Pumped hydropower provides the largest form of grid-scale energy storage. It plays a key role in the integration of variable renewables like wind and solar energy, and contributes to enhancing grid reliability. A great interest exists towards the exploitation of low head differences, particularly in shallow seas and environments with flat topography. This work presents an experimental analysis of the efficiency and operational performance of a 7 kW positive displacement reversible pump-turbine (PD-RPT) designed for low-head hydro applications under steady-state conditions. The PD-RPT features two rotors with three lobes and cycloidal surfaces. The characterisation tests were carried out in the hydraulic laboratory of TU Braunschweig for turbine and pump modes under variable speed conditions. The results highlight experimental peak efficiencies of 74.7% in turbine mode and 74.0% in pump mode for the operational range tested. In addition to challenges in efficiency and operational flexibility, the PD-RPT experiments also showed the need for effective debris management strategies to avoid performance degradation.

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.

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.

Background: The share of renewable energy feeding the European grid has been growing over the years, even though the intermittency of some renewable energy sources can induce electric grid instability. Energy storage has proven to be an effective way of reducing grid instability. Various solutions for large-scale energy storage are being researched nowadays. This study focusses on the innovative low-head pumped hydro storage (LH PHS) technology, a large-scale energy storage scheme suitable for shallow seas (5 – 30 m depth). Implementation of renewable energy technologies, such as wind farms in Europe, Asia and North America, has faced public opposition which has delayed or even cancelled the implementation of renewable energy projects. Literature about public perception of projects highlights the importance of involving stakeholders from the early stages of project planning. Considering this, the present study aims to collect stakeholder opinions (via an online survey) to determine what is necessary for a smooth implementation of LH PHS in the North Sea, both from technical and policy points of view. Results: Stakeholders from commercial parties, government authorities and local groups recognized the potential of LH PHS as a means to increase the share of renewable energies within the European power grid. Economics, bureaucratic burden, and structural safety have emerged as primary aspects of concern respecting the implementation of LH PHS. The impression of the respondents is that a low-head pumped hydro station would not have negative effects on their organizations. Furthermore, most of the engineering firms participating in the study communicated that their knowledge and resources could be involved in the construction of such an energy storage facility. Conclusion: As identified stakeholder concerns such as economics and structural safety are currently being researched, effective communication of the findings of this research is paramount to keep stakeholders informed of the ongoing progress. Two-way communication between researchers and stakeholders is recommended to enhance public acceptance of future technologies. Furthermore, is it advisable to undertake an examination of the available energy policies relevant to LH PHS.

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.