And Two-Well Standing Column Well Geothermal Heat Exchange Systems

applied sciences

Article Comparative Analysis of Geothermal Energy in Korea Based on Closed Borehole and Single- and Two-Well Standing Column Well Geothermal Heat Exchange Systems

Jangyoul You 1 and Changhee Lee 2,*

1 Department of Architecture Engineering, Songwon University, Gwangju 61756, Korea; [email protected] 2 Department of Mechanical and Shipbuilding Convergence Engineering, Pukyong National University, Busan 48547, Korea * Correspondence: [email protected]; Tel.: +82-51-629-7816

Received: 1 March 2020; Accepted: 31 March 2020; Published: 3 April 2020

Abstract: In this study, a mobile measuring device was developed and a thermal response test that applied a standing column well-type heat exchanger was conducted to obtain design parameters from field measurements. The main purpose of this study was to investigate the effects of thermal conductivity and geothermal resistance on site, including the flow and effects of natural convection of groundwater in boreholes. We compared, analyzed, and investigated the effective thermal conductivity of a borehole heat exchanger system and the effective thermal conductivity that was not applied when bleeding single-well standing column wells (SCWs), which is called an open-type standing column well geothermal heat exchanger system. We also investigated the heat transfer characteristics during the bleeding of two-well type SCWs, where water is injected from one clearing hole to the returning hole depending on the bleeding rate. Artificial recharging was used to inject the change of thermal conductivity from the bleeding rate of a geothermal heat exchanger into another SCW type. From the comparison results of the thermal conductivity of the multi-well and single-well underground heat exchangers, four times higher efficiency than the single-well was obtained. The reason for this is considered to be energy utilization utilizing groundwater energy.

Keywords: standing column well (SCW); thermal response test; eﬀective thermal conductivity; bleeding rate; thermal conductivity; two-well SCW system

1. Introduction The development of alternative energy sources is becoming increasingly necessary in Korea to prepare for an era of hefty oil prices, as Korea faces energy and resource shortages and imports approximately 97% of its fossil fuels. The post-Kyoto Protocol, which is in effect since 2013, limits the rate of emission of greenhouse gases. In 2000, Korea’s CO2 emissions amounted to 480.4 million tons, making Korea the ninth highest CO2-emitting country worldwide. Most of the greenhouse gases emitted come from energy consumption [1–3]. The Korean government initiated a program known as law section 11 to reduce the consumption of fossil fuels. The program promotes the supply of renewable energy by enforcing the use of renewable energy systems for more than 5% of the total construction cost for public buildings whose floor space is more than 3000 m2. Moreover, the Korean government set up a plan in 2000 for stimulating the consumption of renewable energy in order to increase the renewable energy share of the total energy consumption to 5% by 2018 [4]. Geothermal energy, defined as thermal energy generated and stored

Appl. Sci. 2020, 10, 2467; doi:10.3390/app10072467 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 2467 2 of 18 in the ground, is considered to be a relatively clean and inexhaustible source of renewable energy. Geothermal energy comes mainly from the breakdown of natural radioactive isotopes of uranium, thorium, and potassium. The average heat flow on the surface of the earth is 82 W/m, and the total heat loss is approximately 42 106 MW. The estimated total thermal energy from the surface of the × earth to a depth of 10 km is 1.3 1027 J, which corresponds to burning of 3 1017 barrels of oil. × × The heat content of rocks on the Earth’s surface is the largest source of geothermal energy. The highest hundreds of meters of crust are not a geothermal reservoir in the classical sense, but the temperature in the area is too low for immediate use. Adjusting only the heat pump gives access to this wide range of resources, which increases the temperature of the heat carrier fluid. The most extensive technique to utilize shallow geothermal resources involves borehole heat exchangers (BHE) with geothermal heat pumps [5,6]. After inserting the BHE into the drilled hole, the borehole is grouted by mixing sand, cement, and bentonite. An important role of boreholes is that thermal contact is a very important factor to ensure sufficient heat transfer properties between the borehole wall and the water circulation system to prevent vertical movement of water. [7]. A thermal response test (TRT) requires suitable measuring equipment to select the thermal performance of the BHE. This test takes into account the overall heat transfer characteristics including groundwater effects and other disturbance factors. The first approach for the constant heat injection or extraction is provided by Morgensen [8] and is based on the line source method [9]. Firstly, mobile TRT devices were deployed in Sweden [10] and the United States [11]. After this period, analytical methods were greatly improved in several countries [10–12]. However, it is more practical than standardized TRT measurements, and it contributes in an easy-to-use way, further reducing TRT costs. The thermal performance characteristics of the ground are measured in a laboratory, but the results are generally inaccurate with some errors. These methods tend to ignore site conditions, such as groundwater effects and grouting materials. [13]. For actual measurement, it is necessary to select a test suitable for local conditions and prepare to compare the thermal performance of various BHE types of underground heat exchangers [14]. The literature allows you to find references to the appropriate number of geothermal system heat pumps (GSHP) and BHEs in the field, design, economic analysis and standard booklets [7,15–19]. Various BHE types (single U type, double U type, coaxial type) and grouting materials are used in GSHP systems. The thermal conductivity and length for BHE in one place should be the same in continuous testing, whereby only BHE type and grouting material can be changed. Ground thermal conductivity (l) and thermal resistance (Rb) for different coaxial BHE types and grouting materials were obtained. To the best of the authors’ knowledge, field studies of this combination are yet to be conducted in the literature. The effectiveness of thermal bore hole resistance was evaluated against the GSHP’s coefficient of performance (COP) and could not be ignored. The previous study’s standing column well (SCW) [20,21] showed that the SCW can reduce the overall borehole length compared to a conventional closed-loop system consisting of a single U-tube heat exchanger. In addition, the improvement of the performance of the SCW can be improved by utilizing groundwater, and some groundwater from the well is discharged (exhausted) without being completely recycled to the SCW system. Groundwater bleeding regulates the well temperature by inducing a flow toward the well. Rees et al.’s [22] parametric research showed that bleeding rate is one of the most important parameters affecting SCW performance, and integrating a bleeding strategy into SCW can significantly reduce depth, capital cost, and life-cycle cost compared to no bleeding. Numerical studies of previous models of standing column wells [23–25] made many assumptions about heat transfer between the various components of the well. There are relatively few design tools and simulation models available for SCW systems [22,26,27]. Standing column wells are in limited use since the development of geothermal heat pump systems; however, in recent years, overall performance improved in areas with adequate hydrological and geological conditions, attracting more attention [22,28–30]. In addition, many studies using single- and multi-well standing column wells were carried out recently, and studies reported that the efficiency is improved by using a plurality of SCW systems and using bleeding of water rather than a sealed heat exchanger. Applied Science 2020, 11, x FOR PEER REVIEW 3 of 18 Appl. Sci. 2020, 10, 2467 3 of 18 reported that the efficiency is improved by using a plurality of SCW systems and using bleeding of water rather than a sealed heat exchanger. In recent years, SCW systems, which are more efficient than sealed type, are increasingly installed In recent years, SCW systems, which are more efficient than sealed type, are increasingly in areas where geological properties and groundwater utilization conditions are suitable [31,32]. The installed in areas where geological properties and groundwater utilization conditions are suitable focus of attention on these systems is not only due to the high thermal efficiency of the SCW system, [31,32]. The focus of attention on these systems is not only due to the high thermal efficiency of the but also because the SCW system requires less construction area than the enclosed system, which is SCW system, but also because the SCW system requires less construction area than the enclosed of primarysystem, which importance is of primary in urban importance areas. In in the urban case ofareas. the In SCW the system,case of the a numerical SCW system, analysis a numerical model is beinganalysis developed model by is applyingbeing developed finite volume by applying and finite finite element volume methods and finite in consideration element methods of complex in undergroundconsideration heat of complex and groundwater underground circulation heat and [ 33groundwater–36]. However, circulation these numerical[33–36]. However, models these cannot benumerical reflected inmodels hourly cannot simulation be reflected programs in hourly or designs simulation because programs calculations or designs are because too complicated calculations and conditionsare too complicated over time areand too conditions heavy. Therefore,over time are it istoo often heavy. simplified Therefore, by it an is empiricaloften simplified approach by an and approximationempirical approach from real and design. approximation This is why from the real direction design. andThis performanceis why the direction of recent and research performance focused onof the recent development research focused of models on the that development simplify the of SCW models system that [simplify37–39]. the SCW system [37–39]. TheThe groundwater groundwater heat heat pump pump system system usingusing groundwatergroundwater extracted extracted from from the the same same well well hole hole and and beingbeing recovered recovered into into the the same same well well hole hole in in aa semi-opensemi-open loop arrangement was was widely widely known known as asthe the SCWSCW (standing (standing column column well) well) system. system. The The groundground heat exchangers in in these these systems systems consist consist of ofvertical vertical boreholesboreholes filled filled with with groundwater groundwater up up to to thethe groundwatergroundwater level [40,41]. [40,41]. In In an an open-loop open-loop pipe pipe circuit, circuit, waterwater is circulatedis circulated from from the the wellwell throughthrough a a heat heat pump pump to toreturn return to th toe thesame same well, well, and, and,for most for mostof the of theyear, year, the the system system works works by recirculating by recirculating water water between between the well the and well the and heat the pump. heat However, pump. However, during duringthe peak the peaktemperature temperature period, period, only a only portion a portion of the offlow the can flow be can returned be returned to the towell, the and well, the and rest the can rest be drained to “bleed” some water from the system [41]. This leads to groundwater flow from the can be drained to “bleed” some water from the system [41]. This leads to groundwater flow from the surrounding formation to the well. This system in case of bleeding cools the wells and surrounding surrounding formation to the well. This system in case of bleeding cools the wells and surrounding grounds during heat removal in the summer and heats the wells and surrounding grounds during grounds during heat removal in the summer and heats the wells and surrounding grounds during heat heat extraction in the winter. The open SCW system according to the circulating water ratio of this extraction in the winter. The open SCW system according to the circulating water ratio of this bleeding bleeding showed that the effective thermal conductivity characteristics are superior to the existing showedSCW [42,43]. that the effective thermal conductivity characteristics are superior to the existing SCW [42,43]. A schematicA schematic diagram diagram of of geothermal geothermal heat heat exchangeexchange systems is is shown shown in in Figure Figure 1.1 .The The geothermal geothermal heatheat exchanger exchanger system system is divided is divided into twointo types:two types: closed closed typeand type open and type. open The type. sealed The heat sealed exchanger heat is dividedexchanger into is divided surface into water surface loops, water wells loops, to groundwater, wells to groundwater, and ground and ground heat exchangers heat exchangers in vertical in boresvertical and bores horizontal and horizontal loops. In loops. addition, In addition, the open th heate open exchanger heat exchanger is divided is divided into ainto single-well a single-well and a multi-welland a multi-well SCW system. SCW system.

