DEVELOPMENT OF PASSIVE ENERGY SOURCE AS EARTH AIR PIPE HEAT EXCHANGERS (EAPHE) SYSTEM -A REVIEW

Diversity in living standards and population growth leads to increased global energy consumption. Human comfort always plays a vital role in using different means to reduce the effects of weather conditions. The building sector captures approximately 40% of the global energy and it is most commonly used for cooling and for heating of the space occupied due to the use of appliances such as room heaters or air coolers, air conditioners, etc. The use of these appliances contributes significantly to global warming, which is a very serious environmental problem. Passive energy sources are used widely to reduce the consumption of energy due to heating and cooling of the building. Earth air pipe heat exchanger is one of the passive cooling/ heating technology used for the indoor thermal comfort of the occupants. In this review article, various discussion has been done on the use of passive energy explained by the earth air pipe heat exchanger and various investigations conducted by authors under different conditions and parameters like air velocity, pipe depth, pipe length, etc. that the thermal conductivity of the soil is the key point to the efficient operation of the earth air pipe heat exchanger system and it is also necessary to maintain the thermal conductivity of the soil in the vicinity of the pipe in order to achieve better performance in operation.

a pipe of 80 m long and a cross-sectional area of 0.53 m 2 where 19 kW power is used for air velocity 4.9 m / s. As a result, the room temperature remained at about 7.63 ° C in the Indian climate. Chel and Tiwari [39] determined the thermal potential of the EAPHE system in which it integrated the adobe IIT Delhi house. EAPHE system was built with a PVC pipe of 6cm diameter buried at a depth -1.5 meters. 0.3 kW air blowers were used to blow air inside the pipe. The analysis leads to significant cooling of the connected system. Al Azmi et al. [40] have developed a theoretical model in order to investigate the energy conservation potential of the EAPHE system in Kuwait's desert climate. It has been reported that the cooling demand reduction is about 30% in the peak season (mid-July). Bojiec et al. [41] investigated the thermal behaviour potential of an EAPHE system used for cooling or heating the building. Within the system, 100% pure air is used as a heating or cooling medium for EAPHE. The system consists of 140 mm diameter circular steel tubes. The complete system has two parallel pipes of 50 meters in length, each buried 2.1 meters below the ground. The system could reduce consumption of energy because of the thermal comfort of the building. Experimental analysis by Nadia et al. [42] used 60 meter long pipe to investigate the thermal performance of the EAPHE in Ouargla, Southeast of Algeria. To achieve the desired effect, the pipe is maintained between 40 and 45 meters, and in the case of a length of fewer than 40 meters, the system cannot reduce the temperature near the ground temperature. It was recommended to maintain the speed of about 2m / sec. Recently Yang et al. investigated the decrease of the temperature on the length of the buried pipe and it was found that for a pipe of 200 meters length, the temperature drop was 5 ˚C for the summer season. Mishra et al., [72] studied the impact of transient analysis and thermal conductivity of soil on the continuous operation of EAPHE system. It was reported that steady-state analysis recorded an increase of 19.6 ° C with a 60 meter length pipe of diameter arrangement of 0.1 meters at a speed of 5 m / s. While in case of analysis the temperature is reduced from 19.6 to 17.2 ° C as a result of a continuous operation of the EAPHE 24-hour system with a thermal conductivity of 0.52 Wm -1 K -1 . After twenty four hours of operation, temperature was reduced from 19.6 to 19.2 ° C and also from 19.6 to 19.5 ° C with thermal conductivity of soil being 2.0 Wm -1 K -1 and 4.0 Wm -1 K -1 . From the output, it was found that for the efficient efficiency of the EAPHE system it is necessary to regenerate the thermal potential of the soil. All the above discussions shows that the performance variation of the EAPHE system is due to climate change, soil thermal properties, the geometric configuration of the system and many other factors. It has been observed in some cases that the climatic condition is largely responsible for the overall performance of the system. In some cases, the small length of the pipe ensures adequate thermal comfort, while for some cases it requires a long pipe. The total thermal performance of the system requires a lot of attention and study needs to be done on the various factors related to it.

