Thermal analysis of horizontal earth-air heat exchangers in a subtropical climate: An experimental study

The earth-air heat exchanger (EHX) has a promising potential to passively save the energy consumption of traditional air conditioning systems while maintaining a high degree of indoor comfort. The use of EHX systems for air conditioning in commercial and industrial settings offers several environmental benefits and is capable of operating in both standalone and hybrid modes. This study tests the performance and effectiveness of an EHX design in a sandy soil area in Baghdad, Iraq. The area has a climate of the subtropical semi-humid type. Ambient air temperatures and soil temperatures were recorded throughout the months of 2021. During the months of January and June, the temperatures of the inlet and outflow air at varying air velocities were monitored concurrently in 10-min increments at each location. Further numerical and thermodynamical analyses of the measurements were conducted to reveal the influencing performance parameters. The highest temperature rises of air between the input and exit sections were determined as 12.3°C (January) and 17.2°C (June). It is found that the maximum values of effectiveness are 0.80 and 0.81, while coefficients of performance are 1.6 and 1.8 for January and June, respectively. It is also found that the EHX shows good functionality and effectiveness, with potential energy savings for equipment for cooling and heating under different weather conditions.

Introduction

Nowadays, the strength of a country is essentially determined by the extent to which it can get a diverse range of energy sources. Otherwise, the large demand for cooling in developing countries where there is no reliable electricity supply and the electricity consumption on the air conditioning systems increased tremendously. As a source of energy, the earth’s tremendous energy storage may be used as a heat sink or source during the summer and winter seasons. Due to the high thermal storage, a time shift between the earth’s surface and the soil temperature with the depth can be observed. Moreover, geothermal energy comes as a promising source due to its high availability and more even distribution in nature. Therefore, at a certain depth, the soil temperature is higher than the ambient temperature in the winter and lower in the summer (Pfafferott and Santamouris, 2007). It is noted that soil can be employed as an energy supply by applying earth heat exchangers (EHX) for purposes of ventilation and air-conditioning. EHX is a kind of thermal heat exchanger that can operate with thermal source energy such as geothermal energy. A basic EHX consists of pipes, which are networks buried at a certain depth under the earth’s surface (Bulut et al., 2015a).

