Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger

Journal of Cleaner Production xxx (xxxx) xxx

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Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger Tiecheng Zhou a, b, Yimin Xiao a, b, *, Haotian Huang a, b, Jianquan Lin a, b a b

Key Laboratory of the Three Gorges Reservoir Region’s Eco-Environment, Ministry of Education, Chongqing University, Chongqing, 400045, China National Centre for International Research of Low-carbon and Green Buildings, Chongqing University, Chongqing, 400045, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 July 2019 Received in revised form 27 September 2019 Accepted 14 October 2019 Available online xxx

The application of renewable energy technologies, such as the earth-air heat exchanger (EAHE), is quite conducive to reducing the energy consumption of buildings. However, the heat transfer process of traditional EAHE may greatly change the soil temperature, which hinders the system from fully exerting its thermal performance. Based on the features of high energy density and stable output temperature in the phase change heat storage process, this paper proposes a cylindrical phase change material-assisted EAHE (CPCM-EAHE) to improve the performance of such systems. An equivalent heat capacity method based 3-D numerical model for this novel system was established on the ANSYS Fluent platform. And its calculation result was verified by an indoor experimental set-up. Under summer high-temperature meteorological conditions of Chongqing, the cooling performance of CPCM-EAHE and traditional EAHE is comparatively studied through this numerical model. The results tell that PCM does enhance the heat transfer of EAHE most of the time, as well as delay its transition from the heating mode to cooling mode but advance its transition from the cooling mode to heating mode. These changes finally make the CPCMEAHE achieve excellent cooling performance. Specifically, compared to traditional EAHE, the daily maximum cooling capacity of CPCM-EAHE is increased by 28.55%e39.74%. Even from the whole 20-day investigation period, its total cooling output is increased by 20.05% as well. Finally, this CPCM-EAHE can damp the temperature fluctuation of fresh air to approximately 1
C. © 2019 Elsevier Ltd. All rights reserved.

Handling editor: Jin-Kuk Kim Keywords: EAHE Cylindrical PCM Cooling performance Numerical simulation Energy-efficient renovation

1. Introduction In recent years, China has been leading the world in energy consumption. For example, in 2017, the primary energy consumption of China was 3132.2 million tons of oil equivalent, accounting for about 23.2% of the global total (BP, 2018). However, just the part of keeping building operation accounts for 20% of the total social primary energy consumption (THUBERC, 2018). Thus, effectively reducing this part of energy consumption can help China to better fulfill its emission-reduction commitments. Studies (Esen and Yuksel, 2013; Kaushal, 2017; Soni et al., 2016) show that reasonable applications of renewable energy technologies, like groundcoupled heat exchanger, in the HVAC system can greatly reduce

* Corresponding author. School of Civil Engineering, Chongqing University, Chongqing, 400045, China. E-mail address: [email protected] (Y. Xiao).

the energy consumption of buildings. Generally, the ground-coupled heat exchanger (GCHE) can be classified into the ground source heat pump (GSHP) and the earthair heat exchanger (EAHE) (Soni et al., 2015). The latter employs shallowly buried air pipes as its key components where fresh air is cooled/heated. Scholars (Jensen-Page et al., 2018; NaranjoMendoza et al., 2018) believed that changes in soil temperature can be summarized by a time-depth based sinusoidal function. With the depth increases, the amplitude of this function is attenuated and finally becomes zero, which means a constant soil temperature (Badache et al., 2016). It is these features of sub-soil that enable EAHE to cool/heat the fresh air. Then Bansal et al. (1983), and Krarti and Kreider (1996), and Liu et al. (2014) and Fazlikhani et al. (2017) provided simplified analytical models and numerical models to explore the cooling/heating capacity of EAHEs. However, all of them have not considered the temperature disturbance at the ground surface, which will introduce deviations in predicting the vertical temperature gradient of sub-soil (Gan, 2015).

