FIRST EXPERIENCES WITH COAXIAL BOREHOLE HEAT
EXCHANGERS
J. Acuña, B. Palm
jose.acuna@energy.kth.se.
Department of Energy Technology/Applied Thermodynamics and Refrigeration/KTH
Brinellvägen 68, 100 44 Stockholm, Sweden
ABSTRACT
Some experiences with coaxial borehole heat exchanger prototypes are discussed here. Four different designs are described as they have been part of a research project at KTH: two pipe-inpipe annular designs, one multi-pipe and one multi-chamber design. A special focus is given to
two of the prototypes, a pipe-in-pipe design with the external flow channel consisting of an annular cross section and partly insulated central pipe, and a multi-pipe design with twelve parallel peripheral pipes and an insulated central channel. The secondary fluid temperature profiles at
low volumetric flow rates are presented for these two prototypes, measured with fiber optic cables during thermal response tests and allowing a detailed visualization of what happens along
the heat exchanger depth. It is the first time this is carried out in these types of borehole heat exchangers. The measurements indicate good thermal performance and point at potential uses for
these heat exchangers in different ground coupled applications.
1. INTRODUCTION
The performance of Ground Source Heat Pumps (GSHP) coupled to vertical energy wells
partially depends on the thermal resistances in the ground and in the borehole. Different
locations may imply different design needs and the thermal response of the ground at a
certain location is difficult to control by other means than changing the position of the
boreholes and controlling the heat rates from and to the ground. However, it is possible to
affect the resistance in the borehole by changing the Borehole Heat Exchanger (BHE).
This resistance depends on the heat transfer between the secondary working fluid and the
borehole wall and depends on the arrangement of and the heat transfer in the flow channels,
possible convection in the borehole, the thermal properties of the BHEs as well as of the
borehole filling material. All these different parts are normally added together and called
borehole thermal resistance, defined as Rb by (Hellström, 1991). It is of great relevance to
design cost effective BHEs characterized by moderate temperature differences (low Rb)
between the secondary fluid and the surrounding ground. A one degree temperature difference flowing from the BHE into the heat pump evaporator may represent 2-3% change
in the COP of the system. This paper shows measurements from an annular coaxial design
with partly insulated central pipe and a so called TIL (Thermal Insulated Leg) design that
present possibilities for solving this issue. These are being tested as part of two research
programs, EFFSYS2 (www.effsys2.se) and EFFSYS+ (http://effsysplus.se).
Sources/Sinks alternative to the outside Air for
Heat Pump and Air-Conditioning Techniques
(Alternative Sources - AS)
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Padua, Italy, April 5-6-7, 2011
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2. WHY COAXIAL BOREHOLE HEAT EXCHANGERS?
The usual method to exchange heat with the ground in GSHP installations is by means of
U-pipe Borehole Heat Exchangers, consisting of two equal cylindrical pipes connected at
the bottom and through which a secondary fluid travels down an upwards while exchanging thermal energy with the ground. U-pipe BHEs are relatively inexpensive and very easy
to install, resulting on their total dominance in the GSHP market for many years. This, although coaxial BHEs consisting of one central and one or several peripheral flow channels
may offer unique advantages (if properly designed) as compared to U-pipes. They may offer good thermal contact between external flow channel(s) and the borehole wall, low
thermal shunt between up and downwards channels, lower pressure drop, among others.
Most of the works presented so far about Borehole Heat Exchangers have been concentrated on U-pipe BHEs. Given that this paper does not include any results from U-pipe
BHE installations, the background about this topic is referred to some of the published
works, such as: (Hellström, 1998), where Rb is presented as a function of filling material
thermal conductivity for three different pipe positions in the borehole; (Hellström and
Kjellsson, 2000) where the influence of free convection heat transfer is discussed by
means Rb at different temperature levels and heat rates; (Gehlin, 2002), where several tests
were carried out on U-pipe designs; (Bose et al., 2002), where bentonite and a thermally
enhanced grout were used as backfill materials and the use of a U-pipe separator (spacers)
was evaluated; (Sanner et al., 2005) who presented a table with results from several TRTs
in grouted boreholes, Rb ranging between 0.06 and 0.20 Km/W depending on the grout
material and the borehole diameter; (Acuña, 2010), where spacers were tested in a
groundwater filled borehole and who identified variations of Rb along the borehole depth
thanks to distributed temperature measurements in U-pipes. All these works generally refer to relatively high borehole thermal resistances ranging between 0.05 and 0.20 Km/W.
