- Email: [email protected]
CFD analysis of EATHE system under transient conditions for intermittent operation
Energy and Buildings 87 (2015) 37–44
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
CFD analysis of EATHE system under transient conditions for intermittent operation Anuj Mathur a,∗ , Ayushman Srivastava b , G.D. Agrawal a , Sanjay Mathur a , Jyotirmay Mathur a a b
Centre for Energy and Environment, Malaviya National Institute of Technology, Jaipur 302017, India Civil Engineering Department, Malaviya National Institute of Technology, Jaipur 302017, India
a r t i c l e
i n f o
Article history: Received 10 September 2014 Received in revised form 3 November 2014 Accepted 6 November 2014 Available online 13 November 2014 Keywords: Earth air tunnel heat exchanger CFD simulation Continuous and intermittent operation Transient thermal performance Heat transfer
a b s t r a c t Thermal performance of earth air tunnel heat exchanger has been investigated under transient conditions for three different soil conditions considering three operating modes. In first operating mode EATHE works continuously for 12 h, in second mode it works for 60 min and then it is off for 20 min and in the last mode EATHE runs for 60 min and remains off for 40 min. In second and third mode EATHE is operated for 12 h intermittently. CFD model was developed in GAMBIT (version 2.2.3), simulated in FLUENT (version 6.3) and then validated with experimental data. CFD analysis has been carried out using a threedimensional transient numerical model. Air temperature drop and heat transfer between air and soil is calculated considering different thermal properties of soils. Higher thermal conductivity (1.28 W m−1 K−1 ) soil should be used in first mode of 12 h continuous operation as it improves the performance by 5.04% as compared to least thermal conductivity soil and soil with poor thermal conductivity (0.52 W m−1 K−1 ) shows performance improvement of 1.81% when operated in intermittent operation as compared to continuous running. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Passive heating/cooling systems consume no or very less energy as compared to active heating and cooling systems. In order to utilize these passive heating/cooling systems with great heat capacity and high thermal inertia, many techniques have been developed in the last decades such as earth air tunnel heat exchanger. Earth air tunnel heat exchanger system uses underground soil as a heat source/sink and air as the heat transfer medium for space heating in winter and cooling in summer because of almost constant sub-soil temperature throughout the year. When air flows in the buried pipes, heat is transferred to/from the soil from/to the air. The outlet air from the buried pipe can be directly used for space heating/cooling. Alternatively, the outlet air may be used with air conditioning machines. Over the last decade, many researchers have carried out a number of performance analyses on EATHE systems to improve its thermal performance, either using numerical modeling or via experiments. It has been established through previous studies
∗ Corresponding author. Tel.: +91 9982000532. E-mail address: [email protected] (A. Mathur). http://dx.doi.org/10.1016/j.enbuild.2014.11.022 0378-7788/© 2014 Elsevier B.V. All rights reserved.
[1–12] that EATHE systems have a good potential to provide cooling effect. Some of these studies also revealed that the thermal performance of EATHE gets deteriorated during continuous operation, due to heat accumulation in the nearby ground, around the EATHE pipe. An experimental study conducted by Darkwa et al. [13] highlighted that the overall performance of the EATHE system gets affected by the soil properties, namely thermal conductivity and heat capacity. Study conducted by Gan [14] has shown that under varying soil and atmospheric conditions, performance of EATHE system greatly depends upon the thermal and physical properties of soil, installation depth and horizontal distance between parallel pipes. To address the issue of thermal saturation of soil, an analytical model was developed by Ozgener et al. [15] to predict the daily soil temperatures depending on depth and duration of operation. Numerical analysis presented by Vaz et al. [16,17] indicated that the soil temperature, near the duct region gets affected by its continuous operation. A correlation between the soil and EATHE temperature variation was developed using linear regression model. Bansal et al. [18] have studied the effect of three different types of soils with thermal conductivities of 0.52, 2 and 4 W m−1 K−1 . It was found that maximum performance deterioration with
38
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
Table 1 Geometrical and simulation parameters. Parameters
Unit
Value
Parameters
Unit
Value
EATHE pipe length Pipe outer diameter Surrounding soil diameter Air density Air thermal conductivity Air specific heat capacity
m m m kg m−3 W m−1 K−1 J kg−1 K−1
40 0.1 1.1 1.225 0.02 1006
Soil density Soil specific heat Soil thermal conductivity PVC pipe density PVC thermal conductivity PVC specific heat capacity
kg m−3 J kg−1 K−1 W m−1 K−1 kg m−3 (W m−1 K−1 ) J kg−1 K−1
2050 1840 0.52 1380 1.16 900
Table 2 Boundaries conditions. Boundaries
Unit
Value
Initial soil temperature Initial pipe temperature Air inlet velocity Air inlet temperature
◦
27 27 5 46.2
C ◦ C ms−1 ◦ C
Grid Independenc e Test
Air Temperatures (°C)
45
Element size = 0.03 m Element size = 0.04 m
40 35 30
Fig. 1. Physical geometry model of EATHE pipe and soil.
25
prolonged operation takes place with soil having lowest thermal conductivity. However in this study, only the effect of thermal conductivity has been investigated, considering same density and specific heat constant for all three soils. It has been clearly seen through the literature review that thermal performance of EATHE system deteriorates upon its continuous operation, especially in case of lower soil thermal conductivity. Thermal conductivity is an inherent property of soil and cannot be changed so easily, if found unfavorable for EATHE development. As an alternative to solve this problem, intermittent operation of EATHE has been investigated; hypothesizing that soil gets regenerated during the period system is not used. The balance between ‘ON’ an ‘OFF’ condition for different types of soil is a matter of investigation. In this paper, a validated transient numerical model was used to investigate the thermal performance of EATHE system for three operation modes such as 12 h continuous, 60 min ON and 20 min rest, 60 min working and 40 min OFF running operation mode. The numerical simulation was done considering three soils with different thermal properties to study the role of different soils in intermittent operation. 2. System description and simulation setup CFD simulations were performed using research CFD code FLUENT (version 6.3) and preprocessor GAMBIT (version 2.2.3) for the geometry and 3D meshing in order to investigate the effect of intermittent operation on EATHE thermal performance. 2.1. Physical model Description of geometrical configuration of the earth air tunnel heat exchanger and soil ambient is presented in Table 1. Physical geometry (Fig. 1) of the EATHE system was created using structured hexahedral meshing for 40 m long and 0.1 m outer diameter PVC
0
5
10
115
20
25
30
35
40
Length of pipe (m)
Fig. 2. Grid independence test.
pipe. The control volume was defined by creating a cylinder volume of soil (soil cylinder diameter of 1.1 m) around the EATHE pipe. 2.2. Simulation model ANSYS FLUENT v 6.3 was used in the study that used finite volume method to convert the governing equations in to numerically solvable algebraic equations. To predict the turbulence inside the pipe, simple k–ε model with standard wall treatment is selected as turbulent model and energy equation is also solved since the computations included heat transfer as explained in ANSYS user guide [19]. The simple k–ε model is built in within FLUENT itself and one of the most common turbulence model and gives good results for internal flows. The numerical investigation was based on the following assumptions: i. Thermo-physical properties of solids and fluids remain constant over the range of soil and air temperature during operation. ii. Inlet air velocity is constant throughout the operation of EATHE. The far-field distances were carefully selected to ensure that the far-field boundary temperature would not have any effect over the period of the simulations. Therefore, far-field boundaries in the soil environment were treated as adiabatic surfaces. Coupled boundary conditions were used for the PVC pipe and surrounding soil so that heat can be transfer through pipe to surrounding soil and were initialized with 300 K i.e. 27 ◦ C as shown in Table 2. ‘Velocity inlet’ boundary condition was specified for the inlet air velocity of 5 m/s and as air is a compressible fluid so ‘pressure outlet’ condition for the outlet for the air.
