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Applied Thermal Engineering 30 (2010) 2574e2583

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Applied Thermal Engineering

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

Experimental measurement and numerical simulation of horizontal-coupledslinky ground source heat exchangers

Yupeng Wu a,*, Guohui Gan a, Anne Verhoef b, Pier Luigi Vidale c, Raquel Garcia Gonzalez b

aDepartment of the Built Environment, Faculty of Engineering, University of Nottingham, Nottingham NG7 2RD, UKbDepartment of Soil Science, School of Human and Environmental Sciences, University of Reading, Reading RG6 6DW, UKcNCAS-Climate, Department of Meteorology, University of Reading, Reading RG6 6BB, UK

a r t i c l e i n f o

Article history:Received 19 March 2010Accepted 8 July 2010Available online 15 July 2010

Keywords:Ground source heat pumpHorizontal-coupledSlinkySpecific heat extraction

* Corresponding author. Tel.: þ44 (0) 1158467312.E-mail addresses: Yupeng.Wu@nottingham.ac.uk

(Y. Wu).

1359-4311/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.applthermaleng.2010.07.008

a b s t r a c t

Results from both experimental measurements and 3D numerical simulations of Ground Source HeatPump systems (GSHP) at a UK climate are presented. Experimental measurements of a horizontal-coupled slinky GSHP were undertaken in Talbot Cottage at Drayton St Leonard site, Oxfordshire, UK.The measured thermophysical properties of in situ soil were used in the CFD model. The thermalperformance of slinky heat exchangers for the horizontal-coupled GSHP system for different coildiameters and slinky interval distances was investigated using a validated 3D model. Results from a twomonth period of monitoring the performance of the GSHP system showed that the COP decreased withthe running time. The average COP of the horizontal-coupled GSHP was 2.5. The numerical predictionshowed that there was no significant difference in the specific heat extraction of the slinky heatexchanger at different coil diameters. However, the larger the diameter of coil, the higher the heatextraction per meter length of soil. The specific heat extraction also increased, but the heat extraction permeter length of soil decreased with the increase of coil central interval distance.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

To reduce dependence on fossil fuel energy resources andenvironmental degradation resulting from their combustion,renewable energy sources must play an increasingly significantrole. Geothermal energy, one of the renewable energy resources,can be used to provide electricity, heating and cooling for buildings.Shallow Ground Source Heat Pump systems (GSHP) which makeuse of the relatively constant temperature of the earth are mainlyused for space conditioning. However, the installation of theground Heat Exchanger (HE) is costly, which is one of the maincauses preventing the wide adoption of the GSHPs in the UK.Therefore, the horizontal-coupled ground source heat pump hasbecome increasingly important. It can reduce installation costcompared to that of other GSHP systems as no drilling is necessaryand only a trench 1e2 m in depth is required. Nevertheless, a singletier ground loop requires a large area of ground to lay the pipenetwork. This problem can be alleviated to some extent byemploying double tier pipe arrangement or slinky ground loop.

, jackwuyp@googlemail.com

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A slinky ground loop requires horizontal trench lengths of 20e30%of those for a horizontal-coupled single pipe configuration,however the thermal performance of the slinky ground loop maybe doubled. A first recorded slinky heat exchanger coupled GSHPsystem was developed by Bose and Smith [1] at Oklahoma StateUniversity. They indicated that the slinky heat exchanger enhancedthe heat transfer area within a limited space when compared toa horizontal buried heat exchanger.

The performance of a heat pump is usually expressed in terms ofits Coefficient Of Performance (COP) which is the ratio of energyoutput to supplied energy (electricity for the compressor, pumpetc) of GSHP. COPs in the heating and cooling mode are identifiedby COPh and COPc, respectively. A typical heat pump has a COP ofaround 4 which indicates that the heat pump produces four unitsof heating energy for every unit of electrical energy input. The COPof the GSHP depends on the soil type at the installation, etc.Research has been carried out to determine the performance ofhorizontal-coupled GSHP. Metz [2] used a horizontal-coupled GSHPto provide heating and cooling for a 104 m2 house in Long Island,New York. During a period of one year monitoring, the obtainedaverage COPh and COPc of the GSHP system were 2.46 and 1.91,respectively. _Inallı and Esen [3] carried out experimentalmeasurements of a GSHP system with horizontal-coupled heatexchanger in selected depths of 1 and 2 m in Elazi�g, Turkey.

