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Distributed thermal response tests on pipe-in-pipe borehole heat exchangers
Applied Energy 109 (2013) 312–320
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Distributed thermal response tests on pipe-in-pipe borehole heat exchangers José Acuña ⇑, Björn Palm KTH, Royal Institute of Technology, Brinellvägen 68, 100 44 Stockholm, Sweden
h i g h l i g h t s " Distributed thermal response tests are for first time carried out on pipe-in-pipe BHEs. " Published data sets serve as reference for researchers to validate pipe-in-pipe BHE models. " The borehole wall temperature is measured along the borehole during all DTRTs. " Discussion on measured local and effective borehole resistances in pipe-in-pipe BHEs. " Thermal performance of tested pipe-in-pipe BHEs is about 100% better than U-pipes.
a r t i c l e
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Article history: Received 4 July 2012 Received in revised form 2 January 2013 Accepted 7 January 2013 Available online 8 February 2013 Keywords: Borehole thermal resistance Borehole heat exchanger Coaxial Pipe-in-pipe Distributed thermal response test
a b s t r a c t Borehole Thermal Energy Storage systems typically use U-pipe Borehole Heat Exchangers (BHE) having borehole thermal resistances of at least 0.06 K m/W. Obviously, there is room for improvement in the U-pipe design to decrease these values. Additionally, there is a need for methods of getting more detailed knowledge about the performance of BHEs. Performing Distributed Thermal Response Tests (DTRT) on new proposed designs helps to fill this gap, as the ground thermal conductivity and thermal resistances in a BHE can be determined at many instances in the borehole thanks to distributed temperature measurements along the depth. In this paper, results from three heat injection DTRTs carried out on two coaxial pipe-in-pipe BHEs at different flow rates are presented for the first time. The tested pipe-in-pipe geometry consists of a central tube inserted into a larger external flexible pipe, forming an annular space between them. The external pipe is pressed to the borehole wall by applying a slight overpressure at the inside, resulting in good thermal contact and at the same time opening up for a novel method for measuring the borehole wall temperature in situ, by squeezing a fiber optic cable between the external pipe and the borehole wall. A reflection about how to calculate borehole thermal resistance in pipe-in-pipe BHEs is presented. Detailed fluid and borehole wall temperatures along the depth during the whole duration of the DTRTs allowed to calculate local and effective borehole thermal resistances and ground thermal conductivities. Local thermal resistances were found to be almost negligible as compared to U-pipe BHEs, and the effective borehole resistance equal to about 0.03 K m/W. The injected power was found to be almost evenly distributed along the depth. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Borehole Thermal Energy Storage (BTES) systems are normally designed based on simple thermal response tests that give average information about ground thermal conductivity, undisturbed ground temperature, and borehole thermal resistance. Also, BTES systems typically use U-pipe borehole heat exchangers having borehole thermal resistances (between 0.06 and 0.12 K m/W, or
⇑ Corresponding author. Tel.: +46 8 7908941. E-mail address: [email protected] (J. Acuña). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.01.024
even higher). However, the rapid growth of Ground Source Heat Pumps (GSHP) installations for BTES systems makes more detailed knowledge about ground thermal properties desirable, and also calls for more efficient Borehole Heat Exchangers (BHE). The relevance of this research topic has been recently confirmed by Spitler and Bernier [1] and by the European Platform for Renewable Heating and Cooling [2]. Promoting these areas is the main objective of the present study, focusing on in situ Distributed Thermal Response Tests (DTRT) carried out on two pipe-in-pipe coaxial BHEs. Such tests and proper temperature measurements in this type of heat exchanger has been missing until recently.
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In a Distributed Thermal Response Test (DTRT), the ground thermal conductivity and thermal resistance in a BHE can be determined at many locations along the borehole, as demonstrated by [3,4] on a U-pipe BHE and a coaxial Multi-pipe BHE, respectively. Similar studies, but only of the ground thermal conductivity, have been presented by [5,6]. Laboratory scale tests using distributed temperature measurements not necessarily with fiber optics are also possible and have been done by Eslami-nejad and Bernier [7] and Beier et al. [8]. Two independent U-pipe systems are used in [7] to study methods to thaw surrounding ground in order to reduce the borehole depth. About 20 thermocouples and a six point temperature profile probe was used. The experiment results verify a numerical model that can account for multiple radial ground layers as the freezing of the ground in the vicinity of boreholes is studied. In [8], laboratory experiments including a large amount of measurement points inside a sand box provide data for U-pipe BHE testing and for validating ground thermal conductivity estimation models. In this paper, results from three DTRTs carried out on two coaxial pipe-in-pipe BHEs are presented for the first time, and their thermal performance is studied in detail, opening up new knowledge windows within BTES systems that clarify how measured temperatures during a thermal response test should be interpreted in terms of thermal borehole resistances. Local and effective borehole resistances are separated from each other in order to distinguish how the surrounding ground and a heat pump evaporator can be affected, respectively, when using this type of BHE. This coaxial pipe-in-pipe geometry consists of a central pipe inserted into a larger tube (external pipe), together forming an annular flow channel between them. What is new in particular with the tested designs is that the external pipe is flexible and pressed against the whole perimeter of the rock wall inside the borehole by a slight overpressure. Other coaxial and/or pipe-in-pipe geometries have been studied to some extent by many authors, as follows below. Mei and Fischer [9] presented an experimental and theoretical (short term response model) study of a 50 m deep borehole with a polyvinylchloride (PVC) pipe. The borehole had a diameter of 200 mm and it was backfilled with sand. Spacers were used in order to center the central pipe, and temperature sensors were installed along the depth approximately every 7.5 m inside and outside the annular channel as well as at the inlet and outlet. The measurements had certain limitation (for validating their short term model) due to the high thermal resistance in the external PVC pipe, resulting in differences of up to 0.9 K from the measurements during continuous operation. During cyclic operation, predicted fluid temperatures at 16.8 m depth in the annulus matched well with the measurements. Outside the outer PVC pipe, an asymptotic behavior of the temperatures showed that it almost did not feel the cyclic temperature behavior, just decreasing or increasing slowly during heat extraction or injection, respectively. A discussion on the effects of geometry variations, flow rate, and ground thermal response are also included in [9]. Oliver and Braud [10] presented an analytical solution of pipein-pipe heat exchangers under steady state operation using a constant temperature boundary condition 1 m out from the borehole wall, having limited practical application due to the purely transient process around the borehole in reality. Other studies are those by Hellström [11,12]. [11] is more theoretical and points shortly at the effects of laminar flow and eccentricity in pipe-inpipe BHEs. [12] describes field tests on several closed and open annular coaxial BHEs with and without external pipe, having a borehole resistances as low as 0.01 K m/W. Flexible pipes (so called liners) having a different geometrical configuration than the one tested in this paper were also documented in [12].
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Sanner et al. [13] describe the GROUNDHIT design, which consisted of one PE 63 5.3 mm outer pipe with an inner channel with dimensions PE 40 3.7 mm. Installation and assembling methods were tested and presented. However, the thermal resistances of this BHE were high due to material thermal properties, thermal shunting flow between the up- and downgoing flows, and a relatively large distance to borehole wall. The effects of flow rate and of thermal shunting (and methods to avoid it) are also studied based on a numerical model in [14], but no experiments are done; while in [15], an outer pipe made of stainless steel is driven into the ground without having any grouting to study these effects. Neither backfilling/grouting material nor spacers were used by us in our tested BHEs and, instead of cyclic operation, we are focused on quantifying the result of three distributed thermal response tests at different flow rates, presenting data which can be used as reference for validating models. The need for measured data for validation of borehole heat exchanger models has many times been pointed out, as for example in [16]. An example on the use of the measured data presented in this paper is [17]. Most of the above mentioned publications point at the potential for efficiency improvements that pipe-in-pipe BHEs can bring. There are also other coaxial BHE geometries under development, as the multi-chamber [18] and multi-pipe types [19], consisting of one central tube and several outer chambers or pipes, respectively. The latter was studied by [20,4], yielding thermal resistances between 0.009 and 0.028 K m/W in [20] and of around 0.04 K m/W in [4]. The latter work also evidenced the potential of thermally insulating the central leg of this type of BHE. Recently, Witte [21] presented the development of another coaxial BHE called GEOTHEX, consisting of a pipe-in-pipe design having an insulated central tube with helical vanes on its outer part. This BHE showed a high borehole thermal resistance, but a rather good installation method. A similar pipe-in-pipe geometry consisting of a steel helix around the central pipe was studied in [22]. The helix was welded around a central steel pipe which contained an insulated polyethylene tube inside. The fluid circulated in direct contact with the ground. The BHE was studied extending the approach of [23] and carrying out simple temperature measurements. This coaxial BHE showed a good performance. In our opinion, more work on coaxial BHEs is necessary in order to promote their introduction into the market and thereby increase the efficiency of GSHP and BTES systems. This paper should contribute to a better understanding of BTES design by revealing more advantages of coaxial BHEs and DTRTs. Detailed fluid and borehole wall temperature measurements along the depth of two new BHE designs are interpreted. Global as well as local ground thermal conductivities and borehole thermal resistances are quantified together a discussion on how to evaluate borehole resistance in coaxial BHEs, with the global (effective) and local results being of relevance for the performance of the heat pump located above the ground and for the temperature response of the ground surrounding the borehole, respectively.