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
Air Temperature aer 1 hr
Air Temperature aer 3 hr 50
50 Air outlet Temperature (°C)
Air outlet Temperature (°C)
Present Numerical Soluon
45
Misra et al numerical Soluon
40 35 30 25
Present Numerical Soluon
45
Misra et al numerical Soluon
40 35 30 25 20
20 0
10
20
30
0
40
10
Length of Pipe (m)
Misra et al numerical Soluon
40 35 30 25
0
10
20
40
50
Present Numerical Soluon
45
30
Air Temperature aer 12 hr
Air outlet Temperature (°C)
Air outlet Temperature (°C)
50
20
Length of Pipe (m)
Air Temperature aer 6 hr
20
39
30
40
Present Numerical Soluon
45
Misra et al numerical Soluon
40 35 30 25 20
0
10
20
30
40
Length of Pipe (m)
Length of Pipe (m)
Fig. 3. Air outlet temperature comparison between numerical solutions.
Fig. 4. Variation in air outlet temperature in continuous operation mode
The convergence criteria were achieved for continuity, xvelocity, y-velocity, z-velocity, k, epsilon and the energy equation at the each time step. The convergence criteria except for the energy was set at a level of 1e−3 and for energy, it was 1e−6. 3. Grid independence test Grid independence test (Fig. 2) was conducted to assess the quality of developed CFD model. If the mesh is refined (i.e. the
cells are made smaller in size hence larger in number), then the behavior observed by the post processing should remain unchanged if the solution is grid independent. To have grid independent solution, simulations are run for two grid sizes i.e. 0.04 m and 0.03 m taking operating parameters same for both cases. It is observed from Fig. 2 that there is no or minimum effect on the air temperature (main operating parameter) when grid size changed from 0.04 m to 0.03 m. Therefore, 0.04 m element size is
Table 3 Thermal properties of three different soils. Soil
Location
Density (kg m−3 )
Specific heat capacity (J kg−1 K−1 )
Thermal conductivity (W m−1 K−1 )
Thermal diffusivity (m2 /s)
Reference
Soil-1 Soil-2 Soil-3
Ajmer (India) Jodhpur (India) Presles (France)
2050 1470 1500
1840 1553 880
0.52 1.00 1.28
1.37 × 10−7 4.37 × 10−7 9.69 × 10−7
[18] [21] [22]
40
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
Fig. 5. (a) EATHE outlet air temperature in intermittent (mode-2) and continuous operation with soil-1. (b) EATHE outlet air temperatures in intermittent (mode-2) and continuous operation with soil-2. (c) EATHE outlet air temperatures in intermittent (mode-2) and continuous operation with soil-3. (d) EATHE outlet air temperature in mode-2 (intermittent) operation mode.
Table 4 Hourly radial temperature variation of soil at 10 m section of EATHE pipe from inlet. Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface 0.05 m
1 3 6 12
Table 5 Hourly temperature variation of soil layers at section 10 m length from EATHE inlet. Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface (mode-1 operation) 0.05 m
0.25 m
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
28.46 30.07 31.34 32.61
28.81 30.15 31.09 31.95
29.21 30.46 31.25 31.88
27.00 27.00 27.00 27.01
27.00 27.06 27.29 27.65
27.01 27.29 27.75 28.15
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
1 3 6 12
28.46 30.07 31.34 32.61
28.81 30.15 31.09 31.95
29.21 30.46 31.25 31.88
27.00 27.00 27.00 27.01
27.00 27.06 27.29 27.65
27.01 27.29 27.75 28.15
Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface (mode-2 operation)
chosen as the model grid size as it gave good accuracy and lesser computation time. 4. Validation of CFD model The developed CFD model in the present study was validated against the numerical solution (which was experimentally verified) suggested by Mishra et al. [20]. Maximum temperature difference at one point along the EATHE length was found to be 0.72 ◦ C from the two studies whereas, for most of the other points, temperatures were almost same as shown in Fig. 3. There is very good agreement between the two numerical solutions and thus the proposed CFD model is validated. 5. Selection of soil for analysis Three soils with different thermal properties were selected as shown in Table 3. These soils were considered to investigate role of
0.25 m
0.05 m
1 3 6 12
0.25 m
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
28.46 29.36 30.50 31.58
28.81 29.42 30.34 31.08
29.21 29.69 30.55 31.16
27.00 27.00 27.00 27.00
27.00 27.03 27.20 27.38
27.01 27.23 27.55 27.80
thermal diffusivity and thermal conductivity on the EATHE performance in intermittent operation. 6. Simulation on continuous 12 h operation (mode-1) In the simulation process, air inlet temperature and flow rate were taken to be constant of 319.2 K (46.2 ◦ C) and 5 ms−1 , respectively. Whereas, soil and pipe temperature was initialized at 300 K i.e. 27 ◦ C and it increases as the hot air (in summer) passes through the buried EATHE pipe. These initial conditions taken for air and
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
41
Fig. 6. (a) EATHE outlet air temperatures in intermittent (mode-3) and continuous operation with soil-1. (b) EATHE outlet air temperatures in intermittent (mode-3) and continuous operation with soil-2. (c) EATHE outlet air temperatures in intermittent (mode-3) and continuous operation with soil-3. (d) EATHE outlet air temperature in mode-3 (intermittent) operation mode.
soil were same as the actual condition considered by Mishra et al. [20]. EATHE outlet air temperature variation for 12 h of continuous running for three soils is shown in Fig. 4 and it is observed that the outlet air temperature increases with time due to saturation of adjacent soil. The EATHE outlet air temperature increase was maximum 1.87 K for soil-1 whereas it was 1.01 K and 0.85 K for soil-2 and soil-3, respectively, after 12 h of continuous operation. Hence, the performance of EATHE system gets deteriorated due to saturation of subsoil surrounding the EATHE pipe during continuous running. The performance deterioration in terms of EATHE outlet air temperature increase was the least (Fig. 4) for soil-3 which was having the highest thermal conductivity, and resulted into better thermal performance even after continuous operation of 12 h. Heat penetration into the surrounding soil due to heat transfer between the EATHE air and soil depends on the thermal
conductivity of the soil. Soil temperature in radial direction was measured at six locations i.e. 0.0 m, 0.05 m, 0.1 m, 0.15 m, 0.20 m, and 0.25 m in order to get the idea of radial heat penetration. Heat penetration for soil-2 and soil-3 up to 0.25 m away from the EATHE pipe during 12 h continuous operation due to higher thermal conductivity but for soil-1 heat penetration was up to 0.05 m only due to lower thermal conductivity (Table 4). Better thermal performance of EATHE system was observed with higher thermal conductivity due to easier dissipation of heat through the soil layers situated in the immediate vicinity of EATHE pipe to the soil layers situated away from the soil pipe interface in the radial direction. Regression analysis was performed using Eureqa software for EATHE outlet air temperature in terms of operation time as shown in Eqs. (1), (2) and (3) for soil-1, soil-2 and soil-3, respectively. The
Table 6 Average hourly heat transfer rate in mode-1 and mode-2. Operation time (h)
1 3 6 12
Heat transfer rate (W/m2 ) in mode-1 (continuous operation)
Heat transfer rate (W/m2 ) in mode-2 (intermittent operation)
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
44.50 43.98 43.45 42.92
44.92 44.68 44.49 44.36
44.92 44.69 44.54 44.45
44.92 44.68 44.49 44.36
44.92 44.83 44.67 44.59
44.92 44.84 44.70 44.63
42
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
R-square values for the three equations are 0.9975, 0.9885 and 0.9777, respectively.
