A laboratory experimental study
of high-temperature thermal storage in the
unsaturated soil using a vertical borehole
heat exchanger
..............................................................................................................................................................
Huajun Wang * and Chengying Qi
School of Energy and Environment Engineering, Hebei University of Technology,
Tianjin 300401, PR China
.............................................................................................................................................
Abstract
High-temperature thermal storage (HTTS) in soils is a promising energy-saving technology for space
heating of buildings. Based on a laboratory experimental setup using a vertical borehole heat exchanger
(BHE), dynamic changes of the soil temperature and moisture content during the thermal storage
process are studied. Effects of the heat injection temperature and initial moisture content on the
thermal performance of the BHE are analyzed. The results show that at the first thermal storage stage,
the soil temperature and moisture content near the heat source may appear a temporary peak. Its
occurrence depends on the initial soil moisture content, the heat injection temperature and the distance
from the heat source. As the heat injection temperature increases, the heat transfer rate of the BHE
increases greatly. As the initial soil moisture content increases, the temperature profile near the BHE
tends to be deviated from the results predicted by heat conduction, thereby influencing the thermal
performance of the BHE. The present results can provide useful guidelines for the design of an HTTS
system.
Keywords: high temperature thermal storage; borehole heat exchanger; unsaturated soil; thermal
performance
*Corresponding author:
[email protected]
Received 5 February 2011; revised 14 March 2011; accepted 29 March 2011
................................................................................................................................................................................
1 INTRODUCTION
There has been a growing interest on possible applications of
high-temperature thermal storage (HTTS) for space heating of
buildings, due to an increasing awareness of CO2 emissions
implied by conventional systems based on fossil fuels [1]. This
interest is also motivated by technical advances in a groundsource heat pump (GSHP) based on borehole heat exchangers
(BHEs). In an HTTS system based on BHEs, the extra energy
in summer is transferred to the shallow soil which serves as
heat capacity and is heated up to a required temperature, and
is extracted from the soil to meet the requirement for space
heating in winter.
Compared with low-temperature systems (e.g. GSHP),
HTTS systems have a more appealing potential to apply
various heat source types with a wider temperature range
(e.g. 60– 908C), and obtain higher energy efficiency. For
instance, Reuss et al. [2] designed a thermal storage system
using the industrial waste heat at a temperature level of
808C. The results showed that such a system was able to
obtain a high thermal storage efficiency of 64%. Similar
systems based on solar energy have also been applied in
Sweden, Germany and other countries in the past decades
[3– 5]. Gabrielsson and Bergdahl [6] predicted the performance of a solar heating system with thermal storage in soil
at temperatures reaching 908C. Wang et al. [7,8] analyzed
the performance of a solar-ground coupled thermal storage
system for residential buildings. Test results showed that the
thermal storage efficiency reached over 70% when the heat
source temperature was 608C. In spite of these efforts, there
is still a long way to further improve the energy efficiency
and decrease the cost of HTTS systems.
It is well accepted that for typical GSHP applications, heat
transfer of BHEs in the soil is dominated mostly by heat conduction. These models, which are based on Fourier’s law of
heat conduction, include the analytical line-source and
International Journal of Low-Carbon Technologies 2011, 6, 187– 192
# The Author 2011. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
doi:10.1093/ijlct/ctr006 Advance Access Publication 17 May 2011
187
H. Wang and C. Qi
cylindrical-source model [9,10]. High-temperature thermal
energy into the soil, however, can produce a noticeable temperature gradient and simultaneous moisture migration in the
vicinity of BHEs. The thermal properties of the soil may also
undergo a great variation with water content at high temperatures [11,12]. For the design of HTTS systems, these considerations have to be taken into account. For instance, Reuss et al.
[2] indicated that water losses in soil could be drastic due to
vapor diffusion along the high-temperature gradient, which
could lead to dry-out and cracking in the area surrounding the
BHEs in the worst situation, thereby reducing the heat transfer
rate significantly. With the background above, this paper performs a laboratory experimental investigation of HTTS in the
unsaturated soil using a vertical BHE. The dynamic changes of
the temperature and moisture content in the soil and their
influences on the thermal performance of BHEs are analyzed.
The main purpose of the present work is to provide some
useful guides for the design of HTTS systems.
2 EXPERIMENTAL INVESTIGATIONS
2.1 Experimental setup
Figure 1 shows the schematic view of a small-scale experimental setup of HTTS. The setup mainly consists of a soil container, a BHE, a circulation heating system, a circulation
cooling system, a data acquisition computer, and relevant
temperature and moisture sensors. The cylindrical soil container is made of 3 mm thick stainless steel plates with 120 cm
outside diameter and 100 cm height. In order to reduce the
undesired loss of heat or cold, the whole container is insulated
using polyethylene foam materials with 50 mm thickness. The
vertical BHE is simulated by a copper pipe with 20 mm
nominal diameter and 100 cm length. The circulation heating
and cooling systems are provided by two Julabo-type constant
temperature water baths with +0.28C accuracy. By means of
advanced PID techniques, the heating or cooling system is able
to keep a relatively stable inlet fluid temperature to the BHE or
the water jacket surrounding the soil container. In this experiment, the inlet fluid temperature to the BHE is kept as 60 and
Figure 1. Diagram of the experimental setup to simulate an HTTS system.