Moreover, EHX is one of the solutions to the problem of energy crisis in comparison with the conventional vapor-compression system. This machine has a low manufacturing cost, long operating life, and has no moving parts, which makes it silent with cheap maintenance. In my country, Iraq, about two-thirds of the electricity may be consumed for air-conditioning in buildings, which results in a shortage of electricity, therefore, the development of an air conditioning technology with the concept of energy saving and environmental protection is considered to be very important. Several theoretical and technological breakthroughs have been made in the field of EHXs to better explore their employment in heating and cooling applications (Kasim et al., 2013; Atwan et al., 2016; Alhosainy and Aljubury, 2019). The EHXs also receive other benefits when they are coupled with ground-coupled heat pumps, such as protection against dust accumulation, Sun insolation, windstorms, and air infiltration (Esen et al., 2006; Esen et al., 2007a; Esen et al., 2007b; Esen et al., 2007c; Esen et al., 2008a; Esen et al., 2008b; Balbay and Esen, 2013; Esen and Yuksel, 2013; Esen et al., 2017). The functionality of EHX systems was assessed experimentally in different areas and climates around the globe. Benrachi et al. (Benrachi et al., 2017) studied the relevance of EHX application in some Algerian arid areas, and reported that the soil temperature ranged between 26°C at depth of 6 m and 22.2°C at depth of 5 m for cities of ADRAR and El-OUED, respectively. Mongkon et al. (Mongkon et al., 2014) tested a horizontal EHX system in the tropical climate of Thailand to explore the impact of the tube diameter and air velocity on maximizing the coefficient of performance. They found that the coefficient of performance is better with increasing pipe diameter and air velocity, and this should be considered a routine case for cooling/heating cycles when heat exchange occurs at a higher rate. More studies (Muehleisen, 2012; Bisoniya et al., 2014; Bulut et al., 2015b; Bisoniya, 2015; Chiesa, 2017) investigated through field tests and numerical simulation the effect of different designs of EXH to increase thermal performance, effectiveness, and efficiency. For instance, Chiesa (2017) assessed the use of precise tools and methods for calculating the performance of the required EXH. Furthermore, he suggested a methodology that gives a primary idea of the climate-related best EXH performance. Liu et al. (2017) studied the thermal performance of EXH in hot summer and cold winter where they measured different parameters such as soil temperature, inlet and outlet air temperatures. They demonstrated that at a distance of 20 m, the inlet air temperature approaches the soil temperature, and the COP and Q decreased with increasing inlet air temperature but increased with increasing pipe length. Literature also reported data for the EXH at different design configurations (Jakhar et al., 2015; Jakhar et al., 2016; Hassoon and Ali, 2021). Rouag et al. (2018) studied the thermal design of an EXH using a new developed semi-analytical method with soil temperature distribution in hot and dry climates. They proved that the thermal performance of their system tends to decrease along with the duration of continuous operation. Also, they linked the thermal performance to several parameters, such as the diameter of the pipe, the soil’s ability to move heat, and the diameter of EXH pipes. Mironov et al. (2016) proposed modeling of a horizontal ground heat exchanger that describes the rate of heat transfer between the designed heat exchanger and the soil. Using this model, the results reveal that heat transport rates rapidly decrease when the spacing between pipelines is less than 2 m. Moreover, the developed model can be employed to design a horizontal heat exchanger with optimum operating capability. An 18,000 BTU/hr tube in tube air heat exchanger had been designed and tested under local climate conditions and soil properties of Iran. The results showed that a significant saving in energy consumption and COP of 2.57 was obtained. Knowing soil temperature at a different depth, the ambient air temperature, and solar radiation are important parameters for the design of air heat exchangers (Yari et al., 2005). Singh and Sharma (2017a) presented a numerical analysis of soil temperature. The diurnal temperature variation for the hottest and coldest days and the annual variation for the year 2016 have been computed. Diurnal temperature variation is found up to 0.4 m depth of soil, whereas annual temperature variation is up to a depth of 4 m. An integrated system comprising a solar chimney with an EHX was presented by Maerefat and Haghighi (2010). Theoretical analyses of this study had been introduced for investigating the performance of the proposed system. Without any electricity needed, the results shows that this system has the potential to provide comfortable air conditioning for a long period during the daytime.

An air-to-earth heat exchanger under a transient condition in cooling mode was described and tested at the University of Biskra, Algeria by Belloufi et al. (Belloufi et al., 2017) The performance of the system was analyzed for continuous operation of 71 h based on a prototype of a PVC pipe of 53.16 m long and 110 mm diameter buried at 3 m depth. The experimental results proved that there is no significant effect on the outlet air temperature and thus on the EHE performances during all the continued operations. Besides this finding, a good agreement between their developed model along with the measured temperature has been achieved. The studies on the EHXs are focused on improving thermal performance, which can technically and economically be a serious alternative to conventional systems. Benhammoua and Draouib (2015) applied a transient one-dimensional numerical model for studying the thermal performance of an EHX for summer cooling under the Algerian Sahara conditions. The model is designed for a buried PVC pipe in the ground. The influence of geometrical and dynamical parameters on the thermal performance of EHX is discussed for all experiment phases. The model results showed that the drop in outlet air temperature is higher than the inlet air temperature and is smaller for the longer pipe, and the thermal performance in transient conditions is approached the steady state condition during a short time. The simulation work has been extensively presented in the literature. Most of these studies were aiming at evaluating the performance and studying its dependency on other system parameters (Hollmuller and Lachal, 2001; Tiwari et al., 2014), soil temperature (Singh and Sharma, 2017b), and the exergy efficiency of the whole EAHE (Hepbasli, 2013). A study of the effects of operating parameters on the performance of an earth-air-pipe heat exchanger during the winter season is presented by Bansal et al. (2009). The parameters tested are pipe materials and air velocity with a 34.32 m length pipe. The experimental results have illustrated the ability of the employed system to achieve heating in the range of 4.1–4.8 °C for the flow velocities of 2–5 m/s. Using different materials such as steel and PVC pipes has not affected the performance of the system, on the contrary, air velocity showed a great impact. Later, the same team (Bansal et al., 2010) investigated the performance of an earth heat exchanger during summer cooling. They got a cooling lift temperature of 8.0–12.7 °C and COP of 1.9–2.9 for the flow velocities of 2–5 m/s, higher than in the case of winter heating. A significant decrease in inlet and outlet temperatures was observed, which indicates that the system is efficient. Moreover, these experiments reveal the system is competitive with conventional vapor compression. Greco and Masselli (2020) investigated the thermal performance optimization of a horizontal Earth-Air Heat Exchanger. They reported that the thermal performance increases with the length until the limited distance is attained. A sensibility analysis was executed, by changing the pipe diameter with air velocity affecting the thermal performance of the EAHX. Moreover, they advised the designed pipe with a smaller diameter and low air velocity to obtain closer air outlet temperature to the soil temperature. Mathur et al. (2017) provided experimental results where a design of spiral heat exchanger buried at a depth of 3 m was used with a 60-m pipe length, a diameter of 0.1 m, and a 1-m distance among the spirals. The results obtained revealed that the coefficient of performance for a spiral configuration is 6.24 in the cooling season and 2.11 in the heating season. Sehli et al. (2012) studied the variation of outlet air temperature with tube form factor, L/D, for an EHX buried at depth of 4 m in the Algerian arid climate under the condition of hot mode. They proved that the outlet air temperature decreases the form factor increases because a longer tube provides a longer residence time for the air inside the pipe enhancing the heat exchange rate. Also, they advised employing low values of Reynolds number where according to their calculations the best value of the form factor was 250.

