The increasingly share of renewable power plants within the current energy scenario keeps challenging the electric distribution system, due to the intermittent nature itself of these non-programmable sources. Large-scale energy storage systems (ESS) are of paramount importance within the energy transition since they can guarantee the safe spreading of renewable power plants, ensuring grid stability. Among ESS devices, Vanadium redox flow batteries are one of the best candidates to achieve this objective thanks to their numerous advantages. They are characterized by a net distinction between rated energy and power, high round trip efficiency, fast response time and long lifetime expectancy. However, their commercialization is still hindered by the low power and energy density, and the high costs associated to the electrolytes and the membrane. This work of thesis focuses on water transport, an unwanted phenomenon causing electrolyte imbalance. The first part is dedicated to the description and the analysis of various the transport mechanisms, highlighting the relative impact that they have on the overall net movement of water. The second part is relative to the behavior of the battery under diffusion conditions, thus when the electric circuit is disconnected, and the only aspect that can be observed is the flux of all species moving due to differences in concentration between the positive and negative electrolytes. This kind of operative procedure allowed to shed light on osmosis, a phenomenon on which scientific literature is quite inaccurate and discordant. Finally, the last part of this work studies the impact that water transport has on the cycling operations of the battery. A model was developed for the prediction of the volume variation in time, and it was used to understand how water transport is shared among the transport phenomena. In this last section, a mitigation strategy is proposed and validated during the experimental campaign, proving that an accurate electrolyte preparation is the key to control this complex phenomenon.
Il crescente impatto che le energie rinnovabili hanno assunto all’interno dell’attuale scenario energetico, ha messo fortemente a rischio il sistema di distribuzione elettrico a causa della natura intermittente di queste fonti. I dispositivi di accumulo su larga scala sono il mezzo più efficace per consentire lo sviluppo di impianti rinnovabili, garantendo allo stesso tempo la stabilità della rete elettrica. Tra tutte le tecnologie di accumulo, emergono le batterie al flusso di Vanadio, le quali vantano numerosi vantaggi, come il disaccoppiamento tra potenza ed energia nominale, un’alta efficienza energetica, un breve tempo di risposta e una lunga vita utile. Purtroppo, la loro piena commercializzazione è ancora ostacolata dalla bassa densità di potenza e di energia, e dagli elevati costi della membrana e degli elettroliti. Questa tesi si focalizza sul trasporto d’acqua, un fenomeno indesiderato che porta allo sbilanciamento elettrolitico. La prima parte è dedicata all’introduzione e alla descrizione dei meccanismi di trasporto. Nella stessa, verrà evidenziata la relativa importanza di ciascun fenomeno sullo sbilanciamento globale che viene a verificarsi. La seconda parte si concentra invece sul comportamento della batteria in diffusione, ovvero quando il circuito elettrico è sconnesso al fine di osservare il trasporto di specie dovuto principalmente alla differenza di concentrazione tra i due elettroliti. Questa procedura operativa ha permesso di far luce sulla natura dell’osmosi, un fenomeno su cui la letteratura scientifica è piuttosto approssimativa e discordante. Infine, l’ultima parte di questo lavoro è dedicata allo studio del trasporto d’acqua durante le convenzionali modalità operative della batteria. A questo proposito, è stato sviluppato un modello in grado di simulare la variazione di volume degli elettroliti nel tempo, scindendo la quantità totale di acqua spostata nei suoi vari contributi. In quest’ultima fase, una strategia di mitigazione è stata studiata e validata sperimentalmente per dimostrare che un accurata preparazione delle soluzioni elettrolitiche iniziali consente di minimizzare sostanzialmente l’entità di questo complesso fenomeno.
Experimental and modeling analysis of the water transport mechanisms occurring in vanadium redox flow batteries
NOVACO, SILVIA
2020/2021
Abstract
The increasingly share of renewable power plants within the current energy scenario keeps challenging the electric distribution system, due to the intermittent nature itself of these non-programmable sources. Large-scale energy storage systems (ESS) are of paramount importance within the energy transition since they can guarantee the safe spreading of renewable power plants, ensuring grid stability. Among ESS devices, Vanadium redox flow batteries are one of the best candidates to achieve this objective thanks to their numerous advantages. They are characterized by a net distinction between rated energy and power, high round trip efficiency, fast response time and long lifetime expectancy. However, their commercialization is still hindered by the low power and energy density, and the high costs associated to the electrolytes and the membrane. This work of thesis focuses on water transport, an unwanted phenomenon causing electrolyte imbalance. The first part is dedicated to the description and the analysis of various the transport mechanisms, highlighting the relative impact that they have on the overall net movement of water. The second part is relative to the behavior of the battery under diffusion conditions, thus when the electric circuit is disconnected, and the only aspect that can be observed is the flux of all species moving due to differences in concentration between the positive and negative electrolytes. This kind of operative procedure allowed to shed light on osmosis, a phenomenon on which scientific literature is quite inaccurate and discordant. Finally, the last part of this work studies the impact that water transport has on the cycling operations of the battery. A model was developed for the prediction of the volume variation in time, and it was used to understand how water transport is shared among the transport phenomena. In this last section, a mitigation strategy is proposed and validated during the experimental campaign, proving that an accurate electrolyte preparation is the key to control this complex phenomenon.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/183531