Pipe positions with respect to each other and to the borehole wall, the relatively low thermal conductivity of borehole filling/grouting materials, the low thermal conductivity and
the thickness of the pipes, and the thermal contact between channels (specially at low
volumetric flow rates), are the common cause for the poor performance of U-pipe BHEs.
On the other hand, works on coaxial BHEs have been rather few. A simple design was
discussed by (Hellström, 2002), consisting of an open annular coaxial BHE where the
fluid travels in absolutely direct contact with the rock in the annular channel, with Rb of
circa 0.01 Km/W. Another concepts was presented by (Platell, 2006), consisting of one
central insulated pipe and several outer pipes (a version of this idea, a TIL design manufactured by UPONOR, is actually presented in this paper). The first such prototype consisting of 62 thin pipes (diameter of 3.8 mm and thickness of 0.65 mm) arranged close to
the borehole wall was tested in a laboratory scale with a 104 mm borehole by (Hellström
et al., 2000), resulting on thermal resistances between 0.009 and 0.028 Km/W.
Another coaxial annular BHE, in this case a closed loop, was described during the EU
project GROUNDHIT by (Erdwärme-Systemtechnik GmbH & Co, 2006) and (CRES,
2008), consisting of one PE63x5.3 mm outer pipe and an inner channel with dimensions
PE40x3.7mm. Rather easy installation and assembling methods were as well tested and
presented. Similarly, (Acuña, 2010) presented another annular design consisting on a
flexible external hose that is attached to the rock wall as it is filled with water after instal ling it into the borehole, followed by the insertion of a PE 40mm pipe. The latter work
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shows measurements of the borehole wall, illustrating very low temperature differences as
compared to the temperature of the fluid circulating through the annular channel.
Some of the few studies presented so far about coaxial BHEs point at potentials for improvements as compared to U-pipes, justifying their further study that should not only be
focused on determining their borehole resistance but also fully characterizing them from
the heat transfer point of view for better understanding and knowledge promotion. Many
tests have been done at KTH and this paper is just one of the first steps within this study.
3. COAXIAL BHE INSTALLATIONS DURING THE EFFSYS PROJECT
Four different coaxial borehole heat exchanger installations have been done, including two
annular coaxial designs that consist of pipe-in-pipe configurations (where the external pipe is
a flexible hose attached to the borehole wall), one multi-pipe coaxial design known as TIL
(Thermal Insulated Leg) BHE, and a multi-chamber BHE (named REFLA by its manufacturer). These designs are illustrated in Figure 1, Figure 2, Figure 3,
Figure 4, respectively. What follows is a brief explanation of each of these heat exchangers
and their installation.
3.1 Pipe in pipe designs, Annular Coaxial
The two installed prototypes consist of two 190
m deep BHEs, consisting of a central pipe
(without and with partial insulation, respectively) inserted into a so called energy capsule. The
latter consists of a thin hose (0.4 mm thick) that
is sent into the borehole and subsequently filled
with water. The diameter of the capsule is 114
mm and a sketch of the cross section is shown in
Figure 1 and Figure 2. This collector is at the
moment manufactured and delivered in two
main separate parts, the capsule and the central
pipe, by the Swedish company PEMTEC. The
installations started by inserting the capsule and
a fiber optic cable into the borehole with a bottom weight (the fiber cable is used to measure
temperatures at the interface between the capsules and the rock and thereby estimate the borehole wall temperature). This part of the
process was done in a relatively short time, followed by filling the energy capsule with water.
Figure 1. Annular Coaxial - Standard Pipe
Figure 2. Annular Coaxial - Insulated Pipe
Once the capsule is filled with water, it sticks the borehole wall allowing good physical contact between the fluid flowing inside the annular channel and the rock. This fact has been confirmed by filming from the inner part along the whole borehole depth. The installation procedure until this point was the same for both cases. After inserting the capsule, it was proceeded
to insert the central pipe, also with a bottom weight. The case with partly insulated pipe required a much larger bottom weight in order to overcome the buoyancy forces. The installation procedure was simpler and shorter in time in the case without insulation. In both cases, a
fiber optic cable was also installed along the central and annular flow channels in order to
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measure the secondary fluid temperature along the whole borehole heat exchanger. A detailed
measured temperature profile for this design at a typical volumetric flow rate was presented in
(Acuña, 2010), illustrating its potentials and pointing at possible advantages if using insulation on the central shank. A measurement along the design having partly insulated central
shank during heat injection into the ground is shown in the next section. This type of installation may require a rather hard rock without cracks or fissures so that the energy capsule can
be uniformly attached to the rock wall, specially if working as a closed loop.
