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In-situ thermal response test for ground source heat pump system in Elazığ, Turkey
Energy and Buildings 41 (2009) 395–401
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
In-situ thermal response test for ground source heat pump system in Elazıg˘, Turkey Hikmet Esen a,*, Mustafa Inalli b a b
Department of Mechanical Education, Faculty of Technical Education, Fırat University, 23119, Elazıg˘, Turkey Department of Mechanical Engineering, Faculty of Engineering, Fırat University, 23279, Elazıg˘, Turkey
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 May 2008 Received in revised form 8 October 2008 Accepted 10 November 2008
Ground source heat pump (GSHP) systems exchange heat with the ground, often through a vertical, U-tube, borehole heat exchanger (BHE). The performance of this U-tube BHE depends on the thermal properties of the ground formation, as well as grout or backfill in the borehole. The design and economic probability of GSHP systems need the thermal conductivity of geological structure and thermal resistance of BHE. Thermal response test (TRT) method allows the in-situ determination of the thermal conductivity (l) of the ground formation in the vicinity of a BHE, as well as the effective thermal resistance (Rb) of this latter. Thermal properties measured in laboratory experiments do not comply with data of in-situ conditions. The main goal has been to determine same in-situ ground type of BHE, including the effect of borehole’s depths (60 m: VB2; 90 m: VB3). As shown in these results, l and Rb of the VB2/VB3 boreholes are determined as 1.70/1.70 W m1 K1and 0.05/0.03 K W1 m, respectively. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Ground source heat pump Vertical heat exchanger Thermal response test Thermal conductivity Thermal resistance
1. Introduction The ground is a nearly unlimited source, always available heat source/sink for heat pumps (HPs). Deep boreholes may be used as heat exchanger in the ground. GSHPs take advantage of the relatively stable temperature of the ground. A BHE is a ground heat exchanger (GHE) devised for the extraction or injection of thermal energy from/into the ground. The energy content of the ground is continuously revitalized by solar radiation and other heat transfer processes from the atmosphere. Because of the large heat capacity, the temperature of the ground is slowly varying with small amplitude, being thus close to an ideal heat source for a HP [1]. The performance of a GSHP system is determined by the HP characteristics, and by the thermal process in the ground with its heat exchanger. The temperature of the heat delivered from the ground must lie within a specified range depending on the HP, the performance of which deteriorates with falling temperatures. The BHE must be designed so that the required heat is delivered at proper temperatures. The construction costs of the BHEs are vital for the economical competitiveness of this type of air-conditioning systems [2].
* Corresponding author. Tel.: +90 424 237 0000/4228; fax: +90 424 236 7064. E-mail addresses: hikmetesen@firat.edu.tr, [email protected] (H. Esen), minalli@firat.edu.tr (M. Inalli). Abbreviations: BHE, borehole heat exchanger; GHE, ground heat exchanger; GSHP, ground source heat pump; HP, heat pump; VB1, borehole at 30 m depth; VB2, borehole at 60 m depth; VB3, borehole at 90 m depth; TRT, thermal response test. 0378-7788/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2008.11.004
Although they have been in use for years in developed countries and the performance of the components is well documented, the utilization of GSHPs in residential buildings is new in Turkey [3– 14]. This study presents that establishing vertical GSHP system by using national facilities first time in Turkey. Ground and grout properties were taken fixed in the study but only the depths of GHEs part were changed. The TRTs which are the most important parameters in design of these systems were done. By this way, we obtained the effective ground thermal conductivity (l) and the effective thermal resistance (Rb) values for different boreholes depths. They also have same ground and grout properties. This study aims that giving an idea about movement of underground water. Also, it has contributed to the geothermal mapping of Turkey. A known thermal load is applied to a BHE and the temperature development of the inlet and outlet temperatures are measured over time in a thermal analysis of borehole. This temperature response allows extrapolation of the thermal behaviour in the next time. TRT may be done using a device that is transportable and can be brought on-site to the borehole. One possible conceptual model for the explanation is to presume the ground to be a conductive medium and to determine the apparent l and other thermal parameters of this medium. The basic requirements of a thermal response apparatus are [15–17]: use a power load as steady as possible, record the development of the inlet and outlet temperature of the temperature of the BHE,
H. Esen, M. Inalli / Energy and Buildings 41 (2009) 395–401
396
Nomenclature Q H Rb Rp rb k m Tf Tf,o Tf,i Tsur t
l lp a ap g
