Bulletin of Engineering Geology and the Environment https://doi.org/10.1007/s10064-018-1407-2
ORIGINAL PAPER
A field study on the application of distributed temperature sensing technology in thermal response tests for borehole heat exchangers Dingfeng Cao 1,2 & Bin Shi 1
&
Hong-Hu Zhu 1 & Guangqing Wei 3 & Hainar Bektursen 1 & Mengya Sun 1
Received: 30 June 2017 / Accepted: 13 October 2018 # Springer-Verlag GmbH Germany, part of Springer Nature 2018
Abstract Although the enhanced thermal response test (ETRT) method has been used to determine the distribution of ground temperatures and effective thermal conductivities, there are a number of obstacles which limit the wide application of this technology in the discipline of geoengineering. In this literature, four aspects of ETRT technology were investigated: (a) acquisition of ground temperature, (b) installation of the heat exchange tubes, (c) optimization of the monitoring positions, and (d) the difference in thermal conductivity obtained by the ETRT and numerical simulation. To investigate these issues, a field trial was carried out in Heze, Shandong Province, China, and the corresponding numerical models were built. The results demonstrate that: (i) the conventional methods that infer undisturbed ground temperature using water in tubes have large errors, whereas the distributed temperature sensing (DTS) technique enables the measurement of precise temperature profiles; (ii) the thermal conductivity measured using double U-tubes reflects the soil thermal property more accurately than that for a single U-tube; (iii) it is more reasonable to install optical fibers outside the U-tube sidewall than inside the observation tube; and (iv) it is essential to quantitatively consider various interface thermal impedance when estimating ground thermal conductivities using numerical simulation. Keywords Distributed temperature sensing (DTS) . Thermal response test (TRT) . Ground-coupled heat pump (GCHP) . Thermal conductivity . Fiber optic sensor
Introduction Environmental and geological problems caused by the exploitation and burning of fossil fuels have brought grave threats to human survival and development, i.e., soil metallization, land subsidence in diggings, air haze, ozone holes, and increasing sea levels (Chang and Kim 2016; Guo et al. 2016). These issues have also caused prominent harm to human health (Guo et al. 2016). It is found that 40% of fossil fuel energy * Bin Shi
[email protected] * Hong-Hu Zhu
[email protected] 1
School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
2
Department of Civil and Environmental Engineering, Engineering College, University of Wisconsin–Madison, Madison, WI 53706, USA
3
Suzhou NanZee Sensing Technology Co., Ltd., Suzhou 215123, China
consumption in the world is used to heat, cool, and light buildings. Improving energy usage efficiency and changing energy structure are, therefore, necessary to combat global warming, improve air and soil quality, and save resources (Borinaga-Treviño et al. 2013; Miyara 2015). First, the insulation capacities of buildings should be enhanced to reduce energy losses (Belzile et al. 2016). In addition, fossil fuels should be replaced by renewable energy. Among various renewable energy sources, geothermal energy is the most sustainable and reliable, with the advantages of low cost and zero greenhouse gas emissions (Lee et al. 2013a, b). The most effective equipment for exploiting geothermal energy is the ground-coupled heat pump (GCHP), which allows a building to maintain a constant temperature by exchanging heat with a geothermal source (Chang and Kim 2016; Bandos et al. 2016). To extract geothermal energy, ground thermal conductivity must be determined beforehand to design suitable GCHP devices (Tang et al. 2015). According to the literature, there are four basic approaches to evaluate ground thermal conductivity, i.e., by using the engineering analogy method, laboratory heat probe tests, in