SUBSOIL TEMPERATURE AND IMPACT OF SURFACE PROPERTIES
The subsoil temperature is mainly a key factor for the effectiveness of the EAPHE system hence few researchers mainly focused on the experimental analysis of the same [44, 46, 47, and 68]. Some of them investigated analytically using various empirical models [50][51][52]. It is a fact that thermal behaviour of the earth surface mainly influences the subsoil thermal performance and its temperature varies with respect to season, area and the climatic condition of the particular region.
The first term on RHS of the above equation represents the heat flux as a result of transfer of heat between air and the surface, the 2nd term represents the solar energy which is absorbed by the surface, the 3rd term represents the heat flux as a result of evaporation of water from the surface of the body and the last term represents the net longwave radiation exchange between the sky and the surface (Its value is usually 61.5 W/m 2 ℃ for all types of surfaces) [47]. Mihalakakou et al. [54] investigated the energy balance equation with respect to earth surface and its impact on the temperature distribution of the soil. The investigation concluded that thermo-physical properties of the surface affect the temperature of the subsoil. Derbel and Kanoun [3] performed an investigation of the properties of the soil such as density, thermal conductivity and specific heat and also correlated the impact on the subsoil temperature at various depths. The current study concludes that small depth had a negligible effect on the subsoil temperature which is less than 1℃.
In order to investigate the subsoil temperature with respect to the depth various experimental and numerical studies were performed [8,20,44,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60]. Some of the studies investigated about the impact of daily as well as annual changes in the surface temperature on the depth of the subsoil temperature and it was observed that penetration of the surface temperature variation on daily basis is about 0.5m only while in case of annual basis it is about 4 m. [53,[61][62][63].The temperature variation noted was about 7-8˚C in Bangkok for about 1-meter depth throughout the year [65]. Table 1. Distribution of temperature under the ground (a) [43], (b) [44], (c) [45], (d) [ In Guangzhou, South China, the monthly average temperature variation was investigated by Wen et.al [21] and it was verified that the gradual variation of the basement temperature increased with increase in depth. Popiel et al. [44] investigated the temperature measurement of the car parking area and a lawn in Poznan for depth variation, i.e. 0-2 meters and 0-17 meters. As a result, it was found that at the parking area of the vehicle, the temperature is 1 ° C greater than the surface of the lawn (with short grass). Various researcher shows the earth temperature at different depth and at different time during the year, table 1 shows the relationship between the temperature and the depth of the ground for different locations, it shows the temperature of the soil remains constant at certain depth as per the location and geographic conditions.
It is necessary to optimize the basement temperature between (18-30 ° C) to be used for the thermal comfort of the occupied space. Different manipulations performed in terms of the above equation can help modify the basement and surface temperature in order to enhance the performance of the proposed system. It has been analysed that many implementations vary depending on climatic conditions. Bansal et al. [61] has made various estimates based on the surface area of the earth with various materials for the 4-meter depth in Delhi, India for different climatic conditions. As a result, it was shown that the maximum temperature was 53˚C for the covered black surface and 17.3˚C for the area covered by various plants, shrubs, etc. Ghoshal et al. conducted a study to analyse subsurface temperature variation in two places for the greenhouse and others for the bare surface in Delhi. The daily and monthly study determined that the underground temperature at different depths under the greenhouse was about 7-9˚C and in comparison with the bare surface it was about 3 to 6˚C higher for the daily and the monthly variation of the basement temperature. Some researchers have suggested that the earth is covered with small stones (pebbles), shedding, wetting or lime coating to reduce the basement temperature [56,66,67] .B. Givoni conducted an experiment at the Institute for Disease Research in Israel and concluded that the irrigated soil covered with pebbles thickness 12 cm decreases the temperature by 9 ° C to the undisturbed area. While covering the surface of the earth with dark sheets leads to the rise of the basement temperature. [7] Recently, some experiments were performed by Nassar et al. [68] in Tripoli, Libya, where the underground temperature measurement was analysed at a depth of 4-meter with bare ground and covered with dry glass. It was found that during December and January the maximum temperature was 1.5 ° C and 47 ° C respectively, and the minimum temperature was observed in May and June, that is 19 ° C and 44 ° C, respectively. Impact on subterranean temperature due to EAPHE pipe: Sodha et al. [69] made the calculation of the outlet air of the pipeline taking both the surface of the cylindrical tunnel and the surface of the undisturbed earth in which the temperature was equal, which leads to an infinite thermal conductivity and a limited capacity of the earth around the tunnel. Practically, the impact of tube/tube presence affects the temperature variation of the basement, which affects the thermal potential behaviour of the EAPHE system. Depending on the pipe or tube usage, the convective heat transfer takes place between the inner surface of the pipe and moving air due to the thermal gradient between them. And the transfer of conductive heat between the surrounding basement and the inner surface of the tube/pipe that is responsible for changing the underground temperature and it is necessary to investigate the possible means to increase the thermal performance of the EAPHE. The geometric configuration of the EAPHE system was defined by the dimensional parameter (U * ).
The equation for relation of the geometrical parameter is as follows: In the above equation; = Pipe material thermal conductivity = Outer radius of the buried pipe.
= Inner radius of the buried pipe. ℎ = Heat transfer coefficient due to convection of the inner surface of buried pipe.
In the above equation, the first right-hand term emphasizes the ratio of conductive thermal resistance it possesses from ground or the subsoil to the heat flow beyond it and the conductive thermal resistance possessed by the flow of air to the inner surface of the pipe / tube buried. The value of the second term is much lower compared to the first term and can be neglected.
For an efficient recovery performance by EAPHE, soil and its thermal saturation plays an important role and needs to be investigated significantly. In addition, both the factors can be improved or influenced by a defined operation, which may be continuous or periodic. Recently, Mathur et al. [70] investigated the above two factors for efficient performance, namely thermal saturation and soil recovery around the buried pipeline for continuous and intermittent modes of operation. As a result, soil temperature has been recovered in both ways by adopting the natural heat conductivity (heat extracted from the near-neighbour pipe) and convection (the heat removed from the underground by night air purging). The complete experiment was performed on a 5m long pipe for intermittent and continuous mode for a running time of 1.30, 45.60 days and as a conclusion it was shown that the heat penetration was more in the basement intermittently compared to the module continuously, while the continuous heat recovery mode works efficiently than the intermittent mode.
For efficient EAPHE performance, it is desirable to evaluate the basement temperature. To increase the effectiveness of the EAPHE system, it must adopt an appropriate strategy to increase the effectiveness of the basement. In some reviews, it was estimated that various factors play a key role in surface temperature recovery. Air speed, ambient air temperature and climate condition are largely responsible for improving system performance. Various research has also estimated that the system arrangement includes pipe position, size and pitch between two pipes fluctuating significantly in performance.

