Renewable and Sustainable Energy Reviews 47 (2015) 83–92

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Renewable and Sustainable Energy Reviews

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Ground coupled heat exchangers: A review and applications

Suresh Kumar Soni a,n, Mukesh Pandey a, Vishvendra Nath Bartaria b a Department of Energy Technology, RGPV, Bhopal, India b Department of , LNCT, Bhopal, India article info abstract

Article history: The use of ground coupled (GCHE) systems is increasing worldwide. They are mainly Received 23 September 2014 used for space conditioning, water heating, agricultural drying, bathing, swimming, etc. They reduce Received in revised form cooling load in summer and heating load in winter. GCHE systems make available excellent scope to 6 January 2015 conserve significant amount of primary energy and thus mitigating the impact on environment through Accepted 1 March 2015 emission reduction. This paper reviews the experimental and modeling studies carried out on GCHE systems. The reviewed literature focuses on performance of both types of GCHE systems viz. earth–air Keywords: heat exchanger (EAHE) and ground source (GSHP) systems and brings out their merits and GCHE demerits. EAHE & 2015 Elsevier Ltd. All rights reserved. GSHP Passive heating/cooling

Contents

1. Introduction ...... 83 2. GCHEsystems...... 84 3. Scenario of GCHE systems...... 85 3.1. EAHE systems studies ...... 85 3.1.1. Experimental studies of EAHE systems...... 85 3.1.2. Modeling of EAHE systems ...... 87 3.2. GSHP systems studies ...... 88 3.2.1. Experimental studies of GSHP systems...... 88 3.2.2. Modeling of GSHP systems ...... 89 4. Applications of GCHE systems ...... 89 5. Conclusion ...... 90 References...... 91

1. Introduction

The way towards energy and environment sustainability is the Abbreviations: GCHE, ground coupled heat exchanger; EAHE, earth air heat incremental adoption and promotion of renewable energy tech- exchanger; UAT, underground air tunnel; GSHP, ; DX-

GCHP, direct expansion ground-coupled heat pump; CO2-e, carbon dioxide- nologies, practices and policies [1]. Green building control strate- equivalent; CER, certified emission reduction; HVAC, heating ventilation and air gies use various concepts of natural heating, ventilation and air- conditioning; 1D, one dimensional; 2D, two dimensional; 3D, three dimensional; conditioning [2,3]. GCHE technique is one of them. It is an under- CFD, computational fluid dynamics; GHE, ground heat exchanger; PVC, polyvinyl chloride; PBP, payback period; ANN, artificial neural network; ID, internal dia- ground heat exchanger that can absorb from/release heat to the meter; COP, coefficient of performance; PCM, phase changing material; HETS, ground. The underground temperature remains relatively constant horizontal earth tube system; PV, photo voltaic; AC, air conditioner; EER, energy throughout the year or all year average of sol air temperature of its efficiency ratio; T , temperature at entry of pipe in 1C; T , temperature at exit of in exit ground surface [4,5]. pipe in 1C. n Corresponding author. Tel.:þ91 9826283893. This constant temperature characteristic is due to high thermal E-mail address: [email protected] (S.K. Soni). inertia of the soil and as the depth increases, effect of temperature http://dx.doi.org/10.1016/j.rser.2015.03.014 1364-0321/& 2015 Elsevier Ltd. All rights reserved. 84 S.K. Soni et al. / Renewable and Sustainable Energy Reviews 47 (2015) 83–92

fluctuations of the ground surface is reduced. Due to time lag Fresh between the temperature variations at the surface of the ground Air and below the ground, at a sufficient depth, temperature below Suction Heated or the ground is always higher than that of the outside temperature pump Cooled Air of air in winter and is lower in summer. This temperature Ground difference can be used for pre-heating in winter and pre-cooling Level in summer by installing appropriate GCHE system. Advantages of GCHE systems are high efficiency, stable capacity, good air quality, better , easy control, require simple equipments, low maintenance cost, environment friendly, long term cost effective, tax benefit, and noise free being the underground unit. Drawbacks are higher initial cost, limited availability of trained Fig. 1. Earth air heat exchanger system. technicians and contractors. Performance of GCHE systems depends on air/liquid flow rate, depth and length of buried pipe/ tube (sufficient for air/liquid to lose the heat to certain extent), material and diameter of pipe/tube, temperature difference between earth and ambient, initial soil temperature, rating of blower , and various combinations of pipes. Water/anti- freezing liquid 2. GCHE systems Pump GCHE systems have multiple primary objectives. These are to house achieve the best operational efficiencies, the lowest possible opera- tional cost and run environmentally safe, to have lowest possible Ground loop (High density initial cost and surface area, to increase interior comfort levels and plastic) long-term system durability, to make possible ease of service and maintenance and to earn revenue under certified emission reduc- Fig. 2. Regular geothermal heating/cooling system. tion (CER). Sequestration of one ton of carbon dioxide-equivalent