2. Materials, borehole heat exchangers, measurement instruments, and test rig Two pipe-in-pipe borehole heat exchangers, labeled BH9 and BH10, are installed in two boreholes with a diameter of 115 mm and each being 190 m deep. The boreholes are drilled about 1 m apart from each other. The BHEs consist of an annular 114 mm external flexible pipe and a standard 40 mm central pipe (see Fig. 1), both made of polyethylene. The upper 84 m of the central pipe in BH10 have been thermally insulated as illustrated in Fig. 2. No attempts to center the central pipe were made in these
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Fig. 3. Thermal circuit of a pipe-in-pipe BHE.
Fig. 1. Cross section. BH9 and lower half of BH10.
installations, making an eccentric position likely to occur as indicated in Figs. 1 and 2. Both BHEs use plain water as circulating fluid. The bottom of the central pipe in BH9 and BH10 reaches a depth of 182 m and 168 m, respectively. The groundwater level in both boreholes was 3 m under the ground surface before the BHE installation. A fiber optic cable, type 50/125, having two graded index multimode fibers protected by a thin stainless tube with an outside diameter of 3.8 mm, was inserted into both BHEs. Temperatures along the depth are measured using these cables, which are placed inside and outside the flow channels, allowing a detailed observation of the fluid (Tf) and borehole wall (Tbw) temperature profiles. The cables were connected to a Sensornet-DTS readout equipment of type HALO in BH9 and ORYX in BH10. More details about the BHE installation procedures can be found in [24,25]. A thermal response test rig consisting of a circulation pump, an inductive flow meter, a flow regulation valve, and an electric heater was used for supplying heat and measuring the volumetric flow to the boreholes during the DTRTs.
3. Evaluation method and distributed thermal response tests The heat transfer process in the pipe-in-pipe geometry may be illustrated by two thermal resistances connected in series, one between the inner and annular channel (Rcp-an) and another between the annular channel and the borehole wall (Ran-bw), as shown in Fig. 3. The figure shows that there is no direct thermal connection between the fluid flowing through the central pipe (Tcp) and the borehole wall (Tbw). The thermal resistance study in this type of borehole heat exchanger has thus to be done in a different way
than when both the fluid flowing down and the fluid flowing upwards has a thermal connection with the borehole wall. In BH9, two continuous heat injection DTRTs with different volumetric flow rates (DTRT1 with 0.58 l/s and DTRT2 with 0.50 l/s) were analyzed and compared with one DTRT at a slightly lower volumetric flow rate (0.43 l/s) carried out in BH10. A total heating power of about 6 kW was supplied to both BHEs during the tests, and the circulating fluid (water) was fed to the BHEs through the central pipe, returning upwards through the annular space. During the DTRTs on BH9, the temperature measurements were integrated during 2.5 min periods with a repetition interval of 5 min and a spatial averaging along the fiber cable of 10 m. The flow rate was also logged every 5 min. In BH10, the integration and repetition time were 2 min and the integration length 1 m. The duration and general conditions during all experiments varied from test to test and Table 1 shows a detailed chronology of the activities in the boreholes. As shown in Table 1, two DTRTs were done on BH9 (DTRT1 and DTRT2), both including a pre-circulation, heat injection and thermal recovery phase. No fluid circulation takes place during thermal recovery or during measurements of undisturbed ground conditions ca. 24 h previous to the DTRTs. Fig. 4 shows that the undisturbed ground temperature decreases along the first half of the borehole, having a minimum at about 90 m depth (due to local urbanization effects), and then increases with a gradient of almost 0.01 K/m in the lower half of the borehole. The measurements taken inside and outside the BHE pipes (Tf and Tbw) show the same tendencies and almost the same absolute values. The average standard deviation in these measurements was ±0.05 K. The DTRTs then start with a short fluid pre-circulation phase followed by longer heat injection and thermal recovery periods. The experiments in BH9 lasted about 2 weeks. In BH10, only the first 50 h of the test are considered in this study. Two studies, local and global, are carried out to analyze the measurements. The former considers the local conditions along the depth and the latter (also so called ‘‘effective’’) considers the BHEs as a whole. In the global approach, different thermal resistances, R, are obtained using the line source model (Eq. (1)) applying alternative definitions of the fluid line source temperature Tf as input for the calculation. Tf is a crucial parameter for the calculation of the thermal resistance. The traditional way of analyzing thermal response tests with the line source model is letting Tf be the arithmetic mean between the fluid inlet and outlet temperatures. This may, sometimes, be the wrong approach for U-pipe BHEs, as pointed out by [26,3,27]. Depending on which Tf that is used, the thermal resistance R may have different meanings. The reader is referred to Ingersoll and Plass [28] and to Mogensen [29] for details about the line source model and the thermal resistance R in Eq. (1), respectively.
Tf Tg ¼
Fig. 2. Cross section. Upper part of BH10.
" Z 1 b2 # q 1 e Rþ db L 2pkg prbffiffiffi b 2 at
ð1Þ
In the local approach, vertical temperature profiles along the BHEs during all DTRTs are interpreted and the ground thermal conductivity and thermal resistance are locally evaluated in two 60 m sections, section 1 between 17 and 77 m, and section 2 between 97 and 157 m.
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J. Acuña, B. Palm / Applied Energy 109 (2013) 312–320 Table 1 Sequence of DTRT phases in BH9 and BH10. DTRT phase
Start
Stop
BH9 DTRT1
Undisturbed ground measurements Pre-circulation Heat injection Thermal recovery
05-07-2009 05-08-2009 05-09-2009 05-12-2009
BH9 DTRT2
Pre-circulation Heat injection Thermal recovery
05-19-2009 at 19.05 05-20-2009 at 08.09 05-22-2009 at 14.42
05-20-2009 at 08.09 05-22-2009 at 14.42 05-28-2009 at 23.53
BH10
Undisturbed ground measurements Pre-circulation Heat injection
09-13-2010 at 12.33 09-13-2010 at 19.45 09-13-2010 at 20.39
09-13-2010 at 19.45 09-13-2010 at 20.39 09-17-2010 at 12.05
at at at at
13.57 14.25 10.13 16.18
05-08-2009 05-09-2009 05-12-2009 05-19-2009
at at at at
14.25 10.13 16.18 19.05
responses of all power steps (caused by small power variations) are superposed in time in order to get the total temperature response for the determination of kg and R. Thermal resistances are also calculated directly from the measured data. The supplied power is calculated as the product of the volumetric flow rate, the volumetric heat capacity, and the fluid temperature difference along the borehole or the studied section, starting at 17 m depth. The active borehole length from 17 m depth, L, becomes 165 m and 151 m in BH9 and BH10, respectively (the central pipe reaches different depths in the two BHEs, as explained in section 2). 4. Results and discussion
Fig. 4. Undisturbed ground temperature profile.
First the thermal conductivity of the ground, kg, is found as the value that minimizes the squared error between calculated and measured values when calculating Tf as a function of time (t) with Eq. (1). Local undisturbed ground temperatures Tg are used as input 1.The calculation is done during thermal recovery (phase between DTRT 1 and DTRT 2 in BH9, see Fig. 5), since radial gradients in the borehole are at that time low and any uncertainty in R has little influence on kg. This has been suggested in [18,30,31]. The calculated kg is then used in determining R, again, by the same procedure, this time by adjusting R during a heat injection steady flux period of each DTRT. The temperature response after time t of a step change in thermal power (q) is evaluated and the
Figs. 5 and 6 show measured temperatures and volumetric flow rates during the tests in BH9 and BH10, respectively. The evolution in time for different Tf definitions and a measured average borehole wall temperature (Tbw) are shown. The definitions include the arithmetic mean based on fluid in and outlet temperature measurements at 17 m depth, the average true mean fluid temperature measured along the borehole depth, and the measured average temperature of the fluid in the annular flow channel. The chronology presented in Table 1 can be followed from left to right in Figs. 5 and 6, except for the undisturbed measurements previous to activities in BH9 and the last hours of the DTRT in BH10, which are not included. All temperatures show similar behavior during the pre-circulation periods whilst during heat injection they keep identical slopes but significantly different temperature levels. As expected, the slopes are also similar during the late time of the thermal recovery. The temperature differences between the diverse Tf and Tbw during the heat injection periods in Figs. 5 and 6, indicate that different global fluid to borehole wall thermal resistances may be determined. Hence, for a global analysis, resistances using the arithmetic mean fluid temperature approach (Rb ), the true mean fluid temperature along the borehole depth (Rt b ), and using the average temperature along the annulus (Ran-bw), are calculated. These different resistances can be used at different BTES system design stages. i.e. Rb is used when studying the heat pump system, while Rt b (or Ran-bw in pipe-in-pipe BHEs) for the thermal response of the ground itself. Observe in Fig. 6 that the arithmetic mean temperature (in&out) in BH10 is slightly lower than the true mean value and that this is not the case in Fig. 5 for BH9. The difference between these is slightly higher during DTRT2 than in DTRT1 due to the lower volumetric flow rate (average flow is 0.58 l/s and 0.50 l/s during DTRT1 and DTRT2, respectively). In BH10, it was relatively difficult to sustain a constant flow during the test and it gradually decreased from 0.47 to 0.40 l/s, being about 0.43 l/s during the period used for the calculation of the thermal resistances.
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Fig. 5. DTRTs in BH9: volumetric flow rate, fluid and borehole wall temperature.
Fig. 6. DTRT in BH10: volumetric flow rate, fluid and borehole wall temperatures.
