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Energy and Buildings 41 (2009) 1151–1154
Performance analysis of earth–pipe–air heat exchanger for winter heating
Vikas Bansal *, Rohit Misra, Ghanshyam Das Agrawal, Jyotirmay Mathur
Mechanical Engineering Department, Malaviya National Institute of Technology, Jaipur 302017, India
A R T I C L E I N F O
Article history:
Received 20 March 2009
Received in revised form 14 May 2009
Accepted 29 May 2009
Keywords:
Earth–pipe–air heat exchanger
Computational fluid dynamics
FLUENT
A B S T R A C T
Earth–pipe–air heat exchanger (EPAHE) systems can be used to reduce the heating load of buildings in
winter. A transient and implicit model based on computational fluid dynamics is developed to predict
the thermal performance and heating capacity of earth–air–pipe heat exchanger systems. The model is
developed inside the FLUENT simulation program. The model developed is validated against
experimental investigations on an experimental set-up in Ajmer (Western India). Good agreement
between simulated results and experimental data is obtained. Effects of the operating parameters (i.e.
the pipe material, air velocity) on the thermal performance of earth–air–pipe heat exchanger systems are
studied. The 23.42 m long EPAHE system discussed in this paper gives heating in the range of 4.1–4.8 8Cfor the flow velocities 2–5 m/s. Investigations on steel and PVC pipes have shown that performance of the
EPAHE system is not significantly affected by the material of the buried pipe. Velocity of air through the
pipe is found to greatly affect the performance of EPAHE system.
� 2009 Published by Elsevier B.V.
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1. Introduction
In recent times, air conditioning is widely employed not only forindustrial productions but also for the comfort of occupants. It canbe achieved efficiently by vapor compression machines, but due todepletion of the ozone layer and global warming by chlorofluor-ocarbons (CFCs) and the need to reduce high grade energyconsumption; numerous alternative techniques are being cur-rently explored. One such method is the earth–pipe–air heatexchanger systems. It uses underground soil as a heat source andair as the heat transfer medium for space heating in winter. Coldoutdoor air is sent into the earth-to-air heat exchangers. When airflows in the earth–air–pipes, heat is transferred from the earth tothe air. As a result, the air temperature at the outlet of the earth–air–pipes is much higher than that of the ambient. The outlet airfrom the earth–air–pipes can be directly used for space heating ifits temperature is high enough. Alternatively, the outlet air may beheated further by associated air conditioning machines. Both of theabove uses of earth–air–pipes can contribute to reduction inenergy consumption.
Several researchers have studied the use of the ground as heatsource and sink such as Bansal et al. [1] evaluated a large earth–airtunnel system meant to provide thermal comfort inside the wholebuilding complex at one of the hospitals in India. Goswami andBiseli [2] demonstrated an open loop, underground air tunnelsystem as well as an indirect air tunnel system to improve the COP
* Corresponding author. Tel.: +91 9982378283.
E-mail address: [email protected] (V. Bansal).
0378-7788/$ – see front matter � 2009 Published by Elsevier B.V.
doi:10.1016/j.enbuild.2009.05.010
of a heat pump or a refrigeration system. Mihalakakou et al. [3]presented a parametrical model in which varying parameters werepipe length, pipe radius, velocity of the air inside the tube anddepth of the buried pipe below earth surface. Santamouris et al. [4]investigated the impact of different ground surface boundaryconditions on the efficiency of a single and a multiple parallelearth-to-air heat exchanger system. Kumar et al. [5] evaluated theconservation potential of an earth–air–pipe system coupled with abuilding with no air conditioning. Ghosal et al. [6] developed athermal model to investigate the performance of earth–air heatexchanger (EAHE) integrated with green house. Ajmi et al. [7]studied the cooling capacity of earth–air heat exchangers fordomestic buildings in a desert climate. Badescu [8] developed aground heat exchanger model based on numerical transient bi-dimensional approach. Wu et al. [9] developed a transient andimplicit model based on numerical heat transfer and computa-tional fluid dynamics and then implemented it on the CFD(computational fluid dynamics) platform, PHOENICS, to evaluatethe effects of the operating parameters (i.e. the pipe length, radius,depth and airflow rate) on the thermal performance and coolingcapacity of earth–air–pipe systems. Cucumo et al. [10] proposed aone-dimensional transient analytical model to estimate theperformance of earth-to-air heat exchangers, installed at differentdepths, used for building cooling/heating. Cui et al. [11] developeda finite element numerical model for the simulation of the groundheat exchangers in alternative operation modes over a short timeperiod for ground-coupled heat pump applications.
