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.
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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.

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Large-scale energy storage solutions are crucial to ensure grid stability and reliability in the ongoing energy transition towards a low-carbon, renewable energy based electricity supply. This article presents the evaluation of a novel low-head pumped hydro storage system designed for coastal environments and shallow seas. The proposed system addresses some of the challenges of low-head pumped hydro storage including the need for larger flow rates and reservoirs as well as the requirement of machinery with high efficiencies across a wide operating range to accommodate larger changes in gross head during storage cycles. It includes several units of contra-rotating reversible pump-turbines connected to axial-flux motor generators within a ring dike, as well as dedicated machine- and grid-side control. The technology allows for independent control of each runner, making it possible to adapt to the specific operating conditions of low-head systems. In this work, a numerical approach is used to simulate the system's performance and dynamic behaviour under various operational conditions, including energy generation, storage, and grid support of a 1 GW system with 4 GWh of storage capacity. The potential system performance for energy balancing cycles is evaluated, and a sensitivity analysis is conducted to assess the influence of scaling the motor-generators on performance and footprint of the plant. Additionally, the capability and limitations of the system to respond to grid demand fluctuations and provide frequency regulation services are assessed. The results demonstrate that the low-head pumped hydro storage system is a viable large-scale energy storage solution, capable of round-trip efficiencies above 70% across a wide operating range. By increasing the maximum power of the electric machines, the maximum head range of the whole system is increased which correlates with a threefold increase in energy density per unit area. The dynamic simulations further show that the system can rapidly change its power output allowing it to provide frequency regulation services. Allocating 20% of its nominal power as a reserve, the new power setpoints can be reached within a maximum of 5 s independent of its initial state of charge.

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Recently, contra-rotating reversible pump-turbines (CR RPTs) have been proposed to increase the efficiency of low-head pumped hydropower storage applications, which are promising to provide energy storage for non-mountainous regions. To study the control architectures for these systems, a dual-rotor hardware-in-the-loop (HIL) emulator test-setup is developed. The HIL test-setup employs two induction machines controlled by separate regenerative variable frequency drives to emulate the torques on the two runners. A quasi-steady-state RPT model is developed based on 380 steady-state computational fluid dynamics (CFD) simulations and compared to three transient CFD simulations to analyse the dynamics. Furthermore, the runner torques are adapted to account for the lower friction and higher inertia of the HIL test-setup compared to the prototype CR RPT, ensuring accurate emulation. Finally, it is shown how precise calibration of the drive torque response averts torque errors related to the machine model estimator used in direct torque control. The developed emulator setup offers a cost-effective and controlled environment to optimise and validate control architectures for the novel CR RPT, providing a higher fidelity than theoretical simulation by including the physical effects of the drivetrain, electrical machines and converters that are not entirely captured in mathematical models.

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In an effort to make pumped hydropower storage (PHS) technology feasible for regions with a flat topography, recent research shows promising results using a contra-rotating reversible pump-turbine at low-head. In this study, the impact of dual variable speed and inlet valve control is analyzed to evaluate the effect of these three degrees of freedom (DOFs) on the system efficiency and operating range. To this end, analytical models are described to assess pump-turbine performance, conduit losses and electromechanical losses. Methodologically, optimal efficiency maps are computed for every combination of the three DOFs to evaluate individual and combined effects on the overall efficiency. Furthermore, three energy storage cycles are analyzed to further study the performance in realistic use-cases. Key conclusions include an increase in round-trip efficiency by combining variable speed ratio and inlet valve control of 5.6% and 2.0% compared with only variable speed ratio control and variable inlet valve control, respectively. Furthermore, it is shown that using only 1 DOF significantly limits the operating range, with the addition of a variable inlet valve granting a higher impact than a variable speed ratio. Combining inlet valve and speed ratio control leads not only to the highest efficiency, but also the largest operating range, with a maximum round-trip efficiency of 67.5% and an energy storage capacity of 58.6 Wh/m ². The results confirm that exploiting both dual variable speed operation and inlet valve control yields the maximum efficiency and operating range, and is thus the preferred topology for contra-rotating reversible pump-turbines in low-head operation.

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To provide large-scale energy balancing and ancillary services to grids experiencing rapidly increasing shares of inverter coupled renewable generators, the ALPHEUS project proposes a novel low-head pumped storage system. Aimed at regions where traditional high-head pumped storage systems are not feasible due to topographic constraints, the system consists of a newly designed reversible pump-turbine (RPT), axial-flux motor-generators and a dedicated control and grid coupling. The main aims of ALPHEUS include increased round-trip efficiencies, reduced switching times between pump and turbine mode as well as faster power ramp rates. These directly aid its capability to contribute to grid stability. Aiming to validate simulation results investigating the hydrodynamic performance of the two novel contra-rotating runners making up the RPT as well as the axial-flux motor-generators, an experimental series is planned as part of ALPHEUS. For this, a scaled down version of the RPTs and motor-generators are assembled and connected to two open tanks serving as the upper and lower reservoir. Before the experiments are conducted, preparatory tests are performed to characterise Coulomb and viscous friction in the drivetrain, hydraulic losses in the conduit and other system components. This paper introduces the proposed low-head pumped storage system before presenting the developed experimental setup, approach and the aforementioned characterisation tests.@en

