A prospective assessment of scale effects of energy conversion in ultra-low-head pumped hydro energy storage units

•Bidirectional scale effects of energy conversion in ultra-low-head PHES units are evaluated.•A comprehensive understanding of the technical and economic aspects is presented.•Pump mode is key to restricting the limit size for safe and stable bidirectional operation.•Controlling partial flow separation is the most fundamental way to improve technicality.•Optimal generalized economic scheme is recommended for ultra-low-head PHES units. Ultra-low-head pumped hydro energy storage (PHES) is an attractive solution to the intermittency of sustainable energy in lowland countries and regions. For the development of large-scale ultra-low-head PHES units, tubular pump-turbine is the core equipment, but a comprehensive understanding of the technical and economic aspects of its scale effects of energy conversion is still lacking. In this paper, taking the East Route of the South-to-North Water Diversion Project in China as a prospective case, a comprehensive technical assessment of scale effects of energy conversion in ultra-low-head PHES units is conducted, and then a generalized economic analysis considering both benefits and stability is performed. The following valuable results are obtained: (1) For energy performance, special scale effects are observed in the head, power and hydraulic efficiency in both pump and turbine modes, suggesting more attention should be paid to the energy conversion similarity failure induced by the variations of geometric scale. (2) For hydraulic stability, the rotor force moment and pressure fluctuations increase with the geometric scale in both pump and turbine modes, while the pump mode plays a crucial role in restricting the limit size for safe bidirectional operation. (3) For internal flows, partial flow separation is observed in the guide-vane region in pump mode and in the draft tube in turbine mode, respectively. The fundamental path to improving project technicality lies in controlling the energetic vortices. (4) For generalized economy, the adverse effects caused by the deterioration of stability with the increase of geometric scale must be considered, and the 7 ∼ 9 m runner size is recommended for ultra-low-head PHES units from the perspective of optimal generalized revenue. These findings can provide a comprehensive guidance for the selection, optimization and development of ultra-low-head PHES units with different capacities, and can help the decision-makers effectively avoid technical and economic problems caused by the sc