Performance evaluation of closed-loop vertical ground heat exchangers by conducting in-situ thermal response tests

Renewable Energy 42 (2012) 77e83

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Performance evaluation of closed-loop vertical ground heat exchangers by conducting in-situ thermal response tests Chulho Leea, Moonseo Parka, The-Bao Nguyena, Byonghu Sohnb, Jong Min Choic, Hangseok Choia, * a

School of Civil, Environmental and Architectural Engineering, Korea University, 5 ga, Anam-dong, Seongbuk-gu, Seoul 136-701, South Korea Plant Research Division, Building & Urban Research Department, Korea Institute of Construction Technology, Goyang, South Korea c Department of Mechanical Engineering, Hanbat National University, Daejeon, South Korea b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 December 2010 Accepted 8 September 2011 Available online 29 September 2011

The effective thermal conductivity of six vertical closed-loop ground heat exchangers (GHEXs), which were installed in a test bed located in Wonju, South Korea, has been experimentally evaluated by performing in-situ thermal response tests (TRTs). To compare the thermal efficiency of the GHEXs in field, various installation conditions are considered such as different grouting materials (cement vs. bentonite), different additives (silica sand vs. graphite) and shapes of the circulating pipe-section (conventional U-loop type vs. 3-pipe type). From the test results, it can be concluded that the cement grout has higher effective thermal conductivity than the bentonite grout by 7.4e10.1%, and the graphite outperforms the silica sand by 6.7e9.1% as a thermally-enhancing additive. In addition, the new 3-pipe type heat exchange pipe that yields less thermal interference between the inlet and outlet pipes shows better thermal performance over the conventional U-loop type heat exchange pipe by 14.1 e14.5%. Based on the results from the in-situ thermal response tests, a series of cost analyses has been carried out to show the applicability of the cement grouting, the graphite additive, and the new 3-pipe type of heat exchange pipe section. For the same condition, the cement grouting can reduce the construction cost of GHEXs by around 40% in the given cost analysis scenario. In addition, an addition of graphite and use the new 3-pipe heat exchange pipe lead to about 8% and 6% cost reduction, respectively. Ó 2011 Elsevier Ltd. All rights reserved.

Keywords: Ground heat exchanger In-situ thermal response test Thermal conductivity Thermal interference

1. Introduction The closed-loop vertical ground heat exchangers (GHEXs) with a U-loop are typically used as part of a ground source heat pump (GSHP) system when an available construction space is limited. For common residential houses, a borehole for the GHEXs is usually designed from 100 to 250 m deep with a diameter of 10e15 cm, depending on a level of energy demand and a subsurface condition [3,6,7,11]. After the installation of the U-loop in the borehole, backfilling should be performed to fill an annular gap between the pipe and ground formation. Bentonite and cement grouts are commonly used as backfilling materials [1,3,7]. The well-known influence factors on heat exchange rate in this system are summarized as the thermal conductivity of grout backfilling boreholes, HDPE pipes, and adjacent geologic formations. The thermal efficiency of GHEXs can be improved by * Corresponding author. Tel.: þ82 (2) 3290 3326; fax: þ82 (2) 928 7656. E-mail address: [email protected] (H. Choi). 0960-1481/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2011.09.013

increasing the thermal conductivity of grout and HDPE pipes [12,14]. This leads to saving to some extent of construction cost along with minimizing the design length of the heat exchange pipe. In this paper, another factor, thermal interference between the inlet and outlet pipes, which is additionally discussed in detail later. To investigate the thermal efficiency of GHEXs, six boreholes were constructed with various construction conditions: different grouting materials (cement vs. bentonite), different additives (silica sand vs. graphite), and different shapes of pipe-sections (conventional U-loop type vs. new 3-pipe type) at a test bed in Wonju, South Korea. Thereafter, the thermal performance of each GHEX was evaluated by conducting a series of in-situ thermal response tests (TRTs). Lastly, the design length of GHEXs for an artificial heating/cooling load for an imaginary building is estimated based on the in-situ TRT results of each borehole. A series of cost analyses has been carried out to show the applicability of the cement grouting, the graphite additive, and the new 3-pipe type of heat exchange pipe.