(a)

Figure 1. Cont. Appl. Sci. 2020, 10, 2467 4 of 18 Applied Science 2020, 11, x FOR PEER REVIEW 4 of 18

(b)

FigureFigure 1. 1.Schematic Schematic diagramdiagram of ground-source heat heat pu pumpmp geothermal geothermal heat heat exchange exchange systems: systems: (a) (a) installedinstalled examples; examples; ( b(b)) classiﬁcation classification ofof geothermalgeothermal heat heat exchanger exchanger system. system.

InIn this this study, study, we we compared, compared, analyzed,analyzed, and inve investigatedstigated the the effective effective thermal thermal conductivity conductivity of a of a boreholeborehole geothermal geothermal heat heat exchangerexchanger system and and th thee effective effective thermal thermal conductivity conductivity not not applied applied when when bleedingbleeding single-well single-well SCWsSCWs are are used. used. We We also also investig investigatedated the theheat heat transfer transfer characteristics characteristics during during the thebleeding bleeding of oftwo-well two-well SCWs, SCWs, where where water water is injected is injected from from one one clearing clearing hole hole to the to the returning returning hole hole accordingaccording to to the the bleeding bleeding rate.rate. To compare the the characteristics characteristics of of closed closed and and open open heat heat exchangers, exchangers, single-single- and and two-well two-well heat heat exchangers,exchangers, for the the effect effectiveive thermal thermal conductivity conductivity coefficient coefficient in inthe the same same boreholeborehole with with the the same same depth,depth, wewe intend to generate generate basic basic data data on on the the basis basis and and performance performance of of geothermal heat exchanger design. The geothermal heat exchanger was characterized and the geothermal heat exchanger design. The geothermal heat exchanger was characterized and the thermal thermal performance was compared under these three conditions. performance was compared under these three conditions.

2.2. In In Situ Situ Thermal Thermal Response Response TestTest Several techniques can be employed to predict the thermal conductivity of a GHEX design. The Several techniques can be employed to predict the thermal conductivity of a GHEX design. The easiest approach is to use the standard value of the rock where the GHEX is located. However, this easiest approach is to use the standard value of the rock where the GHEX is located. However, this method may require expensive samples and may not provide a full profile of the field. Mogenson [8] method may require expensive samples and may not provide a full profile of the field. Mogenson [8] presented a thermal response test (TRT) for estimating the ground value of underground thermal presentedconductivity a thermal and the response thermal resistance test (TRT) of for a bore estimatinghole system. the ground The general value method of underground of TRT adopts thermal a conductivitysystem in which and the heat thermal medium resistance fluid circulates of a borehole through system. the Hall The system general at a methodconstant ofheat TRT injection adopts a systemrate, as in whichshown the in heatFigure medium 2. The fluid thermal circulates respon throughse is then the Hallrecorded system continuously. at a constant After heat injectiondata rate,acquisition, as shown the in temperature Figure2. The data thermal are compared response with is then mathematical recorded continuously. models such Afteras the data source acquisition, model the[44–48]) temperature or cylindrical data are source compared model with[44–46,49,50]. mathematical models such as the source model [44–48]) or cylindrical source model [44–46,49,50]. Appl. Sci. 2020, 10, 2467 5 of 18 Applied Science 2020, 11, x FOR PEER REVIEW 5 of 18

Figure 2. Thermal response test apparatus [[51].51].