injected heat power (kW) effective borehole depth (m) borehole thermal resistance (K W1 m) U-pipe thermal resistance (K W1 m) borehole radius (m) slope of the line constant heat carrier mean fluid temperature (K) outlet average heat carrier solution temperature of GHE (K) inlet average heat carrier solution temperature of GHE (K) undisturbed initial ground temperature (K) time from start (min) borehole thermal conductivity (W m1 K1) U-pipe thermal conductivity (W m1 K1) borehole thermal diffusivity (m2 s1) U-pipe thermal diffusivity (m2 s1) Euler’s constant (0.5772)
do this for a minimum time of ca. 50 h, evaluate according to rules set in this guidelines. By means of the increasing commercial use of TRT, the desire for shorter test duration became clear, in particular in the USA. A recommendation for a minimum of 50 h was given [15,17,18], which is compatible with the International Energy Agency (IEA) recommendations, but there is also scepticism [19]. A test time of ca. 12 h desired, which also would allow not having the test apparatus out on the site over night. In general, there are physical limits for the shortening of the measuring period, because a somewhat stable heat flow has to be achieved in the ground. In the first few hours, the temperature development is mainly controlled by the borehole
filling and not by the surrounding soil or rock. A time of 48 h is considered by the authors as a minimum test period [15]. The thermal properties of the ground are often estimated by utilizing a TRT in a ready to operate BHE. This approach was first proposed by Ref. [20] and is based on an infinite line-source model [21]. In 1995, the first mobile measurement devices were introduced in Sweden [22] and in the USA [23]. Since then, the method has been improved and its use has spread to several other countries [16,24–32]. In this study, the TRT means in-situ measurements of the heat transfer capacity of boreholes for energy injection. Two tests were carried out with the apparatus in the garden in Sultanus¸ag˘ı village, Elazıg˘, Turkey. The results of a response test are the in-situ determination of the effective ground thermal conductivity (l) and the effective thermal resistance (Rb) of the tested BHEs. 2. Experimental apparatus and methodology The vertical GSHP system using R-22 as refrigerant have a three single U-tube BHE made of polyethylene pipe with a 40 mm outside diameter. The BHE was placed in a vertical borehole (VB) with 30 m (VB1), 60 m (VB2) and 90 m (VB3) depths and 150 mm diameter. The horizontal distances between the boreholes are 3.4 m (VB3–VB2) and 2 m (VB1–VB2), respectively (see Fig. 1). The measurement points of the ground section are also shown in Fig. 1. A useful tool to do so is a TRT, carried out on a BHE in a pilot borehole (VB2 and VB3). The most common configuration for the vertical loop piping element in the drilled bore is a U-tube, where a 180-bend fitting has been factory fused to join two lengths of GHE pipe, and this inserted into the borehole (see Fig. 2a–e). Fig. 2a shows the operation of the adding bentonite for easily drilling borehole. This bentonite is also providing for not fall down of borehole during the drilling borehole. Fig. 2b depicts the ultimate state of the opening borehole. Fig. 2c illustrates the photograph of the weight (40– 50 kg) for easily down in hole of U-pipe. Fig. 2d presents the releases of U-pipe toward the down hole. Fig. 2e shows the ultimate state put into the holes of the bentonite backfill [14]. Where regulations require the grouting of boreholes, the following
Fig. 1. The sketch of ground section.
H. Esen, M. Inalli / Energy and Buildings 41 (2009) 395–401
397
Fig. 2. (a) The view of the operation adding bentonite. (b) The view of the opening borehole. (c) The photograph of the weight. (d) The photograph of the release down of Upipe into borehole. (e) The view of the bentonite backfill.
measures should be taken to minimize grout impact on thermal performance: reduce borehole diameter as much as possible to minimize the amount of grout needed, use a thermally enhanced grout formulation with a higher ratio of sand to bentonite in the grout mixture, use clips to push the U-tube elements apart, holding them against the borehole walls. Field tests are done in Sultanus¸ag˘ı village, Elazıg˘, Turkey with mobile a TRT apparatus. Turkish mobile TRT equipment has been donated by Lulea˚ University of Technology, Sweden. In this study, the test device has been hired by C¸ukurova University, Adana Turkey for two tests, 48 2 = 96 h. The TRTs were carried out over 4 days from 4 June to 7 June 2007. The in-situ equipment is installed on a mobile trailer and consists of a 1-kW pump circulating the heat carrier fluid (water)
through the borehole collector and through a cross-flow heater with adjustable and stable heating power in the range of 3–12 kW. Fluid temperature is measured at the inlet and outlet of the borehole with thermistors, with an accuracy of 0.2 8C. A data logger records the temperatures at a set time interval. The equipment is powered by 16 A electricity. The circulation pressure of flow is ca. 2–3 bar. Test apparatus of TRT is shown as in Fig. 3. Fig. 4 depicts the TRT flow diagram. The principle of the TRT is to inject a known amount of power into energy bore over a certain period of time, by letting a heat carrier fluid circulate through the energy bore piping system while a certain power rate is transferred to the fluid. The temperature response of the ground is measured by recording the inlet and outlet temperatures. The thermal properties of the ground and collector installation and proportional to the temperature change in the ground over the measurement period [30]. The general layout of a TRT is shown in Fig. 5.
