Parametric study on thermal performance of horizontal earth pipe cooling system in summer

Energy Conversion and Management 114 (2016) 324–337

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Parametric study on thermal performance of horizontal earth pipe cooling system in summer S.F. Ahmed a,b,⇑, M.T.O. Amanullah a, M.M.K. Khan b, M.G. Rasul b, N.M.S. Hassan b a b

School of Engineering, Deakin University, Geelong Waurn Ponds Campus, Victoria 3220, Australia School of Engineering and Technology, Central Queensland University, Rockhampton Campus, Queensland 4702, Australia

a r t i c l e

i n f o

Article history: Received 22 October 2015 Accepted 24 January 2016

Keywords: Parametric study Earth pipe cooling Thermal performance Hot humid climate

a b s t r a c t Rational use of energy and its associated greenhouse gas emissions has become a key issue for a sustainable environment and economy. A substantial amount of energy is consumed by today’s buildings which are accountable for about 40% of the global energy consumption. There are on-going researches in order to overcome these and find new techniques through energy efficient measures. Passive air cooling of earth pipe cooling technique is one of those which can save energy in buildings with no greenhouse gas emissions. The performance of the earth pipe cooling system is mainly affected by the parameters, namely air velocity, pipe length, pipe diameter, pipe material, and pipe depth. This paper investigates the impact of these parameters on thermal performance of the horizontal earth pipe cooling system in a hot humid subtropical climate at Rockhampton, Australia. For the parametric investigation, a thermal model was developed for the horizontal earth pipe cooling system using the simulation program, FLUENT 15.0. Results showed a significant effect for air velocity, pipe length, and pipe diameter on the earth pipe cooling performance, where the pipe length dominated the other parameters. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction The global energy consumption has increased in different forms during the last decades. In 2009, it was about 480 quadrillion Btu that increased to 524 quadrillion Btu in 2012. This signifies an annual average increase rate of 3.06% from 2009 throughout 2012 [1]. Population growth and higher income leads to this high energy demand. The world population is projected to reach 8.3 billion from its current 7 billion by 2030 [2]. Therefore, more energy will be needed for an additional 1.3 billion people. The residential sector is a substantial energy consumer all over the world. Nationally, the energy consumption of this sector accounts for 16-50% and averages roughly 30% worldwide [3]. This use of energy is mainly due to the space cooling and heating the buildings. It is therefore important to apply energy efficient techniques in these buildings through new and novel building designs, which can be developed by employing several passive air cooling strategies. Earth pipe cooling system is one of the passive air cooling systems, which can reduce the cooling loads of the buildings. The earth pipe cooling system operates with long buried pipes in which intake air comes through one end and passes through ⇑ Corresponding author at: School of Engineering, Deakin University, Geelong Waurn Ponds Campus, Victoria 3220, Australia. http://dx.doi.org/10.1016/j.enconman.2016.01.061 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

the buried pipes, and thus gets cooled by the soil. The cooled air is then blown out of the other end into a space. Since the system uses the underground spaces, it offers several additional advantages, for example, noise, protection from dust, partial air infiltration, storms and radiation, etc. It also offers a great potential for energy saving for any hot humid climate, like Queensland, since it can supplement the air conditioning load of many homes [4]. As a reasonable and economical option to ordinary cooling, the earth pipe cooling system is a type of choice, since no customary mechanical units are needed. In this system, the earth’s near constant underground temperature is used for cooling air in industrial, residential and agricultural buildings [5–7]. The infinite thermal capacity of earth has made it a very useful heat sink for building cooling. The rationale behind this is that the daily and regular temperature variation is significantly diminished in the ground below a certain depth where the soil temperature remains constant. The soil temperature decreases in summer with increasing depth, which allows the utilisation of earth as a heat sink [8]. Meanwhile, the soil temperature increases in winter with increasing depth to a certain point, hence the use of earth as a heat source [9]. Thermal performance assessment is very important in order to measure the cooling capacity of the earth pipe cooling system. To assess the thermal performance of this system, several researches

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325

Nomenclature kinetic energy for turbulent flow (m2 s
2) turbulent energy dissipation rate (m2 s
3) air density (kg/m3) qv air flow rate (m3/s) v air velocity (m/s) A area of a region (m2) component of diffusion flux (m
2 s
1) Jj xi component of length (m)



C 3e ¼ tanh
uu12
constant, u1 and u2 are the velocity components k

e q

parallel and perpendicular to the gravitational vector respectively C 1e , C 2e constants YM contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate (kg m
1 s
2) D diameter of the pipe (m) DT difference in temperature (K) i, j, k (=1, 2, 3) direction vector index Dx distance of heat transfer between two surfaces (m) keff effective conductivity (W m
1 K
1) Pef effective fan power (W) _ in energy input rate into the earth pipe cooling system (J) W h enthalpy (J kg
1)