Before being able to quantify the values of Rb , Rt b , and Ran-bw, the thermal conductivity of the ground is obtained during the thermal recovery phase using the evolution in time of the true mean temperature (the arithmetic mean temperature data is not a proper representation of what occurs along the borehole and should not be used for the thermal conductivity calculation in the thermal recovery phase, at least not without fluid circulation). The result was an effective ground thermal conductivity equal to 3.28 W/mK. The heat recovery period gives the best information about the ground thermal conductivity since the radial temperature gradients in the borehole are low, eliminating the uncertainty caused by the unknown radial positions of the fiber cable. Thermal conductivities obtained during the heat injection phases were 3.53 W/m K, 3.21 W/m K for DTRT1 and DTRT2 in BH9 respectively, and 3.25 W/m K in BH10. All values obtained during the heat injection phase are within a normal error margin as compared to the
one calculated during thermal recovery. However, it has been recently observed by [32] that, for U-pipe BHEs in groundwater filled boreholes, thermal conductivities may be slightly overestimated during the heat injection phase as compared to thermal recovery. The effective thermal conductivity found during the thermal recovery period (3.28 W/m K) has thus been used as input in the calculation of Rb , Rt b , and Ran-bw, during a steady flux period of each heat injection phase. The thermal resistance results from the global study are presented in Table 2. As seen in Table 2 that the global borehole resistance is higher in BH9 when based on the arithmetic mean temperature approximation, being at least 10% lower when based on the true mean temperature. This is not the case in BH10 where Rb is slightly lower than Rt b . The two pipe-in-pipe borehole heat exchangers show similar global thermal resistances (similar thermal performance), but the contrast between the arithmetic and true mean temperature
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J. Acuña, B. Palm / Applied Energy 109 (2013) 312–320 Table 2 Global borehole resistances during all DTRTs using three Tf approaches (result of global study). Global borehole resistance
BH9: 0.58 l/s
BH9: 0.50 l/s
BH10: 0.43 l/s
Rb (based on arithmetic mean) [K m/W] Rt b (based on true mean) [K m/W]
0.027 0.024
0.035 0.029
0.029 0.030
Ran-bw (based on temperature in annulus) [K m/W]
0.005
0.007
0.009
approaches indicates that these BHEs actually operate in a different way. Differences between Rb and Rt b are a function of the borehole depth and the volumetric flow rate, so these results indicate that the mean temperature approximation may result in a large uncertainty in the borehole thermal resistance in pipe-in-pipe BHEs. When comparing BH9 with BH10, it is important to keep in mind that the active length of BH10 is 14 m less than BH9, which is perhaps compensated with the lower flow rate in BH10. Regardless of the fluid temperature approaches, the thermal resistances increase as the flow is decreased. In BH9, Rb increases between DTRT1 to DTRT2 from 0.027 to 0.035 K m/W, Rt b from 0.024 to 0.029 K m/W, and Ran-bw from 0.005 to 0.007 K m/W, evidencing the influence of the flow rate on the thermal performance, as the convective heat transfer increases with flow rate. The results for Ran-bw were significantly lower than Rb and Rt b in all cases (as expected), given the small temperature differences between fluid flowing through the annulus and the borehole wall, evidencing that the thermal contact between the fluid and the borehole wall is good along the annular flow channel. Using the temperature differences between the fluid in the annular pipe (Tan) and Tbw (plotted in Figs. 5 and 6), and subsequently dividing it by the heat injection rate per unit length at each time instant (it is later shown in the paper that the injected power is almost evenly distributed along the borehole depth), results in values for Ran-bw of around 0.015 K m/W, being higher than those presented in Table 2. Many, but not all, of the results above from the global DTRT study, apply at a local level along the borehole as well. As mentioned above, the global results for the effective thermal conductivity and Rb are useful when understanding what the heat pump system sees. Only these are of relevance for the heat pump performance. However, especially in this type of coaxial BHE, local analyses based on observation of the measured vertical temperature profiles in BH9 and BH10 allow evaluating what happens along these heat exchangers and to have a better idea of what the surrounding ground (and not the heat pump) feels during their operation, which is more dependent on Ran-bw. Figs. 7–9 present instantaneous measured temperature profiles during the steady flux period of all DTRTs. Borehole wall and fluid temperatures are presented together with an illustration of the instantaneous arithmetic and true mean temperature profiles along the depth, clearly showing what each of the Tf definitions discussed above implies. The observation and comparison between them indicates how the true mean approximation qualitatively differs from the arithmetic mean approach and, even more important, how these two approaches differ from the temperature along the annulus (Tan) which is the only one being in direct thermal contact with the borehole wall, i.e. the fluid temperature in the central pipe (as illustrated in Fig. 3) does not influence the total heat flux going to the ground and it is thus suggested that none of the mean temperatures should be used for the estimation of local resistances. The difference between Tbw and the various fluid mean temperature alternatives varies with depth at the same time as the difference between Tan and Tbw is kept nearly constant, meaning that local thermal resistances will generally decrease with increasing depth if the mean fluid temperature approaches are used, resulting on incorrect local resistances.
Fig. 7. Instantaneous vertical temperature profiles during DTRT1 in BH9.
Fig. 8. Instantaneous vertical temperature profiles during DTRT2 in BH9.
In order to demonstrate this previous statement, the thermal resistances are quantified at a local level in two 60 m long borehole sections (section 1 and section 2, as described in part 3 of this paper), in the same way as during the global study, i.e. first, the local thermal conductivity of the ground is calculated during the thermal recovery period, resulting to be 3.39 W/m K and 2.92 W/m K for section 1 and section 2, respectively. When calculated during the first heat injection phase of DTRT1, the results are 3.49 W/m K in section 1 and 3.09 W/m K in section 2. Observe that the upper part of the borehole has slightly higher thermal conductivity than the lower part. Once the thermal conductivity is found, the thermal resistances for each section are calculated during the heat injection period. The local resistance is calculated for two different cases: between the borehole wall and an imaginary fluid having the true mean fluid temperature (denoted here as Rf-bw), and between the borehole wall and the bulk fluid in the annulus, Ran-bw (see Fig. 3). It is evident from Figs. 7–9 that Ran-bw will result to be sig-
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Fig. 9. Instantaneous vertical temperature profiles during DTRT in BH10.
nificantly lower than Rf-bw. The result from the local line source analyses are shown in Fig. 10 for the upper borehole section, showing that Ran-bw is about 4 times lower than Rf-bw in BH9 and BH10. The fitting between measured and calculated temperatures resulting in Fig. 10 was carried out during the test period 40–95 h during BH9 DTRT1, 300–335 h during BH9 DTRT2, and 25–45 in BH10 (see Figs. 5 and 6). Thermal resistances resulted to be similar in BH9 and BH10 (i.e. both BHEs show comparable local efficiencies). Ran-bw resistances at each time instant during both DTRTs in BH9 were also obtained using the measured data for both borehole sections (dividing the temperature difference between Tan and Tbw by the local injected power), shown in Fig. 11 (the injected power per section is shown in Fig. 12). As for the global study, the results from the direct calculation are somewhat higher than those obtained using parameter estimation with the line source model. As shown in Figs. 10 and 11, the local comparison of Rf-bw and Ran-bw demonstrates that the fluid temperature along the central pipe (Tcp in Fig. 3) should not be considered for local borehole resistance calculation in pipe-in-pipe BHEs. Tcp can change, but the local heat transfer to the ground will not change if the temperature in the annulus does not change, for a given borehole wall temperature. Larger Rf-bw are, for instance, found in section 1 due the larger temperature differences between the imaginary true mean fluid temperature and the borehole wall (see Figs. 5, 7 and 8). On the other hand, all measurements show that Ran-bw does not change much with depth, meaning that Ran-bw is the most appropriate
Fig. 10. Local true mean to borehole wall Rf-bw and Ran-bw in section 1 of both BHEs.
Fig. 11. Instantaneous local borehole resistances in two borehole sections during DTRTs in BH9.