(CO2-e) is represented by one CER unit. Developing country like India can earn additional revenue of 234 107 Euros under CER by using GCHE [6]. GCHE systems are preferred to those locations where fluctuation in ground surface temperature level is high and under the demands of reduction of CO2 released into the atmo- Refregerent sphere. According to connection and orientation, it can be classified as series and parallel, horizontal and vertical. According to flow substance, it can be classified as EAHE or underground air tunnel (UAT) and GSHP system. EAHE system concept is popularly used in Ground loop colder countries for space cooling and heating, respectively in (Copper) summer and winter, shown in Fig. 1 [7]. EAHE systems contain buried pipes in various combinations, in open loop as well as in closed loop. Air is blown through buried pipes using a properly Fig. 3. DX-GCHP system. sized blower fan installed at the entry or exit [8]. EAHE systems are cost effective and having long life but prone to air contamination Copper is a superior heat conductor in comparison to the most from bacterial growth, corrosion due to moist/humid air and plastic materials. DX-GCHP systems are more appropriate for small chemical reaction inside the tunnel may dither public acceptability space heating/cooling. It requires less excavation/drilling to install, in residential buildings which needs further investigations. Above more efficient because one step of heat exchange process is drawbacks in adopting EAHE systems are overcome in GSHP eliminated in comparison to other GSHP systems, no requirement systems which are suitable for space cooling/heating of residential of a circulation pump and increased thermal contact with ground buildings and small commercial offices. that facilitate to overcome freezing problems associated with GSHP systems are electrically powered, circulate a colder climates. Ground coils can be buried in different manners. fluid around a loop of buried pipe, use relatively constant tem- Horizontal layout of coils avoids the difficulty of liquid perature of the earth and transfer heat to or from earth. They can build-up, because there is no low point, but needs a huge size of be classified as regular and direct exchange geothermal heating/ ground. Vertical layout of coils is better than horizontal, but cooling system or direct expansion ground-coupled heat pump requires a condenser receiver and a metering system to overcome (DX-GCHP). liquid refrigerant buildup within buried coils and get maximum In regular geothermal heating/cooling system, water or anti- efficiency by allowing relatively large quantity of refrigerant [11]. freezing liquid or mixture of them flows through buried high density Common problems are overstressed particularly in cooling mode, plastic tube as shown in Fig. 2.Itisefficient technology but requires suction line pressure drop and partially filled up with debris in more space. Regular heating/cooling system can be further classified vertical well that need design improvements [12]. Some other as horizontal and vertical. Vertical boreholes are more effective, drawbacks reported are effect of corrosion on life of copper tube efficient, need less piping area than horizontal loops but more due to acidic property of soil, more quantity of refrigerant required expensive [9]. In vertical boreholes two-layer, two hole systems are due to leakage of refrigerant and adverse effect on soil [13,14]. more beneficial than one hole system [10]. Comparison between EAHE and GSHP systems is given in Table 1. In DX-GCHP system refrigerant flows through buried copper Though GSHP systems are costlier and have lesser life than tube and exchanges heat directly with the soil as shown in Fig. 3. EAHE systems, getting good response all over the world because S.K. Soni et al. / Renewable and Sustainable Energy Reviews 47 (2015) 83–92 85 they are energy efficient and suitable for all seasons. The perfor- heating as well as summer cooling as given in Table 2. Tempera- mance of both systems degrade over time due to increase in the ture dropped more in initial length of buried pipe. Then cost and soil temperature because of prolonged heat dissipation, accumula- durability would be the deciding factor for selection of material of tion around the tubes in cooling process [21,22]. Both systems pipe. It was suggested that plastic pipes require extreme care have their own merits and demerits. Suitability can be optimized during the backfill process to prevent the pipes from mechanical in terms of seasons, climate zones and cost effectiveness [23]. damage whereas galvanized steel pipes overcome from mechan- ical injuries but increase the cost by 25–30% of the project. It was also concluded that there was a fair agreement between the 3. Scenario of GCHE systems simulated and experimental results. Cooling COP is normally higher than heating. Abbaspour-Fard Studies of GCHE systems cover both i.e. EAHE and GSHP systems. et al. [30] tested the performance of an EAHE system on various parameters i.e. burial depth, length of pipe, air velocity, pipe 3.1. EAHE systems studies material in Iran. After 72 experimental trials it was concluded that all parameters were directly related to performance except Air heat exchanger system concepts like wind towers and under- pipe material. Cooling coefficient of performance (COP) of 5.5 was ground air tunnels, were known in ancient times for passive cooling higher than heating COP of 3.5. [24]. Historically, EAHE system related research works and imple- Mogharreb et al. [31] tested the performance of an EAHE system mentations have been confined to Europe, America and other cold by taking two independent variables i.e. area of greenhouse and countries. In the recent years, scope of EAHE systems is being inv- the percentage of vegetation coverage inside the greenhouse during estigated in developing countries also. Many researchers adopted both heating and cooling modes. It was concluded as variables had EAHE system alone or in combination with , conven- significant effect on the performance of EAHE system. Cooling COP tional heating ventilation and (HVAC) system, phase was 4.32 higher than heating COP 1.01. The percentage of vegetation changing material (PCM) and other renewable technologies, using coverage had negative effect during cooling and positive effect experimental, modeling or both methodologies for their research during heating mode on the performance of EAHE system. works [25–27]. Mongkon et al. [32] presented the experimental results of horizontalearthtubesystem(HETS)forcoolinginanagricultural 3.1.1. Experimental studies of EAHE systems greenhouse in the tropical climate of Thailand. Performance of HETS It was concluded by various researchers that thermal perfor- was monitored on typical days in the winter, summer and monsoon manceofEAHEsystemsisnotaffectedbythematerialofburiedpipe seasons. The experiments were conducted in 30 m2 of greenhouse for winter heating as well as summer cooling. Bansal et al. [28,29] volume with the 38.5 m of length and 0.08 m diameter HETS buried made an experimental setup of EAHE system at Ajmer in India, at a depth of 1 m. Readings of performance in various seasons were which had two horizontal cylindrical pipes of 0.15 m inner diameter, distinct. The values of COP were 3.56, 2.04, and 0.77 during summer, 23.42 m buried length each, made up of polyvinyl chloride (PVC) and winter and monsoon days respectively. It was established that coo- mild steel, buried at a depth of 2.7 m in flat land with dry soil. ling COP was higher than heating. Performance comparison of steel and PVC pipes is given in Table 2. Soil conditions significantly affect the performance. Ascion et al. It was concluded from experiment that performance of EAHE [33] tested and concluded that best energy performance could be system was not affected by the material of buried pipe for winter obtained for wet/humid soil by adopting pipe length of more than