IMPACT OF DESIGN PARAMETERS ON EAPHE SYSTEM
Influence of different parameters on the geometry of the EAPHE system results in a variation in system performance. For example, pipe geometry including: pipe length, pipe diameter, operational parameters including air velocity, air pressure, etc. the pipes, etc.

Influence of pipe parameter on the performance of EAPHE system.
For an efficient performance of an EAPHE, different parameters, such as pipe diameter, pipe length and material properties, and depth of buried pipe are responsible. Recently, many researches were conducted according to the parametric variation to investigate the impact significantly in different arrangements [83,92]. Rakesh et al. [64] studied about the behaviour of various factors influencing air outlet temperature, including air velocity, pipe diameter, pipe length and depth of the pipe in the basement. Last F. Niu et al. [114] conducted a parametric analysis study of the EAPHE system to investigate exit temperature, humidity, etc. and also developed a regression model to perform in various operating conditions in order to analyse the required performance.

Influence of velocity of the air in buried pipe
Various research conducted by Mihalakakou et al. [4,71,72] reported the influence of velocity of air on the performance of the EAPHE system to ensure thermal comfort. In their investigations, they analysed the fact that the small variation in air velocity resulted in a notable change in the evaporative temperature of the EAPHE system. It has also been reported that increasing the air velocity reduces the heating capacity of the system. Kumar et al., [73] investigated the system by changing the air flow rate from 2.58 kg/sec to 1.2 kg/sec, resulting in room temperature changes from 7.65 ° C to 50.9 ° C. Sodha et al. observed that there is an increase in air velocity with the decrease in tunnel length. It has been reported that increasing the rate of air velocity results in a low-temperature gradient between the inlet air and the exhaust air of the system [74]. Serageldin et al. [75] studied in the Egyptian climate that increasing fluid velocity from 1-3m / s results in an increase in temperature from 0.4 ° C to 19.4 ° C. The data in table 5 depicts the effect of air velocity on the efficiency, COP temperature and system output. The graph concludes that all three parameters decrease with increasing air velocity inside the buried pipe.

Effect of Pipe Material
Various EAPHE research has been performed using different pipe materials. Santamauris et al., [76] reported the analysis using various pipe materials, such as plastic, aluminium, concrete and cast. Bojie et al., [77]   Selected PVC and steel for their investigation and showed that the material has no effect on the thermal behaviour of the EAPHE system. Table 2 shows the variation of the outlet temperature of EAPHE with regard to pipe material and shows that there are no high-temperature variations. Table 3 compares the temperature variation using steel and PVC pipe for the system. Effect of material properties and parameters on the thermal performance of the EAPHE was analysed by Viorel Badeseu [81].