Fig. 12. Instantaneous power injected at each borehole section during DTRTs in BH9.
way to evaluate local thermal resistances in these pipe-in-pipe BHEs. As shown in Fig. 12, about 35 W/m are transferred through each BHE section to the ground in all DTRTs. The rates are slightly lower in section 1 than in section 2, especially at 0.58 l/s, which partially can explain the differences in the calculation of Ran-bw between the two sections during DTRT1. Although this difference, the power injection is rather evenly distributed along the depth. When determining Rb , Rt b , and Ran-bw, the influence of the flow rate and the thermal shunt between central and annular pipe on the global resistances is more obvious. Both the local and global analyses carried out in this paper show that Ran-bw is low in the tested pipe-in-pipe BHEs, also evident in Figs. 7–9, where Tbw closely follows the same trend as the annular flow in all tests. Although this borehole resistance has been almost eliminated as compared to U-pipe BHEs, a heat pump connected to such pipein-pipe design only feels the effect of Rb (about 0.03 K m/W, see Table 2), which is still low. The low resistances in this type of heat exchanger are mainly thanks to the good physical contact of the external pipe with the borehole wall. A photo the external pipe is shown in Fig. 13, which also serves as link in the electronic version of this article to a video showing how the external pipe looks inside the borehole. It is worth to add here that not only the thermal resistances but also the pressure drop in these pipe-in-pipe BHEs is significantly lower than U-pipes. Although BH9 and BH10 show to have similar performance, it was shown in Figs. 7–9 that their vertical fluid temperature profiles have some differences. Fig. 14 compares the three profiles,
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showing that BH9 has a rather symmetric temperature distribution between down and up-going fluid. In BH10 the temperature drop on the way down is about one fourth of the total temperature change. The total temperature drop downwards is similar for all cases but the derivative dT/dz (a tangent line along the temperature profiles) is rather different. The effect of the insulation in BH10 is barely noticeable (possibly due to deterioration by groundwater intrusion into the insulation pores) but a modest indication of its presence is palpable on the way upwards, where the lower half of the heat exchangers show similar temperature profiles (this part of the BHE normally presents the largest heat transfer rates to the ground). When the fluid in BH10 flows through the upper half of the annular channel, it continues exchanging heat at a high rate in BH10, while the thermal shunt becomes obvious in BH9. Fig. 14 also shows estimated instantaneous thermal power per meter through section 1 and section 2 in the central and annular channels, illustrating how power is distributed. The thermal shunt in BH10 is, although not optimum, significantly lower than in BH9, i.e. the power injected from the central pipe in BH10 is about 15 W/m, value that is almost doubled in BH9. More about the power distribution in BH10 is found in [33]. Finally, we want to emphasize the importance of being consistent in the calculation methods used when comparing different types of borehole heat exchangers. Although Ran-bw has been suggested as the proper heat transfer approach for local thermal resistance in this type of BHE, DTRT results can only be compared between these pipe-in-pipe and other BHEs on a global basis in terms of Rb and/or Rt b . For instance, comparing Ran-bw from a pipe-in-pipe BHE with Rb from a U-pipe would be a mistake and leads to misunderstandings and unfair comparisons. However, comparing Rb and/or Rt b is correct (these statements are also partially valid for Multi-pipe BHEs with insulated central pipe). In any case, large improvements in BTES performance can be expected if pipe-in-pipe BHEs as in BH9 and BH10 (having thermal resistances of about 0.03 K m/W or lower) are used instead of typical U-pipes for which global and local borehole thermal
Fig. 13. External pipe after installation (serves as link to a video in the electronic version of this paper).
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Fig. 14. Instantaneous fluid temperature profiles and specific heat injection during DTRTs in both BHEs.
resistances have been reported to be between 0.06 and 0.12 K m/ W, or even higher. 5. Conclusions The thermal performance of two pipe-in-pipe BHEs, one of them having a partly insulated central pipe, has been studied with regard to global and local borehole thermal resistances during several DTRTs. Analyses of instantaneous vertical fluid and borehole wall temperature profiles are presented. They reveal the detailed thermal behavior for this type of BHE. The thermal resistances calculated along the borehole are similar in both pipe-in-pipe designs, indicating that both BHEs actually operate with similar efficiency. Global (effective) borehole resistances resulted to about 0.03 K m/ W while local resistances are about 0.01 K m/W. The global and local borehole thermal resistances have been calculated with different fluid temperature definitions, including the typical arithmetic mean temperature, a true mean temperature, and just using the fluid temperature in the annular flow channel. The observation and comparison between them indicates how the true mean approximation qualitatively differs from the arithmetic mean approach and how these two approaches differ from the temperature of the fluid in the annulus, the latter being the only fluid in direct thermal contact with the borehole wall. Locally, the thermal resistance corresponding to the true mean fluid temperature is higher than when it is calculated between the annulus and the borehole wall. The local resistances based on the true mean fluid temperature are also higher in the upper than in the lower borehole sections, mainly due to larger temperature differences between the fluid and the borehole wall. It has, therefore, been suggested that none of the mean temperature definitions should be used for the estimation of local borehole resistances and, instead, it would be most logical to take the local borehole resistance between the annulus and the borehole wall. A pipe-in-pipe design having a standard 40 2.4 mm polyethylene central pipe presents a global (effective) thermal resistance calculated with the arithmetic mean fluid temperature of 0.027 and 0.035 K m/W when the fluid flow rate is 0.58 l/s and 0.50 l/s, respectively (for an active depth of 165 m). In case this resistance is instead calculated using the true mean temperature, the corresponding values are 0.024 and 0.029 K m/W. Likewise, the design with a partially insulated central pipe had a global borehole thermal resistance of 0.029 to 0.030 K m/W (based on in and out temperatures and true mean fluid temperatures, respectively) at 0.43 l/ s (for an active borehole depth of 151 m). It is not recommended to use the mean temperature approximation in pipe-in-pipe BHEs as
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there is not thermal contact between the fluid in the central pipe and the borehole wall. Instead, an annulus to borehole wall thermal resistance should be used. A new borehole wall temperature measurement technique along the borehole depth has been used, consisting of a fiber optic cable pressed against the borehole wall with a flexible plastic pipe that is filled with water once it is installed into the ground. The borehole wall temperature measurements during the DTRTs clearly illustrate the meaning of borehole resistance as it has been plotted as a function of time together with fluid temperatures, allowing a straightforward estimation of the borehole thermal resistance by simply dividing the difference between the measured annular fluid and borehole wall temperatures by the supplied heat. This simple estimation method show slightly higher results but rather good accordance with the values obtained with the line source model. The heat transfer per meter in the borehole was measured to be almost evenly distributed along the depth in the tested pipe-inpipe BHEs. The measured borehole wall temperature closely follows the same trend as the temperature in the annular flow in all cases. The thermal resistance between the fluid in the annular flow channel and the borehole wall is almost negligible compared with the result using the fluid mean temperature approach. Such a resistance has shown to be about 0.01 K m/W in the tested pipe-inpipe BHEs. It has been shown that pipe-in-pipe BHEs can, on a global basis, be compared with other BHEs by comparing global thermal resistances, but that the comparison may lead to misunderstandings due to the lack of direct thermal contact between the fluid in the central pipe with the borehole wall. Regardless of this, large improvements in BTES performance are expected if pipe-in-pipe BHEs of the type tested here are used instead of typical U-pipes. Acknowledgements STEM and all industry partners within EFFSYS+ are acknowledged for financing this research. Special thanks to Klas Andersson, Jan Cederström, Bo Jansson, Hans Alexandersson, and Kenneth Weber, for their support related to the experiments in these borehole heat exchangers. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2013. 01.024. References [1] Spitler J, Bernier M. Ground-source heat pump systems: the first century and beyond. HVAC&R 2011;17(6):891–4. [2] Renewable heating and cooling platform (RHC). Strategic research priorities for geothermal technology. Brussels: European Technology Platform on Renewable Heating and Cooling; 2012. [3] Acuña J, Mogensen P, Palm B. Distributed thermal response test on a U-pipe borehole heat exchanger. Effstock – the 11th international conference on energy storage. Stockholm; 2009 [4] Acuña J, Mogensen P, Palm B. Distributed thermal response tests on a multipipe coaxial borehole heat exchanger. HVAC&R 2011;17(6):1012–29.