Table 1 Comparison between EAHE and GSHP systems.

Parameters EAHE systems GSHP systems

Application For small/medium space heating/cooling For small space heating/cooling [13] Initial cost Low High [15] Material Any material can be used Plastic for flow of water/anti-freeze and copper tube for refrigerant [15] Suitability Average about 1.3 times higher heat exchange on summer days than winter days and not preferred For all seasons, but give maximum efficiency in humid environment [16,17] during monsoon season[15] Efficiency Less [18,19] More Operational Low Low to conventional heating/cooling [20] cost Payback period Less More (PBP) Life span More Less

Table 2 Comparison of steel and PVC pipes.

Air flow velocities Temp.(1C) Summer cooling (1C) Winter heating (1C)

Steel pipe PVC pipe Steel pipe PVC pipe

2m/s Tin 43.7 43.7 20.6 20.6

Texit 31.0 33.1 25.4 25.1

Tin–Texit down/(up) 12.7 10.3 (4.8) (4.5)

5m/s Tin 43.6 42.2 20.6 20.6

Texit 33.7 34.2 24.7 24.2

Tin–Texit down/(up) 10.3 8.0 (4.1) (3.6) 86 S.K. Soni et al. / Renewable and Sustainable Energy Reviews 47 (2015) 83–92

50 m, buried at a depth of 3 m. Balghouthi et al. [34] studied Misra et al. [41] made an experimental setup of EAHE system in experimentally thermal and moisture behavior of dry and wet soil hybrid with air conditioner (AC) at Ajmer in India. The experimental heated by buried capillary plaits, on a prototype similar to an setup comprised of 60 m long, 0.10 m ID PVC pipe, buried at a depth agricultural tunnel greenhouse. It was concluded that the surface of 3.7 m in a flat land with dry soil. There were four modes of test, temperature amplitude was superior in wet soil as compared to in mode-I, alone AC supplied the conditioned air to the test room dry soil. that was treated as base mode. In mode-II, AC and EAHE system Li et al. [35] constructed experimental setup at Herbin in China, both supplied their 100% conditioned air to the test room. In mode- for study of a ground sink direct cooling system in cold areas and III, AC supplied conditioned air to room and but 100% conditioned concluded that there was substantial scope of energy conservation air from EAHE system was supplied to AC's condenser coils to cool within a particular region. It was found that geographical and them only. In mode-IV, AC supplied conditioned air to room and climatic conditions affect the performance level. 50% conditioned air from EAHE system was supplied to AC's cond- Efficiency can be improved by reducing pipe diameter, mass enser coils to cool them and remaining 50% air was supplied to test flow rate of air, increasing pipe length, buried depth and buried room. Outcome of different combinations of EAHE system with con- pattern. Goswami et al. [36] constructed an experimental setup at ventional 1.5 TR AC is presented in Table 3. the energy research and education park at the university of It was concluded from experiment that suitable combination of Florida, which consisted of a 0.3 m diameter, 30.5 m long corru- EAHE system with conventional AC in mode-III could conserve gated plastic pipe buried at a depth 2.75 m, a 0.2 kW blower, a 2½- significant amount of energy as given in Table 3. ton heat pump, open loop. It was suggested that 0.3 m diameter Haghighi et al. [42] integrated EAHE system with solar chimney for single pipe and 0.2–0.25 m diameter for multiple pipes in and concluded that 0.5 m diameter, 25 m length of EAHE system, parallel could be suitable for achieving optimum performance but 0.2 m air gap and outlet sizes of solar chimney could enhance the paper is silent on selection criteria for 0.3 m diameter pipe. It was performance. observed that smaller diameter provided higher temperature drop, Tavakolinia [43] integrated EAHE system with wind catcher for but consumed more fan power. cooling and natural ventilation for the single story spaces in Los Bisoniya et al. [37] tested EAHE system for hot and dry summer Angeles area. It was concluded that integrated system could perform conditions at Bhopal in India. The experimental set up had two better than unity