Fig. 1. Image of the horizontal-coupled ground source heat exchanger at TalbotCottage, Drayton St Leonard site, Oxfordshire, UK.

Y. Wu et al. / Applied Thermal Engineering 30 (2010) 2574e2583 2575

After running the GSHP for heating from November 2002 to April2003 for a room with 16.24 m2

floor area, the obtained averageCOPh of the GSHP system was 2.66 and 2.81 for the heat exchangerburied at depths of 1 m and 2 m, respectively. Doherty et al. [4]installed and undertook experimental measurements of a hori-zontal slinky GSHP system in the University of Nottingham, UK. Anaverage COPh of approximately 2.7 was reported. Coskun et al. [5]studied the performance of a horizontal ground source compres-sion refrigeration system in Bursa city, Turkey. The horizontal-coupled heat exchanger was buried at a depth of 2.0 m. The COPc ofthe overall system varied between 2 and 2.5.

Research has also been undertaken for the horizontal-coupledGSHP systems by other investigators including the experimentalcharacterisations of the thermal performance of the horizontal-coupled heat exchanger and different approaches to enhance theheat transfer between the heat exchanger and the soil [6e11].Florides and Kalogirou [12] reviewed the performance and

Fig. 2. 3D sketch of the sensor positions used to meas

numerical model of ground coupled heat exchangers. They indi-cated that the ambient climatic conditions would affect thetemperature profile below the ground surface and this should beconsidered when designing a horizontal-coupled heat exchanger.In the UK, most of the recent studies of the horizontal-coupledGSHP were focused on the heat pump output to maintain dailyindoor air temperature [13e16]. Few studied the long term effect ofhorizontal-coupled heat exchanger on the ground temperature,which is vital for correct sizing of heat pumps to achieve an accu-rate prediction of their long term performance over their usefulworking life. In this work, the performance of a horizontal-coupledslinky GSHP installed in the UK was monitored and measuredresults were used to validate a CFDmodel. The validated model wasthen used to predict the thermal performance of ground coupledslinky heat exchangers for different slinky diameters and slinkyinterval distances.

2. Experimental measurements

2.1. Experimental apparatus installation

Experimental measurements were carried out at Talbot Cottage,Drayton St Leonard site, Oxfordshire, UK where 4 parallel hori-zontal-coupled slinky Heat Exchanger loops were installed ina 80 m long by 20 mwide paddock area at a depth of around 1.2 mbelow the ground surface. Fig. 1 shows an image for one of theloops. The fluid in the heat exchanger was watereethylene glycol(30% by weight) mixture. The ground surface was overgrown withwild flowers. Soft sand was used below and on top of the slinkyheat exchanger.

To monitor the soil temperature distribution and also theperformance of the horizontal-coupled heat exchanger, two holeswere dug in the ground to install thermistors at various depths. Onehole was dug above a portion of the ground coupled heatexchanger, the second hole served as a reference hole dug ata distance of around 2 m from the other hole and with no heatexchanger installed around it. After installing the sensors, all theholes were refilled with the original soil. Monitoring started

ure the slinky and the soil thermal performance.

Fig. 3. Measured ambient air and soil temperatures from 6th Nov 2009 to 31st Dec2009.