Figure3 3 consists consists ofof thethe trailer-typetrailer-type transporttransport systemsystem usedused inin this this study study [ [52–56].52–56]. In addition, a schematic diagram of the experimental set-up of the SCW system showing the thermal response of the test equipmentequipment shown in FigureFigure4 4 is is shown. shown. InIn thethe test test set-up set-up (Agilent (Agilent 34970A),34970A), aa measurement measurement program (LabView)(LabView) and and a a flow flow meter meter were were used used to to measure measure temperature temperature and and flow. flow. Control Control circuits circuits and temperatureand temperature sensors sensors were were installed installed on each on each pipe atpipe the at inlet the/ outlet,inlet/outlet, including includ boilersing boilers and filters. and filters. Each deviceEach device used anused instrument an instrument that was that calibrated was calibrated at the factory at the orfactory regularly. or regularly. The boiler The capacity boiler providedcapacity 84provided kW (42 84 kW kW 2(42 units) kW × of 2 oil units) condensation. of oil condensa The flowtion. rateThe offlow the rate circulating of the circulating fluid was measuredfluid was × usingmeasured a flow using meter a flow (Macnaught). meter (Macnaught). In addition, In aaddi returntion, pipe a return flow pipe meter flow (water meter meter) (water was meter) installed was andinstalled the bleeding and the bleeding rate was controlledrate was controlled to measure to me theasure bleeding the bleeding rate of the rate measuring of the measuring device without device introducingwithout introducing water into water the SCW into system.the SCW Table system.1 shows Table the 1 components shows the installedcomponents in the installed experimental in the andAppliedexperimental measurement Science 2020 and, 11 measurement, devices x FOR PEER used REVIEW indevices this study. used in this study. 6 of 18 Figure 5 shows the behavior of the closed borehole and single/two-well SCW systems according to the exponential and exponential used in this study, as well as the position of the return pipe. The groundwater heat exchanger under test is of type SCW, eight inches in diameter and 350 m and 400 m deep. It has a PVC inner casing and a top extraction/bottom circulation structure. During the drilling process, water intake was observed to increase significantly at depths of 80–100 m and 270– 290 m. The maximum quantity analyzed was found to be in the range of 600–800 tons/day based on the groundwater impact survey results, and the stable withdrawal capacity was found to be in the range of 200–250 tons/day. Table 2 shows the bleeding percentage used in this study. The bleeding rates of 10% and 25% have absorption capacities of 80 tons/day and 200 tons/day (0.06 m3/min and 0.13 m3/min), respectively. In this study, the characteristics of single-well and two-well underground heat exchangers were compared. We also evaluate the thermal performance of a two-well system and a single-well system underground heat exchanger when there is no discharge rate and when returning from the inlet and re-injecting the discharge into the wells [40–42].

Figure 3. Experimental apparatus of a standing column well (SCW) system.

Figure 4. Schematic diagram of the measurement apparatus of the SCW system.

Table 1. Specifications of the thermal response test rig.

No. Items Manufacturer Specification Remarks 1 Boilers Kyungdong 35,300–36,500 kcal/h - 2 Sand filter FeelanTek 400 L/min - 3 Pumps Willo 8 m3/h - 4 Oil tank - 400 L - 5 Flow meter (water) Blue-White 20–200 L/min ±1% 6 Flow meter (oil) Macnaught 35–830 cc/min ±1% 7 Temperature sensor Pt100 Ω, four-wire ±0.5% 8 Data logger AGILENT 34,790 A, 34902MUX ±1% 9 Inverter LS SV015iG5A, 380 V, 30–60 Hz - 10 Measurement program National Instruments LabView 8.6 -

Table 2. Experimental conditions of bleeding rate for SCW system.

No. Bleeding Conditions Water Withdrawal Capacity (m3/day) Remarks

1 No bleeding rate 0 2 10% bleeding rate 80 0.06 m3/min 3 25% bleeding rate 200 0.13 m3/min Applied Science 2020, 11, x FOR PEER REVIEW 6 of 18

Appl. Sci. 2020, 10, 2467 6 of 18 Figure 3. Experimental apparatus of a standing column well (SCW) system.

Figure 4. Schematic diagram of the measurement apparatus of the SCW system.

Table 1. Specifications of the thermal response test rig. Table 1. Specifications of the thermal response test rig. No. Items Manufacturer Specification Remarks No. Items Manufacturer Specification Remarks 11 BoilersBoilers Kyungdong 35,300–36,500 35,300–36,500 kcal/h kcal /h- - 2 Sand filter FeelanTek 400 L/min - 32 Pumps Sand filter FeelanTek Willo 400 L/min8 m3/h - - 43 OilPumps tank Willo - 8 m 4003/h L- - 5 Flow meter (water) Blue-White 20–200 L/min 1% 4 Oil tank - 400 L - ± 6 Flow meter (oil) Macnaught 35–830 cc/min 1% ± 75 Temperature Flow meter sensor (water) Blue-White Pt10020–200Ω L/min, four-wire ±1% 0.5% ± 86 DataFlow loggermeter (oil) Macnaught AGILENT 34,790 35–830 A, cc/min 34902MUX ±1% 1% ± 97 Temperature Inverter sensor LS SV015iG5A,Pt100 Ω, four-wire 380 V, 30–60 Hz ±0.5% - 10 Measurement program National Instruments LabView 8.6 - 8 Data logger AGILENT 34,790 A, 34902MUX ±1% 9 Inverter LS SV015iG5A, 380 V, 30–60 Hz - Figure5 shows the behavior of the closed borehole and single /two-well SCW systems according 10 Measurement program National Instruments LabView 8.6 - to the exponential and exponential used in this study, as well as the position of the return pipe. The groundwater heat exchanger under test is of type SCW, eight inches in diameter and 350 m and 400 Table 2. Experimental conditions of bleeding rate for SCW system. m deep. It has a PVC inner casing and a top extraction/bottom circulation structure. During the drilling process,No. Bleeding water intake Conditions was observed Water to increase Withdrawal significantly Capacity at(m depths3/day) of 80–100Remarks m and 270–290 m. The maximum quantity analyzed was found to be in the range of 600–800 tons/day based on the 1 No bleeding rate 0 groundwater impact survey results, and the stable withdrawal capacity was found to be in the range 3 of 200–2502 tons/day. 10% bleeding Table2 shows rate the bleeding percentage 80 used in this study. 0.06 The m bleeding/min rates of 10% and3 25% 25%have bleeding absorption rate capacities of 80 tons/day 200 and 200 tons/day (0.06 0.13m m33//minmin and 0.13 m3/min), respectively. In this study, the characteristics of single-well and two-well underground heat exchangers were compared. We also evaluate the thermal performance of a two-well system and a single-well system underground heat exchanger when there is no discharge rate and when returning from the inlet and re-injecting the discharge into the wells [40–42].