system. cylindrical PVC pipes of 0.1016 m ID. The length of each cylindrical Ralegaonkar et al. [44] tested EAHE system at Nagpur in India pipe was 9.114 m, connected in series. The total length of burial and concluded that geothermal cooling system i.e. EAHE could pipe assembly including elbows, connector pipe was 19.228 m, save up to 90% of electricity as compared to conventional air buried in black cotton soil at a depth of 2 m. For air flow velocities conditioning systems and 100% of water as consumed by evapora- of 2 m/s, 3.5 m/s and 5 m/s, researchers concluded that decrease in tive cooling systems. air temperature was significant in initial length of pipe than in the Chel et al. [45] made an experimental setup of EAHE system remaining length of pipe. Maximum and minimum drops in air integrated with 2.32 kWp photovoltaic power system at solar temperature were 12.9 1C, 11.3 1C observed at air flow velocities of energy park in New , India. This integration had estimated 2 m/s and 5 m/s respectively. It was felt that lower air velocities potential of up to 50% energy conservation. would lead to higher temperature drop. A simulation model was Appropriate modifications like cross ventilation, earth surface developed on computational fluid dynamics (CFD) platform CFX and soil–cement mixture treatment, wall configuration, etc. pro- 12.0 and simulation results validated satisfactorily with the expe- vide enhanced output. Shukla et al. [46] tested on adobe houses in rimental results. three modes, room with an EAHE system, untreated room and Dubey et al. [38] tested EAHE system with pipes in parallel at room with cross ventilation, at New Delhi in India. In the dry and Bhopal in India. Experimental setup had 3 GI pipes of 64 mm hot climate, the EAHE system cooled the room by 3 1C. In the cold internal diameter (ID), 3 m each, connected in parallel to common climate, the EAHE system heated the room by 6.5 1C more in intake and exhaust, buried at a depth of 1.5 m in a flat land with comparison with untreated room. Room with cross ventilation dry soil. Researchers found that temperature difference of air at gave significantly lower temperature than other rooms in absence the inlet section and exit section of the pipe at 1.5 m depth, varied of sunshine in summer or peak sunshine in winters. from 8.6 to 4.18 1C in the air velocity range of 4.1–11.6 m/s and COP Sodha et al. [47] tested the effects of different earth surface from 6.4 to 3.6. It was observed that lower air velocities resulted treatments on thermal performance of EAHE system for a coupled higher temperature drops and COPs. room (sized 4 m 3m 3 m) in India. Earth surface treatments Woodson et al. [39] constructed experimental setup of EAHE were shading and wetting for hot–dry and hot–humid climate, system at the International Institute for Water and Environmental blackening and glazing for cold–dry climate. It was concluded that Engineering in Ouagadougou, Burkina Faso. It was a horizontal earth surface treatments could conserve lot of energy. open-loop system used 25 m long, 125 mm diameter PVC pipe at a Yassine et al. [48] tested and found that optimal wall config- depth of 1.5 m. The EAHE system had two air inlets, located 15 m uration in each zone integrated with the EAHE system could and 25 m away from the air outlet in the building. But this study reduce 76.7% energy as compared to conventional cooling. was done only on 25 m long pipe which was laid down in Choudhury et al. [49] carried out an experimental study in the serpentine pattern. It was found that 25 m long EAHE buried at North Eastern part of India. Earth pipe was prepared by utilizing local a depth 1.5 m could reduce the temperature of air drawn from available bamboos and soil–cement mixture plaster. Soil–cement outside by more than 7.5 1C. While outdoor temperature varied from 25 1Cto431C and the soil temperature at a depth of 1.5 m remained at about 30.4 1C. Paper does not provide selection crit- Table 3 eria for serpentine pattern and comparison with others. Performance of different modes of operation.