https://doi.org/10.1016/j.jclepro.2019.118907 0959-6526/© 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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Fortunately, Gan (2014, 2015) has overcome this deficiency by taking account of the heat and moisture disturbances at the ground surface due to convection, radiation, evaporation/condensation, and precipitation. Correctly understanding the relationships between parameters and thermal performance of the system is essential for exploring performance improvement schemes for EAHEs. These parameters can be classified into geographical and meteorological conditions, geometric parameters, thermophysical properties, and operational strategies. Firstly, the geographical and meteorological conditions mainly include ground surface conditions and periodic variation features of outdoor air temperature. In general, the wet and shade or shortgrass covered ground surfaces are more conducive to cooling the fresh air flowing through EAHE (Mihalakakou et al., 1994; Sodha et al., 1991), while the blackened and glazed or bare soil surfaces are more favorable for EAHE to perform its heating effect (Mihalakakou et al., 1996; Sodha et al., 1991). Moreover, Ramírezvila et al. (2014) confirmed that climates of extreme and modDa erate temperatures are more appropriate for EAHEs to cool/heat the buildings. Besides, if the annual energy saving is considered, the EAHE is more suitable to use as a cooler in hot-arid climates rather than use as a heater in cold climates (Fazlikhani et al., 2017). Secondly, the most focused geometric parameters are the buried depth, diameter and length of the air pipe, and pipe spacing under the multi-tube arrangement. Yang et al. (2016) and Yang and Zhang (2015) confirmed that, through increasing the buried depth or length of the air pipe, the cooling performance of EAHE can be improved. But these improvements become less significant beyond certain values. Besides, an increase in pipe diameter will reduce the heat transfer area corresponding to per unit volume air, which is detrimental to the heat transfer process between air and soil (Ahmed et al., 2016; Menhoudj et al., 2018). However, increasing the pipe diameter can increase the air volume and finally improve the overall cooling output of EAHE (Niu et al., 2015; Wu et al., 2007). For the system employing a multi-tube arrangement, the pipe spacing should be determined carefully to minimize thermal interaction (Freire et al., 2013; Yoon et al., 2009), because an increase in pipe spacing helps to improve the overall performance of the system. However, a large pipe spacing does not always lead to performance improvement (Peretti et al., 2013). Then, the thermophysical properties are generally used to characterize the soil and pipe material. Cuny et al. (2018) declared that, both the soil type and moisture content have non-negligible influences on the cooling/heating performance of EAHE, and coating soils that have the capacity of storing water are conducive to improving its performance. It is important to recognize that the thermal performance of EAHE can be improved by employing high thermal conductivity soils (Mathur et al., 2015a), and the thermal conductivity of soil can be improved by increasing its water content (Cao et al., 2018; Zhang and Wang, 2017). Therefore, high water content soils are beneficial for EAHE to cool/heat fresh air. This argument also has been proved by Ascione et al. (2011) which discussed the thermal performance of EAHE under different soil types. Although high thermal conductivity is beneficial for heat transfer, the actual influence of pipe material on the thermal performance of EAHE is not noticeable due to the small thickness of the air pipe wall (Menhoudj et al., 2018; Serageldin et al., 2016). Finally, for the operational strategies of EAHE, scholars are more concerned about its air velocity and operating mode. In particular, increasing the air velocity will lead to a decline in the difference between the inlet and outlet temperature of EAHE, however, the final cool/heat output of EAHE will be improved due to the increase in air volume (Wu et al., 2007; Yang et al., 2016). Furthermore, through the numerical simulation results of Mathur et al. (2015b,

2016), it is not hard to argue that the most reasonable strategy to operate an EAHE is the intermittent operation assisted by limited night ventilation. The above studies indicate that the relationships between parameters and the thermal performance of EAHE have been fully clarified, and the results are sufficient to guide a practical project. However, significant temperature changes are inevitable during the charge/discharge processes of sensible heat storage mediums, which will continuously deteriorate the thermal performance of a GCHE system. In GSHP systems, this process can be alleviated through employing phase change materials as backfill materials (Chen et al., 2018; Qi et al., 2016). Similarly, Liu et al. (2019) and Zhou et al. (2018) have tried to improve the traditional EAHE by filling PCMs in the annular space between the air pipes and surrounding soil, and achieved significant performance improvements. However, from the engineering perspective, this improved scheme is unfriendly to existing traditional EAHE systems. In order to make full use of these systems in the process of building energyefficient renovation, new improvement schemes must consider both thermal performance and construction difficulty. Therefore, some changes are made in the combination scheme of PCM and EAHE in this study, specifically, the PCM is filled in a cylindrical container. Thus, existing traditional EAHE systems can be easily reconstructed to novel ones by placing this PCM container in the center of their air pipes. After the reconstruction process, these new systems will have two heat storage units, the soil and the PCM, and the latter has a relatively stable temperature during the heat transfer process. In addition, the effective heat transfer area of the new system will also be increased. So it can be predicted that this new system will achieve better performance. Here, this new system is defined as the cylindrical PCM-assisted EAHE (CPCM-EAHE). In the following study, an equivalent heat capacity method based 3-D numerical model will be built on ANSYS FLUENT 16.0. Its prediction results will be validated through the test data from an indoor experimental set-up. Then, the CPCM-EAHE and a traditional EAHE will be comparatively studied through this model under the continuous high-temperature weather in Chongqing (China) summer to evaluate its actual cooling performance. 2. Numerical model As presented in the previous section, the CPCM-EAHE is an improved system of the shallowly buried traditional EAHE, and it is achieved by placing a cylindrical PCM container in the center of the air pipe in the existing system. Its schematic diagram and key dimensions are shown in Fig. 1. This study assumes that this CPCMEAHE is located in Chongqing (China) and its ground surface is covered by plants. In addition, the following assumptions for this CPCM-EAHE system are also made before conducting practical research: (a). The air cannot be compressed and its thermophysical features are independent of temperature; (b). The soil is a homogeneous and isotropic solid with constant thermophysical features; (c). The PCM is a homogeneous and isotropic solid that has a constant density and specific heat over the discussed temperature range; (d). The air pipe is made of impervious material, and the condensation and evaporation processes on the ground surface and the pipe wall are all neglected; (e). The influence of contact thermal resistance between adjacent materials, the wall thickness of the air pipe and PCM container, and the gravity are not in consideration.