Table 2. Experimental conditions of bleeding rate for SCW system.

No. Bleeding Conditions Water Withdrawal Capacity (m3/day) Remarks 1 No bleeding rate 0 2 10% bleeding rate 80 0.06 m3/min 3 25% bleeding rate 200 0.13 m3/min Appl. Sci. 2020, 10, 2467 7 of 18 Applied Science 2020, 11, x FOR PEER REVIEW 7 of 18

Figure 5. Schematic diagrams of a geothermal heat exchanger system: ( a) closed loop; ( b) SCW with return pipe location; (c) single-well SCWSCW withwith bleedingbleeding rate;rate; ((dd)) two-welltwo-well SCWSCW system.system. 3. Validation of Line-Source Theory and Uncertainty of System 3. Validation of Line-Source Theory and Uncertainty of System 3.1. Line-Source Theory 3.1. Line-Source Theory The line-source theory for calculating the thermal performance of a hermetic heat exchanger is well established,The line-source and ittheory was applied for calculating to thermal the performance thermal performance testing for of decades a hermetic [57,58 heat]. The exchanger theory was is extendedwell established, to comparisons and it was and applied studies ofto energythermal losses performance in different testing heat exchangerfor decades arrangements [57,58]. The [theory59,60]. Inwas this extended study, theto comparisons line-source theory and studies is applied of energy to the losses TRT of in a different closed source heat exchanger heat exchanger, arrangements and the performance[59,60]. In this of study, an open the underground line-source theory heat exchanger is applied is to extended the TRT toof thea closed measurement. source heat exchanger, and theThe performance equation for of the an temperatureopen underground domain he asat aexchanger function is of extended time (t) andto the radius measurement. (r) around the line-sourceThe equation model for with the a temperature constant heat domain injection as a rate functi (Q)on can of betime used (t) and as the radius approach (r) around value the of line- heat injectionsource model from closedwith a andconstant open geothermalheat injection heat rate exchangers (Q) can asbe follows:used as the approach value of heat injection from closed and open geothermal heat exchangers as follows: Z u Q𝑄 ∞𝑒e−𝑑𝑢du T(r, t) To = . (1) T(r, t) −𝑇 = kH u . (1) − 44𝜋𝑘𝐻π x 𝑢 The exponential integral of Equation (1) can be approximated for a large value of a parameter (αtt/r/r22) with the following simple relationship: 𝑟2 ! 4𝛼𝑡 E r =ln4αt −𝛾. (2) E 4𝛼𝑡 = ln 𝑟 γ. (2) 1 4αt r2 − If the infinite series is used for Equation (1), the following summary is obtained: If the infinite series is used for Equation (1), the following summary is obtained: 𝑄 4𝛼𝑡 𝑟 𝑟 T(r, t) −T = ln −𝛾+ 1 − , ( !) (3) 4𝜋𝑘𝐻Q 4𝑟αt 4𝛼𝑡r2 16𝛼𝑡r2 T(r, t) To = ln γ + 1 , (3) where γ is a Euler constant of 0.5772.− The4πkH solutionr2 of− the heat4αt source− 16 toαt the bore hole can be applied only to rr and the heat resistance R can be used in the bore hole. where γ is a Euler constant of 0.5772. The solution of the heat source to the bore hole can be applied only to r r and the heat resistance𝑄 R can be𝑄 used in4𝛼 the bore hole.𝑄 b T = 𝑙𝑛(b𝑡) + 𝑙𝑛 −𝛾+ 𝑅 +𝑇. (4) ≥ 4𝜋𝑘𝐻 4𝜋𝑘𝐻 𝑟 𝐿     Q Q  4α  Q Thus, a time-dependentT f = fluid temperatureln(t) + equationln canγ be + describedRb + T oas. shown in Equation (4). (4) 4πkH 4πkH   r2  −  L The only variable in Equation (4) is ln t. It can be bexpressed by transforming constant ln t in a generalThus, linear a time-dependent equation as follows: fluid temperature equation can be described as shown in Equation (4). y=kx+c, (5) Appl. Sci. 2020, 10, 2467 8 of 18

The only variable in Equation (4) is ln t. It can be expressed by transforming constant ln t in a general linear equation as follows: y = kx + c, (5) Q ( ) 4α Q where k = 4πλH , x = ln t , c = k(ln 2 0.5772) + H Rb + To. ro − Therefore, a linear equation can be obtained by plotting the borehole fluid temperature over time obtained through the heat exchanger system as a measured value, and the fluid temperature and ln t relationship as coordinates. The thermal conductivity (λ) can be determined by calculating using the slope (a) of Equation (5). The effective thermal conductivity and thermal resistance can be obtained from TRT data as given in Equations (6) and (7), respectively [45–55].

Q k = . (6) 4πλH Thermal conductivity (λ) can be expressed using the slope (k) calculated in Equation (5), the length of the borehole (H), and heat energy (Q). In Equation (6), k is proportional to the eﬀective thermal conductivity, and ! ! H 1 4α R = T T ln(t) + ln γ . (7) b f sur 2 Q − − 4πλ ro −

3.2. Uncertainty Utilizing Mass Heat Balance Method A thermal balance approach was used as a verification method to confirm that it was reasonable to ensure the accuracy of the experiment on the fluid volume and temperature of the entire system as shown in Table3. The simplest representation of the heat balance equation is

q = V Cp (Tout T ), (8) in · · − in where qin (W) is the calorific value obtained through the measured temperature and the flow rate of the pump. V (liters per minute, LPM) is the pump flow rate. Cp is the specific heat of water. Tin and Tout are the inlet and outlet temperatures of circulating water. The uncertainty of the temperature measurement for the temperature sensor was 0.01 C, and the signal conditioner of the digital display ± ◦ with analog signals was 0.04 C. The total uncertainty for temperature measurement is given in ± ◦ Equation (9). q q ∆T = ( 0.01)2 + ( 0.04)2 + ( 0.01)2 + ( 0.04)2 0.0825 °C. (9) ± in ± in ± out ± out ≈ ± 0.0825 °C Error = ± 100% = 1.65%. (10) 5 °C × ≈ ±

Table 3. Heat balance check.

Location Transducer Reading (W) Average q (W) Diﬀerence (W) % of Average Power A 2506.6 2657.8 101.2 3.88 B 3207.2 3302.5 93.3 2.82

Using the highest error for the ﬂowmeter taken from Table4 of 2.03%, the total uncertainty in ± the heat balance equation was computed as q Total Error = ( 0.0165)2 + ( 0.0203)ˆ2 2.62%. (11) ± ± ≈ Appl. Sci. 2020, 10, 2467 9 of 18

Table 4. Results from the ﬂowmeter calibration.