[5] Fujii H, Hiroaki O, Itoi R. Thermal response tests using optical fiber thermometers, vol. 30. Kyushu University, Fukuoka: GRC Transactions; 2006. [6] Fujii H, Hiroaki O, Nishia K, Itoi R, Ohyamab K, Shibatac K. An improved thermal response test for U-tube ground heat exchanger based on optical fiber thermometers. Geothermics 2009;38(4):399–406. [7] Eslami-nejad P, Bernier M. Freezing of geothermal borehole surroundings: a numerical and experimental assessment with applications. Appl Energy 2012;98:333–45. [8] Beier R, Smith M, Spitler J. Reference data sets for vertical borehole ground heat exchanger models and thermal response test analysis. Geothermics 2011;40:79–85. [9] Mei V, Fischer S. Vertical concentric tube ground-coupled heat exchangers. ASHRAE Trans 1983;89(part 2B):391–406. [10] Oliver J, Braud H. Thermal exchange to earth with concentric well pipes. Trans ASABE 1981;24(4):0906–10. [11] Hellström G. Ground heat storage, Thermal analyses of duct storage systems. Doctoral thesis. University of Lund, Sweden; 1991. [12] Hellström G. Borehole heat exchangers: state of the art. In: Hummelshöj R, Hansen J, Lorenzen K, editors. ECES: Annex 13. Subtask 2. International, Energy Agency (IEA); 2002. [13] Sanner B, Karytsas K, Abry M, Coelho L, Goldbrunner J, Mendrinos D. GROUNDHIT, advancement in ground source heat pumps through EU support. EU Geothermal Congress; 2007. [14] Zanchini E, Lazzari S, Priarone A. Improving the thermal performance of coaxial borehole heat exchangers. Energy 2010;35:657–66. [15] Zanchini E, Lazzari S, Priarone A. Effects of flow direction and thermal shortcircuiting on the performance of small coaxial ground heat exchangers. Renew Energy 2010;35:1255–65. [16] Yang H, Cui P, Fang Z. Vertical-borehole ground-coupled heat pumps: a review of models and systems. Appl Energy 2010;87(1):16–27. [17] Beier R, Acuña J, Mogensen P, Palm B. Borehole resistance and vertical temperature profiles in coaxial borehole heat exchangers. Appl Energy 2013;102:665–75. [18] Acuña J. Improvements of U-pipe borehole heat exchangers. Licentiate thesis, stockholm. Sweden: The Royal Institute of Technology, KTH; 2010 [19] Platell P. Developing work on ground heat exchangers. In: ECOSTOCK conference proceedings 7A-1. New Jersey; 2006. [20] Hellström G, Kjellsson E. Laboratory measurements of heat transfer properties of different types of borehole heat exchangers. In: Proc. of terrastock. Stuttgart, Germany; 2000. [21] Witte H. The GEOTHEX geothermal heat exchanger, characterisation of a novel high efficiency heat exchanger design. In: The proceedings of the 12th international conference on energy storage (ss. INNO-U-32). Lleida: ECES; 2012. [22] Zarrella A, Scarpa M, De Carli M. Short time step analysis of vertical groundcoupled heat exchangers: the approach of CaRM. Renew Energy 2011;36(9):2357–67. [23] De Carli M, Tonon M, Zarrella A, Zecchin R. A computational capacity resistance model (CaRM) for vertical ground coupled heat exchangers. Renew Energy 2010;35(7):1537–50. [24] Acuña J, Palm B. A novel coaxial BHE – description and first distributed thermal response test measurements. Bali: IGA World Geothermal Congress; 2010. [25] Acuña J, Palm B. First Experiences with coaxial borehole heat exchangers. IIR conference on sources/sinks alternative to the outside air for HPs and AC techniques; 2011. [26] Marcotte D, Pasquier P. On the estimation of thermal resistance in borehole thermal conductivity test. Renew Energy 2008;33:2407–15. [27] Beier R. Vertical temperature profiles in ground heat exchanger during in-situ test. Renew Energy 2011;36:1578–87. [28] Ingersoll LR, Plass HJ. Theory of the ground pipe heat source for the heat pump, vol. 54. Madison, Wis: ASHVE Transactions; 1948. [29] Mogensen P. Fluid to duct wall heat transfer in duct system heat storages. International conference on subsurface heat storage in theory and practice; 1983. p. 652-7 [Appendix, Part II]. [30] Raymond J, Therrien R, Gosselin L. Borehole temperature evolution during thermal response tests. Geothermics 2011;40:69–78. [31] Raymond J, Therrien R, Gosselin L, Lefebvre R. A review of thermal response test analysis using pumping test concepts. Ground Water 2011;49(6):932–45. [32] Monzó P, Acuña J, Palm B. Analysis of the influence of the heat power rate variations in different phases of a distributed thermal response test. INNOSTOCK; 2012 [33] Guillaume F. Analysis of a novel pipe in pipe coaxial borehole heat exchanger. Master of science thesis. Energy Technology EGI-2011-081MSC. KTH, Stockholm; 2011.
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Applied Energy journal homepage: www.elsevier.com/locate/apenergy
Distributed thermal response tests on pipe-in-pipe borehole heat exchangers José Acuña ⇑, Björn Palm KTH, Royal Institute of Technology, Brinellvägen 68, 100 44 Stockholm, Sweden
h i g h l i g h t s " Distributed thermal response tests are for first time carried out on pipe-in-pipe BHEs. " Published data sets serve as reference for researchers to validate pipe-in-pipe BHE models. " The borehole wall temperature is measured along the borehole during all DTRTs. " Discussion on measured local and effective borehole resistances in pipe-in-pipe BHEs. " Thermal performance of tested pipe-in-pipe BHEs is about 100% better than U-pipes.
a r t i c l e
i n f o
Article history: Received 4 July 2012 Received in revised form 2 January 2013 Accepted 7 January 2013 Available online 8 February 2013 Keywords: Borehole thermal resistance Borehole heat exchanger Coaxial Pipe-in-pipe Distributed thermal response test
a b s t r a c t Borehole Thermal Energy Storage systems typically use U-pipe Borehole Heat Exchangers (BHE) having borehole thermal resistances of at least 0.06 K m/W. Obviously, there is room for improvement in the U-pipe design to decrease these values. Additionally, there is a need for methods of getting more detailed knowledge about the performance of BHEs. Performing Distributed Thermal Response Tests (DTRT) on new proposed designs helps to fill this gap, as the ground thermal conductivity and thermal resistances in a BHE can be determined at many instances in the borehole thanks to distributed temperature measurements along the depth. In this paper, results from three heat injection DTRTs carried out on two coaxial pipe-in-pipe BHEs at different flow rates are presented for the first time. The tested pipe-in-pipe geometry consists of a central tube inserted into a larger external flexible pipe, forming an annular space between them. The external pipe is pressed to the borehole wall by applying a slight overpressure at the inside, resulting in good thermal contact and at the same time opening up for a novel method for measuring the borehole wall temperature in situ, by squeezing a fiber optic cable between the external pipe and the borehole wall. A reflection about how to calculate borehole thermal resistance in pipe-in-pipe BHEs is presented. Detailed fluid and borehole wall temperatures along the depth during the whole duration of the DTRTs allowed to calculate local and effective borehole thermal resistances and ground thermal conductivities. Local thermal resistances were found to be almost negligible as compared to U-pipe BHEs, and the effective borehole resistance equal to about 0.03 K m/W. The injected power was found to be almost evenly distributed along the depth. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Borehole Thermal Energy Storage (BTES) systems are normally designed based on simple thermal response tests that give average information about ground thermal conductivity, undisturbed ground temperature, and borehole thermal resistance. Also, BTES systems typically use U-pipe borehole heat exchangers having borehole thermal resistances (between 0.06 and 0.12 K m/W, or
⇑ Corresponding author. Tel.: +46 8 7908941. E-mail address: [email protected] (J. Acuña). 0306-2619/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2013.01.024
even higher). However, the rapid growth of Ground Source Heat Pumps (GSHP) installations for BTES systems makes more detailed knowledge about ground thermal properties desirable, and also calls for more efficient Borehole Heat Exchangers (BHE). The relevance of this research topic has been recently confirmed by Spitler and Bernier [1] and by the European Platform for Renewable Heating and Cooling [2]. Promoting these areas is the main objective of the present study, focusing on in situ Distributed Thermal Response Tests (DTRT) carried out on two pipe-in-pipe coaxial BHEs. Such tests and proper temperature measurements in this type of heat exchanger has been missing until recently.
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In a Distributed Thermal Response Test (DTRT), the ground thermal conductivity and thermal resistance in a BHE can be determined at many locations along the borehole, as demonstrated by [3,4] on a U-pipe BHE and a coaxial Multi-pipe BHE, respectively. Similar studies, but only of the ground thermal conductivity, have been presented by [5,6]. Laboratory scale tests using distributed temperature measurements not necessarily with fiber optics are also possible and have been done by Eslami-nejad and Bernier [7] and Beier et al. [8]. Two independent U-pipe systems are used in [7] to study methods to thaw surrounding ground in order to reduce the borehole depth. About 20 thermocouples and a six point temperature profile probe was used. The experiment results verify a numerical model that can account for multiple radial ground layers as the freezing of the ground in the vicinity of boreholes is studied. In [8], laboratory experiments including a large amount of measurement points inside a sand box provide data for U-pipe BHE testing and for validating ground thermal conductivity estimation models. In this paper, results from three DTRTs carried out on two coaxial pipe-in-pipe BHEs are presented for the first time, and their thermal performance is studied in detail, opening up new knowledge windows within BTES systems that clarify how measured temperatures during a thermal response test should be interpreted in terms of thermal borehole resistances. Local and effective borehole resistances are separated from each other in order to distinguish how the surrounding ground and a heat pump evaporator can be affected, respectively, when using this type of BHE. This coaxial pipe-in-pipe geometry consists of a central pipe inserted into a larger tube (external pipe), together forming an annular flow channel between them. What is new in particular with the tested designs is that the external pipe is flexible and pressed against the whole perimeter of the rock wall inside the borehole by a slight overpressure. Other coaxial and/or pipe-in-pipe geometries have been studied to some extent by many authors, as follows below. Mei and Fischer [9] presented an experimental and theoretical (short term response model) study of a 50 m deep borehole with a polyvinylchloride (PVC) pipe. The borehole had a diameter of 200 mm and it was backfilled with sand. Spacers were used in order to center the central pipe, and temperature sensors were installed along the depth approximately every 7.5 m inside and outside the annular channel as well as at the inlet and outlet. The measurements had certain limitation (for validating their short term model) due to the high thermal resistance in the external PVC pipe, resulting in differences of up to 0.9 K from the measurements during continuous operation. During cyclic operation, predicted fluid temperatures at 16.8 m depth in the annulus matched well with the measurements. Outside the outer PVC pipe, an asymptotic behavior of the temperatures showed that it almost did not feel the cyclic temperature behavior, just decreasing or increasing slowly during heat extraction or injection, respectively. A discussion on the effects of geometry variations, flow rate, and ground thermal response are also included in [9]. Oliver and Braud [10] presented an analytical solution of pipein-pipe heat exchangers under steady state operation using a constant temperature boundary condition 1 m out from the borehole wall, having limited practical application due to the purely transient process around the borehole in reality. Other studies are those by Hellström [11,12]. [11] is more theoretical and points shortly at the effects of laminar flow and eccentricity in pipe-inpipe BHEs. [12] describes field tests on several closed and open annular coaxial BHEs with and without external pipe, having a borehole resistances as low as 0.01 K m/W. Flexible pipes (so called liners) having a different geometrical configuration than the one tested in this paper were also documented in [12].
