Applied Energy 102 (2013) 418–426

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Applied Energy

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‘Derating Factor’ new concept for evaluating thermal performance of earth airtunnel heat exchanger: A transient CFD analysis

Vikas Bansal a,⇑, Rohit Misra b, Ghanshyam Das Agarwal b, Jyotirmay Mathur a

a Mechanical Engineering Department, Govt. Women Engineering College Ajmer, Indiab Mechanical Engineering Department, Malaviya National Institute of Technology, Jaipur, India

h i g h l i g h t s

" Transient 3-D Computational Fluid Dynamics analysis of Earth Tunnel Air Heat Exchanger." Introduction of new concept ’Derating factor’ for evaluating thermal performance of Earth Tunnel Air Heat Exchanger." Effect of period of continuous operation on thermal performance of Earth Tunnel Air Heat Exchanger." Effect of thermal conductivity of soil on thermal performance of Earth Tunnel Air Heat Exchanger.

a r t i c l e i n f o

Article history:Received 16 April 2012Received in revised form 30 June 2012Accepted 20 July 2012Available online 30 August 2012

Keywords:Earth air tunnel heat exchangerDerating factorComputational fluid dynamicsFLUENT

0306-2619/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.apenergy.2012.07.027

Abbreviations: EATHE, Earth Air Tunnel Heat Echloride; RPM, revolution per minute; CFD, Computadry bulb temperature of air (�C); RTD, resistance teground source heat pump; GCHP, ground coupled heexchangers.⇑ Corresponding author. Address: Govt. Women

Mechanical Engineering Department, Nasirabad RoadIndia. Tel.: +91 9982378283.

E-mail addresses: bansal_vikas1@yahoo.com (V.com (R. Misra), gdagrawal2@gmail.com (G.D. Agarwcom (J. Mathur).

a b s t r a c t

A new term ‘Derating Factor’ is devised for evaluating deterioration in thermal performance of Earth AirTunnel Heat Exchanger (EATHE) under transient operating conditions in predominantly hot and dry cli-mate of Ajmer (India) using experimental and computational fluid dynamics modeling with FLUENT soft-ware. Maximum air temperature drop obtained using steady state approach for EATHE of pipe length100 m, pipe diameter 0.2 m and at air velocity of 5 m s�1 is 18.4 �C, 18.7 �C and 18.4 �C for soil thermalconductivity of 0.52, 2.0 and 4.0 W m�1 K�1 respectively. However, the maximum air temperature dropobtained using transient approach during 24 h of operation vary between 18.3 �C and 14.0 �C, 18.3 �Cand 17.2 �C and 18.6 �C and 18.0 �C for soil thermal conductivity of 0.52, 2.0 and 4.0 W m�1 K�1 respec-tively. The derating factor is found to be a function of thermal conductivity of soil, duration of continuousoperation of EATHE and length of pipe. The analyzed cases have shown the range of derating to be as min-imal as 0.2% to as high as 68%, which if ignored while designing may lead to poor performance of earth airheat exchangers. Maximum value of derating factor is observed after continuous operation of EATHE for24 h.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Earth Air Tunnel Heat Exchanger (EATHE) has been used as apassive heating/cooling method for reducing consumption of pri-

ll rights reserved.

xchanger; PVC, poly vinyltional Fluid Dynamics; DBT,mperature detectors; GSHP,at pump; GHEs, ground heat

Engineering College Ajmer,, Makhupura, Ajmer 305 001,

Bansal), rohiteca@rediffmail.al), jyotirmay.mathur@gmail.

mary energy resources in space heating/cooling. EATHE uses theground as heat source/ sink for space heating/cooling.

A lot of research work on ground heat exchangers has beencarried out. Khatry et al. [1] presented an analysis of the periodicvariation of ground temperature with depth which takes into ac-count the periodicity of solar radiation and atmospheric tempera-ture. Explicit expression for the temperature as a function of timeand depth was derived. Jacovides et al. [2] studied several statisti-cal characteristics of the soil temperature in Athens/Greecethrough Fourier analysis of a 74 year record (1917–1990) of soiltemperatures at the surface and at various depths and for bothbare and short-grass-covered areas. Bau [3] presented analyticalsolutions for heat losses from a buried pipe. Puri [4] performedparametric study of a single pipe carrying warm fluid buried inmedium wet sand for pipe diameter, initial soil moisture concen-tration and temperature, and fluid-tube interface temperature

V. Bansal et al. / Applied Energy 102 (2013) 418–426 419

using finite element model. Svec et al. [5] studied steady-state,transient and cyclic behavior of several configurations of EATHE.Sensitivity analysis for evaluating the performance of various pas-sive and hybrid cooling techniques such as earth-to-air heatexchangers, direct and indirect evaporative coolers as well as nightventilation techniques was carried out by Agas et al. [6]. Santa-mouris et al. [7] suggested that an increase of the buried-pipe’slength and soil height above the pipes resulted in an increase ofthe system potential cooling capacity. Bojic et al. [8] evaluatedthe technical and economic performance of an Earth Air TunnelHeat Exchanger (EATHE) which consists of pipes buried in soil cou-pled to the system for heating or cooling of a building that uses100% fresh air as heating or cooling medium during winter andsummer respectively using mathematical modeling.