398
H. Esen, M. Inalli / Energy and Buildings 41 (2009) 395–401
Fig. 3. TRT apparatus, coupled to a borehole heat exchanger. Fig. 5. Test set-up for a TRT.
For good results, it is crucial to set up the system correctly and to minimize external influences. This is done easier with heating the ground (electric resistance heaters) than with cooling (heat pumps). However, even with resistance heating, the fluctuations of voltage in the grid may result in fluctuations of the thermal power injected into the ground. Another source of deviation is climatic influences, affecting mainly the connecting pipes between test apparatus and BHE, the interior temperatures of the test apparatus, and sometimes the upper part of the BHE in the ground. Heavy insulation is required to protect the connecting pipes (see Fig. 1). With open or poorly grouted BHE, also rainwater intrusion may cause temperature changes. A longer test duration allows for statistical correction of power fluctuations and climatic influence, and results in more trustworthy evaluation [15]. In this study, two single boreholes which have 0.15 m diameter with 60 m (VB2) and 90 m (VB3) depths are drilled for TRT. The geological formation consists of mainly marn. A single polyethylene U-pipe (SDR-11) was installed in the boreholes. The pipe diameter and pipe wall thickness are 40 and 2.4 mm, respectively. The main characteristic of high-density polyethylene tube is shown in Table 1.
A mixture of bentonite and soil (marn) was used as grouting material in the field test. The composition of the grout pumped to the borehole is 70% water, 5% bentonite and 25% dried mud or soil. Inlet and outlet fluid temperatures and also ambient temperatures are recorded every 10 min by the data logger. The power supply is also recorded during measurement in order to determine the actual injected power. To determine undisturbed ground temperature, the heat carrier is initially circulated through the system without heating for 20–30 min. The mean fluid temperature along the piping will then show, and this temperature corresponds to the temperature of the undisturbed ground. After this procedure, the heater is switched on and the measurements are continued for minimum 48 h. 3. Data calculation and test evaluation The evaluation method widely accepted for simplicity and reasonable accuracy is based on the solution of the Line Source problem. A common difficulty quite often encountered when applying this model to analyse experimental data is that different
Fig. 4. TRT flow diagram.
H. Esen, M. Inalli / Energy and Buildings 41 (2009) 395–401 Table 1 Main characteristics of U-pipe SDR 11. External diameter Internal diameter Density Melting temperature Working temperature Peak temperature (momentary) Thermal conductivity (lp) Thermal resistance (Rp) Coefficient of linear expansion
40 mm 35.2 mm 0.96 g cm3 130 8C 90 8C 125 8C 0.4 (W m1 K1) 0.0815 (K W1 m) 0.17 mm/(mK)
time intervals lead to different slopes and this turn, leads to different values for the soil properties sought [27]. In the line source modelling, the equation for the temperature field as a function of time and radius around a line source with constant heat injection rate may be used as an estimation of the heat injection from BHE [30]. In this model, it is considered sufficient to quote the two formulate used to analyze the data. In Eq. (2), k is determined from
399
the slope of the line in the plot of ln time versus mean fluid temperature. All equations used to calculate thermal conductivity (l) and thermal resistance (Rb) are follows: T f ¼ k lnðtÞ þ m
(1)
where m and k are constants. Q 4pkH ( " ! # ) Q 1 4a m¼ ln 2 g Rb þ T sur H 4pl rb
l¼
( " * ! + # ) Q 1 4a lnðtÞ þ ln 2 g þ Rb þ T sur Tf ¼ 4plH H 4pl rb Q
(2) (3)
(4)
where l is the thermal conductivity (W m1 K1), Q the injected heat power (kW), Tf the heat carrier mean fluid temperature (K), rb the borehole radius (m), Tsur the denotes the undisturbed initial ground temperature (K) in borehole, a the thermal diffusivity
Fig. 6. Mid- to late-stage time/temperature data for the experiment. Slope (k) of linear relationship is 3.6687. This value is substituted into Eq. (1).