D. Cao et al.
situ thermal response tests (TRTs), and numerical simulation (Ramstad et al. 2015; Lhendup et al. 2014; Zhang et al. 2014; Homuth et al. 2013; Liebel et al. 2012a, b). If ground thermal conductivity is evaluated referring to existing data or using heat probes, the results are generally not representative (Nguyen et al. 2017; Zhang et al. 2016; Wang et al. 2016; Jha et al. 2016; Liuzzo-Scorpo et al. 2015; Dehkordi et al. 2015a; Platts et al. 2015; Lhendup et al. 2014; Ramstad et al. 2007; Fujii et al. 2005; Nusier and Abu-Hamdeh 2003). The tested soil samples have different properties from in situ soil in terms of structures, temperature, moisture content, and stress state. Although numerical simulation is a powerful tool for analyzing complicated problems that cannot be solved by conventional testing methods, the numerical results are highly dependent on the initial and boundary conditions (Soriano et al. 2017; Tordrup et al. 2017; You et al. 2017; Biglarian et al. 2017; Wołoszyn and Gołaś 2016; Dai et al. 2016; Lee 2016; Priarone and Fossa 2015; Rees 2015). Therefore, it only provides qualitative results of heat transfer characteristics in ground but can hardly perform accurate calculations for a specific case. It was reported by Lhendup et al. (2014) that the TRT was first proposed by Mogensen in 1983, which is the most commonly used method in determining ground thermal conductivity at present. The factors affecting TRT have been extensively studied, including the features of test devices, measurement of undisturbed ground temperature, heat input rate, and test duration (Spitler and Gehlin 2015). After being improved, TRT can be used to accurately measure the effective thermal conductivity of the entire borehole and describe the thermal transfer properties of all the strata where the borehole is located. However, the thermal conductivity of each soil stratum cannot be determined. Luo et al. (2016) proposed that the heat transfer process in the subsurface is largely affected by ground heterogeneity; therefore, it is indispensable to develop a new distributed technology with small spatial and temporal intervals to investigate the soil heterogeneity. In recent years, the distributed temperature sensing (DTS) technology has developed rapidly. With the use of corrosionresistant and anti-electromagnetic optical fibers as sensors, DTS has been successfully applied in ground temperature monitoring (Coleman et al. 2015). Some scholars have conducted an enhanced thermal response test (ETRT), in which optical fibers are installed on (or in) U-tubes to record real-time borehole temperatures. Shim et al. (2009) found that the ground thermal conductivity obtained using the ETRT method is consistent with that from theoretical calculations and laboratory experiments. By comparing ETRT results and geological data in Tokyo, Japan, Fujii et al. (2009a, b) demonstrated the practicability and reliability of ETRTs. Acuña (2010) and Acuña and Palm (2013) designed a new pipe-in-pipe equipment and inserted optical fibers into the space between the external pipe and the borehole wall, and recorded the instantaneous vertical
temperature profiles during ETRTs. Luo et al. (2015) proposed that natural ground is generally layered with different geological strata and groundwater flow is often limited to aquifers, defined as a stratum with high hydraulic conductivity. Luo et al. (2015) also quantitatively computed the thermal conductivity of each soil layer at the test site and found that the thermal conductivity of an aquifer with obvious groundwater can be 4.4 times larger than that of the geological stratum with negligible groundwater flow. Radioti et al. (2016) measured the thermal conductivity of bedrock on the campus of the University of Liège and pointed out that the long-term heat transfer behavior of GCHPs in this area can be predicted by combining ETRT results and some numerical (or mathematical) models. Although ETRTs can be used to obtain the thermal conductivities of individual stratum and capture ground temperature profiles, there are four unsolved problems that limit the popularization of this technology. The first problem pertains to the difference between the undisturbed geothermal field obtained by the ETRT and that achieved using conventional methods— which method is more reliable and accurate? The second issue is how to determine the installation position of the optical fibers. Luo et al. (2015) installed an optical fiber on the outside sidewall of a tube, whereas Acuña and Palm (2013) inserted it into a tube, but neither discussed which approach is more suitable. The third problem is what the difference is between the thermal conductivity obtained by numerical simulation based on DTS data and linear fitted results—which method is more accurate or how to improve numerical models. Luo et al. (2015) used the former and Fujii et al. (2009a) adopted the latter, but neither analyzed the differences. The last problem is whether a double U-tube or a single U-tube is more suitable for ETRTs. In view of the above problems, the purpose of this study was to investigate these problems by carrying out a field ETRT in Heze, Shandong Province, China. Furthermore, several numerical models were established for evaluating the long-term ground heat exchange potential and providing a more comprehensive basis for the design of GCHPs.
Basic principle of ETRTs The measurement principle of ground thermal conductivity is based on the rate of radial transmission of heat. A U-tube is placed in the ground through a borehole and the two ends of the U-tube are connected to a heat exchange heater. An optical fiber fixed on the outer surface of the U-tube (or in it) is used to retrieve temperature readings along the borehole. Based on the slopes of the temperature–time curves, the thermal conductivity of each stratum can be calculated, as shown in Fig. 1. Among the models used to determine thermal conductivity, the line-source models are the most commonly adopted in ETRTs (Lhendup et al. 2014). The stratum is assumed to be homogeneous, isotropic, and boundless, and the temperature
A field study on the application of distributed temperature sensing technology in thermal response tests...