were conducted in different hot humid climatic conditions. It’s thermal performance was evaluated in a subtropical climate in Queensland, Australia by the authors [10–13]. A 1–2 °C reduction in temperature was attained in those studies for a 27.23 m3 room. The cooling performance of the horizontal earth pipe cooling system was investigated in an agriculture greenhouse in Thailand by Mongkon et al. [14]. The study shows that this system has the potential to cool the greenhouse during daytime. In most of the cases, the earth pipe cooling system is supported by a heat pump as a heat exchanger positioned within the buried pipe [15]. This is also identified as an earth pipe air heat exchanger, which can be used for cooling the buildings during summer and for heating in winter [16–19]. Bansal et al. [20] evaluated the cooling capacity of the earth pipe air heat exchanger by a numerical model. The model was developed to assess the impact of different pipe materials and air velocities on the thermal performance of the heat exchanger using FLUENT. The results showed that the pipe materials have no noticeable impact whereas the air velocity has greater influence. Another numerical model was also developed for the earth pipe air heat exchanger [21]. It was found that the model is computationally fast and simple to be implemented into building thermal insulation programs. As mentioned earlier, the cooling performance of the earth pipe cooling system is mainly influenced by pipe length, pipe radius, buried underground pipe depth and air flow rate used in the earth pipe cooling system. The impact of these parameters on the performance of the earth pipe cooling system was assessed by an implicit and transient model using PHOENICS in Southern China [22]. The results revealed that a daily cooling capacity up to 74.6 kW h can be achieved using the system. Many researchers found that the resulting outlet temperature at the buried pipe decreases with decreasing pipe radius, decreasing mass flow rate in the pipe, increasing pipe length and increasing depths up to 4 m [23]. Various pipe diameters produce different cooling rates in an earth pipe cooling system. A study was carried out to investigate the earth pipe cooling performance using three different buried

fluid velocity components (m s
1) fluid viscosity (kg m
1 s
1) generation of turbulence kinetic energy due to buoyGb ancy (kg m
1 s
2) Gk generation of turbulence kinetic energy due to the mean velocity gradients (kg m
1 s
2) Q heat flow rate (J/s) r  ðkeff rTÞ heat transfer due to convection r  ðseff  ~ v Þ heat transfer due to viscous diffusion l length of the pipe (m) m mass of a substance (kg) t ¼ l=q molecular kinetic viscosity of the fluid (m2 s
1) p pressure (Pa) DP t pressure loss (Pa)   P ! r species diffusion j hj J j ui ; uj

l

cp T t Sh

e rk re

SK ; Se

specific heat capacity (J/kg K) temperature (K) time (s) total entropy (J K
1) turbulent energy dissipation rate (m2 s
3) turbulent Prandtl numbers for k turbulent Prandtl numbers for e user-defined source terms

pipe radius of 0.125 m, 0.180 m and 0.250 m [24]. The outlet temperature of the buried pipe got higher with increased pipe radius. A buried pipe of small radius allows air to transfer excess heat to the soil quickly, since the centre point of the pipe gets closer to the outside soil [25,26]. Ghosal and Tiwari [23] agreed with these and reported that the pipe outlet temperature can be decreased with reducing the pipe radius. Length of the buried pipes is one of the major factors that influence the earth pipe cooling performance. A longer buried pipe produces lower air temperature at the buried pipe outlet, which has been proved by several researches [23–25]. But, in some cases the longer pipes are not acceptable from the economic point of view. Moreover, the pipes need to be cost effective in case of an efficient cooling system. Their cost efficiency was evaluated for a hot, arid climate in Kuwait [27]. It was measured by calculating the payback time of the system. The payback time for the optimum configuration was obtained as 7.24 years, where the pipe diameter, the pipe length, and the pipe depth were 0.35 m, 56.97 m and 5.47 m respectively. Material of the pipe is another factor that also affects the performance of the earth pipe cooling system. Each material has different thermal conductivity. The materials of higher thermal conductivity have higher heat transfer rate, and therefore can reduce the buried pipe outlet temperature. The impact of different pipe materials were analysed through a number of studies [28]. It was observed that the pipe material has no noticeable impact on the cooling performance. Mihalakakou et al. investigated the impact of different pipe depths of 1.2 m, 2 m and 3 m on the earth pipe cooling performance [24]. The deeper pipe depth of 3 m provided the lowest temperature at the pipe outlet. They also conducted a similar study with different pipe depths of 2.5 m, 4 m and lower than 4 m [29]. The outcomes of this study also gave similar result. Air velocity is another key factor that influences the earth pipe cooling performance. To analyse the impact of different air velocities, 2 m/s and 5 m/s were considered for a study conducted in summer [20].

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The pipe outlet temperature was increased in this study by increasing the air velocity at the pipe inlet. The low energy cooling techniques using the earth became progressively popular in America and Europe after the oil emergency in 1973 [26]. It has been commonly applied in Germany, Denmark, Austria and India since the 1990s, and are gradually being implemented in North America [30]. However, there have been a few researches on the earth pipe cooling system comprising both experimental and numerical studies. In particular, no extensive parametric study is seen to have been undertaken on the performance of the earth pipe cooling system. Moreover, it has not been practiced in any hot and humid climate in Australia. Therefore, the parametric study for the earth pipe cooling system is very important and timely for the Australian economy and environment. This study aims to assess the horizontal earth pipe cooling performance by a parametric analysis in a hot humid climate in Rockhampton, Australia. 2. Experimental design and measurement An excavation of dimension 8.1 m  2.0 m was made (Fig. 1) using an excavator for installing the horizontal earth pipe cooling (HEPC) system. The HEPC piping layout involves two simple Polyvinyl Chloride (PVC) pipes of diameter 0.125 m with thickness 0.004 m, which are also known as manifolds. The atmospheric air comes through one of the manifolds, goes down under the ground and passes through a series of buried pipes, and finally moves into the room via another manifold. Each manifold contains 20 holes of 21 mm diameter each to accept 20 PVC tubes of 20 mm each. The tubes, each of length 7.5 m with diameter 20 mm, were connected with the manifolds. These PVC tubes with a wall thickness of 2 mm were pressed (friction fitting) into the manifold horizontally, i.e. all the PVC tubes were aligned in a single row. Each tube in the row was separated from its neighbour by approximately 20 mm. Fig. 2 shows the horizontal earth pipe cooling diagram. A fan was fitted inside one of the manifolds as shown in Fig. 2. The fan sucks intake air from the pipe inlet and pushes it through the series of buried pipes, and finally into the room. Small grasses and trees were planted to shade the soil as well as to cover the underground pipes to increase the cooling effect of the system. This was planned to reduce the solar radiation absorbed by the ground surface [31].

