The exploitation of low enthalpy geothermal energy through borehole heat exchangers is a system with low environmental impact that is increasingly being applied in Italy and, in particular, in the city of Milan. The focus of the present study is the new Connected Linear Network Package (CLN), implemented in the last version of the modular finite-difference computer code MODFLOW, called MODFLOW-USG, developed by USGS (U.S. Geological Survey) in 2013 (with subsequent enlargements). The new code simulates groundwater flow though aquifers and tightly coupled processes (such as heat and pollutants transport) introducing the possibility of creating unstructured grids, more flexible in their structure than the classic grids used in the previous versions. The CLN package has been developed for incorporating one-dimensional connected features into a three dimensional grid and it is identified as any hydrogeologic or hydrologic water conveyance feature that has a cross-sectional dimension which is much smaller than the longitudinal flow dimension and the size of the encompassing cell. The CLN package may represent wells, pipes, fractures, streams, or other linear features within a simulation domain that need to be represented by flow connections that are separate from those of the aquifer. The study of this new package is here deepened with the aim of adapting it to the numerical representation of borehole heat exchangers, which are pipes with a “U” shape installed underground (depth between 50-200 m) in which flows a fluid able to exchange thermal energy with the surrounding aquifer. So far, the numerical representation of these elements, exploited for the heat exchange in aquifers, was fulfilled in MODFLOW/MT3DMS through a very strong discretization of the modelling domain (Angelotti et al., 2014), which, however, results in a not very rapid method from the computational point of view, numerically unstable and applicable only to a single probe and not to real cases where the borehole heat exchangers are dozens. To understand the methodology to be used to adapt the CLN package to the representation of a borehole heat exchanger, the study starts with the analysis of a preliminary numerical model developed by Panday, mean author of MODFLOW-USG, at the conference 46th IAH Congress - Malaga, Spain 2019. Then it is created a new numerical model characterized by the same hydrogeological, physical and boundary conditions of the model of a vertical borehole heat exchanger developed and validated by Angelotti et al. (2014) through the comparison with analytical solutions (Infinite Line Source, Moving Line Source). The new numerical model uses the CLN package to represent the vertical borehole heat exchanger, its operation and the energetic load imposed. Then the model has been replicated identical to itself, unless for the spatial discretization, different in each of the models on the base of the minimal cell dimension of the grid of the domain, this operation was made in order to investigate the different results with the variation of both the cell size dimension and the new refinement grid method (introduced in MODFLOW-USG and called “quadtree refinement”). The results of the implemented models are compared with the ones generated from the model validated by Angelotti et al. (2014) to verify both the correct functioning of the CLN package within the representation of a vertical borehole heat exchanger and the effects on the results generated from the different spatial discretization and the different method of the grid construction. At last, once the CLN package and the model related were validated, the phase of implementation of the model for a real case as part of the model-MI project supported by Metropolitana Milanese (MM). The CLN package was used for the numerical modeling of a closed loop geothermal plant, made by 60 vertical borehole heat exchangers, in Milan, with the aim of assessing the thermal influence of the plant towards the extraction wells for drinking use of the Armi Station located downstream of it.
Lo sfruttamento dell’energia geotermica a bassa entalpia mediante sonde geotermiche a circuito chiuso è un sistema a basso impatto ambientale che sempre più sta trovando applicazione in Italia ed in particolare nella città di Milano. Questo elaborato di tesi si focalizza sullo studio dell’impatto ambientale e dell’efficienza di tali sistemi mediante la loro modellazione matematica attraverso il nuovo pacchetto Connected Linear Network (CLN) introdotto nell’ultima versione del codice di calcolo alle differenze finite MODFLOW, denominata MODFLOW-USG, sviluppato dall’USGS (U.S. Geological Survey) nel 2013 (con successive integrazioni). Il nuovo codice nasce per simulare il deflusso idrico sotterraneo e processi ad esso connessi (ad esempio trasporto di calore e di contaminanti), introducendo la possibilità di creare griglie non strutturate, più flessibili dal punto di vista strutturale rispetto alle classiche griglie alle differenze finite utilizzate nelle versioni precedenti. Il pacchetto CLN è stato sviluppato per introdurre elementi di connessione monodimensionali in una griglia tridimensionale ed è identificato come un elemento idrogeologico o idrologico di trasporto dell’acqua, caratterizzato da una sezione trasversale molto più piccola rispetto alla dimensione di flusso longitudinale e della cella in cui è collocato. Il pacchetto CLN nasce per rappresentare pozzi, condotte idrauliche, condotti carsici, canali idrici, e qualsiasi altro elemento lineare che necessita di una rappresentazione del flusso differenziata rispetto a quella dell’acquifero. Lo studio di questo nuovo pacchetto viene qui approfondita con lo scopo di adattarlo alla rappresentazione numerica di sonde geotermiche verticali nel sottosuolo, ovvero tubi con forma ad “U” installati nel sottosuolo (con profondità da 50 a 200 m) ed in cui scorre un fluido di lavoro in grado di scambiare energia termica con l’acquifero circostante. La rappresentazione numerica di questi elementi, utilizzati per lo scambio di calore in acquifero, era sino ad ora ben assolta in MODFLOW/MT3DMS attraverso una discretizzazione estremamente spinta del dominio di modellazione (Angelotti et al. 2014), che tuttavia è poco speditiva dal punto di vista computazionale, numericamente instabile ed applicabile ad una singola sonda e non a casi