To tackle the growing demand for grid-scale energy storage, the ALPHEUS project proposes a novel low-head pumped hydro storage system aimed for coastal application in countries where the topography does not allow for traditional high-head storage. This system consists of a reversible pump-turbine technology with two contra-rotating runners coupled to their respective axial-flux motor-generators as well as a dedicated control, optimising for energy balancing and the provision of ancillary services. To better understand the integration and dynamic interaction of the individual components of the plant and to allow for the simulation of a wide variety of operating conditions and scenarios, this research aims at developing a system model coupling the hydraulic, mechanical and electrical components. Numerical results are compared and verified based on CFD simulations. While some inaccuracies have to be expected, the comparison shows an overall good match with only minor deviations in dynamic behaviour and steady state results.

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The pan-European power grid is experiencing an increasing penetration of Variable Renewable Energy (VRE). The fluctuating and non-dispatchable nature of VRE hinders them in providing the Ancillary Service (AS) needed for the reliability and stability of the grid. Therefore, Energy Storage Systems (ESS) are needed along the VRE. Among the different ESS, a particularly viable and reliable option is Pumped Hydro Storage (PHS), given its cost-effective implementation and considerable lifespan, in comparison to other technologies. Traditional PHS plants with Francis turbines operate at a high head difference. However, not all regions have the necessary topology to make these plants cost-effective and efficient. Therefore, the ALPHEUS project will introduce low-head PHS for regions with a relatively flat topography. In this paper, a grid-forming controlled converter coupled with low-head PHS that can contribute to the grid stability is introduced, emphasising its ability to provide different AS, especially frequency control, through the provision of fast Frequency Containment Reserve (fFCR) as well as synthetic system inertia. This paper is an extended version of the paper “The Contribution of Low-head Pumped Hydro Storage to a successful Energy Transition”, which was presented at the 19^th Wind Integration Workshop 2020.

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To counteract a potential reduction in grid stability caused by a rapidly growing share of intermittent renewable energy sources within our electrical grids, large scale deployment of energy storage will become indispensable. Pumped hydro storage is widely regarded as the most cost-effective option for this. However, its application is traditionally limited to certain topographic features. Expanding its operating range to low-head scenarios could unlock the potential of widespread deployment in regions where so far it has not yet been feasible. This review aims at giving a multi-disciplinary insight on technologies that are applicable for low-head (2-30 m) pumped hydro storage, in terms of design, grid integration, control, and modelling. A general overview and the historical development of pumped hydro storage are presented and trends for further innovation and a shift towards application in low-head scenarios are identified. Key drivers for future deployment and the technological and economic challenges to do so are discussed. Based on these challenges, technologies in the field of pumped hydro storage are reviewed and specifically analysed regarding their fitness for low-head application. This is done for pump and turbine design and configuration, electric machines and control, as well as modelling. Further aspects regarding grid integration are discussed. Among conventional machines, it is found that, for high-flow low-head application, axial flow pump-turbines with variable speed drives are the most suitable. Machines such as Archimedes screws, counter-rotating and rotary positive displacement reversible pump-turbines have potential to emerge as innovative solutions. Coupled axial flux permanent magnet synchronous motor-generators are the most promising electric machines. To ensure grid stability, grid-forming control alongside bulk energy storage with capabilities of providing synthetic inertia next to other ancillary services are required.

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The pan-European power grid is experiencing an increasing penetration of Variable Renewable Energy (VRE). The fluctuating and non-dispatchable nature of VRE hinders them in providing the Ancillary Service (AS) needed for the reliability and stability of the grid. Today’s grid is reliant on synchronous generators. In case of sudden frequency deviations, the inertia of their rotating masses contributes significantly to the stabilisation of the system. However, as the modern power grid is gravitating towards an inverter-dominated system, these must also be able to replicate this characteristic. Therefore, Energy Storage Systems (ESS) are needed along the VRE. Among the different ESS, Pumped Hydro Storage (PHS) can be identified as particularly convenient, given its cost-effective implementation and considerable lifespan, in comparison to other technologies. PHS is reliant on difference in altitudes, which makes this technology only available if suitable topographic conditions exist. The ALPHEUS project will introduce a low-head PHS for a relatively flat topography. In this paper, a grid-forming controlled inverter coupled with low-head PHS that can contribute to the grid stability is introduced, emphasising its ability to provide different AS, especially frequency control, through the provision of synthetic system inertia, as well as fast Frequency Containment Reserves (fFCR).@en