Fig. 2. Diagram for horizontal earth pipe cooling (HEPC) system.

underground, transfers excess heat to the earth and thus gets cooler. A CFD model, namely the realisable k
e turbulence model was used for modelling the heat transfer process. This model deals with an extensive class of turbulent flows in heat transfer and industrial flow simulations. The turbulence model was selected for modelling the HEPC system because of the turbulent flow occurred at the outlet. Using the CFD code ‘‘FLUENT in ANSYS 15.0”, the problem was solved numerically that performs with the finite volume method for discretisation of the computational domain. A 2D geometry was created for the HEPC model using DesignModeller in ANSYS 15.0 that consists of all the pipes used in the HEPC system. A typical mesh was generated for the model as shown in Fig. 3. The element size of 0.01 m was used to generate the mesh, which produced 46,245 elements in the meshing of the model. To check the impact of grid variation and to establish an optimum mesh size, a study was carried out to ensure consistent results for every mesh size as discussed in Section 3.3. 3.1. Modelling equations The Realisable k
e model is derived from the Navier–Stokes equations. The Navier–Stokes equation of motion and the transport equations for the Realisable k
e model are given by [34]:

@ui @ui 1 @p @ 2 ui þ uj ¼
þt @t @xj q @xi @xj @xj

ð1Þ

and 3. CFD model description Heat transfer process is used in the horizontal earth pipe cooling system, where the air passes through the buried pipes

@ðqkÞ @ðqkuj Þ @ þ ¼ @t @xj @xj



lþ





lt @k þ Gk þ Gb
qe
Y M þ SK rk @xj ð2Þ

@ðqeÞ @ðqeuj Þ @ ¼ þ @t @xj @xj



lþ

e





lt @ e e2 pffiffiffiffiffi þ qC 1 Se
qC 2 re @xj k þ te

þ C 1e C 3e Gb þ Se k

ð3Þ

where ui ; uj are the fluid velocity components (m s
1), xi is the component of length (m), p is the pressure (Pa), t ¼ l=q is the molecular kinetic viscosity of the fluid (m2 s
1), l is the fluid viscosity (kg m
1 s
1), q is the fluid density (kg m
3), t is the time (s), k is the kinetic energy (m2 s
2), e is the dissipation rate (m2 s
3), Gk is the generation of turbulence kinetic energy due to the mean velocity gradients (kg m
1 s
2), Gb is the generation of turbulence kinetic energy due to buoyancy (kg m
1 s
2), Y M represents the contribution of the fluctuating dilatation in compressible turbulence to the overall dissipation rate (kg m
1 s
2), C 1e and C 2e are constants,



C 3e ¼ tanh
uu12
where u1 and u2 are the velocity components parallel Fig. 1. Excavation made for installing HEPC system.

and perpendicular to the gravitational vector respectively, rk and re are the turbulent Prandtl numbers for k and e respectively, SK and Se

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Fig. 3. Mesh created for HEPC model. (a) Showing inlet. (b) Showing outlet.

are user-defined source terms, and i, j, k (=1, 2, 3) are the direction vector index. The energy equation for this heat transfer problem is solved throughout the entire domain and is given by:

X ! @ðqEÞ þ r  ð~ v ðqE þ PÞÞ ¼ r  ðkeff rT
hj J j þ ðseff  ~ v ÞÞ þ Sh @t j ð4Þ where keff is the effective conductivity (W m
1 K
1), kt is the thermal conductivity for turbulent flow, Jj is the component of diffusion flux (m
2 s
1), T is the temperature (K), r  ðkeff rTÞ is the heat P ! transfer due to convection, h is the enthalpy (J kg
1), r  ð j hj J j Þ

v Þ is the heat transfer due to visis the species diffusion, r  ðseff  ~ cous diffusion and Sh is the total entropy (J K
1). 3.2. Solver approaches

3.3. Grid Independence study Three grids were generated for the HEPC model using three different element sizes as summarised in Table 1. The model consists of the element sizes of 0.01 m, 0.005 m and 0.003 m for generating the mesh of the model, where 0.01 m element size was used for this study. Grid 2 and Grid 3 in the table represent different mesh size, which was obtained by changing 0.01 m element size to 0.005 m and 0.003 m respectively. The temperature profiles for different mesh size are shown at the outlet of the pipe model (Fig. 4). The simulated pipe outlet temperature using Grid 1 agrees well with the outlet temperature using Grid 2 and Grid 3. The simulated outlet temperature using Grid 2 and Grid 3 lie within 1–2% of values attained with Grid 1. Consequently, this study was progressed using Grid 1 for simulating the model as this grid size takes less time for numerical computations. 4. Results and discussion

A 2D pressure-based-coupled solver was used for the simulations of the model. The solver solves a coupled system of equations along with the pressure-based continuity and momentum equations [32,33]. Although it has some limitations, the solver offers some additional advantages over a segregated or non-coupled approach. The coupled scheme allows a robust and efficient single phase execution for steady-state flows with high performance compared to the other solution schemes [34]. Pressure was discretised with a PRESTO scheme because of its strong convergence ability [35]. This scheme is available for all types of meshes such as triangular, tetrahedral, hexahedral, quadrilateral and hybrid meshes. Since the second-order discretisation of the viscous terms is always accurate in FLUENT, spatial discretisation with secondorder upwind scheme was used for the turbulent dissipation rate, turbulent kinetic energy and momentum. Moreover, the differencing scheme of second-order upwind was utilised to overcome the numerical dispersion. The standard initialisation in the entire domain used in this study allows setting the initial values of the flow variables and initialising the solution with these values.

