Energy Conversion and Management 88 (2014) 189–198

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Energy Conversion and Management

journal homepage: www.elsevier .com/ locate /enconman

Energy and economic savings using geothermal heat pumps in differentclimates

http://dx.doi.org/10.1016/j.enconman.2014.08.0070196-8904/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +39 081 5010284; fax: +39 081 5010204.E-mail addresses: biagio.morrone@unina2.it (B. Morrone), coppola.gbe@gmail.

com (G. Coppola), enzo.raucci@gmail.com (V. Raucci).

Biagio Morrone ⇑, Gaetano Coppola, Vincenzo RaucciDepartment of Industrial and Information Technology Engineering, Seconda Università di Napoli, Via Roma 29, 81031 Aversa, Italy

a r t i c l e i n f o

Article history:Received 16 April 2014Accepted 7 August 2014Available online xxxx

Keywords:Geothermal heat pumpsEnergy savingHeating and coolingEnergy management in buildingsNumerical simulations

a b s t r a c t

A technical and economic feasibility study is performed on residential buildings, heated and cooled bygeothermal heat pumps (GHPs) equipped with energy piles. The analysis is carried out for two differentclimate locations and building energy needs, which have been evaluated following the current Europeanstandard ISO 13790. The energy pile system performance coupled with the GHP has been numericallycalculated by using the PILESIM2 software over 20 years of operation.

The Primary Energy Saving (PES) indices were calculated comparing the actual GHPs systems with tra-ditional cooling and heating systems, together with their sensitivity to thermal and cooling loads for twodifferent climate locations. Also, economic savings and greenhouse gases (GHG) reduction have beencalculated resulting from the GHPs use.

The results show that in mild climates, where the GHPs are mainly used as HP, the annual averagetemperature of the ground around the energy piles can increase up to about 10 �C after many years ofoperation, whereas in cold climates the increase is nearly negligible.

Thus, the economical profit of GHPs is more difficult to achieve in mild climates than in cold ones.Conversely, GHG emission reduction is found to be larger in mild climates than in cold ones.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The percent energy final consumption in Italy for 2011 and eachsector is reported in Fig. 1(a). It can be observed that the Residen-tial–Tertiary sector accounts for about 35% of the total final energyconsumption. Observing in details the share of the Residential sec-tor, Fig. 1(b), the final heating energy accounts for about 70% of theenergy usage [1]. In addition, considering that part of the electricalenergy can be employed for driving electric air conditioners, thiscan sum up to about 80% of the final energy employed inResidential–Tertiary sector in Italy.

Similar values of the energy usage can be found in othercountries with mild climates, whereas in cold climate countriesbuilding heating needs can account for more than 80% of the entireenergy consumption in Residential and Tertiary sector.

This is the main reason why efficient systems are required forbuilding heating and cooling operations.

An effective device for heating and cooling buildings is the HeatPump (HP). Those available on the market are usually reversible

(RHP for short), which means that they can heat during the coldseason and refrigerate during the hot one. Although HPs are usu-ally driven by electrical motors (EHP), they can represent a veryeffective solution instead of classical boilers, even if condensingones are considered.

When the Reversible HP works as a Heating Machine isindicated with HP, whereas when operating as Cooling Machineis indicated as CM, in the following. These devices can be employedeither in small dwellings with low to medium thermal loads, or inlarge buildings, such as hospitals, hotels or commercial centreswith large centralized systems.

The most employed internal emission systems can be radiantpanels, fans or radiators. Each of them has typical temperaturesof the heating fluid in winter, of about 35 �C, 45 �C and 60 �C,respectively [2], using water as energy carrier fluid.

The external source can be outside air, water (where available)or the ground. Air is the most widely used external source, but itpresents several problems. In fact, its temperature is highly vari-able during the year and also during the day, and it is usually verylow in cold climates and very high during the summer in warm cli-mates. This implies that the efficiency of the HPs using air can bequite low. In addition, in cold climates with a high level of humid-ity, water vapor can freeze on the external heat exchanger causinga severe decrease of the performance.

Nomenclature

CM Cooling Machine (–)COP Coefficient of Performance (–)DD Degree Days (K d/y)DPB Discounted Pay-Back time (y)EER Energy Efficiency Ratio (–)EHP electric heat pump (–)Eheat heating energy need (MJ/MW h)Ecool cooling energy need (MJ/MW h)Ep primary energy (MJ/MW h)Fel en factor emission for electric energy production (kgCO2/

kW hel)FNG factor emission for burning Natural Gas as a fuel (kgCO2/

kgNG)GHG greenhouse gases (–)GHP geothermal heat pump (–)HP heat pump (–)i discount rate (–)IRR Internal Rate of Return (–)N time horizon of the investment (y)

NPV Net Present Value (€)OC over cost (€)PI index of profitability (–)P power (kW)Pel heat pump electrical power supply (kW)PES Primary Energy Saving value (–)Qc heat rate from low temperature source (kW)Qh heat rate to high temperature source (kW)RHP reversible heat pumpS annual cash flow (€/y)SEER seasonal EER (–)T temperature (K)Th high temperature reservoir (K)Tc low temperature reservoir (K)gboil efficiency of the boiler (–)gCOP heat pump second principle efficiency (–)gEER cooling machine second principle efficiency (–)ggrid average efficiency of the national grid (–)

190 B. Morrone et al. / Energy Conversion and Management 88 (2014) 189–198

It is well known that the larger the difference between the tem-perature of the fluid in the secondary circuit and the externalsource, the less efficient the HP is [3]. Thus, in order to attain largevalues of the performance coefficients, the temperature differencebetween the two thermal reservoirs needs to be the lowest and themost constant possible.