Deglin et al. [9] described a three-dimensional non-steady-stateheat flow model for studying the influence of type of ground, airspeed and characteristics of the pipes (diameter, length, depthand spacing) on the efficiency of heat exchange between the soiland the air flowing through the pipe. A mathematical model basedon the representation of temperature in the form of the Fourierintegral for calculating the temperature of the soil and air in a soilheat exchanger for ventilation systems was developed byKabashnikov et al. [10]. A transient three-dimensional heat con-duction model was presented by Cui et al. [11] to describe the tem-perature response in the ground caused by heat transfer in theground heat exchangers (GHEs) with multiple boreholes. It wasfound by Badescu [12] that the energy delivered by the groundheat exchanger depends significantly on different design parame-ters like pipe’s depth, diameter and material. Increasing the pipeexternal radius yielded the decrease of both the heating energyand the cooling energy. Bansal et al. [13,14] studied the thermalperformance of EATHE system in summer and winter climatic con-ditions using CFD simulations and experimental observations.Investigations by Givoni [15] have shown that the potential ofthe system in hot climates can be improved using various soil cool-ing strategies to lower the natural subsurface soil temperaturesuch as shading, surface irrigation, surface treatment using plantsand pebbles. As reported by Bansal et al. [16], EATHE system inte-grated with evaporative cooling could deliver thermal comfortconditions in hot and dry climates. Ozgener and Ozgener [17–21]evaluated the performance of an underground air tunnel systemusing exergy analysis and experimental data. They determinedthe optimal design of a closed loop earth to air heat exchangerfor greenhouse heating by using exergo-economics. Bi et al. [22]presented exergy analysis of ground source heat pump (GSHP)for both heating and cooling mode and found that potential energysaving components for GSHP is compressor and ground heat ex-changer. It was also found that exergy loss of a GSHP system forbuilding heating mode is bigger than that of cooling mode. Manet al. [23,24] found experimentally that the performance of GSHPsystem in both heating and cooling mode was dependent on itsoperating conditions and mode of operation. They also studied hy-brid GSHP system, which was used for air-conditioning. Michopo-ulos et al. [25] observed that the size and performance of GSHPsystems depends on the building (geometry, construction materi-als, orientation etc. but also usage and internal gains), ground ther-mo-physical characteristics, climatology of the area and on theground heat exchanger design and construction. Yang et al. [26]observed that vertical ground heat exchanger (GHE) is a vital com-ponent for the design and simulation of ground coupled heat pump(GCHP). Lee [27,28] highlighted the merit of employing a variable-speed part-load in a GSHP system as the initial cost might also besaved besides the running costs under a wide range of climaticconditions. It was also observed that adoption of an effectiveground thermal conductivity and an effective ground volumetricheat capacity for a multi-layer ground is essential for evaluating

the performance of the system. Numerical simulation of a solar as-sisted ground coupled heat pump (SAGCHP) system was presentedby Chen and Yang [29] which was used for both space heating anddomestic hot water (DHW). Sagia et al. [30] studied hybrid GSHPsystem using numerical modeling on TRNSYS. Thermal interactionof multiple vertical ground heat exchangers was investigated byFayegh and Rosen [31] using a finite volume numerical modeldeveloped on FLUENT. Li and Lai [32] analyzed influence of anisot-ropy of anisotropic soil on the processes of heat transfer by groundheat exchangers used in ground-coupled heat pump systems.