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C. Lee et al. / Renewable Energy 42 (2012) 77e83

Fig. 1. Layout of test bed with six GHEXs.

Table 1 Configuration of boreholes in test bed. Borehole

Grout

Additives

Pipe type

Borehole depth (m)

1 2 3 4 5 6

Cement Bentonite Cement Bentonite Cement Bentonite

Silica Silica Silica Silica Silica Silica

U-loop U-loop U-loop U-loop 3-pipe type 3-pipe type

81 142 150 148 137 148

sand sand sand þ Graphite sand þ Graphite sand sand

2. Field experiment

constructed to compare the thermal performance of each type. The configuration of boreholes in the test bed is summarized in Table 1. Grout mixture designs for the cement and bentonite grouts are shown in Table 2. The mixing ratio of cement grout is expressed as a weight ratio of water to cement (i.e, W/C). The mixing ratio of bentonite grout is expressed as a weight ratio of water to neat bentonite mixture (i.e., W/(B þ W)). The thermal conductivity of the grout materials that were sampled from the boreholes was measured in the laboratory and also summarized in Table 2 (in the last column). From the laboratory test result, the cement grout shows higher thermal conductivity than the bentonite grout. In addition, it can be shown that an addition of graphite significantly increased the thermal conductivity of the cement grout. A new 3-pipe type GHEX is devised to reduce thermal interference between the inlet and outlet pipes. In a cooling mode, the circulating fluid should be cooled down enough during circulating through the GHEX. However, if the inlet and outlet pipes are located so close that one pipe thermally interferes with another pipe, the relatively higher temperature in the inlet pipe may adversely heat up the circulating fluid in the outlet pipe especially near the ground surface. In a heating mode, it is vice versa. Such thermal interference will lessen thermal performance efficiency of GSHP systems. In addition, thermally enhanced grout materials may result in severer interference. In order to overcome such adverse condition, the new 3-pipe type GHEX has been devised, which is equipped with a dummy pipe between the inlet and outlet pipes. The dummy pipe is expected to function as a thermal insulation zone. Water is filled in the dummy pipe to overcome the buoyant force during inserting a bundle of the heat exchange pipes in the borehole. The schematic diagram of the new 3-pipe type GHEX is presented in Fig. 2. 2.2. In-situ thermal response test

2.1. Construction of GHEXs To evaluate relative thermal efficiency of ground heat exchangers (GHEXs) along with different installing conditions, six boreholes were constructed for different construction conditions such as grouting materials (cement vs. bentonite), different additives (silica sand vs. graphite), and the shape of pipe-sections (conventional U-loop type vs. new 3-pipe type) as shown in Fig. 1. To investigate the effect of grouting materials on the overall thermal performance of GHEXs, Boreholes 1, 3, and 5 are grouted with cement, and Boreholes 2, 4, and 6 are grouted with bentonite. To study the effect of additives, both graphite and silica sand are mixed in Boreholes 3 and 4, while only silica sand was mixed with grout in Boreholes 1 and 2 (for the conventional U-loop type GHEXs) and in Boreholes 5 and 6 (for the new 3-pipe type GHEXs). The conventional U-loop type has been designed in Boreholes 1, 2, 3, and 4, and the new 3-pipe type in Boreholes 5 and 6. Lee et al. [5] reported that graphite can be a promising alternative to silica sand as a thermally enhancing additive to the bentonite grout. In addition, two different types of the cross-section of GHEXs were

The thermal response test (TRT) method is commonly recognized as an effective means for the in-situ determination of the effective thermal conductivity of the ground formation. In this test, a known thermal load is applied to the GHEX along with accurate measurements of temperature and flow rate of the circulating fluid. The concept and analysis method for the TRT based on the line source model can be referred to numerous references [2,10,13,15]. Hence the effective ground thermal conductivity (l) based on the line source model can be evaluated from the slope (m) by plotting the average fluid temperature (Tf) of the inlet and outlet versus the natural logarithm of time (ln t) as follows.