This is the main reason for considering the ground as the exter-nal thermal source/sink. In fact, the temperature is nearly constantthroughout the year starting 10 m below the surface of the groundand is fairly equal to the average annual temperature of the zone[4–8], quite independent of the geological characteristics of the

Fig. 1. Percent final energy consumption per sector (a) and final energy consump-tion share for the domestic sector (b) in Italy 2011 [1].

ground. In addition, if groundwater is present, the heat transferto and from the soil improves largely, also reducing the tempera-ture variation of the ground due to the heat injection and extrac-tion during the winter and summer seasons. [9,10].

Ground source or geothermal heat pumps (GHPs) are a highlyefficient energy technology for room heating and cooling. The sys-tem includes three main components – an earth connection sub-system, heat pump subsystem and distribution subsystem. Heatcollecting pipes in a closed loop, containing water, or water/anti-freeze mixture, are used to extract the geothermal energy storedin the ground, which can then be used to provide room heatingand domestic hot water [4]. Anyway, as pointed out in [11], thesustainability of the GHPs should be precisely scrutinized beforeinstallation because of the plausible unexpected temperature riseof the near ground due to the presence of many packed groundheat exchangers.

Two different types of ground heat exchangers are used: bore-holes and energy piles. The first type is largely employed. It is usu-ally made of a hollow cylinder filled with grout with a U-type pipein which the fluid flows thermally interacting on one side with thesoil and on the other side with the heat exchanger of the HP.Boreholes can be buried in the ground anywhere there is spaceand the pipes can be placed either vertically or horizontally [7].

Recently, different shapes of ground heat exchangers have beeninvestigated to improve the injected/removed heat per meter ofpipe [12].

Instead, energy piles may comprise base slabs, piles barrettes,slurry trench systems, concrete columns and grouted stone col-umns [13]. The geothermal energy piles fulfil two purposes [14];they are designed both as structural foundation elements andground heat extraction/injection. Furthermore, reinforced concretepiles have been found to be advantageous due to the material’shigh thermal storage capacity and enhanced heat transfer capabil-ities respect to borehole heat exchangers; in fact, the grout of thegeothermal probes is typically bentonite mixture, which has a highthermal resistance [3,14]. Energy piles are the most common wayto use the energy foundations in civil engineering. There are someadvantages in using the foundations of buildings as energy piles:one reason is the space already used by the foundations, whichcan go deep in the ground, between 10 and 30 m, related to theheight of the building and to the type of terrain. The general layoutof the energy piles is reported as a sketch in Fig. 2(a), and typicalreinforcement piles equipped with the pipes, the heat exchanger

Fig. 2. (a) Sketch of an energy pile and (b) reinforcement pile with pipes.

B. Morrone et al. / Energy Conversion and Management 88 (2014) 189–198 191

component, are shown in Fig. 2(b). The main drawback of energypiles is their length which is usually smaller than that of the bore-holes, implying a greater number of equipped piles to attain therequired heat rate. In addition, due to their limited depth theyare more influenced by the temperature variations of the ground.Last, the thermal stresses induced by the carrier fluid in the energypiles should be taken into account for a correct thermo-mechanicaldesign of the system [15,16].

As already pointed out in the Introduction section, one of themain problems for boreholes and energy piles is the probable nearground temperature rise, implying a limited number and density ofground heat exchangers to set up, either boreholes or energy piles.Thus heat injection/removal rate should be carefully estimated todesign a correct system spacing. Numerical or analytical modelingof such systems is strongly required to determine the potentialthermal interaction between the ground heat exchangers, whichcan severely affect the performance of GHPs [17].

In literature, very little information seems to be available aboutthe global thermal performance of energy piles during the wholeoperational life, whereas many papers deal with the local heattransfer characteristics of these devices [18–20]. Instead, severalnumerical investigations have been carried out on borehole heatexchangers global characteristics, such as in [21].

In this paper a numerical study has been conducted on residen-tial buildings, heated and cooled by geothermal heat pumpsequipped with energy piles to evaluate their technical as well aseconomical feasibility during 20 years of operations.

The analysis has been carried out for two different climate loca-tions and building energy needs. Building energy needs evaluationshave been performed according to the current European standardISO 13790:2008 [22], and the energy pile system performancehas been numerically calculated by means of the PILESIM2 soft-ware [23]. The degradation of the system performance has beenconsidered since the temperature rise of the ground due to theoperations of heat injection/removal has been numerically evalu-ated during 20 years of operation. Results show that differentclimates give different temperature rise of the ground and, thus,

divergent degradation performance of the GHPs systems. Thus acorrect evaluation of injected and extracted heat during manyyears of operation is required to obtain realistic evaluations ofthe GHP performances. The results show that in mild climates,where the GHPs are mainly used as HP, the average annual temper-ature of the ground around the energy piles can increase up toabout 10 �C after many years of operation, whereas in cold climatesthe increase is nearly negligible.