Hybrid systems give increased end results. Rodrigues et al. [40] Modes of operation Power consumption as compared to base mode experimented combination of EAHE with PCM as alternative of conventional air-conditioning. It was concluded that combined I Base mode effect of EAHE with PCM conserved significant amount of energy II Reduced by 6% III Reduced by 18.1% as compared to conventional air conditioning system. It could enh- IV Increased by 16% ance cooling up to 47%. S.K. Soni et al. / Renewable and Sustainable Energy Reviews 47 (2015) 83–92 87 mixtures were able to decrease the by 30–40%. EAHE sys- Bansal et al. [58] presented an integrated model of EAHE tem was able to reduce outlet temperature up to 30–35%. Important system with . It was concluded that length of observations of EAHE systems are presented in Table 4. pipe could be reduced up to 93.5% in comparison to alone EAHE It is observed from Table 4 that all parameters (i.e. burial depth, system. length of pipe, air velocity, wet/humid soil, etc.) are important and 2D models are advanced than 1D and were adopted during directly related to the performance of an EAHE system except pipe nineties, it could calculate temperatures of ground and different material. Combinations with other renewable options and mod- depths. Tittelein et al. [59] developed a 2D numerical model and ifications increase the overall system efficiency. used to reduce computational time. Pfafferott [60] tested for one year on small diameter EAHE sys- tem in Germany. Surrounding soil temperature of pipes was cons- 3.1.2. Modeling of EAHE systems idered as a design input parameter. It was concluded that ther- Models can be classified as one dimensional (1D), two dimen- mal recovery of soil was adequate to retain the performance level. sional (2D) and three dimensional (3D). 1D model is used to derive Trzaski et al. [61] simulated the same experimental results and a relation between pipes inlet and outlet temperatures [50]. Freire concluded that there was little change in surrounding soil tem- et al. [51] studied on multiple layer configurations. It was con- perature after one year of operation. cluded that 1.5 m was the optimum distance between layers and Kumar et al. [62] worked on 2D model using artificial neural very useful in urban areas. network (ANN) concept and developed two models, deterministic Abdelkrim et al. [52] considered two parameters, Reynolds and intelligent and then validated experimentally. Deterministic number and the form factor for evaluating the performance of the model accuracy was 75.3% whereas intelligent model accuracy EAHE system and experimentally validated the results of 1D ste- was 72.6%. ady numerical model. 3D models are more dynamic, advanced in technology and Singh [53] developed a 1D mathematical model to evaluate the used in recent years. 3D models allow any type of grid geometry performance of an earth-tunnel used for space cooling. It was con- that helps to analyze the temperature variations around the pipes cluded that longer plastic pipe could perform similar to conc- and in the depth of ground. rete pipe. Various types of commercial computer modeling tools are Sethi et al. [54] worked on an agricultural greenhouse integrated available. Energy Plus and TRANSYS are used for analysis but not with an aquifer coupled cavity flow heat exchanger system and quickly for design. CFD is popular tool for 2D and 3D studies. Some developed a 1D thermal model using Cþþ then experimentally popular commercial codes are FLUENT, CFX, STAR, CD, FIDAP, validated at Chandigarh in India. It was observed that the predicted ADINA, CFD2000, PHOENICS, etc. [63,64]. and measured values were in close agreement. Breesch et al. [65] developed a 3D simulation model TRNSYS‐ Ghosal et al. [55] studied the thermal performance of two buried COMIS at Belgium, compared with experimental results and con- pipes integrated with greenhouse at Indian Institute of Technology, cluded that natural light ventilation was more effective than EAHE New Delhi, India and developed a 1D complete numerical model. It system for improving summer thermal comfort. was concluded that ground air collector was the better option than Hybrid systems enhance the overall efficiency. Khalajzadeh EAHE for heating of greenhouse in composite climate of India. et al. [66] studied thermal performance of a hybrid system of Ghosal et al. [56] tested and developed a model to investigate the ground heat exchanger (GHE) with indirect evaporative cooling thermal performance of an EAHE system coupled with a green house during summer at Tehran, Iran. Hybrid system simulated on 3D at Indian Institute of Technology, New Delhi, India. It was concluded CFD model and concluded that hybrid system was more efficient. that green house air temperature increased in winter with reducing Bansal et al. [67] validated combination of EAHE system with eva- pipe diameter and mass flow rate of air, increasing pipe length and porative cooler by using 3D multiphase CFD modeling with FLUENT depth of buried pipe up to 4 m. A fair agreement was found between 6.3, at Ajmer in India. Combined cooling effect was 7609 MJ in com- measured values and predicted values of green house air tempera- parisonto4500MJinEAHEsystem. ture in terms of statistical analysis i.e. root mean square of percentage Bansal et al. [68] developed a transient and implicit CFD based 3D deviation and correlation coefficient. model for economic study of combination of EAHE system with Shukla et al. [57] tested performance of EAHE system buried at a evaporative cooling and concluded that PBP with energy efficient depth of 1.5 m at New Delhi, India and developed quasi-steady state blower was minimum 2 years which was less than any other blowers. mathematical model. It was observed that 8.9 and 5.9 1Ctempera- Bansal et al. [69] introduced a new concept, de-rating factor for tures rise and fall during winter and summer. It was concluded from evaluating thermal performance of EAHE system through 3D transient statistical analysis that there was fair agreement between theoretical CFD and experimental analysis. A new term de-rating factor was and experimental observations. related to evaluate deterioration in thermal performance of EAHE

Table 4 Highlights of experimental studies of EAHE systems.