3.2. Multi-pipe, Thermal Insulated Leg
The installed TIL prototype consists of 100
m deep water filled BHE with 12 thin peripheral tubes placed around an insulated central pipe. A cross section view is shown in
Figure 3. This design is, in contrast to the
annular prototypes, delivered as a whole
package with all 13 pipes rolled around a
drum. The installation procedure consists of
just rolling out the heat exchanger as it is
inserted into the borehole.
Figure 3. TIL BHE
Our installation was considerably more special, given that a fiber optic cable was also inserted
along the central pipe and through one of the peripheral channels, also for measurement of the
secondary fluid along the whole BHE. Inserting the fiber cables in the peripheral pipe added a
considerable amount of extra time to the installation. Moreover, extra bottom weight was
added at the bottom part of the heat exchanger for overcoming the buoyancy forces and facilitating the installation into the well. The procedure of inserting the BHE into the borehole is
relatively simpler at the beginning, but gets more difficult as more pipe volume is inserted into the well. A special tool such as the one used for U-pipe BHEs but adapted to this design
may facilitate the installation. A measurement of the secondary fluid temperature along this
prototype during heat injection into the ground is shown in the next section.
3.3. Multi-chamber design, REFLA
This prototype is installed in a 250 m deep water filled borehole and it consists of one central
pipe and five external chambers, as shown in
Figure 4, all in a single pipe package that can be installed in a similar way as U-pipe BHEs.
The installation, as for the previous case, requires bigger efforts for inserting the collector into
the well as more pipe volume is sent down. This is due to large buoyancy forces and also to
the stiffness of the pipe package.
Theoretical and experimental results in a 140
mm diameter borehole where this prototype
was tested and presented in (Acuña, 2010),
pointing at high heat extraction rates
through the outer wall of the peripheral
channels but somewhat high thermal shunt
effects. The resulting Rb was within the same range as for U-pipe BHEs and the pressure drop at all flow rates was significantly lower than in a U-pipe.
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wall
140
Central pipe
Figure 4. First REFLA prototype
Figure 5. Upgraded REFLA design
First Experiences with Coaxial Borehole Heat Exchangers
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After these results, a second prototype has been released (see Figure 5) with smaller contact
area between the external chambers and the central channel, and a more compact pipe package, possibly uppgrading the thermal performance and facilitating the installation.
4. TEMPERATURE MEASUREMENTS AND DISCUSSION
Several Distributed Thermal Response Tests have been carried out using the borehole heat
exchangers presented in section 3 and their analysis is ongoing. Figure 6 and Figure 7 present
an illustration of some of the measurements in two of these designs: the annular coaxial with
partly insulated central channel and the multi-pipe (TIL) design, respectively. The fluid is injected through the central pipe in the measurement along the annular case, while in the TIL
case the injection takes place through the peripheral channels. Both insulations are of the
same type and the span between the 40 mm pipe and the 68 mm diameter shown in Figure 2
and Figure 3 includes a plastic protection.
The measurements have been taken with different instruments (ORYX and HALO, respectively, both from SENSORNET) during so called Distributed Thermal Response Tests
(DTRT), where temperatures are measured at different depths in the borehole while running a
thermal response test. The temperature is measured by sending laser pulses through optical fiber cables and interpreting the backscattered light. The intensity ratio between up- and downshift is a function of the temperature, and this ratio and the delay time are converted to temperatures and positions by the instruments. The accuracy of this type of measurement depends
on integration time, the measuring length interval, the laser features, among other factors.
Measurements of this type have so far only been presented by (Fujii, et al., 2006) with an optical fiber cable located on the external part of a U-pipe, by (Fujii et al., 2009) with a cable
along one of the two U-pipe shanks, and by (Acuña, 2010) who measured along both shanks
inside a U-pipe and in the non insulated annular coaxial BHE design shown in Figure 1.