Y. Wu et al. / Applied Thermal Engineering 30 (2010) 2574e25832576

1 month after switching on the GSHP system. A sketch of thepositions of the sensors used to monitor the soil and the heatexchanger thermal performance with detailed sensor positions isshown in Fig. 2. T-type thermocouples were installed at the inletand outlet of the ground coupled heat exchangers to monitor thethermal energy extracted by the ground coupled slinky heatexchanger. Simultaneously, a mini weather station was installed tomonitor the ambient air temperature, wind speed, global solarradiation, relative humidity and rain fall. All the sensors werecalibrated before installation.

2.2. Analysis of experimental results

The sensor readings were taken every 30 min. Ambient airtemperature and soil temperatures above a portion of heatexchanger and in the reference hole for a period of around twomonths from 6th Nov 2009 to 31st Dec 2009 are shown in Fig. 3.The measured soil temperature at a depth of 0.25 m had a similarcharacteristic to ambient air temperature. There were short termstrong and irregular fluctuations of temperature, caused by thedaily change of weather conditions. However, the temperature

fluctuation ceased at a larger depth, below the depth of 0.5 m, thetemperature fluctuations were hardly discernible. It can be seenthat soil with and without heat exchanger below had a similartemperature variation pattern at the same depth. However, thesoil temperatures with a heat exchanger installed below werelower than those of the soil in the reference hole without heatexchanger under it. With the soil depth increase, the soiltemperature increased without heat exchanger below it, whereasthe soil temperature decreased where the heat exchanger wasinstalled below it. This is due to the thermal energy beingextracted by the heat exchanger. At a depth of 0.02 m below theground surface, there were no significant temperature differencesfor the soil with and without heat exchanger below them. Around20th Dec 2009, the soil without heat exchanger installed belowhad a higher temperature of around 2 �C than that with heatexchanger installed below. This deviation may be caused by thedifferent soil physical properties or the heat exchanger. At a largerdepth, the temperature difference between the soil with andwithout heat exchanger installed was larger. The temperaturedifference at a depth of 0.5 m was around 1.5 �C, and 3 �C ata depth of 1 m.

The ground temperature distribution was analysed in moredetail for two different days-7th Nov and 28th Dec 2009. It wassunny on 7th Nov 2009, and a partly cloudy day and the groundwascovered by snow on 28th Dec 2009. The measured solar radiationintensity, ambient air temperature and wind speed for the two daysare shown in Fig. 4(a). The measured solar radiation intensity on7th Nov. 2009 was significantly higher than that on 28th Dec. 2009.The highest solar radiation intensity was around 350 W/m2 on 7thNov 2009 and approximately 150 W/m2 on 28th Dec 2009. Themeasured ambient temperature was also higher on 7th Nov 2009.The measured average wind speed was around 1 and 2 m/s on 7thNov and 28th Dec 2009, respectively.

The various ground surface temperatures with ambient airtemperature are shown in Fig. 4(b), and the ground temperaturedistribution above the heat exchanger and without heat exchangerinstalled below on 7th Nov 2009 is illustrated in Fig. 5. From Figs.4(b) and 5, it can be seen that the soil with and without heatexchanger installed, their surface temperature had a similarcharacteristic to the ambient air temperature on that day,however, the ground surface temperature was slightly higher withno heat exchanger installed below. This might be caused bydifferent soil physical properties. They all decreased in the nightfrom 00:00 to 8:00 due to convective and long wave radiative heatlosses, and reached the lowest value of around 3 �C of the day at8:00 in the morning, and then increased to around 13.5 �C (no HE)and 11.5 �C (HE installed), respectively at noon. After 13:30 they allbegan to decrease due to the reduced solar radiation intensity.Because the thermal energy in the ground was extracted by theheat exchanger, the soil temperature varied between 7 and 8 �C ata depth of around 1.14 m below the ground. The environmentalconditions influenced the soil temperature up to a depth of around0.5 m. It can also be seen that at the same depth of around 1 mfrom the ground surface, the soil with no heat exchanger installedbelow had a constant temperature around 11 �C for the whole day,which was over 3 �C higher than the soil temperature with heatexchanger installed below. At a depth of 0.25 m from the groundsurface, the soil without heat exchanger installed below hada temperature around 1 �C higher than that with heat exchangerinstalled below. It can be concluded that the extraction of thermalenergy by the heat exchanger affected the soil temperature ata distance of around 0.9 m from the heat exchanger during theoperation period.

