Performance Analysis of Slinky Horizontal Ground Heat Exchangers for a Ground Source Heat Pump System

Transcription

1 resources Article Performance Analysis of Slinky Horizontal Ground Heat Exchangers for a Ground Source Heat Pump System Md. Hasan Ali 1,2, * ID, Keishi Kariya 3 Akio Miyara 3,4 1 Graduate School of Science Engineering, Saga University, 1 Honjo-machi, Saga , Japan 2 Department of Energy Science Engineering, Khulna University of Engineering & Technology, Khulna 9203, Bangladesh 3 Department of Mechanical Engineering, Saga University, 1 Honjo-machi, Saga , Japan; kariya@me.saga-u.ac.jp (K.K.); miyara@me.saga-u.ac.jp (A.M.) 4 International Institute for Carbon-Neutral Energy Research, Kyushu University, Fukuoka-shi , Japan * Correspondence: mhakuet@yahoo.com; Tel.: Received: 26 July 2017; Accepted: 9 October 2017; Published: 13 October 2017 Abstract: This paper highlights rmal performance of reclined (parallel to ground surface) sting (perpendicular to ground surface) slinky horizontal ground heat exchangers (HGHEs) with different water mass flow rates in heating mode of continuous intermittent operations. A copper tube with an outer surface protected with low-density polyethylene was selected as tube material of ground heat exchanger. Effects on ground temperature around reclined slinky HGHE due to heat extraction effect of variation of ground temperatures on reclined HGHE performance are discussed. A higher heat exchange rate was experienced in sting HGHE than in reclined HGHE. The sting HGHE was affected by deeper ground temperature also a greater amount of backfilled s in sting HGHE (4.20 m 3 ) than reclined HGHE (1.58 m 3 ), which has higher rmal conductivity than site soil. For mass flow rate of 1 L/min with inlet water temperature 7 C, 4-day average heat extraction rates increased 45.3% 127.3%, respectively, when initial average ground temperatures at 1.5 m depth around reclined HGHE increased from 10.4 C to 11.7 C 10.4 C to 13.7 C. In case of intermittent operation, which boosted rmal performance, a short time interval of intermittent operation is better than a long time interval of intermittent operation. Furrmore, from viewpoint of power consumption by circulating pump, intermittent operation is more efficient than continuous operation. Keywords: horizontal ground heat exchanger; slinky coil; rmal performance; ground temperature; operation mode 1. Introduction The conventional sources of energy for heating cooling of a building have environmental pollution issues. To reduce emissions of carbon dioxide or greenhouse gases around world, re is a good opportunity to produce energy from sustainable sources such as solar, wind, biomass, hydro, ground that produce low or no emissions. In contrast to many or sources of energy for heating cooling, which need to be transported over long distances, geormal energy is available on-site. A ground source heat pump (GSHP) transforms this ground energy into useful energy to heat cool buildings. Even with higher initial cost, GSHPs are most efficient heating cooling technology since y use 25% to 50% less electricity [1] than or traditional heating cooling systems. In order to gain an understing of how well GSHPs function after installation, analysis of ir performance needs to be conducted [2]. Resources 2017, 6, 56; doi: /resources

2 Resources 2017, 6, 56 2 of 18 In a GSHP system, heat is extracted from or returned to ground via a closed-loop i.e., ground heat exchanger (GHE) buried in horizontal trenches or vertical boreholes. Horizontal ground heat exchangers (HGHEs) are common choice of GHEs for small buildings if re are no major limitations on l since y require a larger area in comparison with vertical borehole heat exchangers. HGHEs are usually laid in shallow trenches at a depth of 1.0 to 2.0 m from ground surface [3 6]. The performance of GSHP strongly depends on GHE performance. Therefore, for improving overall efficiency of GSHPs, heat transfer efficiency of GHE needs to be improved by adopting more advanced shapes devices. The shallow HGHEs give lower energy output than vertical GHEs; it is best to improve efficiency of HGHEs by selecting different geometries of single pipes, multiple pipes, coiled pipes. Since single multiple pipes require greatest amount of ground area if l area is limited, slinky or spiral GHEs can be placed vertically in narrow trenches or laid flat at bottom of wide trenches. These slinky or spiral GHEs may be used in order to fit more piping into a small trench area. Also, employing high rmal conductivity materials for HGHEs backfilling shallow trench by moderate s soil, required trench lengths are only 20% to 30% compared to single pipe horizontal GSHPs, but trench lengths may increase significantly for equivalent rmal performance [2]. While slinky or spiral GHE reduces amount of l used, it requires more pipes, which results in additional costs. Therefore, slinky ground heat exchangers are subject of many studies that are both experimental as well as numerical. The heat transfer between GHE adjoining ground depends strongly on ground type moisture gradient [7], rmal performance of GHEs is affected by change of ground temperature [7,8]. Ground temperature is a function of soil rmal properties such as rmal diffusivity, rmal conductivity heat capacity [9]. These properties need to be known to predict rmal behavior of GHEs. Many researchers concluded that rmal conductivity of soil, velocity of heat transfer fluid [10 12] pipe rmal conductivity [10] are key factors for rmal performance of ground heat exchanger. Tarnawski et al. [11] mentioned that heat transfer process of a GHE depends on pipe diameter, as well as on density specific heat capacity of heat transfer fluid. Consequently, more accurate soil data will allow designer to minimize safety factor reduce trench length of HGHE installation. Although numerous analytical numerical models have been developed for rmal analyses of slinky HGHEs, only a few experimental analyses of slinky HGHEs have been conducted. Most numerical studies assumed that soil rmal hydraulic properties are constant [6,13,14]. Wu et al. [15] Congedo et al. [12] numerically simulated performance analysis of different slinky HGHEs for GSHPs. Fujii et al. [16] simulated performance of slinky HGHEs for optimum design. Because of complexity of slinky heat exchanger configurations, Demir et al. [17] calculated heat transfer through a horizontal parallel pipe ground heat exchanger using a numerical method. Selamat et al. [18] numerically investigated HGHE operation in different configurations to predict outlet fluid temperature heat exchange rate. Chong et al. [19] presented rmal performance of slinky HGHE with various loop pitch, loop diameter, soil rmal properties in continuous intermittent operation by numerical simulation. Their results indicate that system parameters have a significant effect on rmal performance of system. A real scale numerical simulation of present experimental set-up was done by Selamat et al. [6] to analyze optimize rmal performance of different layouts of slinky HGHEs. Thermal performance of slinky heat exchangers for GSHP systems was investigated experimentally numerically by Wu et al. [13] for UK climate. They showed that rmal performance of slinky heat exchangers decreased with running time. Adamovsky et al. [5] experimentally compared linear slinky HGHEs in terms of soil temperature, heat flows, energy transferred from soil massif, y determined energy recovery capabilities of ground massif during stagnation (off) period. Also, due to lack of information on heat exchange capacity long-term performance of slinky coils, Fujii et al. [20] experimentally performed long-term tests on two types of slinky coil HGHEs compared ir results. Esen Yuksel [21] experimentally investigated greenhouse heating

