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.

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.

Purpose: Europe aims to decarbonize its economy by 2050, which implies a significant deployment of renewables and energy storage technologies. Offshore low-head pumped hydro storage (O-PHS) is presented as an alternative solution for coastal countries with shallow seas and flat topography as a technology for grid-scale energy storage. Methods: We conduct a Life Cycle Assessment (LCA) for the construction, operation, and maintenance stages of an O-PHS plant located in the North Sea, with a rated installed power of 2 GW and an average daily storage capacity of 8 GWh. We further compare O-PHS with conventional pumped hydro storage (C-PHS) in two inland European locations and lithium iron phosphate (LFP) batteries. Due to the location of the O-PHS plant, offshore wind electricity generation is assumed. Although the study focuses on climate change, the results for all 16 environmental impact categories of the European Product Environmental Footprint methodology are provided. Results and discussion: We find that the O-PHS plant’s construction, maintenance, and operation emits around 33 gCO2eq/kWh. When comparing technologies, O-PHS greenhouse gas (GHG) emissions are slightly higher than C-PHS in the Alpine region and LFP batteries. In contrast, C-PHS results in the non-Alpine region are twice as high as the rest of the technology values. From these emissions, we see that the impacts related to electricity storage are roughly the same as those related to electricity generation. In other words, the use of O-PHS technology doubles the emissions from offshore wind farms. Although this may seem a high premium to pay, it becomes a relatively low value when comparing it to the GHG emissions from the electricity mix from surrounding countries like Germany or the Netherlands. On the other hand, the high demand for steel, copper, and magnets, together with efficiency losses, makes turbines a hotspot for the O-PHS plant in all environmental indicators. Conclusion: This article urges engineers working in the O-PHS technology to focus on the turbines, increasing efficiency and considering circularity strategies during the design phase, including lifetime extension and recycling to reduce emissions across all impact categories.

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.

Tidal energy is increasingly recognized as a viable renewable energy source with applications extending beyond electricity generation, including seawater desalination for coastal and off-grid communities. This study investigates a novel direct-driven tidal desalination system that eliminates intermediate electrical conversion by mechanically coupling a horizontal-axis tidal turbine to a high-pressure positive displacement pump and a Seawater Reverse Osmosis system with an integrated energy recovery device. The system converts the mechanical power of the turbine into hydraulic energy, driving pressurized seawater through reverse osmosis membranes to produce freshwater, while the brine is recirculated through the Energy Recovery Device to improve efficiency. A time-domain numerical model simulates the system's performance in the below-rated, variable-speed operational range for two configurations under steady-state and turbulent tidal flow conditions.

Results indicate that the proposed variable displacement configuration maintains a constant tip speed ratio and offers more stable operation. A 140 kW rotor is capable of producing up to 88.3 m3/h of freshwater at rated current speed, with a specific energy consumption of 3.2 kWh/m3. Under turbulent flow, it achieved an 8% increase in freshwater production with respect to the fixed pumped configuration, as well as lower pressure fluctuations while maintaining a constant recovery rate. These findings suggest that active hydraulic control enhances the efficiency, stability, and freshwater output of direct-driven tidal desalination systems. However, practical constraints such as membrane flow limits, cavitation risk, and rotor fatigue at high speeds must be addressed in future design considerations.

This work highlights the potential and challenges of integrating tidal energy directly into desalination processes and provides insights into system optimization under realistic operating conditions.

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.

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.

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.

The increasing development of floating wind turbines has paved the way for exploiting offshore wind resources at locations with greater depth and energy potential. The study presents a novel Subsea Buoyancy Gravity Energy Storage System (SBGESS) that combines buoyancy energy storage and gravity energy storage technologies to overcome the intermittent nature of wind energy. The proposed system is assessed for time-shifting power delivery applications in two Brazilian offshore wind farm sites with varying wind conditions and water depths. The performance of the SBGESS is evaluated by considering different numbers of units, water depths, and control strategies. The results demonstrate that the SBGESS can effectively enhance offshore wind farms’ capacity factor and power output during peak times, particularly in regions with lower wind potential and higher profundity.

