In this study, a three-dimensional hydrodynamic model is developed to investigate diurnal thermal dynamics induced by pumped hydropower storage operations. At this stage, the focus is on thermal mixing in a generic reservoir, with the aim of providing a methodology that can be adapted to various reservoir scenarios. Key issues include enhancing the understanding of how numerical grid resolution impacts modeling results and demonstrating a method for conducting mesh studies in standing water bodies influenced by flow fields, such as those generated by pumping. The model, being implemented in Delft3D FM, is designed to simulate the upper reservoir of a pumped hydropower plant under initial conditions of thermal stratification. A systematic mesh study was conducted by varying cell sizes in different directions to evaluate their influence on the modeling results. A Richardson analysis shows that the longitudinal resolution, along the main reservoir direction, has minimal impact, while the vertical and the lateral resolutions are critical to avoid thin layers in the mesh and prevent oscillations and numerical inaccuracies. The research demonstrates that pumped hydropower operations alter the thermal regime in the upper reservoir, leading to thinning and temperature fluctuations in the epilimnion, as well as weakening the thickness and strength of the thermocline. Additionally, these operations promote the formation of a large-scale recirculation zone. The adaptable model framework allows for changes in bathymetry, initial stratification conditions, and pumping scenarios, enabling new insights into general temperature dynamics and mixing patterns. 

The increasing integration of fluctuating renewable energy into our energy systems has created a high demand for efficient electricity storage solutions, aiming to meet demand at intermittent supply conditions and maintain grid stability. Pumped hydropower storage (PHS) can be seen as a key solution, already accounting for the majority of the electricity storage today. However, the impact from PHS on reservoir ecology remains poorly explored, with open questions regarding how the physics within the reservoir is affected by pumping. Thermal stratification is a main factor influencing the ecological state of a lake ecosystem. To exemplify, stratification is a decisive factor for both nutrient recycling and deoxygenation. Therefore, this numerical study investigates the influence of PHS on thermal stratification in reservoirs. A 3D hydrodynamic model of a generic PHS reservoir is used to test how different types of stratification phenology are disrupted by a diurnal pumping cycle. The study demonstrates that thermocline phenology may influence the ecological effects of PHS operations by determining the extent of vertical mixing and thereby nutrient redistribution and oxygen availability. Scenarios with a well-established thermocline exhibit different responses to pumping compared to weakly stratified conditions, leading to variations in thermal structure and potential biological impacts. This adds to the comparability of existing case studies and, moreover, improves understanding of the thermal dynamics in hydropower reservoirs, necessary to optimize PHS operations as to the status of the ecosystem and enhance water quality management.