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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specific heats corresponding to the states before and after melting, respectively; l1 and l2 are the thermal conductivity corresponding to the states before and after melting, respectively; L is the latent heat of fusion; T1 and T2 represent the extrapolated onset temperature and extrapolated end temperature of the phase transformation process, respectively. 2.2. Material properties The thermophysical properties of the air and soil employed in this study are presented in Table 1. 2.3. Initial and boundary conditions 2.3.1. Initial condition Since the CPCM-EAHE is a shallowly buried system, knowledge of the undisturbed temperature field of sub-soil is necessary. However, few institutions can offer such information, and on-site measurements are quite difficult. Thus, Barakat et al. (2016) and Ozgener et al. (2013) proposed a theoretical model, described as Equ. (6), to evaluate the temperature field of sub-soil.

Fig. 1. Schematic diagram and key dimensions of the CPCM-EAHE.

2.1. Governing equations The 3-D numerical model used to simulate the heat transfer process of CPCM-EAHE is built on ANSYS Fluent 16.0. The equations are as follows: Continuity equation:

vui ¼0 vxi

(1)

Momentum equation:

vui vu 1 vp m v2 ui þ uj i ¼  þ vt vxj r vxi r vxj vxj

(2)

In addition, the turbulent flow in the air pipe is calculated by the RNG k-ε model. Energy equation:

rC

vT vT þ uj vt vxj

! ¼

v vxj

l

vT vxj

! (3)

In the above equations, r, C, l, and m mean the density, specific heat, thermal conductivity and dynamic viscosity of materials, respectively. Generally, both the enthalpy method (Esen et al., 1998) and the equivalent heat capacity method (Bonacina et al., 1973) can be used to numerically simulate the phase change process. In this study, the latter will be employed to assist the above equations to simulate the heat transfer process of PCM. The equivalent heat capacity of PCM is described as (Niyas et al., 2017):

8 > > > <

lpcm ¼

> > > :

l1 þ

rffiffiffiffiffiffiffiffiffi y

p

as Ty

l1 l2  l1 T2  T1

l2

T1  T  T2

(4)

T > T2 T < T1

,ðT  T1 Þ

T1  T  T2

(5)

T > T2

In the above equations, Ceq and lpcm are the equivalent specific heat and thermal conductivity, respectively; C1 and C2 are the

y !

p

as Ty

þ uy t  fy0

 cos (6)

In this study, the Ts , Ags , fy0 , and as are defined in Table 2. Besides, a temperature of 24.51
C is assigned to the air and PCM regions, which is the average soil temperature within the depth of 1.35 me1.65 m of the undisturbed sub-soil. 2.3.2. Boundary conditions Before the detailed description, an overview of all boundaries is displayed in Table 3. (a) Air-inlet The meteorological data from July 1st to August 19th in the TMY of Chongqing are employed as the temperature condition at Airinlet, as shown in Fig. 2. In addition, the average velocity of airflow on this boundary is 2.67 m/s. (b) Ground surface Considering previous assumptions, the heat transfer process on the ground surface is simplified as follows (Khatry et al., 1978):

 ls

vT vy

 y¼0

  ¼ hgs Tsolair  Tgs

(7)

εs DR hgs

(8)

T < T1

C1

C1 þ C2 Ceq ¼ þ f ðTÞ,L > 2 > > : C2 8 > > > <

rffiffiffiffiffiffiffiffiffi ! Tðy; tÞ ¼ Ts þ Ags exp

Tsolair ¼ Tair þ

a0 I hgs



Here, y means the depth; hgs means the composite heat transfer coefficient of convection and radiation at the ground surface; Tsolair is the sol-air temperature; a0 and εs are the solar-radiation absorptivity and long-wave emissivity of the ground surface, respectively; I represents the solar radiation intensity; DR means the difference between the long-wave radiation emitted by the ground surface and its received long-wave radiation from the sky and surroundings. For a horizontal surface, it is approximately 63 W=m2 (ASHRAE, 2009). Combining with previous assumptions, the ground surface in

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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Table 1 The thermophysical properties. Parameter

Value Density (kg=m3 ) Specific heat (J=ðkg ,
CÞ) Thermal conductivity (W=ðm ,
CÞ) Dynamic viscosity (kg=ðm ,sÞ) Density (kg=m3 ) Specific heat (J=ðkg ,
CÞ) Thermal conductivity (W=ðm ,
CÞ)

Air

Soil (Zhou et al., 2018)

Thermal diffusivity (m2 =s)

2.4. Solution method

Table 2 The values of Tms , As , fy0 , and as . Parameter Value

Ts (
C) 19.66

Ags (
C) 15.95

fy0 (rad) 3.58

as (m2/s) 7.1  10

7

Table 3 An overview of all boundaries. Boundary

Type

Air-inlet Ground surface Bottom Air-outlet Air-pipe wall PCM-tube wall Front, Back, and Sides