S. No
Outlet air temperature = −0.0134 × Time2 + 0.3364 × Time + 301.20
(1)
Outlet air temperature = −0.0105 × Time2 + 0.2192 × Time + 300.82
1 2 3 4 5 6
Operation mode Mode-1 Mode-2 Mode-1 Mode-2 Mode-1 Mode-2
R2 value
Soil
Equation
Soil-1 Soil-1 Soil-2 Soil-2 Soil-3 Soil-3
y = −0.0006x + 0.0061x + 301.1 y = 302 − 1.42 cos(20.5x) y = −0.0006x2 + 0.0044x + 300.68 y = 301 − cos(0.0777x) y = −0.0006x2 + 0.0043x + 300.66 y = 301 − cos(0.0779x) 2
0.994 0.960 0.972 0.974 0.940 0.978
(2)
Outlet air temperature = −0.0099 × Time2 + 0.1944 × Time + 300.85
Table 7 Equations and R-square values.
(3)
where outlet air temperature is in K and operation time in h. 7. Simulation on short period operation Earth air tunnel heat exchanger was operated for short period in two modes; in mode-2 EATHE was operated for 60 min and remained OFF for 20 min in one cycle and these cycles continued for 12 h. In mode-3, it worked for 60 min and remained OFF for 40 min and these cycles also continued for 12 h. During the simulation, inlet air temperature was set constant at 319.2 K i.e. 46.2 ◦ C, soil and pipe temperature was initialized at 300 K i.e. 27 ◦ C and air flow rate was maintained at 5 m s−1 during ON mode and 0 m s−1 during OFF mode.
be lower (Table 5) in mode-2 (intermittent operation) than the mode-1 (continuous operation) i.e. better performance because in intermittent operation there is 20 min OFF time available to dissipate heat to the surrounding sub-soil layers. For soil-1 (lower thermal conductivity) improvement in the thermal performance (Table 6) is better than for soil-2 and soil-3 (higher thermal conductivity) in mode-2 as compared to mode-1. Simulation results of Table 6 shows that heat transfer rate after 12 h of operation increases by 3.35 % for soil-1, 0.45% for soil-2 and 0.40% for soil-3 in mode-2 as compared to mode-1 which shows that EATHE system in intermittent mode gives best thermal performance with soil-1 having least thermal conductivity. Using regression analyses, following equations were deducted in terms of ‘outlet air temperature (K) (denoted as y)’ and ‘time of operation (h) (denoted as x)’ comprising second order polynomial and trigonometric equations. Continuous operation of EATHE (mode-1) follows the second order polynomial trend whereas intermittent operation follows trigonometric trend. The obtained equations and R-square values are shown in Table 7.
7.1. EATHE operation in mode-2 7.2. EATHE operation in mode-3 In mode-2 EATHE operated for 60 min and remained OFF for 20 min and similar EATHE working cycles continued for 12 h. EATHE outlet air temperature variation in mode-2 for the three different soils is presented in Fig. 5(a)–(d). In mode-1 (continuous 12 h operation) the performance of EATHE system in terms of EATHE outlet air temperatures was better in the beginning and then outlet air temperature starts increasing and performance decline. The air flowing through EATHE pipe loses heat to the surrounding soil and soil gets saturated with time. In order to study the effect of soil saturation on EATHE performance, EATHE system is studied in two intermittent modes i.e. mode-2 and mode-3. It is noticed that the thermal performance of EATHE system improved in mode-2 as compared to mode-1 (Fig. 5(a)) because during the OFF period of EATHE system, surrounding soil dissipates the stored heat to the next sub soil region and also regains its heat absorbing capacity which was not possible in mode-1. For soil-1 (lowest thermal conductivity), improvement in the outlet air temperature (Fig. 5(a)) during mode-2 (intermittent operation) was greater than the soil-2 and soil-3 (highest thermal conductivity) (Fig. 5(b) and (c)). Due to higher thermal conductivity of soil (soil-3), it can transfer heat to the surrounding sub-soil layers even in continuous operation of EATHE (mode-1) but lower thermal conductivity soil (soil-1) get saturated faster so it requires some regain time for heat transfer to the surrounding sub-soil. It is observed from simulation results that outlet air temperature in mode-2 decrease 1.81% for soil-1, 0.94% for soil-2 and 0.76% for soil-3 as compared to mode-1. This indicates that operating EATHE system in intermittent operation (mode-2) improves its thermal performance only with lower thermal conductivity soil (soil-1) (Fig. 5(a)–(c)). Table 5 shows the soil temperature variation at 10 m section of EATHE pipe length from inlet at radial distances of 0.05 m and 0.25 m, respectively, from the pipe surface in mode-1 and mode2. The soil temperature at both the radial distances was found to
Similarly, EATHE was operated for 60 min running and 40 min OFF (mode-3) in one cycle and similar cycles continued for 12 h. EATHE outlet air temperature variation in mode-3 for three types of soils is presented in Fig. 6(a)–(d). Simulation result shows that the outlet air temperature decreases by 2.90 % for soil-1, 1.66 % for soil-2 and 1.39 % for soil-3 in mode-3 as compared to mode-1. This also indicates that the thermal performance of EATHE improves only with lower thermal conductivity soil (soil-1) (Fig. 6(a)–(c)) as in mode-2. Table 8 shows the soil temperature variation at 10 m section of EATHE pipe length from inlet at radial distance of 0.05 m and 0.25 m in mode-1 and mode-3. The soil temperatures at both the radial distances were found to be lower (Table 8) in mode-3 Table 8 Hourly temperature variation of soil layers at section 10 m length from EATHE inlet. Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface (mode-1 operation) 0.05 m
0.25 m
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
1 3 6 12
28.46 30.07 31.34 32.61
28.81 30.15 31.09 31.95
29.21 30.46 31.25 31.88
27.00 27.00 27.00 27.01
27 27.06 27.29 27.65
27.01 27.29 27.75 28.15
Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface (mode-3 operation) 0.05 m
1 3 6 12
0.25 m
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
28.46 29.23 30.14 30.90
28.81 29.44 30.07 30.50
29.21 29.78 30.31 30.62
27.00 27.00 27.00 27.00
27.00 27.00 27.12 27.24
27.01 27.15 27.38 27.59
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
43
Table 9 Average hourly heat transfer rate in mode-1 and mode-3. Operation time (h)
1 3 6 12
Heat exchange rate (W/m2 ) in mode-1 (continuous operation)
Heat exchange rate (W/m2 ) in mode-3 (intermittent operation)
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
44.50 43.98 43.45 42.92
44.92 44.68 44.49 44.36
44.92 44.69 44.54 44.45
44.92 44.69 44.54 44.45
44.92 44.90 44.72 44.65
44.92 44.92 44.74 44.70
Table 10 Equations and R-square values. S. No
Operation mode
Soil
Equation
R2 value
1 2 3 4 5 6
Mode-1 Mode-3 Mode-1 Mode-3 Mode-1 Mode-3
Soil-1 Soil-1 Soil-2 Soil-2 Soil-3 Soil-3
y = −0.0006x2 + 0.0062x + 301.09 y = 301 + 0.0542x + sin(5.8 + 3.78x) y = −0.0006x2 + 0.0045x + 300.66 y = 301 + 0.0269x + 0.593 cos(2.17 − 3.79x) y = −0.0006x2 + 0.0043x + 300.64 y = 301 + 0.0209x + 0.558 sin(5.69 + 3.79x)
0.994 0.740 0.971 0.750 0.940 0.763
(intermittent operation) than the mode-1 (continuous operation) i.e. better operation because of 40 min OFF time available to dissipate the heat. For soil-1 (lower thermal conductivity) improvement in the thermal performance (Table 9) is better than for soil-2 and soil-3 (higher thermal conductivity) in mode-3 as compared to mode-1. This suggests that soil-1 (lower thermal conductivity) gives best thermal performance in mode-3 because 40 min OFF time helps the soil to spread the accumulated heat and regain the soil heat absorbing capability. Using regression analyses, following equations and R-square values are obtained as shown in Table 10.