Effect of depth of pipe in subsoil
Investigations were carried out by Derbel and Kanoun [55] in order to analyse the efficiency of the system at various depth of the pipe inside the earth. It was also revealed that inside air temperature of the pipe decreases with increase in depth of the pipe up to certain limit. [82] Wu et al. [21] investigated the effectiveness of the EAPHE system at various depths of buried pipe. As a result, variation in temperature of air was found to be between 7.2 ˚C -31.7˚C and between 5.7˚C to 30.7˚C at a depth of 1.6 m and 3.2 m of the pipe. Badescu [81] investigated that depth of the buried pipe increases the thermal potential of the system but limiting up to 4 meters only.

Effect of spacing between the tubes/pipes
Various experimental analysis suggested that single pipe with short length could not provide adequate thermal comfort. One of the investigations reported that use of four parallel pipes instead of two pipes of less than 17 m is more effective than single pipe. Sodha et al. [84] concluded the results of spacing between the pipes. In their conclusion it was addressed that heating potential increases with the increase in spacing and it starts decreasing in case of cooling. Hence it was suggested to keep small spacing between the pipes for effective heating potential. Yoon et al. [85] explained the effect of interval of pipe on the heat transfer rate. Investigation shows that result were independent from pipe diameter. Reduction of the spacing to 1m resulted in 15 to 25% loss of power. Effect of pipe length At certain depths, the basement temperature remains constant and the temperature is high in the winter season and low in the summer season. Lee KH et al., [112] noticed that the air temperature of the EAPHE system drops during the summer and increases in the winter season with the increase of the length of the pipeline. However, the performance rate varies depending on climatic conditions and location. In the experiments it was reported that for the length of the 50 meter pipeline, the outlet temperature of the EAPHE system remains equal to the basement temperature. [106] In recent research, Derbel and Kanoun [81] investigated the energy load of the EAPHE system and reported that the load of the system depends on the pipe length and in this case the variation in length of the buried pipes from 10 meters to 30 meters increases the load Consequently. It has been increased due to the air time inside the buried pipe, which requires a long period of time for the heat exchange process. Basically, it was noticed that there was no effect on the air temperature for a pipe length of 5 meters. [106] various investigators have noticed that increasing the length of the pipe is responsible for the high temperature difference between intake and ventilation air. [4,72,86] Mihalakakou reported a high heat capacity of the EAPHE system during the length of the pipeline from 30 meters to 70 meters. Recent research by Serageldin et al. [75] observed that the increase in length of pipe from 5.45 meters to 7.0 meters results in temperature rise from 19.7 ° C to 19.9 ° C.Wu et al. [21] conducted analysis for three variations of pipe lengths in Guangzhou, southern China. Based on the results, they observed the exit temperature in between 6.1-33.6 ˚C 4.7-31.2˚c and 3.8-29.5˚C for the length of 20 meters, 40 meters and 60 meters when the inlet temperature varied from 7.4˚C to 36 ° C. Ahmad et al. [87] in their investigation found that length of pipe is the most important parameter of EAPHE systems. Table 4, shows the variation in mean efficiency and COP of the system with the length of the pipe. It also shows that the performance of EAPHE increases with length of pipe and saturates after 50m length. Table 5 shows the variation in mean efficiency and COP of the system with respect to the velocity of air.

Effect of pipe's radius
Many researchers have investigated the fact that the radius of the EAPHE pipe system significantly influences the performance of the system [14,72,71,73,81,82,37]. Increasing in the inside diameter of the buried pipe reduces the coefficient of convective heat transfer which results in high temperatures during the summer season [4] and low temperature during the winter season [72]. Mihalakakou et al. [71] performed a survey to verify the effect of the pipe diameter on the EAPHE exhaust air temperature. Their study showed that reducing the inside diameter of the pipe from 0.50 to 0.25 meters results in the exhaust air temperature increasing from 4.5 ° C to 1.5 ° C. In the later study, it was discovered that lowering the radius of pipe from 150 mm to 100 mm resulted in an increase in the outlet temperature from 1.9 to 1.8˚C in the winter season. While Kabahnikov analysed only a small temperature difference after changing the pipe diameter from 0.1 to 0.4 meters. Serageldin et al., [75] reported an increase of temperature from 0.4 ° C to 18.7 ° C, with pipe diameters increasing from 0.0508 meters to 0.0762 meters. Rakesh et al. [73] reported the thermal potential of the tunnel with variation in diameter. In their investigation, they analysed the temperature increase of the occupied space (room) of 1.5 ° C with the increase of the tunnel radius. Investigation by Sodha et al. [88] showed experimentally that increase of tunnel width decreases system cooling load (tunnel geometry changed from 105x0.5 meters to 40 x 0.2 meters). Table 6 shoes the variation in performance of system with respect to the tunnel radius.