Fig. 7. Mean fluid temperature from response test on U-pipe adjusted to Eq. (4). l = 1.70 W m1 K1, Rb = 0.05 K W1 m.
400
H. Esen, M. Inalli / Energy and Buildings 41 (2009) 395–401
Fig. 8. Mid- to late-stage time/temperature data for the experiment. Slope (k) of linear relationship is 3.5182. This value is substituted into Eq. (1).
(m2 s1), H the effective borehole depth (m), t the time from start, Rb the thermal resistance (K W1 m), and g is the Euler’s constant (0.5772). The slope of the mean temperature data versus the natural log of time in seconds given in Fig. 6 is proportional to the thermal conductivity of the rock and filled material through which the heat is transferred. According to the line sources model for these measurements, l and Rb are calculated with an iterative approach where l is given an initial estimated value and Rb is calculated from Eq. (4). The iteration is continued until calculated fluid temperature distribution fits the experimental distribution for 48 h in each measurement. Fig. 7 depicts the best iterative approach for the thermal conductivity (l) and thermal resistance (Rb) for VB2 state. The error value for during the TRT of VB2 is obtained as 1.85%. This test was made between 4 June 2007 and 5 June 2007. The slope of the mean temperature data versus the natural log of time in seconds given in Fig. 8 is proportional to the thermal
conductivity of the rock and filled material through which the heat is transferred. According to the line sources model for these measurements, l and Rb are calculated with an iterative approach where l is given an initial estimated value and Rb is calculated from Eq. (4). The iteration is continued until calculated fluid temperature distribution fits the experimental distribution for 48 h in each measurement. Fig. 9 depicts the best iterative approach for the thermal conductivity (l) and thermal resistance (Rb) for VB3 state. The error value for during the TRT of VB3 is obtained as 2.22%. This test was made between 6 June 2007 and 7 June 2007. The parameters of the TRT for VB2 and VB3 states are shown in Table 2. The in-situ response test has been used mainly to obtain accurate estimates of the thermal characteristics of the ground. However, there are a number of other important applications of such a test facility. In the design of a GHE, many decisions have to be made beforehand: drilling method and depth, type of loop (concentric or U-loop), type of backfilling to use, etc. Even though increasing borehole depths the ground temperature increases and
Fig. 9. Mean fluid temperature from response test on U-pipe adjusted to Eq. (4). l = 1.70 W m1 K1, Rb = 0.03 K W1 m.
H. Esen, M. Inalli / Energy and Buildings 41 (2009) 395–401 Table 2 Parameters of the TRT at VB2 and VB3 states. Test parameters
Test duration Undisturbed ground temperature Injected heat Depth of borehole Borehole diameter Power of heater Collector type Specific heat (Cp) Thermal diffusivity (a)
States VB2
VB3
48 h 15.7 8C
48 h 15.7 8C
4.90 kW 60 m 150 mm 3 kW Single U-tube SDR11 2,200,000 J kg1 K1 7.73 107 m2 s1
4.90 kW 90 m 150 mm 6 kW Single U-tube SDR11 2,200,000 J kg1 K1 7.73 107 m2 s1
is stable. The same thermal conductivity values are obtained for two regions (60 and 90 m), but the values of the thermal resistances are different. These differences in thermal resistances can be attributed to the flowing groundwater movement, the difference of boreholes depths, the weather variations during the measuring period (even if the tests for VB2 and VB3 are carried out consecutive days), and the error of the construction process. 4. Conclusions The in-situ TRT provides an effective method to determine the ground thermal characteristics required for the design of a GSHP system installation. When the ground-type and the environmental confines are favourable, the use of grouts for the filling material is to be recommended. From experience and the results obtained we draw the following conclusions: The effective values of 1.70 W m1 K1, 0.05 K W1 m were determined for the thermal conductivity l and borehole thermal resistance Rb respectively for VB2 state. The effective values of 1.70 W m1 K1, 0.03 K W1 m were determined for the thermal conductivity l and borehole thermal resistance Rb respectively for VB3 state. The accuracy of the evaluation depends on the care taken when performing the test. It may be stated that thermal response measurements with mobile apparatus may have many potential applications in the future for our country, such as testing of new U-pipe materials, characteristic, standardization, quality control and certification of GSHP applications, and in-situ pre-research of thermal properties for large GSHP systems. Acknowledgements The authors would like to acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK) for its financial support through contract no. 106Y188 (2006–2007) and Dr. Bekir Turgut for his contribution to this project. References [1] M.T. Kangas, Thermohydraulic analysis of ground as a heat source for heat pumps using vertical pipes, Transactions of the ASME 118 (December) (1996) 300–305.