As per the literature survey above, several attempts have been made by researchers to test the applicability of EHX systems under diverse climatic conditions. However, the applicability verification of EHX systems under very hot subtropical climate conditions, such as that of Iraq, is hard to find. The major phenomenon unique to such a climatic zone is the high-temperature difference between summer and winter, which exceeds 40°C, whereas the previous research has addressed relatively lower temperature differences. Furthermore, the previous research focused mostly on the performance during the cooling mode rather than the heating mode. This study aims to fill in the research gaps identified above by conducting full-scale experimental work in a field setting to explore the effects of the soil and the surrounding temperatures on the performance of earth-heat exchangers running with open-air circulation modes. A parametric analysis was performed to determine the impact of pipe length and air flow rate on the effectiveness of the system. The responsiveness of the designed EHX system to the seasonal changes in atmospheric and subsurface temperatures during the winter and summer seasons was also investigated to identify the heating and cooling potentials of the proposed system. These would be useful for highlighting the adaptation strategies for EHX systems in sandy soil area settings and hot subtropical climatic conditions.

System description

Figure 1 shows a schematic of the experimental earth heat exchanger prototype. The prototype is designed to test various operating air conditions in the summer and winter seasons. The whole system is consisting of a blower (300 W) and two PVC pipes with 0.1 m-diameter and 0.005 m-thickness. The length of each pipe was 8 m and both pipes were tied in series by a pipe of 2 m. The total length of the buried pipe was 18 m which was entombed at a depth of 4 m in the flat ground with sandy soil at the back area of Al-Khwarizmi engineering college, Iraq. The open end of each buried pipe was connected through a vertical pipe of 4 m to a centrifugal blower which was fixed at the beginning of one open end. The velocity of the ambient air can be controlled by changing a gate installed at the entrance air inlet of the blower. The air outlet was pushed to a caravan which was installed for monitoring air conditions results.

FIGURE 1

FIGURE 1. Schematic diagram of the experimental earth heat exchanger prototype.

For ambient air, two main parameters were measured during the experiment: temperature and air velocity. Along the length of the buried pipe, seven thermocouples (type K) were inserted at a constant interval to measure the temperature of inlet and outlet air (T_in, T₁, T₂, T₃, T₄, T₅, T_out). Another set of five thermocouples was used in the soil to measure the temperature of the soil at different levels (T6, T₇, T₈, T₉, T_soil). All thermocouples were connected to a data logger of 32 channels type CKT4000 series multi-channel temperature recorder uses 32-bit high-speed CPU for data processing and recorded every 60 s. Pressure drop was not measured to focus on the estimation of the thermal performance of the heat exchanger. The data measurements were taken every 24 h. A computer was used to collect and process the measurement data acquired by the data logger. The air speeds were measured by a vane type of anemometer at the outlet of the pipe and controlled manually. The different stages of the system installation are shown in Figure 2. The unburied part of the pipe was thermally isolated from the surface of the ground until it was connected to the caravan.