Actual Flow (LPM) Calibration Flow (LPM) Error (%) 3.316 3.292 0.73 Applied Science 2020, 11, x FOR PEER15.87 REVIEW 16.032 1.01 9 of 18 100.9 102.99 2.03 4. Investigations and Discussion350.62 355.41 1.35

4.1.4. Thermal Investigations Conductivity and DiscussionCharacteristics of Various Rocks in a Closed Borehole Heat Exchanger System

4.1.Figure Thermal 6 shows Conductivity the average Characteristics temperature of Various and the Rocks diffe inrence a Closed of inlet/outlet Borehole Heat temperatures Exchanger over System time for alluvial deposit, gneiss, and granite rocks using a ground heat exchanger system with a borehole depth Figureof 200 6m shows using thea closed average loop temperature borehole heat and theexchanger di fference system of inlet[3,6,13,41,49,53]./outlet temperatures The initial over groundwatertime for alluvial temperature deposit, from gneiss, the three and granite types of rocks rock was using approximately a ground heat 15–16 exchanger °C, with system an average with a temperatureborehole depth distribution of 200 mof usingapproximately a closed loop28–35 borehole °C during heat the exchangertest period. system The average [3,6,13 temperatures,41,49,53]. The obtainedinitial groundwater from the three temperature types of rock from are theshown three in types Figure of 7, rock linearized was approximately according to 15–16ln (t). The◦C, withslope an ofaverage the graph temperature is linearized distribution by the line-source of approximately method using 28–35 means◦C during of k. theThe test effective period. heat The transfer average coefficienttemperatures can be obtained obtained from by substituting the three types k for of rockEquati areon shown (6). If heat in Figure quantity7, linearized (Q), borehole according depth to (H), ln (t). andThe slope slope (k) of are the substituted graph is linearized into Equation by the (1), line-source effective heat method transfer using and meansslope values of k. Thecan be eff ectiveobtained heat astransfer shown in coe Tablefficient 5. From can be Equation obtained (6), by it substituting can be seen kthat for k Equation and λ are (6). in a If trade-off heat quantity relationship (Q), borehole with eachdepth other. (H), The and thermal slope (k)performance are substituted evaluation into Equation results of (1), the e ffgroundective heatheat transferexchanger and for slope the three values typescan beof rock obtained show as that shown the effective in Table5 thermal. From Equation conductivity (6), itranges can be from seen 2.5 that to k 3.2 and W/(m·K).λ are in Based a trade-o on ff therelationship results of this with study, each other. it is possible The thermal to grasp performance the thermal evaluation conductivity results of of daughters the ground in heat the exchanger case of for the three types of rock show that the effective thermal conductivity ranges from 2.5 to 3.2 W/(m K). the same borehole depth according to the characteristics of the rock. In addition, the influence of· groundwaterBased on the is results considered of this to study, be an it influential is possible fa toctor grasp on thethe thermaleffective conductivity thermal conductivity. of daughters In inthe the future,case ofthe the effect same of borehole groundwater depth accordingflow on the to theeffective characteristics thermal ofconductivity the rock. In of addition, underground the influence heat exchangersof groundwater will be is studied. considered to be an influential factor on the effective thermal conductivity. In the future, the effect of groundwater flow on the effective thermal conductivity of underground heat exchangers will be studied.

40 10

9 35 8 C] C]

30 o o 7 25 6

20 5

4 15 3 10 Average temperature [ Average temperature Alluvial deposit Alluvial deposit T-DFF 2 [ temperature Different Gneiss 5 Gneiss T-DIFF Granit Granit T-DIFF 1

0 0 0 500 1000 1500 2000 2500 3000 Time (min)

Figure 6. Average temperature distribution of various of rocks. Figure 6. Average temperature distribution of various of rocks. Applied Science 2020, 11, x FOR PEER REVIEW 10 of 18 Appl. Sci. 2020, 10, 2467 10 of 18

38 Granit 150m Gneiss 150m 36 Alluvial deposit 150m Granit 200m C]

o Gneiss 200m 34 Alluvial deposit 200m

28 Average temperature [ 26

24 5.5 6.0 6.5 7.0 7.5 8.0 ln(t)

Figure 7. Linearization of line-source method for various rock distributions.

Table 5. EvaluationFigure of 7. eff Linearizationective thermal of conductivity line-source andmethod experimental for various conditions rock distributions. at borehole depth of 200 m in various rocks. Table 5. Evaluation of effective thermal conductivity and experimental conditions at borehole depth of 200 m in various rocks.Unit Alluvium #1 Alluvium #2 Gneiss #1 Granite #1 Granite #1 Granite #2 Initial temperature ◦C 15.2 15.4 16.2 16 14.5 16.3 Average temperature C 28.5Alluvium Alluvium 31 31Gneiss 33.2Granite 28.4Granite 30.1Granite ◦ Unit Heat injection kW 14.5#1 13.9#2 13.8#1 14#1 14.1#1 14,0#2 Slope (K) - 2.151 2.175 1.584 2.094 1.945 2.094 EInitialffective temperature thermal °C 15.2 15.4 16.2 16 14.5 16.3 W/(m K) 2.614 2.663 3.213 2.648 2.813 2.518 conductivityAverage · °C 28.5 31 31 33.2 28.4 30.1 temperature 4.2. CharacteristicsHeat injection of the Effective kW Thermal Conductivity14.5 of13.9 SCW System 13.8 at Different 14 Locations 14.1 of Return 14,0 Pipe Slope (K) - 2.151 2.175 1.584 2.094 1.945 2.094 FiguresEffective8 and thermal9 show the evaluation results of thermal performance by temperature for a W/(m·K) 2.614 2.663 3.213 2.648 2.813 2.518 ground heatconductivity exchanger with the return pipe positioned as shown in Figure5b, using an SCW underground heat exchanger with a borehole depth of 400 m, as shown in References [31–41]. This study4.2. Characteristics analyzed the characteristicsof the Effective Thermal of this inConducti accordancevity of with SCW the System effective at Different thermal Locations conductivity of Return and thermalPipe resistance as a study on the heat transfer characteristics according to a position above the submerged pump [52,56,57,60]. Through the research results of Lee et al. [4,11,41], it was found that Figures 8 and 9 show the evaluation results of thermal performance by temperature for a ground the effective thermal conductivity and the thermal resistance were improved according to the effect of heat exchanger with the return pipe positioned as shown in Figure 5b, using an SCW underground the position of the pump. The k-values obtained by the line-source method for the lower (Figure8) heat exchanger with a borehole depth of 400 m, as shown in References [31–41]. This study analyzed and upper return pipes (Figure9) were 4.26 and 4.56, respectively. Using Equation (6), the e ffective the characteristics of this in accordance with the effective thermal conductivity and thermal resistance thermal conductivities of the lower (Figure8) and upper return pipes (Figure9) were 3.86 and 3.54 as a study on the heat transfer characteristics according to a position above the submerged pump W/(m K), respectively, using a borehole insertion depth of 400 m with injected calories of 48 kW. The [52,56,57,60].· Through the research results of Lee et, Al. [4,11,41], it was found that the effective results are shown in Table6 and confirm that the performance improved by about 9% depending thermal conductivity and the thermal resistance were improved according to the effect of the position on the return pipe position of the circulating water. From the result of Figures8 and9, the e ffective of the pump. The k-values obtained by the line-source method for the lower (Figures 8) and upper thermal conductivity increased at the lower location of the return pipe more than at the upper location. return pipes (Figure 9) were 4.26 and 4.56, respectively. Using Equation (6), the effective thermal Through these results, it was judged that the technology can improve thermal performance. This can conductivities of the lower (Figure 8) and upper return pipes (Figure 9) were 3.86 and 3.54 W/(m · K), be important research data to improve the heat transfer characteristics in the same ground hole by respectively, using a borehole insertion depth of 400 m with injected calories of 48 kW. The results considering the location of the submerged pump. are shown in Table 6 and confirm that the performance improved by about 9% depending on the return pipe position of the circulating water. From the result of Figure 8 and Figure 9, the effective thermal conductivity increased at the lower location of the return pipe more than at the upper location. Through these results, it was judged that the technology can improve thermal performance. Applied Science 2020, 11, x FOR PEER REVIEW 11 of 18