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Sanner et al. [13] describe the GROUNDHIT design, which consisted of one PE 63 5.3 mm outer pipe with an inner channel with dimensions PE 40 3.7 mm. Installation and assembling methods were tested and presented. However, the thermal resistances of this BHE were high due to material thermal properties, thermal shunting flow between the up- and downgoing flows, and a relatively large distance to borehole wall. The effects of flow rate and of thermal shunting (and methods to avoid it) are also studied based on a numerical model in [14], but no experiments are done; while in [15], an outer pipe made of stainless steel is driven into the ground without having any grouting to study these effects. Neither backfilling/grouting material nor spacers were used by us in our tested BHEs and, instead of cyclic operation, we are focused on quantifying the result of three distributed thermal response tests at different flow rates, presenting data which can be used as reference for validating models. The need for measured data for validation of borehole heat exchanger models has many times been pointed out, as for example in [16]. An example on the use of the measured data presented in this paper is [17]. Most of the above mentioned publications point at the potential for efficiency improvements that pipe-in-pipe BHEs can bring. There are also other coaxial BHE geometries under development, as the multi-chamber [18] and multi-pipe types [19], consisting of one central tube and several outer chambers or pipes, respectively. The latter was studied by [20,4], yielding thermal resistances between 0.009 and 0.028 K m/W in [20] and of around 0.04 K m/W in [4]. The latter work also evidenced the potential of thermally insulating the central leg of this type of BHE. Recently, Witte [21] presented the development of another coaxial BHE called GEOTHEX, consisting of a pipe-in-pipe design having an insulated central tube with helical vanes on its outer part. This BHE showed a high borehole thermal resistance, but a rather good installation method. A similar pipe-in-pipe geometry consisting of a steel helix around the central pipe was studied in [22]. The helix was welded around a central steel pipe which contained an insulated polyethylene tube inside. The fluid circulated in direct contact with the ground. The BHE was studied extending the approach of [23] and carrying out simple temperature measurements. This coaxial BHE showed a good performance. In our opinion, more work on coaxial BHEs is necessary in order to promote their introduction into the market and thereby increase the efficiency of GSHP and BTES systems. This paper should contribute to a better understanding of BTES design by revealing more advantages of coaxial BHEs and DTRTs. Detailed fluid and borehole wall temperature measurements along the depth of two new BHE designs are interpreted. Global as well as local ground thermal conductivities and borehole thermal resistances are quantified together a discussion on how to evaluate borehole resistance in coaxial BHEs, with the global (effective) and local results being of relevance for the performance of the heat pump located above the ground and for the temperature response of the ground surrounding the borehole, respectively.
2. Materials, borehole heat exchangers, measurement instruments, and test rig Two pipe-in-pipe borehole heat exchangers, labeled BH9 and BH10, are installed in two boreholes with a diameter of 115 mm and each being 190 m deep. The boreholes are drilled about 1 m apart from each other. The BHEs consist of an annular 114 mm external flexible pipe and a standard 40 mm central pipe (see Fig. 1), both made of polyethylene. The upper 84 m of the central pipe in BH10 have been thermally insulated as illustrated in Fig. 2. No attempts to center the central pipe were made in these
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Fig. 3. Thermal circuit of a pipe-in-pipe BHE.
Fig. 1. Cross section. BH9 and lower half of BH10.
installations, making an eccentric position likely to occur as indicated in Figs. 1 and 2. Both BHEs use plain water as circulating fluid. The bottom of the central pipe in BH9 and BH10 reaches a depth of 182 m and 168 m, respectively. The groundwater level in both boreholes was 3 m under the ground surface before the BHE installation. A fiber optic cable, type 50/125, having two graded index multimode fibers protected by a thin stainless tube with an outside diameter of 3.8 mm, was inserted into both BHEs. Temperatures along the depth are measured using these cables, which are placed inside and outside the flow channels, allowing a detailed observation of the fluid (Tf) and borehole wall (Tbw) temperature profiles. The cables were connected to a Sensornet-DTS readout equipment of type HALO in BH9 and ORYX in BH10. More details about the BHE installation procedures can be found in [24,25]. A thermal response test rig consisting of a circulation pump, an inductive flow meter, a flow regulation valve, and an electric heater was used for supplying heat and measuring the volumetric flow to the boreholes during the DTRTs.
3. Evaluation method and distributed thermal response tests The heat transfer process in the pipe-in-pipe geometry may be illustrated by two thermal resistances connected in series, one between the inner and annular channel (Rcp-an) and another between the annular channel and the borehole wall (Ran-bw), as shown in Fig. 3. The figure shows that there is no direct thermal connection between the fluid flowing through the central pipe (Tcp) and the borehole wall (Tbw). The thermal resistance study in this type of borehole heat exchanger has thus to be done in a different way
than when both the fluid flowing down and the fluid flowing upwards has a thermal connection with the borehole wall. In BH9, two continuous heat injection DTRTs with different volumetric flow rates (DTRT1 with 0.58 l/s and DTRT2 with 0.50 l/s) were analyzed and compared with one DTRT at a slightly lower volumetric flow rate (0.43 l/s) carried out in BH10. A total heating power of about 6 kW was supplied to both BHEs during the tests, and the circulating fluid (water) was fed to the BHEs through the central pipe, returning upwards through the annular space. During the DTRTs on BH9, the temperature measurements were integrated during 2.5 min periods with a repetition interval of 5 min and a spatial averaging along the fiber cable of 10 m. The flow rate was also logged every 5 min. In BH10, the integration and repetition time were 2 min and the integration length 1 m. The duration and general conditions during all experiments varied from test to test and Table 1 shows a detailed chronology of the activities in the boreholes. As shown in Table 1, two DTRTs were done on BH9 (DTRT1 and DTRT2), both including a pre-circulation, heat injection and thermal recovery phase. No fluid circulation takes place during thermal recovery or during measurements of undisturbed ground conditions ca. 24 h previous to the DTRTs. Fig. 4 shows that the undisturbed ground temperature decreases along the first half of the borehole, having a minimum at about 90 m depth (due to local urbanization effects), and then increases with a gradient of almost 0.01 K/m in the lower half of the borehole. The measurements taken inside and outside the BHE pipes (Tf and Tbw) show the same tendencies and almost the same absolute values. The average standard deviation in these measurements was ±0.05 K. The DTRTs then start with a short fluid pre-circulation phase followed by longer heat injection and thermal recovery periods. The experiments in BH9 lasted about 2 weeks. In BH10, only the first 50 h of the test are considered in this study. Two studies, local and global, are carried out to analyze the measurements. The former considers the local conditions along the depth and the latter (also so called ‘‘effective’’) considers the BHEs as a whole. In the global approach, different thermal resistances, R, are obtained using the line source model (Eq. (1)) applying alternative definitions of the fluid line source temperature Tf as input for the calculation. Tf is a crucial parameter for the calculation of the thermal resistance. The traditional way of analyzing thermal response tests with the line source model is letting Tf be the arithmetic mean between the fluid inlet and outlet temperatures. This may, sometimes, be the wrong approach for U-pipe BHEs, as pointed out by [26,3,27]. Depending on which Tf that is used, the thermal resistance R may have different meanings. The reader is referred to Ingersoll and Plass [28] and to Mogensen [29] for details about the line source model and the thermal resistance R in Eq. (1), respectively.
Tf Tg ¼
Fig. 2. Cross section. Upper part of BH10.
" Z 1 b2 # q 1 e Rþ db L 2pkg prbffiffiffi b 2 at
ð1Þ
In the local approach, vertical temperature profiles along the BHEs during all DTRTs are interpreted and the ground thermal conductivity and thermal resistance are locally evaluated in two 60 m sections, section 1 between 17 and 77 m, and section 2 between 97 and 157 m.
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J. Acuña, B. Palm / Applied Energy 109 (2013) 312–320 Table 1 Sequence of DTRT phases in BH9 and BH10. DTRT phase
Start
Stop
BH9 DTRT1
Undisturbed ground measurements Pre-circulation Heat injection Thermal recovery
05-07-2009 05-08-2009 05-09-2009 05-12-2009
BH9 DTRT2
Pre-circulation Heat injection Thermal recovery
05-19-2009 at 19.05 05-20-2009 at 08.09 05-22-2009 at 14.42
05-20-2009 at 08.09 05-22-2009 at 14.42 05-28-2009 at 23.53
BH10
Undisturbed ground measurements Pre-circulation Heat injection
09-13-2010 at 12.33 09-13-2010 at 19.45 09-13-2010 at 20.39
09-13-2010 at 19.45 09-13-2010 at 20.39 09-17-2010 at 12.05
at at at at
13.57 14.25 10.13 16.18
05-08-2009 05-09-2009 05-12-2009 05-19-2009
at at at at
14.25 10.13 16.18 19.05
responses of all power steps (caused by small power variations) are superposed in time in order to get the total temperature response for the determination of kg and R. Thermal resistances are also calculated directly from the measured data. The supplied power is calculated as the product of the volumetric flow rate, the volumetric heat capacity, and the fluid temperature difference along the borehole or the studied section, starting at 17 m depth. The active borehole length from 17 m depth, L, becomes 165 m and 151 m in BH9 and BH10, respectively (the central pipe reaches different depths in the two BHEs, as explained in section 2). 4. Results and discussion
Fig. 4. Undisturbed ground temperature profile.
