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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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.

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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

Many sea dikes along the coast of the North Sea are protected against wave loading and currents by riprap revetments that are grouted with mortar. The mortar bonds the individual stones of the top layer, thereby forming a coherent structure that is able to withstand normal forces and shear forces as well as momentums, thus leading to a planar load distribution. While this kind of revetment has been built for decades, its design is yet solely based on empirical knowledge. On the one hand, the current design practice of mortar-grouted riprap revetments may therefore potentially lead to an uneconomic design exceeding the load and safety criterion for a particular site. On the other hand, it is also possible that the current design practice will lead to a weaker revetment than is required for the load and safety criterion for a particular site. Therefore, the objective of the project "Wave Toad and Stability of Mortar-Grouted Riprap Revetments" is to derive a scientific basis for the design of mortar-grouted riprap revetments. In order to describe the structural integrity of mortar-grouted riprap revetments, the results of an assessment of the condition of revetments in the field and a literature research are used to describe mechanisms leading to damage of the revetments. For the mechanism "crack development in the top layer" a structural model is set up in order to describe the load and resistance in the limit state. The hydraulic load due to wave action was measured duringfull-scale model tests in the Targe Wave Flume in Hannover, Germany. The resistance and structural parameters were determined using mechanical and fracture mechanical tests with the individual components as well as with the compound material of mortar and stone. Furthermore, pull-out tests to determine the force for debonding of an individual stone were carried out in the field and under laboratory conditions. The models for the structural stability of mortar-grouted riprap revetments presented in this study describe the processes relevant for designing a mortar-grouted riprap revetment, namely "crack development in the top layer" and "debonding of an individual stone". For the functional dimensioning, the wave run-up height was determined and reduction coefficients for the EurOtop wave run-up formula have been established.

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