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Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
In-situ thermal response test for ground source heat pump system in Elazıg˘, Turkey Hikmet Esen a,*, Mustafa Inalli b a b
Department of Mechanical Education, Faculty of Technical Education, Fırat University, 23119, Elazıg˘, Turkey Department of Mechanical Engineering, Faculty of Engineering, Fırat University, 23279, Elazıg˘, Turkey
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 May 2008 Received in revised form 8 October 2008 Accepted 10 November 2008
Ground source heat pump (GSHP) systems exchange heat with the ground, often through a vertical, U-tube, borehole heat exchanger (BHE). The performance of this U-tube BHE depends on the thermal properties of the ground formation, as well as grout or backfill in the borehole. The design and economic probability of GSHP systems need the thermal conductivity of geological structure and thermal resistance of BHE. Thermal response test (TRT) method allows the in-situ determination of the thermal conductivity (l) of the ground formation in the vicinity of a BHE, as well as the effective thermal resistance (Rb) of this latter. Thermal properties measured in laboratory experiments do not comply with data of in-situ conditions. The main goal has been to determine same in-situ ground type of BHE, including the effect of borehole’s depths (60 m: VB2; 90 m: VB3). As shown in these results, l and Rb of the VB2/VB3 boreholes are determined as 1.70/1.70 W m1 K1and 0.05/0.03 K W1 m, respectively. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Ground source heat pump Vertical heat exchanger Thermal response test Thermal conductivity Thermal resistance
1. Introduction The ground is a nearly unlimited source, always available heat source/sink for heat pumps (HPs). Deep boreholes may be used as heat exchanger in the ground. GSHPs take advantage of the relatively stable temperature of the ground. A BHE is a ground heat exchanger (GHE) devised for the extraction or injection of thermal energy from/into the ground. The energy content of the ground is continuously revitalized by solar radiation and other heat transfer processes from the atmosphere. Because of the large heat capacity, the temperature of the ground is slowly varying with small amplitude, being thus close to an ideal heat source for a HP [1]. The performance of a GSHP system is determined by the HP characteristics, and by the thermal process in the ground with its heat exchanger. The temperature of the heat delivered from the ground must lie within a specified range depending on the HP, the performance of which deteriorates with falling temperatures. The BHE must be designed so that the required heat is delivered at proper temperatures. The construction costs of the BHEs are vital for the economical competitiveness of this type of air-conditioning systems [2].
* Corresponding author. Tel.: +90 424 237 0000/4228; fax: +90 424 236 7064. E-mail addresses: hikmetesen@firat.edu.tr, [email protected] (H. Esen), minalli@firat.edu.tr (M. Inalli). Abbreviations: BHE, borehole heat exchanger; GHE, ground heat exchanger; GSHP, ground source heat pump; HP, heat pump; VB1, borehole at 30 m depth; VB2, borehole at 60 m depth; VB3, borehole at 90 m depth; TRT, thermal response test. 0378-7788/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2008.11.004
Although they have been in use for years in developed countries and the performance of the components is well documented, the utilization of GSHPs in residential buildings is new in Turkey [3– 14]. This study presents that establishing vertical GSHP system by using national facilities first time in Turkey. Ground and grout properties were taken fixed in the study but only the depths of GHEs part were changed. The TRTs which are the most important parameters in design of these systems were done. By this way, we obtained the effective ground thermal conductivity (l) and the effective thermal resistance (Rb) values for different boreholes depths. They also have same ground and grout properties. This study aims that giving an idea about movement of underground water. Also, it has contributed to the geothermal mapping of Turkey. A known thermal load is applied to a BHE and the temperature development of the inlet and outlet temperatures are measured over time in a thermal analysis of borehole. This temperature response allows extrapolation of the thermal behaviour in the next time. TRT may be done using a device that is transportable and can be brought on-site to the borehole. One possible conceptual model for the explanation is to presume the ground to be a conductive medium and to determine the apparent l and other thermal parameters of this medium. The basic requirements of a thermal response apparatus are [15–17]: use a power load as steady as possible, record the development of the inlet and outlet temperature of the temperature of the BHE,
H. Esen, M. Inalli / Energy and Buildings 41 (2009) 395–401
396
Nomenclature Q H Rb Rp rb k m Tf Tf,o Tf,i Tsur t
l lp a ap g
injected heat power (kW) effective borehole depth (m) borehole thermal resistance (K W1 m) U-pipe thermal resistance (K W1 m) borehole radius (m) slope of the line constant heat carrier mean fluid temperature (K) outlet average heat carrier solution temperature of GHE (K) inlet average heat carrier solution temperature of GHE (K) undisturbed initial ground temperature (K) time from start (min) borehole thermal conductivity (W m1 K1) U-pipe thermal conductivity (W m1 K1) borehole thermal diffusivity (m2 s1) U-pipe thermal diffusivity (m2 s1) Euler’s constant (0.5772)