EATHE system with higher thermal conductivity soil (soil-3) can be operated continuously (mode-1) while EATHE system with lower thermal conductivity soil (soil-1) must be used in intermittent mode (mode-2 and mode-3).
Acknowledgement We acknowledge financial support provided by the Department of Science and Technology, Government of India under US-India Centre for Building Energy Research and Development (CBERD) project, administrated by Indo-US Science and Technology Forum (ISSUTF).
8. Conclusions The EATHE air outlet temperatures have been numerically determined using CFD software FLUENT v 6.3 and comparison were made between three different types of soils in mode-1 (continuous 12 h operation), mode-2 (intermittent operation i.e. 60 min ON and 20 min OFF cycle for 12 h operation) and mode-3 (intermittent operation i.e. 60 min ON and 40 min OFF cycle for 12 h operation). The main concluding remarks of the study are as follows: 1. Using EATHE system for 12 h continuous operation does not allow the heat stored in the soil to dissipate away and the temperature of the soil near the pipe increases continuously which decrease the temperature drop from EATHE inlet to outlet. This effect depends on the thermal conductivity of the soil i.e. soil with lower thermal conductivity will saturate at a faster rate compare to soil with higher thermal conductivity. 2. Thermal performance of EATHE system in mode-2 (60 min ON, 20 min OFF intermittent operation) increases by 1.81%, 0.94% and 0.76% in terms of EATHE outlet air temperature and 3.35%, 0.45% and 0.40% in terms of heat transfer rate for soil-1, soil-2 and soil-3, respectively. These results show that during off time period allows the accumulated heat in the soil around the pipe to dissipate away from the pipe it contributes to improve thermal performance of EATHE. 3. EATHE system operated in mode-3 (60 min ON and 40 min OFF intermittent operation) in cooling also increases its performance by 2.90%, 1.66% and 1.39% in terms of outlet air temperature and 3.56%, 0.65% and 0.56in terms of heat transfer rate. 4. Operating the EATHE in intermittent mode found to be very useful. Heat accumulation in the nearby soil during continuous operation of EATHE system can be minimized by running the system in intermittent mode. This study also revealed that
References [1] N. Bansal, M. Sodha, S. Singh, A. Sharma, A. Kumar, Evaluation of an earth air tunnel system for cooling/heating of a hospital complex, Build. Environ. 20 (1985) 115–122. [2] F. Ajmi, D. Loveday, V. Hanby, The cooling potential of earth air heat exchangers for domestic buildings in a desert climate, Build. Environ. 41 (2006) 235–244. [3] N. Thanu, R. Sawhney, D. Buddhi, An experimental study of the thermal performance of an earth–air–pipe system in single pass mode, Sol. Energy 71 (2001) 353–364. [4] M. Santamouris, G. Mihalakakou, C. Balaras, A. Argiriou, D. Asimakopoulos, M. Vallindras, Use of buried pipes for energy conservation in cooling of agricultural greenhouses, Sol. Energy 55 (1995) 111–124. [5] V. Bansal, R. Mishra, G. Agarwal, J. Mathur, Performance analysis of integrated earth–air-tunnel-evaporative cooling system in hot and dry climate, Energy Build. 47 (2012) 525–532. [6] S. Said, M. Habib, E. Mokheimer, M. El-Sharqawi, Feasibility of using groundcoupled condensers in A/C systems, Geothermics 39 (2010) 201–204. [7] R. Mishra, V. Bansal, G. Agarwal, J. Mathur, T. Aseri, Thermal performance investigation of hybrid earth air tunnel heat exchanger, Energy Build. 49 (2012) 531–535. [8] F. Niu, Y. Yu, D. Yu, H. Li, Heat and mass transfer performance analysis and cooling capacity prediction of earth to air heat exchanger, Appl. Energy 137 (2015) 211–221. [9] R. Brum, J. Vaz, L. Rocha, L. Santos, L. Isoldi, A new computational modeling to predict the behavior of earth–air heat exchangers, Energy Build. 64 (2013) 395–402. [10] H. Su, X. Liu, L. Ji, J. Mu, A numerical model of a deeply buried air–earth-tunnel heat exchanger, Energy Build. 48 (2012) 233–239. [11] B. Bouhacina, R. Saim, H. Benzenine, H. Oztopc, Analysis of thermal and dynamic comportment of a geothermal vertical U-tube heat exchanger, Energy Build. 58 (2013) 37–43. [12] J. Xamán, I. Hernández-Péreza, J. Arce, G. Álvarez, L. Ramírez-Dávila, F. Noh-Pat, Numerical study of earth-to-air heat exchanger: the effect of thermal insulation, Energy Build. 85 (2014) 356–361. [13] J. Darkwa, G. Kokogiannakis, C. Magadzire, K. Yuan, Theoretical and practical evaluation of an earth-tube (E-tube) ventilation system, Energy Build. 43 (2011) 728–736. [14] G. Gan, Dynamic interactions between the ground heat exchanger and environments in earth–air tunnel ventilation of buildings, Energy Build. 85 (2014) 12–22.
44
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
[15] O. Ozgener, L. Ozgener, J. Tester, A practical approach to predict soil temperature variations for geothermal (ground) heat exchangers applications, Int. J. Heat Mass Transfer 62 (2013) 473–480. [16] J. Vaz, M. Sattler, E. Santos, L. Isoldi, Experimental and numerical analysis of an earth–air heat exchanger, Energy Build. 43 (2011) 2476–2482. [17] J. Vaz, M. Sattler, R. Brum, E. Santos, L. Isoldi, An experimental study on the use of earth–air heat exchangers (EAHE), Energy Build. 72 (2014) 122–131. [18] V. Bansal, R. Misra, G. Agarwal, J. Mathur, Transient effect of soil thermal conductivity and duration of operation on performance of earth air tunnel heat exchanger, Appl. Energy 103 (2013) 1–11.
[19] ANSYS, ANSYS Fluent User’s Guide Release 14.5, 2012 http://www.ansys.com/ Support/Documentation [20] R. Mishra, V. Bansal, G.D. Agrawal, J. Mathur, T.K. Aseri, CFD analysis based parametric study of derating factor for earth air tunnel heat exchanger, Appl. Energy 103 (2013) 266–277. [21] L. Chandra, P. Garg, R. Maitri, A. Agarwal, K. Shweta, A Stepwise Modeling Approach for Designing an Earth-Air Heat Exchanger in Jodhpur Region of Rajasthan, 2012. [22] F. Boithias, J. Zhang, M. Mankibi, F. Haghighat, P. Michel, Simple model and control strategy of earth-to-air heat exchangers, in: ACTEA 2009, ZoukMosbeb, Lebanon, July 15–17, 2009.
