Anacreonte, Vitaliano Alessandro (2025) Impact of Inlet Fluctuations on the Stratification of a Solar-Coupled Thermocline Thermal Energy Storage: Analysis and Mitigation Approaches. [Tesi di dottorato]

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Abstract

Thermal Energy Storage (TES) systems play a pivotal role in the integration of renewable energy sources, enabling a more flexible and efficient management of thermal energy across a wide range of applications. Among them, stratified sensible heat storages are particularly attractive for solar thermal plants and hybrid energy systems, as they allow the simultaneous coexistence of hot and cold fluid regions within the same volume, maximizing exergy efficiency. However, their real operation often deviates from ideal conditions due to thermal disturbances arising from the fluctuating nature of solar input and from the dynamic interaction with the user side. This doctoral thesis, co-founded by TVP Solar SA and developed in collaboration with the National Research Council (CNR) of Italy, focuses on the impact of temperature oscillations on the inlet of a stratified thermal energy storage. Such oscillations are typically observed when the storage is coupled with solar thermal fields or other intermittent heat sources. While the tank is expected to act as a buffer, mitigating short-term variations, these fluctuations can still affect the thermal stratification, leading to mixing phenomena, degradation of the stored exergy, and a reduction in the overall system performance. The research was developed along three main stages. First, experimental characterization was performed on a real-scale solar industrial installation, to identify the frequency, amplitude, and recurrence of the inlet temperature oscillations under realistic operating conditions. Second, a numerical investigation was conducted to quantify their effects on stratification quality and storage efficiency. Appropriate Performance Indicators (PIs) were defined and applied to capture the degradation mechanisms induced by these disturbances. Finally, two mitigation strategies were proposed and evaluated. The first, of a passive nature, introduces an innovative PCM–TPMS composite insert, developed in collaboration with the University of Connecticut (UCONN), within the diffuser of the tank, acting as a thermal filter to smooth the inlet fluctuations. The second, of an active nature, employs forecasting and Model Predictive Control (MPC) techniques, designed in collaboration with the Technical University of Denmark (DTU), to dynamically adjust the mass flow rate and proactively suppress oscillations in the solar and storage loops. The thesis is organized as follows: • Chapter 1 presents the techno-economic background of the study and positions the problem within the broader context of solar thermal applications and industrial energy management. • Chapter 2 introduces the methodological background, reviewing the state of the art of the modeling and control tools employed throughout the work. • Chapter 3 details the adopted methodology, from experimental data analysis to modeling, simulation, and optimization of the proposed mitigation strategies. • Chapter 4 presents and discusses the results, structured into three main parts: the characterization of the oscillation problem, the design and assessment of the PCM–TPMS diffuser, and the development and testing of the grey-box and MPC-based control framework. Overall, this work aims to contribute to a deeper understanding of the interaction between dynamic boundary conditions and storage behavior, and to propose innovative and practical solutions for enhancing the performance and robustness of stratified thermal energy storages in real-world applications.

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