42°
Heat exchange equipment
DTS interrogator
T(°C)
D(m) -50
200
U-tube Inlet
Optical fiber
Water bath
Heat Outlet
Soil 1 Soil 2 Soil 3 Soil 4 Soil 5 Soil 6 Soil 7
Fig. 1 Basic principle of the enhanced thermal response test (ETRT) to evaluate ground heat exchange capability
(T) recorded by DTS satisfies the following relationship (Hwang et al. 2010; Lhendup et al. 2014): T¼
P P 1 4α 5r2 for t ≥ b lnt þ ln 2 −γ þ Rb þ T 0 α 4πλD D 4πλ rb
ð1Þ where T is the measured temperature (K), P is the heat (or cool) power (W), λ is the effective thermal conductivity (W/mK), D is the borehole depth (m), t is the elapsed time from the start of heating (s), α is the thermal diffusivity (m 2 /s), r b is the borehole radius (m), γ is Euler’s constant (0.5776), Rb is the effective thermal resistance (mK/W), and T0 is the undisturbed ground temperature (K). On the right side of Eq. (1), the first term is a variable and the remaining terms are constant. Thus, Eq. (1) can be rewritten as: T¼
P lnt þ b 4πλi D
ð2Þ
Equation (2) can be further simplified to: T ¼ kx þ b
ð3Þ
P where b is a constant and k (k ¼ 4πλD ) is the slope between T and lnt. Thus, the effective thermal conductivity λ can be calculated as:
λi ¼
P 1 : 4πD k
ð4Þ
During the process of determining λ of an ETRT, the borehole temperature needs to be measured using DTS equipment. The working principle of DTS is based on the thermal sensitivity of the relative intensities of Raman Stokes and anti-Stokes light caused by collisions between the photons and electrons of the optical fiber (Grattan and Sun 2000; Tyler et al. 2009).
In situ ETRTs Site description A field trial was carried out in a borehole located in Heze, Shandong Province, China (35°14′ N, 115°26′ E). The 120m-long borehole passes through 22 strata, which can be divided into four types, namely, fine sand, silty clay, silt, and clay. The geological conditions and related parameters of each stratum are shown in Table 1.
Materials and methods A schematic diagram of the field ETRT devices is shown in Fig. 2. The heat exchange devices are mainly made up of a U-
D. Cao et al. Table 1
Geological conditions and basic parameters of the strata of the test site
No.
Depth (m)
Lithology
Density (g/cm3)
Porosity
Moisture content (%)
Degree of saturation (%)
Heat capacity (J/kg·K)
Diffusion coefficient (×10− 6 (m2/s))
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
0–5.8 5.8–13.0 13.0–15.6 15.6–17.5 17.5–29.2 29.2–35.2 35.2–39.3 39.3–41.7 41.7–44.8 44.8–47.0 47.0–53.9 53.9–56.5 56.5–65.8 65.8–71.0 71.0–78.1 78.1–83.6 83.6–93.4 93.4–101.0 101.0–104.0 104.0–107.0 107.0–114.8 114.8–120.0
Silt Clay Silty clay Fine sand Silty clay Silt Clay Fine sand Silt Fine sand Silt Fine sand Silt Clay Fine sand Silty clay Fine sand Clay Silty clay Silt Silty clay Clay
1.97 1.89 1.80 2.04 2.04 1.99 2.06 1.99 1.99 2.09 1.92 2.02 1.98 2.04 2.05 2.12 2.10 2.00 2.10 2.11 2.09 2.03
0.41 0.46 0.50 0.38 0.38 0.41 0.38 0.41 0.43 0.37 0.46 0.40 0.43 0.42 0.40 0.38 0.37 0.40 0.39 0.39 0.37 0.39
23.6 28.0 32.6 21.4 21.5 23.7 22.1 23.9 29.3 21.4 28.3 24.2 26.4 30.0 24.8 25.3 23.0 19.8 23.7 25.7 22.0 21.9
92.0 89.0 88.0 95.0 94.0 93.0 99.0 95.0 100 100 93.0 97.0 97.0 100 100 100 100 100 100 100 100 95.0
1465 1240 1235 1230 1235 1465 1240 1230 1465 1230 1465 1230 1465 1240 1230 1235 1230 1240 1235 1465 1235 1240
0.65 0.72 0.67 0.75 0.67 0.65 0.72 0.75 0.65 0.75 0.65 0.75 0.65 0.72 0.75 0.67 0.75 0.72 0.67 0.65 0.67 0.72
tube with an inner diameter of 2.9 cm, a buffer with a volume of 0.125 m3, three valves, a pump, a pressure gauge, a flow meter, and several thermal meters. The specific functions of these components can be found in Lhendup et al. (2014). The borehole has a diameter of 15 cm, and two U-tubes (A and B) are installed in it. The borehole is upright, and tubes A and B are at right angles to each other, as shown Fig. 2. The distance between the inlets and outlets of the U-tube was 6 cm, which was fixed by pump clamps. The distance from the inner wall of the borehole to the outside of the U-tube was 1.3 cm. An observation tube with an inner diameter of 2.9 cm was installed between the inlets and outlets of the U-tube, which is located in the center of the borehole. Two optical fibers are attached to the surfaces of the U-tubes using nylon ties. Another optical fiber is inserted into the observation tube. Figure 2 shows the installation positions. After the installation is completed, the borehole was backfilled with sand, and the observation tube and U-tube were filled with water. Finally, the temperature sensing optical fibers were connected to a DTS interrogator (model NZS-DTS-M06, produced by Suzhou NanZee Sensing Technology Co., Ltd., Suzhou, China). The basic NZS-DTS-M06 parameters are shown in Table 2. Prior to measurements, the water in the U-tubes and observation tube stood for 48 h to ascertain that the temperatures inside and outside the tubes attained equilibrium. The entire test was divided into five stages. The first stage was to measure the ground temperature using DTS, with a total measurement time of 24 h. After the 48-h equilibration was completed, the DTS interrogator was opened to measure the true values of the undisturbed ground temperature, with a 30-min collection time.