FIGURE 2

FIGURE 2. Earth heat exchanger prototype.

Analysis method

The effectiveness of the system can be estimated by dividing its actual temperature difference by the maximum temperature difference, as follows:A

$\mathrm{\epsilon =}\left({\mathrm{T}}_{\mathrm{in}}\mathrm{-}{\mathrm{T}}_{\mathrm{out}}\right)/\left({\mathrm{T}}_{\mathrm{in}}\mathrm{-}{\mathrm{T}}_{\mathrm{soil}}\right)(1)$

COP of the system is investigated by calculating its heating capacity (q) using the following equation:

$\mathrm{q=m}\mathrm{.}{\mathrm{C}}_{\mathrm{p}}\mathrm{.\Delta T}(2)$

where.

$\mathrm{m}$ = the mass flow rate of air in kg/s.

$\mathrm{Cp}$ = the specific heat of the air in kJ/kg K and.

$\mathrm{\Delta T}$ = the difference between inlet and outlet air temperature.

Effectiveness can be formulated as shown in Eq. 1:

$\mathrm{COP}\mathrm{=}\mathrm{q}/\mathrm{W}(3)$

where W (watt) is the power electricity of the blower.

The relative humidity (RH) of the air has an impact on the level of thermal comfort that can be achieved indoors during the summer mode. As a result, it is essential to parameterize its influence to prevent the air from condensing in the EHX pipes. For instance, if the atmospheric air that enters the system has a dew-point temperature lower than the temperature of the soil, then the air that passes through the buried underground pipes will become undesirably moist (Lindblom and Nordell, 2006). In this work, the dew point of the air temperature during the summer mode was measured to be 11.3°C at the minimum conditions of 48°C and RH of 12% of the atmospheric air entering the system. This dew-point temperature is higher than the soil temperature. Therefore, there was no water vapor condensation from the air entering the system.

Results and discussion

In order to build and apply the earth heat-exchanger systems in hot summer and cold winter seasons, it is critical to have a thorough understanding of their model designs and operating conditions. This section describes how the possible changes in subterranean soil temperature, outside air temperature, and inlet air velocity affect the thermal performance of a building’s underground soil heater.

Figure 3 shows the change of the ambient temperature along with each month of the year under the climatic conditions of Iraq. As can be seen, the ambient temperature ranges from 6.6 to 50.3°C which indicated a significant difference in air temperature. Moreover, the air temperature for the hottest and coolest months was observed in June and January, respectively. Figure 4 shows the change of the soil temperature at the depth of 4 m along with the different months of the year. The lowest soil temperature was recorded in March at 21.71°C which corresponds to maximum and minimum ambient temperatures of 29.1 and 15.7°C, respectively. While the highest soil temperature was observed in October at 30.5°C, with the ambient temperature variations ranging between 43 and 29.4°C. It is an unexpected state that how the variation of the soil temperatures is not incompatible with ambient temperatures. For example, the soil temperature in March was 6°C higher than the minimum soil temperature while in October was higher by 1.1°C. This significant difference can be attributed to the moisture content of the soil in March being higher than in October.

FIGURE 3

FIGURE 3. Variation of the ambient temperature with time.

FIGURE 4

FIGURE 4. Variation of the soil temperature with time.

Corresponding to a temperature greater than the surrounding temperature, the average soil temperature at different depths has been measured and shown in Figure 5. It can be seen from the figure that as the depth of the soil grows, the soil temperature rises in January and declines in June. For January month the ground temperatures at 0 m, 1 m, 2 m, 3 m, and 4 m were 14.9, 11.2, 16.6, 20.9, and 23.8°C respectively. It can be noted that the soil temperature increases in the range of 3–4°C at each meter depth. For June month the ground temperatures at 0 m, 1 m, 2 m, 3 m, and 4 m were recorded at 44.1, 41.4, 32.6, 28.5, and 25.7°C respectively. Compared with the soil temperature in June month, January month showed a significant difference in temperature at depth of 1 m, which is lower than at 0 and 2 m. This can be attributed to the moisture content in the soil at 1 m being still a little higher than other nearby layers. At 4 m depth, the soil temperature in January month shows the closest value to that in June month.