In this paper, the authors aim to study the effect of thermal con-ductivity of soil surrounding the EATHE pipe and the effect oflength of operation of EATHE on its thermal performance. As dis-cussed above, in previous researches, most of the work was carriedout to evaluate the performance of the EATHE. Some of theresearchers discussed the deterioration in thermal performanceof EATHE qualitatively but not quantitatively. Similarly most ofthe work has been carried out in cold climates. The main objectiveof this paper is to develop an accurate EATHE model to evaluate thedeterioration in the performance of the EATHE due to continuousoperation of the system in extreme summer weather conditionsby introducing a new term ‘Derating Factor’. Effect of duration ofcontinuous operation and type of soil on derating factor has beendetermined for extreme summer conditions. In the present worksoil temperature field have also been studied in detail. Thermalperformance of the EATHE is evaluated in terms of derating factor,which is defined as the ratio of the difference between the drop inair temperature obtained by EATHE in steady state (soil surround-ing the EATHE pipe is assumed as infinite heat sink) and in tran-sient state to the drop in air temperature obtained by EATHE insteady state. Derating factor is an indirect measure to ascertainthe maximum deterioration in the thermal performance of EATHEoperating under transient conditions. Therefore, the deteriorationin thermal performance of EATHE under transient operating condi-tions is investigated and presented in a format suitable for de-signer’s use. Results of the CFD model which were validated bythe experimental observation have been used to evaluate the der-ating factor of EATHE for three soil thermal conductivities.

2. Description of CFD model

Computational Fluid Dynamics (CFD) tools are well known fortheir capability to carry out in-depth analysis of fluid flow, heattransfer, mass transfer and several other problems. They providenumerical solutions of partial differential equations governingfluid flow and heat transfer in a discretized form. CFD employs avery simple principle of resolving the entire system in small cellsor grids and applying governing equations on these discrete ele-ments to find numerical solutions regarding pressure distribution,temperature gradients and flow parameters in a shorter time at alower cost.

To examine the complicated airflow and heat transfer processesin an EATHE system, CFD software FLUENT 6.3 was used in thisstudy. In the present analysis, CFD simulations have been per-formed using an unstructured grid. CFD software has been em-ployed to resolve the transient temperature field around thehorizontal buried pipe of EATHE. The element type and the griddensity were selected to be variable because the temperaturechanges more sharply around the pipe wall therefore the gridwas designed to be more dense in that area, while it was moresparse farther away from the pipe wall. In the present study ithas been assumed that air is incompressible and the soil is homo-geneous and its physical properties are constant. It was also as-sumed that the thermo physical properties of pipe and soil donot change with temperature and engineering materials used in

Fig. 1. Four different views of CFD model for EATHE.

Table 1Thermo-physical parameters used in simulation.

Material Density(kg m�3)

Specific heat capacity(J kg�1 K�1)

Thermal conductivity(W m�1 K�1)

Air 1.225 1006 0.0242Soil (S1) 2050 1840 0.52Soil (S2) 2050 1840 2.0Soil (S3) 2050 1840 4.0PVC 1380 900 1.16

420 V. Bansal et al. / Applied Energy 102 (2013) 418–426

the CFD model are isotropic and homogeneous. It has also been as-sumed that thermal conductivity of the soil remains constant. TotalNo. of elements used in the present CFD simulation are 364989 forEATHE pipe and 1075849 for surrounding soil.

The fundamental equations of fluid flow and heat transfer havebeen used in the analysis. The geometric modeling and meshinghave been prepared using GAMBIT version 2.3 as shown in Fig. 1.The main objective of the CFD study was to investigate the transientbehavior of simple EATHE system used in continuous cooling modeand compare it’s thermal performance with EATHE operating understeady state condition (assuming that the temperature of soil sur-rounding the pipe remains constant) in terms of derating factor.

The thermo-physical parameters of materials used in the simu-lation are listed in Table 1. The thermal properties of soil S1 (for Aj-mer) viz. thermal conductivity, specific heat capacity and densityhave been determined through physical testing of the soil sampletaken from a depth of 3.7 m. The test was conducted followingstandard procedures in laboratory. Actual determined values ofabove mentioned soil properties for soil S1 were used for the val-idation of CFD model. Thereafter the validated model was usedfor parametric variation. For soil S2 and S3, the values of thermalconductivity of soil were selected from available literature suchas Cucumo et al. [33] and Beier et al. [34] for studying the effectof soil thermal conductivity on thermal performance of EATHEkeeping specific heat capacity and density of soil S2 and S3 as thatof S1. The CFD model of EATHE system was validated using anexperimental set up having pipe diameter of 0.1 m and pipe lengthof 60 m, as shown in Fig. 2.

The validated CFD model has been extended to analyze EATHEsystem having pipe diameter of 0.2 m and 100 m length. Ajmiet al. [35] assumed that the thermal effect of soil surroundingthe pipe is negligible after a distance ‘r’ from the pipe outer surface,where ‘r’ is the pipe radius. In order to be on safer side, in CFD mod-eling outer diameter of the soil cylinder surrounding the EATHEpipe has been taken as four times the pipe diameter. Air–pipeinterface in CFD simulation has been considered as coupled bound-ary. Different soil layers surrounding the pipe have also been pre-pared to study the transient temperature field of the soil. Effect ofthermal conductivity of soil on the thermal performance of EATHEsystem has also been studied by using three different types of soilnamely S1, S2 and S3 having thermal conductivity of 0.52, 2.0 and4.0 W m�1 K�1 respectively.