After measuring the true values, the water in the U-tubes was circulated at 1.62 L/s. During the second stage, U-tubes A and B were heated for 120 h at 6000 W. During the third stage, Utubes A and B were cooled for 48 h at 1200 W. During the fourth stage, U-tube A was cooled for 48 h at 1200 W and Utube B was disconnected. During the fifth stage, U-tube A was heated for 120 h at 6000 W. Throughout the process, DTS interrogation was opened to collect uninterrupted data.
Numerical model As shown in Fig. 3, a numerical model was built using COMSOL Multiphysics software. The purpose was to compare and analyze the thermal conductivities obtained by the ETRT and numerical simulation, calculate the temperature disturbance radius of the strata in Heze during the testing and GCHP operational process, and provide basic parameters for the design of GCHPs. When the thermal conductivity of each stratum was determined using the numerical model, the temperature recorded by DTS was taken as the verification condition. An empirical thermal conductivity value was input to the model to calculate the temperatures of the stratum during heating or cooling. If the calculated temperature was consistent with the measured results of DTS, the input value was considered to be the true value. If they were inconsistent, the input value was adjusted until they became completely consistent. During the calculation of the temperature disturbance radius, the average value for all the strata was used to comprehensively calculate the heat transfer characteristics.
A field study on the application of distributed temperature sensing technology in thermal response tests...
Electric heater/cooler
Fig. 2 Experimental setup and instrumentation details of the field ETRT
Buffer
Value Pump
Value
Value
Optical fiber
T( )
P
Pressure gauge
F
Flow meter
U-tube
D(m)
Temperature sensors Grout
DTS interrogator Optical fiber
Borehole
Inlet A B
O
B
A Observation tube
Outlet
Results and discussions Undisturbed ground temperature Both ground thermal conductivity and borehole thermal resistance are very sensitive to the initial undisturbed ground temperature field, and it is, therefore, very important to obtain an Table 2
Basic parameters of the NZS-DTS-M06 interrogator
Items
Performance parameters
Distance measurement range (km) Temperature measurement range (°C) Fiber type Temperature accuracy (°C) Response time (s) Spatial resolution (m) Operation humidity (%) Channel number Optical connector Communication protocol Power consumption (W)
50 − 40 to 120 Multimode (50/125) 0.1 10–300 1 0–95 4 E2000/APC MODBU 300
accurate measurement (Spitler and Gehlin 2015; Lee et al. 2013a, b). According to Spitler and Gehlin (2015) and Zhang et al. (2014), there are six common methods used to test undisturbed ground temperatures. Austin (1998) advocated installing a temperature probe in the borehole to measure the temperature at intervals and calculating the average results to best estimate the undisturbed ground temperature (Spitler and Gehlin 2015). Gehlin and Nordell (2003) suggested taking rapid measurements (every 10 s for their test) during the first transit of the fluid out of the borehole. Lim et al. (2007) recommended circulating the water in a U-tube for 20– 30 min without heating and measuring the average water temperature during that period for use as the undisturbed ground temperature. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) advised recording the temperature of the water in the U-tube immediately following circulation pumping (Zhang et al. 2014). Another method is to place a temperature sensor in the U-tube and test the temperature at different depths (Zhang et al. 2014). Spitler and Gehlin (2015) recommended measuring the water temperature during the initial circulation and using the lowest temperature as the ground undisturbed temperature. To evaluate the above commonly used methods, the
D. Cao et al. Fig. 3 Numerical model used in the study: a longitudinal profile (120 m deep and 12 m wide, axisymmetric), b cross section of the borehole
borehole temperatures during a 24-h circulation without heating or cooling are shown in Fig. 4. As can be seen in Fig. 4, once the water in the U-tubes began to circulate without heating or cooling, the exchange of heat between the upper and lower parts could not be avoided, especially in the first hour. The gradient in the water temperature gradually disappeared, and, finally, the entire borehole tended to be at a constant temperature. As shown in Fig. 4a, the true undisturbed ground temperature has a regular gradient, and, therefore, if Austin’s method is adopted to monitor this gradient Fig. 4 Borehole temperature change process under the action of water circulation in the U-tubes
distribution, a large number of probes are required and, so, the cost will be very high. The ASHRAE’s suggestion is too simple and does not illustrate how to determine the time interval. The advice to select the lowest temperature can only reflect the temperature at a specific position but not the average of the entire borehole (Spitler and Gehlin 2015). Theoretically, a temperature sensor dropped into a U-tube can accurately measure the ground temperature distribution, but this method requires a long collection time and cannot be used after water circulation begins. Furthermore, it is difficult to drop a sensor into a U-tube if the
A field study on the application of distributed temperature sensing technology in thermal response tests...