l ¼

q q lnðt2 Þ  lnðt1 Þ ¼ 4pm 4p Tf ðt2 Þ  Tf ðt1 Þ

(1)

where, q is the heat injection rate per active length of borehole [W/m]. A schematic diagram of an in-situ thermal response test (TRT) apparatus is shown in Fig. 3. The TRT apparatus is composed of a water supply, an electric heater, a circulating pump, measuring

Table 2 Mixture design for grouts. Grout Mixture Design, kg (Mixing ratio)

Cement

Raw material

Water

Silica sand

120

90e100 (W/C ¼ 0.79)

120 (S/C ¼ 1.0)

Graphite

Plasticizer (ml) e

5 (G/C ¼ 0.04) Bentonite

25

120e130 (W/(B þ W) ¼ 0.17)

60 (S/(B þ W) ¼ 0.40)

500 10 (G/(B þ W) ¼ 0.07)

*C, B, W, S, G and w ¼ Cement, Bentonite, Water, Silica sand, Graphite and Water content of mixture.

Thermal conductivity (W/m$K) 2.10 2.59 0.73 (w ¼ 178%) 0.74 (w ¼ 280%)

C. Lee et al. / Renewable Energy 42 (2012) 77e83

Fig. 2. Schematic diagram of 3-pipe type.

instruments, and a data acquisition system. In this study, the TRTs have been performed for 48 hours following 1 hour of precirculation for guaranteeing thermal equilibrium conditions. A series of in-situ TRTs has been performed to evaluate the effective thermal conductivity of the six vertical closed-loop ground heat exchangers, which had been constructed at a test bed in Wonju, South Korea. The two columns of Borehole 1, 3, and 5 and Borehole 2, 4, and 6 are located 5 m apart, and the distance between each row is 2.5 m as illustrated in Fig. 1.

3. Experimental results and discussion The slope (m) of the relationship between the average fluid temperature and the natural logarithm of time (ln t) was measured 480 min after starting the TRTs to obtain a linear relationship between the temperature and ln t. The slope was used to calculate

79

the effective thermal conductivity of boreholes along with Eq. (1). The experimental results are shown in Fig. 4 and the calculated effective thermal conductivity is summarized in Table 3. For the bentonite-grouted boreholes (Boreholes 2, 4, and 6), the in-situ thermal response tests were performed at three days after grouting was finished. On the other hand, the cement-grouted boreholes (Boreholes 1, 3, and 5) needed at least 14 days elapsing for the in-situ thermal response tests due to the hydration heat of cement. It can be seen from the TRT results that the cement grout (Boreholes 1, 3, and 5) overall provides higher effective thermal conductivities than that of the bentonite grout (Boreholes 2, 4, and 6) by 7.4e10.1% with the same additive and pipe type. For example, with the same conventional U-loop pipe type GHEX, the effective thermal conductivity of Borehole 1 (i.e., 3.20 W/m$K) grouted with the cement-silica sand mixture is much higher than that of Borehole 2 (i.e., 2.97 W/m$K) grouted with the bentoniteesilica sand mixture. In comparison of the additives, the effective thermal conductivity of Boreholes 3 and 4 grouted with both graphite and silica sand additives is higher by 6.7e9.1% than that of Boreholes 1 and 2 at which only silica sand was added to the grout. The thermally enhancing effect of graphite over silica sand on grout materials was reported based on laboratory experiments by [5]. Note that the performance of GHEXs can be improved by using thermally enhanced grout. With respect to the shape of GHEX pipe sections, overall the new 3-pipe type provides higher thermal conductivity than the conventional U-loop pipe by 14.1e14.5%. For example, in comparison between Boreholes 1 and 5 which were grouted with the cement-silica sand mixture, the effective thermal conductivity of Borehole 1 with the conventional U-loop pipe was 3.20 W/m$K, while Borehole 5 equipped with the new 3-pipe type yielded the effective thermal conductivity of 3.65 W/m$K. It can be inferred from this comparison that the newly developed 3-pipe type GHEX

Fig. 3. Schematic diagram of in-situ TRT apparatus.