In the present study, transient analysis of earth–pipe–air heatexchanger (EPAHE) has been done using FLUENT. EPAHE issimulated for studying the performance during winter for heating.
Nomenclature
cp specific heat capacity of air
Cd coefficient of discharge of the pipe
COP coefficient of performance
d diameter of the pipe
EPAHE Earth–pipe–air heat exchanger
m mass flow rate of air through the pipe
PVC poly vinyl chloride
Qh total hourly heat gain from the EPAHE system
Tinlet temperature at the inlet of earth pipe air heat
exchanger
Texit temperature at the exit of earth pipe air heat
exchanger
v mean velocity of air through the pipe
V. Bansal et al. / Energy and Buildings 41 (2009) 1151–11541152
The model developed is validated with experimental resultsobtained on an experimental set-up installed in Ajmer (WesternIndia). Effect of pipe material and flow velocity of air on theperformance of the EPAHE is studied.
2. Description of the earth tube heat exchanger system
The EPAHE as shown in Fig. 1 comprises of two horizontalcylindrical pipes of 0.15 m inner diameter with buried length of23.42 m, made up of PVC and mild steel pipes and buried at a depthof 2.7 m in a flat land with dry soil. The two pipes viz. PVC and steelare connected to common intake and outlet manifold for airpassage. Globe valves are fitted for each pipe assembly for flowcontrol of air. At the inlet, the open end of this single pipe isconnected through a vertical pipe to a 1 HP, single phasemotorized, 2800 rpm, 0.033 m3/s (70 cfm) blower. The air fromthe ambient is forced through the earth air pipe system with thehelp of blower. Velocity of the air through the pipe can be varied bychanging the rpm of the blower with the help of an autotransformer whose range is 0–270 V, 2 A maximum, type: 2D-1PHASE with a least count of 1 V. Six thermocouples are inserted atfixed distances along the length of each pipe at T1, T2, T3, T4, T5 andT6, to measure temperature of the air along the length of each pipe.Thermocouples are of K-type with temperature indicator having aleast count of 0.1 8C. Flow of air through the individual pipe can becontrolled with the help of valves. For flow of air through one pipeonly one valve is kept open at a time. Observations were taken fordifferent velocities and flow of air through both the pipes
Fig. 1. Experimental set-up of EPAHE.
separately. Airflow velocities are measured with the help of avane probe type anemometer having range of 0.4–30.0 m/s with aleast count of 0.1 m/s.
3. Description of CFD model
Computational fluid dynamic (CFD) is well known as a powerfulmethod to study heat and mass transfer for many years. CFD codesare structured around the numerical algorithms that can tackle fluidflow problems. It provides numerical solutions of partial differentialequations governing airflow and heat transfer in a discretised form.To examine the complicated airflow and heat transfer processes inan earth–pipe–air heat exchanger system, CFD software, FLUENT6.3, was used in this study. In order to provide easy access to thesolving power of CFD codes, FLUENT 6.3 packages includesophisticated user interfaces to input problem parameters and toexamine the results. CFD codes in FLUENT contain three mainelements: (i) a pre-processor, (ii) a solver and (iii) a post-processor.Pre-processing consists of the input of a flow problem to a CFDprogram by means of definition of the geometry of the region ofinterest: the computational domain, Grid generation – the sub-division of the domain into a number of smaller, non-overlappingsub-domains: a grid (or mesh) of cells (or control volumes orelements), selection of the physical and chemical phenomena thatneed to be modeled, definition of fluid properties, specification ofappropriate boundary conditions at cells which coincide with ortouch the domain boundary. Solver uses the finite control volumemethod for solving the governing equations of fluid flow and heattransfer. Post-processor shows the results of the simulations usingvector plots, contour plots, graphs, animations etc.
Thermal modeling of the earth–pipe–air heat exchanger systemshown in Fig. 1 is done using FLUENT 6.3. The model was developedinside the FLUENT simulation program using GAMBIT.