3 Resources 2017, 6, 56 3 of 18 with slinky HGHEs. To evaluate optimal parameters of GHE, effect of mass flow rate, length, buried depth inlet temperature of water were examined experimentally analytically with different horizontal configurations [8]. However, most studies on GHE were carried out for continuous operation, some researchers [19,22 24] also investigated performance of GHE in intermittent operations. The results indicated that intermittent operations are successful to improve rmal performance of GHE compared to continuous operations. In present work, experimental investigations have been performed to analyze performance of slinky HGHEs in heating mode of continuous intermittent operations. In order to compare rmal performance of slinky HGHEs, GHEs were installed at a depth of 1.5 m in ground with two orientations: reclined (parallel to ground surface) sting (perpendicular to ground surface). Water was considered as working fluid. Thermocouples were installed in different loops of reclined GHE at 0.5 m, 1.0 m 1.5 m depths to monitor analyse ground temperature behavior with operation time. In addition, to underst undisturbed ground temperature distribution, rmocouples were placed at different depth positions up to 10 m depth between two orientations (reclined, sting) of GHEs. At same time, atmospheric air temperature was also measured. 2. Description of Experimental Set-Up 2.1. Material Selection Ground Soil Characteristics High-density polyethylene (HDPE) tube is a recommended choice in terms of performance durability for GHEs. However, copper tubing has been successfully used in some applications since copper tubes have a very high rmal conductivity. Regardless of high rmal conductivity, copper tubes do not have durability corrosion resistance of HDPE [2]. Hence, copper tubing has to be protected from corrosion. Low density polyethylene (LDPE) has similar properties to HDPE, but LDPE is easy to film wrap, which can be used as a surface protection. Furrmore, analysis [25] shows that effect of different tube materials is more significant in slinky configuration of GHE. Therefore, instead of HDPE tubes, which are generally used for horizontal ground heat exchanger, copper tubes protected with a thin coating of LDPE were selected as tubing of slinky HGHEs in present study. The present research was conducted at Saga University, Saga city, Japan. The ground sample in Fukudomi area of Saga city consists of clay from 0 to 15 m in depth, water content of 30% to 150% varies with depth [26]. The rmo-physical properties of GHE tube materials ground soil are listed in Table 1. Table 1. Tube sizing rmo-physical properties of materials. Material Inner Diameter (mm) Outer Diameter (mm) Density (kg/m 3 ) Specific Heat (J/(kg K)) Thermal Conductivity (W/(m K)) Copper (inner) LDPE (outer) Ground: clay [27] Details of Experimental Set-Up Experimental measurements were conducted in Saga University, Japan, for slinky HGHEs in two orientations: reclined (parallel to ground surface) sting (perpendicular to ground surface). A schematic diagram of slinky HGHE system is shown in Figure 1. The reclined sting slinky HGHEs were installed 2.0 m apart from each or. The copper tube with its outer surface coated with LDPE was considered as heat exchanger material. The detailed dimensions of LDPE-coated copper tube is shown in Figure 2a. The loop diameter, length of trench number of loop for both GHEs are 1.0 m, 7.0 m 7, respectively. Each GHE consists of 39.5 m in tube length. The reclined GHE was laid in ground at a 1.5 m depth 1.0 m wide trench. On or h, centers of sting GHE loops are located at 1.5 m depth 0.5 m wide trench in ground. Figure 2b shows photograph of installation of both GHEs. After placing GHEs inside

4 Resources 2017, 6, 56 4 of 18 Resources 2017, 6, 56 4 of 18 trenches, GHEs were covered with typical Japanese s; water was sprayed on s to reduce s voidto space reduce hence void space rmal hence resistance rmal aroundresistance GHEs. around The remaining GHEs. upper The remaining parts of trenches upper were parts of n backfilled trenches were withn site backfilled soil compressed with site soil by a power compressed shovel. by a power shovel. Pure Resources Pure water 2017, water was 6, 56 was considered considered as as heat carrier heat liquid carrier flowing liquid through flowing through GHEs. TheGHEs. experimental 4 of 18 The setup experimental also included setup a water also included bath (consists a water ofbath pump, (consists heater of & pump, cooler), heater flow& controller, cooler), flow mixing controller, chamber etc. mixing s The water chamber to reduce batc. maintains The void water space a constant bath hence maintains rmal temperature a constant resistance watertemperature around supply to water GHEs. system. supply The remaining Theto flow system. controller measures The upper flow controller parts of mass flow measures trenches were rate also mass n backfilled controls flow rate with site flowalso soil rate. controls compressed The inlet flow by outlet rate. a power The water inlet shovel. temperatures outlet Pure water was considered as heat carrier liquid flowing through GHEs. The of each water GHE temperatures were measured of each using GHE Pt100, were measured which were using installed Pt100, close which to were ground installed surface. close to In order experimental setup also included a water bath (consists of pump, heater & cooler), flow controller, ground surface. In order to obtain a uniform temperature inside tube, mixing chambers were to obtain mixing a uniform chamber temperature etc. The water inside bath maintains tube, a constant mixing temperature chambers were water installed supply to before system. each Pt100. installed before each Pt100. To monitor undisturbed ground temperature distributions, a monitoring To monitor The flow undisturbed controller measures ground temperature mass flow rate distributions, also controls a monitoring flow rate. hole The inlet middle outlet of hole two GHEs water in was temperatures middle of dug in of two ground each GHEs was towere dug install measured in ground T-type rmocouples using Pt100, to install which T-type at were rmocouples various installed depthclose positions to at various (0.1 m, depth ground positions surface. (0.1 In m, order 0.5 m, to 1.0 obtain m, a 1.5 uniform m, 2.5 temperature m, 5.0 m, 7.5 inside m 10.0 tube, m mixing depth) chambers up to 10.0 were m depth. 0.5 m, 1.0 m, 1.5 m, 2.5 m, 5.0 m, 7.5 m 10.0 m depth) up to 10.0 m depth. After installation of After installed installation before each of Pt100. rmocouples, To monitor undisturbed hole was ground refilled temperature by soil. For distributions, analysis a of monitoring trench ground rmocouples, hole was refilled by soil. For analysis of trench ground temperature variations, temperature hole in variations, middle of T-type two GHEs rmocouples was dug in were ground placed to install T-type 1st, 4th rmocouples 7th loops at various at 0.5 m, 1.0 T-type m depth rmocouples 1.5 positions m depth, (0.1 respectively, were m, 0.5 placed m, 1.0 of m, in1.5 reclined m, 1st, 2.5 m, 4th GHE 5.0 m, (shown 7.57th m loops in Figure 10.0 at m 0.5 depth) 1). m, At up 1.0to same m10.0 m time, depth. 1.5 a m digital depth, respectively, rmometer After of installation was reclined installed of rmocouples, GHE about (shown 1.0 m above inhole Figure was ground refilled 1). At by surface soil. same For to time, record analysis a ambient digital of trench rmometer temperature. ground was It installed is also temperature about possible 1.0variations, to mmeasure abovet-type ground rmocouples pressure surface inside were to record placed tube in by ambient using 1st, temperature. 4th a pressure 7th loops sensor. It at is0.5 also The m, possible pressure 1.0 to measure m difference 1.5 between pressure m depth, inside respectively, inlet tube of outlet byreclined of using GHE GHE a pressure (shown is measured sensor. in Figure by a differential The 1). At pressure same time, pressure difference a digital sensor. between Data rmometer was installed about 1.0 m above ground surface to record ambient temperature. It from inletdifferent outlet measuring of GHE points is measured are recorded by by a differential a Agilent 34972A pressuredata sensor. logger Data from stored different in a is also possible to measure pressure inside tube by using a pressure sensor. The pressure measuring centrally points located are PC. recorded by a Agilent 34972A data logger stored in a centrally located PC. difference between inlet outlet of GHE is measured by a differential pressure sensor. Data from different measuring points are recorded by a Agilent 34972A data logger stored in a centrally located PC. Figure Figure Schematic Schematic of of of ground groundheat heat exchangers. Figure 1. Schematic of experimental set-up of slinky horizontal ground exchangers. exchangers. (a) (b) (a) (b) Figure 2. (a) Outer surface LDPE-coated copper tube; (b) Installation of slinky-coil ground heat exchangers. Figure 2. (a) Outer surface LDPE-coated copper tube; (b) Installation of slinky-coil ground Figure 2. (a) Outer surface LDPE-coated copper tube; (b) Installation of slinky-coil ground heat exchangers. heat exchangers.

5 Resources 2017, 6, 56 5 of Experimental Data Analysis To investigate performance of GHEs, experimental heat exchange rate is calculated by following equation: Q = ṁc p (T o T i ) (1) where ṁ is flow rate of water (kg/s), C p is specific heat of water (J/(kg K)), T i T o are inlet outlet temperatures of water, respectively. The heat exchange rate per unit tube length of GHE is simplified by following equation: Q = Q L (2) where L in total tube length of GHE. The overall heat transfer coefficient U is defined by relation: Q = UA T LM (3) where Q is heat exchange rate (W), U is overall heat transfer coefficient (W/(m 2 C)), A is heat transfer area (m 2 ) of GHE, T LM is logarithmic mean temperature difference ( C). To calculate overall heat transfer coefficient from Equation (3) for GHE, T LM proposed by Naili et al. [8] was adopted as: T T LM = ( ) Tg (4) T Ln i T g T o where T is temperature difference between inlet outlet of GHE ( C), T g is ground soil temperature ( C) Uncertainty Analysis In present study, temperatures of water, ground soil ambient air were measured by Pt100, T-type rmocouple digital rmometer, respectively. Water mass flow rates were measured by a flow controller. The accuracy of measured data results are important issues for reliability of data results. Therefore, uncertainty is considered for experimental measurements. The experimental uncertainties in this study are estimated according Holman (pp , [28]). Table 2 summarizes technical specifications experimental uncertainties. Table 2. Total maximum uncertainty of measured calculated parameters. Item The water temperatures at inlet outlet of sting reclined GHEs Soil temperature in ground Ambient air temperature Mass flow rate of water in sting reclined GHE Name Technical Specifications of Measured Equipment Pt100 Temperature range: 200 to 600 C Sensor type: Class A, 4 wire T-type rmocouple Temperature range: 200 to 200 C Lutron SD Card Data Logger Model: HT-3007SD Range: 0 50 C Resolution: 0.1 C TOFCO flow meter Model: FLC-605 Range: L/min Uncertainty ±0.15 C ±0.5 C ±0.8 C ±5% Heat exchange rate (W/m) - ±5.8% Overall heat transfer coefficient, UA-value (W/ C) - ±5.5%