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
CFD analysis of EATHE system under transient conditions for intermittent operation Anuj Mathur a,∗ , Ayushman Srivastava b , G.D. Agrawal a , Sanjay Mathur a , Jyotirmay Mathur a a b
Centre for Energy and Environment, Malaviya National Institute of Technology, Jaipur 302017, India Civil Engineering Department, Malaviya National Institute of Technology, Jaipur 302017, India
a r t i c l e
i n f o
Article history: Received 10 September 2014 Received in revised form 3 November 2014 Accepted 6 November 2014 Available online 13 November 2014 Keywords: Earth air tunnel heat exchanger CFD simulation Continuous and intermittent operation Transient thermal performance Heat transfer
a b s t r a c t Thermal performance of earth air tunnel heat exchanger has been investigated under transient conditions for three different soil conditions considering three operating modes. In first operating mode EATHE works continuously for 12 h, in second mode it works for 60 min and then it is off for 20 min and in the last mode EATHE runs for 60 min and remains off for 40 min. In second and third mode EATHE is operated for 12 h intermittently. CFD model was developed in GAMBIT (version 2.2.3), simulated in FLUENT (version 6.3) and then validated with experimental data. CFD analysis has been carried out using a threedimensional transient numerical model. Air temperature drop and heat transfer between air and soil is calculated considering different thermal properties of soils. Higher thermal conductivity (1.28 W m−1 K−1 ) soil should be used in first mode of 12 h continuous operation as it improves the performance by 5.04% as compared to least thermal conductivity soil and soil with poor thermal conductivity (0.52 W m−1 K−1 ) shows performance improvement of 1.81% when operated in intermittent operation as compared to continuous running. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Passive heating/cooling systems consume no or very less energy as compared to active heating and cooling systems. In order to utilize these passive heating/cooling systems with great heat capacity and high thermal inertia, many techniques have been developed in the last decades such as earth air tunnel heat exchanger. Earth air tunnel heat exchanger system uses underground soil as a heat source/sink and air as the heat transfer medium for space heating in winter and cooling in summer because of almost constant sub-soil temperature throughout the year. When air flows in the buried pipes, heat is transferred to/from the soil from/to the air. The outlet air from the buried pipe can be directly used for space heating/cooling. Alternatively, the outlet air may be used with air conditioning machines. Over the last decade, many researchers have carried out a number of performance analyses on EATHE systems to improve its thermal performance, either using numerical modeling or via experiments. It has been established through previous studies
∗ Corresponding author. Tel.: +91 9982000532. E-mail address: [email protected] (A. Mathur). http://dx.doi.org/10.1016/j.enbuild.2014.11.022 0378-7788/© 2014 Elsevier B.V. All rights reserved.
[1–12] that EATHE systems have a good potential to provide cooling effect. Some of these studies also revealed that the thermal performance of EATHE gets deteriorated during continuous operation, due to heat accumulation in the nearby ground, around the EATHE pipe. An experimental study conducted by Darkwa et al. [13] highlighted that the overall performance of the EATHE system gets affected by the soil properties, namely thermal conductivity and heat capacity. Study conducted by Gan [14] has shown that under varying soil and atmospheric conditions, performance of EATHE system greatly depends upon the thermal and physical properties of soil, installation depth and horizontal distance between parallel pipes. To address the issue of thermal saturation of soil, an analytical model was developed by Ozgener et al. [15] to predict the daily soil temperatures depending on depth and duration of operation. Numerical analysis presented by Vaz et al. [16,17] indicated that the soil temperature, near the duct region gets affected by its continuous operation. A correlation between the soil and EATHE temperature variation was developed using linear regression model. Bansal et al. [18] have studied the effect of three different types of soils with thermal conductivities of 0.52, 2 and 4 W m−1 K−1 . It was found that maximum performance deterioration with
38
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
Table 1 Geometrical and simulation parameters. Parameters
Unit
Value
Parameters
Unit
Value
EATHE pipe length Pipe outer diameter Surrounding soil diameter Air density Air thermal conductivity Air specific heat capacity
m m m kg m−3 W m−1 K−1 J kg−1 K−1
40 0.1 1.1 1.225 0.02 1006
Soil density Soil specific heat Soil thermal conductivity PVC pipe density PVC thermal conductivity PVC specific heat capacity
kg m−3 J kg−1 K−1 W m−1 K−1 kg m−3 (W m−1 K−1 ) J kg−1 K−1
2050 1840 0.52 1380 1.16 900
Table 2 Boundaries conditions. Boundaries
Unit
Value
Initial soil temperature Initial pipe temperature Air inlet velocity Air inlet temperature
◦
27 27 5 46.2
C ◦ C ms−1 ◦ C
Grid Independenc e Test
Air Temperatures (°C)
45
Element size = 0.03 m Element size = 0.04 m
40 35 30
Fig. 1. Physical geometry model of EATHE pipe and soil.
25
prolonged operation takes place with soil having lowest thermal conductivity. However in this study, only the effect of thermal conductivity has been investigated, considering same density and specific heat constant for all three soils. It has been clearly seen through the literature review that thermal performance of EATHE system deteriorates upon its continuous operation, especially in case of lower soil thermal conductivity. Thermal conductivity is an inherent property of soil and cannot be changed so easily, if found unfavorable for EATHE development. As an alternative to solve this problem, intermittent operation of EATHE has been investigated; hypothesizing that soil gets regenerated during the period system is not used. The balance between ‘ON’ an ‘OFF’ condition for different types of soil is a matter of investigation. In this paper, a validated transient numerical model was used to investigate the thermal performance of EATHE system for three operation modes such as 12 h continuous, 60 min ON and 20 min rest, 60 min working and 40 min OFF running operation mode. The numerical simulation was done considering three soils with different thermal properties to study the role of different soils in intermittent operation. 2. System description and simulation setup CFD simulations were performed using research CFD code FLUENT (version 6.3) and preprocessor GAMBIT (version 2.2.3) for the geometry and 3D meshing in order to investigate the effect of intermittent operation on EATHE thermal performance. 2.1. Physical model Description of geometrical configuration of the earth air tunnel heat exchanger and soil ambient is presented in Table 1. Physical geometry (Fig. 1) of the EATHE system was created using structured hexahedral meshing for 40 m long and 0.1 m outer diameter PVC
0
5
10
115
20
25
30
35
40
Length of pipe (m)
Fig. 2. Grid independence test.
pipe. The control volume was defined by creating a cylinder volume of soil (soil cylinder diameter of 1.1 m) around the EATHE pipe. 2.2. Simulation model ANSYS FLUENT v 6.3 was used in the study that used finite volume method to convert the governing equations in to numerically solvable algebraic equations. To predict the turbulence inside the pipe, simple k–ε model with standard wall treatment is selected as turbulent model and energy equation is also solved since the computations included heat transfer as explained in ANSYS user guide [19]. The simple k–ε model is built in within FLUENT itself and one of the most common turbulence model and gives good results for internal flows. The numerical investigation was based on the following assumptions: i. Thermo-physical properties of solids and fluids remain constant over the range of soil and air temperature during operation. ii. Inlet air velocity is constant throughout the operation of EATHE. The far-field distances were carefully selected to ensure that the far-field boundary temperature would not have any effect over the period of the simulations. Therefore, far-field boundaries in the soil environment were treated as adiabatic surfaces. Coupled boundary conditions were used for the PVC pipe and surrounding soil so that heat can be transfer through pipe to surrounding soil and were initialized with 300 K i.e. 27 ◦ C as shown in Table 2. ‘Velocity inlet’ boundary condition was specified for the inlet air velocity of 5 m/s and as air is a compressible fluid so ‘pressure outlet’ condition for the outlet for the air.