Subsea buoyancy gravity energy storage systems (SBGESS) could take advantage of large water depth to store energy in the form of potential energy. In the proposed system, drum hoists mounted on a semisubmerged support structure simultaneously lift concrete cylinders with hundreds of tonnes and lower floaters with equivalent buoyancy force, which can be released with high round trip efficiencies by inverting the motor operation. The present study addresses the potential effect of the vortex-induced vibration (VIV) produced by current velocities on the behaviour of the energy storage modules. In order to analyse the system response, a state-of-the-art VIV model was integrated with a spherical pendulum and tuned with experimental results from the literature. The numerical model allows estimating amplitudes and frequencies of oscillation for a single module in both in-line and cross-flow directions. Results are used to assess the risk between modules of collisions on a previously designed SBGESS.

Underwater noise from offshore pile driving has raised significant concerns over its ecological impact on marine life. To protect the marine environment and maintain the sustainable development of wind energy, strict governmental regulations are imposed. Assessment and mitigation of underwater noise are usually required to ensure that sound levels stay within the noise thresholds. The air-bubble curtain system is one of the most widely applied noise mitigation techniques. This paper presents a multi-physics approach for modeling an air-bubble curtain system in application to offshore pile driving. The complete model consists of four modules: (i) a compressible flow model to account for the transport of compressed air from the offshore vessel to the perforated hose located in the seabed; (ii) a hydrodynamic model for capturing the characteristics of bubble clouds in varying development phases through depth; (iii) an acoustic model for predicting the sound insertion loss of the air-bubble curtain; and (iv) a vibroacoustic model for the prediction of underwater noise from pile driving which is coupled to the acoustic model in (iii) through a boundary integral formulation. The waterborne and soilborne noise transmission paths are examined separately, allowing us to explore the amount of energy channeled through the seabed and through the bubble curtain in the water column. A parametric study is performed to examine the optimal configuration of the double bubble curtain system for various soil conditions and pile configurations. Model predictions are compared with measured data. The model allows for a large number of simulations to examine different configurations of a single bubble curtain and a double big bubble curtain

Wave-to-Wire models play an important role in the development of wave energy converters. They could provide insight into the complete operating process of wave energy converters, from the power absorption stage to the power conversion stage. In order to cover a set of relevant nonlinear effects, wave-to-wire models are predominately established in the time domain. However, the low computational efficiency of time-domain modeling is hindering the extensive application of wave-to-wire models, especially in early-stage design and optimization where a large number of iterations are required. To address this issue, a spectral-domain wave-to-wire model is proposed, and the nonlinear effects are incorporated by stochastic linearization. This model can significantly reduce the computational load and maintain good accuracy. The reference concept studied in this paper is defined as a heaving point absorber coupled with a linear permanent-magnet generator. Four representative nonlinear effects involved in both the hydrodynamic stage and the electrical stage of the concept are considered. The proposed model is verified against a corresponding nonlinear time-domain wave-to-wire model, and a good agreement is observed. The relative error of the proposed spectral-domain wave-to-wire model is around 2 % in typical operational regions and is still within 7 % for wave states with large significant wave heights, regarding the estimate of the power conversion efficiency. Meanwhile, the computational load of the spectral-domain wave-to-wire model is reduced by 2 to 3 orders of magnitudes compared with the conventional time-domain approach. Finally, a case study of tuning the PTO damping to maximize power production is conducted to demonstrate the performance of the proposed spectral-domain wave-to-wire model.

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.

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.

The integration of seawater desalination and wind energy technologies has allowed for the development of a directly wind-driven desalination system, with the potential to address freshwater scarcity issues without contributing to CO2 emissions. The system described in this manuscript consists of a wind turbine rotor, which employs a hydraulic transmission to directly pressurise seawater into a reverse osmosis desalination plant and a Pelton turbine generator. After building and commissioning of a 44 m hydraulic wind turbine prototype in the port of Rotterdam in the Netherlands, an experimental campaign was conducted to evaluate the operational range and performance of the hydraulic system. A combination of hardware-in-the-loop tests where used to get insight into the behaviour of the integrated system. The control philosophies used for automatic operation and safety of the system are compared and discussed, as well as the system's behaviour in response to different wind conditions using dummy elements to replace the desalination module. Technical challenges and achievements of commissioning and testing the system are also described, along with lessons learned.

