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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. [ABSTRACT FROM AUTHOR]

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 19th Wind Integration Workshop 2020. [ABSTRACT FROM AUTHOR]