FIGURE 5

FIGURE 5. Variation of average soil temperature with depth.

When the depth increases, the difference between the soil temperatures for January and June decreases, and it is at a rate of 24–25, which indicates that the soil at this age is suitable for heating and cooling purposes. Figures 6A,B depicts the daily variation of soil temperatures at different depth during the January and June months. It can be seen that the soil temperature is still constant after the depth of 1 m for both January and June. It may be concluded that the used soil is with a low thermal conductivity property. Moreover, poor thermal diffusivity may be another reason for keeping the soil at a constant temperature at different depths. The variation of temperatures of soil, inlet air, outlet air and ambient with time is given during January and June months in Figures 7A,B . There is a significant difference between the ambient and inlet air temperatures which can be attributed to the generated heat from the friction of air with the blower impeller. Also, it can be noted that the difference in temperatures of soil and the air outlet is higher during June than during January indicating that the system has a higher cooling potential than the heating potential. While the ambient air temperature varies during the day, the outlet air and soil temperatures keep nearly constant, this means that the heating performance of earth-air heat exchangers (EAHX) was stable in this experimental period.

FIGURE 6

FIGURE 6. The daily variation of soil temperature with time during (A) January and (B) June.

FIGURE 7

FIGURE 7. Variation of temperature with time during (A) January and (B) June.

The variation of the outlet air temperature under different air velocities is shown in Figures 8A,B. As seen, when the air velocity is increased, the outlet air temperature lowers during January and increases during June, respectively. It can be noticed that the outlet air temperature virtually maintains a constant in January while swings in June. This indicates that the performance of this prototype during the heating mode was more stable than that of the cooling mode. The rates of rising in output air temperature among the five intake velocity variations are statistically substantially different. The maximum rate occurs when the intake air velocity is 1 m/s, and the lowest rate occurs when the inlet air velocity is 5 m/s. The inlet air velocity has the greatest influence on the rate. In fact, practically all of the outlet air temperatures increase or drop at or near the soil temperature at a depth of 4 m down, which is a significant increase or reduction. This implies that the heat transport rate between the surrounding air and the soil has been sufficient at the depth in question. The use of such a heat exchanger has the potential to keep the temperature close to the comfort level without the use of air-conditioning. Monitoring of this system has shown that the soil was very effective at heating and cooling the ambient air.

FIGURE 8

FIGURE 8. Variation of outlet temperature with air velocity during (A) January and (B) June.

Figures 9A,B illustrate the relationship between the air output temperature and the input temperature. In both winter and summer, we noticed that when the temperature of the air entering the system increases, the temperature of the air exiting the system also increases. Also, it can be seen that temperatures in June were more fluctuating than temperatures in January. These results are consistent with those observed in the work of Chiesa et al. (Chiesa et al., 2014). Figures 10A,B show the change of effectiveness with inlet air temperatures. There is a significant difference in the behavior of effectiveness in January month, curve shape, and in June month, a linear relationship which causes the effectiveness in January higher than in June. This state can be attributed to the properties of the soil, particularly thermal diffusivity, which indicates that the heat transfer rate from the soil to the pipe in winter is higher than from the air to the soil in summer. That’s why the difference between inlet and outlet air temperature is less in June than in January (Figure 11). It became clear that this system works more efficiently in summer than in winter, and as the results proved, the heat potential is transferred better than the cooling potential, which becomes a crucial characteristic of this system. Figure 12 shows the change of air temperature along the length of the pipe under an air velocity of 1 m/s. The change in air temperature is quicker for the first 18 m of the pipe and it becomes small at the rest of the pipe. This is because of the fact that as the airflow upward from the location T5 to the surface of the ground (4 m), the soil temperature affected the air temperature in a reverse process.

FIGURE 9

FIGURE 9. Variation of outlet temperature with inlet temperature during (A) January and (B) June.

FIGURE 10

FIGURE 10. Variation of effectiveness with inlet air temperature during (A) January and (B) June.

FIGURE 11

FIGURE 11. Variation of temperature raise with inlet air temperature during (A) January and (B) June.

FIGURE 12

FIGURE 12. Variation of air temperature with the pipe length during January and June.