Velocity-inlet Wall (Convection) Wall (Constant temperature) Outflow Wall (Coupled and Non-slip) Wall (Coupled and Non-slip) Wall (Adiabatic)

this study can be treated as a dry-shade surface. Thus, it can be defined that a0 ¼ 0.6, εs ¼ 1, hgs ¼ 14 W=ðm2 ,KÞ and I ¼ Idiffuse (Sodha et al., 1991). Here, Idiffuse means diffuse radiation that the sun incident on a surface. Based on these parameters, the hourly sol-air temperature for the TMY of Chongqing from July 1st to August 19th has been calculated and plotted in Fig. 2. (c) Bottom The analysis of the undisturbed soil temperature field in Chongqing indicates that the soil temperature can basically keep constant when the depth is greater than 10 m. Therefore, the temperature at this boundary is defined as:

Tðbottom; tÞ ¼ Ts ¼ 19:66
C

1.165 1005 0.0267 1.90  105 1800 860 1.1 7.1  107

(9)

By comparing the simulation results of the CPCM-EAHE system using different meshes, this study finally chose the structured mesh with sizes of 4 million to conduct the numerical calculation works, as shown in Fig. 3. Because the convective boundary layer near pipe walls can be fully resolved through this mesh under the Enhanced Wall Treatment, meanwhile the computational efficiency is still acceptable. Besides, in the transient pressure-based solver of ANSYS Fluent 16.0, the solution scheme of SIMPLEC is employed to perform the actual numerical calculation process of this 3-D model for CPCMEAHE. The scheme of second-order upwind is employed to spatially discrete the k-ε equations and the scheme of second-order implicit is employed to discrete the transient formulation. A convergence criterion of 108 is assigned for the energy equation, while default ones for others. After verifying the time-step independence of this numerical model, a time-step size of 60 s is finally employed to perform the following numerical calculations. 3. Model validation 3.1. Experimental system To check the reliability of the 3-D numerical model of CPCMEAHE, an indoor experimental system was built as Fig. 4 shows. Here, the CPCM-EAHE test rig was built up with three concentric 2m-long pipes. They are the stainless steel pipe 1, R1 ¼ 25.4 mm & d1 ¼ 0.9 mm; stainless steel pipe 2, R2 ¼ 50.8 mm & d2 ¼ 1.1 mm; UPVC pipe 3, R3 ¼ 158.4 mm & d3 ¼ 3.9 mm (R is the outer radius, d is the wall thickness). The enclosed spaces between pipes from inside to outside are used as the PCM container, air channel and wet sand container, respectively. The whole test rig was wrapped in 60 mm thick insulation cotton. T-type thermocouples were fixed at the inlet and outlet centers of the CPCM-EAHE test rig to record real-time air temperature. Besides, T-type thermocouples were arranged on sections I, II and III as well. Before the test, all thermocouples were calibrated in the

Fig. 2. Hourly outdoor dry-bulb and sol-air temperatures of Chongqing TMY during Jul. 1st to Aug. 19th.

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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Fig. 3. Diagram of the 4 million structured mesh.

Fig. 4. Photograph of the indoor experiment system and diagram of material locations and temperature measuring point layout.

air by a mercury thermometer whose accuracy is ±0.1
C at 0e50
C. A Testo 480 was employed to monitor the air velocity at the outlet of the CPCM-EAHE test rig, so as to provide a reference for controlling air volume constant during the experiment process. Moreover, under the Log-Tchebycheff rule, the actual air volume during the experimental process was evaluated by this anemometer as well. The PCM employed in this experiment is a laboratory-prepared SSPCM, which is a composite material of capric acid (CA)-palmitic

acid (PA) eutectic mixture/expanded graphite (EG). It was prepared from the CA-PA (97 wt%-3 wt%) binary eutectic mixture and EG in a mass ratio of 10:1 through the vacuum absorption method. The measured thermophysical parameters of this SSPCM and wet sand are all listed in Table 4. 3.2. Validation result In this experiment, low-temperature outdoor air was inhaled

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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T. Zhou et al. / Journal of Cleaner Production xxx (xxxx) xxx Table 4 Parameters of the laboratory-prepared SSPCM and wet sand. Parameter

Value

SSPCM Density (kg=m3 ) Thermal conductivity (W= ðm ,
CÞ) Heat capacity (J= ðkg ,
CÞ) Latent heat of fusion (kJ= kg) Extrapolated onset temperature (
C) Peak temperature (
C) Extrapolated end temperature (
C) Wet sand Density (kg=m3 ) Thermal conductivity (W= ðm ,
CÞ) Heat capacity (J= ðkg ,
CÞ)