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
Air Temperature aer 1 hr
Air Temperature aer 3 hr 50
50 Air outlet Temperature (°C)
Air outlet Temperature (°C)
Present Numerical Soluon
45
Misra et al numerical Soluon
40 35 30 25
Present Numerical Soluon
45
Misra et al numerical Soluon
40 35 30 25 20
20 0
10
20
30
0
40
10
Length of Pipe (m)
Misra et al numerical Soluon
40 35 30 25
0
10
20
40
50
Present Numerical Soluon
45
30
Air Temperature aer 12 hr
Air outlet Temperature (°C)
Air outlet Temperature (°C)
50
20
Length of Pipe (m)
Air Temperature aer 6 hr
20
39
30
40
Present Numerical Soluon
45
Misra et al numerical Soluon
40 35 30 25 20
0
10
20
30
40
Length of Pipe (m)
Length of Pipe (m)
Fig. 3. Air outlet temperature comparison between numerical solutions.
Fig. 4. Variation in air outlet temperature in continuous operation mode
The convergence criteria were achieved for continuity, xvelocity, y-velocity, z-velocity, k, epsilon and the energy equation at the each time step. The convergence criteria except for the energy was set at a level of 1e−3 and for energy, it was 1e−6. 3. Grid independence test Grid independence test (Fig. 2) was conducted to assess the quality of developed CFD model. If the mesh is refined (i.e. the
cells are made smaller in size hence larger in number), then the behavior observed by the post processing should remain unchanged if the solution is grid independent. To have grid independent solution, simulations are run for two grid sizes i.e. 0.04 m and 0.03 m taking operating parameters same for both cases. It is observed from Fig. 2 that there is no or minimum effect on the air temperature (main operating parameter) when grid size changed from 0.04 m to 0.03 m. Therefore, 0.04 m element size is
Table 3 Thermal properties of three different soils. Soil
Location
Density (kg m−3 )
Specific heat capacity (J kg−1 K−1 )
Thermal conductivity (W m−1 K−1 )
Thermal diffusivity (m2 /s)
Reference
Soil-1 Soil-2 Soil-3
Ajmer (India) Jodhpur (India) Presles (France)
2050 1470 1500
1840 1553 880
0.52 1.00 1.28
1.37 × 10−7 4.37 × 10−7 9.69 × 10−7
[18] [21] [22]
40
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
Fig. 5. (a) EATHE outlet air temperature in intermittent (mode-2) and continuous operation with soil-1. (b) EATHE outlet air temperatures in intermittent (mode-2) and continuous operation with soil-2. (c) EATHE outlet air temperatures in intermittent (mode-2) and continuous operation with soil-3. (d) EATHE outlet air temperature in mode-2 (intermittent) operation mode.
Table 4 Hourly radial temperature variation of soil at 10 m section of EATHE pipe from inlet. Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface 0.05 m
1 3 6 12
Table 5 Hourly temperature variation of soil layers at section 10 m length from EATHE inlet. Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface (mode-1 operation) 0.05 m
0.25 m
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
28.46 30.07 31.34 32.61
28.81 30.15 31.09 31.95
29.21 30.46 31.25 31.88
27.00 27.00 27.00 27.01
27.00 27.06 27.29 27.65
27.01 27.29 27.75 28.15
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
1 3 6 12
28.46 30.07 31.34 32.61
28.81 30.15 31.09 31.95
29.21 30.46 31.25 31.88
27.00 27.00 27.00 27.01
27.00 27.06 27.29 27.65
27.01 27.29 27.75 28.15
Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface (mode-2 operation)
chosen as the model grid size as it gave good accuracy and lesser computation time. 4. Validation of CFD model The developed CFD model in the present study was validated against the numerical solution (which was experimentally verified) suggested by Mishra et al. [20]. Maximum temperature difference at one point along the EATHE length was found to be 0.72 ◦ C from the two studies whereas, for most of the other points, temperatures were almost same as shown in Fig. 3. There is very good agreement between the two numerical solutions and thus the proposed CFD model is validated. 5. Selection of soil for analysis Three soils with different thermal properties were selected as shown in Table 3. These soils were considered to investigate role of
0.25 m
0.05 m
1 3 6 12
0.25 m
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
28.46 29.36 30.50 31.58
28.81 29.42 30.34 31.08
29.21 29.69 30.55 31.16
27.00 27.00 27.00 27.00
27.00 27.03 27.20 27.38
27.01 27.23 27.55 27.80
thermal diffusivity and thermal conductivity on the EATHE performance in intermittent operation. 6. Simulation on continuous 12 h operation (mode-1) In the simulation process, air inlet temperature and flow rate were taken to be constant of 319.2 K (46.2 ◦ C) and 5 ms−1 , respectively. Whereas, soil and pipe temperature was initialized at 300 K i.e. 27 ◦ C and it increases as the hot air (in summer) passes through the buried EATHE pipe. These initial conditions taken for air and
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
41
Fig. 6. (a) EATHE outlet air temperatures in intermittent (mode-3) and continuous operation with soil-1. (b) EATHE outlet air temperatures in intermittent (mode-3) and continuous operation with soil-2. (c) EATHE outlet air temperatures in intermittent (mode-3) and continuous operation with soil-3. (d) EATHE outlet air temperature in mode-3 (intermittent) operation mode.
soil were same as the actual condition considered by Mishra et al. [20]. EATHE outlet air temperature variation for 12 h of continuous running for three soils is shown in Fig. 4 and it is observed that the outlet air temperature increases with time due to saturation of adjacent soil. The EATHE outlet air temperature increase was maximum 1.87 K for soil-1 whereas it was 1.01 K and 0.85 K for soil-2 and soil-3, respectively, after 12 h of continuous operation. Hence, the performance of EATHE system gets deteriorated due to saturation of subsoil surrounding the EATHE pipe during continuous running. The performance deterioration in terms of EATHE outlet air temperature increase was the least (Fig. 4) for soil-3 which was having the highest thermal conductivity, and resulted into better thermal performance even after continuous operation of 12 h. Heat penetration into the surrounding soil due to heat transfer between the EATHE air and soil depends on the thermal
conductivity of the soil. Soil temperature in radial direction was measured at six locations i.e. 0.0 m, 0.05 m, 0.1 m, 0.15 m, 0.20 m, and 0.25 m in order to get the idea of radial heat penetration. Heat penetration for soil-2 and soil-3 up to 0.25 m away from the EATHE pipe during 12 h continuous operation due to higher thermal conductivity but for soil-1 heat penetration was up to 0.05 m only due to lower thermal conductivity (Table 4). Better thermal performance of EATHE system was observed with higher thermal conductivity due to easier dissipation of heat through the soil layers situated in the immediate vicinity of EATHE pipe to the soil layers situated away from the soil pipe interface in the radial direction. Regression analysis was performed using Eureqa software for EATHE outlet air temperature in terms of operation time as shown in Eqs. (1), (2) and (3) for soil-1, soil-2 and soil-3, respectively. The
Table 6 Average hourly heat transfer rate in mode-1 and mode-2. Operation time (h)
1 3 6 12
Heat transfer rate (W/m2 ) in mode-1 (continuous operation)
Heat transfer rate (W/m2 ) in mode-2 (intermittent operation)
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
44.50 43.98 43.45 42.92
44.92 44.68 44.49 44.36
44.92 44.69 44.54 44.45
44.92 44.68 44.49 44.36
44.92 44.83 44.67 44.59
44.92 44.84 44.70 44.63
42
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
R-square values for the three equations are 0.9975, 0.9885 and 0.9777, respectively.