A series of experiments and measurements were carried out to assess the cooling performance of the HEPC system. Average air velocity and air temperature were measured at the pipe inlet and outlet of the HEPC system, and soil temperature data was recorded at 5 different depths under the ground and 2 different ground surface conditions with bare soil and short grass covered soil. All the measurements were conducted during summer (December 2013– February 2014), where the fan was running 24 h daily.

Table 1 Different grid sizes for HEPC model. Grid size

Element size (m)

Nodes

Elements

Grid 1 Grid 2 Grid 3

0.01 0.005 0.003

59,066 179,759 447,920

127,367 199,464 509,296

Fig. 4. Temperature profiles for different mesh size at the pipe outlet of HEPC system.

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4.1. Soil temperature investigation Soil temperature investigation is a key part for evaluating the HEPC performance. The soil temperature was investigated to seek the cooling potential using the HEPC system buried in the soil of Rockhampton, Australia. Soil temperatures were recorded at every 20 min interval, when the ground surface was covered by grass. The temperature sensors of the data logger, Lutron 12 channels temperature recorder was buried into the ground at different depths of 0.60 m, 0.73 m, 0.85 m, 0.97 m, and 1.10 m. The soil temperatures were recorded at these points to compare them with the outdoor temperature. Fig. 5 shows a typical soil temperature profile over a 24 h period at different depths. It is noted that the buried pipes were fitted at the maximum depth of 1.10 m in this study to keep the installation cost within the budget. Fig. 5 illustrates the hourly average soil temperature, which ranges from 20.72 °C to 21.75 °C at the various depths. The lowest average soil temperature distribution was observed at the depth of 1.10 m underground, while the maximum occurred at 0.6 m depth. The maximum average temperature reduction was found between these two depths that was 0.70 °C. This reduction occurred at the middle of the day (12:20 pm) on 12 January 2014. Usually, soil temperature gets cooler during the hot peak hours than during the off peak hours in summer. However, the soil temperature at 0.60 m depth (where the pipes were buried) was compared with the outdoor air temperature to find the maximum temperature difference as shown in Fig. 6. It is seen that the underground soil temperatures decreases with the increasing depth. However, from various literature, it is seen that this temperature reduction continues up to a depth of 4 m underground as the soil temperature is fairly constant and stable at that depth [25]. Fig. 6 shows the hourly average temperature on a typical day at 0.6 m depth and at the outdoor, where the outdoor temperature varies from 21.22 °C to 35.10 °C. The maximum diurnal temperature during this period was 14.9 °C, which occurred on 7 December 2013. Meanwhile, the minimum diurnal temperature was 3.6 °C, which occurred on 19 December 2013. The outdoor air temperature was found as lower after midnight from 4:20 am to 5:40 am. The outdoor air temperature normally falls during the late night and increases during the day. The average temperature reduction between the outdoor and soil temperature at 0.6 m depth was observed as 5.25 °C, while the temperature reduction varies from 0.1 °C to 13.35 °C. The maximum temperature reduction occurred at middle of the day (12:40 pm), whereas the minimum reduction was observed at late

Fig. 5. Hourly average soil temperature distribution during summer 2013–2014.

Fig. 6. Temperature distribution of soil at 0.6 m depth and outdoor air.

night (4:20 am). This reduction contributes to cool the room during the day. As the soil temperature was below the outdoor minimum temperature during the peak warming hours of the day, it worked as an effective heat sink to cool the room. Surface condition of the ground is an important factor that affects the earth pipe cooling performance. A bare ground surface allows exposure to solar radiation especially in a hot, humid climate like Rockhampton, Australia. The ground surface with bare soil is comparatively warmer than the ground surface with covered grass soil as the heat generated due to the solar radiation dissipated into the soil. Fig. 7 shows the temperature distribution of the ground surfaces between the grass covered soil and bare soil. Short grass covered soil offers more cooling potential than the bare soil [24]. An average temperature reduction of 3.12 °C was found between the two ground surfaces of bare soil and grass covered soil, where the grass covered soil temperature was found to be lower. This result also agrees with the other studies [24,36]. This difference occurred due to the effect of high solar radiation on uncovered or unshaded soil.

4.2. Horizontal earth pipe cooling performance 4.2.1. Experimental investigation To evaluate the horizontal earth pipe cooling performance, the air temperature and velocity were measured at the pipe inlet and outlet of the HEPC system. For measuring the inlet air temperature,

Fig. 7. Ground surface temperatures of grass covered soil and bare soil.

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the waterproof data logger of HOBO U23-001 Pro v2 was set at the pipe inlet to protect it from heavy rain. Meanwhile, HOBO U10-003 data logger was placed at the pipe outlet to observe the outlet air temperature. The inlet and outlet temperature profiles are shown in Fig. 8 over 24 h duration, where both the profiles present a similar trend. The inlet temperature varies from a minimum of 21.22 °C at 3:40 am to a maximum of 35.10 °C at 12:20 pm, while the outlet temperature varies from 19.74 °C at 4:40 am to 27.42 °C at 12:40 pm. The inlet air temperatures get lower during the night and started to rise during the day. The rising temperatures reach to peak from around 10:00 am to 5:00 pm. The inlet temperature was found quite lower during the late night and early morning from 2:35 am to 2:50 am and 3:35 am to 6:00 am respectively. Usually, the outdoor temperature in Rockhampton falls at the late night and early morning. When the cooler outdoor temperature comes to the inlet and goes through the buried pipes, it gains heat from the soil as the soil works as a heat source in cool weather. Then the heated air moves to the pipe outlet and hence the higher temperature arises during this period. The inlet and outlet data were recorded over the summer 2013– 2014 (92 days). The average of these data showed 3.08 °C temperature reduction at the pipe outlet. This reduction increases to 5.45 °C during the hot peak hours of the day from around 10:00 am to 5:00 pm. The temperature profiles at the pipe inlet and outlet is shown in Fig. 9. As the outside temperature warmed up due to the sun light, the soil temperature remained cooler. The higher temperature differential between the inlet air temperature and the soil temperature improves the cooling process and thus produces more temperature reduction during this period. Although the pipe outlet temperature dropped more during the warming hours, a 24 h data collection and measurement were taken for evaluating the overall performance of the earth pipe cooling system during the whole summer. This approach is consistent with other studies undertaken elsewhere. The higher temperature reduction at the pipe outlet indicates that the earth pipe cooling system has the potential to reduce more temperature in the room, and hence save more energy during the warming hours from 10:00 am to 5:00 pm. Air velocity plays an important role to reduce the air temperature at the pipe outlet. The velocity profiles at the pipe inlet and outlet are shown in Fig. 10. As seen from Fig. 10, the air velocity at the pipe inlet ranges from 0.2 m/s to 0.6 m/s and the mean value of these velocities of 0.41 m/s was taken for the simulation of the model. Meanwhile, the outlet velocity varies from 1 m/s to 1.1 m/s, while the average

