Low-Head Pumped Hydro Storage with Contra-Rotating Pump-Turbines

Abstract

As the energy transition gains momentum globally, energy storage systems are becoming increasingly critical to balance intermittent energy sources such as solar and wind. Traditional pumped hydro storage has historically played a dominant role in energy storage due to its cost effectiveness, efficiency, and long lifecycle. However, its reliance on high head applications limits its deployment to regions with specific topographical features. With an increasing demand for the provision of energy balancing and ancillary services, novel approaches are being developed to adapt pumped storage technologies for applications in regions where high-head installations are not feasible, including areas with limited elevation change or coastal regions. This study sets out to explore the potential of one such novel approach, low-head pumped hydro storage.

This work employs both experimental and numerical methodologies to evaluate the technical capabilities of low-head pumped hydro storage to provide energy balancing and frequency regulation services. Initially, a review is carried out, assessing different technologies including pump-turbine designs, electric machines, control and grid integration approaches for their applicability to low-head applications. Aside from this review, the majority of the work is centered around a new system concept developed by the ALPHEUS research project. The project proposes the use of contra-rotating reversible pump-turbines with adjustable speeds. These units are designed to handle the unique demands of low-head applications by using two independently controlled runners to achieve high efficiencies across a broad operating range. The integration of axial-flux permanent magnet synchronous machines allows for high torques at low speeds, making them well-suited for the large volumes of water required in low-head systems. By combining these novel pump-turbines and electric machines with state-of-the-art control systems, the major goals of the project are improved roundtrip efficiencies as well as fast power ramp rates and mode switching times.

To evaluate the technical potential of this technology and low-head pumped hydro storage as a whole, a medium-fidelity numerical system model has been developed. It integrates and couples a pump-turbine model with the dynamics of the hydraulic, mechanical, electrical and control subsystems. This model introduces a novel approach by incorporating the interaction between the two independent runners and their coupling with the other system components. A defining feature of this model is its flexibility and computational efficiency, enabling accurate simulations of various operational scenarios and dynamic responses without the high resource demands of high-fidelity computational fluid dynamics simulations. The individual model components are specifically tailored to the unique challenges of low-head applications, such as the increased inertia of the water column, which increases the risk of significant pressure transients. By incorporating the two coupled independent runners, as well as tailoring the approach to the demands of low-head systems, this model fills a critical gap in literature, providing a robust tool for advancing the development of low-head pumped hydro storage.

To ensure the accuracy of the numerical model results, experiments are conducted with a 50 kW laboratory setup, featuring a 1:22 scale version of the reversible pump turbine. The experimental campaign serves to evaluate the performance of the newly designed pump-turbine, explore the effects of utilising two independent runners and, crucially, benchmark the model. The setup is noteworthy for its gravity-fed design, aiding to create realistic in- and outflow conditions, reducing the risk of swirl and pressure pulsations occurring. These are typically induced by pumps used to create the required head for testing. Both steady-state and dynamic tests are performed in turbine and pump modes and compared to their corresponding numerical results. The benchmarking tests show that a medium-fidelity numerical model can effectively integrate the performance of two independent runners with the hydraulic and mechanical subsystems, capturing with sufficient accuracy the steady-state and dynamic behaviour as well as the interaction of the different subsystems of low-head pumped hydro storage.

Finally, the model is applied to a hypothetical grid-scale plant in the North Sea, providing, for the first time, detailed insights into the potential performance and operational capabilities of an integrated low-head pumped hydro storage plant. The results showed that the system could achieve a roundtrip efficiency of 73% during energy balancing as well as rapid power ramp rates suitable for providing frequency containment reserves. Sensitivity analyses further highlighted the potential for optimising the reservoir footprint and power input/output by scaling the electric machines.

Future work should focus on optimising energy management strategies, further refining control systems as well as detailed economic and environmental assessments. All of which can be aided by the developed numerical modelling approach. If a largescale demonstration confirm its viability, low-head pumped hydro storage could play a transformative role in stabilising renewable energy dominated grids. This innovative approach may thus become a crucial component in advancing the global energy transition.