904.5 5.43/3.26 1750 135.70 28.16 32.06 32.68 Fig. 6. Equivalent heat capacity and thermal conductivity curves of SSPCM. 1452 0.76 1150

into the experimental system by the fan under constant air volume, and its temperature was controlled to follow a preset temperature curve by changing heater power. Then, the preheated air was pumped into the CPCM-EAHE test rig. The experimental system had continuously operated for approximately 21 h under a mean air velocity of 1.22 m/s. And temperatures of the measuring points, especially on the inlet and outlet of the CPCM-EAHE test rig, were recorded every 10 s. The simulated and real-time record air temperatures at the center of the outlet are all plotted in Fig. 5. The figure tells that, at the center of the outlet in CPCM-EAHE, the measured and simulated air temperatures have similar variation trends, and the difference between them is not significant. And it is not hard to find that this difference tends to decrease as the experiment progresses. Considering the difference in the initial temperature field and the influence of thermal contact resistance, it is reasonable to suggest that the calculation result of this 3-D numerical model of the CPCM-EAHE is reliable enough to support the following research work. 4. Results and discussion According to Equ. (4) and (5), the equivalent heat capacity and thermal conductivity curves of the laboratory-prepared SSPCM are calculated and shown in Fig. 6. To understand the cooling performance of the CPCM-EAHE system more intuitively, a traditional EAHE is introduced to conduct a comparative study. The structures of air channels in both systems are shown in Fig. 7. In addition, the air velocity of the traditional EAHE is defined as 2.0 m/s to ensure it

has the same air volume as the CPCM-EAHE. Then, these EAHEs have been simulated for continuous operation of 50 days under the same conditions. 4.1. Outlet temperature and cooling capacity In order to fully understand the cooling performance of this CPCM-EAHE system, the simulation result has also been compared with the PCM assisted EAHE proposed by Zhou et al. (2018) which has an air channel structure as shown in Fig. 7. Fig. 8 shows the hourly average values of outlet temperature and cooling capacity in three different EAHEs during days 19e40. It is seen, these EAHEs have experienced the examination of continuous high-temperature weather in these days. Fig. 8 (a) shows that the outlet temperature of CPCM-EAHE is obviously lower in most time of the daytime during days 20e39, compared to traditional EAHE and the EAHE proposed by Zhou et al. (2018). In other words, the heat transferred from the airflow to heat storage mediums in CPCM-EAHE is greater, as seen in Fig. 8(b). These figures also demonstrate that the outlet temperature of CPCM-EAHE is higher than that of the other two EAHEs during the heat-release period. Thus, the heat absorbed from the air and deposited in the heat storage mediums during the daytime can be released faster and more at night, which finally helps the CPCM-EAHE to keep efficient operation the following day. Through analyzing the hourly outlet temperature and cooling capacity of these EAHEs, their cooling performance will be further discussed. As seen in Fig. 9(a), the changing trends of daily average outlet temperature for all EAHEs are consistent, and the values are similar as well. However, Fig. 9(b) shows that the daily amplitudes of outlet temperature of CPCM-EAHE are significantly lower than

Fig. 5. Simulated and record air temperatures at the center of the outlet.

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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Fig. 7. Diagram of air channels in different EAHEs.

Fig. 8. Hourly outlet temperature and cooling capacity for EAHEs. (a) Outlet temperature; (b) Cooling capacity.

those of traditional EAHE. Even compared with the EAHE system proposed by Zhou et al. (2018), the CPCM-EAHE still has better performance in damping temperature fluctuation. As seen, the daily amplitude of outlet temperature of CPCM-EAHE is attenuated to less than 1
C except for the 20th day. Fig. 10 (a) shows, compared to traditional EAHE, the daily total cooling capacity of CPCM-EAHE is improved by 8.36%e32.12% in studied days excluding day 33. By contrast, the same index of the EAHE proposed by Zhou et al. (2018) is improved by 11.92%e 20.40%. Finally, throughout the whole studied period, these improvements result in increasing the total cooling output by 20.05% for CPCM-EAHE, while only by 17.68% for the EAHE proposed by Zhou et al. (2018). Furthermore, the daily total heat releases shown in Fig. 10(b) demonstrate that CPCM-EAHE releases more heat through the low-temperature air at night than the other two EAHEs, which is beneficial for CPCM-EAHE to restore its cooling potential and maintain its high efficient operation state. Fig. 11(a) shows, under daily outdoor peak temperatures, temperature drops of CPCM-EAHE are significantly greater than those of the other two EAHEs. Especially under the outdoor peak temperature of day 39, when the temperature drop of traditional EAHE is 5.86
C, the same index of the EAHE proposed by Zhou et al. (2018) is 6.79
C, just an improvement of 0.93
C. However, the corresponding value of CPCM-EAHE is as high as 8.10
C, an improvement of 2.24
C. Even for the whole 20 days, the CPCMEAHE still has an average improvement of 1.67
C. Moreover,

Fig. 11(b) indicates that the cooling capacity of CPCM-EAHE under outdoor peak temperature is greater than 1 kW in most days, while the same index of traditional EAHE is approximately 0.8 kW. By contrast, the corresponding values of the EAHE proposed by Zhou et al. (2018) are mainly between 0.9 kW and 1.0 kW. Overall, during the studied 20 days, the cooling capacity of CPCM-EAHE under daily outdoor peak temperature is increased by 28.55%e39.74% compared to traditional EAHE.

