable sources necessitates the need for modernization of PS units. This paper studies the effect of five PS technologies with different operational flexibility in Qinghai Province's energy system. Firstly, PS models considering the difference in minimum stable generation & pumping, unit efficiency, configurations of turbine & pump are established. Then, the energy mixes, planning capacity, inflow, reservoir operation curve and other data are input into the energy system model. Finally, we quantify the effects of different PS forms on the energy system based on technical, economic, and emission indicators, also identify the optimal PS considering cost preference and emission preference. Results show that the investment costs for various pumped storage technologies depend on the motor type and pump capacity. Fixed-speed pump turbine technologies have a lower cumulative power variation of up to 11,225 GWh and the largest number of start-ups in pumping conditions, at 10,857 times. Increasing operational flexibility stabilizes the reservoir water level, and enhancing pump capacity reduces net load fluctuations for ternary PS and variable-speed quaternary PS. Improving wind-solar curtailment is limited by pump capacity, generating flexibility, and reservoir water level. Systems equipped with fixed-speed PS technologies emit more than carbon emissions other PS technologies. Our research outcomes have the potential to provide valuable insights, enabling stakeholders to optimize the deployment of pumped storage flexibility in low-carbon energy systems, taking into account the energy mix and hydrological conditions that are specific to each region.