6 Resources 2017, 6, 56 6 of 18 If u(x i ) is uncertaninty of independent N set of mesurements in any result represented by R = R(x 1, x 2,..., x N ), n uncertainity of result can be calcutated by: 3. Results Discussion u R = [ N i=1 ( ) ] R 2 1/2 u 2 (x i ) (5) x i Based on experiments of HGHEs in winter season, water temperatures in inlet outlet, water flow rates trench loop ground temperatures of reclined orientation at 0.5 m, 1.0 m 1.5 m depth were measured for different heating mode. The undisturbed temperatures at various depth positions of ground soil up to 10.0 m depth were also measured. All data were recorded in 5-minute intervals by using a data logger connected to a computer. In addition, ambient temperature was also measured using a digital rmometer. The experimental conditions in continuous operations are shown in Table 3. All experiments were performed without prior operation of system. Table 3. Experimental conditions in continuous operations. Case Operation Period Inlet Water Temperature ( C) Flow Rate (L/min) Average Initial Ground Temperature around GHE at 1.5 m Depth ( C) Reynolds Number February March March April Daily Average Ground Thermal Behavior (Undisturbed Ground Temperature) The temperature distributions in ground are very important for performance [21] sizing of ground heat exchanger [3]. For optimum performance of GHEs, it is necessary to know minimum maximum ground temperature to decide at which depth GHE should be installed. Since ambient climatic conditions affect temperature profile below ground surface, ambient temperature also needs to be considered when designing a GHE [3]. Consequently, analysis of ground temperature distribution as well as ambient air temperature is required for implementation of GHEs. The ground temperature distribution from 1 February 2016 to 31 March 2017 at various depths up to 10.0 m is shown in Figure 3. This figure also includes measured ambient temperature. Figure 3 shows that ground temperature very close to ground surface (0.1 m depth) has similar characteristic to ambient temperature fluctuates strongly irregularly caused by change of ambient temperature. Ground temperature fluctuations decrease with increasing ground depth. In zone 1.0 m to 2.5 m, ground temperature variation depends mainly on seasonal wear conditions. Below a certain depth (about 5 m), ground temperature remains relatively constant, about 18 C at 7.5 to 10.0 m depth, for example. But at depth 1.5 m where GHEs were installed in present study, ground temperature changes seasonally, which is an important parameter for present GHE performance.

7 Resources 2017, 6, 56 7 of 18 Resources 2017, 6, 56 7 of 18 Figure Figure 3. Measured 3. Measured daily daily average average ground ground temperature ambient temperature from 1 February 2016 to March to March Comparison of Performance of Sting Reclined Slinky Horizontal Heat Exchangers 3.2. Comparison of Performance of Sting Reclined Slinky Horizontal Heat Exchangers Inlet Outlet Temperatures of Circulating Water Inlet Outlet Temperatures of Circulating Water The efficiency of a GSHP system depends on temperature difference between inlet The outlet efficiency of circulating of a GSHP fluid through system depends GHE. Therefore, on temperature it is desirable difference to have a higher between temperature inlet outletdifference of circulating between fluid through inlet outlet GHE. for Therefore, higher performance it is desirable of GHEs. to have The a higher inlet temperature outlet difference temperatures between for case inlet 1 case 2 outlet are shown for in higher Figure performance 4. In case 1 of case GHEs. 2, undisturbed The inletground outlet temperatures for between case m case to m are depth shown were inalmost Figure same 4. (Figure In case3), 1with little casechange. 2, After undisturbed ground start temperatures of experiment, between inlet 1.0 mwater to 2.5temperatures m depth were should almost quickly same approach (Figure 3), water with little bath change. set After temperature start of (7 C). experiment, At beginning inlet of water experiment, temperatures outlet should temperatures quickly approach of water are high water for bath both cases 1 2 gradually decrease. The outlet temperatures approach a nearly stable value set temperature (7 C). At beginning of experiment, outlet temperatures of water are high after about 12 h of operation. Then, re are no significant changes in outlet temperatures for both GHEs, for both cases 1 2 gradually decrease. The outlet temperatures approach a nearly stable value experiments were continued until 4 days of continuous operation to observe steady state after performances about 12 h of of operation. GHEs. Then, re are no significant changes in outlet temperatures for both GHEs, For experiments same operating werecondition, continued until sting 4 days GHE of continuous showed a greater operation temperature to observe difference steady state between performances outlet of GHEs. inlet than reclined oriented GHE for both operation periods. After 4 days For of operation, same operating temperature condition, differences between sting GHE outlet showed inlet a greater of sting temperature reclined difference between GHEs are outlet 0.84 C inlet 0.76 than C, respectively, reclined for oriented case 1. For GHE case for 2, both temperature operationdifferences periods. between After 4 days of operation, outlet temperature inlet of sting differences reclined between GHEs are outlet 1.48 C inlet 1.28 of C, respectively. sting A higher reclined GHEs temperature are 0.84 C difference 0.76 between C, respectively, outlet for case inlet 1. leads For case to a 2, higher temperature heat extraction differences rate. In Figure between 4, it is also seen that outlet water temperature is higher for 1 L/min than 2 L/min. The lower mass outlet inlet of sting reclined GHEs are 1.48 C 1.28 C, respectively. A higher flow rate reduces velocity of water inside tube. Consequently, convective heat transfer temperature difference between outlet inlet leads to a higher heat extraction rate. In Figure 4, coefficient inside tube as well as overall heat transfer coefficient decreased. Consequently, it is also heat seen extraction that will outlet be lower water for lower temperature mass flow israte. higher The for effects 1 L/min of lower than heat 2 L/min. extraction The reduce lower mass flow degradation rate reduces of ground velocity soil temperature of water inside around tube. GHE. Consequently, The higher change in convective water temperature heat transfer is coefficient affected inside by this low tube degraded as well ground as soil overall temperature. heat transfer As a result, coefficient outlet decreased. water temperature Consequently, is heat extraction will be lower for lower mass flow rate. The effects of lower heat extraction reduce degradation of ground soil temperature around GHE. The higher change in water temperature is affected by this low degraded ground soil temperature. As a result, outlet water temperature is

