11
Energy and Buildings 79 (2014) 12–22 Contents lists available at ScienceDirect Energy and Buildings j ourna l ho me pa g e: www.elsevier.com/locate/enbuild Possible applications of ground coupled heat pumps in high geothermal gradient zones Antonio Galgaro a , Giuseppe Emmi b,, Angelo Zarrella b , Michele De Carli b a Department of Geoscience, University of Padova, Via G. Gradenigo 6, 35131 Padova, Italy b Department of Industrial Engineering, University of Padova, Via Venezia 1, 35131 Padova, Italy a r t i c l e i n f o Article history: Received 18 February 2014 Received in revised form 14 April 2014 Accepted 17 April 2014 Available online 2 May 2014 Keywords: Ground coupled heat pumps Geoexchange Geothermal Borehole Heat pump Energy COP a b s t r a c t Geothermal energy is increasingly being used to heat and cool buildings. However its use has been slow to catch on due to the high installation costs of the most common Borehole Heat Exchangers (BHE), which constitute the major cost item of an air geothermal conditioning system. In areas where the shallow subsoil temperature is higher than normal (geothermal basins), the average temperature of the carrier water in the BHE is higher than in systems located in areas with a normal geothermal gradient, thus improving heat pump Coefficient Of Performance (COP). This paper investigates a range of solutions that use geoexchange systems coupled with BHE in anomalous geothermal zones. This study evaluates a residential building’s heating system when it is directly coupled with BHEs and compares the results with a range of thermal plant solutions. In Northern Italy, as in many other parts of the world, there are places where the ground’s thermal conditions are anomalous, with temperatures reaching around 35–85 C, instead of normal values of about 13–15 C. The energy of the building was analyzed by means of the TRNSYS, coupled with the CaRM model—designed by the authors—which provides detailed thermal behaviour readings for Ground Heat Exchangers (GHE). © 2014 Elsevier B.V. All rights reserved. 1. Introduction Ground Coupled Heat Pumps (GCHP) have been increasingly used in the last decade around Europe, and companies produc- ing heat pumps, or drilling wells and boreholes, are devoting more time and energy to this field. GCHP systems can be used in a wide range of applications, from small residential buildings to large-scale commercial ones, and are considered to be one of the most highly efficient heating and cooling technologies. The core of the system is the Ground Heat Exchanger (GHE), which comes in a range of models and comprises a closed loop of buried pipes. The GHE may be a simple pipe system buried in the ground; it may also com- prise a horizontal network or, more commonly, vertically drilled boreholes filled with double U (or single U) pipe and grout (i.e. Borehole Heat Exchangers [BHE]) [1]. These systems, when cou- pled with heat pumps, ensure far greater energy efficiency than air-source systems. However, they are rarely used because of their high initial installation costs. Corresponding author. Tel.: +39 49 827 6884; fax: +39 49 827 6896. E-mail addresses: [email protected] (A. Galgaro), [email protected], [email protected] (G. Emmi), [email protected] (A. Zarrella), [email protected] (M. De Carli). Ground temperatures usually vary from 7 C to 20 C, depend- ing on outdoor mean annual air temperature. Geothermal areas are therefore a highly promising field of application, as sub- soil temperatures can reach 35–85 C. When low-temperature geothermal energy is employed (known as low-enthalpy use) the ground normally performs as an energy source, energy sink or storage device [2–4]. For the purposes of this study, the ground will be considered only as an energy source due to the high mean temperature level of the subsoil. Accordingly, the GCHP systems in geothermal areas must be used in heating mode only. A number of European regions are well known as low tempera- ture (i.e. liquid dominated) shallow geothermal sites [5], and many of them are home to famous tourist destinations. In a wide vari- ety of cases, however, it might be difficult to heat houses directly with water and indirectly with open circuit water-to-water heat pumps due to local regulations and restrictions. Nevertheless, even if the ground reaches mean temperatures of about 25–30 C, its energy can be exploited through the use of vertical closed loop heat exchangers in GCHP, which, if properly designed and installed, do not damage the groundwater. Low-temperature geothermal energy has recently attracted more attention because it can be used in geographical areas that are not typically associated with it. Its appeal includes, but is not http://dx.doi.org/10.1016/j.enbuild.2014.04.040 0378-7788/© 2014 Elsevier B.V. All rights reserved.