3. Experimental set-up

The schematic diagram of room integrated EATHE is shown inFig. 2. Experimental test set up comprises of 60 m long horizontalPVC pipe of inner diameter 0.10 m, buried in flat land with dry soilat a depth of 3.7 m. Inlet end of EATHE pipe is connected through avertical pipe to a 0.75 kW, single phase, variable speed motorizedblower (maximum flow rate of 0.0945 m3/s and maximum speedof 2800 rpm).

Ambient air was forced through the earth air pipe system withthe help of blower and air flow velocity was changed with the helpof an auto transformer (single phase, 0–270 V, 2 A maximum cur-rent, with a least count of 1 V). Seven RTD (Pt-100) temperaturesensors viz. T0–T6 were mounted at a depth of 0 m, 0.62 m,1.24 m, 1.86 m, 2.48 m, 3.10 m and 3.7 m respectively from theground surface on inlet vertical pipe to measure soil temperaturesat different depths. One additional temperature sensor was in-serted at a distance of 10 m away from the EATHE system at adepth of 3.7 m in the ground to measure the undisturbed soil tem-perature. Nine RTD (Pt-100) temperature sensors viz. T7–T15 werealso inserted at the center of EATHE pipe along the length at a hor-izontal distance of 0.2 m, 1.7 m, 4.7 m, 9.3 m, 15.1 m, 24.2 m,34.0 m, 44.4 m and 60.0 m respectively from the upstream end to

Fig. 2. Schematic of room integrated EATHE system.

V. Bansal et al. / Applied Energy 102 (2013) 418–426 421

measure air temperature. A group of four RTD (Pt-100) tempera-ture sensors at axial distance of 6.4 m, 27.4 m and 48.8 m fromthe inlet of EATHE were also provided to measure the temperatureof pipe-soil interface, temperature of soil at a distance of 0.2 m,0.4 m and 0.6 m from pipe surface respectively. Properly cali-brated, digital temperature display devices (accuracy of ±0.1 �Cand resolution 0.1 �C) have been used. Dry bulb temperature andrelative humidity of ambient air were recorded hourly using RTD(Pt-100) temperature sensor and capacitive transducer mountedon weather station. Temperature and relative humidity of air in-side the test room were also measured accurately with the helpof calibrated thermo hygrometer (make – Fluke-971, temperatureaccuracy of ±0.1 �C, temperature resolution of 0.1 �C and relativehumidity resolution of 0.1%). Air flow velocity is measured withthe help of a vane probe type anemometer (make – Lutron, mod-el-AM-4201, range 0.4 to 30.0 m/s and least count of 0.1 m/s).

Electrical energy consumed by the centrifugal blower was mea-sured with the help of calibrated digital energy meter (make –Power tech measurement system, type – PTS-01, least count of0.1 kWh and an accuracy of ±0.1 kWh). dimensions of researchroom is 4.3 m � 3.8 m � 3.05 m, having two windows(1.52 m � 1.22 m each, located on east and north facing wallsrespectively) and a door (1.82 m � 0.91 m, located on west facingwall).

4. Test data and error analysis

Test data are shown in Table 2. Based on the analysis of errors inthe experimental measurements through various instruments em-ployed, the uncertainties in the measurement of temperature andvelocity are estimated as ±0.38% and ±2% [36].

Table 2Air temperature along pipe length after different hours of continuous operation ofEATHE (test data).

Pipe length Hourly air temperature variation along pipe length

6 h 12 h 24 h 36 h 48 h

0.2 317.8 316.3 309.0 316.3 309.01.7 317.2 315.9 308.8 315.9 308.84.7 315.1 314.3 308.1 314.6 308.39.3 312.5 312.3 307.2 312.9 307.5

15.1 309.4 309.9 306.1 310.7 306.524.2 306.1 307.0 304.6 308.1 305.234.0 303.7 304.7 303.4 305.8 304.144.4 302.3 303.1 302.6 304.1 303.160.0 301.1 301.7 301.7 302.4 302.1

300

302

304

306

308

310

312

314

316

318

320

0 10 20

Tem

pera

ture

of

air

in p

ipe

(K)

Length

Simulated temperature after 1 hourSimulated temperature after 4 hourSimulated temperature after 7 hour

Fig. 3. Validation of CFD results

Table 3Variation in air temperature along the EATHE pipe length for

Pipe length (m) Air temperature along the EATHE

S1 S

Inlet 319.1 35 317.8 3

10 314.6 315 312.0 320 309.8 325 308.0 330 306.6 340 304.6 350 303.1 360 302.2 370 301.4 380 301.1 390 300.9 3

