With the rising need for flexible energy storage, recent research shows the potential of contra-rotating reversible pump–turbines (CR RPT) to enable low-head pumped hydropower storage. This study presents a dual variable-speed control architecture for CR RPTs, aimed at providing grid frequency control. The proposed control maximizes the efficiency and shapes the power response to minimize the rise time while averting excessive fluctuations. The control architecture is experimentally validated on a 45 kW reduced-scale CR RPT. The results show that for full reserve activation in frequency containment reserve (FCR), the rise times are <3.92s in turbine mode and <0.23s in pump mode. When scaled to a 10 MW system, with a factor of 1.53 to 2.46, the rise times remain well below the regulatory limit of 30 s. Furthermore, the power response stays within the allowed limits, with root mean square deviations of <58% in turbine mode and <39% in pump mode, relative to the allowed limits. Additionally, the system effectively tracks the varying power setpoints in an actual FCR use case. These findings demonstrate that the proposed control methodology can successfully provide frequency control by dynamically varying the power within imposed power constraints.

Low-head pumped hydro storage technology has been identified as a promising contributor to grid-scale energy storage and the provision of ancillary services. Low-head systems have differing characteristics compared to conventional high-head systems, including larger relative head ranges and increased inertias of both, the water column and the pump–turbines. These differences require new designs as well as a detailed evaluation of their steady-state performance and transient behaviour. For this purpose, an experimental 50kW setup incorporating a 1:22 scale version of a novel reversible pump–turbine, with two contra-rotating runners and independent drivetrains, is designed and constructed. Steady-state performance tests are conducted in turbine and pump modes for several speed ratios between runners. Using head and torque coefficients, the results are compared to a numerical pump–turbine model based on a range of computational fluid dynamics simulations. Additionally, the transient response for a change of operating points is tested and used to benchmark a 1-D numerical model covering dynamic effects including coupling between the conduit and drivetrains. The developed numerical model is then used to simulate the transient behaviour during a shutdown sequence in turbine mode. During the steady-state tests a maximum efficiency of 89% was measured in turbine mode and 92% in pump mode. The test results show that the steady-state RPT characterisation accurately predicts the RPT performance, particularly in turbine mode, with correlation coefficient values between 0.9–0.97. The comparison of the pump mode results shows a minor offset and difference in the correlation between experimental and numerical results. Similarly, the comparison of the transient test case shows a good agreement between the experimental and the simulated dynamic response of the flow rate and rotational speeds. The results have shown the capability of the numerical modelling approach to provide accurate results for steady-state and dynamic performance evaluations. Finally, the simulation of the shutdown sequence indicates that there is no risk of dangerous pressure transients during the desired deceleration of the runners and concurrent closure of the valve.

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.

The energy transition requires large-scale storage to provide long-term supply and short-term grid stability. Though pumped hydro storage is widely used for this purpose, regions without natural topography do not have the potential for traditional high-head pumped hydro storage. To address this, multiple projects for low-head and seawater pumped hydro storage have been proposed, though few have been implemented. Here, we review the state of the art of the components of low-head seawater pumped hydro storage projects, for construction in shallow seas or integrated into coastal defenses. We reference all civil infrastructure components, in addition to legal, environmental/biological, and financial constraints, drawing knowledge from proposed, planned, and constructed tidal power and seawater pumped hydro storage projects worldwide. Combining this knowledge, we make a preliminary evaluation of the feasibility for low-head seawater pumped hydro storage in the North Sea. We find that an elevated storage basin is more economical than an excavated one in shallow bathymetry (10 m deep or less), while the reverse is true in deeper water. Corrosion and fouling prevention are already well developed due to implementation of these measures at tidal power plants. Dam construction is feasible if measures are taken to address piping, macro-instability (primarily from rapid drawdown), and bursting of the clay layer. Within the context of Europe, legal and environmental regulations may be the most formidable hurdles to such projects.