The Primary Energy Saving (PES) indices have been calculatedcomparing the actual GHP performance during 20 years of opera-tion to the boiler and air-to-air heat pumps (in the following iden-tified as traditional systems), together with their sensitivity toheating and cooling loads for different climate locations. In addi-tion, economical and greenhouse gases (GHG) emission savingshave been calculated resulting from the GHPs use.

The cost effectiveness compared to the traditional systemsaccording to the usual benefit-cost ratio standards has beenestimated.

2. Main existing energy pile GHPs installations

Austria, Switzerland and Germany can be regarded as the firstcountries that investigated this technology for decades. Extensiveuse of ground-source heat exchangers has been featured in Austria.Two interesting examples are lot LT24 of the Lainzer tunnel andUniqa Tower in the centre of Vienna. Both of these have receivedattention from several author [13–16,22–25]. The section of theLainzer tunnel employs energy piles in its side walls. Uniqa toweris founded on a raft foundation with two diaphragm walls reachingdepths of 35 m [24]. Switzerland has many years of experiencewith ground-source heat pumps. The technology is used in manyresidential houses as borehole heat extractors for heat distribution.Two notable examples of projects using energy piles are at theSwiss Federal Institute of Technology in Lausanne and DockMidfield Airport. The Lausanne project has also been part of anextensive performance monitoring program. Conclusions drawn

192 B. Morrone et al. / Energy Conversion and Management 88 (2014) 189–198

from the investigations reported by Laloui et al. [26] showed thatthermo-elastic strains were more pronounced than mechanicallyinduced strains. More than 300 foundation piles at Dock MidfieldZurich airport were converted into energy piles for heating andcooling purposes [26,27]. Performance assessment concluded that85% of the annual heating demand was supplied by the heat pumpwhile a combination of energy piles and conventional cooling sys-tems were able to meet annual cooling demands.

In Germany, the oldest seasonal thermal energy storage system,which was built in 1998 is located in Neckarsulm. Energy pileshave been used in some commercial projects such as the 200 mhigh Frankfurt Main tower [14]. Berlin’s International Solar Centeremploys 200 energy piles to meet 20% of heating and 100% of cool-ing demands through seasonal heat storage [28].

Energy piles are undergoing extensive research in the UK. Thefirst energy pile project in the UK began in 2001 at Keble College,Oxford University [13]. Both standard foundation piles and secantpiles for retaining walls were outfitted with absorber pipes. A totalof 146 energy piles at 25 m depth were installed for heating andcooling [14]. Bourne-Webb et al. [16] conducted an investigationon the impact of heating and cooling processes on the geotechnicalperformance of energy piles.

3. Mathematical model

Vapor compression inverse heat cycle is the most employedsystems to cool confined environments. It can also be used as veryefficient heater [29]. The simplest thermodynamic sketch of HPstakes into account the interaction with two thermal reservoirs:one at high temperature and the other at low temperature. Theyrepresent the inside and outside environments interacting withthe inverse cycle. The actual simplest cycle is made of four maincomponents: the compressor, the evaporator, the condenser andthe expansion valve. The condenser releases heat to the environ-ment to be heated if the system works as an HP; the evaporatordraws heat from the ambient that should be cooled when operat-ing as a CM [30].

3.1. Theoretical analysis

The energy balance of an HP for a steady state condition is:

Q h ¼ Q c þ P ð1Þ

where Qh is the heat rate released by the HP, Qc the heat rateabsorbed from the low temperature source and P the powerabsorbed by the compressor.

The main parameters used in the analysis of reversible heatpumps are the Coefficient of Performance (COP) when used inheating mode, and the Energy Efficiency Ratio (EER) in coolingmode. The first coefficient is defined as:

COP ¼ Q h

Pð2Þ

whereas the EER is defined as:

EER ¼ Qc

Pð3Þ

If the corresponding energies are used in Eqs. (2) and (3), eval-uated over the entire heating/cooling season, the coefficients rep-resent the so called seasonal performance indices, which takeinto account the changes of the RHP performance due to differentworking loads.

By means of the thermodynamic theory, the highest and purelytheoretical values of the two previous coefficients that can beattained are directly related to the higher and lower reservoir tem-peratures between which the HP operates:

COPth ¼Th

Th � Tcð4Þ

EERth ¼Tc

Th � Tcð5Þ

3.2. Actual analysis

The previous analysis is limited to the theoretical modeling ofRHPs. The actual RHPs present lower performance values com-pared to the ideal ones for several reasons, given by:

COP ¼ gCOPCOPth ð6Þ

EER ¼ gEEREERth ð7Þ

The g coefficients, Eqs. (6) and (7), called second principle effi-ciencies, take into account the departure of the actual performancefrom the ideal one. The reasons why the actual performance coef-ficients are lower than the ideal ones are several: first the heat istransferred to and from the HP through a finite difference of tem-perature, which implies an entropy production [31], then the heatexchangers efficiency is lower than one, third the compressor is notisentropic and the work required is greater than the ideal one. Inaddition, there are pressure drops in the circuit which reduce theperformance of the HP. Last, the working point is usually differentfrom the nominal one. For all these reasons, the values of gEER andgCOP can range from 30% to 70%.