tube is not vertical. Therefore, the above conventional methods only yield approximate estimates of the average borehole temperature, but it is difficult to obtain accurate measurements over time. DTS technology enables the accurate measurement of undisturbed ground temperatures and can record the temperature distributions during the heating and cooling processes. The undisturbed ground temperatures for Heze as measured by DTS are shown in Fig. 5. It can be seen from Fig. 5 that, at 0–4-m depth, the ground temperature decreased rapidly as the depth increased. The temperature in this zone was dominated by solar radiation. Homuth et al. (2013) observed the same phenomenon in Hanau-Seligenstadt Basin (in Germany) by using borehole heat exchange (BHE) logs. The temperature from 4 to 16 m was affected by both solar radiation and heat from deeper ground. For the depth of 16–120 m in soil, the influence of solar radiation can be ignored, and the temperature is, therefore, only determined by the temperature of the underlying layers. As the depth increased, it is shown that the ground temperature increased linearly, and the correlation coefficient (R2) of the linear fit reached 0.991. According to the regular temperature distribution, the soil can be divided into two sections with respect to depth (I and II). The temperature in the first section is dominated by solar radiation and airflow, which changes with weather and season, and cannot be regarded as a reference for GCHP designs. The temperature in the second section is very stable and can be used to design GCHPs. For different regions and geological conditions, the boundary depth between the two sections may vary. For example, at
this site, the boundary depth is 16 m, and Fujii et al. (2009a) found that it was 30 m in Tokyo. As can be seen from the linearly fitted result in Fig. 5, the temperature gradient in the second section was 2.27 °C/100 m, which is reasonable according to the conclusions of Zhang et al. (2014).
Heat transfer characteristic of double U-tubes and single U-tube In the study of Wagner et al. (2013) and Oppelt et al. (2010), double U-tubes numerical models were built. Lee et al. (2013a, b) developed a single U-tube numerical model. The differences between these two methods in actual applications should be clarified. To compare the two methods, the borehole temperatures using double heated U-tubes and a single heated U-tube are shown in Fig. 6. As shown in Fig. 6, the recorded temperature profiles shapes were identical, which illustrates that both methods can be used to qualitatively evaluate the thermal conductivity distribution and to determine the best stratum for exchanging heat. Under the same heating power, the temperature of the outside sidewall surface of the double U-tubes was lower than that of the single U-tube, which indicates that the heat transfer rate of the double U-tubes was larger than that of the single U-tube. This is because the double U-tubes had a larger contact area with the surrounding soil. In addition, compared with a single U-tube, the heat transfer for double U-tubes was more isotropic and better aligned with the linear heat source. Furthermore, the borehole-installed double U-tubes have lower thermal resistance, which is beneficial for the TRTs and GCHPs. Hence, Sandler et al. (2017) suggested increasing the pipe diameters, Chang and Kim (2016) recommended inserting more tubes in the borehole, and Dehkordi et al. (2015b) advised decreasing the space between U-tube and borehole wall. Therefore, in this paper, the double U-tubes data were selected to calculate the thermal conductivities of the strata.
Monitoring position selection
Fig. 5 Undisturbed ground temperature profile measured by distributed temperature sensing (DTS) and the linearly fitted result
There are certain advantages and disadvantages when installing optical fibers in observation tubes or on the outside sidewall of U-tubes. If the temperature sensing fiber is installed in the observation tube, scratches and damage of the fiber can be effectively avoided during installation. If any problems occur, the optical fiber in the observation tube can be easily replaced. However, when using this installation method, the sensitivity of the sensor will reduce due to inner water convection in the vertical direction, which leads to reduced temperature differences at different depths. If the optical fiber is installed on the outside sidewall of U-tubes, the sensing fiber is very sensitive to the surrounding soils, but it is vulnerable during installation. To analyze the differences between the two kinds of installation technologies, the
D. Cao et al. Fig. 6 Borehole temperatures under a constant heating power (500 W/m)
temperatures recorded by different optical fibers in different positions are plotted in Fig. 7. As shown in Fig. 7, the temperatures recorded by the optical fiber fixed on the outside sidewall of U-tube A were higher than those recorded in the observation tube. This phenomenon can be explained as follows. The size of the heat source was thought to be so small that its influence was negligible when calculating the ground thermal conductivity. However, the Fig. 7 Comparison of the temperature recorded in different positions under double U-tubes heating
monitoring results show that the temperature field was not homogeneous. The temperature of the outside sidewall of the U-tube should be closer to that of the borehole sidewall, and, therefore, it is more rational to use the outside sidewall temperature to calculate the ground thermal conductivity than the internal temperature. In addition, it can be seen that the temperature profile measured by the optical fiber in the observation tube was smoother than the outside temperature, which
A field study on the application of distributed temperature sensing technology in thermal response tests...