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Page 1: Possible applications of ground coupled heat pumps in high geothermal gradient zones

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Energy and Buildings 79 (2014) 12–22

Contents lists available at ScienceDirect

Energy and Buildings

j ourna l ho me pa g e: www.elsev ier .com/ locate /enbui ld

ossible applications of ground coupled heat pumps in higheothermal gradient zones

ntonio Galgaroa, Giuseppe Emmib,∗, Angelo Zarrellab, Michele De Carlib

Department of Geoscience, University of Padova, Via G. Gradenigo 6, 35131 Padova, ItalyDepartment of Industrial Engineering, University of Padova, Via Venezia 1, 35131 Padova, Italy

r t i c l e i n f o

rticle history:eceived 18 February 2014eceived in revised form 14 April 2014ccepted 17 April 2014vailable online 2 May 2014

eywords:round coupled heat pumpseoexchange

a b s t r a c t

Geothermal energy is increasingly being used to heat and cool buildings. However its use has been slow tocatch on due to the high installation costs of the most common Borehole Heat Exchangers (BHE), whichconstitute the major cost item of an air geothermal conditioning system. In areas where the shallowsubsoil temperature is higher than normal (geothermal basins), the average temperature of the carrierwater in the BHE is higher than in systems located in areas with a normal geothermal gradient, thusimproving heat pump Coefficient Of Performance (COP). This paper investigates a range of solutionsthat use geoexchange systems coupled with BHE in anomalous geothermal zones. This study evaluatesa residential building’s heating system when it is directly coupled with BHEs and compares the results

eothermaloreholeeat pumpnergyOP

with a range of thermal plant solutions. In Northern Italy, as in many other parts of the world, thereare places where the ground’s thermal conditions are anomalous, with temperatures reaching around35–85 ◦C, instead of normal values of about 13–15 ◦C. The energy of the building was analyzed by meansof the TRNSYS, coupled with the CaRM model—designed by the authors—which provides detailed thermalbehaviour readings for Ground Heat Exchangers (GHE).

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Ground Coupled Heat Pumps (GCHP) have been increasinglysed in the last decade around Europe, and companies produc-

ng heat pumps, or drilling wells and boreholes, are devoting moreime and energy to this field. GCHP systems can be used in a wideange of applications, from small residential buildings to large-scaleommercial ones, and are considered to be one of the most highlyfficient heating and cooling technologies. The core of the systems the Ground Heat Exchanger (GHE), which comes in a range of

odels and comprises a closed loop of buried pipes. The GHE maye a simple pipe system buried in the ground; it may also com-rise a horizontal network or, more commonly, vertically drilledoreholes filled with double U (or single U) pipe and grout (i.e.orehole Heat Exchangers [BHE]) [1]. These systems, when cou-

led with heat pumps, ensure far greater energy efficiency thanir-source systems. However, they are rarely used because of theirigh initial installation costs.

∗ Corresponding author. Tel.: +39 49 827 6884; fax: +39 49 827 6896.E-mail addresses: [email protected] (A. Galgaro),

[email protected], [email protected] (G. Emmi),[email protected] (A. Zarrella), [email protected] (M. De Carli).

ttp://dx.doi.org/10.1016/j.enbuild.2014.04.040378-7788/© 2014 Elsevier B.V. All rights reserved.

Ground temperatures usually vary from 7 ◦C to 20 ◦C, depend-ing on outdoor mean annual air temperature. Geothermal areasare therefore a highly promising field of application, as sub-soil temperatures can reach 35–85 ◦C. When low-temperaturegeothermal energy is employed (known as low-enthalpy use) theground normally performs as an energy source, energy sink orstorage device [2–4]. For the purposes of this study, the groundwill be considered only as an energy source due to the highmean temperature level of the subsoil. Accordingly, the GCHPsystems in geothermal areas must be used in heating modeonly.

A number of European regions are well known as low tempera-ture (i.e. liquid dominated) shallow geothermal sites [5], and manyof them are home to famous tourist destinations. In a wide vari-ety of cases, however, it might be difficult to heat houses directlywith water and indirectly with open circuit water-to-water heatpumps due to local regulations and restrictions. Nevertheless, evenif the ground reaches mean temperatures of about 25–30 ◦C, itsenergy can be exploited through the use of vertical closed loop heatexchangers in GCHP, which, if properly designed and installed, do

not damage the groundwater.