The technical analysis carried out to compare the geothermalsystem with the traditional ones considers as the main parameterthe Primary Energy Saving (PES) index, which accounts for the sav-ing of the primary energy brought by more efficient systems. It isdefined, on annual basis, as:

PES ¼ ETRADP � EGEO

P

ETRADP

¼ 1� EGEOP

ETRADP

¼ 1�EGEO

P;heat þ EGEOP;cool

ETRADP;heat þ ETRAD

P;cool

ð8Þ

considering the geothermal system working both as HP and CM,where Ep are the primary energies required by each system.

The primary energy values for electrical geothermal HPs can beobtained as a function of the heating and cooling loads required bythe users:

EGEOP;heat ¼

EGEOheat

COPGEO � ggrid

; EGEOP;cool ¼

EGEOcool

EERGEO � ggrid

ð9Þ

where ggrid is the average efficiency of the national electric grid, andthe primary energy required by the traditional systems, boiler andair-to-air cooling machine, can be given by:

ETRADP;heat ¼

ETRADheat

gboil; ETRAD

P;cool ¼ETRAD

cool

EERTRAD � ggrid

ð10Þ

with gboil the efficiency of the boiler employed for comparison withthe HP and EERTRAD refers to the traditional air-to-air CM index.

The economical evaluation is obtained using the parameterssuch as the Discounted Pay-Back time (DPB), the Net Present Value(NPV), the Internal Rate of Return (IRR) and the Profitability Index(PI), defined as:

XDPB

k¼1

Skð1þ iÞ�k ¼ OC ð11Þ

NPV ¼XN�1

k¼1

Skð1þ iÞ�k � OC ð12Þ

0 ¼XN�1

k¼1

Skð1þ IRRÞ�k � OC ð13Þ

B. Morrone et al. / Energy Conversion and Management 88 (2014) 189–198 193

PI ¼ NPVOC

ð14Þ

where Sk is the economical saving per year brought by a more effi-cient system (€/y), OC is the over cost of the alternative systems tothe traditional ones (€), i is the nominal annual discount rate and Nis the time horizon of the investment (y) [32].

4. Numerical methods

4.1. Building energy needs software

The analysis has been carried out for two different climate loca-tions and building energy needs. Building energy needs evaluationshave been performed according to the current European standardISO 13790:2008 [22], specified for Italy by the UNI/TS 11300 stan-dards [33,34] by means of the DOCET program [35], developed byCNR-ITC and ENEA. The software is based on standards frameworkCEN defined to support of the EPBD. DOCET software allows for theanalysis of different energy needs either for residential buildings orsingle apartment. The overall structure of software is divided intofour modules of calculation.

4.2. PILESIM2 software

PILESIM2 [23] software simulates the behavior of geothermalheat pump systems equipped with energy piles or boreholes. Itwas developed by the Laboratory of Energy Systems (LASEN), atthe Swiss Federal Institute of Technology in Lausanne (EPFL) [36]and created using the transient simulation program TRNSYS [37].

Its development has been carried out with the help of measure-ments from existing systems for comparison and validation pur-poses. A non-standard simulation model, devised for heat storagein the ground with borehole heat exchangers, has been adaptedfor energy piles [23].

In Fig. 3 a schematic view of the modeled energy pile system isshown, as it is simulated by PILESIM2. The system border indicatesthe limits of the thermal simulation, which does not take intoaccount the distribution system.

PILESIM2 can calculate the energies transferred between thedifferent components of the system, the global heat balance ofthe system, monthly or yearly, the temperature levels of theground as a function of the time, the heat pump performance

Fig. 3. PILESIM2 model of energy p

coefficient (COP) and the actual cooling machine efficiency (SEER);the auxiliary thermal energy for heating and cooling, the electricenergy used by heat pump, the cooling machine and auxiliaries.The fluid flowing inside the energy piles is a mixture of waterand antifreeze.

Once assessed the thermal loads of the building for the chosenlocation by means of hourly local weather and solar radiation data,the calculation was performed with a time-step of one hour for the20 years of operation. The performances of the HP or CM weredetermined dynamically by assuming a design inlet fluid tempera-ture of the heat carrier fluid in the evaporator equal to 7 �C and anoutlet heat carrier fluid temperature from the condenser equal to45 �C. Anyway, these values were dynamically adjusted based onthe thermal performance of the energy pile system.

The parameters required to run PILESIM2 simulation can sum-marized as: the ground characteristics (initial ground temperature,thermal conductivity, volumetric thermal capacity of the ground,Darcy velocity of ground water, etc.); the energy piles (number,length, thermal resistance of the piles, spacing between the piles,etc.); the ground-building interface; the heat pump and coolingmachine; the loading conditions for heating and cooling.

The main hypotheses for the simulations regard the number ofenergy piles that should be rather large, the arrangement quiteregular and the ground area where the energy piles are buriednearly circular or square shaped. In addition, the model allows toconsider up to three layers of ground with different thermophysi-cal properties.