Parameters Important observations

Thermal performance Temperature dropped more in initial length of buried pipe. The thermal performance of EAHE system was not significantly affected by the material of the buried pipe [28–30]. COP Cooling COP was higher than heating [30–32]. Depth Normally 3 m was optimum buried depth [33]. Soil Best energy performance obtained for wet/humid soil and for the coldest climate, adopting pipe length more than 50 m [33,34]. Climate Geographical and climatic conditions determined the performance [35]. Pipe dimensions, air velocity, pipe length Reducing pipe diameter, mass flow rate of air, increasing pipe length, buried depth up to 4 m and buried pattern provided and buried depth better thermal performance [36–39]. Efficiency Combined EAHE system with PCM enhanced cooling up to 47% [40], and with HVAC, evaporative cooling solar chimney, solar photo voltaic (PV),etc., gave better result than unity [41–45]. Wall configuration, cross-ventilation, surface treatment, soil–cement mixture plaster gave better results [46–49]. 88 S.K. Soni et al. / Renewable and Sustainable Energy Reviews 47 (2015) 83–92 under various operating conditions. It was observed that de-rating ton modified AC that used the ground as a heat sink factor played key role in evaluating the performance of EAHE systems. in Thailand. The lengths of copper coil fabricated and used as It was concluded that de-rating factor mainly depended on thermal condenser coil of modified AC set at 67, 50, 40 and 30 m and conductivity of soil, duration of continuous operation and length required amount of R-134a refrigerant were 5.8, 4.6, 3.8, 2.4 kg of pipe. respectively. Whereas one refrigeration ton conventional (original) Bansal et al. [70] prepared an experimental setup at Ajmer in split-type AC had about 22 m long condenser coil and required India and investigated thermal performance regarding pipe length, quantity of R-134a refrigerant was 1.2 kg. All condenser coils were soil thermal conductivity and time period of continuous operation buried at 1 m deep in the ground. It was found that the condenser and then validated with CFD and found experimental and simu- coil of original AC could be replaced by buried coil of modified AC. lated values had a minor difference of 3.4–8%. It was concluded The COP of original AC was about 2.5. Values of COP of modified AC that initial length of buried pipe and thermal conductivity of soil, with condensing buried coils of 67, 50 and 40 m long were about play important role in the thermal performance of EAHE systems. 6.9, 5.5 and 3.3 respectively. For 30 m long condensing buried coil Important observations of EAHE systems are presented in Table 5. result was found lower than original AC that was about 2.1. The It is observed from Table 5 that there is close agreement bet- electrical power consumption of modified AC systems were found ween experimental and simulated results. EAHE system with other 11.5–15.5% lower than the original AC. Life of underground copper renewable technology options i.e. evaporative cooler give higher coil as condensing coil of modified AC was found to be over efficiency and lower PBP. 7 years. Said et al. [81] experimented viability of using ground coupled 3.2. GSHP systems studies condensers for air-conditioning systems in the Kingdom of Saudi Arabia. Closed loop vertical ground heat exchanger was used. Concept of GSHP system technology was introduced by Lord Results reflected increase in COP and reduced electrical power Kelvin in 1852 and then was modified by Robert Webber in the consumption as compared to original air cooled condenser. How- 1940s. They got commercial recognition in the 1960 s and 1970 s. ever due to high drilling cost and low electricity prices in Saudi, GSHP systems work on same principle like refrigerator system. economic viability became prominent. They absorb the heat and change the form of fluid into gaseous Pahud et al. [82] explained thermal performance of a borehole state then give away heat and return to liquid form. Now GSHP heat exchanger with a response test. For a borehole heat exchan- systems are gaining market in all over the world with United ger and a typical ground conditions, length of borehole was sized States, Canada, Switzerland, Sweden, Austria and Germany have for a heat extraction rate of 50 W/m. Test concluded that thermal emerged as the leaders [71–76]. GSHP systems had the largest resistance could be reduced by 30% when quartz sand was used energy use 68.3% and installed capacity 47.2% of the worldwide instead of bentonite. capacity and use of direct-utilization of geothermal energy. Annual Wang et al. [83] tested performance of a DX-GCHP system in energy utilization growth rate was reached to 18% around 2010 heating mode. The DX-GCHP system used R134a refrigerant and three which was fastest of all the direct-utilization of geothermal energy single U-copper tubes placed in 30 m vertical holes. The COP values of applications [77]. It emits greenhouse gases at almost zero rate the heat pump and the whole system on the basis of energy based [78]. Growth of GSHP systems technology is slower in developing heating were 3.55, 3.28 and exergetic 0.599, 0.553 respectively. countries than other conventional and non-conventional technol- Luo et al. [84] examined the performance of a GSHP system at ogies because of many factors i.e. non-standardized system design, Nuremberg in Germany. COP was calculated to be 3.9 for a typical significant capital cost and limited trained man power. winter day and energy efficiency ratio (EER) was projected to be As per environmental protection agency 1993, geothermal heat 8.0 for a typical summer day. It was concluded that cooling efficiency pump systems have ability to decrease energy consumption and was higher than heating. consequent emissions up to 44% in comparison with air source Hybrid systems are the economically feasible option for heat pump systems and upto72% as compared to electric resis- increasing performance. Ozgener et al. [85] tested the heating tance heating with standard air-conditioning equipment. It has the performance of solar-assisted GSHP system for greenhouse with a capacity to reduce cooling energy by 30–50% and heating energy 50 m vertical 0.032 m nominal diameter U-bend GHE at Izmir in by 20–40% [79]. They have underground condensing units and Turkey. The heating COP of the heat pump was about 2.13 and 2.84 hence there is no noise outside home resulting in reduction in size at the end of a cloudy day, sunny day respectively. The COP values of conventional AC. for the whole system were 5–15% lower than heating COP. Momin et al. [86] experimented geothermal air conditioning 3.2.1. Experimental studies of GSHP systems systemwithcombinationofevaporativecoolingatPuneinIndia. DX-GCHP system is efficient but economic viability plays key Experimental set up consisted of a closed loop system with continuous role. Permchart et al. [80] tested the cooling performance of a one supply of water. The evaporative coil was continuously kept wet with

Table 5 Highlights of modeling of EAHE systems.