4.2. Heat transfer characteristics of CPCM-EAHE In this subsection, the heat transfer process of the CPCM-EAHE and traditional EAHE under continuous operation will be analyzed and compared. It should be known that a positive value of heat indicates that the wall absorbs heat from the airflow, a negative value of heat indicates that the wall releases heat to the airflow. Looking back at Fig. 7, in traditional EAHE, the heat transfer between air and heat storage medium can only perform on Wall-S, but in CPCM-EAHE, this thermal process can perform both on Wall-S and Wall-P. Here, the hourly transferred heat on these walls has been counted, as shown in Fig. 12, to further discuss the heat transfer happening on the walls of CPCM-EAHE. It is seen in Fig. 12, the difference between the transferred heat on Wall-S of two EAHEs is not significant. However, in CPCM-EAHE, significant heat transfer also happens on Wall-P. It is this part of the heat that makes CPCM-EAHE have better cooling performance than

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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Fig. 9. Daily average values and amplitudes of the outlet temperatures in EAHEs. (a) Daily average temperatures; (b) Daily amplitudes.

traditional EAHE. Additionally, these heat transfer curves also indicate that, for the soil in each EAHE, there is a serious imbalance between its heat absorption and heat release in one day, but for the PCM in CPCM-EAHE, there is no such serious imbalance. In order to quantify this imbalance of a heat storage unit, a new parameter named balance-index is introduced and defined as:

Balance index ¼

Qabsorbed þ Qreleased minfjQabsorbed j; jQreleased jg , jQabsorbed þ Qreleased j maxfjQabsorbed j; jQreleased jg (10)

where, for a heat storage unit, Qabsorbed and Qreleased refer to the heat absorbed from and released to air in one day, respectively. This balance-index ranges from 1 to 1, where values close to 0 means serious imbalance. And positive values mean the absorbed heat is predominant, while negative values mean the released heat is predominant. It is seen in Fig. 13, for soils in both EAHEs, their balance-indexes are greatly close or even equal to 0 in most days. However, for PCM in CPCM-EAHE, its balance-index is between 1.0 to 0.5 or 0.5 to 1.0 in all days except days 20 and 33. Thus, assisted by the PCM, the balance-index of the whole CPCM-EAHE system has been significantly increased compared to traditional EAHE. This can be confirmed by comparing the balance-indexes of CPCM-EAHE and Traditional EAHE in Fig. 13. In a word, CPCM-EAHE can not only achieve greater cooling output at daytime but also release more deposited heat at night, so as to maintain its cooling potential at a high level as much as possible.

Fig. 10. Daily total cooling capacities and Daily total heat releases of EAHEs. (a) Daily total cooling capacities; (b) Daily total heat releases.

4.3. Temperature and heat flux distributions inside CPCM-EAHE Before conducting the following discussion, the period from the moment of starting heating to that of finishing cooling is defined as one operation cycle of the continuous running EAHE, during which the system sequentially experiences one heating period and one cooling period. Combined with Fig. 8, the period between the 19th to 40th day of CPCM-EAHE can be divided into 22 sequential operation cycles. Finally, the 39th operation cycle, as shown in Fig. 14, is chosen as a representative for the following discussion. As seen in Fig. 14, compared to traditional EAHE, both the daytime cooling effect and nighttime heating effect of the CPCM-EAHE are enhanced with the assistance of PCM. Moreover, its transition from heating mode to cooling mode is delayed, but its transition from cooling mode to heating mode is advanced. Besides, in each operation cycle, the curve for the total transferred heat of CPCMEAHE always contains mode transition points (Point 1, 3, 5), and peaks for heating (Point 2) and cooling (Point 4). Similar features can be found on heat transfer curves of Wall-S in traditional EAHE, and Wall-S and Wall-P in CPCM-EAHE. Next, the temperature and heat flux distributions of CPCMEAHE and traditional EAHE under the heating and cooling peaks (Points 2, 4), as well as the mode transition points (Point 3,5) will be discussed. In this paper, the heat flux on the wall is defined as the transferred heat per unit length of pipe wall in a unit time. Please note that the temperature and heat flux used in this discussion are instantaneous values at the end of each hour, which may have some slight difference from the mean ones. And the values finally plotted on the figures are area/volume-weighted average ones on annular walls/air columns of unit length. As seen in Fig. 15(a), both the curves for temperature

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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9

Fig. 11. Temperature drops and cooling capacities of EAHEs under outdoor peak temperatures. (a) Temperature drops; (b) Cooling capacities.

Fig. 12. Hourly transferred heat on the walls of CPCM-EAHE and traditional EAHE.

distribution in Z-direction of Wall-S and Wall-P in CPCM-EAHE are higher than that of Wall-S in traditional EAHE. Under the same inlet temperature, this makes the air temperature in Z-direction of CPCM-EAHE always higher than that of traditional EAHE at the same position. Besides, unlike the position relationship between

the curves of Wall-S and air in traditional EAHE, these curves in CPCM-EAHE have an intersection. This means, when air flows through CPCM-EAHE, Wall-S does not always heat it. However, the temperature of Wall-P still keeps higher than that of air at this moment. Thus, in the latter half of the pipe, when Wall-S begins to

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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Fig. 13. Balance-indexes of soil and PCM in EAHEs.