5. Discussion of results

The building considered in the paper for the evaluation of theGHP performance is a seven storey building, with a total heattransfer surface of 1609 m2, a gross heated volume of 5991 m3

and window surface of about 200 m2. The same building has beenconsidered for two different climate zones; Naples, located in theSouth of Italy, with a number of Degree Days (DD) equal to 1034,and a winter external design temperature equal to 2.0 �C, andMilan, North of Italy, with a number of Degree Days (DD) equalto 2404, external design temperature of �5.0 �C. The temperatureinside the dwellings has been assumed uniform and equal to20 �C for the whole heating season and to 26 �C for the coolingseason.

iles and reversible heat pump.

0.0

5.0

10.0

15.0

20.0

25.0

1 2 3 4 5 6 7 8 9 10 11 12

Gro

und

Tem

pera

ture

[ °C]

Month of the year

Pel=8 kW-Naples

1st year

20th year (a)

0.0

5.0

10.0

15.0

20.0

25.0

30.0

1 2 3 4 5 6 7 8 9 10 11 12G

roun

d Te

mpe

ratu

re [°

C]

Month of the year

Pel=14 kW-Naples

1st year

20th year

(b)

Fig. 5. Ground temperature as a function of the month of the year at the first and20th year in Naples for two heat pump power supplies: (a) Pel = 8 kW and (b)Pel = 14 kW.

(a) Naples

20

30

40

50

Wh/

y

Heat extractedfrom piles

Heat injectedin piles

194 B. Morrone et al. / Energy Conversion and Management 88 (2014) 189–198

The ground thermal conductivity has been set equal to 2.5 W/(m K) for both the locations, the volumetric thermal capacity equalto 2.5 MJ/(m3 K). These values are typical of a satured sand [28] Thetotal thermal resistance, which takes into account the interactionpile-ground, has been assumed equal to 0.1 (m K)/W, which is rep-resentative of a pre-cast pile with double U-pipe fixed on metallicreinforcement and with a diameter of 0.60 m. The number of pilesconsidered was set to 24 each equipped with 25 m and 4 U-pipesper pile, giving a total length of about 4800 m of pipes. The hori-zontal connections for about 500 m were also considered.

Fig. 4 represents the net energy needed by the building both forheating and cooling needs. The building situated in Naples,Fig. 4(a), shows large values of cooling energy needs, as expected;about 39,500 kW h/y, whereas the energy needs for heating areabout 15,500 kW h/y, with a cooling to the heating energy ratioof about 2.5. The situation in Milan, Fig. 4(b), is quite differentfor the same type of building; in fact, it accounts for about52,700 kW h/y for heating needs and about 25,100 kW h/y for cool-ing needs. The energy needs ratio is about 0.5.

The power supply of the RHP for Naples has been chosen in therange 8–14 kWel, whereas for Milan the RHP power supply was inthe 6–12 kWel range. Such powers can satisfy the greatest part orentirely the heating and cooling loads of the buildings, as shownnext.

The ground temperature as a function of the month of the yearis reported in Fig. 5 for two geothermal system power supplies,8 kWel Fig. 5(a) and 14 kWel Fig. 5(b), at two operating years, thefirst and the 20th year of operations for the same location: Naples.It can be observed that starting from a temperature of the undis-turbed ground of 15 �C, the first year shows nearly a constant valueof the temperature for the first months of service. When the hotseason starts, the temperature starts increasing because duringthe summer heat is injected into the ground to a greater rate thanit is extracted from, and it increases up to about 18–20 �C. At the20th year of operation the average ground temperature is largerthan 20 �C, because in the previous 19 years the heat injected dur-ing the summer season was far larger than the heat extracted fromthe ground.

0

2000

4000

6000

8000

10000

12000

1 2 3 4 5 6 7 8 9 10 11 12

Naples -DD=1034

Net Energy need for Hea�ng Net Energy need for Cooling

(a)

Month of the year

kWh

02000400060008000

10000120001400016000

1 2 3 4 5 6 7 8 9 10 11 12

Milan -DD=2404

Net Energy need for Hea�ng Net Energy need for Cooling

(b)

Month of the year

kWh

Fig. 4. Net heating and cooling energy: (a) Naples, DD = 1034; (b) Milan, DD = 2404.

(b)

0

10

8 10 12 14kWel

M

Naples

0

20

40

60

80

100

8 10 12 14

kWel

%

% coveredHea�ng

% coveredCooling

Fig. 6. (a) Energy extracted from and injected into piles as a function of the powersupply of the HP and (b) coverage of the energy needs of the building.

The annual ratio of Injected to Extracted Heat is more than 300%for all the investigated geothermal system powers in Naples. Thiscan be observed in Fig. 6(a), where the values of the Injected andExtracted heat from the ground are reported for four values ofthe RHP power supply.

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

1 2 3 4 5 6 7 8 9 10 11 12

Gro

und

Tem

pera

ture

[°C]

Month of the year

Pel =8 kW - Milan

1st year

20th year(a)

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

16.0

1 2 3 4 5 6 7 8 9 10 11 12G

roun

d Te

mpe

ratu

re [°

C]

Month of the year

Pel=12 kW - Milan

1st year

20th year(b)

Fig. 8. Ground temperature as a function of the month of the year at the first and20th year in Milan for two heat pump power supplies: (a) Pel = 8 kW and (b)Pel = 12 kW.