Fig. 8. Temperature profile at pipe inlet and outlet of HEPC system.

329

Fig. 9. Temperature profile at pipe inlet and outlet during hot pick hours.

Fig. 10. Trend of air velocity at pipe inlet and outlet.

velocity is 1.01 m/s. A fan was set at the pipe outlet to suck the air from the pipe inlet. This increases the air velocity at the pipe outlet which is cooled by the soil under the ground. This cooler outlet velocity produces cooler temperature, which assists the HEPC system to cool the room. The amount of the cooling rate of the air at the pipe outlet depends on several factors, for example, the residence time of the air flow in the buried pipe, temperature difference between soil temperature and ambient air temperature, and the thermal conductivity of the buried pipe. 4.2.2. Numerical investigation The HEPC performance was calculated numerically using simulation in FLUENT. The flow and thermal variables for the boundary and cell zone conditions were set on the boundaries of the models. No slip boundary conditions were applied on the pipe walls. The cell zone condition for the surface body was defined as fluid. All the simulations were run on an Intel Xeon CPU E3-1225 V3 @ 3.20 GHz processor computer of RAM 16.0 GB (15.8 GB usable). For the HEPC model, the solution was convergent at 139 iterations, which required the total CPU time of 44.20 s. The total CPU time does not include any waiting time for communications or load imbalances. The simulation results were obtained using the boundary conditions which are shown in Table 2. The measured average air velocity of 0.41 m/s and the average air temperature of 26.76 °C at the pipe inlet were set as the inlet velocity and inlet temperature

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Table 2 Parameters used in boundary conditions of the HEPC pipe model. Parameters

Value

Inlet velocity Inlet temperature Soil temperature at 0.60 m Thermal conductivity of PVC pipe Density of PVC pipe Specific heat of PVC pipe Air thermal conductivity Air density Specific heat of air Air viscosity

0.41 m/s 26.76 °C 21.51 °C 0.16 W/m K 1390 kg/m3 1000 J/kg K 0.024 W/m K 1.204 kg/m3 1006.43 J/kg K 1.850387e-05 kg/m s

respectively in the boundary conditions of the model. The exhaust fan operated in the HEPC system was set as the pipe outlet. A soil temperature of 21.51 °C at 0.6 m depth (where the 20 PVC pipes were laid and aligned) was also used in the boundary conditions. Figs. 11–13 show the temperature distribution throughout the pipes, at the pipe inlet and pipe outlet respectively, while Figs. 14– 16 show the velocity distribution through the entire pipes, at the pipe inlet and pipe outlet respectively. The air temperature varies from 21.50 °C and 26.76 °C throughout the pipes, where the maximum occurs at pipe inlet and the minimum occurs in the buried pipe underground as shown in Fig. 11. The reason is that the outside hot air moves to the pipe inlet, goes down to the buried pipes, gets cooler by transferring heat to the soil, and finally moves the cooler air to the pipe outlet. When the air moves through the 7.5 m long buried pipes, it gets sufficient time to transfer heat to the soil, and thus the air is cooled. The outlet air temperature was found to be higher than that in the buried pipes as shown in Fig. 13. This higher outlet temperature occurs because of the heat generated by the motor of the fan set at the pipe outlet. When the cooler air comes through the buried pipes to the pipe outlet, the air absorbs heat from the

atmosphere due to conduction. Air velocity may be another reason for this higher air temperature at the pipe outlet. An average outlet temperature of 22.65 °C and air velocity of 0.89 m/s were found in the simulation results, which shows a good agreement with the measured average outlet temperature of 23.08 °C and velocity of 1.01 m/s. These results make a 1.90% and 11.88% difference with the average outlet temperature and outlet velocity respectively. Differences between the numerical and experimental results at different heights of the pipe outlet (Fig. 17) are shown in Table 3. Fig. 18 shows the temperature profile plotted by experimental and numerical results along the centre of the pipe outlet, where both the outlet shows higher temperature. This is due to the solar impact on the outlet ends. The temperature at the top end of the outlet (0.12 m) is higher than that of the bottom end (0.0 m) as there is a direct solar impact on the top end, while the bottom end is slightly shaded by the pipe. The minimum temperature occurs close to the middle of the pipe outlet. The overall simulated results are in very good agreement with the corresponding experiments as shown in Fig. 18, though there are some slight variations (Table 3) between these results. The probable reason for these variations is the measurement uncertainties due to the experiments as discussed in Section 5. Other reason is the uncertainties and errors in CFD simulations because of the auxiliary physical models [37] of turbulence model used in this study. Additionally, this may also occur due to the initial and boundary conditions, discretization and solution.