Low-temperature geothermal energy has recently attractedmore attention because it can be used in geographical areas thatare not typically associated with it. Its appeal includes, but is not

Page 2: Possible applications of ground coupled heat pumps in high geothermal gradient zones

A. Galgaro et al. / Energy and Bu

Nomenclature

a diffusivity (m2/s)Fo Fourier’s number (at/r2) (–)k slope of the curve (–)q specific heat flow per unit length (W/m)r radius (m)Rb borehole thermal resistance (m K/W)t time (s)Tb borehole wall temperature (◦C)Tf fluid temperature (◦C)T0 undisturbed ground temperature (◦C)� Euler’s constant (–)�ground ground thermal conductivity (W/m K)RppA thermal resistance between adjacent tubes (m K/W)RppB thermal resistance between opposite tubes (m K/W)Rp0 thermal resistance between tubes and borehole wall

li

Eth

wotowtra[

lgeiaap

Aid

gEtwctGiisa

cihp

(m K/W)

imited to, its stable, base-load energy output, low environmentalmpact and high renewability [6].

The north-east Italian town of Abano Terme, which sits in theuganean Geothermal Basin, is one potentially attractive site forhe expansion of geothermal resource utilization due to the area’sigh ground temperatures [7].

The thermal properties of the water in the Abano Terme areaere known to the ancient Romans, but they were only marketed

n a larger scale at the beginning of the 20th century, reachingheir peak in the 1960s, when artesian wells stopped the naturalutflow of thermal water and pumps were used to extract thermalater from the sandy aquifers in the alluvial cover. However, as

his water was not injected back into the aquifers, the extractionate overtook the recharge rate, causing subsidence, which reached

rate of 2 cm/y, and damaged some buildings and infrastructure8].

In general, two systems are used in areas characterized byow enthalpy: open-loop and closed-loop. In open-loop systems,roundwater is the heat-carrier fluid and it enables heat to bexchanged directly with a heat pump or a heat exchanger. Waters extracted from a well and then either injected back intonother well, or discharged at the surface. The same principlepplies when hot groundwater is extracted for therapeutic pur-oses.

The alternative technique is to use closed-loop systems. closed-loop system is generally not subject to bureaucratic

ssues related to mining concessions and does not cause subsi-ence.

This paper discusses an application that exploits low-enthalpyeothermal energy. The feasibility of installing a Borehole Heatxchanger (BHE) in Abano Terme was analyzed and research washen conducted within the Euganean Geothermal Basin (Fig. 1)here thermal resources are exploited for multiple uses. The field

overs a total surface area of about 40 km2 and comprises fourowns (Abano Terme, Montegrotto Terme, Battaglia Terme andalzignano Terme) [9]. The word “Terme” means “Spa” and the area

ncludes more than 130 establishments and 220 thermal pools. Its an important part of the region’s economic, social and healthectors, employing over 5000 people directly and another 6000 inssociated services.

In areas where the temperature is higher than usual, some criti-

al aspects, such as materials and drilling methods, have to be takennto account. Furthermore, proper grouting materials for sealingave to be chosen to obtain good thermal contact between theipe and ground, as have good hydraulic isolation between the

ildings 79 (2014) 12–22 13

different groundwater levels crossed by wells. Last but not least,pipes should be made of special materials due to high groundtemperatures. Therefore, high-strength Peroxide CrosslinkedPolyethylene (PE-Xa) is recommended, as it resists both standardcircuit pressure and high temperatures.

The Euganean Geothermal Basin can be classified as a hydro-thermal convection system, where the water is the dominant phase[10,11]. At present, about 250 wells are active and the total averageextraction flow rate of thermal fluids is about 17 million m3/year.These fluids are exclusively used for health purposes, as requiredby current legislation. The physical and chemical parameters of theEuganean area’s thermal waters have been extensively analyzed,mainly with statistical methods. Temperatures range from 60 ◦C to87 ◦C, and remain practically constant, confirming that the Basinhas “a high-up flow rate”. The total dissolved solids are 6 g/l with aprimary presence of Cl and Na (70%) and a secondary one of SO4, Ca,Mg, HCO3 and SiO2. 3H and carbon-14 measurements suggest a res-idence time of more than 60 years, but the most likely figure is a fewthousand years. Analyses of oxygen isotopes show that the ther-mal waters are of meteoric origin and fall in an area up to 1500 ma.s.l. in the Pre-Alps [12]. Work [9] proposed a good example of thehydrothermal circuit, which explained the genesis and dynamicsof Euganean fluids. Rainwater infiltrates the Pre-Alps and reachesdepths of 3000–4000 m, is warmed up by the normal geothermalgradient, and circulates towards the South East, flowing through acomplex of hills formed by the Lessini, Berici and Euganean Hills.The Permian crystalline-schist bed is the lower limit of the watercirculation system and is conditioned by the structural shape of theregion.