0

10

20

30

40

6 8 10 12

MW

h/y

kWel

MilanHeat extracted from piles

Heat injected into piles

(a)

80

100Milan

% covered Hea�ng

(b)

B. Morrone et al. / Energy Conversion and Management 88 (2014) 189–198 195

The HP with the lowest power supply value, 8 kWel, shows34.7 MW h per year of heat injected into the ground and12.1 MW h/y extracted with a percent ratio of 287%.

The highest HP, the one with 14 kWel, shows values of48.3 MW h/y of the injected heat and 12.1 MW h/y with a ratio ofabout 400%. This means that considering 20 years of operation,the heat injected into the ground is four times that extracted,bringing a large increase of temperature of the ground, as shownin Fig. 6(b), worsening the thermal performance of the HP.

In details, the highest temperature attained during the first yearis 20.0 �C in September, whereas after 20 years of operations thesame temperature reaches 25.6 �C, with an increase of 28%. Theannual average temperature increases from 16.6 �C of the first yearof operations to 23.2 �C at the 20th year, with an increase of morethan 40%. The coverage of the heating as well as cooling loadsrequired for the building in Naples is reported in Fig. 6(b). It canbe observed that in any case the heating load is always completelycovered by the HP whereas the cooling load is only partially cov-ered by the CM. When a 8 kWel power supply is considered, thecoverage of the cooling load is only about 70%. Considering the14 kWel HP the coverage increases up to 98%.

The absorbed electrical energy for the HP and CM is reported inFig. 7 for the four investigated power supplies. The 8 kWel HPabsorbs 3.4 MW h per year for heating operation and 7.5 MW h/yfor cooling operation, with a total of 10.9 MW h/y. The most pow-erful RHP considered, 14 kWel, requires 3.4 MW h/y in the heatingmode and 10.5 MW h/y in cooling mode. These values give rise toseasonal values of COP equal to 4.6 and EER equal to 3.4.

The ground temperature in Milan is reported in Fig. 8 as a func-tion of the month of the year for two power supplies of the RHP,8 kWel Fig. 8(a) and 12 kWel Fig. 8(b), at the first and the 20th yearof operations. It can be observed that starting from a temperatureof the undisturbed ground of 12 �C, soil temperature shows adecrease in the cold season and an increase in the hot one. The val-ues are nearly the same at the first and 20th years of operation, set-tling at the 20th year to an average value of 0.5 �C larger than the1st year. In fact, the injected to extracted energy ratio is aboutequal to 100%, i.e. the quantity absorbed from the ground equalsthat injected into it, letting the ground temperature nearly con-stant throughout the 20 years of operations. The values of theinjected and extracted heat from the ground, for four values ofthe power supplies of the RHP, are reported in Fig. 9(a).

The HP with the lowest value, 6 kWel, shows 34.3 MW h peryear of heat extracted from the ground and 24.9 MW h/y injectedwith a ratio of about 138%. The HP with the highest power supply,12 kWel, shows values of 41 MW h/y of the extracted heat and32.1 MW h/y of the injected with a ratio of about 128%. This meansthat over 20 years the heat extracted from the ground is about 0.75times that injected and this implies that the average values of the

0

2000

4000

6000

8000

10000

12000

8 10 12 14

kWh/

y

kWel

NaplesElectric energy HP

Electric energy CM

Fig. 7. Electrical energy per year absorbed in Naples by HP and CM as a function ofthe power supply of the system.

0

20

40

60

6 8 10 12

%

kWel

% covered Cooling

Fig. 9. (a) Energy extracted from and injected into piles as a function of the powersupply of the HP and (b) percent coverage of the energy needs of the building.

ground temperature to remain nearly constant, as shown inFig. 8(b).

The coverage of the heating and cooling loads for Milan isreported in Fig. 9(b). It can be observed that the heating as wellas the cooling loads are almost totally covered by the RHP with a

Table 1Evaluation of PES and GHG saving for the two geographical locations.

City Eheat/Ecool PES (%) GHG emission saving (%)

Naples 0.39 40.0 20.6Milan 2.09 53.1 5.0

Fig. 11. PES as a function of the ratio of: (a) cooling energy to the heating energy,with parameter values in Table 2 and (b) grid efficiency to the boiler efficiency.

Table 2Set of parameters for evaluation of PES in Fig. 11.

Series Parameter values

ggrid (%) gboil (%) EERTR COPGEO EERGEO

PES1 45.0 80.0 2.5 4.0 4.0PES2 45.0 80.0 2.5 3.5 3.5PES3 45.0 90.0 2.5 3.5 3.5PES4 45.0 90.0 3.0 3.0 3.0PES5 38.0 90.0 3.0 3.0 3.0PES6 35.0 90.0 3.0 3.0 3.0

196 B. Morrone et al. / Energy Conversion and Management 88 (2014) 189–198

power supply of 10 kWel, whereas the coverage is about 78% of thecooling load and the heating coverage is about 83% considering a6 kWel RHP.