S. No
Outlet air temperature = −0.0134 × Time2 + 0.3364 × Time + 301.20
(1)
Outlet air temperature = −0.0105 × Time2 + 0.2192 × Time + 300.82
1 2 3 4 5 6
Operation mode Mode-1 Mode-2 Mode-1 Mode-2 Mode-1 Mode-2
R2 value
Soil
Equation
Soil-1 Soil-1 Soil-2 Soil-2 Soil-3 Soil-3
y = −0.0006x + 0.0061x + 301.1 y = 302 − 1.42 cos(20.5x) y = −0.0006x2 + 0.0044x + 300.68 y = 301 − cos(0.0777x) y = −0.0006x2 + 0.0043x + 300.66 y = 301 − cos(0.0779x) 2
0.994 0.960 0.972 0.974 0.940 0.978
(2)
Outlet air temperature = −0.0099 × Time2 + 0.1944 × Time + 300.85
Table 7 Equations and R-square values.
(3)
where outlet air temperature is in K and operation time in h. 7. Simulation on short period operation Earth air tunnel heat exchanger was operated for short period in two modes; in mode-2 EATHE was operated for 60 min and remained OFF for 20 min in one cycle and these cycles continued for 12 h. In mode-3, it worked for 60 min and remained OFF for 40 min and these cycles also continued for 12 h. During the simulation, inlet air temperature was set constant at 319.2 K i.e. 46.2 ◦ C, soil and pipe temperature was initialized at 300 K i.e. 27 ◦ C and air flow rate was maintained at 5 m s−1 during ON mode and 0 m s−1 during OFF mode.
be lower (Table 5) in mode-2 (intermittent operation) than the mode-1 (continuous operation) i.e. better performance because in intermittent operation there is 20 min OFF time available to dissipate heat to the surrounding sub-soil layers. For soil-1 (lower thermal conductivity) improvement in the thermal performance (Table 6) is better than for soil-2 and soil-3 (higher thermal conductivity) in mode-2 as compared to mode-1. Simulation results of Table 6 shows that heat transfer rate after 12 h of operation increases by 3.35 % for soil-1, 0.45% for soil-2 and 0.40% for soil-3 in mode-2 as compared to mode-1 which shows that EATHE system in intermittent mode gives best thermal performance with soil-1 having least thermal conductivity. Using regression analyses, following equations were deducted in terms of ‘outlet air temperature (K) (denoted as y)’ and ‘time of operation (h) (denoted as x)’ comprising second order polynomial and trigonometric equations. Continuous operation of EATHE (mode-1) follows the second order polynomial trend whereas intermittent operation follows trigonometric trend. The obtained equations and R-square values are shown in Table 7.
7.1. EATHE operation in mode-2 7.2. EATHE operation in mode-3 In mode-2 EATHE operated for 60 min and remained OFF for 20 min and similar EATHE working cycles continued for 12 h. EATHE outlet air temperature variation in mode-2 for the three different soils is presented in Fig. 5(a)–(d). In mode-1 (continuous 12 h operation) the performance of EATHE system in terms of EATHE outlet air temperatures was better in the beginning and then outlet air temperature starts increasing and performance decline. The air flowing through EATHE pipe loses heat to the surrounding soil and soil gets saturated with time. In order to study the effect of soil saturation on EATHE performance, EATHE system is studied in two intermittent modes i.e. mode-2 and mode-3. It is noticed that the thermal performance of EATHE system improved in mode-2 as compared to mode-1 (Fig. 5(a)) because during the OFF period of EATHE system, surrounding soil dissipates the stored heat to the next sub soil region and also regains its heat absorbing capacity which was not possible in mode-1. For soil-1 (lowest thermal conductivity), improvement in the outlet air temperature (Fig. 5(a)) during mode-2 (intermittent operation) was greater than the soil-2 and soil-3 (highest thermal conductivity) (Fig. 5(b) and (c)). Due to higher thermal conductivity of soil (soil-3), it can transfer heat to the surrounding sub-soil layers even in continuous operation of EATHE (mode-1) but lower thermal conductivity soil (soil-1) get saturated faster so it requires some regain time for heat transfer to the surrounding sub-soil. It is observed from simulation results that outlet air temperature in mode-2 decrease 1.81% for soil-1, 0.94% for soil-2 and 0.76% for soil-3 as compared to mode-1. This indicates that operating EATHE system in intermittent operation (mode-2) improves its thermal performance only with lower thermal conductivity soil (soil-1) (Fig. 5(a)–(c)). Table 5 shows the soil temperature variation at 10 m section of EATHE pipe length from inlet at radial distances of 0.05 m and 0.25 m, respectively, from the pipe surface in mode-1 and mode2. The soil temperature at both the radial distances was found to
Similarly, EATHE was operated for 60 min running and 40 min OFF (mode-3) in one cycle and similar cycles continued for 12 h. EATHE outlet air temperature variation in mode-3 for three types of soils is presented in Fig. 6(a)–(d). Simulation result shows that the outlet air temperature decreases by 2.90 % for soil-1, 1.66 % for soil-2 and 1.39 % for soil-3 in mode-3 as compared to mode-1. This also indicates that the thermal performance of EATHE improves only with lower thermal conductivity soil (soil-1) (Fig. 6(a)–(c)) as in mode-2. Table 8 shows the soil temperature variation at 10 m section of EATHE pipe length from inlet at radial distance of 0.05 m and 0.25 m in mode-1 and mode-3. The soil temperatures at both the radial distances were found to be lower (Table 8) in mode-3 Table 8 Hourly temperature variation of soil layers at section 10 m length from EATHE inlet. Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface (mode-1 operation) 0.05 m
0.25 m
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
1 3 6 12
28.46 30.07 31.34 32.61
28.81 30.15 31.09 31.95
29.21 30.46 31.25 31.88
27.00 27.00 27.00 27.01
27 27.06 27.29 27.65
27.01 27.29 27.75 28.15
Time (h)
Soil temperatures (◦ C) at various radial distances from EATHE pipe surface (mode-3 operation) 0.05 m
1 3 6 12
0.25 m
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
28.46 29.23 30.14 30.90
28.81 29.44 30.07 30.50
29.21 29.78 30.31 30.62
27.00 27.00 27.00 27.00
27.00 27.00 27.12 27.24
27.01 27.15 27.38 27.59
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
43
Table 9 Average hourly heat transfer rate in mode-1 and mode-3. Operation time (h)
1 3 6 12
Heat exchange rate (W/m2 ) in mode-1 (continuous operation)
Heat exchange rate (W/m2 ) in mode-3 (intermittent operation)
Soil-1
Soil-2
Soil-3
Soil-1
Soil-2
Soil-3
44.50 43.98 43.45 42.92
44.92 44.68 44.49 44.36
44.92 44.69 44.54 44.45
44.92 44.69 44.54 44.45
44.92 44.90 44.72 44.65
44.92 44.92 44.74 44.70
Table 10 Equations and R-square values. S. No
Operation mode
Soil
Equation
R2 value
1 2 3 4 5 6
Mode-1 Mode-3 Mode-1 Mode-3 Mode-1 Mode-3
Soil-1 Soil-1 Soil-2 Soil-2 Soil-3 Soil-3
y = −0.0006x2 + 0.0062x + 301.09 y = 301 + 0.0542x + sin(5.8 + 3.78x) y = −0.0006x2 + 0.0045x + 300.66 y = 301 + 0.0269x + 0.593 cos(2.17 − 3.79x) y = −0.0006x2 + 0.0043x + 300.64 y = 301 + 0.0209x + 0.558 sin(5.69 + 3.79x)
0.994 0.740 0.971 0.750 0.940 0.763
(intermittent operation) than the mode-1 (continuous operation) i.e. better operation because of 40 min OFF time available to dissipate the heat. For soil-1 (lower thermal conductivity) improvement in the thermal performance (Table 9) is better than for soil-2 and soil-3 (higher thermal conductivity) in mode-3 as compared to mode-1. This suggests that soil-1 (lower thermal conductivity) gives best thermal performance in mode-3 because 40 min OFF time helps the soil to spread the accumulated heat and regain the soil heat absorbing capability. Using regression analyses, following equations and R-square values are obtained as shown in Table 10.