Fig. 6 illustrates ground temperature distribution above the heatexchanger and ground temperature distribution in the reference

Fig. 4. Measured ambient air temperature, ground surface temperature, solar radiation and wind speed on 7th Nov and 28th Dec 2009.

Y. Wu et al. / Applied Thermal Engineering 30 (2010) 2574e2583 2577

hole on 28th Dec 2009. From Figs. 4(b) and 6, it can be seen that thesoil at a depth of 0.02 m had a temperature around 0 �C for thewhole day, because the ground surface was covered with approx-imately 0.05 m thick snow, which prevented the radiative andconvective heat loss, and also the chance to absorb the diffusedsolar radiation at the ground surface. Its temperature was higherthan the ambient air temperature at the same time. At a depth ofaround 1.0 m from the ground surface, the soil with no heatexchanger below had a constant temperature around 5 �C for thewhole day, it was approximately 2.5 �C higher than that of the soilwith heat exchanger installed below. At a depth of 0.1 m from theground surface, the soil with no heat exchanger below hada temperature around 0.5 �C higher than that with heat exchangerinstalled below, similar to characteristics of the soil temperaturedistribution found on 7th Nov 2009.

Fig. 7 illustrates the measured inlet and outlet fluid tempera-tures of the ground coupled heat exchanger. The temperatures ofthe inlet and outlet of the heat exchanger significantly decreasedwith time. However, there were no significant change in temper-ature difference between the inlet and outlet. The GSHP system hadbeen continuously running for nearly two month by the time

(nearly 24 h a day). Only for a few hours (less than 1 h a week), thetemperature difference dropped to approximately 1 �C from 2 �Cduring a two month operation, because the GSHP system stoppedrunning, and the inlet temperature for the heat exchanger began toincrease for a short period. The calculated COP of the ground sourceheat pump system is shown in Fig. 8. It can be seen that the systemCOP decreased slightly with the time from 6th Nov 2009 to 31st Dec2009. It was around 2.7 on 6th Nov 2009, and decreased toapproximately 2.5 on 31st Dec 2009.

2.3. Measurement of site soil properties

Three soil samples at depths of around 0.2, 0.8 and 1 m belowthe ground surface at the test site were taken to the lab at theDepartment of the Built Environment, University of Nottinghamusing 50 mm deep by 50 mm inner diameter aluminium cylindertins. The thermal conductivity, thermal capacity, density andvolumetric moisture content of the in situ undisturbed soil weremeasured using the following facilities

Fig. 5. Measured soil temperature variation on 7th Nov 2009.

Fig. 6. Measured soil temperature variation on 28th Dec 2009.

Fig. 7. Measured variation of inlet and outlet temperatures of heat exchanger.

Y. Wu et al. / Applied Thermal Engineering 30 (2010) 2574e25832578

� KD2 pro used to measure the thermal conductivity andcapacity,

� A thermostatically controlled oven, capable of maintaininga temperature between 105 and 110 �C,

� A balance readable and accurate of 0.01 g,

The measured thermophysical properties of the soil samples areshown in Table 1. The measured thermophysical properties variedat different depths. The thermal conductivity and density werehigher for the soil near the ground surface and heat exchanger thanin the middle but the soil thermal capacity and the volumetricmoisture content decreased with the depth. Average thermophys-ical properties were used in the numerical prediction.

3. Numerical prediction

A commercial CFD software package FLUENTwas used to predictthe thermal performance of a portion of the horizontal-coupledslinky and straight heat exchangers. The transient 3 dimensionalsensible heat transfermodel for the ground coupled heat exchangeris as follows:

vðrTÞvt

þ V$

�r*VT � k

cVT

�¼ q

c(1)

where r is the density (kg/m3), t is the time (s), T is the temperature(K),V is thefluidflowvelocity (m/s), k is the thermal conductivity (W/m K), c is the specific heat (J/kg K) and q is the heat source (W/m3).