Parameters Important observations

Distance between two layers 1.5 m distance was optimum between two layers [51]. Thermal performance The thermal performance of EAHE system was not significantly affected by the material of the buried pipe but concrete pipe was better than plastic pipe [53]. Pipe dimensions, air velocity, pipe length Reducing pipe diameter and mass flow rate of air, increasing pipe length and depth of buried pipe up to 4 m increased the and buried depth performance [56]. Efficiency EAHE system with evaporative cooler could reduce length of pipe up to 93.5% in comparison to alone EAHE system [58]. Hybrid system was more efficient than unity [66]. EAHE system with evaporative cooler had3109 MJ higher cooling effect than alone EAHE system [67] and PBP was 2 years [68]. De-rating factor De-rating factor was mainly related with thermal conductivity, duration of continuous operation, length of pipe [69]. Soil Thermal performance was directly related with initial length of buried pipe, thermal conductivity of soil [70]. S.K. Soni et al. / Renewable and Sustainable Energy Reviews 47 (2015) 83–92 89 thehelpofgunnybagsorgrasspadsandfan.ItwasfoundthatCOP Zheng et al. [94] developed finite element numerical simulation could be almost doubled from 2.2855 to 4.1415 by adding evaporative 3D model by virtue of MATLAB for analyzing the problem of cooling with earth heat exchanger system. underground vertical U-tube heat exchangers. It was obtained Kalema [87] studied the energy saving potential in small from simulation solution that larger center distance of the tube houses through integration of active solar heating and heat pumps legs i.e. 100–200 mm was more apposite to ensure greater heat in the Northern climate of Finland. It was concluded that energy flow and reduce the probability of heat short-circuit occurrence. obtainable could be approximately doubled. Peak power demand Congedo et al. [95] investigated performance for different config- could be decreased noticeably by increasing the area of the earth urations for horizontal type GSHP systems. Calculations were done pipes for extracting the earth energy whole year. with CFD code fluent for whole year for climatic conditions of the Thermal performance of system is directly linked with pipe South of Italy. It was observed that important key factors were material and modifications. Svec et al. [88] presented experimen- thermal conductivity of the ground around the heat exchanger, tal investigation of the pattern of heat flow around buried plastic velocity of the heat transfer fluid inside the tubes. Depth of fitting pipe. It was observed that significantly reduced heat flows were of the horizontal GSHP systems did not play significant role and the found when plastic pipe was used. helical heat exchanger arrangement performed best. Svec et al. [89] developed a high-efficiency copper GHE, spiral Zeng et al. [96] presented quasi-3D model for vertical GSHP shaped could be installed in a vertical borehole backfilled with sand. systems related to the fluid axial convective heat transfer and It was found that significant settlement obtained after the first thermal “short-circuiting” among U-legs. It was found that double freeze-thaw cycle owing to initial collapse of the soil structure. U-tubes in parallel offer enhanced thermal performance than Kara [90] tested the performance of a GSHP system, using those in series. Double U-tube boreholes reduced 30–90% borehole R-134a, single U-tube GHE made of polyethylene pipe with a resistance than those of single U-tube. 16 mm inside diameter, at Erzurum, Turkey. It was concluded that He et al. [97] presented 3D model for analyzing swings in fluid backfilled with sand and one to two meter thick surface plug of temperature during GSHP operation and predicted a lower heat clay could improve the COP of GSHP. Important observations of transfer rate during steady operations. Important observations of GSHP systems are presented in Table 6. GSHP systems are presented in Table 7. It is observed from Table 6 that all parameters (i.e. burial depth, It is observed from Table 7 that predicted results are higher than length of pipe, fluid velocity, wet/humid soil, pipe material, etc.) the experimental results. Vertical GSHP systems are better than the are important and directly related to the performance of a GSHP horizontal. Depth of fitting of the horizontal GSHP systems does not system. Same as EAHE system, GSHP system has higher cooling play significant role and require larger area. efficiency than heating. Hybridization with other renewable tech- nologies and appropriate modifications in the systems increase the fi overall system ef ciency and reduce the PBP. 4. Applications of GCHE systems

3.2.2. Modeling of GSHP systems GCHE systems are very valuable especially in now days due to Most of the researchers opted 2D and 3D models for their studies. energy crisis and global environmental issues. Single building sector Kujawa et al. [91] presented a 1D computational model for utilizes approximately 35% of primary energy for HVAC and heating estimating the temperature of geothermal water extracted to the water in most of the countries [98–101].GCHEsystemsaremostly earth's surface in addition to the temperature of water injected used for direct utilization of geothermal energy. Main applications are into a deposit level. GSHP systems, bathing and swimming, cooling/snow melting, indus- Magraner et al. [92] developed a 2D model then compared the trial uses, agricultural drying, aquaculture pond heating, greenhouse designed and actual energy performance of a HVAC ground coupled heating, space heating and others. Summary of geothermal energy in heat pump system. Predictions were performed with TRNSYS soft- different applications worldwide in 2005, 2010 and 2015 (projected) is ware tool and compared with experimental results. Predicted values presented in Table 8(a) and (b). Applications of geothermal energy in were 15–20% higher. It was concluded that discrepancies between 2015 (Projected) is shown as a percentage of total installed capacity experimental results and simulation outputs were mainly due to heat and total utilization in Fig. 4(a), and (b) respectively. Projected data of pump efficiency degradation for being used at partial load. the year 2015 has been worked out by the authors on the basis of Cocchi et al. [93] presented a model in order to simulate available data of 2005 and 2010 [72]. an air conditioning system integrated with GSHP system. TRNSYS GCHE systems are popularly used in industrial, commercial and 17 software was used in order to refine the sizing. It was concluded domestic applications but have some limitations also. They are that after simulation of system for the period of more than 5 years, mainly space related i.e. restricted and costly space, high tem- the optimum configuration would be 14 vertical heat exchangers perature lift, availability of other economic options i.e. waste heat in series with distance and length of 10 m and 100 m respectively. recovery systems and higher PBP. Since corporate sector always It could reduce 35% of total and operating cost. consider lower PBP options that may rule out the GCHE systems.