8 Resources 2017, 6, 56 8 of 18 Resources 2017, 6, 56 8 of 18 higher with a lower mass flow rate. For instance, outlet temperatures of sting GHE with higher with a lower mass flow rate. For instance, mass flow rates 1 L/min 2 L/min are 8.57 outlet C 7.57 temperatures of sting GHE with mass flow rates 1 L/min 2 L/min are 8.57 C 7.57 C, C, respectively, respectively, after after 4 days 4 days of operation. of operation. Figure 4. Time variation of inlet outlet water temperature ambient temperature in heating Figure 4. Time variation of inlet outlet water temperature ambient temperature in heating mode during cases 1 2. mode during cases 1 2. The set temperatures of supply water bath were 7.0 C for both GHEs, but inlet temperatures The fluctuate set temperatures in a similar fashion of as supply ambient watertemperature. bath were 7.0 The water C for both baths GHEs, were inside but inlet laboratory temperatures fluctuate room, in a similar GHE fashion inlet as outlet ambient temperature temperature. measuring The water points baths were were outside inside room. laboratory The room, distance between GHE inlet water outlet bath temperature measuring measuring point of points water were temperature outside was room. about The 10.0 distance m. All connecting pipes between water bath GHEs were insulated, but it is difficult to ensure 100% between water bath measuring point of water temperature was about 10.0 m. All connecting perfect insulation. Therefore, some heat exchanged between connecting pipes surroundings. pipes between water bath GHEs were insulated, but it is difficult to ensure 100% perfect insulation. From Figure 4, it is shown that inlet temperature fluctuated similarly as ambient temperature, Therefore, as well some as heat outlet exchanged temperature. between connecting pipes surroundings. From Figure 4, it is shown that inlet temperature fluctuated similarly as ambient temperature, as well as outlet temperature. Heat Exchange Rate Figure 5 shows heat exchange rates per unit tube length of sting reclined GHEs Heat Exchange Rate for cases 1 2. At beginning of experiments, heat exchange rates per unit tube length Figure of sting 5 shows reclined heatghe exchange are high rates for both per unit cases. tube This length happens of due sting to higher temperature reclined GHEs for cases difference 1 between 2. At ground beginning soil around of experiments, GHE circulating heatwater exchange at rates beginning. per unit After tube about length of sting 12 h of operation, reclined GHE heat exchange are highrate for both declines cases. slightly This happens tends to due be to constant. higher The reason temperature is because with increases in operation time, heat extraction from vicinity of GHE occurred. difference between ground soil around GHE circulating water at beginning. After about As a result, surrounding ground soil rmal energy degraded decreased temperature 12 h of operation, heat exchange rate declines slightly tends to be constant. The reason is difference between soil around GHEs circulating water inside GHE. because with There increases is a little in change operation in undisturbed time, ground heat extraction soil temperatures from during vicinity cases of 1 GHE 2. occurred. For As a more result, comparable surrounding of cases 1 ground 2, soil overall rmal heat transfer energycoefficient degraded(ua-value) decreased has been calculated temperature difference using between Equation (3). soil The around 12 to 96 h average GHEsheat exchange circulating rates water UA-value inside of cases GHE. 1 2 are There presented is a in little Table change 4 for in when undisturbed system had ground almost soil reached temperatures equilibrium during (12 cases h after 1 starting 2. For more comparable operation). of cases It can 1 be seen 2, that overall heat heat exchange transferrate coefficient (UA-value) overall heat has transfer beencoefficient calculatedof using Equation sting (3). GHE The 12 dominate to 96 h average heat exchange heat exchange rate rates overall UA-value heat transfer ofcoefficient cases 1 of 2 areclined presented in Table 4 for when system had almost reached equilibrium (12 h after starting operation). It can be seen that heat exchange rate overall heat transfer coefficient of sting GHE dominate heat exchange rate overall heat transfer coefficient of reclined GHE in both

9 Resources 2017, 6, 56 9 of 18 Resources 2017, 6, 56 9 of 18 cases. For example, average heat exchange rate overall heat transfer coefficient (from 12 to 96 h operation) GHE in of both cases. sting For example, GHE are 3.17 average W/m heat exchange W/ rate C, respectively, overall heat transfer during coefficient case 1. On (from 12 to 96 h operation) of sting GHE are 3.17 W/m W/ C, respectively, during case or h, for reclined GHE, average heat exchange rate overall heat transfer coefficient 1. On or h, for are 2.66 W/m W/ reclined GHE, average heat exchange rate overall heat transfer C, respectively, for case 1. The average heat exchange rate of sting coefficient are 2.66 W/m W/ C, respectively, for case 1. The average heat exchange rate of GHE is sting 19.1% higher GHE is than 19.1% that higher of than reclined that of GHE reclined duringghe caseduring 1. During case 1. case During 2, case average 2, heat exchange average rate heat ofexchange sting rate of GHE sting is 16.0% GHE higher is 16.0% than that higher ofthan reclined that of GHE. reclined GHE. Figure Figure 5. Comparison 5. Comparison of of heat heat exchange rate rate between sting reclined oriented oriented GHEs GHEs for cases for cases Table 4. Average heat exchange rate UA-value from 12 to 96 h of operation. Table 4. Average heat exchange rate UA-value from 12 to 96 h of operation. Flow Rate Average Heat Exchange Rate (W/m) Average UA-Value (W/ C) Operation (L/min) Sting Average Heat Exchange Reclined Rate (W/m) Sting Average UA-Value Reclined (W/ C) Operation Flow Rate (L/min) Case Sting 2.66 Reclined Sting Reclined Case Case Case From ground temperature distribution in Figure 3, it is seen that ground temperatures at 1.0 m deep below remained almost constant for a short period of time, 4 5 days, for example. However, From ground ground temperature distribution in deeper regions in Figure is higher 3, than it is seen that of that shallower groundregions temperatures in at 1.0 mwinter deepseason. below As remained sting GHE almost was constant positioned for between a short 1.0 period m to 2.0 of m time, vertical 4 5 depth, days, half for of example. However, sting GHE ground lay underneath temperature in depth deeper of regions reclined is GHE. higher In than addition, that from of shallower Figure 3, regions it can be in winter seen season. that As deeper half sting of sting GHE was GHE positioned was affected between by higher 1.0 temperature m to 2.0 mthan vertical shallower depth, half of half. sting Hence, GHE sting lay underneath slinky HGHE was depth affected of by reclined higher ground GHE. In temperature addition, in from deeper Figure 3, it can be region. seenthis thatis one deeper reason why halfa of sting sting slinky HGHE was has a affected higher heat byexchange higher temperature rate. Also, in than present study, after placing slinky coils inside trenches, for sting GHE, 1.2 m 0.5 m shallower half. Hence, sting slinky HGHE was affected by higher ground temperature 7 m = 4.20 m 3 trench volume was backfilled by typical Japanese s. On or h, for in deeper region. This is one reason why a sting slinky HGHE has a higher heat exchange reclined GHE, m 1 m 7 m = 1.58 m 3 volume of trench was backfilled by same type of rate. s. Also, The in remaining present upper study, parts after of placing trenches slinky of both coils GHEs inside were backfilled trenches, by site for soil. Hamdhan sting GHE, 1.2 m 0.5 Clarke m [29] 7 m confirmed = 4.20 mthat 3 trench rmal volume conductivity was backfilled varies with bymaterial typicalcondition Japanese s. soil s On rmal or h, conductivity for reclined was GHE, significantly m influenced 1 m by 7its m saturation = 1.58 m 3 volume dry density. of trench The rmal was backfilled conductivity by sameof type clay of s. with The20% remaining water content upper is parts 1.17 of1.76, trenches at a water of both content GHEs of 40%, were backfilled conductivity by site soil. values Hamdhan are 1.59 Clarke 2.18 W/(m K), [29] confirmed respectively that [30]. rmal Since conductivity backfill volume varies of s withis material higher for condition soil s sting rmal GHE (4.20 conductivity m 3 ) than for was reclined significantly GHE (1.58 influenced m 3 ) by rmal its saturation conductivity dry of density. s is The rmal higher conductivity than that of of soil, clay this may s be anor with 20% reason water for content higher is heat 1.17 exchange 1.76, rate of at a water sting content GHE than reclined GHE. The installation of slinky HGHEs consists of only piping excavation of 40%, conductivity values are W/(m K), respectively [30]. Since backfill volume of s is higher for sting GHE (4.20 m 3 ) than for reclined GHE (1.58 m 3 ) rmal conductivity of s is higher than that of soil, this may be anor reason for higher heat exchange