The model was validated using the experimentally measuredresults using the method described below. The measured envi-ronmental conditions at 00:00 7th Nov 2009 were used as the

Fig. 8. Measured variation of COP of the GSHP system.

Table 1Measured thermophysical properties of the undisturbed soil samples from the site.

Sample 1 (Depth at 1 m) Sample 2 (Depth at 0.6 m) Sample 3 (Depth at 0.2 m) Average

Soil thermal conductivity (W/m K) 1.37 1.02 1.32 1.24Soil thermal capacity (J/kg K) 1383.32 1420.27 1591.07 1464.88Soil density (kg/m3) 1598.70 1431.15 1733.01 1587.62Soil volumetric moisture content (%) 22.69 24.79 30.74 26.07

Y. Wu et al. / Applied Thermal Engineering 30 (2010) 2574e2583 2579

initial conditions for the model. The predicted and the measuredsoil temperature distributions at selected times are shown in Fig. 9.It can be seen that the predictions generally agreed with theexperimental measurements. The maximum difference betweenthe predicted and measured soil temperatures was less than 1 �C,which was found near the ground surface. This might be the resultof using constant and uniform soil thermophysical properties in themodel, whereas the soil thermophysical properties might not beuniform particularly near the surface.

The thermal performance of a portion of a horizontal-coupledslinky and straight heat exchangers was predicted using the vali-dated CFD model. Comparisons of thermal performances of theslinky heat exchanger at different slinky (coil) diameters anddifferent slinky (coil) interval distance were also conducted. Themeasured average thermophysical properties of the in situ soildetailed in Section 2.3 were used in the numerical model. In thepredictions, itwas assumed that thehorizontal-coupled straight andslinky heat exchangerswere buried at a depth of 1.2m in the groundas illustrated in Fig. 10. Due to a large number of meshes required tomodel the 3D transient heat transfer, only half of the slinky heatexchanger was applied in the model and a symmetric surface wasdefined at the vertical central surface as shown in Fig. 10(b).

Further assumptions used to simplify themodel were as follows,

� For a short length of the heat exchanger, the pipe outersurface temperature was assumed constant at 1 �C for

Fig. 9. Predicted and measured soil temperatu

heating mode. Prediction of the thermal performance of theheat exchanger with similar dimension and environmentalconditions with 1 �C refrigerant fluid flow at the inlet of theheat exchanger was also undertaken, and the results showedthat after running the system for more than 140 h, therewas no significant difference in the temperature distributionin the soil or the specific heat extraction between theconditions with fixed pipe temperature and fluid flow insidethe heat exchanger,

� A constant temperature of 10 �C was utilised at a depth of 4 mfrom the ground surface for a short period of operating the GSHP,

� The soil thermal properties (soil thermal conductivity anddiffusivity) were constant and uniform,

� The wind speed was 3 m/s on the ground surface with anambient air temperature of 5 �C.

3.1. Predicted thermal performance of the horizontal-coupledstraight and slinky heat exchanger

Isotherms generated from prediction for the horizontal-coupledstraight and slinky heat exchanger at the elapsed time of 1 and 50 hare shown in Figs. 11 and 12, respectively. For the horizontal-coupled straight heat exchanger, after running the system for 1 h,the temperature of the soil around the heat exchanger decreased to1 �C. The temperature of the soil decreased at a distance within0.1 m from the central long axis of the heat exchanger, due to the

res at selected times on 07th Nov. 2009.

a b

Fig. 10. Sketch of a portion of horizontal-coupled pipe (a) Straight pipe, (b) Slinky pipe.