Air pressure drop inside the pipe
Drop of pressure is an important factor for assessing the performance of EAPHE system. Pressure drop in EAPHE system was investigated by Paepe and Janssens [87] in which they developed a relation between pressure drop and the parameters of the EAPHE system. The relation is as follows: From the relation it is also clear that thermal performance is directly proportional to pipe/tube length. Pipe with large length and smaller diameter in EAPHE system gives high thermal performance [89]. .

Effect of earth surface conditions
Earlier it was explained that the temperature of subsoil remains constant at depth of 4 meters and 6 meters which is equal to mean annual earth surface solar temperature [20,23]. Moreover condition of the earth surface is also responsible for the effective working of the earth pipe heat exchanger system .In their investigation, many researchers depicted the results of various locations for different climate conditions.
Bansal et al. [61] carried out the investigation to analyse the performance of earth pipe heat exchanger for two different climatic conditions (1) In winter season, surface is covered with blackened and glazed surface (2) In summer season, the surface is shaded and wetted to decrease the temperature of soil. Sodhat et al. [90] and Sawhney et al. [91] reported the effect of pipe length under various earth surface condition in three regions (1) hot-dry climate in Jodhpur (Rajasthan) (2) composite climate in Delhi, India and (3) cold-dry climate in Leh. Mihalakakou investigated the performance of the EAPHE with arrangement of single and multiple pipes under two surface conditions (a) covered with bare soil (b) covered with soil having short grass. As a result, it was reported that summer season soil with short glass provided best performance while bare soil surface was much helpful for winter season due to its high heating capacity [94].

Impact of other factors.
Two most important factors were introduced by Singh et al. [110] which included Fan factor and Tunnel factor. Both the factors were responsible for thermal performance of the EAPHE system. Fan factor helps to understand the required size of the fan as per the occupied space requirement while tunnel factor demonstrates the size of tunnel for optimal thermal comfort of the occupied space. Ahmad et al. [87] investigated the influence of the outlet pipe height of the EAPHE on the output of the system which includes different parameters also. In the analysis it was reported that height of outlet had inverse effect on the systems performance. Result shows the influence of outlet pipe height on the thermal performance of the heat exchanger system. As the height of the pipe increases above 0.1meter the thermal performance of the EAPHE decreases. Hence, negative performance of the system can be minimized by keeping the outlet pipe height below 0.1meter. In recent Mathur et al. [120] studied the issue of space for installing the EAPHE system. For this he made comparative study between the two arrangement of pipe i.e. straight type arrangement and spiral arrangement of pipe using CFD model in transient thermal analysis. The results shows the coefficient of performance of EAPHE, in which the spiral arrangement for summer season was more as compared to the straight pipe arrangement. For straight arrangement in summer season it was evaluated as 4.3 and for spiral arrangement it was 4.48. Whereas straight arrangement in winter season it was evaluated as 5.0 for spiral arrangement it was 5.16. Figure 1 shows the common layout diagram for the straight and spiral tube arrangement.