100 300.7 3

422 V. Bansal et al. / Applied Energy 102 (2013) 418–426

5. Experimental validation and performance analysis

CFD model of EATHE system (as shown in Fig. 1) has been devel-oped and validated for summer weather conditions by taking obser-vations on the experimental set-up for the month of June, 2011 atAjmer (Western India). Comparison of simulated and experimentalresults for air temperature in the pipe at various sections along thelength is summarized for air velocity of 5 m s�1 as shown in Fig. 3.Inlet condition of air in CFD simulation was kept same as measuredon experimental set up. Validation of the simulation model werecarried out for pipe diameter of 0.05 m, 0.1 m, 0.15 m and 0.2 mfor which experimental set-ups with smaller pipe lengths wereavailable. Experimental set-up of EATHE for pipe diameter 0.2 mconsisted of a length of 20 m only. Experimental data gathered on20 m length tunnel was in agreement with the simulated values.As the experimental set up of EATHE system consisting of 0.1 m pipe

30 40 50 60

of pipe (m)

Experimental temperature after 1 hourExperimental temperature after 4 hourExperimental temperature after 7 hour

with experimental results.

three different soils under steady state conditions.

pipe length for three different soils (K)

2 S3

19.1 319.117.6 317.614.0 313.911.1 310.908.8 308.607.0 306.805.6 305.403.5 303.502.1 302.201.4 301.601.0 301.000.9 300.700.8 300.500.7 300.4

Fig. 4. Temperature contour of air and soil at a section situated 50 m from for pipe inlet after twelve hour for various soils (a) S1, (b) S2, and (c) S3.

V. Bansal et al. / Applied Energy 102 (2013) 418–426 423

diameter was of larger length (60 m), therefore validation result forthis pipe are presented in the manuscript.

It is observed from Fig. 3 that there is a small difference (3.4–8.0%) between the experimental and simulated values and thusthe CFD model is valid and has been used for further analysis.

6. Derating factor and transient performance of EATHE system

The validated CFD model has been used for determining the tem-perature of air along the EATHE pipe using steady state approach. Insteady state approach, it is assumed that the soil surrounding theEATHE pipe remains at constant temperature of 300.24 K (it is

experimentally observed undisturbed ground temperature at adepth of 3.7 m) irrespective of the duration of operation. The outerboundary of the soil cylinder is taken at a constant temperature ofundisturbed soil temperature i.e. 300.24 K. CFD simulations usingsteady state approach were carried out for EATHE system with soilsS1, S2 and S3. Air temperature along the length of EATHE pipe understeady state condition for soil S1, S2 and S3 is shown in Table 3.

Thermal performance of EATHE assuming the soil surroundingthe pipe to be at constant temperature is evaluated in terms of tem-perature drop obtained from EATHE pipe due to heat transfer be-tween air and the soil. Under steady state operation of EATHE, thetotal temperature drop obtained is independent of thermal conduc-tivity of soil as shown in Table 3. Assumption of soil surrounding

Table 4Hourly variation of drop in air temperature at different pipe length for three type of soils.

Pipe length (m) Drop in air temperature at different pipe length for three type of soils during 24 h of operation (K)

After 1 h After 6 h After 12 h After 24 h

S1 S2 S3 S1 S2 S3 S1 S2 S3 S1 S2 S3

0–10 3.3 4.2 4.5 2.2 3.4 4.0 1.9 3.1 3.8 1.6 2.9 3.70–20 7.5 9.0 9.5 5.2 7.5 8.5 4.4 7.0 8.2 3.8 6.5 8.10–30 10.6 12.2 12.7 7.7 10.5 11.6 6.6 9.8 11.2 5.7 9.3 11.10–40 12.9 14.4 14.8 9.7 12.7 13.8 8.5 12.0 13.4 7.3 11.4 13.20–50 14.7 15.9 16.3 11.5 14.4 15.3 10.1 13.7 15.0 8.9 13.1 14.80–60 15.9 16.9 17.1 12.8 15.5 16.3 11.4 14.8 16.0 10.1 14.3 15.80–70 16.8 17.6 17.8 14.1 16.4 17.1 12.7 15.8 16.8 11.3 15.3 16.70–80 17.5 18.1 18.2 15.1 17.1 17.6 13.7 16.6 17.4 12.3 16.1 17.30–90 18.0 18.3 18.5 15.9 17.6 18.0 14.6 17.2 17.8 13.3 16.8 17.80–100 18.3 18.3 18.6 16.5 17.9 18.2 15.3 17.6 18.1 14.0 17.2 18.0

Fig. 5. Hourly variation of air temperature along pipe length during 24 h operation for soil (a) S1 and (b) S3.

