I cicli Rankine a fluido organico (ORC) sono una tecnologia che viene notoriamente utilizzata per ottenere lavoro utile da quelle fonti energetiche le cui temperatura e potenza termica sono troppo basse per poterle sfruttare con i convenzionali cicli Rankine: questo è possibile sostituendo l’acqua con un fluido organico, caratterizzato da un’inferiore temperatura di ebollizione che quindi ne consente la vaporizzazione e la conseguente espansione per input termici inferiori. Grazie a ciò, tramite il loro utilizzo è possibile ottenere potenza utile da sorgenti termiche come quelle rinnovabili (ad esempio solare e geotermico), processi industriali in cui flussi termici a bassa temperatura sono presenti così come i gas di scarico dei motori a combustione interna. Inoltre, sistemi ORC possono essere utilizzati per applicazioni cogenerative, in modo da poter fornire all’utilizzatore, oltre all’energia elettrica, anche il calore a bassa temperatura scartato dal ciclo a scopo, ad esempio, di riscaldamento domestico o distrettuale. Quando la potenza richiesta o quella disponibile sono limitate, vengono utilizzati i cicli ORC di taglia micro, caratterizzati da una potenza generata nell’ordine dei 10 kWe i quali, rispetto ai sistemi di taglia maggiore, pongono ulteriori vincoli in termini di scelta del layout d’impianto, tipo di componenti e fluido di lavoro, in quanto i costi di produzione e manutenzione devono essere ridotti il più possibile per consentire all’utente di ammortare le spese sostenute pur con tali valori di potenza generata. Per questo motivo, in tali sistemi è solito l’impiego di componenti non specificatamente progettati per queste applicazioni, come ad esempio compressori volumetrici usati come espansori. È pertanto interessante studiare il comportamento di suddetti componenti per questo utilizzo in condizioni fuori progetto, soprattutto durante i transitori. In questa tesi, lo studio dei sistemi ORC è stato condotto focalizzandosi proprio sugli apparati di taglia micro; un approccio basato sullo sviluppo di simulazioni numeriche dei componenti principali del sistema, come espansore e scambiatori di calore, è stato utile per comprendere quali siano i fenomeni principali che condizionano le prestazioni di un intero ciclo, come ad esempio le pulsazioni di pressione del fluido di lavoro o le sue fughe all’interno dell’espansore. È stato possibile osservare soprattutto quest’ultimo aspetto operando su un sistema prototipale ottimizzato per il recupero di energia dai gas di scarico di motori a combustione interna, adottante macchine di tipo scroll sia come espansore, sia come pompa. Prima di mostrare i risultati numerici relativi all’analisi dei singoli componenti del ciclo, verrà riportato un approccio originale riguardante, tramite l’utilizzo dello strumento della fluidodinamica computazionale, la simulazione di un intero sistema. In questo modo, è possibile capire come il di questi layout o le condizioni operative di un certo componente influenzino il funzionamento degli altri componenti del ciclo. Per prima cosa, si è simulato un sistema idealizzato per verificare la fattibilità dell’approccio, poi una porzione di un sistema reale è stata discretizzata e di questa è stato determinato il campo di moto all’interno dei componenti. In quei casi in cui le risorse computazionali per svolgere questo tipo di simulazioni non siano disponibili, verrà proposto un ulteriore metodo d’analisi in cui le simulazioni fluidodinamiche dei singoli componenti sono state interfacciate fra loro tramite uno script Python, in modo tale da riflettere una variazione delle condizioni al contorno di un certo componente sul funzionamento di quello posto a monte o valle dello stesso.

Organic Rankine Cycle systems (ORCs) are a well-known technology to obtain useful work from energy sources whose temperature and thermal power are too low for being exploited with conventional steam Rankine cycles: this is possible because water is replaced by an organic fluid, which is characterized by a lower ebullition temperature, allowing its vaporization and consequent expansion for smaller thermal inputs. Because of this, ORCs make it possible to generate power from a broad series of heat sources, such as low-temperature, renewable energy ones (as in geothermal and solar fields), industrial processes in which hot streams are present and heat rejected by engines through their flue gases. ORCs can also be used for heat and power applications meaning that, in addition to the electrical energy, also the low-grade heat rejected by the cycle (e.g. at the condenser) is used, for example for district heating or just to warm a single flat or house. When the user requires a limited power output, or when the heat source thermal power is limited, micro-scale ORCs have to be used, which are characterized by an electrical power output in the order of 10 kWe and, with respect to the systems of greater size, pose additional issues in terms of choice of the cycle layout, of the components and of the working fluid since manufacturing and maintenance costs have to be kept the lowest possible to allow for an acceptable payback period. Given this, since a few components are specifically designed to operate in this power range, it is common to use off-the-shelf devices, like compressors used as expanders. Then, it would be useful to study the behavior of these single cycle components and their interaction when part of a micro-ORC system, especially when the thermal conditions vary during cycle operations, such as when the electrical load increases or the temperature of the heat source or the cold one decreases. In this thesis, the study of the ORC technology has been conducted by focusing on those systems and their components which are to be used for power generation in the micro range. An approach based on numerical simulations of the main components of the cycle, as the expander and the heat exchangers, has been of great use to understand what are the main phenomena which affect the performance of an entire system, such as pressure pulsations of the working fluid or its leakages inside the expander. It has been possible to observe this latter issue by working on an experimental micro-ORC test stand which has been operated to test the performances of a prototypal scroll expander: the heat source of the cycle was a hot stream of air, simulating engine flue gases, and the working fluid was circulated in the system by means of a prototype scroll pump. Before showing the results of the numerical analysis of the single devices, an innovative approach of modeling via Computational Fluid Dynamics (CFD) simulations the behavior of an entire system is reported, since it can provide useful information about how the system layout or the working conditions of a given component can affect the operations of the other devices in the cycle. At first, an idealized cycle has been simulated to study the feasibility of the solution and then a portion of an ORC system has been discretized and the flow field characteristics computed. In those cases in which great computational resources are not available, another possibility has been investigated. By connecting the simulations of the single devices by means of a Python script, it is shown the feasibility of an approach in which the different parts of the system are simulated by means of different 3D CFD tools, allowing for different grid and solver set-up for each of these devices, and letting the behavior of a given component of being conveniently reflected on the one operating downstream of it.