5. Error analysis The errors and uncertainties can be produced by instrument selection, reading, observation, test planning, environment, condition and calibration. An uncertainty analysis is essential to evaluate the accuracy of the experiment. The percentage uncertainties

Fig. 11. Temperature fields throughout the pipes. (a) Temperature magnitude. (b) Temperature vector.

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Fig. 12. Temperature fields at the pipe inlet. (a) Temperature magnitude. (b) Temperature vector.

Fig. 13. Temperature fields at the pipe outlet. (a) Temperature magnitude. (b) Temperature vector.

of the parameters used in this study are calculated using the instruments’ uncertainties as shown in Table 4. The uncertainty analysis in percentage is performed by the following method [38]. Total uncertainty of the experiment for the HEPC system = square root of [(uncertainty of air temperature)2 + (uncertainty of air velocity)2 + (uncertainty of soil temperature)2] = square root of [{(0.02)2 + (0.01)2} + (0.2)2 + (0.02)2] = ± 0.20%.

6. Parametric study on HEPC performance The parameters affecting the HEPC performance are investigated in this section. The HEPC model is optimised by a parametric study using FLUENT. The parameters, namely air velocity, pipe length, pipe diameter and thickness, pipe material, and pipe depth are considered for this parametric study. 6.1. Air velocity The impact of four different inlet air velocities on the HEPC performance has been studied here. The other variables such as pipe

material, pipe diameter and thickness, pipe length, and pipe depth were kept constant. The average air velocities of 0.41 m/s, 1.0 m/s, 1.5 m/s and 2.0 m/s were taken into account for the investigation. The outlet temperature fields for these air velocities are shown in Fig. 19. As stated before, the average inlet air velocity measured in the field work experiment was 0.41 m/s. As seen from Fig. 19, the temperature at the top of the pipe outlet is comparatively higher than that at the bottom and other points. There is a direct solar impact on the top outlet end whereas the bottom end is slightly shaded by the pipe. As a result, the maximum outlet temperature occurs at 0.12 m height, while the minimum is found at 0.02 m. Fig. 19 shows the pipe outlet temperature for the air velocity of 0.41 m/s, which varies from 21.02 °C to 26.45 °C. This outlet temperature is decreased when the velocity is increased to 1.0 m/s, and ranged from 21.56 °C to 26.16 °C. When the velocity is increased to 1.5 m/s, the temperature is further decreased and varied from 21.53 °C to 26.14 °C. But, when the air velocity is increased to 2.0 m/s, the outlet temperature is increased and varied from 21.70 °C to 26.31 °C. These indicate that an air velocity of 1.5 m/s provides a better outcome whereas a higher air velocity shows less performance. This result is also consistent with Khedari’s thermal comfort chart as the chart demonstrated that thermal comfort can be achieved at 0.5 m/s, 1.0 m/s, or 1.5 m/s depending on the climate conditions [39].

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Fig. 14. Velocity fields throughout the pipes. (a) Velocity magnitude. (b) Velocity vector.

Fig. 15. Velocity fields at the pipe inlet. (a) Velocity magnitude. (b) Velocity vector.

6.2. Pipe length The influence of different pipe lengths (7.5 m, 15.0 m, 30.0 m and 60.0 m) on the HEPC performance is analysed to determine the most efficient length. The air velocity of 1.5 m/s was picked from the previous investigation since it showed the best performance. The other variables such as pipe material, pipe diameter and thickness, and pipe depth remain as constants. The outlet temperatures for the different pipe lengths at the air velocity of 1.5 m/s are shown in Fig. 20. Fig. 20 illustrates that the average outlet temperature increases (Table 5) when the length of the buried pipe is halved or quartered, and it is reduced when the length is increased. For example, when the length is doubled from 30.0 m to 60.0 m long, the average outlet temperature is reduced and becomes more stable. For longer buried pipes, the heat transfer process becomes longer as the air

flow stays for a longer period inside the pipe, which allows more time for the heat transfer to take place between the air and soil. Therefore, the 60.0 m long buried pipe provides more of a cooling effect than by a shorter pipe length. It should be noted that the parametric study for the HEPC system started with a 7.5 m long buried pipe, 0.125 m diameter, 4 mm pipe thickness, and inlet air velocity of 0.41 m/s produced by a 8.0 W powered fan. But, this fan may not have the capacity to draw the air velocity of 1.5 m/s from the pipe inlet. Therefore, this parametric study needs to take the fan power into consideration. For example, it is seen from Table 5 that a length of 60.0 m does give an average temperature of 19.78 °C. However this would mean that a length of this size will need more fan power to produce the air velocity of 1.5 m/s. Hence, it is necessary to calculate the effective fan power (P ef ), which can be calculated using the following formula [40,41]:

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Fig. 16. Velocity fields at the pipe outlet. (a) Velocity magnitude. (b) Velocity vector.

Fig. 17. Height at the pipe outlet in HEPC system.

Table 3 Comparison of temperature between experimental and numerical (simulation) results at pipe outlet. Height (m)

Experimental (°C)

Numerical (°C)

Differences (%)

0.13 0.11 0.10 0.08 0.07 0.06 0.04 0.03 0.01 0.00

26.95 24.18 22.85 22.75 22.35 21.85 22.15 22.18 22.21 23.33

26.45 22.98 22.55 22.27 22.05 21.95 21.92 21.94 22.02 22.40

2.21 5.30 1.32 2.12 1.32 0.41 1.05 1.06 0.85 4.11

Fig. 18. Numerical data plotted against experimental data at the pipe outlet.

