Performance evaluation of a variable-speed contra-rotating pump-turbine for low-head pumped hydro energy storage: An experimental study

• Novel variable-speed contra-rotating pump-turbine for low-head pumped hydro storage. • Maximum pump and turbine mode efficiencies at 88.6% and 86.1%, respectively. • Pump and turbine mode efficiencies over 80% across multiple operating conditions. • Turbine mode power output between 13.0 kW and 15.4 kW for efficiencies above 80 %. • High power density compared to similar machines, simplifying engineering requirements. Pumped Hydro Energy Storage (PHS) provides over 90% of the global long-duration energy storage capacity, yet many regions lack the steep terrain required for conventional high-head PHS. Low-head pumped hydro energy storage (LH PHS) systems address this gap in flat topographic regions but requires efficient pump-turbine technology for operation at variable low heads. This study investigates the use of a variable-speed contra-rotating pump-turbine (CR RPT) for LH PHS applications, presenting experimental results from a model-scale test rig stablished at Technische Universität Braunschweig. This test rig uses two open water surface tanks to provide head, unlike conventional hydraulic test rigs that use pumps. The CR RPT achieved hydraulic efficiencies over 80% for a wide range of operating conditions, peaking at 86.1% and 88.4% for pump and turbine modes, respectively. Additionally, dimensionless analysis revealed that the CR-RPT occupies a unique place in the market and that it achieves the largest power density among comparable hydraulic machines, facilitating greater power output and thus streamlining mechanical and civil engineering requirements for LH PHS.