The absorbed electrical energy for the HP and CM is reported inFig. 10 for the four investigated power supplies. The 6 kWel HPabsorbs 9.5 MW h per year for the heating operation and5.4 MW h/y for the cooling operation, with a total of 14.9 MW h/y. The most powerful HP considered, 12 kWel, uses 11.4 MW h/yfor the heating mode and 7.0 MW h/y for the cooling operationmode. The resulting COP is equal to 4.6 with an EER value of 3.6,on seasonal basis.

The technical as well as environmental evaluations have beenaccomplished using Eqs. (8)–(10) for the two locations, Naplesand Milan, by using the following parameter values forcomparison:

ggrid ¼ 0:45; gboil ¼ 0:80; COPGEO ¼ 4:6; EERGEO ¼ 3:6; EERTRAD

¼ 2:6

The GHG emission savings have been evaluated using the fol-lowing emission factors, valid for Italy in 2010:

Fel en ¼ 0:40 kgCO2=kW hel; FNG ¼ 2:75 kgCO2=kgNG

where the first factor accounts for the emissions of equivalent car-bon dioxide related to the production of 1 kW hel by the Nationalgrid system and the second is related to the combustion of NaturalGas in boilers. The results in terms of PES and GHG emission savingare reported in Table 1.

Table 1 shows large values of PES for both the locations. A sav-ing of 40% is accounted for Naples, whereas it is about 53% forMilan. A different situation is observed in terms of GHG emissionsaving: the RHP located in Naples allows a GHG saving of about20%, reducing 7200–5720 kgCO2/y, whereas in Milan the savingaccounts for only 5%. This trend is observed when the thermal loadratio Eheat/Ecool is large, i.e. for cold climates, whereas mild to hotclimates present lower PES but larger GHG emission saving values.

In fact, by observing Fig. 11(a) where the PES is reported as afunction of the Eheat/Ecool ratio, it can be clearly seen that, for anysets of parameters reported in Table 2, the larger the thermal loadratio the larger the PES values. In detail, when considering the leastfavorable set of parameters for the RHP, identified as PES6, theindex ranges from 1.5% for an Eheat/Ecool = 0.1 to about 12% whenthis ratio is equal to 5.0. In addition, the PES as a function of thesame thermal load ratio attains nearly asymptotic values for ratiosgreater than 5.0, whose values are a function of the set of theparameters. When the most favorable set of parameters is consid-ered, PES1 as reported in Table 2, the PES values are about 40% forEheat/Ecool = 0.1 and 53.3% for the thermal load ratio equal to 5.0.

0

2000

4000

6000

8000

10000

12000

6 8 10 12

kWh/

y

kWel

Milan

Electric Energy HP

Electric energy CM

Fig. 10. Electrical energy absorbed per year in Milan by HP and CM as a function ofthe power supply system.

Other interesting observations can be drawn from Fig. 11(b),where PES is reported as a function of the ggrid/gboil ratio, for differ-ent values of the Eheat/Ecool parameter. It can be seen that when thethermal load ratio is very low, for example 0.1, the increase in PESthat can be attained with a more efficient electric grid compared tothe boiler efficiency, is fair. When the thermal load ratio is high,equal to 5.0, there is a large increase related to the increment ofthe grid efficiency. Numerically, when Eheat/Ecool is 0.1 the lowestvalue is about 35% for an efficiency ratio of 0.3, and the largestvalue is about 44% for the efficiency ratio of 0.9 entailing a 25%increment. Instead, considering a large thermal load ratio, equalto 5.0, the lowest PES value reads about 20%, and the largest isabout 70%. Thus the greater energy benefits in terms of PES areattained for Eheat/Ecool large because in this case the GHP mainlyreplaces a boiler, implying a more efficient system. Instead for

-15,000

-10,000

-5,000

0

5,000

10,000

15,000

0 2 4 6 8 10 12 14 16 18 20

year

Net Present Value -Naples

i = 2.5%i = 5.0%i = 7.5%

(a)

-12,000 -7,000 -2,000 3,000 8,000

13,000 18,000 23,000 28,000 33,000 38,000

0 2 4 6 8 10 12 14 16 18 20year

Net Present Value -Milan

i = 2.5%i = 5.0%i = 7.5%

(b)

Fig. 12. NPV as a function of the year with different interest rates: (a) Naples and(b) Milan.

Fig. 13. GHG emission saving as a function of the boiler efficiency for Naples andMilan.

B. Morrone et al. / Energy Conversion and Management 88 (2014) 189–198 197

small thermal load ratios, the GHP mainly replaces an air-to-airdevice, which is usually already efficient thus entailing a smallenergy benefit. In addition, low values of the efficiency ratio bringsmall benefits since the electric grid is not very efficient comparedto the boiler. Thus, GHP systems do not allow a large primaryenergy saving. On the contrary, if the efficiency ratio is high, it isfavorable to use electrical GHPs to replace boilers and air-to-airCM.

The financial analysis uses as main indicator the Net PresentValue (NPV). The NPV has been defined in Eq. (12) and is evaluatedconsidering the entire life of the project set equal to 20 years andwith an interest rate of 5%. This index represents the incrementalgain generated by the alternative system. So, for an investmentto be worth of considering, the NPV should be positive, and the lar-ger the NPV the better the investment.