Table 6 Highlights of experimental studies of GSHP systems.

Parameters Important observations

Cost Power consumption could be reduced 11.5–15.5% in direct exchange geothermal system [80]. But economic viability became prominent in Saudi Arebia [81]. Thermal Thermal resistance could be reduced up to 30% by adding quartz sand instead of bentonite [82]. resistance Efficiency Cooling efficiency was higher than heating [84]. Solar-assisted GSHP system had higher efficiency [85]. COP could be almost doubled by adding evaporative cooling [86]. Energy obtainable could be approximately doubled [87]. Thermal Pipe material could affect the heat flows, significant reduced when plastic pipe was used [88,89]. Backfilled with sand and 1–2 m thick surface plug performance of clay could enhance the results [90]. 90 S.K. Soni et al. / Renewable and Sustainable Energy Reviews 47 (2015) 83–92

Table 7 Highlights of modeling of GSHP systems.

Parameters Important observations

Load Predicted values were 15–20% higher than actual performance due to heat pump efficiency degradation for being used at partial load [92]. Efficiency Significant energy could be saved through multiple vertical heat exchangers in series [93]. Center distance Larger center distance (i.e. 100–200 mm) of the tube legs was more appropriate for increasing the heat flow and reducing the probability of heat short-circuit occurrence [94]. Soil conductivity, fluid velocity, arrangement of heat Thermal conductivity of the soil, velocity of the fluid, helical heat exchanger arrangement played important role exchanger and depth of fitting but depth of fitting of the horizontal GSHP systems did not play significant role [95]. Thermal performance Double U-tubes in parallel offered improved thermal performance than series and double U-tube boreholes decreased 30–90% borehole resistance than those of single [96]. State of operations State of operations affected the heat transfer rate. Steady state of operations provided lower heat transfer rate [97].

Table 8 Applications of geothermal energy.

(a) Installed capacity (MWt)

Categories 2005 2010 2015 (Projected)

GSHP systems 15,384 35,236 80,706 Space heating 4366 5391 6657 Greenhouse heating 1404 1544 1698 Aquaculture pond heating 616 653 692 Agriculture drying 157 127 103 Industrial uses 484 533 587 Bathing and swimming 5401 6689 8284 Cooling/snow melting 371 368 365 Others (include animal husbandry) 86 41 20 Total 28,269 50,583 99,112 (b) Utilization (TJ/year)

GSHP systems 87,503 214,782 527,197 Space heating 55,256 62,984 71,793 Greenhouse heating 20,661 23,264 26,195 Aquaculture pond heating 10,976 11,521 12,093 Agriculture drying 2013 1662 1372 Industrial uses 10,868 11,746 12,695 Bathing and swimming 83,018 109,032 143,198 Cooling/snow melting 2032 2126 2224 Others (include animal husbandry) 1045 956 875 Total 273,372 438,071 797,642

Requirement of specific application and associated factors direct friendly and renewable in nature. Authors reviewed past the designing of GCHE system and selection of equipment. Tempera- researches in the form of experimental and modeling studies ture difference between geothermal resource and the process is and presented key issues and performance features of EAHE and important for selecting appropriate equipment. Higher temperature GSHP systems. It is observed that EAHE systems technology is well difference lowers the cost of equipments i.e. heat exchange device. established in all over world whereas GSHP systems technology Temperature difference and flow rate of heat transfer fluid are needs to be accepted in developing countries, though it has 10– inversely proportional to each other. Temperature difference also 20% higher saving potential than EAHE systems. In EAHE systems plays key role in selection of heat transfer fluid whereas corrosion all parameters are linked with the system's performance except and scaling related problems depend on quality of fluid. pipe material. Hence it can also be concluded that convective heat Use of appropriate hybrid system is the latest trend in the transfer performs more vital role than the conductive heat transfer application of GCHE systems. EAHE systems can be used for both in EAHE systems. In GSHP systems all parameters are coupled with heating/cooling and ventilation. Combining with other technolo- system's performance except depth of fitting of the horizontal gies like solar chimney, solar PV, PCM, HVAC, etc. boots the GSHP systems. Vertical GSHP systems are better than the hori- efficiency, shown in Fig. 5. zontal regarding performance and area. Some researchers have GSHP systems can also be used for heating and cooling. proved that combination of GCHE systems with conventional Combining with refrigeration, evaporative cooling, solar PV, etc. HVAC, solar chimney, solar PV, PCM and other renewable options provides better output, shown in Fig. 6. are better. It was observed from reviewed literature that EAHE system with PCM enhanced the cooling up to 47% and with 5. Conclusion evaporative cooler reduced the pipe length up to 93.5%. GSHP system with evaporative cooler almost doubled the COP. Now EAHE and GSHP systems are drawing attention of the research- further research works are required to make them cost effective ers and designers as they are efficient, economic, environmental and commercially viable. S.K. Soni et al. / Renewable and Sustainable Energy Reviews 47 (2015) 83–92 91

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