424 V. Bansal et al. / Applied Energy 102 (2013) 418–426

the EATHE pipe being at constant temperature does not hold goodpractically for soil having poor thermal conductivity as the air flow-ing through pipe continuously dissipates heat to surrounding soilthereby increasing its temperature. EATHE operating under steadystate condition is taken as reference case for comparing its thermalperformance with the EATHE system operating under transientcondition, however, most of the researchers viz. Ajmi et al. [35]and Bansal et al. [13,14] evaluated the thermal performance ofEATHE assuming soil to be at constant temperature.

Fig. 4 shows temperature contours of air and soil at section sit-uated 50 m from pipe inlet using transient state approach for threesoils S1, S2 and S3 after continuous operation of twelve hours.

Total drop in air temperature obtained under steady state con-dition is 18.4 �C, 18.4 �C and 18.7 �C for soil S1, S2 and S3 respec-tively. Drop in air temperature obtained using transient approachat a section situated at 50 m from pipe inlet for soil S1 after 1, 6,12, and 24 h of continuous operation is 14.7 �C, 11.5 �C, 10.1 �Cand 8.9 �C respectively. Similarly for S2 and S3 under transient

Fig. 6. Hourly variation in derating factor along the length of pipe during 24 h ofoperation for soil (a) S1, (b) S2, and (c) S3.

V. Bansal et al. / Applied Energy 102 (2013) 418–426 425

condition the temperature drop obtained at 50 m pipe length after1, 6, 12, and 24 h of continuous operation is 15.9 �C, 14.4 �C,13.7 �C, 13.1 �C and 16.3 �C, 15.3 �C, 15.0 �C, 14.8 �C respectively,as shown in Table 4. As shown in Figs. 4 and 5 and Table 4, signif-icant deterioration in the thermal performance of EATHE system isobserved with longer period of continuous operation under tran-sient condition. It is also observed that deterioration in thermalperformance of EATHE system is lesser for soil having higher ther-mal conductivity (4.0 W m�1 K�1). However, the total drop in tem-perature under transient conditions varied between 18.3 and14.0 �C during 24 h of continuous operation for S1 as shown inTable 4. Value of the same for S2 and S3 is 18.3–17.2 �C and18.6–18.0 �C respectively, as also shown in Table 4. Temperaturedrops obtained under steady state condition and transient condi-tion are used to determine the derating factor which is defined as,

DFl;t ¼ 1�ðTi� ToÞl;transient

ðTi� ToÞl;steadyð1Þ

where DFl,t: derating factor at length ‘l’ from EATHE pipe inlet, aftertime ‘t’. Ti is air temperature at inlet of EATHE pipe and To is airtemperature at a section situated at a distance of ‘l’ m from the pipeinlet.

(Ti � To)l, transient: difference between air temperature at inletand at length ‘l’ after time ‘t’ in transient state.

(Ti � To)l, steady: difference between air temperature at inlet andat length ‘l’ after time ‘t’ in steady state.

Fig. 6 shows the derating factors calculated (on the basis of Eq.(1)) at different length of pipe of EATHE after different hours ofoperation under transient condition for soil S1, S2 and S3. Sameare also shown in Fig. 6. Physical significance of derating factor isthat it gives a broad comparison between thermal performanceof EATHE operating under steady state and transient conditions.Generally performance of EATHE operating under steady stategives the performance similar to first hour performance undertransient conditions. Therefore, it also shows the deterioration inthe thermal performance of EATHE system due to prolonged con-tinuous use. While evaluating derating factor, the maximum ambi-ent air temperature (46.1 �C) observed during summer season hasbeen taken as the inlet air temperature in simulation model, there-fore, Fig. 6 gives an information about maximum derating in ther-mal performance of EATHE operating under transient mode in hotand dry climate of Ajmer (India).

It is observed from Fig. 6 that irrespective of the length of pipe,as the time of continuous operation increases derating factor alsoincreases. Highest derating factor after 24 h of continuous opera-tion is observed as 0.68, 0.47 and 0.32 for S1, S2 and S3 respectivelyand at a pipe length of 5 m from inlet. Larger values of derating fac-tor reflect greater deterioration in thermal performance of EATHEoperating in transient mode. As shown in Fig. 6, as the thermal con-ductivity of soil increases the slope of derating curve decreases.Curves for derating factor for soil S1are much steeper than thecurves for S2 and S3, as shown in Fig. 6. Thus, it is concluded thedeterioration in thermal performance of EATHE is more prominentand rapid in soil having low thermal conductivity i.e. soil S1.