Fractures and faults in the structure of the Euganean GeothermalBasin lead to a rapid ascent of the fluids and to temperature homog-enization, which is linked to the presence of convective motions.Other factors facilitate upwards movement, such as the side ofthe system being sealed with low permeability sediments and thehydraulic load being generated by cold groundwater seepage fromthe surface of the Euganean Hills [10,11].

The main aquifer is formed by Red Scaglia and Jurassic lime-stone, but the others, which are located in the alluvial quaternarysequence, are formed by sands interlayered with clay and silt. In thissequence, the deep waters mix with the surface waters, loweringsalinities and temperatures. Until late last century, Abano Terme’sspa-resort waters originated from springs or lakes. Later, they werepumped from wells that drained the quaternary aquifers. Later still,the extraction of sand, along with the formation of subsidence, ledto the wells being deepened so that water could be drawn directlyfrom the fractured rock [13,14].

2. Case study

GCHP systems use the ground or the groundwater flow as a heatsource-sink to provide space heating and cooling and are gener-ally more energy efficient than heat pumps that use outdoor air asa heat source-sink [15]. As aforementioned, in zones affected bythermal anomalies the use of heat pumps was especially investi-gated in terms of direct use of groundwater flow [16] in heatingmode. This paper estimates the energy requirement of differentbuilding heating systems coupled with GCHP and BHE (i.e. closedloop system) in ground with an anomalous temperature gradi-ent.

Research was divided into two parts. The first involved the defi-nition of a building model and the subsequent calculation of heatingloads during the year. Ground and BHE properties were then

investigated to estimate thermal exchange capacity. A ThermalResponse Test (TRT) [17–20] was performed to calculate groundproperties in the study area with one BHE at a depth of about125 m.
Page 3: Possible applications of ground coupled heat pumps in high geothermal gradient zones

14 A. Galgaro et al. / Energy and Buildings 79 (2014) 12–22

12]; (B

2

gwfit

TmE

Fig. 1. (A) Test site geographical location [

.1. Building properties and climatic data

The building simulated was a two-story residential home: theround floor is adjacent to the ground, the temperature of whichas equal to the yearly average external air temperature, and therst floor adjacent to an unheated attic. The geometry and orienta-ion of the building are illustrated in Fig. 2.

2

Each floor has a useful area of 60 m and a net height of 2.7 m.he insulation level of the external surfaces complies with theinimum requirements in force in Italy in accordance with the

uropean Union’s Energy Performance Building Directive (EPBD).

Fig. 2. Scheme of the building.

) Euganean gethermal circuit scheme [9].

The insulation of the building envelope depends on the climaticzone [21,22]. Italy is divided into various climatic zones in terms ofDegree Days (DD): the letter A indicates places with the hottest cli-mate and the letter F places with the coldest climates. The locationinvestigated belongs to climatic zone E and has 2383 DD.

The emission system used for heating is a typical radiant floorwith a supply water temperature of about 29.5 ◦C. The indoorheating system was divided into two sections, each with separatecirculator: one for the ground floor and one for the first floor. Thesystem was switched on for the entire heating period and the setpoint temperature for the radiant floor control system was kept at20 ◦C with a dead band of ±0.5 ◦C. The control was based on theoperative temperatures of the heating spaces.

Internal loads were taken from ISO 13790 [23] for both the living(ground floor) and sleeping zone (first floor).

The Test Reference Year (TRY) for Venice [24] was used to esti-mate building heating loads and behaviour, as it is the officialweather reference for places close to the study area. Fig. 3 is a graph

of the external temperature together with the heating loads of thesimulated building.

Table 1 shows the thermal properties of the building envelopeand the boundary conditions used for the computer simulations.

Table 1Building properties.