10 Resources 2017, 6, of 18 rate of sting GHE than reclined GHE. The installation of slinky HGHEs consists of only piping excavation work. Though piping of both GHEs is same, excavation work differs, as = 10.5 m 3 is required for reclined orientation = 7.0 m 3 for sting orientation. In contrast to excavation work, sting GHE is cost effective. Figure 5 also shows that heat exchange rate for case 1 is greater than that for case 2. This is obviously due to increasing mass flow rate, which increases velocity of water inside tube. As a result, both convective heat transfer coefficient overall heat transfer coefficient UA-value increased. Consequently, with higher mass flow rate, heat exchange rate is also higher. For sting GHE, average heat exchange rate of case 1 is 21.5% higher than that of case 2. The average heat exchange rate for reclined GHE is 18.2% higher in case 1 than in case 2. Even though ground temperature changes from case 1 to case 2, this result is comparable because changes in average ground temperature are small (about 0.25 C), GHE heat exchange rate is always dominated by low rmal conductivity of ground soil Effect of Heat Extraction on Ground Temperature around GHE with Reclined Slinky HGHE To underst how ground temperature around GHE is changed with heat exchanged by GHE, reclined slinky GHE is considered for analysis. The ground temperature distributions of loop 1, 4 7 around reclined GHE at 0.5 m, 1.0 m 1.5 m depth (T 1, T 4 T 7 ) for cases 1 2 are shown in Figure 6. Figure 6 also includes undisturbed ground temperature T g at 0.5 m, 1.0 m 1.5 m depth as well as ambient temperature T a. Since circulating water absorbs heat in heating mode of operation from surrounding ground, surrounding ground temperatures around GHE gradually decrease with operation time. Also, when heat exchange fluid enters GHE, re is short-term, strong heat extraction in leading loops rar than subsequent loops. As a result, ground temperatures around leading loops decrease quicker than or subsequent loops. From Figure 6a c, initial trench ground soil temperatures around GHE at 0.5 m, 1.0 m, 1.5 m depths are different in 1st, 4th 7th loops. The reason is that se three loops are located in different horizontal positions (shown in Figure 1), energy potential capacities of loops 1, 4 7 are different. Also, T 1, T 4 T 7 at 0.5 m, 1.0 m 1.5 m depth are different from undisturbed ground soil temperatures of T g at 0.5 m, 1.0 m 1.5 m depth. This might be because undisturbed temperatures were measured inside original soil, but trench was first backfilled by s site soil. Therefore, rmo-physical properties compactness of undisturbed ground soil trench ground soil are obviously different. The ground surface is affected by grasses on surface by building shading beside experimental location. Subsurface variables under ground at different locations also play an important role in variation of se temperatures. It is also seen that T 1 T 7 at 0.5 m 1.0 m depths for case 2 are higher than those of T g at 0.5 m 1.0 m depths. This is due to bare surface may be due to lower compactness of trench ground soil than undisturbed ground soil. The average ambient temperature is about 18.2 C, which is higher than ground soil temperature. As a result, T 1 T 7 are more sensitive to ambient temperature. Despite bare surface lower compactness of trench ground soil, T 4 is lower than T g because T 4 is affected by shading from a tree. From Figure 6a, it can be observed that changing tendency of trench ground temperatures T 1, T 4 T 7 at 0.5 m depth are almost similar to undisturbed ground temperature T g at 0.5 m depth to ambient temperature T a for both cases 1 2. The change of all of se trench ground temperatures slightly lagging behind changes in ambient temperature is due to rmal resistance of ground material. T 1 at 0.5 m depth for case 1 remains almost similar to initial trench ground temperature until about 40 h of operation. After that, it starts to decrease is greater than decreases in T g at 0.5 m depth. However, from Figure 6b, it is seen that T 1 at 1.0 m depth for whole period of case 1 remains almost constant. Therefore, it can be said that changes in 1st loop ground temperature T 1 at 0.5 m depth occurred only due to change in ambient temperature surrounding subsurface soil temperature. Similarly, increase in T 1 at 0.5 m depth during

11 Resources 2017, 6, of 18 case 2 occurs only due to increase of ambient temperature surrounding subsurface ground Resources 2017, 6, of 18 soil temperature. Also, it can be seen from Figure 6a that change in trench ground temperatures at 0.5m depth depthin in 4th 4thloop loop 7th 7thloop loopoccurs occurs only only due due to to changes changes in ambient in ambient temperature temperature surrounding surrounding subsurface subsurface ground ground soil soil temperature during both both cases cases 1 1 2, but 2, butimpact impact is different is different in in different loops. (a) (b) (c) Figure 6. Variation of ground temperature around reclined GHE with operation time in different Figure 6. Variation of ground temperature around reclined GHE with operation time in different loops undisturbed ground temperature during cases 1 2: (a) at 0.5 m depth ambient; (b) loops undisturbed ground temperature during cases 1 2: (a) at 0.5 m depth ambient; at 1.0 m depth; (c) at 1.5 m depth. (b) at 1.0 m depth; (c) at 1.5 m depth. In Figure 6b for case 1, temperatures T,T,T at 1.0 m depth remain constant do In not Figure decrease 6bdue for case to heat 1, extraction temperatures from surrounding T 1, T 4, T 7 soil by Tcirculating g 1.0 m water depththrough remain constant GHE. do not However, decrease for due case to2, heat trench extraction ground from temperatures surrounding T,T soil by T circulating 1.0 m depth water gradually through increase GHE. with running time display almost similar trends of increases in undisturbed ground temperature However, for case 2, trench ground temperatures T 1, T 4 T 7 at 1.0 m depth gradually increase at 1.0 m depth. In contrast to variation in trench loop ground temperatures at 1.0 m depth with running time display almost similar trends of increases in undisturbed ground temperature during cases 1 2, up to 1.0 m depth, which is 0.5 m above reclined GHE, trench ground T g at 1.0 m depth. In contrast to variation in trench loop ground temperatures at 1.0 m depth temperature is not affected by GHE heat extraction, or effect is much smaller than effect of during ambient casestemperature 1 2, up to 1.0 surrounding m depth, which subsurface is 0.5 variables. m above reclined GHE, trench ground temperature From isfigure not affected 6c, it is seen by GHE that heat trench extraction, ground or temperatures effect is T much T smaller at 1.5 m depth than start effect to of ambient decrease temperature from Tbeginning a surrounding of experiments, subsurfacebut variables. T remains constant from start of From experiments Figurefor 6c, both it is cases seen 1 that2. This trench is because ground measuring temperatures points Tof 1 T T 7 Tat at m depth start to decrease were located fromin beginning ground just of outside experiments, of GHE coil but surface T 4 remains as shown constant in Figure from 1. On start or of experiments h T for at 1.5 both m depth cases was 1 measured 2. This is at because center of measuring 4th loop. points Since ofloop T diameter is 1.0 m, 1 T 7 at 1.5 m depth T at 1.5 m depth is located 0.5 m lateral distance from coil surface of GHE. As a result, it takes were located in ground just outside of GHE coil surface as shown in Figure 1. On or time for rmal energy to be extracted by GHE from center of 4th loop. T decreases slowly h T compared 4 at 1.5 m depth was measured at center of 4th loop. Since loop diameter is 1.0 m, to T at 1.5 m depth. Decreasing rates of T T gradually decrease with operation T 4 at 1.5 m depth is located 0.5 m lateral distance from coil surface of GHE. As a result, it time. This fact indicates that heat is exchanging from surrounding ground soil to circulating takeswater. time for Thus, rmal ground energy soil temperature to be extracted degrades by with GHE operation from time, center of efficiency 4th loop. of T 7 decreases GHE slowly compared to T 1 at 1.5 m depth. Decreasing rates of T 1 T 7 gradually decrease with operation