Maximum value of derating factor is observed after continuousoperation of EATHE for 24 h. The section of EATHE pipe where der-ating factor approaches to zero (i.e. 0–0.05) signifies that the ther-mal performance of EATHE in transient mode at that particularsection approaches the thermal performance of EATHE operatingunder steady state condition.

7. Conclusion

This paper presents a transient CFD simulation model of theEATHE system for analyzing the heat transfer process between soiland air flowing in EATHE pipe on hourly basis. Maximum air tem-perature drop obtained under steady state operation of EATHE forpipe length 100 m is 18.4 �C, 18.7 �C and 18.4 �C for soil thermalconductivity of 0.52, 2.0 and 4.0 W m�1K�1 respectively. However,the maximum air temperature drop under transient conditionsfor 24 h of operation varies between 18.3 �C and 14.0 �C, 18.3 �Cand 17.2 �C and 18.6 �C and 18.0 �C for soil thermal conductivityof 0.52, 2.0 and 4.0 W m�1K�1 respectively. Analysis shows thathigher soil thermal conductivity results into better thermal perfor-mance of EATHE system under transient conditions even after long-er period of operation. Derating factor varies between 0.29 and 0.07and 0.68 and 0.24 after 1 and 24 h of continuous operation of EATHErespectively for soil thermal conductivity of 0.52 W m�1 K�1. Thevalue of the same for soil thermal conductivity of 4.0 W m�1 K�1

is 0.01–0.15 and 0.03–0.32. Larger values of derating factor showgreater deterioration in thermal performance of EATHE. Thus, it isconcluded that under transient conditions, thermal performanceof EATHE deteriorates due to continuous operation of EATHE forlong durations. Deterioration in thermal performance is largerand rapid for soil having poor thermal conductivity. Derating factorat a section 50 m from inlet of EATHE pipe during 24 h of continu-ous operation varies between 0.08 and 0.44, 0.07 and 0.23 and 0.04

426 V. Bansal et al. / Applied Energy 102 (2013) 418–426

and 0.12 for soil thermal conductivity of 0.52, 2.0 and4.0 W m�1 K�1 respectively. The value of derating factor at a sectionsituated at 20 m from inlet of EATHE pipe varies between 0.20 and0.59 for soil S1 during 24 h of operation. The value of the same is0.01–0.24 for section situated at 90 m from EATHE pipe inlet. Thisshows that larger deterioration in thermal performance of EATHEis observed for early lengths of pipe.

It is concluded that for better thermal performance of EATHE,the soil situated in the immediate vicinity of the EATHE pipeshould have higher thermal conductivity and at the same timewhile optimizing the thermal performance of EATHE, derating fac-tor should be taken into account so as to ascertain that the EATHEwould be able to give a consistent thermal performance for longerperiod of operation. The derating factor is also affected by air flowrate, pipe diameter, ambient conditions, type of operation (inter-mittent or continuous), period of intermittency, etc. A lot of workis required to be carried out to study the effect of these parameterson derating factor.

References

[1] Khatry AK, Sodha MS, Malik MAS. Periodic variation of ground temperaturewith depth. Sol Energy 1978;20:425–7.

[2] Jacovides CP, Mihalakakou G, Santamouris M, Lewis JO. On the groundtemperature profile for passive cooling applications in buildings. Sol Energy1996;57:167–75.

[3] Bau HH. Heat losses from a fluid flowing in a buried pipe. Int J Heat MassTransfer 1982;22(11):1621–9.

[4] Puri VM. Heat and mass transfer analysis and modeling in unsaturated groundsoils for buried tube systems. Energy in Agri 1987;6:179–83.

[5] Svec OJ, Goodrich LE, Palmer JHL. Heat transfer characteristics of in-groundheat exchangers. Energy Res 1983;7:265–8.

[6] Agas G, Matsaggos T, Santamouris M, Argiriou A. On the use of the atmosphericheat sinks for heat dissipation. Energy Build 1991;17:321–9.

[7] Santamouris M, Mihalakakou G, Balaras C, Argiriou A, Asimakopoulos D,Vallindras M. Use of buried pipes for energy conservation in cooling ofagricultural greenhouses. Sol Energy 1995;55(2):111–4.

[8] Bojic M, Papadakisb G, Kyritsisb S. Energy from a two-pipe, earth-to-air heatexchanger. Energy 1999;24:519–23.

[9] Deglin D, Caenegem LV, Dehon P. Subsoil heat exchangers for the airconditioning of livestock buildings. J Agri Eng 1999;73:179–88.

[10] Kabashnikov VP, Danilevskii LN, Nekrasov VP, Vityaz IP. Analytical andnumerical investigation of the characteristics of a soil heat exchanger forventilation systems. Int J Heat Mass Transfer 2002;45:2407–8.