Element Property Value Unit

External walls U-value 0.288 W/(m2 K)Internal floor U-value 1.793 W/(m2 K)No-heating floor U-value 0.225 W/(m2 K)Ground floor U-value 0.330 W/(m2 K)Roofs U-value 0.236 W/(m2 K)Windows U-value 1.24 W/(m2 K)

Area 1.4 m2

Air change ratio n 0.5 h−1

Mean internal loads – 5.83 W/m2

Page 4: Possible applications of ground coupled heat pumps in high geothermal gradient zones

A. Galgaro et al. / Energy and Buildings 79 (2014) 12–22 15

eratur

Thr

s

2

ChatTcccrsm

“pvw

Fig. 3. Zones operative temperature vs external temp

he energy simulation of the building provides an overall peakeating load of about 7 kW (i.e. 58 W/m2) and an overall energyequirement of 10 MWh (i.e. 83 kWh/(m2 y)).

The heating loads calculated were used for each type of heatingystem investigated, as described below.

.2. CaRM model and TRT

The geoexchange simulation tool used for this research is calledaRM (CApacity Resistance Model) [25,26] and it takes into accounteat transfer within the ground by heat conduction. A BHE has

resistance system, and thermal resistances and capacitanceshat use electrical analogy are buried in the surrounding ground.he ground is modelled into vertical regions, each of which isharacterized by different thermophysical properties (e.g. thermalonductivity, specific heat capacity and density). The thermophysi-al properties are assumed to be independent of time. Each verticalegion is then divided into several annular regions. In this casetudy, the axial heat transfer (e.g. along the depth direction) is notodelled.The model can be used on a range of soil compositions (defined

sub-regions”), each of which has a given undisturbed ground tem-erature. Consequently, this model can also take into account aertical temperature profile, which may be helpful when dealingith geothermal sites with anomalous temperature gradients. The

Fig. 4. Scheme of

e (GF: ground floor, FF: first floor) and heating loads.

undisturbed ground temperatures values are assumed to be inde-pendent of time. Fig. 4 shows a graph of the model.

The ground properties used as CaRM input to simulate BHE ther-mal behaviour are the results of a TRT conducted in MontegrottoTerme, a small town in the Euganean Geothermal Basin.

The TRT enables equivalent thermal conductivity and averageundisturbed ground temperature to be estimated. The ground’sthermal properties can actually be estimated with the values avail-able in literature, but the range of values found for specific soiltypes is very wide. TRT is therefore a more accurate method. Thetest can be done in cooling mode (the system uses an electricalresistance to heat the water supplied to the borehole) or in heat-ing mode (i.e. with a chiller cooling the ground). When the fluidmoves through the heating/cooling system, the inlet and the out-let temperatures are recorded by a data-logger. To determine theundisturbed ground temperature, the fluid is initially circulatedthrough the system without heat injection or extraction.

When the heater is switched on, a suitable power per unit lengthhas to be injected into the ground. The test usually lasts four days. Acontinuous line source model, which issues a constant heat flux perunit time, enables the equivalent thermal conductivity of the soil to

be calculated. Calculation of the test data with the line source modelproduces a systematic error due to test duration; the error value islower than ±2.5% for tests lasting longer than four days. The meanfluid temperature (between supply and return) is plotted against

CaRM [25].

Page 5: Possible applications of ground coupled heat pumps in high geothermal gradient zones

1 and Buildings 79 (2014) 12–22

tWrw

trBgfia(

T

i

E

st

T

b

T

6 A. Galgaro et al. / Energy

he natural logarithm of time, which provides a linear equation.hen the slope k of this linear equation and the constant power

ate per unit length are used, thermal conductivity can be calculatedith the equation:

ground = q

4�k(1)

Equation (1) is based on the infinite line source model [27]hat represents the BHE with a line of uniform and constant heatate exchanged with the ground. Moreover, the ground around theHE is assumed to be homogenous and isotropic with no movingroundwater. On the basis of these assumptions, the temperatureeld around the BHE is a function of time and distance from thexis of the BHE (i.e. radius), which is expressed in the following Eq.2):

q(r, t) = q

4��

∞∫r24at

e−u

udu = q

4��groundE1

(r2

4at

)(2)

For large values of time t, defined as when Fo > 5, the exponentialntegral E1 was approximated with the following simple equation:

1

(r2

4at

)∼= ln

(4at

r2

)− � (3)

Fluid temperature was measured during the TRT. The relation-hip between the fluid temperature and the wall temperature ofhe BHE is shown in the next Eq. (4).

fq(t) = Tb

q(t) + q · Rb (4)

In accordance with Eqs. (3) and (4), the fluid temperature can

e written as follows:

f(t) = q

4��ground

[ln

(4at

r2b

)− �

]+ q · Rb + T0 (5)

Fig. 6. Vertical undisturbed ground temperature, realized in

Fig. 5. TRT machine for heating and cooling purposes.