would greatly increase errors in the analyses. In this study, the temperature recorded by the optical fiber on the outside sidewall of U-tube A was adopted to calculate the ground thermal conductivity. It is recommended to install the temperature sensing optical fiber on the outside sidewall in GCHPs for long-term temperature monitoring. Scraping damage of the fibers can be avoided by improving the strength of the cable jacket or by smearing lubricating oil on the cable surface.
Borehole temperature distribution during the heating and cooling processes To compare the heat transfer characteristics of different strata and verify the effectiveness of the DTS results, the temperature profiles after heating and cooling for 48 h, respectively, are presented in Fig. 8. Figure 8 shows that, at depths of 4– 120 m, there is a strong negative correlation between the temperatures recorded during the heating and cooling processes. At depths of 0–4 m, the temperatures in the heating and cooling processes were quite high, which is attributed to solar radiation and air flow, and was consistent with the results of undisturbed ground temperature in the section titled BUndisturbed ground temperature^. In addition, it can be clearly seen that, for the fine sand (between the depths of 39.3 and 41.7 m), the temperature was much higher than those of the other layers during the cooling process, but it was much lower during the heating process. This abnormal phenomenon means that there was water seepage in this layer, which was confirmed by geotechnical investigation work in this area. To further accurately quantify the ground thermal properties of each layer, an effective time period (teff) should be appropriately selected before fitting the temperature–log time curve. Because the borehole temperatures during the initial Fig. 8 Borehole temperatures after heating (or cooling) by the double U-tubes for 48 h
few hours are significantly affected by a large number of factors, many scholars have advised to disregard the initial few hours of data (Lhendup et al. 2014). Although different discard times (or the start time of teff) have been proposed by different scholars, the principles are the same. All of them considered that a time period within which the thermal conductivity does not change with elapsed time should be chosen. Currently, there are few studies on the end time of teff, because most TRTs are heating tests, and the test duration is generally less than 120 h (Poulsen and Alberdi-Pagola 2015). Under such circumstances, there is a good linear relationship between temperature and logarithm of time, and no end time of teff is needed. However, in this paper, both heating and cooling tests were conducted. During the cooling process, an end time of teff should be determined though the relationship between temperature rise (or drop) and elapsed time. The end time refers to the maximum heating (or cooling) time. If the test duration exceeds the end time of teff, the temperatures of the strata will remain constant, and, under that condition, the relationship between temperature and log time is not linear. Therefore, how to determine the appropriate lower and end time is very important. Five typical strata temperatures and the average temperature of the inlet and outlet were selected to determine teff, as shown in Fig. 9. The five strata include a clay layer, a silty clay layer, a silt layer, a fine sand layer with seepage, and a fine sand layer without seepage. Figure 9a, b shows that the temperature–time curves are very smooth, which indicates that the heating and cooling powers were very stable. Figure 9a, c shows that the temperature and logarithm of time have a good linear relationship during the 5–120-h heating period. Figure 9b, d shows that the logarithm of temperature deviated from a linear trend after 20 h. Therefore, teff during the heating process was selected
D. Cao et al.
Fig. 9 Temperature changes for different layers during heating and cooling processes: a relationship between temperature and time during the heating process, b relationship between temperature and time during
the cooling process, c relationship between temperature and the logarithm of time during the heating process, and d relationship between temperature and the logarithm of time during the cooling process
as 5–120 h, and in the cooling process, it was chosen as 1– 20 h.