do this for a minimum time of ca. 50 h, evaluate according to rules set in this guidelines. By means of the increasing commercial use of TRT, the desire for shorter test duration became clear, in particular in the USA. A recommendation for a minimum of 50 h was given [15,17,18], which is compatible with the International Energy Agency (IEA) recommendations, but there is also scepticism [19]. A test time of ca. 12 h desired, which also would allow not having the test apparatus out on the site over night. In general, there are physical limits for the shortening of the measuring period, because a somewhat stable heat flow has to be achieved in the ground. In the first few hours, the temperature development is mainly controlled by the borehole
filling and not by the surrounding soil or rock. A time of 48 h is considered by the authors as a minimum test period [15]. The thermal properties of the ground are often estimated by utilizing a TRT in a ready to operate BHE. This approach was first proposed by Ref. [20] and is based on an infinite line-source model [21]. In 1995, the first mobile measurement devices were introduced in Sweden [22] and in the USA [23]. Since then, the method has been improved and its use has spread to several other countries [16,24–32]. In this study, the TRT means in-situ measurements of the heat transfer capacity of boreholes for energy injection. Two tests were carried out with the apparatus in the garden in Sultanus¸ag˘ı village, Elazıg˘, Turkey. The results of a response test are the in-situ determination of the effective ground thermal conductivity (l) and the effective thermal resistance (Rb) of the tested BHEs. 2. Experimental apparatus and methodology The vertical GSHP system using R-22 as refrigerant have a three single U-tube BHE made of polyethylene pipe with a 40 mm outside diameter. The BHE was placed in a vertical borehole (VB) with 30 m (VB1), 60 m (VB2) and 90 m (VB3) depths and 150 mm diameter. The horizontal distances between the boreholes are 3.4 m (VB3–VB2) and 2 m (VB1–VB2), respectively (see Fig. 1). The measurement points of the ground section are also shown in Fig. 1. A useful tool to do so is a TRT, carried out on a BHE in a pilot borehole (VB2 and VB3). The most common configuration for the vertical loop piping element in the drilled bore is a U-tube, where a 180-bend fitting has been factory fused to join two lengths of GHE pipe, and this inserted into the borehole (see Fig. 2a–e). Fig. 2a shows the operation of the adding bentonite for easily drilling borehole. This bentonite is also providing for not fall down of borehole during the drilling borehole. Fig. 2b depicts the ultimate state of the opening borehole. Fig. 2c illustrates the photograph of the weight (40– 50 kg) for easily down in hole of U-pipe. Fig. 2d presents the releases of U-pipe toward the down hole. Fig. 2e shows the ultimate state put into the holes of the bentonite backfill [14]. Where regulations require the grouting of boreholes, the following
Fig. 1. The sketch of ground section.
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Fig. 2. (a) The view of the operation adding bentonite. (b) The view of the opening borehole. (c) The photograph of the weight. (d) The photograph of the release down of Upipe into borehole. (e) The view of the bentonite backfill.
measures should be taken to minimize grout impact on thermal performance: reduce borehole diameter as much as possible to minimize the amount of grout needed, use a thermally enhanced grout formulation with a higher ratio of sand to bentonite in the grout mixture, use clips to push the U-tube elements apart, holding them against the borehole walls. Field tests are done in Sultanus¸ag˘ı village, Elazıg˘, Turkey with mobile a TRT apparatus. Turkish mobile TRT equipment has been donated by Lulea˚ University of Technology, Sweden. In this study, the test device has been hired by C¸ukurova University, Adana Turkey for two tests, 48 2 = 96 h. The TRTs were carried out over 4 days from 4 June to 7 June 2007. The in-situ equipment is installed on a mobile trailer and consists of a 1-kW pump circulating the heat carrier fluid (water)
through the borehole collector and through a cross-flow heater with adjustable and stable heating power in the range of 3–12 kW. Fluid temperature is measured at the inlet and outlet of the borehole with thermistors, with an accuracy of 0.2 8C. A data logger records the temperatures at a set time interval. The equipment is powered by 16 A electricity. The circulation pressure of flow is ca. 2–3 bar. Test apparatus of TRT is shown as in Fig. 3. Fig. 4 depicts the TRT flow diagram. The principle of the TRT is to inject a known amount of power into energy bore over a certain period of time, by letting a heat carrier fluid circulate through the energy bore piping system while a certain power rate is transferred to the fluid. The temperature response of the ground is measured by recording the inlet and outlet temperatures. The thermal properties of the ground and collector installation and proportional to the temperature change in the ground over the measurement period [30]. The general layout of a TRT is shown in Fig. 5.
