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.

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.

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).