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Table 4 Instruments used in the HEPC experiment and the accuracy, range and percentage uncertainties. Instruments

Parameters

Accuracy

Range

Percentage Uncertainties

HOBO U10-003 HOBO U10-001 Pro V2 Reed Vane Anemometer BTM-4208SD

Air temperature Air temperature Air velocity Soil temperature

±0.4 °C ±0.2 °C ±(2% reading + 0.2 m/s) ±(0.4% + 0.5)


20 °C to 70 °C
40 °C to 70 °C 0.4–30.0 m/s
50 °C to 400 °C


0.02 to +0.02
0.01 to +0.01
0.2 to +0.2
0.02 to +0.02

where f ; D; l; q; v are the constant, pipe diameter, pipe length, air density, and air velocity respectively. After substituting the value of DPt in Eq. (5), it becomes

Pef ¼

flqv 2 qv 2D go

ð7Þ

Let the effective fan power for the buried pipe length of 7.5 m with 0.41 m/s air velocity and the buried pipe length of 60.0 m with 1.5 m/s air velocity be Pef 1 and P ef 2 respectively. All the variables in the Eq. (7) except the air velocity (v), pipe length (l) and diameter (D) are considered as constant. Then Eq. (7) becomes

lv D

2

Pef ¼ constant x

ð8Þ

Eq. (8) can be written in terms of Pef 1 and Pef 2 as

Pef 1 l1 D2 ¼ Pef 2 l2 D1 Fig. 19. Temperature profiles for different air velocities at pipe outlet.



v1 v2

2 ð9Þ

The variable D1 and D2 are calculated from the following formula:

vi ¼

qi Ai

ð10Þ  2

where qi is the air flow (m3/s), and Ai ¼ p

Di 2

is the area of the

2

pipe (m ). Assuming a constant flow, the following expression can be obtained from the above equation,

D1 ¼

4

sffiffiffiffiffiffi 1

p v1

and D2 ¼

4

sffiffiffiffiffiffi 1

p v2

After substitution the values of D1 and D2 in Eq. (9), the equation becomes

Pef 1 7:5 ¼ 60 Pef 2



v1 v2

1=2 

v1 v2

2

After substituting the values of Fig. 20. Temperature profiles for different pipe lengths at pipe outlet.

Pef 2 ¼ 204:81  Pef 1

Table 5 Pipe outlet temperature for different pipe lengths. Pipe length (m)

7.5 15.0 30.0 60.0

Pef ¼

DP t q v

go

Outlet temperature (°C) Min

Max

Avg

21.53 20.68 20.16 19.80

26.14 25.15 24.79 24.40

22.34 21.43 20.94 20.58

ð5Þ

where P ef is the effective fan power (W), DPt is the pressure loss (Pa), qv is the volume air flow (m3/s), and go is the efficiency.The pressure loss, DPt is given by:

DP t ¼

flqv 2 2D

ð6Þ

ð11Þ

v 1 and v 2

in Eq. (11),

ð12Þ

It is found that the effective fan power, Pef 2 is 204.81 times greater than P ef 1 . This is equal to 8  204.81 = 1638.48 W. Therefore, a 1638.48 W fan is needed to generate 1.5 m/s air velocity at the pipe inlet for the 60.0 m long pipe. As the 60.0 m long pipe provides the lowest outlet temperature, this has been considered for the next parametric study to continue the analysis, although it is expensive to operate a 1638.48 W fan in the HEPC system. 6.3. Pipe diameter and thickness The third parametric study measures the impact of different pipe size in terms of pipe diameter and thickness. Each pipe size has its own diameter, area and thickness as shown in Table 6. The most efficient outcomes from the previous investigations (1.5 m/s air velocity and 60.0 m pipe length) have been used for this study. The other variables such as PVC pipe material and 0.6 m pipe depth were kept constant. The temperature profile for various pipe diameters is shown along the centre of the pipe outlet (Fig. 21). As seen, the pipe outlet temperature increases with increasing pipe inlet diameter. For the

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S.F. Ahmed et al. / Energy Conversion and Management 114 (2016) 324–337 Table 6 Different configuration of pipes in terms of pipe diameter, area and thickness. Pipe size

Pipe size-1

Diameter (m) Inlet Area (m2) Thickness (m)

Pipe size -2

Pipe size -3

Pipe size -4

Manifold

Buried

Manifold

Buried

Manifold

Buried

Manifold

Buried

0.062 0.003 0.003

0.0100 0.0001 0.0020

0.125 0.012 0.004

0.0200 0.0003 0.0020

0.200 0.031 0.008

0.040 0.001 0.003

0.400 0.126 0.012

0.080 0.005 0.005

6.4. Pipe material

Fig. 21. Temperature profiles for different pipe diameters.

0.062 m pipe inlet diameter, the outlet temperature ranges from 19.55 °C to 24.29 °C. When this diameter is increased to 0.125 m, the outlet temperature increases and varies from 19.79 °C to 24.40 °C. For an increased diameter of 0.20 m, the outlet temperature is further increased, and ranges from 19.92 °C to 24.48 °C. The same result is also observed for the 0.40 m inlet diameter, where the outlet temperature varies from 20.02 °C to 24.55 °C. Thus, the pipe inlet having a diameter 0.062 m provides the lower outlet temperature than that for the other pipe diameters used in this study. These results demonstrate that the pipe of smaller diameter can increase the cooling rate. For a pipe of smaller diameter, air stays in the centre of the pipe, gets closer to the surrounding soil, allowing more heat transfer to the soil, and therefore the air temperature tends to get closer to the surrounding soil temperature. The impact of the pipe thickness is another reason. A smaller pipe has less thickness, which enables to transfer more heat to the soil quickly. The 0.062 m pipe diameter has the lowest thickness of 0.003 m and therefore, this pipe size provides the efficient result for the HEPC system.