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Fig. 3. TRT apparatus, coupled to a borehole heat exchanger. Fig. 5. Test set-up for a TRT.
For good results, it is crucial to set up the system correctly and to minimize external influences. This is done easier with heating the ground (electric resistance heaters) than with cooling (heat pumps). However, even with resistance heating, the fluctuations of voltage in the grid may result in fluctuations of the thermal power injected into the ground. Another source of deviation is climatic influences, affecting mainly the connecting pipes between test apparatus and BHE, the interior temperatures of the test apparatus, and sometimes the upper part of the BHE in the ground. Heavy insulation is required to protect the connecting pipes (see Fig. 1). With open or poorly grouted BHE, also rainwater intrusion may cause temperature changes. A longer test duration allows for statistical correction of power fluctuations and climatic influence, and results in more trustworthy evaluation [15]. In this study, two single boreholes which have 0.15 m diameter with 60 m (VB2) and 90 m (VB3) depths are drilled for TRT. The geological formation consists of mainly marn. A single polyethylene U-pipe (SDR-11) was installed in the boreholes. The pipe diameter and pipe wall thickness are 40 and 2.4 mm, respectively. The main characteristic of high-density polyethylene tube is shown in Table 1.
A mixture of bentonite and soil (marn) was used as grouting material in the field test. The composition of the grout pumped to the borehole is 70% water, 5% bentonite and 25% dried mud or soil. Inlet and outlet fluid temperatures and also ambient temperatures are recorded every 10 min by the data logger. The power supply is also recorded during measurement in order to determine the actual injected power. To determine undisturbed ground temperature, the heat carrier is initially circulated through the system without heating for 20–30 min. The mean fluid temperature along the piping will then show, and this temperature corresponds to the temperature of the undisturbed ground. After this procedure, the heater is switched on and the measurements are continued for minimum 48 h. 3. Data calculation and test evaluation The evaluation method widely accepted for simplicity and reasonable accuracy is based on the solution of the Line Source problem. A common difficulty quite often encountered when applying this model to analyse experimental data is that different
Fig. 4. TRT flow diagram.
H. Esen, M. Inalli / Energy and Buildings 41 (2009) 395–401 Table 1 Main characteristics of U-pipe SDR 11. External diameter Internal diameter Density Melting temperature Working temperature Peak temperature (momentary) Thermal conductivity (lp) Thermal resistance (Rp) Coefficient of linear expansion
40 mm 35.2 mm 0.96 g cm3 130 8C 90 8C 125 8C 0.4 (W m1 K1) 0.0815 (K W1 m) 0.17 mm/(mK)
time intervals lead to different slopes and this turn, leads to different values for the soil properties sought [27]. In the line source modelling, the equation for the temperature field as a function of time and radius around a line source with constant heat injection rate may be used as an estimation of the heat injection from BHE [30]. In this model, it is considered sufficient to quote the two formulate used to analyze the data. In Eq. (2), k is determined from
399
the slope of the line in the plot of ln time versus mean fluid temperature. All equations used to calculate thermal conductivity (l) and thermal resistance (Rb) are follows: T f ¼ k lnðtÞ þ m
(1)
where m and k are constants. Q 4pkH ( " ! # ) Q 1 4a m¼ ln 2 g Rb þ T sur H 4pl rb
l¼
( " * ! + # ) Q 1 4a lnðtÞ þ ln 2 g þ Rb þ T sur Tf ¼ 4plH H 4pl rb Q
(2) (3)
(4)
where l is the thermal conductivity (W m1 K1), Q the injected heat power (kW), Tf the heat carrier mean fluid temperature (K), rb the borehole radius (m), Tsur the denotes the undisturbed initial ground temperature (K) in borehole, a the thermal diffusivity
Fig. 6. Mid- to late-stage time/temperature data for the experiment. Slope (k) of linear relationship is 3.6687. This value is substituted into Eq. (1).
Fig. 7. Mean fluid temperature from response test on U-pipe adjusted to Eq. (4). l = 1.70 W m1 K1, Rb = 0.05 K W1 m.