reali dove sono presenti decine di sonde. Per comprendere la metodologia da impiegare per adattare il pacchetto CLN alla rappresentazione di una sonda geotermica si è partiti dall’analisi di un modello numerico preliminare implementato da Panday, principale autore di MODFLOW-USG e presentato nell’ambito del convegno 46th IAH Congress - Malaga, Spagna 2019. Successivamente è stato creato un nuovo modello numerico caratterizzato dalle stesse proprietà idrogeologiche, fisiche e condizioni al contorno proprie del modello numerico di una sonda geotermica verticale validato da Angelotti et al. (2014) attraverso il confronto con soluzioni analitiche (Infinite Line Source, Moving Line Source). Il nuovo modello numerico ha adattato il pacchetto CLN al fine di correttamente riprodurre la sonda geotermica verticale, il suo funzionamento e il carico energetico imposto. Successivamente sono state create diverse versioni al variare della discretizzazione spaziale, con lo scopo di valutare l’influenza sui risultati sia della dimensione delle celle sia di un nuovo metodo di infittimento della griglia (introdotto in MODFLOW-USG e denominato quadtree). I risultati dei modelli implementati sono stati confrontati con i risultati del modello validato da Angelotti et al. (2014) per verificare sia il corretto funzionamento del pacchetto CLN nell’ambito della rappresentazione di una sonda geotermica verticale sia gli effetti sui risultati indotti della diversa discretizzazione spaziale e dei diversi metodi di costruzione della griglia. Una volta validato il pacchetto CLN e il relativo modello, si è passati alla fase di implementazione del modello ad un caso reale nell’ambito del progetto Model-MI finanziato da Metropolitana Milanese. Attraverso il pacchetto CLN è stata realizzata la modellazione numerica di un impianto geotermico a circuito chiuso composto da 60 sonde verticali presente nella città di Milano nei pressi di City Life, con lo scopo di valutare l’influenza termica dell’impianto nei confronti dei pozzi della Centrale idropotabile di Armi posizionata idrogeologicamente a valle.
Adattamento del pacchetto connected linear network per la modellazione numerica di scambiatori geotermici in MODFLOW-USG
Barbieri, Sara
2019/2020
Abstract
The exploitation of low enthalpy geothermal energy through borehole heat exchangers is a system with low environmental impact that is increasingly being applied in Italy and, in particular, in the city of Milan. The focus of the present study is the new Connected Linear Network Package (CLN), implemented in the last version of the modular finite-difference computer code MODFLOW, called MODFLOW-USG, developed by USGS (U.S. Geological Survey) in 2013 (with subsequent enlargements). The new code simulates groundwater flow though aquifers and tightly coupled processes (such as heat and pollutants transport) introducing the possibility of creating unstructured grids, more flexible in their structure than the classic grids used in the previous versions. The CLN package has been developed for incorporating one-dimensional connected features into a three dimensional grid and it is identified as any hydrogeologic or hydrologic water conveyance feature that has a cross-sectional dimension which is much smaller than the longitudinal flow dimension and the size of the encompassing cell. The CLN package may represent wells, pipes, fractures, streams, or other linear features within a simulation domain that need to be represented by flow connections that are separate from those of the aquifer. The study of this new package is here deepened with the aim of adapting it to the numerical representation of borehole heat exchangers, which are pipes with a “U” shape installed underground (depth between 50-200 m) in which flows a fluid able to exchange thermal energy with the surrounding aquifer. So far, the numerical representation of these elements, exploited for the heat exchange in aquifers, was fulfilled in MODFLOW/MT3DMS through a very strong discretization of the modelling domain (Angelotti et al., 2014), which, however, results in a not very rapid method from the computational point of view, numerically unstable and applicable only to a single probe and not to real cases where the borehole heat exchangers are dozens. To understand the methodology to be used to adapt the CLN package to the representation of a borehole heat exchanger, the study starts with the analysis of a preliminary numerical model developed by Panday, mean author of MODFLOW-USG, at the conference 46th IAH Congress - Malaga, Spain 2019. Then it is created a new numerical model characterized by the same hydrogeological, physical and boundary conditions of the model of a vertical borehole heat exchanger developed and validated by Angelotti et al. (2014) through the comparison with analytical solutions (Infinite Line Source, Moving Line Source). The new numerical model uses the CLN package to represent the vertical borehole heat exchanger, its operation and the energetic load imposed. Then the model has been replicated identical to itself, unless for the spatial discretization, different in each of the models on the base of the minimal cell dimension of the grid of the domain, this operation was made in order to investigate the different results with the variation of both the cell size dimension and the new refinement grid method (introduced in MODFLOW-USG and called “quadtree refinement”). The results of the implemented models are compared with the ones generated from the model validated by Angelotti et al. (2014) to verify both the correct functioning of the CLN package within the representation of a vertical borehole heat exchanger and the effects on the results generated from the different spatial discretization and the different method of the grid construction. At last, once the CLN package and the model related were validated, the phase of implementation of the model for a real case as part of the model-MI project supported by Metropolitana Milanese (MM). The CLN package was used for the numerical modeling of a closed loop geothermal plant, made by 60 vertical borehole heat exchangers, in Milan, with the aim of assessing the thermal influence of the plant towards the extraction wells for drinking use of the Armi Station located downstream of it.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/165609