Numerical approaches to the study of organic Rankine cycle systems for decentralized applications

RANDI, Saverio
2020-04-15T00:00:00+02:00

Abstract

Organic Rankine Cycle systems (ORCs) are a well-known technology to obtain useful work from energy sources whose temperature and thermal power are too low for being exploited with conventional steam Rankine cycles: this is possible because water is replaced by an organic fluid, which is characterized by a lower ebullition temperature, allowing its vaporization and consequent expansion for smaller thermal inputs. Because of this, ORCs make it possible to generate power from a broad series of heat sources, such as low-temperature, renewable energy ones (as in geothermal and solar fields), industrial processes in which hot streams are present and heat rejected by engines through their flue gases. ORCs can also be used for heat and power applications meaning that, in addition to the electrical energy, also the low-grade heat rejected by the cycle (e.g. at the condenser) is used, for example for district heating or just to warm a single flat or house. When the user requires a limited power output, or when the heat source thermal power is limited, micro-scale ORCs have to be used, which are characterized by an electrical power output in the order of 10 kWe and, with respect to the systems of greater size, pose additional issues in terms of choice of the cycle layout, of the components and of the working fluid since manufacturing and maintenance costs have to be kept the lowest possible to allow for an acceptable payback period. Given this, since a few components are specifically designed to operate in this power range, it is common to use off-the-shelf devices, like compressors used as expanders. Then, it would be useful to study the behavior of these single cycle components and their interaction when part of a micro-ORC system, especially when the thermal conditions vary during cycle operations, such as when the electrical load increases or the temperature of the heat source or the cold one decreases. In this thesis, the study of the ORC technology has been conducted by focusing on those systems and their components which are to be used for power generation in the micro range. An approach based on numerical simulations of the main components of the cycle, as the expander and the heat exchangers, has been of great use to understand what are the main phenomena which affect the performance of an entire system, such as pressure pulsations of the working fluid or its leakages inside the expander. It has been possible to observe this latter issue by working on an experimental micro-ORC test stand which has been operated to test the performances of a prototypal scroll expander: the heat source of the cycle was a hot stream of air, simulating engine flue gases, and the working fluid was circulated in the system by means of a prototype scroll pump. Before showing the results of the numerical analysis of the single devices, an innovative approach of modeling via Computational Fluid Dynamics (CFD) simulations the behavior of an entire system is reported, since it can provide useful information about how the system layout or the working conditions of a given component can affect the operations of the other devices in the cycle. At first, an idealized cycle has been simulated to study the feasibility of the solution and then a portion of an ORC system has been discretized and the flow field characteristics computed. In those cases in which great computational resources are not available, another possibility has been investigated. By connecting the simulations of the single devices by means of a Python script, it is shown the feasibility of an approach in which the different parts of the system are simulated by means of different 3D CFD tools, allowing for different grid and solver set-up for each of these devices, and letting the behavior of a given component of being conveniently reflected on the one operating downstream of it.
PINELLI, Michele
File in questo prodotto:
File Dimensione Formato  
Randi_PhD_Thesis.pdf

accesso aperto

Descrizione: Randi_PhD_Thesis
Tipologia: Tesi di dottorato
Dimensione 12.57 MB
Formato Adobe PDF
12.57 MB Adobe PDF Visualizza/Apri

I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.

Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11392/2488075
 Attenzione

Attenzione! I dati visualizzati non sono stati sottoposti a validazione da parte dell'ateneo

Citazioni
  • ???jsp.display-item.citation.pmc??? ND
  • Scopus ND
  • ???jsp.display-item.citation.isi??? ND
social impact