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Fig. 8. Mid- to late-stage time/temperature data for the experiment. Slope (k) of linear relationship is 3.5182. This value is substituted into Eq. (1).
(m2 s1), H the effective borehole depth (m), t the time from start, Rb the thermal resistance (K W1 m), and g is the Euler’s constant (0.5772). The slope of the mean temperature data versus the natural log of time in seconds given in Fig. 6 is proportional to the thermal conductivity of the rock and filled material through which the heat is transferred. According to the line sources model for these measurements, l and Rb are calculated with an iterative approach where l is given an initial estimated value and Rb is calculated from Eq. (4). The iteration is continued until calculated fluid temperature distribution fits the experimental distribution for 48 h in each measurement. Fig. 7 depicts the best iterative approach for the thermal conductivity (l) and thermal resistance (Rb) for VB2 state. The error value for during the TRT of VB2 is obtained as 1.85%. This test was made between 4 June 2007 and 5 June 2007. The slope of the mean temperature data versus the natural log of time in seconds given in Fig. 8 is proportional to the thermal
conductivity of the rock and filled material through which the heat is transferred. According to the line sources model for these measurements, l and Rb are calculated with an iterative approach where l is given an initial estimated value and Rb is calculated from Eq. (4). The iteration is continued until calculated fluid temperature distribution fits the experimental distribution for 48 h in each measurement. Fig. 9 depicts the best iterative approach for the thermal conductivity (l) and thermal resistance (Rb) for VB3 state. The error value for during the TRT of VB3 is obtained as 2.22%. This test was made between 6 June 2007 and 7 June 2007. The parameters of the TRT for VB2 and VB3 states are shown in Table 2. The in-situ response test has been used mainly to obtain accurate estimates of the thermal characteristics of the ground. However, there are a number of other important applications of such a test facility. In the design of a GHE, many decisions have to be made beforehand: drilling method and depth, type of loop (concentric or U-loop), type of backfilling to use, etc. Even though increasing borehole depths the ground temperature increases and
Fig. 9. Mean fluid temperature from response test on U-pipe adjusted to Eq. (4). l = 1.70 W m1 K1, Rb = 0.03 K W1 m.
H. Esen, M. Inalli / Energy and Buildings 41 (2009) 395–401 Table 2 Parameters of the TRT at VB2 and VB3 states. Test parameters
Test duration Undisturbed ground temperature Injected heat Depth of borehole Borehole diameter Power of heater Collector type Specific heat (Cp) Thermal diffusivity (a)
States VB2
VB3
48 h 15.7 8C
48 h 15.7 8C
4.90 kW 60 m 150 mm 3 kW Single U-tube SDR11 2,200,000 J kg1 K1 7.73 107 m2 s1
4.90 kW 90 m 150 mm 6 kW Single U-tube SDR11 2,200,000 J kg1 K1 7.73 107 m2 s1
is stable. The same thermal conductivity values are obtained for two regions (60 and 90 m), but the values of the thermal resistances are different. These differences in thermal resistances can be attributed to the flowing groundwater movement, the difference of boreholes depths, the weather variations during the measuring period (even if the tests for VB2 and VB3 are carried out consecutive days), and the error of the construction process. 4. Conclusions The in-situ TRT provides an effective method to determine the ground thermal characteristics required for the design of a GSHP system installation. When the ground-type and the environmental confines are favourable, the use of grouts for the filling material is to be recommended. From experience and the results obtained we draw the following conclusions: The effective values of 1.70 W m1 K1, 0.05 K W1 m were determined for the thermal conductivity l and borehole thermal resistance Rb respectively for VB2 state. The effective values of 1.70 W m1 K1, 0.03 K W1 m were determined for the thermal conductivity l and borehole thermal resistance Rb respectively for VB3 state. The accuracy of the evaluation depends on the care taken when performing the test. It may be stated that thermal response measurements with mobile apparatus may have many potential applications in the future for our country, such as testing of new U-pipe materials, characteristic, standardization, quality control and certification of GSHP applications, and in-situ pre-research of thermal properties for large GSHP systems. Acknowledgements The authors would like to acknowledge the Scientific and Technological Research Council of Turkey (TUBITAK) for its financial support through contract no. 106Y188 (2006–2007) and Dr. Bekir Turgut for his contribution to this project. References [1] M.T. Kangas, Thermohydraulic analysis of ground as a heat source for heat pumps using vertical pipes, Transactions of the ASME 118 (December) (1996) 300–305.
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