BASIC GEOMETRY OF EAPHE AND VARIOUS MODELLING APPROACHES.
In a simplified form, the EAPHE system uses indoor and outdoor air through the buried pipes where the exchange of heat takes place. Under this arrangement, the pipe of a desired size is buried inside the soil, where the temperature remains constant (about 4 meters) and the air passes through it using the blower or blower. In the system, one end acts as an air intake, while the other end is an exit. The entire system decides its performance according to weather conditions. The air is cooled in summer season while in the winter season the air heats up for thermal comfort. Let us assume that due to extraction of heat from the soil variation in temperature near the vicinity of buried pipe does not take place.
The energy balance equation for the width of the pipe (dy) written as: where: Journal of Thermal Engineering, Review Article, Vol. = ℎ ( − ) (5) where: = Surface temperature of tunnel or pipe. Assumption; > On solving both equation (3) & (4) and combining them with initial condition = at y=0 The equation will be: The rise in temperature of pipe expressed in: where L= Length of the pipe.
If > -ve ∆ will obtain which indicate cooling of the air. In case the pipe inner surface is in wet condition, transfer of vapours also take place and can be evaluated as: = ℎ ( − ) Where = (1.005 + 10884 ) 10 3 represents the specific heat of water vapour addition of the dry air. During the investigations many model were proposed [104,87,21,105,79,4,90,92,106,107,108,109] to demonstrate the transient phenomena of the soil above the buried pipe and the EAPHE system. Mihalakakou et al. [4] evaluated the thermal behaviour of earth surface using numerical model in case of EAPHE. On the other hand Krarti and Krieder also investigated analytically to analyse the thermal performance using a model developed by them. During the study assumption was taken that the performance of the system reaches quasi static stage with certain interval of operation.
One dimensional model was developed by Benhammou and Draoui [77] in order to investigate the transient behaviour of EAPHE system. They validated the model against theoretical and experimental approach by Bansal et al. [90] using the approach of computational fluid dynamics under Fluent. Mihalakakou et al. [4] found out the model for both heat and mass transfer simultaneously and also made evaluation of the model for thermal performance due to turbulent flow of the air in it. Niu et al. [59] compared the numerical approach with the experimental data in which two dimensional dynamic heat transfer mechanism was analysed using transient control volume method with the division of soil computing domain into control units along axial and radial direction. In the model by Yang et al. [67] performance of EAPHE was considered by harmonic temperature signal observed from both pipe inlet and the ground surface of proposed model. In whole investigation model was validated using CFD simulation to ensure the accuracy of result obtained experimentally. Figure 2 shows the cross section view of underground pipe used in EAPHE system. In an open loop or ventilated system, EAPHE uses total ambient air to achieve thermal comfort in the occupied space [52] and eventually is exhausted again in the atmosphere. Goswami and Bisnoi demonstrated both the open loop system and the indirect system in which the COP of both systems were compared. In the closed-loop system, the air is continuously recirculated from the pipe / tunnel buried in the occupied space. [5,39] Sawhney et al. has analysed how EAPHE is being recirculated in Ghoshi, India's unconventional research institute for cooling eight of its chambers. In another study, the recirculation system combines with the conventional air conditioning system to analyse the performance (COP) of the system. [95] It can be seen that the performance of the EAPHE system varies significantly according to the following parameters like: (a) pipe diameter (b) pipe length (c) pipe depth (d) soil properties (e) the meteorological condition of the locality. In the partial opening loop configuration, the air blown through the pipe is a mixture of both ambient air and room air in a suitable proportion to the suction point of the pipe to achieve better performance. In some analyses, air conditioning is preconditioned before it is fed into the occupied space. In this process, a conventional vapour compression cycle is used to preheat the air blowing through the buried pipe. Moreover, each arrangement or system had single or multiple ducts such as (a) parallel to one or more layers of pipes and connected to the suction and discharge antenna that were common to all parallel conduits [85] b) Vertical or horizontal comb structure [96] (c) spiral tubular structure [97] for system analysis, various analytical and theoretical models were developed and analysed using different simulation and analysis software (TRANSYS, MATLAB, Energy plus). Different research has been investigated and concluded by several researchers, despite the need to investigate possible means to recover soil temperature for the desired production.

EAPHE system in ventilation mode
Investigations have been conducted to increase the thermal comfort of the system with low energy potential. The integration of EAPHE with the ventilation system was another approach that was investigated by Darkwa et.al. [99] where they installed the system in (CSET) at the University of Nottingham. Their study concluded that the integrated system has enough energy-saving potential with a higher efficiency of the system where its system reached the COP of approximately 3.2-3.53 for March and July respectively.