The investment costs have been reported in Table 3 both for theGeothermal and the traditional systems. The costs of the systems,both the GHP and Cooling and Boiler devices have been obtainedby major European producers. It can be observed that the over costof the GHP system, compared with the traditional one is estimatedequal to about 11,000 €. The cost of pipes has been evaluated con-sidering 24 piles each equipped with 25 m and 4 U-pipes per pile,giving a total length of about 4800 m of pipes. Considering a cost of0.6 €/m it gives a total of 2900 € and considering also the horizontalconnections (about 500 m) the total cost of the pipes has been esti-mated about 4000 €. The cost of pipes installation was determinedconsidering the labor time to accomplish the task. No costs of thehydraulic distribution system inside the buildings are reportedsince they have been assumed the same for both the systems.Maintenance costs were not reported, because they were assumednearly the same for the two systems under comparison.

Taking into account the initial over cost investment which hasbeen estimated about 11,000 €, the annual savings achieved bythe geothermal system in Naples and Milan and discount rates of2.5%, 5% and 7.5%, for a time horizon of 20 years’ investment, theNPVs are shown in Fig. 12. As it can be seen from Fig. 12(a) forNaples, the NPV has positive values for all the three considereddiscount rates. The Discounted Pay-Back times are in the range8–11 years. The Profitability Index, Eq. (14), of the investment witha discount rate of 5% is about 70%, which means a quite goodfinancial performance of this investment. The IRR index has beenestimated equal to 12.4% showing that the margin of revenue isquite limited, for the assumed discount rates.

Fig. 12(b) shows the NPVs for Milan. The NPV trend is some-what similar to the previous, but in this case the financial perfor-mance is much better than in Naples. In fact, the DiscountedPay-Back time occurs at about the 4th year, which is an excellentresult for this investment without considering any public incen-tives. The profitability index, for a discount rate of 5%, is 243%,which means an excellent financial performance and the IRR givesa large value of 28.2%.

Also, an evaluation was conducted to estimate the saving ofgreenhouse gases emissions (GHG) using the geothermal reversibleheat pumps compared with traditional systems. Fig. 13 shows theGHG emission saving estimate that could be achieved using

Table 3Investment costs (in euros) used for the economic evaluation of the systems.

Investment costs Geothermal system Traditional system

Pipes 4000.00 –Labor (installation costs) 3500.00 –GHP machine 19000.00 –Cooling machine – 8500.00Boiler – 7000.00Total (€) 26500.00 15500.00

geothermal HPs for different values of boiler efficiency. It can beobserved that the lower the boiler efficiency the greater the GHGsaving, for both the locations. Anyway, Naples with a low thermalload ratio Eheat/Ecool, accounts for a potential reduction of 22–19%when the boiler efficiency is in the 0.7–0.9 range.

The situation for Milan is quite different; in fact, in the samerange of boiler efficiency variation, the potential saving is between11% and 0%. This is due to the fact that Milan presents a very highthermal load ratio, about 2, which accounts for a small GHG emis-sion reduction when compared to a very efficient condensing boi-ler, as the one considered for comparison in this paper.

6. Conclusion

In this paper a numerical technical and economical feasibilitystudy has been conducted on residential buildings, heated andcooled by geothermal heat pumps equipped with energy piles, con-sidering 20 years of operation.

198 B. Morrone et al. / Energy Conversion and Management 88 (2014) 189–198

The analysis has been carried out for two different climatelocations and building energy needs, whose evaluations have beenperformed according to the current European standard ISO 13790,and the energy pile system performance has been calculated by thePILESIM2 software.

The results show that in mild climates the annual average tem-perature of the ground can increase up to about 10 �C after manyyears of operation, whereas in cold climates it is nearly negligible.This can entail a worsening of GHP performance, that needs to beaccounted for when designing such systems.

The Primary Energy Saving (PES) indices have been calculatedcomparing the actual GHP systems to the traditional ones, togetherwith their sensitivity to heating and cooling loads for two differentclimate locations. In addition, economic savings and reduction ofgreenhouse gases (GHG) emissions have been calculated resultingfrom the GHP use.

Calculated Primary Energy Saving (PES) indices are always posi-tive and large. By considering average seasonal COP of 4.6 and aSEER of 3.6, the PES as a function of the thermal load ratio rangesfrom 38% to 55%, whereas lower seasonal COP and EER values giverise to smaller, but always positive, PES values. Even in the worstcase considered, a PES of about 10% has been evaluated.

The financial performance of the investment is strictly relatedto the geographical location. In fact, the financial indices show anexcellent performance when referred to Milan, with 2040 DegreeDays, showing an index of profitability of 243% and IRR = 28.4%.Naples, with 1034 DD, on the other hand, shows worse financialindicators, with an IP of 68% and a lower IRR = 12.4%. But, as faras the GHG emission saving is considered, the situation totallychanges. In fact, assuming an indicative and typical set of parame-ters for the calculations, the saving can be about 20% compared tothe traditional systems in Naples, whereas in Milan the saving hasbeen estimated not greater than 10%.

Acknowledgment

This research was partially supported by the Second Universityof Naples with the PRIST 2008 grant.

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