In a graph of temperature change against the logarithm of time,Eq. (5) can be used to calculate a straight line where the line slopeis the coefficient function of ground conductivity:

Tf(t) = k · ln t + m where k = q

4��ground

Consequently, once the value of the heat flow q (expressed inW/m of linear development of the BHE) and the curve slope areknown, equivalent ground conductivity can be calculated with Eq.(1).

For this study, a TRT Machine was developed on the basis of

ASHRAE Handbook recommendations [28]; the machine comprisesa central unit for data acquisition, a regulation unit and a heat gen-erator; and a second unit made up of a chiller (Fig. 5). The system hasa wireless connection, which allows measurements to be remote

side the BHE pipe and linear equation from log data.

Page 6: Possible applications of ground coupled heat pumps in high geothermal gradient zones

A. Galgaro et al. / Energy and Buildings 79 (2014) 12–22 17

eratur

cc

t

Fig. 7. Logarithmic profiles of temp

ontrolled in the event of a fault and measurement trends to behecked in real time.

The objective of this experimental test was to estimate groundemperature, as well as to measure and record the average

Fig. 8. Schematic diagrams of the

es in the heating and cooling tests.

temperature of the heat transfer fluid (water) in the heat exchangeras a function of time for a vertical geothermal heat exchanger. Thiswas done to ensure that the thermal power exchanged with theground was constant.

analyzed heating systems.

Page 7: Possible applications of ground coupled heat pumps in high geothermal gradient zones

18 A. Galgaro et al. / Energy and Buildings 79 (2014) 12–22

profi

wDaGimst

Ttta

2

lzft

TG

building load profile. The behaviour of the building was simulatedwith a dynamic code called TRNSYS. The systems were tested withthe energy required by the building’s distribution system.

Table 3BHE properties.

Type 2U –

Fig. 9. Vertical temperature

To determine the undisturbed ground temperature, the fluidas initially circulated through the system without heat injection.ue to the unusual situation of the Euganean Geothermal Basin,

thermometer was used to establish the temperature log of theHE filling water, which produced the temperature profile shown

n Fig. 6; this reading was carried out 10 days after the geother-al heat exchanger had been installed so the exchanger was in a

teady-state condition. The undisturbed ground temperature washen used as input for the simulations.

Fig. 7 shows the average temperatures of the heat-transfer fluid.able 2 summarizes the experimental results used in the simula-ions and Table 3 shows the BHE characteristics. The water used inhe simulations had a mean specific heat value of 4366 J/(kg K) and

density of 996 kg/m3.

.3. The heating system

One aim of this study was to estimate the energy efficiency of

ow-temperature heating systems coupled with BHE installed inones with anomalous geothermal temperature gradients. Four dif-erent heating generation systems were investigated at the sameime.

able 2round properties used in the simulation tool CaRM.

Thermal conductivity 1.8 W/(m K)

Density 1285 kg/m3

Specific heat 2614 J/(kg K)Mean gradient of temperature in depth 0.5 ◦C/mMean annual surface temperature 12 ◦C

le of the ground and water.

The first was a Ground Heat Exchanger directly coupled witha building’s heating system through a 95% efficiency water–waterheat exchanger (Case 1). The second had a smaller BHE field con-nected in series with an air–water heat pump as a back-up devicefor the building’s heating load (Case 2); the third had the smallestBHE field and was coupled with a water-water heat pump (WWHP)(Case 3). And the fourth had an air-water heat pump (AWHP) (Case4).

Fig. 8 shows a schematic diagram of the four systems investi-gated.