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C. Lee et al. / Renewable Energy 42 (2012) 77e83

Fig. 4. Experimental results of in-situ thermal response test.

C. Lee et al. / Renewable Energy 42 (2012) 77e83 Table 3 Effective thermal conductivities of boreholes.

81

Table 4 Input data for GLD program.

Borehole

Effective thermal conductivity, W/m$K

1 2 3 4 5 6

3.20 2.97 3.49 3.17 3.65 3.40

can significantly reduce the thermal interference between the inlet and outlet pipes, which improves the thermal performance of GHEXs. 4. Cost analysis for GHEXs based on in-situ test results

Length of simulation Design heat pump inlet temperatures Design system flow rate Ethanol concentration Undisturbed ground temperature Ground thermal conductivity Ground loop Ground loop pipe size and type Borehole diameter Grout thermal conductivity Borehole vertical grid arrangement Borehole spacing Heat pump nominal cooling capacity & COP Heat pump nominal heating capacity & COP

20 years 30  C for cooling, 5  C for heating 11.3 lpm/RT 12.9% 15.8  C Refer to Table 3 Single U-loop HDPE 32 mm and SDR11 150 mm 0.73, 0.74, 2.10, 2.59 W/m$K, respectively 8  9 (72 holes) 5 m (3-10m variance) 142.5 kW and 4.9 139.3 kW and 3.8

4.1. Design procedure Ingersoll [8] proposed a practical method to design the required GHEX pipe length for the given heating/cooling load. The method started with a simple steady-state heat transfer equation as follows:

(2)

where q ¼ heat capacity (W), L ¼ required vertical bore length (m), tg ¼ ground temperature (K), tw ¼ fluid temperature (K), and R ¼ thermal resistance of borehole (m$K/W). The Eq. (2) was transformed to represent the variable heat rate of a ground heat exchanger by using a series of constant heat rate pulses. The required vertical bore length, L, was then solved by [9] with consideration of the thermal resistance of the ground (per unit length), thermal resistance of the pipe wall, and interfaces between the fluid and pipe and between the pipe and the ground. The thermal resistance of the ground was calculated as a function of time corresponding to the time when a particular heat pulse occurs. The formulation for calculating the required vertical bore length, L, is as follows:

L ¼

  qa Rga þ ðql  WÞ Rb þ PLFm Rm þ Rgd Fsc tg 

twi þ two  tp 2

(3)

where qa ¼ net annual average heat transfer to the ground (W), Rga ¼ effective thermal resistance of the ground, annual pulse (m$K/W), ql ¼ building design block load (W), W ¼ power input at design load (W), Rb ¼ thermal resistance of borehole (m$K/W), PLFm ¼ part-load factor during design month, Rm ¼ effective thermal resistance of the ground, monthly pulse (m$K/W), Rgd ¼ effective thermal resistance of the ground, daily pulse (m$K/W), Fsc ¼ shortcircuit heat loss factor, tg ¼ undisturbed ground temperature (K), twi and two ¼ liquid temperature at heat pump inlet and outlet, respectively (K), and tp ¼ temperature penalty for interference of adjacent boreholes (K). To account for the long-term heat imbalance, average monthly heat rates, and maximum heat rates for a shortterm period, Eq. (3) considers three different pulses of heat. The designed pipe length of GHEX should be determined corresponding to the maximum heat load which is needed to operate the system. The pipe length of the GHEX for an imaginary building with an artificial heating/cooling load was designed along with the effective thermal conductivities of the boreholes that were measured by the TRTs. The program of Ground Loop Design (GLD) version 5.0 [4] was used in this study to obtain the required pipe length of the GHEX. The input data are summarized in Table 4. The imaginary building was assumed to be constructed using reinforced concrete and to