[11] Cui P, Yang H, Fang Z. Heat transfer analysis of ground heat exchangers withinclined boreholes. Appl Therm Eng 2006;26:1169–75.

[12] Badescu V. Simple and accurate model for the ground heat exchanger of apassive house. Renew Energy 2007;32:845–55.

[13] Bansal V, Misra R, Agrawal GD, Mathur J. Performance analysis of earth-pipe-air heat exchanger for summer cooling. Energy Build 2010;42:645–8.

[14] Bansal V, Misra R, Agrawal GD, Mathur J. Performance analysis of earth-pipe-air heat exchanger for winter heating. Energy Build 2009;41:1151–4.

[15] Givoni B. Cooled soil as a cooling source for buildings. Sol Energy2007;81:316–8.

[16] Bansal V, Misra R, Agrawal GD, Mathur J. Performance analysis of integratedearth–air-tunnel-evaporative cooling system in hot and dry climate. EnergyBuild 2012;47:525–32.

[17] Ozgener O, Ozgener L. Exergoeconomic analysis of an underground air tunnelsystem for greenhouse cooling system. Int J Refrig. 2010;33:995–1005.

[18] Ozgener O, Ozgener L. Exergetic assessment of EAHEs for building heating inTurkey: a greenhouse case study. Energy Policy 2010;38:5141.

[19] Ozgener L, Ozgener O. An experimental study of the exergetic performance ofan underground air tunnel system for greenhouse cooling. Renew Energy2010;35:2804–11.

[20] Ozgener O, Ozgener L. Determining the optimal design of a closed loop earth toair heat exchanger for greenhouse heating by using exergoeconomics. EnergyBuild 2011;43(4):960–5.

[21] Ozgener O, Ozgener L, Goswami DY. Experimental prediction of total thermalresistance of a closed loop EAHE for greenhouse cooling system. Int CommunHeat Mass Transfer 2011;38(6):711–6.

[22] Bi Y, Wang X, Liu Y, Zhang H, Chen L. Comprehensive exergy analysis of aground-source heat pump system for both building heating and coolingmodes. Appl Energy 2009;86(12):2560–5.

[23] Man Y, Yang H, Wang J. Study on hybrid ground-coupled heat pump system forair-conditioning in hot-weather areas like Hong Kong. Appl Energy2010;87(9):2826–33.

[24] Man Y, Yang H, Spitler JD, Fang Z. Feasibility study on novel hybrid groundcoupled heat pump system with nocturnal cooling radiator for cooling loaddominated buildings. Appl Energy 2011;88(11):4160–1.

[25] Michopoulos A, Papakostas KT, Kyriakis N. Potential of autonomous ground-coupled heat pump system installations in Greece. Appl Energy2010;88(6):2122–9.

[26] Yang W, Shi M, Liu G, Chen Z. A two-region simulation model of vertical U-tube ground heat exchanger and its experimental verification. Appl Energy2009;86(10):2005–12.

[27] Lee CK. Dynamic performance of ground-source heat pumps fitted withfrequency inverters for part-load control. Appl Energy 2010;87(11):3507–13.

[28] Lee CK. Effects of multiple ground layers on thermal response test analysis andground-source heat pump simulation. Appl Energy 2011;88(12):4405.

[29] Chen X, Yang H. Performance analysis of a proposed solar assisted groundcoupled heat pump system. Appl Energy 2012;97:888–916.

[30] Sagia Z, Rakopoulos C, Kakaras E. Cooling dominated hybrid ground sourceheat pump system application. Appl Energy 2012;94:41–7.

[31] Fayegh SK, Rosen MA. Examination of thermal interaction of multiple verticalground heat exchangers. Appl Energy 2012;97:962–9.

[32] Li M, Lai ACK. Heat-source solutions to heat conduction in anisotropic mediawith application to pile and borehole ground heat exchangers. Appl Energy2012;96:451–8.

[33] Cucumo M, Cucumo S, Montoro L, Vulcano A. One-dimensional transientanalytical model for earth-to-air heat exchangers, taking into accountcondensation phenomena and thermal perturbation from the upper freesurface as well as around the buried pipes. Int J Heat Mass Transfer2008;51:506–16.

[34] Beier RA. Vertical temperature profile in ground heat exchanger during in-situtest. Renew Energy 2011;36(5):1578–87.

[35] Ajmi FA, Loveday DL, Hanby V. The cooling potential of earth–air heatexchangers for domestic buildings in a desert climate. Build Environ2006;41:235–44.

[36] Holman JP. Experimental methods for engineers. New Delhi: Tata McGraw-Hill; 2004.