underground and then transform it into other forms of energy, during the energy transformation, some energy emitted from the electric current was lost, which was not considered when estimating cool power. Therefore, it is necessary to develop new methods to accurately assess cool power. It can be determined from the thermal conductivity curves that all the parts in the same layer are supposed have identical heat transfer properties during the heating and cooling processes. The average thermal conductivities of the fine sand (39.3–41.7 m depth) measured during the heating and cooling processes reached 1.9 W/mK, which indicates that the thermal conductivity of this layer was mainly dependent on seepage instead of soil temperature. It can be seen from Fig. 10c, d that the thermal conductivities calculated by the numerical method for most strata were larger than those calculated using Eq. (4) because the numerical model constructed in this research cannot consider the impacts of the soil interface and inhomogeneities. There are interfacial thermal resistances between the water, pipe walls, grout, and borehole sidewalls, together with various soil structures. These thermal resistances prevent heat transfer during the heating or cooling processes (Sourbeer and Loheide
Ground thermal conductivity The thermal conductivities measured by the ETRT and calculated using the numerical method are shown in Fig. 10. Here, the temperature measured by the ETRT were collected during the second and third experimental stages. Water circulation in the double U-tubes occurred during these stages, and the temperature was measured by the sensing cable fixed on the surface of U-tube A. It can be seen from Fig. 10a, b that the thermal conductivity obtained in the cooling process was lower than that in the heating process, which was determined by the influence of heat (or cool) power. The heat and cool powers were calculated by the electric current and resistance; however, this is an approximate method, especially when estimating the cool power. During the heating process, almost the entire heat emitted by electric current was absorbed by the ground, so, under this case, the heating power can be accurately calculated by the current and resistance. While during the cooling process it was needed to absorb heat from
A field study on the application of distributed temperature sensing technology in thermal response tests...
Fig. 10 Ground thermal conductivity distribution in Heze: a thermal conductivity measured by ETRT, b comparison of the thermal conductivity measured by ETRT during the heating and cooling
processes, c comparison of the thermal conductivities measured by ETRT and that calculated using the numerical method, and d error analysis
2016). As can be seen from Fig. 10d, the error from the numerical method decreased with depth [taking the results obtained by Eq. (4) as the true values]. This is because the lateral earth pressure increases with depth and the gap between the soil and U-tube (or in the soil) decreases, so the interface effects are weaker. Therefore, it is essential to take interfacial thermal resistance into account when estimating soil thermal conductivity using numerical simulation, especially during the long-term operation of GCHPs (Sourbeer and Loheide 2016). Figure 10 shows that the average thermal conductivities of all the strata were 1.84 W/mK during the heating process and 1.68 W/mK during the cooling process. Considering the average temperature of the inlet and outlet, the calculated average thermal conductivities of all the strata were 1.82 W/mK during the heating process and 1.69 W/mK during the cooling process. The absolute errors were 0.02 W/mK and 0.01 W/mK, respectively. Although ETRTs can provide us with continuous vertical thermal information, if we want to know the relevant parameters on the cross section, a numerical model must be built. Among those cross parameters, active heat disturbance radiuses of ETRTs and GCHPs are indispensable for designing the heat pump well spacing.
thermal conductivities of the surrounding strata, and heating (or cooling) time. It can be seen from Fig. 11 that, after heating for 40, 80, and 120 h, the disturbance radii were 0.6, 0.92, and 1.32 m, respectively. Therefore, for future ETRTs under similar geological conditions, if an additional observation borehole is needed, the appropriate distance from the main heat exchange borehole to the observation hole should be in the range 0.5–1 m. If the distance is less than 0.5 m, the requirements on drilling technology are very rigorous. If the distance exceeds 1 m, it is very difficult to observe ground temperature changes, or the observed results will have large errors. The difference in heat transfer between a single U-tube and double U-tubes has been discussed in the section titled BHeat transfer characteristic of double U-tubes and single U-tube^, but the heat disturbance radii of a single U-tube and double Utubes were not determined. To calculate these radii, numerical simulations were conducted, and the results are shown in Fig. 12. It can be seen from Fig. 12 that, under the same power, the difference in the heat disturbance radius between a single Utube and double U-tubes was only 0.15 m, which indicates that this difference was mainly reflected in the heat source itself and that the impact of the surrounding strata was very small. However, if the temperature obtained by a single Utube is adopted to calculate the ground thermal conductivity, the calculation results will be larger than the true values; therefore, the double U-tubes data were selected for this research. It should be pointed out that the heat disturbance radii observed here are only suitable for ETRTs under a constant power condition. However, during a practical GCHP operational process, the heat disturbance radius of a double U-tube will be
Active heat disturbance radius Figure 11 shows the ground temperatures after double U-tubes heating for 40, 80, and 120 h in the ETRT. In the following analyses, a temperature change of 1 C° is used as the criterion to determine the active heat disturbance zone. The disturbance radius is affected by many factors, including heating power, borehole diameter, grout properties,
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Fig. 11 Average ground temperature increase under double U-tubes heating for 120 h: a temperature contour after heating for 40 h, b temperature contour after heating for 80 h, c temperature contour after heating
for 120 h, d temperature profile of the surrounding soil after heating for 40 h, e temperature profile of the surrounding soil after heating for 80 h, and f temperature profile of the surrounding soil after heating for 120 h
larger than that of a single U-tube because the GCHP usually works at a constant temperature (CT) rather than a constant power (CP). The long-term heat disturbance radii for this ground were also calculated using a finite element method. In this study area, buildings are typically cooled for 6, 7, 8, and 9 months per year, and a cooling period of 120 days was, therefore,
taken as the total calculation period. During actual GCHP operational processes, ground temperature changes are very complex and are affected by many factors, such as climate changes, extracting intensity, exploitation technology, air temperature, and people’s lifestyles. Herein, the extreme case was chosen, assuming that the temperature in this region was maintained at its maximum recorded value of 39.5 °C for 6,
Fig. 12 Average strata temperature under a constant heating power for 120 h
A field study on the application of distributed temperature sensing technology in thermal response tests...