In first step of the work, each simulation was based on the same

BHE connection Parallel –Borehole length 120 mBorehole diameter 0.14 mWheelbase distance 0.07 mInside diameter of pipe 0.026 mOutside diameter of pipe 0.032 mPipe connection Parallel –Thermal conductivity of the pipe 0.35 W/(m K)Thermal conductivity of the filling material 2 W/(m K)Total water flow rate 0.18 kg/sSpacing between BHEs 10 mRppA (reference to the model CaRM) 0.437 m K/WRppB (reference to the model CaRM) 0.589 m K/WRp0 (reference to the model CaRM) 0.267 m K/W

Page 8: Possible applications of ground coupled heat pumps in high geothermal gradient zones

A. Galgaro et al. / Energy and Buildings 79 (2014) 12–22 19

F ratureo

ts

cttmttBwyaig

Ab

TS

ig. 10. Results of Case 1 for the 1st and 20th year: (a) and (b) mean ground tempef March.

In all cases, the radiant floor system had the same configura-ion, i.e. two circulators; both had 50 W nominal power and waterupplied at 29.5 ◦C.

Fig. 8 (Case 1) shows a “free heating” system with a directonnection between the working fluids in the BHE and a low-emperature heating system. The BHE field covers a wider areahan a standard GSHP field (see Case 3). Case 1 investigated the

inimum number of BHE that could be used. To evaluate this sys-em, the pipes were buried at the maximum depth permitted byheir materials and the boreholes were 10 m apart. The number ofHE was then chosen to raise the minimum inlet radiant systemater temperature used in the building simulation (29.5 ◦C). Anal-

sis was carried out with the CaRM simulation code and three BHEst a depth of 120 m. The tests simulated the use of electrical energyn the system’s three circulation pumps. The two circulators for theround floor and first floor had 50 W of power.

In Case 2, the BHE field was reduced to two boreholes and anWHP was used to supplement the heat demanded by the distri-ution pipes of the building heating system. As in Case 1, each BHE

able 4ystem properties.

Case number 1 2 3 4

Number of BHE (–) 3 2 1 0Total BHE fluid flow (kg/s) 0.54 0.36 0.18 –BHE pump power (W) 170 160 150 –Mean COP of AWHP/GCHP (–) – 2.8 4.1 3.3Nominal Power of AWHP/GCHP (kW) – 8.5 10.9 8.5Nominal COP of AWHP/GCHP (–) – 3.54 3.54 3.54

, (c) and (d) ground and water temperatures and ground power over the first week

was buried at 120 m and spaced 10 m apart. Machine performancewas calculated with the simulation tool provided in [29].

Case 3 had only one BHE coupled with a WWHP; the propertiesof the BHE were the same as in Cases 1 and 2, but this time involveda borehole in a free field. In Case 4, an AWHP supplied the requiredheating loads. The main properties for all four cases are summarizedin Table 4.

3. Results and discussion

3.1. Energy evaluation

The energy consumption of the four systems was estimatedon the basis of the computer simulation results. Table 5 showsthe results as electrical and primary energy consumption. Primaryenergy was estimated with an electric efficiency factor of 0.46, assuggested by the Italian Electrical Energy and Gas Authority [30].

In terms of primary energy, the best solution is Case 1, i.e. the“free heating” system. In all of the other cases, the energy consump-tion required to ensure a controlled temperature inside the buildingis more than double.

3.2. Ground temperature time variation

The behaviour of the ground near and far from the BHE wasinvestigated with the CaRM model. The ground around the bore-

holes was divided into 20 annular regions (see Fig. 4), each with amaximum diameter of 10 m. In all four cases, the ground tempera-ture profile was plotted for the end of the first, sixth and twentiethyear of system operation. In Case 1, the BHEs were distributed
Page 9: Possible applications of ground coupled heat pumps in high geothermal gradient zones

20 A. Galgaro et al. / Energy and Buildings 79 (2014) 12–22

Table 5Electrical and Primary Energy Consumptions after 20 years working.

Case study Description [kWhe/y]([kWhe/(m2 y)])

kWhp/ykWhp/(m2 y)

BHE pump Radiant systempumps

Air–water HP Water–waterHP

Total Total

Case 1 3 BHEs directlyconnected

461(3.8)

181(1.5)

–(–)

–(–)

643(5.4)

1397(11.6)

Case 2 2 BHEs withAWHP

434(3.6)

181(1.5)

459(3.8)

–(–)

1075(9.0)

2337(19.5)

Case 3 1 BHE with 407 181 – 2061 2664 5790

ltp

1apfcfisIto

Fo

GCHP (3.5) (1.5)Case 4 AWHP –

(–)181(1.5)

inearly in the field and the diagrams detail the most disadvan-aged BHE, i.e. the one in the centre. In Case 2, the temperaturerofile was the same for the two BHEs.