This can be important research data to improve the heat transfer characteristics in the same ground hole by considering the location of the submerged pump. Applied Science 2020, 11, x FOR PEER REVIEW 11 of 18

This can be important research data to improve the heat transfer characteristics in the same ground hole by considering40 the location of the submerged pump. Average fluid temperaturer Appl. Sci. 2020, 10, 2467 11 of 18 36 Linear Fit of A1scw_Averagefluidtemp

C) o 4032 Average fluid temperaturer Linear Fit of A1scw_Averagefluidtemp 3628 y=4.26001x + 19.50694 C)

o R=0.99999

3224

2820 y=4.26001x + 19.50694 R=0.99999

2416

Average Temperature ( 2012

168 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Average Temperature ( 12 Ln(t)

8 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Figure 8. Linearization of the average temperature at the location of the lower return pipe. Ln(t)

Figure 8. Linearization of the average temperature at the location of the lower return pipe.

Figure40 8. Linearization of the average temperature at the location of the lower return pipe.

T_Ave 36 Linear Fit of scw_Averagefluidtemp

C) o 4032 T_Ave 36 28 y=4.5464x Linear Fit + 18.6575of scw_Averagefluidtemp C)

o R=0.99989

3224

2820 y=4.5464x + 18.6575 R=0.99989

2416

Average Temperature ( Average Temperature 2012

168 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Average Temperature ( Average Temperature 12 Ln(t)

Figure8 9. Linearization of the average temperature at the location of the upper return pipe. -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Figure 9. LinearizationTable of 6. theResults average of etemperatureffective conductivity at the location calculation. of the upper return pipe. Ln(t) Single SCW (Lower TypeTable 6. Results Unit of effective conductivity calculation.Single SCW (Upper Location of Pump) Location of Pump) Initial temperature CSingle 15.27 SCW (Lower Single SCW 16.13 (Upper Location FigureType 9. Linearization of theUnit average◦ temperature at the location of the upper return pipe. Average temperature ◦CLocation 34.45 of Pump) 32.44of Pump) InitialHeat temperature injection Table 6. Results °C kW of effective conductivity 48 15.27 calculation. 48 16.13 AverageSlope temperature (K) °C - 4.146 34.45 4.545 32.44 Effective thermal conductivity W/m K 3.86 3.54 · Single SCW (Lower Single SCW (Upper Location Type Unit Ratio of bleeding rate to no bleeding %Location 109 of Pump) - of Pump) Initial temperature °C 15.27 16.13 FigureAverage 10 shows temperature the thermal resistance °C in the SCW 34.45 system [6,53,54]. Borinaga-Tervino 32.44 et al.’s research [6] suggested two main objectives: (i) to determine the variability of the predicted ground thermal conductivity for the same evaluated ground, and (ii) to determine the influence of the type of grout used in the estimated borehole thermal resistance. As shown in Reference [6], the effective Applied Science 2020, 11, x FOR PEER REVIEW 12 of 18

Heat injection kW 48 48 Slope (K) - 4.146 4.545 Effective thermal conductivity W/m·K 3.86 3.54 Ratio of bleeding rate to no % 109 - bleeding

Figure 10 shows the thermal resistance in the SCW system [6,53,54]. Borinaga-Tervino et al.’s research [6] suggested two main objectives: (i) to determine the variability of the predicted ground thermal conductivity for the same evaluated ground, and (ii) to determine the influence of the type of grout used in the estimated borehole thermal resistance. As shown in Reference [6], the effective Appl.thermal Sci. 2020 conductivity, 10, 2467 of this system was obtained by substituting the values into Equation (6). 12Finally, of 18 the thermal resistance value was obtained, as shown in Figure 11. After 60 min at the initial condition, the thermal resistance value showed sudden and repetitive falling and rising before converging to a thermalconstant conductivity value. Despite of this the system short wasmeasurement obtained by time substituting of 12 h, thethe valuesoverall into thermal Equation resistance (6). Finally, value theconverged thermal resistance to a constant. value wasAt the obtained, lower asreturn shown pi inpe, Figure the thermal11. After resistance 60 min at thevalue initial also condition, generally theconverged thermal resistanceto a constant value value. showed The average sudden thermal and repetitive resistance falling values and obtained rising before using converging Equation (7) to at athe constant lower value. return Despitepipe and the at shortthe upper measurement return pipe time were of12 approximately h, the overall 0.0118 thermal K/(W/m) resistance and value 0.0053 convergedK/(W/m), to respectively. a constant. At It thecan lowerbe observed return pipe, that the thermalthermal resistance resistance value value also at the generally lower convergedreturn pipe towas a constant higher than value. that The at averagethe upper thermal return resistancepipe by more values than obtained 44.91%. usingConsidering Equation these (7) results, at the lower it was returnfound pipe that andthe thermal at the upper resistance return value pipe acts were asapproximately an important variable 0.0118 Kamong/(W/m) the and design 0.0053 factors K/(W in/m), the respectively.standing column It can bewell-type observed underground that the thermal heat resistance exchanger. value In at thethis lowerstudy, return the pipeeffective was higherthermal thanconductivity that at the upperand thermal return piperesistance by more values than 44.91%.of the standing Considering column these results,well-type it was underground found that theheat thermalexchanger resistance were measured value acts asby anusing important a thermal variable response among test the device design for factors the case in where the standing the return column pipe well-typeis located underground at the bottom heat and exchanger. the return In pipe this is study, located the at e ﬀtheective top. thermal The effective conductivity thermal and conductivity thermal resistanceand thermal values resistance of the standing values columnof the case well-type where underground the return pipe heat is exchanger located at were the measured bottom were by usingincreased a thermal by about response 6.56% test and device 44.91%, for the respectively, case where compared the return to pipe the is case located where at thethe bottomreturn andpipe thewas returnlocated pipe at isthe located top. at the top. The eﬀective thermal conductivity and thermal resistance values of the case where the return pipe is located at the bottom were increased by about 6.56% and 44.91%, respectively, compared to the case where the return pipe was located at the top.