12 Resources 2017, 6, of 18 time. This fact indicates that heat is exchanging from surrounding ground soil to circulating water. Thus, ground soil temperature degrades with operation time, efficiency of GHE decreases. On or h, T 4 at 1.5 m depth starts to decrease at about h after start of Resources experiments 2017, 6, 56 for cases 1 2, respectively. In case 1, T 4 starts to decrease earlier than 12 of 18 case 2 because a higher mass flow rate through GHE causes more heat extraction. For instance, drop decreases. On or h, T in temperatures T 1, T 4, T 7 is 0.75 at 1.5 m depth C, 0.32 starts to decrease C 0.36 at about h after start C, respectively, during case 1. On or of experiments for cases 1 2, respectively. h, for case 2, drop in T 1, T 4, T 7 is 0.89 In case 1, C, 0.32 T starts to decrease C 0.42 earlier than case 2 because a higher mass flow rate through GHE causes more heat extraction. C, respectively. For instance, Though drop massin flow temperatures rates for cases T,T, 1 T 2is are 0.75 only C, L/min C C, L/min, respectively, if mass during flow case rate 1. On would or increase, n re h, is for a case possibility 2, drop that in T temperature,t, T is 0.89 of C, ground 0.32 C around 0.42 C, GHE respectively. is affected Though by GHE heat extraction mass flow at longer rates for distances cases 1 from2 are only GHE2 L/min coils. 1 L/min, if mass flow rate would increase, The n significant re is a possibility findingthat from Figure temperature 6 is that of ground slinky around HGHE GHE can be is affected installed by GHE withheat variable intervals extraction of loop at longer pitches distances or by reducing from GHE coils. loop diameter from starting loop to end loop because The impact significant of temperature finding from degradation Figure 6 is that decreases slinky from HGHE starting can be installed loop to subsequent with variable loops. intervals of loop pitches or by reducing loop diameter from starting loop to end loop This has potential to reduce excavation work also installation l area. because impact of temperature degradation decreases from starting loop to subsequent loops Effect This has of Variation potential of Ground to reduce Temperature excavation on Thermal work Performance also installation of Reclined l area. Slinky HGHE In 3.4. GHE, Effect of re Variation are many of Ground factors Temperature such ason temperature Thermal Performance of of ground Reclined soil, Slinky rmal HGHE conductivity, moisture content, In GHE, rainfall, re are amount many factors of heat such exchange as temperature etc. thatof influence ground soil, GHErmal performance. conductivity, Therefore, experimental moisture results content, are rainfall, compared amount for of different heat exchange ground etc. soil that temperatures influence withghe a constant performance. mass flow rate through Therefore, xperimental reclined HGHE. results Figure are compared 7 showsfor different heat exchange ground soil rates temperatures for cases 2, with 3 a constant 4. The initial average mass trench flow rate ground through temperatures reclined at HGHE. 1.5 mfigure depth7 around shows heat GHE exchange are 10.4rates C, for 11.7 cases C 2, C. Average 4. The heat initial exchange average rates trench within ground 4 day temperatures from beginning at 1.5 m depth experiment around are GHE 2.36 are W/m, 10.4 C, W/m 5.36 C W/m, 13.7 respectively, C. Average heat for cases exchange 2, 3 rates 4. within The average 4 day from increased beginning heat exchange experiment rates are are % W/m, 3.43 W/m 5.36 W/m, respectively, for cases 2, 3 4. The average increased heat exchange 127.3% for cases 3 4 respectively, compared to case 2. With lower ground soil temperature, rates are 45.3% 127.3% for cases 3 4 respectively, compared to case 2. With lower ground soil ground becomes saturated quickly (case 2), prone to heat extraction. But when ground soil temperature, ground becomes saturated quickly (case 2), prone to heat extraction. But when temperature ground soil is higher, temperature GHE is higher, is capable GHE of continued is capable higher of continued heat extraction higher heat (case extraction 3 & 4). (case It is 3 obvious & that 4). It heat is obvious extraction that rate increases heat extraction with rate increases in with ground increase temperature, in ground buttemperature, not linearly but because variation not linearly pattern because of trench variation groundpattern soil temperature of trench ground is notsoil similar temperature to variation is not similar of undisturbed to ground variation soil temperature. of undisturbed ground soil temperature. Figure Figure 7. Heat 7. Heat exchange rate of reclined GHEfor for cases cases 2, 32, For better understing of effect of variation in ground temperature on rmal For performance better understing of GHE, Figure 8 of shows effect temperature of variation distribution in ground of trench temperature ground soil on around rmal performance reclined GHE of GHE, Figure undisturbed 8 shows ground temperature soil temperature. distribution Figure 8 of also trench includes ground ambient soil around reclined temperature GHE temperature undisturbed difference groundof soil water temperature. between outlet Figure 8 also inlet. includes The undisturbed ambient temperature ground temperature temperature 1.5 difference m depth increases of waterby between about 2.9 C outlet from 4 March inlet. to 20 The April undisturbed 2016.

13 Resources 2017, 6, of 18 ground temperature T g at 1.5 m depth increases by about 2.9 Resources 2017, 6, 56 C from 4 March to 20 April However average values of T g at 1.5 m depth are 10.4 C, 10.8 C of 18 C from 4 7 March, 12 15However March average April, values respectively. of at 1.5 m depth are 10.4 C, 10.8 C 13.2 C from 4 7 March, FromMarch Figure 7a c for cases April, 2respectively. 3, it is seen that T 1, T 4 T 7 at 0.5 m depth change almost same as T From g at 0.5 Figure m depth 7a c for cases T 2 a. In addition, 3, is seen that variation T,T patterns T 0.5 of m T depth 1, T change 4 T almost 7 at 1.0 m depth same as are similar to undisturbed at 0.5 m depth ground soil. In addition, variation patterns of T temperature T g at 1.0 m depth., T Therefore, T at 1.0 m depth trench ground are similar to undisturbed ground soil temperature soil up to 1.0 m depth might not be affected or only slightly at 1.0 m depth. Therefore, trench ground affected by heat extraction in GHE. soil up to 1.0 m depth might not be affected or only slightly affected by heat extraction in GHE. However, for case 4, T 1, T 4 7 at 0.5 depth remain almost similar compared to T g at 0.5 m However, for case 4, T,T T at 0.5 m depth remain almost similar compared to at 0.5 m depth depth T a but not similar to trend of T g at 1.0 depth. but not similar to trend of at 1.0 m depth. (a) (b) (c) Figure 8. Variation in ground temperature from 4 March to 20 April 2016 around reclined GHE Figure 8. Variation in ground temperature from 4 March to 20 April 2016 around reclined GHE undisturbed ground: (a) at 0.5 m depth ambient; (b) at 1.0 m depth; (c) at 1.5 m depth undisturbed ground: (a) at 0.5 m depth ambient; (b) at 1.0 m depth; (c) at 1.5 m depth water water temperature difference between GHE outlet inlet. temperature difference between GHE outlet inlet. Average temperature differences of water between outlet inlet from beginning of experiment Average temperature within 4 days differences are ΔTcase-2 of = 1.4 water C, ΔTcase-3 between = 1.9 C outlet ΔTcase-4 inlet = 2.8 from C, respectively. beginning Since of ΔTcase-4 > ΔTcase-3 experiment within> 4ΔTcase-2, days are higher T heat exchange was experienced in case 4. The T, T T at 1.0 m case-2 = 1.4 C, T case-3 = 1.9 C T case-4 = 2.8 C, respectively. depth are affected by this higher heat exchange in case 4. As higher heat extraction affects a longer Since T case-4 > T case-3 > T case-2, higher heat exchange was experienced in case 4. The T 1, T 4 T 7 distance around GHE, attention should be paid to maintain an optimum distance between GHEs at 1.0 m depth are affected by this higher heat exchange in case 4. As higher heat extraction affects for installation of multiple slinky HGHEs. The behaviors of drop in T,T T at 1.5 m a longer distance around GHE, attention should be paid to maintain an optimum distance between depth for cases 2, 3 4 are similar to discussion pointed out for Figure 6. GHEs foralso, installation from Figure of 7b,c, multiple after stopping slinky HGHEs. experiment The behaviors in case 3, of average drop in trench T 1, Tground 4 T 7 at 1.5 mtemperatures depth for cases at 1.52, m 3 depth 4on aredays similar 1, 2, to 3, 4 are discussion 10.5 C, 10.8 pointed C, 11.1 out C for Figure 11.3 C, 6. respectively. On Also, from or Figure h, 7b,c, average afterundisturbed stopping ground experiment temperature in case at 3, 1.5 m depth average on days trench 1, 2, ground 3 temperatures 4 is 11.2 at C, m depth C, 11.2 on C days 1, , C, 3, 4respectively. are 10.5 C, The 10.8trench C, 11.1 ground C temperatures 11.3 C, at respectively. 1.5 m On or h, average undisturbed ground temperature T g at 1.5 m depth on days 1, 2, 3