Y. Wu et al. / Applied Thermal Engineering 30 (2010) 2574e25832580

thermal energy being extracted by the heat exchanger. At 50 h, theheat exchanger affected the temperature of the soil withina distance of around 0.6 m from the central long axis of the heatexchanger. With further running of the system, the soil tempera-ture continuously decreased, due to the convective heat loss from

Fig. 11. Isotherm generated from prediction for

the soil to the ambient air and heat extraction by the heatexchanger. The slinky heat exchanger and its surrounding soil hadsimilar thermal characteristics as that for the straight heatexchanger under similar conditions. However, the temperature ofthe soil within a distance of around 0.8 m from the central long axis

the straight heat exchanger at 1 and 50 h.

Fig. 12. Isotherm generated from prediction for the slinky heat exchanger at 1 and 50 h.

Y. Wu et al. / Applied Thermal Engineering 30 (2010) 2574e2583 2581

of the heat exchanger was influenced by the heat exchanger, it waslarger than that of the single straight heat exchanger system. This isbecause the slinky system comprised of a straight and coil heatexchangers with a small portion of the heat exchanger being inparallel located.

A parameter used to calculate the required length of boreholeheat exchangers is the specific heat extraction, expressed in Watt

Fig. 13. Predicted variation of specific heat extraction with time for the horizontal-coupled straight and slinky heat exchangers.

per meter length. Typical values are in the range from 40 to70 W/m for borehole heat exchangers, dependent on soil thermalconductivity, heat pump annual operation hours, number ofneighbouring boreholes, etc [17]. The variation of the predictedspecific heat extraction with time for a portion of the horizontal-coupled straight and slinky heat exchangers, and the difference

Fig. 14. Predicted variation of the heat extraction per meter length of soil by straightand slinky heat exchanger with time.

Table 2Dimensions of domain for selected models.

Model dimension Slinky(d ¼ 1 m)

Slinky(d ¼ 0.8 m)

Slinky(d ¼ 0.6 m)

Width (m) 3 2.8 2.6Length (m) 1.5 1.5 1.5Depth (m) 4 4 4Diameter of the

heat exchanger (m)0.04 0.04 0.04

Total length of theheat exchanger (m)

1.5 (straightpart) þ 1.57(coil part)

1.5 (straightpart) þ 1.256(coil part)

1.5 (straightpart) þ 0.942(coil part)

Fig. 16. Predicted variation of specific heat extraction with time for slinky heatexchangers at different coil interval distances.

Y. Wu et al. / Applied Thermal Engineering 30 (2010) 2574e25832582

of the specific heat extraction between the two systems areillustrated in Fig. 13. It can be seen that the specific heatextraction for both the horizontal-coupled straight and slinkyheat exchangers was approximately at 46 W/m initially,decreased to approximately 33 and 30 W/m respectively aftercontinuously running the GSHP system for 10 h, and furtherdecreased to around 18 and 15 W/m after running the GSHPsystem for 140 h. Thus, it can be concluded that the difference ofthe specific heat extraction between the straight and the slinkyheat exchangers increased with running time. It was around1.5 W/m for the first hour, and increased to around 3 W/m at140 h. The predicted specific heat extraction was lower than thatcommonly used for designing GSHPs. If the total length of theheat exchanger was estimated from the over optimistic value ofthe specific heat extraction, the output and COP of the heat pumpwould be lower than expected.

The variation of the heat extraction per meter length of the soiloccupied by heat exchangers is illustrated in Fig. 14. Although thespecific heat extraction of the slinky heat exchanger was lower thanthat of the straight one, the heat extraction per meter length of thesoil for the slinky heat exchanger was significantly higher than thatof the straight heat exchanger. At the beginning, the differencebetween the slinky and straight heat exchanger was about 45 W/mlength of soil, and decreased to approximately 12 W/m length ofsoil after running the simulation for 140 h.

3.2. Effect of the coil diameter on the thermal performance of theslinky heat exchangers

The thermal characteristics of the ground coupled slinky heatexchangers at different coil diameters of 0.6, 0.8 and 1.0 m weredetermined by running the transient state simulation for thesimilar environmental conditions used in Section 3.1. The dimen-sions of the domain and the heat exchangers are shown in Table 2.