Integration of EAPHE system with Solar Chimney
Various experimental analyses have been carried out for integrated system investigations. The integration of the EAPHE system with the solar chimney was introduced by Maerafet and Haghighi [100]. Figure 3 shows the diagram of the integrated system. Breesch et al. [98] performed comparative analyses in office building between EAPHE system and night mode. In their investigation they reported that the effectiveness of night ventilation is much better as compared to EAPHE system in summer season. Figure 3 shows the day and night operation mode.
The integrated system contains a surface of a glass facing the south wall. The wall was responsible for capturing solar radiation during the day. The EAPHE system consists of parallel pipes, with spacing between them considered only at the penetration depth of the pipe, to change the large amount of heat between the air and the ground. The solar chimney contributes to energy saving for the EAPHE system during the day as air circulation takes place continuously due to the chimney chimney effect. Also, the above investigation concluded that the length of the pipe should be more than 20 meters for effective thermal comfort in the interior. Table 5 shows the pipe length effect on the performance of the EAPHE system. Recently, Niu et al. [93] performed the analysis of the integrated chimney system using the two-dimensional model. The result of the analysis was concluded by comparing the experimental data that was collected from the existing system that was used to develop indoor thermal comfort. Preheating or pre-cooling of the air also consumes energy for its operation [101,118].
Thier and Peuportier [103] investigated the system to preheat/pre-cool the fresh air economically. Hence they utilized EAPHE system in order to consume passive energy of the earth. The system was assumed to be working as heat sink in day time for cooling the indoor. The next investigation was carried out in the Greater Paris area where installation of the system was carried out in two buildings. Amongst them, a building was used by elders with connected EAPHE system. The EAPHE system consisted of eight 50-meter polyethylene pipeline under the 1.6 meter depth and the second was installed in the tertiary building where the office that supplied EAPHE-supplied air, comprising six long pipes 25 meters buried at a depth of 1.6 meters. Table 8 shows effect of different length of EAPHE system and number of solar chimney on the indoor comfort of building.

EAPHE system assisted by wind tower
Wind tower EAPHE system was investigated by Benhammou et al. [102] for thermal comfort in dry hot arid region of Algeria. Figure 4 shows the schematic diagram of the system. Table 7 depicts the variation in thermal behaviour of the integrated system with variation in parameters of EAPHE and Wind tower system.

Utilization of EAPHE system in gas turbine power
Investigation was done by Barakat et al. [104] in which EAPHE system was coupled with gas power plant. Figure 5 shows EAPHE system coupled with gas turbine. For analysis they developed a 1-dimensional model of the integrated gas power plant system with EAPHE to evaluate the performance, efficiency and fuel consumption by gas power plant. As a result, it was concluded that the output power was increased by 9% and thermal efficiency about 4.8% of gas power plant after the implementation of EAPHE system with it. Thermal efficiency increased by about 4.8% of gas power plant after the implementation of EAPHE system with it.

SOIL MODELLING APPROACH IN EAPHE SYSTEM
In EAPHE, properties of soil play an important role in improving the thermal performance of the system. Among the different properties, the thermal conductivity of the soil contributes the major effect to provide the desired output while leaving the system. Mishra et al. [111] investigated the impact of moisture content on system performance. For analysis, they proposed a system for semi-arid regions in Ajmer (India) with dry and wet soil. System performance was investigated based on system exit temperature. As a result, it was found that the temperature drop was 90% with a 26-meter pipe length and a moisture content of 15% and the same temperature drop of 90% was observed for a 40meter length of the dry soil pipeline. The result concluded that moisture content in the soil must maintain the effectiveness of the EAPHE system. Table 9, 10 shows the system performance based on dry and wet soil along the length of the pipe. In the following approach, Agarwal et al. [112] studied the effect of soil humidity in the basement in the vicinity of the buried pipe. Within EAPHE, researchers have proposed a system to maintain moisture content in the system and dry soil system. The results showed that in the case of dry soil, the system requires a long pipe length for the same performance as wet soil. It was also concluded that the average COP was high for wet soil compared to that of dry soil. Table 9 and 10 show the heat transfer rate and the performance coefficient for soil and wet soil. Recently, Mathur et al. [119] investigated the different ways and its influence on the operation of the EAPHE system with three different soils in three different areas using the CFD transitory analysis. In the three modes, the first mode is taken as a continuous mode with a continuous operation of 12 hours, while the other two modes have a continuous operation of 60 minutes, with a residence time of 20 and 40 minutes. Soils with different thermal properties were taken from Ajmer, Jodhpur and Presles (France) with a thermal conductivity of 0.52, 1.00 and 1.28 Wm -1 K -1 respectively. CFD analysis was performed in different operating modes with three different soils and the EAPHE performance after 12 hours continuous thermal conductivity of 1.28 Wm -1 K -1 was found to be 5.04%, while in the case of low thermal conductivity it was found 1.81%. Performance of the system analysed in both operating conditions i.e. continuous and intermittent. In the research it was concluded that mode of operation greatly affects the thermal performance of EAPHE system because in continuous operation, soil reaches the saturation stage and leads to a deterioration of the thermal performance. While in the case of an intermittent mode the soil has time to accumulate the soil temperature in the neighbourhood, therefore for a lower thermal conductivity it is beneficial to operate intermittently compared to the continuous mode. Table 11 shows efficiency of the wet and dry soil EAPHE systems along the length of pipe with 15 percentage wetness. It reaches at constant value after certain length of the pipe. Figure 6 average variation in heat transfer rate and COP with dry soil and wet soil at different moisture content wet soil with 15% moisture shows improved performance. Table 12 shows the air outlet temperature under continuous mode and intermittent mode for three different soil conditions. Table 9. Daily average heat transfer between pipe surface and soil [112] Daily average heat trasfer rate between pipe surface and soil.   Furthermore investigation was done by Mishra et al. [117] in which 3D transient CFD analysis of EAPHE system was performed for three different soil conditions having a thermal conductivity of 0.52,2.0,4.0 W −1 −1 under hot arid climate of Ajmer. In the analysis, parametric study of the derating factor was analysed on three different soils with different pipe diameter, pipe length and air velocity. The evaluated result concluded that the thermal conductivity of the soil is the main factor which can assign the thermal performance of EAPHE. Graph in Figure  7 shows that derating factor of the soil with lower thermal conductivity is more in comparison to the soil with high thermal conductivity under transient operating condition with respect to pipe length which is between 0.2-0.3 after twenty four hours of operation of thermal conductivity of soil 4.0 W −1 −1 as compared to approx. 0.4 for thermal conductivity of soil 2.0 W −1 −1 . Figure 7 shows derating factors of the system under different pipe length and soil thermal conductivity.