The CFD simulations were performed using FLUENT andconsidering 3-D transient turbulent flow (standard k–e model)with heat transfer enabled. In this transient analysis time step istaken as 100 s with 20 iterations in each step. Total numbers of thecontrol volume used for the CFD analysis were about 3.8 million.CFD analysis is carried out for two different pipe material viz. mildsteel and PVC.
The main objective of the CFD study was to investigate theeffect of buried pipe material on the performance of the EPAHEsystem (for this two materials, mild steel and PVC wereconsidered) and also to study the effect of air velocity on theperformance of the EPAHE system.
In the study it was assumed that air is incompressible andsubsoil temperature remains constant since the penetration of theheat from the surface of the soil is very slow. It was also assumedthat engineering materials used are isotropic and homogeneous.
The physical and thermal parameters of different engineeringmaterials used in the present simulation are listed in Table 1.
4. Experimental validation and performance analysis
CFD based EPAHE modeling is validated by taking observationson an actual EPAHE fabricated at Ajmer (Western India) as shown
Table 1Physical and thermal parameters used in simulation.
Material Density
(kg/m3)
Specific heat
capacity (J/kg K)
Thermal conductivity
(W/mK)
Air 1.225 1006 0.0242
Soil 2050 1840 0.52
Steel 7833 465 54
PVC 1380 900 0.16
Fig. 2. Temperature distribution along the length of the pipe for exit velocity 2.0 m/s
for (a) steel pipe and (b) PVC pipe. Fig. 3. Simulated temperature along the length of the pipe for various exit velocities
for (a) steel pipe and (b) PVC pipe.
V. Bansal et al. / Energy and Buildings 41 (2009) 1151–1154 1153
in Fig. 1. Observations were taken on January 12, 2008 at Ajmer andfurther repeated on January 21, 2008. Both in experiments andsimulations, flow of air is made through PVC and steel pipesseparately. Observations were taken for flow velocities 2.0, 3.2, 4.0and 5.0 m/s. Total hourly heat gain has been calculated for flowvelocities 2.0, 3.2, 4.0 and 5.0 m/s by the following equation:
Qh ¼ 3600mCdcpðTexit � T inletÞ (1)
where m ¼ p=4 d2 vIn Figs. 2 and 3, points Tinlet and Texit represent the inlet and
outlet of the buried pipe of the earth–pipe–air heat exchangersystem respectively. Fig. 2 represents the comparison of theresults of the simulation and experiments for air velocities of 2.0,3.2, 4.0 and 5.0 m/s at the outlet of the steel and PVC pipesrespectively. Tables 2 and 3 show the validation of simulatedtemperatures with experimental results. This is apparent thatvariation in simulated and experimental results ranges from 0% to2.07% of simulated results. This variation may occur due to
Table 2Comparison of experimental and simulated temperature at different sections along the
Section Air velocity = 2 m/s Air velocity = 3.2 m/s
Simulated
temperature
Experimental
temperature
%
difference
Simulated
temperature
Experimental
temperature
%
differenc
T(inlet) 20.6 20.6 0.00 20.6 20.6 0.00
T(1) 23.6 23.5 0.42 23.1 23 0.43
T(2) 24.2 24.1 0.41 23.7 23.5 0.84
T(3) 24.4 24.3 0.41 24 23.8 0.83
T(4) 24.7 24.6 0.40 24.3 24.1 0.82
T(5) 25 25 0.00 24.6 24.4 0.81
T(6) 25.3 25.2 0.40 24.9 24.8 0.40
T(exit) 25.5 25.4 0.39 25.2 25.1 0.40
variation in coefficient of friction of engineering material used insimulation and experiment, irregularities such as joints inexperimental set-up and improper insulation at the risers ofexperimental set-up. Tables 2 and 3 also depict that as the velocityof air is increased, the temperature of the air at the outlet of thepipe gets reduced. The reduction in temperature of the air at theexit of pipe due to increment in air velocity occurs because whenthe air velocity is increased from 2.0 to 5.0 m/s, the convectiveheat transfer coefficient is increased by 2.3 times, while theduration to which the air remains in contact with the ground isreduced by a factor of 2.5. Thus the later effect is dominant andtherefore, less rise in temperature is obtained at air velocity 5.0 m/s than the 2.0 m/s. At higher velocities though the rise intemperature of air is less yet the total heating effect achievedper unit time is much more. It can be seen that the maximum risein the temperature occurs at air velocity 2 m/s for both PVC andsteel pipe. The maximum rise in temperature for PVC and steelpipe is 4.5 and 4.8 8C, respectively.
length of steel pipe.