Fig. 15. Predicted variation of the heat extraction per meter length of soil for slinkyheat exchanger at different coil diameter.

From the prediction, it was found that there was no significantdifference of the specific heat extraction of the slinky heatexchanger at different coil diameters of 0.6, 0.8 and 1.0 m. Thismight be because the horizontal-coupled straight part of the heatexchanger occupied over 50% of the total length in the simulatedslinky heat exchanger. Thus by changing the diameters of the coil,there was no obvious differences of the specific heat extraction.However, the larger the diameter of the coil, the higher the heatextraction per meter length of soil. The variation of the heatextraction per meter length of the soil occupied by heat exchangersat different coil diameters is illustrated in Fig. 15. At the beginning,the heat extraction per meter length of soil was around 91W/m forthe coil diameter of 1 m, approximately 82 W/m for the coildiameter of 0.8 m and 72 W/m for the coil diameter of 0.6 m. Itdecreased to approximately 30 W/m with a coil diameter of 1 m,around 27 W/m with a coil diameter of 0.8 m and 24.6 W/m witha coil diameter of 0.6 m at 140 h.

3.3. Effect of the coil central interval distance on the thermalperformance of the slinky heat exchangers

The thermal characteristics of the ground coupled slinky heatexchangers at different coil central interval distance of 1.2, 1.6, 2.0and 3.0 m were predicted using the similar environmental condi-tions used in sections 3.1 and 3.2.

The variation of the specific heat extraction for the slinky heatexchangers and the variation of the heat extraction per meterlength of the soil occupied by heat exchangers at different coilcentral interval distances are illustrated in Figs. 16 and 17, respec-tively. From the prediction, it was observed that the specific heat

Fig. 17. Predicted variation of the heat extraction per meter length of soil for slinkyheat exchanger at different coil interval distances.

Y. Wu et al. / Applied Thermal Engineering 30 (2010) 2574e2583 2583

extraction increased with the increase in the coil central intervaldistance. At the beginning, the specific heat extraction was 44.6 W/m for a slinky with coil central interval distance of 3 m, 44.2 W/mfor that at 2 m, 44.0 W/m at 1.6 m and 43.8 W/m at 1.2 m. At 140 h,the specific heat extraction decreased to 14.9 W/m for a slinky withcoil central interval distance at 3 m, 14.0 W/m for that at 2 m,13.3 W/m at 1.6 m and 11.8 W/m at 1.2 m. However, the heatextraction per meter length of soil for a slinky decreased with theincrease of coil central interval distance. At the beginning, the heatextraction per meter length of soil was 158.4 W/m for a slinky withcoil central interval distance of 1.2 m, 130.5 W/m for that at 1.6 m,113.7 W/m at 2.0 m and 91.3 W/m at 3.0 m. At 140 h, the heatextraction per meter length of soil decreased to 42.5 W/m fora slinky with coil central interval distance of 1.2 m, 39.3 W/m forthat at 1.6 m, 36.1 W/m for that at 2.0 m and 30.5 W/m at 3.0 m.

4. Conclusions

The thermal performance of horizontal-coupled slinky GSHPwas determined both experimentally and numerically in a UKclimate. During a twomonth period of monitoring the performanceof the GSHP system, the COP of the GSHP decreased with therunning time. The average COP of the horizontal-coupled GSHPwas2.5. To increase the COP of the system, it required longer heatexchanger and a larger land area for heat exchanger installation.