First the thermal conductivity of the ground, kg, is found as the value that minimizes the squared error between calculated and measured values when calculating Tf as a function of time (t) with Eq. (1). Local undisturbed ground temperatures Tg are used as input 1.The calculation is done during thermal recovery (phase between DTRT 1 and DTRT 2 in BH9, see Fig. 5), since radial gradients in the borehole are at that time low and any uncertainty in R has little influence on kg. This has been suggested in [18,30,31]. The calculated kg is then used in determining R, again, by the same procedure, this time by adjusting R during a heat injection steady flux period of each DTRT. The temperature response after time t of a step change in thermal power (q) is evaluated and the
Figs. 5 and 6 show measured temperatures and volumetric flow rates during the tests in BH9 and BH10, respectively. The evolution in time for different Tf definitions and a measured average borehole wall temperature (Tbw) are shown. The definitions include the arithmetic mean based on fluid in and outlet temperature measurements at 17 m depth, the average true mean fluid temperature measured along the borehole depth, and the measured average temperature of the fluid in the annular flow channel. The chronology presented in Table 1 can be followed from left to right in Figs. 5 and 6, except for the undisturbed measurements previous to activities in BH9 and the last hours of the DTRT in BH10, which are not included. All temperatures show similar behavior during the pre-circulation periods whilst during heat injection they keep identical slopes but significantly different temperature levels. As expected, the slopes are also similar during the late time of the thermal recovery. The temperature differences between the diverse Tf and Tbw during the heat injection periods in Figs. 5 and 6, indicate that different global fluid to borehole wall thermal resistances may be determined. Hence, for a global analysis, resistances using the arithmetic mean fluid temperature approach (Rb ), the true mean fluid temperature along the borehole depth (Rt b ), and using the average temperature along the annulus (Ran-bw), are calculated. These different resistances can be used at different BTES system design stages. i.e. Rb is used when studying the heat pump system, while Rt b (or Ran-bw in pipe-in-pipe BHEs) for the thermal response of the ground itself. Observe in Fig. 6 that the arithmetic mean temperature (in&out) in BH10 is slightly lower than the true mean value and that this is not the case in Fig. 5 for BH9. The difference between these is slightly higher during DTRT2 than in DTRT1 due to the lower volumetric flow rate (average flow is 0.58 l/s and 0.50 l/s during DTRT1 and DTRT2, respectively). In BH10, it was relatively difficult to sustain a constant flow during the test and it gradually decreased from 0.47 to 0.40 l/s, being about 0.43 l/s during the period used for the calculation of the thermal resistances.
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Fig. 5. DTRTs in BH9: volumetric flow rate, fluid and borehole wall temperature.
Fig. 6. DTRT in BH10: volumetric flow rate, fluid and borehole wall temperatures.
Before being able to quantify the values of Rb , Rt b , and Ran-bw, the thermal conductivity of the ground is obtained during the thermal recovery phase using the evolution in time of the true mean temperature (the arithmetic mean temperature data is not a proper representation of what occurs along the borehole and should not be used for the thermal conductivity calculation in the thermal recovery phase, at least not without fluid circulation). The result was an effective ground thermal conductivity equal to 3.28 W/mK. The heat recovery period gives the best information about the ground thermal conductivity since the radial temperature gradients in the borehole are low, eliminating the uncertainty caused by the unknown radial positions of the fiber cable. Thermal conductivities obtained during the heat injection phases were 3.53 W/m K, 3.21 W/m K for DTRT1 and DTRT2 in BH9 respectively, and 3.25 W/m K in BH10. All values obtained during the heat injection phase are within a normal error margin as compared to the
one calculated during thermal recovery. However, it has been recently observed by [32] that, for U-pipe BHEs in groundwater filled boreholes, thermal conductivities may be slightly overestimated during the heat injection phase as compared to thermal recovery. The effective thermal conductivity found during the thermal recovery period (3.28 W/m K) has thus been used as input in the calculation of Rb , Rt b , and Ran-bw, during a steady flux period of each heat injection phase. The thermal resistance results from the global study are presented in Table 2. As seen in Table 2 that the global borehole resistance is higher in BH9 when based on the arithmetic mean temperature approximation, being at least 10% lower when based on the true mean temperature. This is not the case in BH10 where Rb is slightly lower than Rt b . The two pipe-in-pipe borehole heat exchangers show similar global thermal resistances (similar thermal performance), but the contrast between the arithmetic and true mean temperature
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J. Acuña, B. Palm / Applied Energy 109 (2013) 312–320 Table 2 Global borehole resistances during all DTRTs using three Tf approaches (result of global study). Global borehole resistance
BH9: 0.58 l/s
BH9: 0.50 l/s
BH10: 0.43 l/s
Rb (based on arithmetic mean) [K m/W] Rt b (based on true mean) [K m/W]
0.027 0.024
0.035 0.029
0.029 0.030
Ran-bw (based on temperature in annulus) [K m/W]
0.005
0.007
0.009
approaches indicates that these BHEs actually operate in a different way. Differences between Rb and Rt b are a function of the borehole depth and the volumetric flow rate, so these results indicate that the mean temperature approximation may result in a large uncertainty in the borehole thermal resistance in pipe-in-pipe BHEs. When comparing BH9 with BH10, it is important to keep in mind that the active length of BH10 is 14 m less than BH9, which is perhaps compensated with the lower flow rate in BH10. Regardless of the fluid temperature approaches, the thermal resistances increase as the flow is decreased. In BH9, Rb increases between DTRT1 to DTRT2 from 0.027 to 0.035 K m/W, Rt b from 0.024 to 0.029 K m/W, and Ran-bw from 0.005 to 0.007 K m/W, evidencing the influence of the flow rate on the thermal performance, as the convective heat transfer increases with flow rate. The results for Ran-bw were significantly lower than Rb and Rt b in all cases (as expected), given the small temperature differences between fluid flowing through the annulus and the borehole wall, evidencing that the thermal contact between the fluid and the borehole wall is good along the annular flow channel. Using the temperature differences between the fluid in the annular pipe (Tan) and Tbw (plotted in Figs. 5 and 6), and subsequently dividing it by the heat injection rate per unit length at each time instant (it is later shown in the paper that the injected power is almost evenly distributed along the borehole depth), results in values for Ran-bw of around 0.015 K m/W, being higher than those presented in Table 2. Many, but not all, of the results above from the global DTRT study, apply at a local level along the borehole as well. As mentioned above, the global results for the effective thermal conductivity and Rb are useful when understanding what the heat pump system sees. Only these are of relevance for the heat pump performance. However, especially in this type of coaxial BHE, local analyses based on observation of the measured vertical temperature profiles in BH9 and BH10 allow evaluating what happens along these heat exchangers and to have a better idea of what the surrounding ground (and not the heat pump) feels during their operation, which is more dependent on Ran-bw. Figs. 7–9 present instantaneous measured temperature profiles during the steady flux period of all DTRTs. Borehole wall and fluid temperatures are presented together with an illustration of the instantaneous arithmetic and true mean temperature profiles along the depth, clearly showing what each of the Tf definitions discussed above implies. The observation and comparison between them indicates how the true mean approximation qualitatively differs from the arithmetic mean approach and, even more important, how these two approaches differ from the temperature along the annulus (Tan) which is the only one being in direct thermal contact with the borehole wall, i.e. the fluid temperature in the central pipe (as illustrated in Fig. 3) does not influence the total heat flux going to the ground and it is thus suggested that none of the mean temperatures should be used for the estimation of local resistances. The difference between Tbw and the various fluid mean temperature alternatives varies with depth at the same time as the difference between Tan and Tbw is kept nearly constant, meaning that local thermal resistances will generally decrease with increasing depth if the mean fluid temperature approaches are used, resulting on incorrect local resistances.
Fig. 7. Instantaneous vertical temperature profiles during DTRT1 in BH9.
Fig. 8. Instantaneous vertical temperature profiles during DTRT2 in BH9.
In order to demonstrate this previous statement, the thermal resistances are quantified at a local level in two 60 m long borehole sections (section 1 and section 2, as described in part 3 of this paper), in the same way as during the global study, i.e. first, the local thermal conductivity of the ground is calculated during the thermal recovery period, resulting to be 3.39 W/m K and 2.92 W/m K for section 1 and section 2, respectively. When calculated during the first heat injection phase of DTRT1, the results are 3.49 W/m K in section 1 and 3.09 W/m K in section 2. Observe that the upper part of the borehole has slightly higher thermal conductivity than the lower part. Once the thermal conductivity is found, the thermal resistances for each section are calculated during the heat injection period. The local resistance is calculated for two different cases: between the borehole wall and an imaginary fluid having the true mean fluid temperature (denoted here as Rf-bw), and between the borehole wall and the bulk fluid in the annulus, Ran-bw (see Fig. 3). It is evident from Figs. 7–9 that Ran-bw will result to be sig-
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Fig. 9. Instantaneous vertical temperature profiles during DTRT in BH10.
nificantly lower than Rf-bw. The result from the local line source analyses are shown in Fig. 10 for the upper borehole section, showing that Ran-bw is about 4 times lower than Rf-bw in BH9 and BH10. The fitting between measured and calculated temperatures resulting in Fig. 10 was carried out during the test period 40–95 h during BH9 DTRT1, 300–335 h during BH9 DTRT2, and 25–45 in BH10 (see Figs. 5 and 6). Thermal resistances resulted to be similar in BH9 and BH10 (i.e. both BHEs show comparable local efficiencies). Ran-bw resistances at each time instant during both DTRTs in BH9 were also obtained using the measured data for both borehole sections (dividing the temperature difference between Tan and Tbw by the local injected power), shown in Fig. 11 (the injected power per section is shown in Fig. 12). As for the global study, the results from the direct calculation are somewhat higher than those obtained using parameter estimation with the line source model. As shown in Figs. 10 and 11, the local comparison of Rf-bw and Ran-bw demonstrates that the fluid temperature along the central pipe (Tcp in Fig. 3) should not be considered for local borehole resistance calculation in pipe-in-pipe BHEs. Tcp can change, but the local heat transfer to the ground will not change if the temperature in the annulus does not change, for a given borehole wall temperature. Larger Rf-bw are, for instance, found in section 1 due the larger temperature differences between the imaginary true mean fluid temperature and the borehole wall (see Figs. 5, 7 and 8). On the other hand, all measurements show that Ran-bw does not change much with depth, meaning that Ran-bw is the most appropriate
Fig. 10. Local true mean to borehole wall Rf-bw and Ran-bw in section 1 of both BHEs.
