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 multidisciplinary 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.
Despite the increase in computational power of HPC clusters, it is in most cases not possible to include the entire hydraulic system when doing detailed numerical studies of the flow in one of the components in the system. The numerical models are still most often constrained to a small part of the system and the boundary conditions may in many cases be difficult to specify. The headLossPressure boundary condition is developed in the present work for the OpenFOAM open-source CFD code to include the main effects caused by a large hydraulic system onto a component in the system. The main motivation is to provide a boundary condition for incompressible hydraulic systems where known properties are specified by the user and unknown properties are calculated. This paper is a guide to the developed headLossPressure boundary condition. It is based on the extended Bernoulli equation to calculate the kinematic pressure on the patch. An arbitrary number of minor and friction losses are considered to describe the system in terms of head losses. The boundary condition also provides the opportunity to specify the head (difference in height) in relation to a reference elevation. System changes during operations are modelled through Function1 variables, which enables time-varying inputs. The developments are validated against experimental test data, where the varying head between two free surfaces and a valve closing and opening sequence are modelled with the boundary condition. The main effects of the system are well captured by the headLossPressure boundary condition. It is thus a useful and trustworthy boundary condition for incompressible flow simulations of components in a hydraulic system.
Renewable sources of energy are on the rise and will continue to increase the coming decades . A common problem with the renewable energy sources is that they rely on effects which cannot be controlled, for instance the strength of the wind or the intensity of the sunlight. The ALPHEUS Horizon 2020 EU project has the aim to develop a low-head hydraulic pump-turbine which can work as a grid stabilising unit. This work presents numerical results of an initial hub-driven counter-rotating pump-turbine design within ALPHEUS. Computational fluid dynamics simulations are carried out in both prototype and model scale, for pump and turbine modes, and under steady-state and unsteady conditions. The results indicate that the initial design have a hydraulic efficiency of roughly 90 % in both modes and for a wide range of operating conditions. The unsteady simulations reveal a complex flow pattern downstream the two runners and frequency analysis show that the dominating pressure pulsations originates from the rotor dynamics. Given the promising high efficiency, this initial design makes an ideal platform to continue the work to optimise efficiency and transient operations further.
With the rise of renewable energy production in the pan-European grid, the need for flexible energy storage is experiencing a rapid increase. Pumped hydropower storage has proven viability due to its long lifespan and cost-effectiveness. The ALPHEUS project will implement pumped hydropower storage for flat topographies to augment grid stability in adjacent regions. To ensure optimal efficiency and fast switching times in these low head applications, a contra-rotating axial Reversible Pump-Turbine (RPT) is designed. The runners will be driven by two separate Axial-Flux Permanent Magnet Synchronous Motors (AF-PMSM) to ensure optimal efficiency and flexibility at variable speed and flow rate. In this new setup, great attention is needed for the drivetrain architecture. The AF-PMSMs can be placed either outside or inside the water tube, using respectively tube elbows or bulbs. Furthermore, coaxial shafts allow the machines to be placed together, on one side of the RPT. This paper proposes four drivetrain architecture concepts, which are evaluated qualitatively based on their influence on RPT and AF-PMSM performance as well as bearing arrangement.
Pumped Hydropower Storage (PHS) is the maturest and most economically viable technology for storing energy and regulating the electrical grid on a large scale. Due to the growing amount of intermittent renewable energy sources, the necessity of maintaining grid stability increases. Most PHS facilities today require a geographical topology with large differences in elevation. The ALPHEUS H2020 EU project has the aim to develop PHS for flat geographical topologies. The present study was concerned with the initial design of a low-head model counter-rotating pump-turbine. The machine was numerically analysed during the shutdown and startup sequences using computational fluid dynamics. The rotational speed of the individual runners was decreased from the design point to stand-still and increased back to the design point, in both pump and turbine modes. As the rotational speeds were close to zero, the flow field was chaotic, and a large flow separation occurred by the blades of the runners. Rapid load variations on the runner blades and reverse flow were encountered in pump mode as the machine lost the ability to produce head. The loads were less severe in the turbine mode sequence. Frequency analyses revealed that the blade passing frequencies and their linear combinations yielded the strongest pulsations in the system.
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).