14 Resources 2017, 6, of 18 4 is 11.2 C, 11.2 C, 11.2 C 11.3 C, respectively. The trench ground temperatures at 1.5 m depth start to increase rapidly after stopping experiment continue until about 4 days, after which trench ground temperature at 1.5 m depth increases gradually similar to undisturbed ground temperature. Thus, this interval can be considered heat recovery period of ground soil, but Resources 2017, 6, of 18 it will depend on change in undisturbed ground temperature as well as ambient temperature. depth start to increase rapidly after stopping experiment continue until about 4 days, after 3.5. Intermittent Operation of Reclined HGHE which trench ground temperature at 1.5 m depth increases gradually similar to undisturbed ground From Figures temperature. 5 Thus, 7, it isthis seen interval that heat can be exchange considered rates of heat slinky recovery HGHEs period areof high ground in soil, initial 6 12 but h it will decline depend gradually. on change Sincein HGHEs undisturbed are installed ground temperature in shallowas ground, well as ambient rmal temperature. performance is prone to limitation by rmal saturation in ground region [6] is affected by ambient air 3.5. Intermittent Operation of Reclined HGHE temperature. Selamat et al. [6] concluded that GHEs operate effectively before rmal saturation becomes From dominant Figures 5 suggested 7, it is seen that that GHEs heat exchange should operate rates of slinky in cycles HGHEs or alternate are high in cooling-heating initial 6 12 h decline gradually. Since HGHEs are installed in shallow ground, rmal performance mode to recuperate ground rmal balance. In order to investigate heat exchange characteristics is prone to limitation by rmal saturation in ground region [6] is affected by ambient air of GHEs with intermittent operation, experiments were conducted for different intermittent temperature. Selamat et al. [6] concluded that GHEs operate effectively before rmal saturation operations becomes of dominant heating suggested mode with that mass GHEs flow should rate operate 4 L/min. in cycles The or intermittent alternate cooling-heating operations were performed mode to for recuperate 12 h, 6 hground 2 hrmal intervals balance. fromin order February, to investigate heat February exchange characteristics on 6 March 2017, respectively, of GHEs with for intermittent reclined operation, slinky HGHE. experiments Before that, were conducted experiment for different was performed intermittent under continuous operations operation of heating from 29 mode January with mass to 2 February flow rate L/min. with The intermittent same mass operations flow rate. were Then, rmal performed performances 12 h, 6 were h compared 2 h intervals between from continuous February, different February intermittent on 6 March operations. 2017, It was already respectively, noticed for in Figure reclined 7 that slinky rmal HGHE. Before performance that, of GHE experiment depends was onperformed ground temperature. under Fujii continuous et al. [17] introduced operation from a parameter 29 January to T/Q 2 February to eliminate 2017 with this same effect. mass Since flow in rate. our Then, experiment, rmal performances were compared between inlet set temperature was fixed to 7.0 continuous different intermittent operations. It was C, we are considering following parameter for intermittent already noticed in Figure 7 that rmal performance of GHE depends on ground temperature. performance analysis of GHE more accurately: Fujii et al. [17] introduced a parameter to eliminate this effect. Since in our experiment, inlet set temperature was fixed to 7.0 C, we are considering Q/ T = Q/ ( ) following parameter for intermittent T g T o (6) performance analysis of GHE more accurately: where Q is heat exchange rate per unit tube = length, ( T g is ) undisturbed ground temperature (6) at 1.5 mwhere depth is T o is heat exchange outlet water rate temperature unit tube length, of Tg GHE. is This undisturbed parameter ground can be temperature called at overall heat 1.5 transfer m depth response To with is respect outlet water to temperature change in of temperature GHE. This difference parameter between can called undisturbed ground overall heat outlet transfer water. response Figure 9with shows respect time to variation change in of Q/ T temperature values for difference differentbetween intermittent operations. undisturbed It isground seen that outlet Q/ T water. for Figure all intermittent 9 shows time operations variation is of significantly values for higher different than for continuous intermittent operations. It is Inseen that intermittent operation, for all intermittent offoperations period reduces is significantly effect higher of heat than for continuous operations. In intermittent operation, off period reduces effect of degradation of ground soil around GHE; thus, ground is allowed to recuperate its rmal heat degradation of ground soil around GHE; thus, ground is allowed to recuperate its condition during this off period. The heat regeneration in off time period contributed significantly rmal condition during this off period. The heat regeneration in off time period contributed to significantly increase in to heat increase exchange in heat rate. exchange rate. (a) (b) Figure 9. Time variation of value in different intermittent operations: (a) 6 h 12 h interval Figure 9. Time variation of Q/ T value in different intermittent operations: (a) 6 h 12 h interval (b) 2 h interval. (b) 2 h interval.

15 Resources 2017, 6, of 18 In order to observe benefit of intermittent operation in contrast to continuous operation, results presented in Figure 9 are integrated n compared between continuous different intermittent operations. The integral of results presented in Figure 9 is calculated by using Equation (7). Resources 2017, 6, of 18 t2 ( ) In order to observe S benefit = of Q/ T intermittent dt operation in contrast to continuous operation, (7) results presented in Figure 9 are integrated t 1 n compared between continuous different intermittent operations. The integral of results presented in Figure 9 is calculated by using Equation (7). where S (W h/(m C)) is total value of Q/ T in a cycle; t is time (h). Table 5 shows calculated S values for continuous = ( ) intermittent operations also (7) includes integral of continuous cycle over only same on-periods of intermittent operation. It is apparent where that S (W h/(m C)) S values is total (enclosed value of by ellipses) in a cycle; of t is continuous time (h). cycle are always Table 5 shows calculated S values for continuous intermittent operations also higher than those of intermittent cycle. However, S values (enclosed by rectangles) of over includes integral of continuous cycle over only same on-periods of intermittent only on-periodsoperation. of a cycle It is for apparent all of that intermittent S values (enclosed operations by ellipses) areof always continuous highercycle thanare for always continuous operation. higher Therefore, than those of merits intermittent of intermittent cycle. However, cycle S can values be(enclosed achievedby ifrectangles) integral of over of continuous cycleonly is considered on-periods over of a only cycle for all same of on-periods intermittent of operations intermittent are always operation. higher than for continuous operation. Therefore, merits of intermittent cycle can be achieved if integral of continuous cycle is considered over only same on-periods of intermittent operation. Table 5. Calculated value of S. Table 5. Calculated value of S. Cycle Operation Time Operation Cycle Operation Time Period of Integral Operation Period of Integral S (W h/(m C)) S (W h/(m C)) 120 h Continuous 120 h Continuous Over Over whole period whole period Over every Over 6 hevery interval 6 h interval 66.2 Over every Over 12every h interval 12 h interval 67.5 Intermittent Intermittent 6 h interval 6 h interval h interval 12 h interval h Continuous 24 h Continuous Over Over whole period whole period 37.0 Over every Over 2 hevery interval 2 h interval 19.6 Intermittent Intermittent 2 h interval 2 h interval 28.2 Furrmore, to show benefits of intermittent operation from viewpoint of power Furrmore, to consumption show by benefits circulating of intermittent pump, cycle operation integral value froms can be viewpoint compared on of power basis of consumption by pump circulating power consumption pump, to circulate cycle integral water through value S can GHE. bethe compared active operation on time basis for all of intermittent operations is one-half of continuous operations. Consequently, pump of pump power consumption to circulate water through GHE. The active operation time power consumptions of intermittent operations are 50% of continuous operations. Therefore, for all of intermittent S value operations of any intermittent is one-half operation of is less continuous than of continuous operations. operation, Consequently, but not less than 50%. pump power consumptions Thus, intermittent of intermittent operation is more operations efficient from are 50% view of of continuous pump power operations. consumption. Therefore, S value For example, of any intermittent at 12 h intermittent operation, is less S value thanis of 70.4% continuous of continuous operation, operation. but At not same time, pump power consumption for intermittent operation is 50% of continuous less than 50%. Thus, intermittent operation is more efficient from view of pump power operation. consumption. For example, With considering at 12 h intermittent only on-periods operation, of a cycle, Figure S10 value shows is % h average ofpercentage continuous increases operation. At same in time for, 12 h pump 6 h power interval consumption operations based for on continuous intermittent operation. operation From this figure, is 50% it can of continuous operation. be seen that percentage increases in during 6 h interval operation are higher than those of 12 h interval operation. For example, 12 h average of value increased 66.3% in 6 h With considering only on-periods of a cycle, Figure 10 shows 12 h average percentage increases in 38.9% in 12 h interval of operations on 2nd day. In comparison, with respect to continuous Q/ T for 12 h 6operation, h interval operations increased based 43.0% on continuous 25.2% for 2 h operation. 6 h interval Fromoperations, this figure, respectively, it can be on seen that percentage first increases day of operation. in Q/ T The during physical 6significance h interval of operation is that are a higher value than indicates thosea quicker of 12 h interval operation. overall Forheat example, transfer response. 12 h average From this of comparison, Q/ Tit value can be increased concluded that 66.3% re inis 6 ha good opportunity to operate slinky HGHEs in intermittent mode, which will significantly increase 38.9% in 12 h interval of operations on 2nd day. In comparison, with respect to continuous performance. During off period, supplemental sources (air source, for example) can be used to operation, Q/ Tmeet increased continuous 43.0% heat dem. 25.2% for 2 h 6 h interval operations, respectively, on first day of operation. The physical significance of Q/ T is that a higher value indicates a quicker overall heat transfer response. From this comparison, it can be concluded that re is a good opportunity to operate slinky HGHEs in intermittent mode, which will significantly increase performance. During off period, supplemental sources (air source, for example) can be used to meet continuous heat dem.