Fig. 11. Instantaneous local borehole resistances in two borehole sections during DTRTs in BH9.
Fig. 12. Instantaneous power injected at each borehole section during DTRTs in BH9.
way to evaluate local thermal resistances in these pipe-in-pipe BHEs. As shown in Fig. 12, about 35 W/m are transferred through each BHE section to the ground in all DTRTs. The rates are slightly lower in section 1 than in section 2, especially at 0.58 l/s, which partially can explain the differences in the calculation of Ran-bw between the two sections during DTRT1. Although this difference, the power injection is rather evenly distributed along the depth. When determining Rb , Rt b , and Ran-bw, the influence of the flow rate and the thermal shunt between central and annular pipe on the global resistances is more obvious. Both the local and global analyses carried out in this paper show that Ran-bw is low in the tested pipe-in-pipe BHEs, also evident in Figs. 7–9, where Tbw closely follows the same trend as the annular flow in all tests. Although this borehole resistance has been almost eliminated as compared to U-pipe BHEs, a heat pump connected to such pipein-pipe design only feels the effect of Rb (about 0.03 K m/W, see Table 2), which is still low. The low resistances in this type of heat exchanger are mainly thanks to the good physical contact of the external pipe with the borehole wall. A photo the external pipe is shown in Fig. 13, which also serves as link in the electronic version of this article to a video showing how the external pipe looks inside the borehole. It is worth to add here that not only the thermal resistances but also the pressure drop in these pipe-in-pipe BHEs is significantly lower than U-pipes. Although BH9 and BH10 show to have similar performance, it was shown in Figs. 7–9 that their vertical fluid temperature profiles have some differences. Fig. 14 compares the three profiles,
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showing that BH9 has a rather symmetric temperature distribution between down and up-going fluid. In BH10 the temperature drop on the way down is about one fourth of the total temperature change. The total temperature drop downwards is similar for all cases but the derivative dT/dz (a tangent line along the temperature profiles) is rather different. The effect of the insulation in BH10 is barely noticeable (possibly due to deterioration by groundwater intrusion into the insulation pores) but a modest indication of its presence is palpable on the way upwards, where the lower half of the heat exchangers show similar temperature profiles (this part of the BHE normally presents the largest heat transfer rates to the ground). When the fluid in BH10 flows through the upper half of the annular channel, it continues exchanging heat at a high rate in BH10, while the thermal shunt becomes obvious in BH9. Fig. 14 also shows estimated instantaneous thermal power per meter through section 1 and section 2 in the central and annular channels, illustrating how power is distributed. The thermal shunt in BH10 is, although not optimum, significantly lower than in BH9, i.e. the power injected from the central pipe in BH10 is about 15 W/m, value that is almost doubled in BH9. More about the power distribution in BH10 is found in [33]. Finally, we want to emphasize the importance of being consistent in the calculation methods used when comparing different types of borehole heat exchangers. Although Ran-bw has been suggested as the proper heat transfer approach for local thermal resistance in this type of BHE, DTRT results can only be compared between these pipe-in-pipe and other BHEs on a global basis in terms of Rb and/or Rt b . For instance, comparing Ran-bw from a pipe-in-pipe BHE with Rb from a U-pipe would be a mistake and leads to misunderstandings and unfair comparisons. However, comparing Rb and/or Rt b is correct (these statements are also partially valid for Multi-pipe BHEs with insulated central pipe). In any case, large improvements in BTES performance can be expected if pipe-in-pipe BHEs as in BH9 and BH10 (having thermal resistances of about 0.03 K m/W or lower) are used instead of typical U-pipes for which global and local borehole thermal
Fig. 13. External pipe after installation (serves as link to a video in the electronic version of this paper).
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Fig. 14. Instantaneous fluid temperature profiles and specific heat injection during DTRTs in both BHEs.
resistances have been reported to be between 0.06 and 0.12 K m/ W, or even higher. 5. Conclusions The thermal performance of two pipe-in-pipe BHEs, one of them having a partly insulated central pipe, has been studied with regard to global and local borehole thermal resistances during several DTRTs. Analyses of instantaneous vertical fluid and borehole wall temperature profiles are presented. They reveal the detailed thermal behavior for this type of BHE. The thermal resistances calculated along the borehole are similar in both pipe-in-pipe designs, indicating that both BHEs actually operate with similar efficiency. Global (effective) borehole resistances resulted to about 0.03 K m/ W while local resistances are about 0.01 K m/W. The global and local borehole thermal resistances have been calculated with different fluid temperature definitions, including the typical arithmetic mean temperature, a true mean temperature, and just using the fluid temperature in the annular flow channel. The observation and comparison between them indicates how the true mean approximation qualitatively differs from the arithmetic mean approach and how these two approaches differ from the temperature of the fluid in the annulus, the latter being the only fluid in direct thermal contact with the borehole wall. Locally, the thermal resistance corresponding to the true mean fluid temperature is higher than when it is calculated between the annulus and the borehole wall. The local resistances based on the true mean fluid temperature are also higher in the upper than in the lower borehole sections, mainly due to larger temperature differences between the fluid and the borehole wall. It has, therefore, been suggested that none of the mean temperature definitions should be used for the estimation of local borehole resistances and, instead, it would be most logical to take the local borehole resistance between the annulus and the borehole wall. A pipe-in-pipe design having a standard 40 2.4 mm polyethylene central pipe presents a global (effective) thermal resistance calculated with the arithmetic mean fluid temperature of 0.027 and 0.035 K m/W when the fluid flow rate is 0.58 l/s and 0.50 l/s, respectively (for an active depth of 165 m). In case this resistance is instead calculated using the true mean temperature, the corresponding values are 0.024 and 0.029 K m/W. Likewise, the design with a partially insulated central pipe had a global borehole thermal resistance of 0.029 to 0.030 K m/W (based on in and out temperatures and true mean fluid temperatures, respectively) at 0.43 l/ s (for an active borehole depth of 151 m). It is not recommended to use the mean temperature approximation in pipe-in-pipe BHEs as
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there is not thermal contact between the fluid in the central pipe and the borehole wall. Instead, an annulus to borehole wall thermal resistance should be used. A new borehole wall temperature measurement technique along the borehole depth has been used, consisting of a fiber optic cable pressed against the borehole wall with a flexible plastic pipe that is filled with water once it is installed into the ground. The borehole wall temperature measurements during the DTRTs clearly illustrate the meaning of borehole resistance as it has been plotted as a function of time together with fluid temperatures, allowing a straightforward estimation of the borehole thermal resistance by simply dividing the difference between the measured annular fluid and borehole wall temperatures by the supplied heat. This simple estimation method show slightly higher results but rather good accordance with the values obtained with the line source model. The heat transfer per meter in the borehole was measured to be almost evenly distributed along the depth in the tested pipe-inpipe BHEs. The measured borehole wall temperature closely follows the same trend as the temperature in the annular flow in all cases. The thermal resistance between the fluid in the annular flow channel and the borehole wall is almost negligible compared with the result using the fluid mean temperature approach. Such a resistance has shown to be about 0.01 K m/W in the tested pipe-inpipe BHEs. It has been shown that pipe-in-pipe BHEs can, on a global basis, be compared with other BHEs by comparing global thermal resistances, but that the comparison may lead to misunderstandings due to the lack of direct thermal contact between the fluid in the central pipe with the borehole wall. Regardless of this, large improvements in BTES performance are expected if pipe-in-pipe BHEs of the type tested here are used instead of typical U-pipes. Acknowledgements STEM and all industry partners within EFFSYS+ are acknowledged for financing this research. Special thanks to Klas Andersson, Jan Cederström, Bo Jansson, Hans Alexandersson, and Kenneth Weber, for their support related to the experiments in these borehole heat exchangers. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apenergy.2013. 01.024. References [1] Spitler J, Bernier M. Ground-source heat pump systems: the first century and beyond. HVAC&R 2011;17(6):891–4. [2] Renewable heating and cooling platform (RHC). Strategic research priorities for geothermal technology. Brussels: European Technology Platform on Renewable Heating and Cooling; 2012. [3] Acuña J, Mogensen P, Palm B. Distributed thermal response test on a U-pipe borehole heat exchanger. Effstock – the 11th international conference on energy storage. Stockholm; 2009 [4] Acuña J, Mogensen P, Palm B. Distributed thermal response tests on a multipipe coaxial borehole heat exchanger. HVAC&R 2011;17(6):1012–29.
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