Four different pipe materials have been considered here to investigate their impact on the thermal performance of the HEPC system. PVC, polyethylene, concrete and clay were taken into account for this parametric study (Fig. 22). The other parameters, air velocity (=1.5 m/s), pipe length (=60.0 m), pipe diameter and thickness (=0.062 m and 0.003 m), and pipe depth (=0.6 m) were kept constant. Usually, the earth pipe cooling performance depends on the thermal conductivity of the particular material. The higher the thermal conductivity, the more is the heat transfer. The pipes of different materials that are widely used for the earth pipe cooling system were considered for this study. The thermal conductivity of each of the pipe materials is summarised in Table 7. Table 8 summarises the average outlet temperature for the different pipe materials, PVC, polyethylene, concrete and clay. The PVC pipe produces higher temperature at the pipe outlet that varies from 19.55 °C to 24.29 °C, which is greater than that for the other materials. The PVC pipe has the lowest thermal conductivity while the pipe of clay material has the highest. The materials with high thermal conductivity assist in transferring more heat from the air inside the buried pipe to the soil, while the materials with low thermal conductivity transfer less heat. Therefore the clay pipe provides the lowest average outlet temperature as 20.05 °C (Table 8), and so clay has been considered here. Similar result was found in other studies, for example in the study by Sanusi [41]. Although the literatures suggest that the materials of high thermal conductivity give more cooling effect, those have not been considered in this study as they are too expensive and not commonly used. So the cheaper pipe materials of PVC, polyethylene, concrete and clay have been considered. Furthermore, these four materials were used by other researchers as stated before. 6.5. Pipe depth The buried pipe depths of 0.6 m, 2.0 m, 4.0 m and 8.0 m have been taken into consideration in order to evaluate their impact on the thermal performance of the HEPC system. The other parameters such as air velocity (1.5 m/s), pipe length (60.0 m), pipe diameter and thickness (0.062 m and 0.003 m), and pipe material (clay) remain as constants. At a 0.6 m buried pipe depth, the pipe outlet temperature varies from 19.28 °C to 23.92 °C as shown in Fig. 23. This outlet temperature is decreased and ranges from 18.34 °C to 22.90 °C for the increased buried pipe depth of 2.0 m. For the depth of 4.0 m, the temperature is further reduced and varies from 18.09 °C to 22.75 °C. The lowest average outlet temperature was found as 18.88 °C at 8.0 m depth, which ranges from 18.08 °C to 22.72 °C. It is seen that the outlet temperature decreases with increasing

Table 7 Thermal conductivity of different pipe materials.

Fig. 22. Temperature profiles for various pipe materials at the pipe outlet.

Material

PVC

Polyethylene

Concrete

Clay

Thermal conductivity, W/(m K)

0.19

0.42

0.70

1.80

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Table 8 Outlet temperature ranges for various materials. Pipe Material

PVC Polyethylene Concrete Clay

Outlet Temperature (°C) Min

Max

Avg

19.55 19.40 19.32 19.28

24.29 24.09 24.04 23.92

20.35 20.21 20.12 20.05

A noticeable effect of air velocity, pipe length and pipe diameter was observed on the thermal performance of the HEPC system by the parametric analysis, where the pipe length showed a significant influence. Although the parameters of 1.5 m/s air velocity, 60.0 m pipe length, 0.062 m pipe diameter, 0.003 m pipe thickness, clay pipe material, and 8.0 m depth were found as the most efficient configuration, an optimum length and air velocity are required to be adopted for the optimal performance in terms of energy savings. The HEPC system contributed to reduce an average outlet temperature of 4.11 °C, which will assist to cool a room and save energy in the buildings during summer. The outcomes of this study are recommended to be utilised as guidelines by the manufacturers, regulators, and inhabitants for the deployment of the HEPC system in all hot humid climates. Acknowledgement The authors gratefully acknowledge the support provided by Central Queensland University, Australia, and Ergon Energy, Australia for supplying earth pipe cooling system and the installation cost of the measuring tools. References

Fig. 23. Temperature profiles at different pipe depths.

depth, where 8.0 m pipe depth provides the best result. The outlet temperature depends on the soil temperature, which is decreased with the increasing depths. It was also found that the temperature range at the depths of 4.0 m and 8.0 m are nearly the same as the soil temperature is fairly constant and stable at 4.0 m depth as mentioned earlier. Moreover, the laying of pipes at such depths can be expensive and operationally cumbersome.

7. Conclusion Thermal performance of the horizontal earth pipe cooling system was evaluated through a good number of experiments and numerical investigations. A CFD model for the HEPC system was developed using FLUENT. In particular, a parametric study was carried out to investigate the impact of the parameters, namely air velocity, pipe length, pipe diameter and thickness, pipe material, and pipe depth on the thermal performance of the HEPC system. The parametric study commenced with four different air velocities of 0.41 m/s, 1.0 m/s, 1.5 m/s and 2.0 m/s, where 1.5 m/s showed the better cooling performance. Four different buried pipe lengths of 7.5 m, 15.0 m, 30.0 m and 60.0 m were considered for the second parametric study. The 60.0 m long buried pipe provided the maximum performance at the inlet velocity of 1.5 m/s, while a smaller pipe offered inferior performance. The impact of different pipe diameters (0.062 m, 0.125 m, 0.200 m, and 0.400 m) with different pipe thickness (0.01 m, 0.02 m, 0.04 m and 0.08 m) was investigated in the third parametric study. The smaller pipe diameter with smaller thickness presented the most efficient cooling effect. The different pipe materials (PVC, polyethylene, concrete and clay) having different thermal conductivity were considered for the fourth parametric study, where the higher thermal conductivity material provided better cooling performance. The last parametric study assessed the impact of various pipe depths of 0.6 m, 2.0 m, 4.0 m and 8.0 m on the HEPC performance. The study revealed that the deeper pipe depths can deliver more cooling effect compared to the smaller depths.

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