ReHydro [1] is a European research project funded by the Horizon Europe program of the European Commission. Its objective is to demonstrate how existing hydropower plants can be modernized and made more flexible in order to play a leading role in future energy systems, while considering sustainability, climate protection, and societal requirements. Concepts such as hybridization and modernization are intended to improve the adaptability of hydropower to volatile electricity markets. In addition, new fish-friendly turbines and digital tools are being tested. The expected results aim not only to increase efficiency, but also to create up to 700-1150 new jobs and to increase the market value by approximately 275 million Euro. To achieve the desired flexibility of older hydropower plants, the integration of energy storage systems represents a promising approach. It enables both the utilization of existing flexibility in the plant and the use of existing grid infrastructure. Frequency containment reserve (FCR) plays a crucial role in grid stability but leads to numerous small control actions of the turbine. These dynamic load changes can reduce the lifetime of components and increase wear on bearings and moving parts. To enhance flexibility while simultaneously reducing mechanical stress, innovative control strategies are required that effectively limit these effects [2–6]. Within the ReHydro project, a dedicated task addresses how battery storage systems can be optimally integrated into cascades of run-of-river hydropower plants. Previous results from the completed European research project XFLEX Hydro [7] show that integrating a battery significantly reduces turbine control movements [8, 9]. Against this background, the central question arises as to which battery size is optimal for retrofitting existing plants with respect to cost, benefit, and wear reduction. The objective of this work is therefore to develop a methodology that allows determining the optimal battery size for a specific hybrid system and selected use cases. It is essential to distinguish the intended purpose of the battery integration, such as wear reduction, increased flexibility, or provision of additional services. Furthermore, the methodology aims to provide a prediction of how many years the lifetime of the overall system can be extended by using a battery. Based on real operational data, an algorithm is developed that defines the dimensioning of the storage system. The wear of the turbine is evaluated using a finite element method (FEM) model, which is based on control movements and load distributions resulting from the optimization algorithm. The hybrid system consists of a turbine and a battery energy storage system (BESS). An extension of the methodology to cascades of run-of-river power plants is also planned. Therefore, the focus lies on an algorithm that can later be transferred to a full cascade. Within the ReHydro project, three hydropower plants in the Rhine cascade (Vogelgrun, Marckolsheim, and Rhinau) were selected. Since these plants have relatively similar hydraulic characteristics, an additional virtual power plant was introduced in order to investigate the impact of optimization on different turbine dimensions.