Figure 6 shows, for the annular coaxial configuration, the secondary fluid temperature profile
and a measurement that estimates the borehole wall temperature, evidencing that the fluid
traveling upwards has good thermal contact with the borehole wall, with almost no temperature difference between them. Along the downwards pipe, the fluid changes its temperature
with about 1 K, while the change along the annular channel is about 3 K. The mean Reynolds
number (estimated at 15.5°C and 13.5°C for the central and annular channel, respectively) for
these specific working conditions is circa 13 500 along the central pipe, whilst around 3000
and 2600 along the lower half for the upper half the annular channel (the insulation in the upper part reduces the hydraulic diameter), respectively. Transition conditions between laminar
and turbulent flow may occur along the annular channel. The possible presence of laminar
flow along the annular channel is also in some way evident in the temperature profile that oscillates around a relatively constant slope, possibly showing the effect of the placement of the
cable in cross sections having a specific temperature gradient. Still, the temperature is always
decreasing, meaning that no significant thermal shunt flow takes place and indicating positive
insulation effects at this low flow rate conditions (although there is a temperature gradient
along the central pipe).
The good thermal contact with the rock along the annular channel has raised the initial idea of
testing this design with water as a circulating fluid, instead of with an antifreeze aqueous solution, offering possible advantages from the hydraulic, thermal and environmental point of
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view. There is, however, a risk for fluid freezing in cold places where the undisturbed ground
temperature is close to the water freezing point conditions. The performance of this type of
system is being studied at the moment with a variable speed heat pump. It is of paramount
importance to properly design these systems in order to avoid unnecessarily low secondary
fluid temperature levels.
Temperature [°C]
11
12
13
14
15
16
17
0
20
40
Depth [m]
60
80
100
120
140
160
180
Borehole wall
Flow downwards
Flow upwards
Figure 6. Secondary fluid and borehole wall temperature profile along the annular coaxial design
with partly insulated central pipe
Tem p er a tu r e [°C ]
11
12
13
14
15
16
17
0
D e p th [m ]
20
40
60
80
100
F lo w d o w n w a r d s
F lo w u p w a r d s
Figure 7. Secondary fluid temperature profile along the Thermal Insulated Leg (TIL) design
Figure 7 shows a typical temperature profile for a relatively low volumetric flow rate in the
TIL design. The total fluid temperature change (about 4 K) only takes place along the downwards pipe. The hydraulic diameter of the peripheral pipes is small, allowing for laminar
flows and still having high heat transfer coefficients. The upwards flow has almost constant
temperature, confirming a high thermal resistance between the thermal insulated leg (TIL) and
the peripheral channels. An estimation of the Reynolds numbers at the mean temperatures for
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each of the shanks (about 14°C and 12°C for the peripheral and central channel, respectively)
at these specific working conditions results in 3900 for the central pipe and 1000 along the peripheral channels. The thermal properties for the Reynolds estimation are obtained from (Melinder, 2007). The latter Reynolds numbers differ from the reality due to the presence of the
optical fiber cable. A complete analysis about the issues discussed in this paragraph is ongoing and will be analyzed in detail and presented in a different paper.
As it can be inferred, TIL collectors allow having much different temperature levels at different depths in the ground as compared to other designs, pointing at possibilities for using different temperature levels for different applications and making it suitable for different types of
ground loop connections as, for example, with solar collectors and/or ventilation systems.
As it can be observed in Figure 6 and Figure 7, the boreholes have different depths. The scales
in both figures are the same whilst the depth is almost half in the TIL case. Moreover, the
volumetric flow rate during these tests was lower for the TIL case than for the annular case,
meaning that no direct comparison should be done between these two figures. However, the
measurements are good indicators about the shape of the temperature profiles. Given the low
Reynolds numbers along the external channels, it can be stated that both measurements were
done at relatively volumetric low flow rates without having significant risks for thermal shunt
flow, especially in the TIL case. The temperature difference over the inlet and outlet flow at
the top of the heat exchanger is about 4 K for both cases. However, the temperature profiles
are extraordinarily different, resulting in different possibilities for exchanging heat with the
ground. This partially depends on the shape of the undisturbed ground temperature profile.
(Acuña, 2010) shows undisturbed ground temperature profiles having different shapes at different locations within the same city due to urbanization effects. Given this fact and the fluid
temperature profiles presented here, it is possible the these BHEs may in the long term affect
the ground temperatures in different ways, pointing at the need for accounting for this in design programmes, not only regarding long but also for short term response for the systems.
Preliminary thermal resistances in these types of BHEs in groundwater filled boreholes
range between 0.00 and 0.04 Km/W, depending on the flow rates. Putting these preliminary numbers in perspective as compared to typical thermal resistances, it can be believed
that they offer potential for improving BHE performance in general.
