Nowadays buildings energy efficiency is characterized by several factors. One of these is the heating/cooling system of the building, and in particular the type of heat generator. Heating/cooling systems with high energy efficiency and low pollutant emissions are one of the main subjects in the building sector of today, and they are often promoted by government incentives, derived from guidelines and standards for the improvement of buildings energy efficiency. One of them is Ground Source Heat Pump system: heating and cooling are provided by a machine which uses the ground as a cold source or a hot sink, in alternative to the most common Air Source Heat Pumps. The nearly constant ground temperature at certain depths during all the year leads to high performances of the heat pump, so that this can be considered a high energy efficiency system. There are several typologies of ground heat exchangers. One of them is the vertical ground loop heat exchanger: one or more boreholes are realized in the ground, and a closed circuit of pipes in which a thermal-vector fluid flows is entered here. According to the type of ground and the building heating/cooling loads, these boreholes should have a certain length. One of the main tasks of the designer is to determine this length. There are several methods to do it. In this thesis two sizing methods are described: the first is called “ASHRAE method”. It is an analytical procedure proposed by ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers), and described also by the Italian Standard UNI 11466. The second method is the software GLHEPRO, created by the Oklahoma State University. In this thesis the two methods are widely described and two sizing examples, obtained by using both the procedures, are illustrated. According to the fact that the sizing of this type of system leads often to some uncertainties, due to the difficulty of determining the ground thermal-physical parameters, the two methods and the results of the two examples are compared. The purpose of this thesis is to understand the reason of the found differences and, also with the help of literature, to define the pros and cons of the two methods. In addition, for one of the two examples, an economic comparison with traditional heating/cooling systems is made, to determine the convenience of this type of solution with the parameter of the “payback time” of the initial investment.
Al giorno d’oggi l’efficienza energetica degli edifici è caratterizzata da diversi fattori. Uno di questi è l’impianto di riscaldamento/raffrescamento dell’edificio, e in particolare la tipologia di generatore di calore. Gli impianti di riscaldamento/raffrescamento con un’alta efficienza energetica e basse emissioni inquinanti sono uno dei principali argomenti nel settore edilizio odierno. Spesso essi sono promossi da incentivi statali, derivati da linee guida e leggi per il miglioramento dell’efficienza energetica degli edifici. Uno di questi impianti è rappresentato dai sistemi geotermici a pompa di calore: il riscaldamento e il raffrescamento sono forniti da una macchina che utilizza il terreno come sorgente fredda o come pozzo caldo, in alternativa alle più comuni pompe di calore a sorgente aria. La temperatura del terreno pressoché costante a certe profondità durante tutto l’anno porta ad alte prestazioni della pompa di calore, così che questo tipo di impianto può essere considerato ad alta efficienza energetica. Ci sono diverse tipologie di scambiatori di calore con il terreno. Una di queste è lo scambiatore verticale a circuito chiuso: si realizzano uno o più fori nel terreno, e in essi si inserisce un circuito chiuso di tubi nei quali scorre un fluido termo-vettore. In base al tipo di terreno e ai carichi termici dell’edificio, questi scambiatori devono avere una determinata lunghezza. Uno dei principali compiti del progettista è quello di determinare questa lunghezza. Ci sono diversi modi per fare ciò. In questa tesi vengono descritti due metodi: il primo si chiama “metodo ASHRAE”, ed è una procedura analitica proposta dall’ASHRAE (Società Americana di Ingegneri per il Riscaldamento, il Raffrescamento e il Condizionamento d’aria), descritta anche dalla norma italiana UNI 11466. Il secondo metodo è costituito dal software GLHEPRO, creato dall’Oklahoma State University. In questa tesi i due metodi sono accuratamente descritti e si illustrano due esempi di dimensionamento, ottenuti utilizzando entrambe le procedure. Partendo dal fatto che questo tipo di dimensionamento spesso porta ad alcune incertezze, dovute alla difficoltà di determinare i parametri termo fisici del terreno, si confrontano i due metodi e i risultati dei due esempi. Lo scopo di questa tesi è di capire il motivo delle differenze riscontrate e, anche con l’aiuto della letteratura, di definire i pro e i contro dei due metodi. Inoltre, per uno dei due esempi si effettua un confronto economico con sistemi di riscaldamento/raffrescamento tradizionali, per determinare la convenienza di questo tipo di soluzione attraverso il parametro del “tempo di ritorno” dell’investimento iniziale.
The sizing of borehole heat exchangers for ground source heat pumps : a comparison between two methods
STAITI, MATTEO
2012/2013
Abstract
Nowadays buildings energy efficiency is characterized by several factors. One of these is the heating/cooling system of the building, and in particular the type of heat generator. Heating/cooling systems with high energy efficiency and low pollutant emissions are one of the main subjects in the building sector of today, and they are often promoted by government incentives, derived from guidelines and standards for the improvement of buildings energy efficiency. One of them is Ground Source Heat Pump system: heating and cooling are provided by a machine which uses the ground as a cold source or a hot sink, in alternative to the most common Air Source Heat Pumps. The nearly constant ground temperature at certain depths during all the year leads to high performances of the heat pump, so that this can be considered a high energy efficiency system. There are several typologies of ground heat exchangers. One of them is the vertical ground loop heat exchanger: one or more boreholes are realized in the ground, and a closed circuit of pipes in which a thermal-vector fluid flows is entered here. According to the type of ground and the building heating/cooling loads, these boreholes should have a certain length. One of the main tasks of the designer is to determine this length. There are several methods to do it. In this thesis two sizing methods are described: the first is called “ASHRAE method”. It is an analytical procedure proposed by ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers), and described also by the Italian Standard UNI 11466. The second method is the software GLHEPRO, created by the Oklahoma State University. In this thesis the two methods are widely described and two sizing examples, obtained by using both the procedures, are illustrated. According to the fact that the sizing of this type of system leads often to some uncertainties, due to the difficulty of determining the ground thermal-physical parameters, the two methods and the results of the two examples are compared. The purpose of this thesis is to understand the reason of the found differences and, also with the help of literature, to define the pros and cons of the two methods. In addition, for one of the two examples, an economic comparison with traditional heating/cooling systems is made, to determine the convenience of this type of solution with the parameter of the “payback time” of the initial investment.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/92828