Fig. 14. Hourly heat transfers on walls during the 39th operation cycle of CPCM-EAHE.

Fig. 15. Temperature and wall heat flux distributions in Z-direction at the heating peak of t ¼ 918 h.

absorb heat from the air, Wall-P can greatly weaken its adverse effect on heating air. This change can be observed in Fig. 15(b), where curves for the heat flux distributions of Wall-S and Wall-P in CPCM-EAHE appear rising tendency along the Z-axis. But, the former curve shows that the heat flux of Wall-S changes from negative to positive at approximately Z ¼ 12.5 m. Which makes the curve for the total heat flux distribution of walls in CPCM-EAHE higher than that curve of Wall-P after this point. Namely, through Wall-P, the heat released by PCM is employed to heat the air as well as offset the heat loss caused by soil absorbing heat from the air. When the heat released by PCM cannot completely offset this heat loss, the final effect of CPCM-EAHE on air is to decrease its temperature. This well explains the falling at the end of the air temperature curve in CPCM-EAHE in Fig. 15(a). Fig. 16(a) shows that, under the same inlet temperature, the air temperature in CPCM-EAHE is always lower than that in traditional

Fig. 16. Temperature and wall heat flux distributions in Z-direction at the cooling peak of t ¼ 927 h.

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EAHE at the same position along the Z-axis. As Z-value grows, the difference in air temperatures at the same position of two EAHEs increases sharply first, then keeps smoothly, and finally decreases slightly. Through checking the temperature distributions on the walls at this moment, it is found that within approximately Z ¼ 0e9 m, the wall temperature of Wall-S in CPCM-EAHE is still higher than that in traditional EAHE. However, in CPCM-EAHE there is another Wall-P for exchanging heat with air. And at almost all positions on the Z-axis, the temperature of this Wall-P is lower than that of Wall-S in traditional EAHE. It is the joint effect of these two low-temperature walls that makes the outlet temperature of CPCM-EAHE be 2.24
C less than that of traditional EAHE. As seen in Fig. 16(b), the heat exchanges between air and walls are greatly different within Z ¼ 0e10 m in CPCM-EAHE and traditional EAHE. Under the joint effect of Wall-S and Wall-P, CPCMEAHE absorbs nearly twice the amount of heat that traditional EAHE absorbs from the air. Thus, when air flows through this area, the air temperature difference at the same position in two EAHEs increases sharply. But, at approximately Z ¼ 19 m, the heat that walls absorb from the air in CPCM-EAHE begins to lower than that in traditional EAHE. This makes the curve for air temperature difference in two EAHEs fall down in this area as seen in Fig. 16(a). Fig. 17(a) shows that, at the moments of 921 h and 940 h, under the joint effect of Wall-S and Wall-P, the air temperature curves in Z-direction rise first and then decrease as Z-value grows. And finally, CPCM-EAHE shows a non-cooling effect on air. In Fig. 17(b), at the above moments, almost all positions in Z-direction of Wall-S have positive heat flux values, in other words, the Wall-S is in the state of cooling air. Conversely, the Wall-P is in the state of heating air at these moments. Finally, with the assistance of Wall-P, the cooling effect that the soil imposed on air through Wall-S is completely offset, which makes CPCM-EAHE at non-cooling air states. Combined with Fig. 14, it is reasonable to argue that, in the CPCM-EAHE system, the application of PCM delays its transition from heating mode to cooling mode but advances its transition from cooling mode to heating mode.

4.4. Liquid fraction of PCM Since PCM plays an extremely important role in the heat transfer process of CPCM-EAHE, it is necessary to discuss the variation of the liquid fraction of PCM during the continuous operation process of this PCM-assisted system. Here, the liquid fraction of PCM is defined as:

11

Fig. 18. Hourly liquid fraction of the PCM unit in CPCM-EAHE.

fl ¼

ðT 

Ceff  C



, L T1  T  T2

(11)

T1

Based on this definition, the hourly liquid fraction of the PCM unit in CPCM-EAHE during days 19e40 is calculated and shown in Fig. 18. It is seen, the heat absorption process of PCM usually lasts from late morning to midnight in each day. And the absorbed heat will be discharged through low-temperature airflow in the early morning of the next day. Therefore, the PCM heat storage unit in CPCM-EAHE usually reaches its peak liquid fraction near midnight of each day. Additionally, the PCM unit reaches the maximum liquid fraction (fl ¼ 0:34) of the whole 20 days at 1:00 a.m. (t ¼ 937 h) on the 40th day after experiencing 20 days of continuous hightemperature weather. Fig. 19(a) and (b) are the contours for the liquid fraction distributions of PCM on the vertical and horizontal symmetry planes of the PCM region at t ¼ 937 h, respectively. As seen, the liquid fraction of PCM appears a decreasing tendency with Z-value grows. In particular, within approximately the first 1 m of the PCM unit, the liquid fraction of PCM rapidly decreases from 0.95 to 0.65. Then this decreasing tendency slows down and drops to 0.5 at about Z ¼ 5e6 m. In the remaining part of the PCM unit, the liquid fraction slowly drops to 0.1. When contours for the liquid fractions of PCM on the cross-sections of Z ¼ 0, 5, 10, 15, 20 and 25 m are considered, as shown in Fig. 19(c) ~ (h), it is seen that the liquid fractions of PCM are higher in the middle but lower in the outside on these vertical sections, however, these differences are not significant. Further, from these liquid fraction contours of PCM, it can be inferred that the PCM located in the front of the container has an extremely high utilization rate, but the farther from the inlet, the lower the utilization rate of PCM is. Thus, it is reasonable to believe that the amount of PCM can be properly reduced under the given operating condition of v ¼ 2.67 m/s to improve its overall utilization rate. In other words, the container with equal cross-sections employed in this study cannot fully develop the potential of the PCM unit, and further studies are still necessary to optimize the structure of the PCM container to increase the utilization rate of the PCM unit in CPCM-EAHE. 5. Conclusions