0.040

0.035 a t u p p e r re tu rn p ip e K/W]

. 0.030 a t lo w e r re tu rn p ip e

0.025

0.020

0.015 Thermal resistance : 0.0095 [K/(W/m)] 0.010

Thermal resistance [m resistance Thermal 0.005 Thermal resistance : 0.0054 [K/(W/m)] 0.000 012345678910111213 Elase time [h]

Figure 10. Resistance value at the lower and upper locations of the return pipe. Figure 10. Resistance value at the lower and upper locations of the return pipe. 4.3. Characteristics of Effective Thermal Conductivity of SCW System with Single- and Two-Well Types 4.3.Figure Characteristics 11 shows of the Effective results Thermal of thermal Conductivi performancety of SCW according System to with the Single- bleeding and rate Two-Well of groundwater Types using single-well and two-well SCW systems as in References [38–40,54–56]. Figure 11a,b show the Figure 11 shows the results of thermal performance according to the bleeding rate of thermal performance of a single-well SCW system, as shown in Figure5c. A study on the e ffective groundwater using single-well and two-well SCW systems as in References [38–40,54–56]. Figures thermal conductivity characteristics was carried out according to the bleeding effect in the single-well SCW system. Figure 11c,d show the thermal performance of a two-well SCW system, as shown in Figure5d. As shown in the schematic of Figure5d, a study was conducted to optimize heat transfer characteristics through energy circulation caused by groundwater flow by injecting the groundwater bleeding amount from one intake hole. Therefore, thermal performance improvement through effective thermal conductivity was analyzed through the thermal performance evaluation of the SCW underground heat exchanger. Applied Science 2020, 11, x FOR PEER REVIEW 13 of 18

11a,b show the thermal performance of a single-well SCW system, as shown in Figure 5c. A study on the effective thermal conductivity characteristics was carried out according to the bleeding effect in the single-well SCW system. Figures 11c,d show the thermal performance of a two-well SCW system, as shown in Figure 5d. As shown in the schematic of Figure 5d, a study was conducted to optimize heat transfer characteristics through energy circulation caused by groundwater flow by injecting the groundwater bleeding amount from one intake hole. Therefore, thermal performance improvement through effective thermal conductivity was analyzed through the thermal performance evaluation of the SCW underground heat exchanger. From the results of Figures 11a,b, obtained through the average inlet/outlet temperature according to ln (t) using the line-source method, the k-values were 4.88 and 2.94, respectively. The effective thermal conductivity values shown in Table 7 were 3.45 and 5.59 W/(m·K). From this result, the improvement of thermal performance was improved by 62% when bleeding rate was performed. In the case of single-well SCW, the heat transfer characteristics were improved by minimizing the energy change of the groundwater because of the amount of water flowing from the underground hole into the groundwater. Table 7 also shows the line-source method results of Figures 11c,d using two-well SCW underground heat exchangers. Finally, the thermal performance of the system improved by about four times compared to the bleeding rate. This improvement of the effective thermal conductivity was the result of the groundwater being well used as the circulating water. Figure 11d shows the characteristic that the value of k (i.e., the exact slope) changed using the IIT (initial ignoring time) method. It was confirmed that the experiments need to be carried out through a longer-term performance evaluation relative to the current measurement method, as this affects the final effective thermal conductivity. As shown in Figures 11a,b, the k value and the effective heat transfer value were almost constant. However, in the two-well SCW of Figures 11c,d, the value of k decreased with time (Figure 12). The reason for this is because the groundwater temperature uses energy without the change of the Bleeding water injected from the intake hole to the return hole. It was judged that the two-well SCW system can maximize the heat consumption in a narrow area by using a geothermal heat exchanger. Appl. Sci. 2020, 10, 2467 13 of 18

32 (a) 30 y=4.9x+17.3 C]

o 28 (b)

26 y=2.93x+20.7

20 Average temperature [ 18 Applied Science 2020, 11, x FOR PEER REVIEW 14 of 18 16 0.00.51.01.52.02.53.0 Ln(t) (h) 44

40 (c) C] o

32 (d)

Average temperature [ temperature Average 24

20 5.05.25.45.65.86.06.26.46.66.87.07.27.4 Ln(t)

Figure 11. Linearization of the average temperature of the single- and two-well type SCW systems: (a) single-well type (no bleeding); (b) single-well type (10% bleeding); (c) two-well type (no bleeding); (d) Figure 11. Linearization of the average temperature of the single- and two-well type SCW systems: two-well type (25% bleeding). (a) single-well type (no bleeding); (b) single-well type (10% bleeding); (c) two-well type (no bleeding); From(d) two-well the results type (25% of Figure bleeding). 11a,b, obtained through the average inlet/outlet temperature according to ln (t) using the line-source method, the k-values were 4.88 and 2.94, respectively. The effective thermalFigure conductivity 12 shows valuesthe characteristic shown in Table of effective7 were 3.45thermal and conductivity 5.59 W /(m K). under From no this bleeding result, and the · improvementbleeding conditions of thermal for single- performance and two-well was improved type SCW by systems 62% when with bleeding respect rateto the was influences performed. of the In thekey case factors, of single-well including SCW,injection the heatrate, transferinlet temperat characteristicsure, injection–production were improved by interval, minimizing and thegeological energy changeconditions, of the on groundwater the open-loop because geotherm of theal system amount (OLGS) of water performance flowing from [43]. the underground hole into the groundwater.From the results of Figure 12, the application of bleeding to the single-well SCW system showed that the effective thermal conductivity was 162% higher than that without bleeding. In addition, the two-wellTable SCW 7. Comparison system also on theshowed measuring a 400% of thermal improvement conductivity in inperformance various bleeding when conditions. bleeding was performed. Based on the thermal performance results of the single-well and two-well SCW system, Single SCW Single SCW Two-Well SCW Two-Well SCW Type Unit it may be advantageous to select and(No use Bleeding) an appropriate(10% Bleeding) SCW. In(No particular, Bleeding) the(25% two-well Bleeding) SCW system is expected to be applicable in a very efficient underground heat exchanger design if the Initial temperature ◦C 16.13 16.13 17.25 15.64 influenceAverage of groundwater temperature is used◦C well. 34.45 32.44 40.92 30.91 Heat injection kW 40 40 78 81 Slope (K) - 2.937 2.937 7.525 1.88 Effective thermal conductivity W/m K 3.45 5.59 2.4 9.59 · Ratio of bleeding to no bleeding14 % - 162 -14 400 K-slop Thermal conductivity No bleeding (Two well) No bleeding (Two well) 12 (a) 25% of Bleeding (Two well) (b) 25% of Bleeding (Two well) 12 No bleeding (single well) No bleeding (single well) 10% of Bleeding (single well) 10% of Bleeding (single well) 10 10