Figure 7.
Derating factor as per the thermal conductivity of the soil. [117], with permission from Elsevier.

DISCUSSION
In the review, the EAPHE system was explained as a powerful option for indoor thermal comfort in the building area. Many researchers have been interested in developing the various models to optimize the efficiency of the system, as well as to significantly use passive energy. The overall analysis has concluded the best operational condition of the EAPHE system through different experimental and analytical models. In experiments, most researchers completed the soil depth between 0.5 meters and 4 meters for an optimal outcome / result. The study shows that EAPHE is the best way to use passive energy and reduce greenhouse gas emissions through the conventional air conditioning system. To optimize the efficiency and effectiveness of the EAPHE system it is necessary to treat the soil so as to increase the thermal conductivity of the soil. For this, different techniques of shading, soil soaking, use of short grass, etc. were applied. Analytical analysis utilized different dimensional and three-dimensional models and analysis using similar software (ANSYS, Energyplus, Fluent, Matlab, etc.). Table 13 below summarizes the EAPHE comparative study based on design parameters including pipe length, pipe diameter, pipe quantity, pipe depth, pipe material, pipe distance. All of the factors mentioned above are responsible for the efficient operation of the system to ensure the interior thermal comfort.

CONCLUSION AND RECOMMENDATION
The earth air pipe heat exchanger system becomes one of the most widely used as passive energy source to provide thermal comfort in the occupied space. It is the best way to use geothermal energy to reduce the consumption of conventional resources for heating / cooling indoor spaces. The general study of various experimental or analytical research using different bi-dimensional or three-dimensional models has helped to understand the approach of technology under different circumstances. The whole article demonstrated all the factors responsible for the effectiveness of the EAPHE system and also demonstrated the advantages and disadvantages of different system parameters to enhance the thermal potential of the system.
The study concluded that the depth of the buried pipe should vary between 0.5 and 4 meters. Increasing air speed reduces the thermal performance of the system; therefore, it is necessary to operate the system with an optimal air speed. Performance of EAPHE system depends upon thermal properties of soil above the buried pipe. After continuous operation, performance of EAPHE deteriorated due to thermal saturation of soil, so it is recommended to adopt such methods that maintain moisture content of soil to retain the thermal properties.
Performance of the EAPHE system is independent of the pipe material hence use of cheap PVC pipes are advised. Long pipes with small diameter gives better performance compare to the larger diameter.
Output pipe height is also an important factor affecting the thermal efficiency of the system; it is recommended to ensure an optimum outlet height for a better output.
To optimize the current system, other systems are coupled with EAPHE, such as the solar basket, photovoltaic cells, the wind turbine, etc. and which are significantly demonstrated in this article.
After extensive study of literature related to EAPHE system, it is advised that study should be carried out with different layout design of the pipe and cross section of pipe to enhance the performance of the system. Further modelling of the soil above the buried pipe should be carried out to enhance the effective working duration of the system. As the air passed through the EAPHE system its temperature, relative humidity and humidity ratio were reduced. The heat exchange for summer was 1.3 times higher than winter season. Thermal conductivity of the soil directly influence the value while spacing between pipe did not effect the performance.