Air velocity = 4 m/s Air velocity = 5 m/s
e
Simulated
temperature
Experimental
temperature
%
difference
Simulated
temperature
Experimental
temperature
%
difference
20.6 20.6 0.00 20.6 20.6 0.00
22.9 22.8 0.44 22.7 22.6 0.44
23.5 23.3 0.85 23.3 23 1.29
23.9 23.7 0.84 23.7 23.3 1.69
24.2 24.1 0.41 24.1 23.6 2.07
24.5 24.4 0.41 24.4 23.9 2.05
24.8 24.7 0.40 24.7 24.3 1.62
25.1 24.9 0.80 25 24.7 1.20
Table 3Comparison of experimental and simulated temperature at different sections along the length of PVC pipe.
Section Air velocity = 2 m/s Air velocity = 3.2 m/s Air velocity = 4 m/s Air velocity = 5 m/s
Simulated
temperature
Experimental
temperature
%
difference
Simulated
temperature
Experimental
temperature
%
difference
Simulated
temperature
Experimental
temperature
%
difference
Simulated
temperature
Experimental
temperature
%
difference
T(inlet) 20.6 20.6 0.00 20.6 20.6 0.00 20.6 20.6 0.00 20.6 20.6 0.00
T(1) 23.2 23 0.86 22.7 22.6 0.44 22.6 22.2 1.77 22.3 22.1 0.90
T(2) 23.8 23.6 0.84 23.3 23.1 0.86 23.1 22.7 1.73 22.8 22.5 1.32
T(3) 24.1 24 0.41 23.6 23.6 0.00 23.5 23.2 1.28 23.2 22.9 1.29
T(4) 24.4 24.3 0.41 24 23.8 0.83 23.8 23.5 1.26 23.5 23.3 0.85
T(5) 24.7 24.5 0.81 24.3 24.1 0.82 24.1 23.8 1.24 23.8 23.6 0.84
T(6) 25 24.9 0.40 24.6 24.3 1.22 24.4 24.1 1.23 24.2 24 0.83
T(exit) 25.3 25.1 0.79 24.9 24.7 0.80 24.7 24.3 1.62 24.5 24.2 1.22
V. Bansal et al. / Energy and Buildings 41 (2009) 1151–11541154
It can be concluded from Fig. 3 that there is a very smalldifference in temperature of the air at the outlet of pipe betweenPVC and steel pipe if all other input conditions are same. Thisvariation occurs because the material with higher coefficient offriction marginally improves the performance of earth–pipe–airheat exchanger system due to reduction in thickness of laminarsub-layer which results in variation in Nusselt number but thevariation in Nusselt number is only 4–5%. Though the steel hashigher thermal conductivity than PVC, yet the variation intemperature of the air at the outlet of pipe between steel andPVC is very small. Therefore, it can be concluded that in EPAHEsystem, convective heat transfer plays a more important role thanconductive heat transfer. This indicates that the performance of theEPAHE system does not depend upon the material of the pipe. Totalhourly heat gain obtained from this EPAHE system varies from423.36 to 846.72 kW h. Maximum hourly heat gain is observed atair velocity 5 m/s. The EPAHE system discussed in this paper savesabout 38% of electrical energy in comparison to an electric heater ofefficiency 95% for the same heating effect.
5. Conclusion
There is a fair agreement between the experimental andsimulation results for modeling of EPAHE system with maximumdeviation of 2.07%. Rise in air temperature is found to decreasewith increase in flow velocity. It can be concluded from thisanalysis that performance of the EPAHE system is not affected bythe material of the buried pipe, therefore a cheaper material pipecan be used for making the pipe. For the pipe of 23.42 m length and0.15 m diameter, temperature rise of 4.1–4.8 8C has been observedfor the flow velocity ranging from 2 to 5 m/s. The hourly heat gain
through the system is found to be in the range of 423.36–846.72 kW h.
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