5. CONCLUSIONS
Four different borehole heat exchanger prototypes are described as they have been part of a research project at KTH. They consist of two pipe-in-pipe annular designs, one multi-pipe and
one multi-chamber design.
Secondary fluid temperature profiles at low volumetric flow rates measured with fiber optics
during heat injection show how the heat is distributed along the heat exchanger depth for two of
the prototypes: the annular design and a so called TIL (Thermal Insulated Leg) design, both
with partial and full insulated central channel, respectively. Both collectors required extra bottom weight for overcoming the buoyancy forces of the insulated pipe.
For the borehole wall temperature measurements in the annular coaxial design, it is possible
to infer the presence of efficient heat transfer between the fluid flowing through the external
channel and the borehole wall thanks to proper physical contact between the channel walls
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and the rock, which was confirmed by a film made from the inner part of the pipe along the
borehole heat exchanger depth.
The advantages with a fully insulated central pipe are evidenced with the temperature profile
in the TIL design, where the whole temperature change takes place along 12 peripheral channels as the fluid flows at laminar regime, confirming the existence of a high thermal resistance
between the down and upwards flow channels, and a low resistance between the latter and the
ground. The results open a wide range of possibilities for the use of this borehole heat exchanger.
ACKNOWLEDGEMENTS
The Swedish Energy Agency, the EFFSYS2 and EFFSYS+ projects, and all our industry
partners, are greatly acknowledged for financing this project. Special thanks to Hans Alexanderson from PEMTEC, John Ljunqvist and Klaus from UPONOR, Peter Platell from
LOWTE, and to Klas Andersson, Bo Jansson, Kenneth Webber, Benny Sjöberg and Kalle for
their help during the installations. JM AB and SUST (Sustainable Innovation AB) are also
greatly acknowledged for the cooperation through the project in Hålludden, Värmdö.
REFERENCES
1. Acuña J. Improvements of U-pipe Borehole Heat Exchangers. Licentiate Thesis, Stockholm:
The Royal Institute of Technology KTH. Sweden.
2. Bose, J., Smith, M., Spitler J., 2002, Advances in Ground Source Heat Pump Systems - An
International Overview. Proc. 7th IEA Heat Pump Conference. 1:313-324, Beijing.
3. CRES, 2008, GROUNDHIT – Ground Couples Heat Pumps of High Technology, specific
Targeted Research Project, deliverable 24, Borehole Heat Exchangers Take-up study, Greece.
4. Erdwärme-Systemtechnik GmbH & Co. KG, 2006, GROUNDHIT, Brief Reporton on Deliverable no. 8 – Design drafts - Description of pre-prototype Borehole Heat Exchanger.
5. Fujii H., Hiroaki O., Ryuichi I., 2006, Thermal Response Tests Using Optical Fiber Thermometers, GRC Transactions - Vol. 30, Kyushu University.
6. Fujii H., Okubo H., Nishi K., Itoi R., Ohyama K., Shibata K., 2009, An improved thermal response test for U-tube ground heat exchanger based on optical fiber thermometers, Geothermics
38, 399-406.
7. Gehlin S., 2002 Thermal Response Test - Method, Development and Evaluation, PhD thesis,
Luleå University of Technology, Sweden.
8. Hellström G., 1998. Thermal Performance of Borehole Heat Exchangers. The Second Stockton International Geothermal Conference.
9. Hellström G., Kjellsson E., 2000. Laboratory measurements of heat transfer properties of different types of borehole heat exchangers. Proc. of Terrastock. Stuttgart, Germany.
10. Hellström, Göran. Ground Heat Storage - Thermal Analyses of Duct Storage Systems. PhD
thesis. LTH, Sweden.
11. Hellström G., 2002. Borehole Heat Exchangers - State of the art. IEA-Annex 13.
12. Platell, Peter, 2006. Developing work on ground heat exchangers. ECOSTOCK Conference Proceedings 7A-1. New Jersey.
13. Sanner B., Hellström G., Spitler J., Gehlin S., 2005, Thermal Response Test – Current
Status and World-Wide Application, Proceedings World Geothermal Congress. Turkey.
14. Melinder, Åke, 2007, Thermophysical Properties of Aqueous Solutions Used as Secondary Working Fluids, PhD Thesis, Stockholm: The Royal Institute of Technology KTH. Sweden.
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