18 Total pipe lengths Cooling Heating

16 Total pipe length, L (km)

q ¼ Lðt0  tw Þ=R

have offices, meeting rooms, restaurants, etc. The dominant heat exchange process applied to the building was the cooling operation. The maximum cooling and heating loads were artificially selected as 522 and 451 kW, respectively. However, the total heating load was assumed greater than the total cooling load. Thus, the ground temperature around the GHEXs may gradually decrease with time every year. The highest and lowest entering water temperatures (EWT) were designed to be 30 and 5  C, respectively. To find the optimum design, the EWT was calculated for 20 years which is the design lifespan of the system. In order to evaluate the efficiency of the entire GSHP system, the total designed pipe lengths of the GHEXs were compared. The monthly energy load was calculated using the maximum heating load and time during the operation. The annual equivalent full load hours were then estimated using the monthly energy load. The initial temperature of ground was set as 15.8  C. The effective thermal conductivities for the six boreholes were obtained from the in-situ TRTs (refer to Table 3). The laboratory test procedure proposed by [3] was adopted in this study to evaluate the thermal conductivity of the grout

Borehole #2 Borehole #4

14

Borehole #6

12

10

8

U-tube configuration : B Borehole spacing : 5m Grout thermal conductivity, kg = 0.73 W/mK

6 1

1.5 2 2.5 3 3.5 Ground thermal conductivity, ks (W/mK)

Fig. 5. Designed total pipe length of bentonite-grouted GHEX.

4

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C. Lee et al. / Renewable Energy 42 (2012) 77e83

cementesilica sandegraphite mixture showed the thermal conductivity equal to 2.10 and 2.59 W/m$K, respectively.

15 Test borehole #4 Ground thermal conductivity, ks = 3.07W/mK Grout : bentonite&silica-sand mixture Grout thermal conductivity, kg = 0.74 W/mK

Total pipe length, L (km)

14 13

4.2. Designed pipe length and cost analysis

12 11 10 9 8 Total pipe lengths Cooling Heating

7 6 2

3

4 5 6 7 8 9 Space between boreholes (m)

10

11

Fig. 6. Designed total pipe length of GHEX depending on space between boreholes. (Borehole 4).

20 Grout thermal conductivity, kg (W/mK) Ground thermal conductivity, ks (W/mK)

18

kg = 0.73, ks = 2.00 kg = 0.73, ks = 3.00 kg = 0.73, ks = 4.00

Total pipe length, L (km)

16

kg = 2.59, ks = 2.00 kg = 2.59, ks = 3.00 kg = 2.59, ks = 4.00

14 12 10 8 6 4 2

3

4 5 6 7 8 9 Space between boreholes (m)

10

11

Fig. 7. Parametric study results for cooling operation.

mixtures. The thermal conductivity of the bentoniteesilica sand mixture measured 0.73 W/m$K, and the thermal conductivity of the bentoniteesilica sandegraphite mixture measured 0.74 W/ m$K. On the other hand, the cementesilica sand mixture and the