Fig. 9 compares the temperature profile of annular regions nos.1,0 and 15 with ground depth. In these diagrams, the plotted valuesre for 12 pm on 31 December of every year. The previous diagramsrove that the ground temperature profile along the depth changesrom the first to the sixth year of operation, but soil behaviour isonstant for the subsequent years. The latter consideration is con-rmed by the fact that ground and water temperatures are notubject to significant changes from the sixth to the twentieth year.

n Case 3, the ground is under more stress, as the temperatures ofhe BHE fluid inlet are lower than the other cases. The temperaturef the fluid flowing through the BHE passage increases by about

ig. 11. Results of Case 2 for the 1st and 20th year: (a) and (b) mean ground temperaturef March.

(–) (17.2) (22.2) (48.3)3201(26.7)

–(–)

3382(28.2)

7353(61.3)

1.5–2.0 ◦C in Cases 1 and 2 and by 5.5–6.0 ◦C in Case 3. By way ofexample, the temperature profile with a radius R equal to 3.576 hasa steeper slope in Cases 1 and 2, which would lead to a lower aver-age temperature than in Case 3. This is due to the borefield shape.In Case 3, the BHE is in an open field, but in the other two cases, theBHE is positioned between or next to other BHEs, which penalizesits exchange with the ground.

Figs. 10–12 show the trend of the average ground temperatureduring a whole year at different distances from the BHE. The dia-grams are for the first and twentieth year of operation. They alsoshow the profiles of the main results for the first week of March in

each year analyzed.

In all cases, the mean ground temperature during the year andin the extension of the temporal window to the first week of March

, (c) and (d) ground and water temperatures and ground power over the first week

Page 10: Possible applications of ground coupled heat pumps in high geothermal gradient zones

A. Galgaro et al. / Energy and Buildings 79 (2014) 12–22 21

F mperw

wttdstd

4

spaw

irtsdmft

tcflpt

ig. 12. Results of Case 3 for the 1st and the 20th year: (a) and (b) mean ground teeek of March.

as higher at the end of the first year of operation than at the end ofhe twentieth year. From the first to the twentieth year, the meanemperature of each annular region decreased by about 1 ◦C. Thisecrease occurred over the first six years of operation; after theixth year no significant difference are noted in terms of groundemperature. As a consequence, after six years ground behaviouroes not change when subjected to thermal stress by BHEs.

. Conclusions

This study shows that the energy requirements of the variousystems under investigation vary vastly, therefore costs will changeroportionally. From Case 4 to Case 1, energy requirement fell by

factor of six. Note that the primary energy requirement of Case 1as the lowest of the four systems studied.

The results of Case 2 suggest that this solution is particularlynteresting when a compromise is being sought between energyequirement and practical aspects. This system was the only onehat allowed a building to be heated with relatively reduced con-umption and, at the same time, allowed the building to be cooleduring summer and the BHE field to be bypassed, without furtherachines being added. By way of example, the energy requirement

or cooling in the case study is 0.9 MWh, i.e. 8 kWh/(m2year), whilehe cooling peak load is 2.4 kW (20 W/m2).

Ground analysis revealed that the systems investigated guaran-eed long-term constant efficiency. In the case with one BHE and a

oupled heat pump (Case 3), the temperature variation in the BHEuid had a negligible effect on the global performance of the heatump because after 20 years the difference was about 1 ◦C, similaro the variation in ground temperature.

ature, (c) and (d) ground and water temperatures and ground power over the first

Analysis also showed that these geoexchange solutions had ahigh degree of renewability and sustainability, which means thatantifreeze need not be used in the BHE carrier fluid, thus main-taining the subsoil’s thermal structure substantially undisturbed.It also enables the subsurface’s favourable geothermal propertiesto be exploited, thus reducing local emissions of greenhouse gasesand avoiding the problems associated with the direct use of thermalwaters. To conclude, the abovementioned results suggest that areassuch as the Euganean Geothermal Basin may be highly suitable forheating buildings with direct heat exchange or GCHP.

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