EATHE system with higher thermal conductivity soil (soil-3) can be operated continuously (mode-1) while EATHE system with lower thermal conductivity soil (soil-1) must be used in intermittent mode (mode-2 and mode-3).
Acknowledgement We acknowledge financial support provided by the Department of Science and Technology, Government of India under US-India Centre for Building Energy Research and Development (CBERD) project, administrated by Indo-US Science and Technology Forum (ISSUTF).
8. Conclusions The EATHE air outlet temperatures have been numerically determined using CFD software FLUENT v 6.3 and comparison were made between three different types of soils in mode-1 (continuous 12 h operation), mode-2 (intermittent operation i.e. 60 min ON and 20 min OFF cycle for 12 h operation) and mode-3 (intermittent operation i.e. 60 min ON and 40 min OFF cycle for 12 h operation). The main concluding remarks of the study are as follows: 1. Using EATHE system for 12 h continuous operation does not allow the heat stored in the soil to dissipate away and the temperature of the soil near the pipe increases continuously which decrease the temperature drop from EATHE inlet to outlet. This effect depends on the thermal conductivity of the soil i.e. soil with lower thermal conductivity will saturate at a faster rate compare to soil with higher thermal conductivity. 2. Thermal performance of EATHE system in mode-2 (60 min ON, 20 min OFF intermittent operation) increases by 1.81%, 0.94% and 0.76% in terms of EATHE outlet air temperature and 3.35%, 0.45% and 0.40% in terms of heat transfer rate for soil-1, soil-2 and soil-3, respectively. These results show that during off time period allows the accumulated heat in the soil around the pipe to dissipate away from the pipe it contributes to improve thermal performance of EATHE. 3. EATHE system operated in mode-3 (60 min ON and 40 min OFF intermittent operation) in cooling also increases its performance by 2.90%, 1.66% and 1.39% in terms of outlet air temperature and 3.56%, 0.65% and 0.56in terms of heat transfer rate. 4. Operating the EATHE in intermittent mode found to be very useful. Heat accumulation in the nearby soil during continuous operation of EATHE system can be minimized by running the system in intermittent mode. This study also revealed that
References [1] N. Bansal, M. Sodha, S. Singh, A. Sharma, A. Kumar, Evaluation of an earth air tunnel system for cooling/heating of a hospital complex, Build. Environ. 20 (1985) 115–122. [2] F. Ajmi, D. Loveday, V. Hanby, The cooling potential of earth air heat exchangers for domestic buildings in a desert climate, Build. Environ. 41 (2006) 235–244. [3] N. Thanu, R. Sawhney, D. Buddhi, An experimental study of the thermal performance of an earth–air–pipe system in single pass mode, Sol. Energy 71 (2001) 353–364. [4] M. Santamouris, G. Mihalakakou, C. Balaras, A. Argiriou, D. Asimakopoulos, M. Vallindras, Use of buried pipes for energy conservation in cooling of agricultural greenhouses, Sol. Energy 55 (1995) 111–124. [5] V. Bansal, R. Mishra, G. Agarwal, J. Mathur, Performance analysis of integrated earth–air-tunnel-evaporative cooling system in hot and dry climate, Energy Build. 47 (2012) 525–532. [6] S. Said, M. Habib, E. Mokheimer, M. El-Sharqawi, Feasibility of using groundcoupled condensers in A/C systems, Geothermics 39 (2010) 201–204. [7] R. Mishra, V. Bansal, G. Agarwal, J. Mathur, T. Aseri, Thermal performance investigation of hybrid earth air tunnel heat exchanger, Energy Build. 49 (2012) 531–535. [8] F. Niu, Y. Yu, D. Yu, H. Li, Heat and mass transfer performance analysis and cooling capacity prediction of earth to air heat exchanger, Appl. Energy 137 (2015) 211–221. [9] R. Brum, J. Vaz, L. Rocha, L. Santos, L. Isoldi, A new computational modeling to predict the behavior of earth–air heat exchangers, Energy Build. 64 (2013) 395–402. [10] H. Su, X. Liu, L. Ji, J. Mu, A numerical model of a deeply buried air–earth-tunnel heat exchanger, Energy Build. 48 (2012) 233–239. [11] B. Bouhacina, R. Saim, H. Benzenine, H. Oztopc, Analysis of thermal and dynamic comportment of a geothermal vertical U-tube heat exchanger, Energy Build. 58 (2013) 37–43. [12] J. Xamán, I. Hernández-Péreza, J. Arce, G. Álvarez, L. Ramírez-Dávila, F. Noh-Pat, Numerical study of earth-to-air heat exchanger: the effect of thermal insulation, Energy Build. 85 (2014) 356–361. [13] J. Darkwa, G. Kokogiannakis, C. Magadzire, K. Yuan, Theoretical and practical evaluation of an earth-tube (E-tube) ventilation system, Energy Build. 43 (2011) 728–736. [14] G. Gan, Dynamic interactions between the ground heat exchanger and environments in earth–air tunnel ventilation of buildings, Energy Build. 85 (2014) 12–22.
44
A. Mathur et al. / Energy and Buildings 87 (2015) 37–44
[15] O. Ozgener, L. Ozgener, J. Tester, A practical approach to predict soil temperature variations for geothermal (ground) heat exchangers applications, Int. J. Heat Mass Transfer 62 (2013) 473–480. [16] J. Vaz, M. Sattler, E. Santos, L. Isoldi, Experimental and numerical analysis of an earth–air heat exchanger, Energy Build. 43 (2011) 2476–2482. [17] J. Vaz, M. Sattler, R. Brum, E. Santos, L. Isoldi, An experimental study on the use of earth–air heat exchangers (EAHE), Energy Build. 72 (2014) 122–131. [18] V. Bansal, R. Misra, G. Agarwal, J. Mathur, Transient effect of soil thermal conductivity and duration of operation on performance of earth air tunnel heat exchanger, Appl. Energy 103 (2013) 1–11.
[19] ANSYS, ANSYS Fluent User’s Guide Release 14.5, 2012 http://www.ansys.com/ Support/Documentation [20] R. Mishra, V. Bansal, G.D. Agrawal, J. Mathur, T.K. Aseri, CFD analysis based parametric study of derating factor for earth air tunnel heat exchanger, Appl. Energy 103 (2013) 266–277. [21] L. Chandra, P. Garg, R. Maitri, A. Agarwal, K. Shweta, A Stepwise Modeling Approach for Designing an Earth-Air Heat Exchanger in Jodhpur Region of Rajasthan, 2012. [22] F. Boithias, J. Zhang, M. Mankibi, F. Haghighat, P. Michel, Simple model and control strategy of earth-to-air heat exchangers, in: ACTEA 2009, ZoukMosbeb, Lebanon, July 15–17, 2009.