8 8

6 6 K-slope

4 4

2 2 Effective thermal conductivity (W/mK) conductivity thermal Effective 0 0 235710 Initial ignoring timing [h]

Figure 12. Effective thermal conductivity characteristic under no bleeding and bleeding conditions for single- and two-well type SCW systems: (a) k-slope; (b) effective thermal conductivity. Applied Science 2020, 11, x FOR PEER REVIEW 14 of 18

40 (c) C] o

32 (d)

Average temperature [ temperature Average 24

20 5.05.25.45.65.86.06.26.46.66.87.07.27.4 Ln(t)

FigureTable7 also12 shows shows the line-sourcecharacteristic method of effective results of thermal Figure 11 conductivityc,d using two-well under SCW no bleeding underground and bleedingheat exchangers. conditions Finally, for single- the thermal and two-well performance type of SC theW system systems improved with respect by about to the four influences times compared of the keyto the factors, bleeding including rate. Thisinjection improvement rate, inlet temperat of the effure,ective injection–production thermal conductivity interval, was the and result geological of the conditions,groundwater on beingthe open-loop well used geotherm as the circulatingal system (OLGS) water. Figureperformance 11d shows [43]. the characteristic that the valueFrom of k (i.e.,the results the exact of Figure slope) 12, changed the application using the IITof bleeding (initial ignoring to the single-well time) method. SCW It system was confirmed showed thatthat the the effective experiments thermal need conductivity to be carried was out 162% through higher a longer-term than that without performance bleeding. evaluation In addition, relative the to two-wellthe current SCW measurement system also method, showed as thisa 400% affects im theprovement final effective in performance thermal conductivity. when bleeding As shown was in performed.Figure 11a,b, Based the k on value the andthermal the eperformanceffective heat transferresults of value the single-well were almost and constant. two-well However, SCW system, in the ittwo-well may be SCWadvantageous of Figure 11toc,d, select the and value use of an k decreased appropriate with SCW. time In (Figure particular, 12). The the reason two-well for thisSCW is systembecause is the expected groundwater to be temperatureapplicable in uses a very energy efficient without underground the change heat of the exchanger Bleeding waterdesign injected if the influencefrom the intakeof groundwater hole to the is return used hole.well. It was judged that the two-well SCW system can maximize the heat consumption in a narrow area by using a geothermal heat exchanger.

14 14 K-slop Thermal conductivity No bleeding (Two well) No bleeding (Two well) 12 (a) 25% of Bleeding (Two well) (b) 25% of Bleeding (Two well) 12 No bleeding (single well) No bleeding (single well) 10% of Bleeding (single well) 10% of Bleeding (single well) 10 10

8 8

6 6 K-slope

4 4

2 2 Effective thermal conductivity (W/mK) conductivity thermal Effective 0 0 235710 Initial ignoring timing [h]

Figure 12. Effective thermal conductivity characteristic under no bleeding and bleeding conditions for Figuresingle- 12. and Effective two-well thermal type SCW conductivity systems: ( acharacteristic) k-slope; (b) eunderffective no thermal bleeding conductivity. and bleeding conditions for single- and two-well type SCW systems: (a) k-slope; (b) effective thermal conductivity. Figure 12 shows the characteristic of effective thermal conductivity under no bleeding and bleeding conditions for single- and two-well type SCW systems with respect to the influences of the key factors, including injection rate, inlet temperature, injection–production interval, and geological conditions, on the open-loop geothermal system (OLGS) performance [43]. From the results of Figure 12, the application of bleeding to the single-well SCW system showed that the effective thermal conductivity was 162% higher than that without bleeding. In addition, the two-well SCW system also showed a 400% improvement in performance when bleeding was performed. Based on the thermal performance results of the single-well and two-well SCW system, it may be advantageous to select and use an appropriate SCW. In particular, the two-well SCW system is expected to be applicable in a very efficient underground heat exchanger design if the influence of groundwater is used well.

4.4. Characteristics of Effective Thermal Conductivity of SCW System Single/Two-Well SCW System with/without Bleeding Water Table7 presents the results of e ffective thermal conductivity measurements of the closed borehole single-well SCW system and single- and two-well SCW system for each return pipe location. In these results, the position of the single-well SCW return pipe without bleeding increased by approximately 9% compared to the lower return pipe installation position. In the case of the single-well type SCW system, the effective thermal conductivity value obtained by evaluating thermal performance in bleeding and non-bleeding conditions was 1.6 times higher in the non-bleeding condition than in the 10% bleeding condition. However, as a result of evaluating the thermal performance of the geothermal Appl. Sci. 2020, 10, 2467 15 of 18 heat exchanger of the two-well SCW system, the effective thermal conductivity value was evaluated as approximately four times higher with 25% bleeding than without bleeding. From these results, it was proven that high thermal performance can be obtained by improving the groundwater flow of the groundwater suction at the well drainage well.

5. Conclusions This paper presented the effective thermal conductivity of closed boreholes and single- and two-well type SCW systems with bleeding conditions using a TRT device specially prepared for SCW system. The comparative analysis of the effective thermal conductivity was as follows: 1. The effective thermal conductivity values and thermal resistance increased by 9.03% and 44.91%, respectively, compared with those of the installed upper return pipe of the SCW system. This result indicates that the thermal resistance and conductivity values varied with the installed location of the return pipe. These results also show that the thermal resistance is an important design parameter. Hence, it can be concluded that the location of the return pipe in the SCW should be properly selected. 2. In the case of the single-well SCW system, from the results of evaluating thermal performance in the bleeding and non-bleeding state, the effective heat conductivity value of the underground heat exchanger improved 1.6 times in the 10% bleeding state compared to the bleeding-free state. However, the stain-free condition of the two-well type SCW system did not significantly affect the initial removal time, but the linear equation was changed by applying the initial removal time at 25% bleeding condition. When injecting 25% discharge into the SCW inlet, it is considered that the linear equation needs to be changed due to the influence of the groundwater flow, and it is necessary to change the stable measurement time according to the groundwater flow. 3. Finally, from the results of the thermal performance evaluation of the two-well type SCW system, the bleeding condition was improved by about four times compared to the non-bleeding condition. From these results, it was possible to achieve high thermal performance through improvement of the groundwater flow of the groundwater suction well at the drainage well.

Author Contributions: J.Y. and C.L. conceived and designed the experiments; J.Y. performed the experiments; C.L. analyzed the data; J.Y. contributed reagents/materials/analysis tools; C.L. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding or This research was funded by [the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT)] grant number [No. 2019R1A2C1010055] And The APC was funded by [the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT)]. Acknowledgments: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2019R1A2C1010055). Conﬂicts of Interest: The authors declare no conﬂict of interest.

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