An adjustable draft point absorber was recently proposed as a novel approach to improve power absorption with constrained power take-off (PTO) capacities. The key feature of the novel wave energy converter (WEC) concept is to adjust the buoy draft by regulating the ballast water inside the buoy, which aims to enable variation of the natural frequency of the WEC. Although previous research has shown benefits for the energy absorption stage, the impact of the draft adjustment on the power conversion efficiency and overall performance has not been examined yet. Therefore, a wave-to-wire model is established to provide an in-depth insight into the systematic performance of the adjustable draft point absorber integrated with a linear permanent magnet generator. Both a nonlinear hydrodynamic model and an analytical generator model are derived, thus the complete process from the wave power input through the whole WEC system to the usable electricity is covered. Based on the established model, wave-to-wire responses of the novel concept are obtained and analyzed. The negative effects of the draft adjustment on the stroke and overlap between the stator and translator are demonstrated. Moreover, a comparison is made between this novel WEC and conventional fixed draft WEC, and both regular and irregular wave states are considered. The results show that the adjustable draft system could increase not only the absorbed power but also the generator conversion efficiency. In specific conditions, the delivered electrical power of the adjustable draft WEC was over 20 % and 10 % higher than a traditional fixed draft system for regular and irregular waves respectively.

The power take-off (PTO) system is a main component in wave energy converters (WECs), and it accounts for a notable proportion in the total cost. Sizing the PTO capacity has been proven to be significant to the cost-effectiveness of WECs. In the numerical modeling, the PTO size is normally represented by a force constraint. Therefore, to accurately evaluate the power performance of WECs with various PTO sizes, it is necessary to take the PTO force limitation, a nonlinear effect, into consideration. In this paper, a computationally-efficient spectral domain model of the PTO force saturation is developed for a heaving point absorber, and the nonlinear term is included by statistical linearization. For comparison, a frequency domain and nonlinear time domain model are implemented, and the developed spectral model is verified with the results of the nonlinear time domain model. Compared with the frequency domain model, the spectral domain model remarkably reduces the relative errors in predicting the power performance of WECs with force constraints, while the computational demand is much lower than the nonlinear time domain model. Furthermore, a case study is conducted to size the PTO capacity for reducing the levelized cost of energy (LCOE) in a chosen wave site. Three different numerical models are applied respectively. The frequency domain model could lead to a misestimate of the optimal PTO capacity, with a maximum relative error on the prediction of the annual energy production (AEP) of 24%. In contrast, the spectral domain model indicates the same optimal PTO size with the time domain modeling, and its relative errors on the prediction of the AEP are within 4.3%.

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.

This article presents a preliminary assessment of a subsea buoyancy and gravity energy storage system (SBGESS). The storage device is designed to power an off-grid subsea water injection system to be installed at the Libra oil field in Brazil at 2000 m below sea level. Two 12MW floating wind turbines provide the energy supply. The system performance is evaluated according to historical wind data from reanalysis models, the water injection pumps power curves, the required daily water flow rate, and the maximum number of shutdowns allowed per year. A control strategy with three different operation modes and one energysave sub-mode was implemented to optimise the size of the proposed energy storage system.

The current focus of offshore wind industry and academia lies on regions with strong winds, neglecting areas with mild resources. Photovoltaics' cost reductions have shown that even mild resources can be harnessed economically, especially where electricity prices are high. Here, we study the technical and economic potential of offshore wind power in Indonesia as an example of mild-resource areas, using bias-corrected ERA5 data, turbine-specific power curves, and a detailed cost model. We show that low-wind-speed turbines could produce up to 6,816 TWh/year, which is 25 times Indonesia's electricity generation in 2018 and 3 times the projected 2050 generation, and up to 166 PWh/year globally. Although not yet competitive against current offshore turbines, low-wind turbines could become a crucial piece of the global climate mitigation effort in regions with vast marine areas and high electricity prices. As low-wind-speed turbines are not yet on the market, we recommend prioritizing their development.