16 Resources 2017, 6, of 18 Resources 2017, 6, of 18 Figure 10. Percentage increasesin in Q/ T (12 h average) for 12 h 6 h interval operations based on continuous operation Conclusions Conclusions The The experimentalrmal rmal performancesof ofslinky slinkyhghes (sting (sting reclined orientation) have have been been measured measured in different in different heating heating modes. modes. The experimental The experimental results highlighted results highlighted comparison comparison of performances of performances of sting of sting reclined orientation, reclined orientation, effects on ground effects temperature on ground temperature around around reclined HGHE reclined duehghe to heatdue extraction, to heat extraction, effect of variation effect of variation in groundin temperature ground temperature on reclined on reclined HGHE performance. HGHE performance. The rmal The rmal performance performance improvements improvements by by intermittent intermittent operations operations of of GHE GHE are are also also discussed. Moreover, Moreover, temperature distributionsof of undisturbedground ground ambient temperature temperatureare arealso alsomeasured. The Themeasured measuredundisturbed undisturbedground groundtemperature temperatureinformation informationprovides auseful usefulindicator indicatorof of installation installationof ofghes GHEsat at asuitable suitabledepth for for heating heating cooling purposes. A higher heat heat exchange exchange rate rate of of sting sting HGHE HGHE compared compared to reclined to reclined HGHE was HGHE observed. was observed. The average The heat average exchange heat rate exchange is 16.0% rate higher is 16.0% for higher sting for slinky sting HGHEslinky than HGHE reclined than slinky reclined HGHE at slinky a flowhghe rate of at 1a L/min. flow rate Forof 1 L/min. mass flow For ratemass 2 L/min, flow rate average 2 L/min, heat exchange average heat rate exchange of sting rate of GHE sting is 19.1% GHE higher is 19.1% than higher reclined than GHE. reclined In addition, GHE. In addition, calculated overall calculated heat overall transferheat coefficient transfer UA-value coefficient signifies UA-value signifies customary sizing customary of slinky sizing GHEs of slinky for different GHEs for ground different soil ground temperatures soil temperatures operating conditions. operating It can conditions. be suggested It can thatbe slinky suggested HGHEsthat in sting slinky HGHEs orientation in sting would require orientation more significant would require backfilling more significant by using high backfilling rmalby conductivity using high material. rmal conductivity With respect material. to excavation With work, respect sting to excavation slinky HGHEs work, sting are costslinky effective HGHEs compared cost wiffective reclinedcompared slinky HGHEs. with reclined Theslinky trenchhghes. ground temperature variation of different loops around GHE decreased from starting The loop trench to subsequent ground temperature loops. Since variation impact of different of heat loops exchange around rate on GHE ground decreased temperature from starting variation loop decreases to subsequent from loops. starting Since loop to impact subsequent of loops, heat exchange slinky HGHEs rate on can ground be installed temperature with a variation gradually decreases sinking loop from pitch starting from loop starting to subsequent loop to loops, end loop. slinky This HGHEs has can potential be installed to reduce with a gradually installation sinking l loop area pitch as well from as excavation starting loop work. to end loop. This has potential to reduce installation For mass l flow area rate as well of 1 as L/min excavation with inlet work. water temperature 7 C, 4-day average heat extraction For rates mass increased flow rate 45.3% of 1 L/min 127.3%, with respectively, inlet water when temperature initial 7 average C, ground 4-day average temperatures heat extraction at 1.5 m depth rates around increased reclined 45.3% HGHE increased 127.3%, from respectively, 10.4 C to 11.7 when C 10.4 initial C average to 13.7 C. ground This is temperatures not a linear relationship at 1.5 m depth because around reclined variation HGHE pattern increased of trench from 10.4 ground C to soil 11.7 temperature C 10.4 is C not to 13.7 similar C. to This variation is not a linear in relationship undisturbed because ground soil temperature. variation pattern of trench ground soil temperature is not similar to variation in undisturbed ground soil temperature. The ground temperature degradation can be recovered by intermittent operation of GHE. The intermittent operation exhibited a great potential to boost rmal performance of slinky HGHE. From viewpoint of overall heat transfer response parameter, a short time

17 Resources 2017, 6, of 18 The ground temperature degradation can be recovered by intermittent operation of GHE. The intermittent operation exhibited a great potential to boost rmal performance of slinky HGHE. From viewpoint of overall heat transfer response parameter Q/ T, a short time interval of intermittent operation is better than a long time interval of intermittent operation. It is apparent that cycle integral value of Q/ T for continuous operation is always higher than that of intermittent operation. However, merits of intermittent cycle can be achieved if integral of Q/ T for continuous cycle is compared over only same on-periods of intermittent operation. Furrmore, from viewpoint of power consumption by circulating pump, intermittent operation is more efficient than continuous operation. Acknowledgments: This work was sponsored by project Renewable energy-heat utilization technology & development project of New Energy Industrial Technology Development Organization (NEDO), Japan. Author Contributions: All authors conceived designed experiments; Md. Hasan Ali performed experiments; Md. Hasan Ali analyzed data in cooperation with both of co-authors; Keishi Kariya contributed to provide materials; Md. Hasan Ali wrote paper. Akio Miyara provided guidance, supervision contributed to revision of this paper. Conflicts of Interest: The authors declare no conflict of interest. References 1. Sarbu, I.; Sebarchievici, C. General review of ground-source heat pump systems for heating cooling of buildings. Energy Build. 2014, 70, [CrossRef] 2. Kavanaugh, S.P.; Rafferty, K. Ground-Source Heat Pumps, Design of Geormal Systems for Commercial Institutional Buildings; ASHRAE: Atlanta, GA, USA, 1997; ISBN Florides, G.; Kalogirou, S. Ground heat exchangers-a review of systems, models applications. Renew. Energy 2007, 32, [CrossRef] 4. Chiasson, A.D. Modeling horizontal ground heat exchangers in geormal heat pump systems. In Proceedings of COMSOL Conference, Boston, MA, USA, 7 9 October Adamovsky, D.; Neuberger, P.; Adamovsky, R. Changes in energy temperature in ground mass with horizontal heat exchangers-the energy source for heat pumps. Energy Build. 2015, 92, [CrossRef] 6. Selamat, S.; Miyara, A.; Kariya, K. Numerical study of horizontal ground heat exchangers for design optimization. Renew. Energy 2016, 95, [CrossRef] 7. Leong, W.H.; Tarnawski, V.R.; Aittomaki, A. Effect of soil type moisture content on ground heat pump performance. Int. J. Refriger. 1998, 21, [CrossRef] 8. Naili, N.; Hazami, M.; Attar, I.; Farhat, A. In-field performance analysis of ground source cooling system with horizontal ground heat exchanger in Tunisia. Energy 2013, 61, [CrossRef] 9. Florides, G.; Theofanous, E.; Iosif-Stylianou, I.; Tassou, S.; Christodoulides, P.; Zomeni, Z. Modeling assessment of efficiency of horizontal vertical ground heat exchangers. Energy 2013, 58, [CrossRef] 10. Song, Y.; Yao, Y.; Na, W. Impacts of soil pipe rmal Conductivity on performance of horizontal pipe in a ground-source heat pump. In Proceedings of Sixth International Conference for Enhanced Building Operations, Shenzhen, China, 6 9 November Tarnawski, V.R.; Leong, W.H.; Momose, T.; Hamada, Y. Analysis of ground source heat pumps with horizontal ground heat exchangers for norrn Japan. Renew. Energy 2009, 34, [CrossRef] 12. Congedo, P.M.; Colangelo, G.; Starace, G. CFD simulations of horizontal ground heat exchangers: A comparison among different configurations. Appl. Therm. Eng. 2012, 33 34, [CrossRef] 13. Wu, Y.; Gan, G.; Verhoef, A.; Vidale, P.L.; Gonzalez, R.G. Experimental measurement numerical simulation of horizontal-coupled slinky ground source heat exchangers. Appl. Therm. Eng. 2010, 30, [CrossRef] 14. Xiong, Z.; Fisher, D.E.; Spitler, J.D. Development validation of a slinky ground heat exchanger model. Appl. Energy 2015, 141, [CrossRef] 15. Wu, Y.; Gan, G.; Gonzalez, R.G.; Verhoef, A.; Vidale, P.L. Prediction of rmal performance of horizontal-coupled ground source heat exchangers. Int. J. Low Carbon Technol. 2011, 6, [CrossRef]

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