Fig. 17. Temperature and wall heat flux distributions in Z-direction at mode transition points, heating mode to cooling mode (t ¼ 921 h) and cooling mode to heating mode (t ¼ 940 h), of CPCM-EAHE.

This study proposes the CPCM-EAHE which is a novel passive system reconstructed from the traditional EAHE. And an equivalent heat capacity method based 3-D numerical model is developed in ANSYS FLUENT 16.0 to calculate the outlet temperature and cooling capacity of this novel system. Its calculation result is verified through an indoor experimental system. Then, through this numerical model, a comparative study has been conducted between

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

Fig. 19. Liquid fraction distributions of PCM on the cross-sections of CPCM-EAHE at t ¼ 937 h (a) Section X ¼ 0 m; (b) Section Y ¼ 1.5 m; (c) Section Z ¼ 0 m; (d) Section Z ¼ 5 m; (e) Section Z ¼ 10 m; (f) Section Z ¼ 15 m; (g) Section Z ¼ 20 m; (h) Section Z ¼ 25 m.

Please cite this article as: Zhou, T et al., Numerical study on the cooling performance of a novel passive system: Cylindrical phase change material-assisted earth-air heat exchanger, Journal of Cleaner Production, https://doi.org/10.1016/j.jclepro.2019.118907

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CPCM-EAHE and traditional EAHE under the meteorological conditions of Chongqing summer. The conclusions are: 1. The cooling performance of CPCM-EAHE in summer daytime is significantly better than that of traditional EAHE. Under the daily outdoor peak temperature, its temperature drop is improved by 2.24
C on maximum and by 1.67
C on average, and its cooling capacity is increased by 28.55%e39.74%, compared with traditional EAHE. Even for the whole 20 days, its total cooling output is still increased by 20.05%. And the temperature fluctuation of the fresh air introduced by this novel system can be damped to about 1
C. 2. The main contributor to the performance improvement of CPCM-EAHE is the PCM heat storage unit. And assisted by PCM, the CPCM-EAHE not only has a great improvement in cooling capacity but also achieves a significant improvement in the balance between daily absorbed and released heat, compared to traditional EAHE. That is to say, its heat release in the night is improved as well, which is conducive to keeping its high efficient operation state in the following day. 3. The contributions of PCM to the performance improvement of CPCM-EAHE mainly focuses on strengthening its daytime cooling effect and nighttime heating effect, delaying its transition from heating mode to cooling mode, but advancing its transition from cooling mode to heating mode. 4. The PCM located in the front of the container has an extremely high utilization rate, but the farther from the inlet, the lower the utilization rate of PCM is. Therefore, it is necessary to further optimize the structure of PCM container and the system operational strategy (both air volume and operating mode) to improve the utilization rate of PCM in this CPCM-EAHE. Acknowledgments This work was supported by the National Natural Science Foundation of China [grant number 51678088]; and the Chongqing (China) Science & Technology Commission [grant number cstc2018jcyjAX0072]. Nomenclature Ags as C Ceq fl hgs I L p Q

DR T Ts Ty y

amplitude of the annual periodic fluctuated temperature at the ground surface,
C soil thermal diffusivity, m2 =s specific heat, J=ðkg ,
CÞ equivalent specific heat of PCM, J=ðkg ,
CÞ liquid fraction of PCM composite heat transfer coefficient of convection and radiation at the ground surface, W=ðm2 ,
CÞ solar radiation intensity, W=m2 latent heat during the phase transformation process of PCM, J=kg pressure, Pa heat, J difference between the long-wave radiation emitted by the ground surface and its received long-wave radiation from the sky and surroundings, W=m2 temperature,
C annual average temperature at ground surface,
C year period, s depth in sub-soil, m

Greek letters a0 solar-radiation absorptivity of the ground surface

εs

uy

long-wave emissivity of the ground surface thermal conductivity, W=ðm ,
CÞ dynamic viscosity, kg=ðm ,sÞ density, kg=m3 phase constant of the ground surface, rad annual fluctuating frequency, s1

Subscripts air gs pcm s 1 2

air ground surface phase change material soil before melting after melting

l m r fy0

13

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