A comprehensive parametric study has been performed to design the GHEX pipe length of for the given imaginary building. Fig. 5 shows the designed total pipe length of the GHEX for the bentonite-grouted boreholes (Boreholes 2, 4, and 6) with various effective ground thermal conductivities. Fig. 6 shows the effect of adjacent borehole distance (i.e., space between boreholes) on the designed total pipe length of the GHEX for the conditions of Borehole 4 (U-loop; bentoniteesilica sandegraphite mixture). Fig. 7 shows results of the parametric study for the cooling operation that the designed total pipe length of the GHEX is rendered as a function of the effective ground thermal conductivity, the thermal conductivity of the grout material and the space between boreholes. Based on the preceding design, the construction cost of the GHEXs were estimated and compared in Table 5. The typical construction cost for a GHEX is about 66.7 USD/m according to the Korean market standard (at the exchange rate of 1200 Korean Won to the U.S. dollar). The construction cost includes a drilling and material (i.e., pipe, grout, etc.) cost of 16.7 USD/m) and a labor cost of 50.0 USD/m. For Borehole 6 which is fabricated with the 3-pipe type GHEX, the construction cost was assumed to be equal to the typical cost because the additional dummy pipe is not filled with the grout and so the cost of grout can be compensated. When 72 GHEXs are constructed with a borehole spacing of 5 m, the designed total pipe lengths of the GHEX are also presented in Table 5. In addition, Fig. 8 shows the required total pipe length design for the cooling and heating operations of 72 (9  8) boreholes with the six borehole conditions, respectively. For the bentonite-grouted boreholes (Boreholes 2, 4, and 6), the total construction costs are 81,599, 76,023 and 74,835 USD, respectively. Borehole 6 (3-pipe type; bentoniteesilica sand mixture) results in the lowest construction cost. However, the difference in the construction costs is marginal because the designed total pipe lengths of the GHEX become similar when the ground thermal conductivity is greater than 3.0 W/m$K as can be seen in Fig. 5. For smaller ground thermal conductivities, the total pipe lengths of the GHEX are considerably influenced by the magnitude of the ground thermal conductivity. In other words, the slope of relationship between the total pipe length and the ground thermal conductivity decreases as the ground thermal conductivity increases. Because the cement-grouted boreholes (Boreholes 1, 3, and 5) possess higher grout and ground thermal conductivities than that of the bentonite-grouted boreholes, the designed total pipe length of the GHEX is overall smaller than the case of bentonite-grouted boreholes. The construction cost for Borehole 3 (U-loop; cementesilica sandegraphite mixture) is the lowest (i.e., 43,589 USD) which is almost half of the construction cost for the dummy borehole.

Table 5 Comparison of construction costs. Borehole

dummy

Borehole 1

Borehole 2

Borehole 3

Borehole 4

Borehole 5

Borehole 6

Grouting Pipe section Ground k (W/m$K) Grout k (W/m$K) Total length for Cooling (m) Total length for Heating (m) Total construction cost based on cooling (USD)

BþS U-loop 2.0 0.73 12,954.2 10,633.0 86,361

CþS U-loop 3.20 2.10 7163.3 5992.4 47,755

BþS U-loop 2.97 0.73 11,864.8 9448.4 81,599

CþSþG U-loop 3.49 2.59 6538.4 5479.8 43,589

BþSþG U-loop 3.17 0.74 11,403.4 8980.7 76,023

CþS 3-pipe 3.65 2.10 6856.4 5658.6 45,709

BþS 3-pipe 3.40 0.73 11,225.2 8752.5 74,835

B: Bentonite, C: Cement, S: Silica sand, G: Graphite.

C. Lee et al. / Renewable Energy 42 (2012) 77e83

16

Total pipe length, L (km)

14

Total pipe lengths Cooling Heating

kg1 = 2.10 kg2 = 0.73 kg3 = 2.59 kg4 = 0.74 kg5 = 2.10 kg6 = 0.73

12 Bentonite

10 8

Cement

6 4

Borehole spacing : 5m

83

(3) The new 3-pipe type provides higher thermal conductivity than the conventional U-loop pipe by 14.1e14.5% because the new 3-pipe type GHEX can reduce the thermal interference between the inlet and outlet pipes, which improves the thermal performance of GHEXs. (4) From the imaginary design and cost analysis in this paper, the cement grouting can reduce the construction cost of GHEXs by around 40% compared to the bentonite grouting. In addition, an addition of graphite and use the new 3-pipe heat exchange pipe lead to about 8% and 6% cost reduction, respectively. The GHEX grouted with the cementesilica sandegraphite mixture and fabricated with the 3-pipe type heat exchange pipe thermally outperforms the other types. (5) In order to optimize the GHEX design, it is highly recommended simultaneously to enhance the grout thermal conductivity and to minimize thermal interference between the pipes.

kgN : Grout thermal conductivity of Nth borehole(W/mK) Acknowledgement

2 1

2

3 4 Borehole number

5

6

Fig. 8. Total pipe length for each borehole (borehole spacing ¼ 5 m).