Fig. 13 Average ground temperatures after heating for 120 days: a comparison of temperature–time curves under constant power (500 W/ m) and constant temperature (39.5 °C) heating conditions, b average soil
temperatures under constant power (500 W/m) heating for 120 days, and c average soil temperatures under constant temperature (39.5 °C) heating for 120 days
7, 8, and 9 months, and the heat transfer process was then simulated. The advantage of this assumption is to make GCHP designs more conservative and to achieve maximum energy usage efficiencies. For comparison with the constant temperature condition, the heat transfer process at a constant power was also simulated, and the simulation results are shown in Fig. 13. It can be seen from Fig. 13 that the rate of temperature increase under a constant temperature condition is much larger than that under a constant power condition, which is consistent with the findings of Yu et al. (2016). Figure 13b, c shows that, after operating GCHPs for 120 days, the heat disturbance radius under a constant power (500 W/m) was 6 m, but under a constant temperature condition (39.5 °C), it was 8 m. Therefore, in the design of GCHPs, the recommended spacing between two adjacent heat exchange wells should be at least 16 m to effectively avoid interaction effects.
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Conclusions In this paper, undisturbed ground temperature profiles in Heze, Shandong Province, China were determined using the distributed temperature sensing (DTS) system, and the distribution of strata thermal conductivity was obtained and analyzed. By combining the DTS technique with the finite element method, four aspects of the enhanced thermal response test (ETRT) were investigated: undisturbed ground temperature acquisition, U-tube installation, monitoring position selection, and thermal conductivity reliability evaluation. Finally, the heat disturbance radii in ETRT and groundcoupled heatpump (GCHP) operational processes were calculated, which provided the basis parameters for designing GCHPs for this area in the future. The following are the primary findings of this study:
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The common methods used to indirectly infer geothermal fields by measuring water temperatures in U-tubes produce large errors. These methods cannot be used to obtain ground temperature profiles, and only DTS can achieve the goal of obtaining continuous geothermal profiles in real time. The undisturbed ground temperature determined by DTS showed that the depth can be divided into two sections according to the geotemperature distribution. For the upper section (0–16 m depth), the temperature is significantly affected by solar radiation, air temperature, and flow. For the lower section (16–120 m depth), the ground temperature increases linearly with depth, and the temperature gradient is 2.27 °C/100 m. For different areas, the boundary of these two sections may be different, so before designing GCHPs, temperature profiles should be measured by DTS. For an ETRT, the ground thermal conductivity determined from double U-tubes is closer to the true value than that from a single U-tube. The numerical simulation results show that, although the heat disturbance radius difference between double U-tubes and a single U-tube was confined to 0.15 m, the thermal conductivities determined using a single Utube were larger than the true values, which is detrimental when designing GCHPs. Therefore, double U-tubes are recommended for future ETRTs. It is preferable to install an optical fiber on the outside sidewall of the U-tube than to insert it into an additional observation tube because this approach can avoid the influence of water circulation in the tubes and record more accurate borehole wall temperatures. It is essential to quantitatively consider various interface thermal impedance when estimating ground thermal conductivities using numerical simulation. Otherwise, the numerically calculated thermal conductivities will be much larger than the true values, which is harmful for GCHP design.
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Among the strata under Heze, due to the impact of seepage, the thermal conductivity in the fine sand (39.3–41.7 m depth) was larger than those for the other fine sand layers during the heating and cooling processes. The finite element calculation results show that the heat disturbance radius of this ETRT was 1.2 m, but it may be as large as 8 m in future GCHP operational processes, so the suggested distance between two adjacent heat wells should be greater than 16 m to avoid interaction effects. For some other sites, this distance should be calculated using numerical simulation based on the results obtained by DTS. Acknowledgements The authors would like to thank all the participants of the experimental studies. The financial support provided by the National Natural Science Foundation of China (grant nos. 41230636, 41427801, and 41722209), Research Funds for the Central Universities (grant no. 020614380050), the Key Laboratory of Earth Fissures Geological Disaster, Ministry of Land and Resources, and Geological Survey of Jiangsu Province (grant no. 201401) are gratefully acknowledged. The first author is grateful for the scholarship provided by the China Scholarship Council.
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