The thermal performance of a portion of slinky GSHP at differentcoil diameters and different coil central interval distances waspredicted using a validated 3D model. The performance of slinkyheat exchangers was also compared with that of straight heatexchangers. The specific heat extraction for both the straight andslinky heat exchangers initially at 46 W/m decreased withincreasing system running time but at different rates. After runningthe systems for 140 h, the specific heat extraction of the straightpipe would be 3.5 W/m higher than that of the slinky pipe.However, the heat extraction per meter length of soil for the slinkyheat exchanger was significantly higher than that of the straightsystem. There was no significant difference in the specific heatextraction of the slinky heat exchanger at different coil diameters of0.6, 0.8 and 1.0 m. However, the larger the diameter of coil, thehigher the heat extraction per meter length of soil. The specific heatextraction also increased with the increase in the coil centralinterval distances. On the other hand, the heat extraction per meterlength of soil for a slinky decreased with the increase of coil centralinterval distance.

Acknowledgements

This work has been supported by the Natural EnvironmentResearch Council through the funding of the project. Also, theauthors would like to thank Mr Alan Johns for allowing us to accessthe Ground Source Heat Exchanger used in this work.

References

[1] J.E. Bose, M.D. Smith, Performance of new ground heat exchanger configura-tions for heat pump, Solar Engineering 1 (1992).

[2] P.D. Metz, Ground Coupled Heat Pump System Experimental Results. Broo-khaven National Laboratory, Upton, 1983, Report No. BNL-33540.

[3] M. _Inallı, H. Esen, Experimental thermal performance evaluation of a hori-zontal ground source heat pump system, Applied Thermal Engineering24 (2004) 2219e2232.

[4] P.S. Doherty, S. Al-Huthaili, S.B. Riffat, N. Abodahab, Ground source heat pumpe description and preliminary results of the Eco House system, AppliedThermal Engineering 24 (2004) 2627e2641.

[5] S. Coskun, E. Pulat, K. Ünlü, R. Yamankaradeniz, Experimental performanceinvestigation of a horizontal ground source compression refrigerationmachine, International Journal of Energy Research 32 (2008) 44e56.

[6] W.V. Jones, J. Taylor Beard, R.J. Ribando and Barry K. Wilhelm, Thermalperformance of horizontal closed-loop ground coupled heat pump systemsusing flowable-fill, Proceedings of the 1996 IECEC Meeting, Washington, D.C.,1996.

[7] M. Piechowski, Heat and mass transfer model of a ground heat exchanger:validation and sensitivity analysis, International Journal of Energy Research22 (1998) 965e979.

[8] M. Piechowski, Heat and mass transfer model of a ground heat exchanger:theoretical development, International Journal of Energy Research 23 (1999)571e588.

[9] H. Esen, M. Inalli, M. Esen, Numerical and experimental analysis of a hori-zontal ground- coupled heat pump system, Building and Environment42 (2007) 1126e1134.

[10] H. Demir, A. Koyun, G. Temir, Heat transfer of horizontal parallel pipe groundheat exchanger and experimental verification, Applied Thermal Engineering29 (2009) 224e233.

[11] A. Koyun, H. Demir, Z. Torun, Experimental study of heat transfer of buriedfinned pip for ground source heat pump applications, International Commu-nications in Heat and Mass Transfer 36 (2009) 739e743.

[12] G. Florides, S. Kalogirou, Ground heat exchangers e a review of systems,models and applications, Renewable Energy 32 (2007) 2461e2478.

[13] R. Rawlings, Ground source heat pump: a technology review Available from:www.bsria.co.uk.

[14] H. Singh, A. Muetzeb, P.C. Eames, Factors influencing the uptake of heat pumptechnology by the UK domestic sector, Renewable Energy 35 (2010) 873e878.

[15] Energy Saving Trust, Heat Pumps in the UK e A Monitoring Report (2000Edition) GIR72, London Available from: http://www.energysavingtrust.org.uk/business/Global-Data/Publications/Heat-Pumps-in-the-UK-a-monitoring-report-GIR72.

[16] Energy Saving Trust, Domestic Ground Source Heat Pumps: Design andInstallation of Closed-loop Systems (2007 Edition) (2007) CE82/GPG339,London Available from: www.est.org.uk/housingbuildings.

[17] European Geothermal Energy Council, Ground Source Heat Pump: a GuideBook (2008).