For the same condition, the cement grouting can reduce the construction cost of GHEXs by around 40% in the given cost analysis scenario. In addition, an addition of graphite and use the new 3pipe heat exchange pipe lead to about 8% and 6% cost reduction, respectively. Consequently, it can be expected that the GHEX grouted with the cementesilica sandegraphite mixture and fabricated with the 3-pipe type heat exchange pipe thermally outperforms the other types. Note that not only the thermal enhancement of the grout material but also the thermal interference between the pipes should be considered to improve the thermal performance of the GHEX. Therefore, an optimum design of GHEXs can be achieved when both the thermally-enhanced grout materials and the thermal interference between the pipes are considered.

5. Conclusions The following conclusions are drawn based on the findings from the in-situ thermal conductivity tests for different construction conditions and the cost analysis: (1) The cement grout (Boreholes 1, 3, and 5) overall provides higher effective thermal conductivities than that of the bentonite grout (Boreholes 2, 4, and 6) by 7.4e10.1% with the same additive and pipe type. In addition, the cement-grouted boreholes require much smaller total pipe lengths than the bentonite-grouted boreholes. (2) The effective thermal conductivity of the boreholes grouted with both graphite and silica sand additives (Boreholes 3 and 4) is higher by 6.7e9.1% than that of Boreholes 1 and 2 at which only silica sand is added to the grout. This shows that the thermal performance of GHEXs can be improved by using thermally enhanced grout such as graphite.

The authors appreciate the support partially by the Construction Technology Innovation Program (Grant No. 06CTIPD04) from KICTEP, The Ministry of Land, Transport and Maritime Affairs and by National Research Foundation of Korea Grant funded by the Korean Government (#2010-0011159). References [1] Allan ML. Material characterization of superplasticized cementesand grout. Cement and Concrete Research 2000;30:937e42. [2] Carslaw SH, Jaeger JC. Conduction of heat in solids. 2nd ed. Oxford: Claremore Press; 1993. [3] Choi H, Lee C, Choi H-P, Woo S. A study on the physical characteristics of grout material for backfilling ground heat exchanger. Journal of Korea Geotechnical Society 2008;24(1):37e49. in Korean. [4] Gaia Geothermal LLC. Ground loop design version 5.0 manual, US; 2007. [5] Lee C, Lee K, Choi H, Choi H-P. Characteristics of thermally-enhanced bentonite grouts for geothermal heat exchanger in South Korea. Science China: Technological Sciences 2010;53(1):123e8. [6] Han J, Han G, Han H, Han C. Geothermal heat pump (GHP) system. Han Lim Won; 2005. in Korean. [7] IGSPHA. Grouting for vertical geothermal heat pump systems: engineering design and field procedures manual. Stillwater. US: International Source Heat Pump Association; 2000. [8] Ingersoll LR, Zobel OJ, Ingersoll AC. Heat conduction with engineering. Geological and other applications. New York: McGraw-Hill; 1954. [9] Kavanaugh SP, Rafferty K. Ground-source heat pumps - design of geothermal systems for commercial and institutional buildings. Atlanta: American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE); 1997. [10] Mogenson P. Fluid to duct wall heat transfer in duct heat storages. In: Proceedings of the International Conference on Subsurface Heat Storage in Theory and Practice. Swedish council for building research; 1983. [11] Pahud D, Matthey B. Comparison of the thermal performance of double Upipe borehole heat exchangers measured in situ. Energy and Buildings 2001; 33:503e7. [12] Paul ND, Remund CP. Physical, thermal and hydraulic properties of bentonitebased grouts. Electric Power Research Institute; 1997. Final Report No.TR109160, ERI Project RP3881e1. [13] Sharqawy MH, Mokheimer EM, Habib MA, Badr HM, Said NA, Al-Shayea SA. Energy, energy and uncertainty analyses of the thermal response test for a ground heat exchanger. International Journal of Energy Research 2009;33: 582e92. [14] Sohn B, Effect of grouting materials on ground effective thermal conductivity, KSME, 2007 Conference, pp. 1333e1338, 2007, in Korean [15] Wagner R, Clauser C. Evaluating thermal response tests using parameter estimation for thermal conductivity and thermal capacity. Journal of Geophysics and Engineering 2005;2:349e56.