Moreover, the change in air temperature at the vertical section of the pipe (i.e., 0–4 m) is the biggest due to the temperature difference between the soil and air being the highest, and thereby the heat transfer rate in this segment is the highest too. In addition, the increase in air temperature rates is significantly different among all the thermocouples. Although the inlet and outlet temperatures are different, the outlet air temperatures are still not close to the soil temperature indicating that the used 18 m of length pipe is not sufficient to exchange heat between the soil and the surrounding air.

Figure 13 shows the change of effectiveness under the influence of air velocity during January and June month. As seen in Figure 13, the effectiveness decreased along with the increase in air velocity. When the air velocity is 1 m/s, the effectiveness is the highest at 0.8 and 0.82 while the lowest effectiveness of 0.21 and 0.19 are recorded at the air velocity of 4 m/s and 5 m/s in January and June months respectively. It can be seen that the effectiveness decreases along with time and reaches the minimum values at 15:00 for January month and at 5:30 for June month, after this time, it increases again.

FIGURE 13

FIGURE 13. Variation of effectiveness with air velocity during (A) January and (B) June.

The statistical values for parameters tested in January and June are shown in Tables 1 and 2. As can be seen, the average COP values are low for both low and high air velocity. Higher velocity results in a lower average effectiveness of (0.6) in January and (0.46) in June. This occurs because the amount of time available for air to remain in the pipe is shorter. The greatest coefficient of performance (COP) was recorded is 1.6 at a velocity of 5 m/s in January and 1.8 in June. Although the temperatures change during the day and in the month, the maximum temperature difference between inlet and outlet air is estimated as 12.3°C (January) and 17.2°C (June) and the mean difference is found at 7.2 and 9.4°C in the EAHX system. While inlet temperature and air velocity change during the day and the month, the average outlet air temperature change in the range of (17.9–23.9) ^oC (January) and in the range of (28.2–36.4) ^oC (June).

TABLE 1

TABLE 1. Computed parameters at different experimental temperatures during January.

TABLE 2

TABLE 2. Computed parameters at different experimental temperatures during June.

Table 3 compares the results of this study to those of three previous studies carried out by Bulut et al. (2015b), Bisoniya et al. (2014), and Liua et al. (2017). Compared to previous systems, this system exhibits a significant improvement in the temperature difference between the air input and air output. This increase in temperature difference would be attributed to the higher rate of heat exchange inside the EHX pipes. It can also be noted that this EHX design has a lower COP than both of its predecessors. This might be explained by the lower soil temperature, which was influenced by the soil type and surrounding environmental conditions. Regarding the COP results, this system exhibits a higher value of heating load as compared to the one designed by Liu et al. (2017). This is because the greater the difference in temperature increase, the greater the gain in the overall system effectiveness.

TABLE 3

TABLE 3. Results of this work with the results of similar works.

Conclusion

In this work, an EAHE was designed, established and tested in sandy soil in Baghdad. The influence of different variables including ambient air temperature, soil temperature, and air velocity on the thermal performance (COP) and effectiveness (ε) was analyzed during the hottest and coldest months of the year 2020. The outlet temperature decreases along with air velocity and the supply air temperature. COP and effectiveness decrease with air velocity but increase with the inlet air temperature. The COP and ε of the EAHE during January could reach up to the maximum of 1.8 and 0.81 respectively, when the inlet air temperature is 48.8°C, and decrease to the minimum of 0.2 and 0.16 respectively when the inlet air temperature is 32.3°C. It was concluded that this system can be used effectively to reduce the cooling and heating duties of buildings in hot and cold weather conditions. A significant amount of electrical power can be conserved if it is used instead of conventional air conditioning systems. A further amount of energy consumption can be decreased by combining it with different renewable energy sources such as wind turbines, solar panels, and biogas to meet its energy demand. Our next work includes experimentally investigating the performance of EAHE under more influencing parameters, such as pipe length, pipe diameter and pipe material. Moreover, mathematical models will be modified to optimize the system’s performance under different operation conditions. It is expected that this system will be consecutively applied in different areas in Iraq.

Data availability statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Ethics statement

Written informed consent was obtained from the individual(s) for the publication of any potentially identifiable images or data included in this article.

Author contributions

All authors listed have made a substantial, direct, and intellectual contribution to the work and approved it for publication.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

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