188 International Journal of Low-Carbon Technologies 2011, 6, 187– 192
808C, respectively, to simulate the HTTS process, while the
temperature of the outside water jacket is kept at 18.58C as a
simplification of the actual far-field soil boundary.
As shown in Figure 1, three groups of temperature and
moisture sensors are embedded at the depth of 20, 40 and
60 cm, respectively. Each group includes four copperconstantan thermocouples with +0.18C accuracy and four
MP406B-type moisture sensors with +2% accuracy. They are
embedded at the horizontal distance of 10, 20, 35 and 50 cm,
respectively, counting from the outer wall of the BHE. These
moisture sensors are based on the technique of frequency
domain reflectometer (FDR) [13]. Compared with the conventional time domain reflectometers (TDRs), FDR moisture
sensors have a faster response time and are able to effectively
remove the soil type sensitivity using a high frequency of 100 –
150 Hz. The inlet and outlet fluid temperature through the
BHE are measured by two Pt1000 sensors with +0.18C accuracy, respectively. All signals of temperature and volumetric
moisture content in the soil are recorded by an Aglient
34970-type data logger, and then sent to a data computer.
Prior to the installation, all temperature sensors are calibrated
by an XLR-1 type constant-temperature bath with +0.018C
accuracy, and all moisture probes are calibrated using a standard weighting method in a GXZ-type air-blow drying oven.
In addition, the top of the soil container is covered with a
plastic layer in order to avoid the undesired evaporation loss of
moisture contents from the top soil. Figure 2 shows a photo of
the present experimental setup.
2.2 Soil sample and data treatment
In this experiment, river sand is selected by a screen sizer for
the preparation of experimental soil samples. After the
measurement, the representative size of sand grains was
0.15 mm, which is in the range of fine sand (0.1 – 0.25 mm)
according to the soil classification method recommended by
the US Department of Agriculture (USDA). The thermal conductivity of fine sand is determined using a thermal needle
probe as 0.24 W/mK at a dry state and 1.78 W/mK at half saturation, respectively, and the corresponding dry density is
Figure 2. Photo of the HTTS experimental setup.
Experimental study of HTTS in the unsaturated soil
1540 kg/m3. In addition, the saturated hydraulic conductivity
of fine sand is measured using a constant-head method as
14 1026 m/s at 208C, which is about 35 times than that of
clay soil [14].
Before the experiments, fine sand is pre-mixed fully to
reach a given initial moisture content, and then filled into the
container. The mass of dry sand is kept as the same in each
experiment. After a settling period of 12 –16 h, the heating and
cooling systems start to work simultaneously.
The heat transfer rate q of the BHE per unit depth can be
calculated by measuring the inlet and outlet fluid temperature
and the flow rate through the BHE. It can be expressed as
q¼
mcp ðtj tc Þ
H
ð1Þ
where m is the mass flow rate (kg/s), cp the specific heat
(J/kg K), H the BHE’s length (m), tj and tc are the inlet and
outlet fluid temperature of the BHE (8C), respectively.
The effective saturation degree S of the unsaturated sand is
defined as
S¼
u ur
us ur
ð2Þ
where us is the saturated moisture content, which is equivalent
to porosity, and ur the residual moisture content. In this experiment, the values of us and ur are measured as 0.3433 and
0.007 m3/m3, respectively. All experiments were performed
under an initial moisture content of lower than 0.135 m3/m3
(38.1% of saturation).
Figure 3. Variations of the soil temperature and moisture content under
different heat injection temperatures. (a) Temperature; (b) moisture content.
3 RESULTS AND DISCUSSION
3.1 The soil temperature and moisture content
profile during the HTTS process
Figure 3 compares the soil temperature and moisture content
near the pipe wall of the BHE under different heat injection
temperatures. It can be seen that the soil temperature and
moisture content increase during the first thermal storage
stage, and then decrease gradually. This is very different from
the case with a dry soil condition, as shown in Figure 4, where
there is no peak soil temperature, or rather that the soil temperature increases with time until finally reaching an approximate steady-state value.
In this experiment, the peak phenomenon of the soil temperature and moisture content appears a complex behavior
varying in time and space field. Its occurrence depends mainly
on the initial soil moisture content, the heat injection temperature and the distance from the heat source. For the initial soil
moisture content, there exists a dominant range. It is found
that when the initial soil moisture content is over 0.08 m3/m3
(or 21.7% of saturation), the peak moisture content is not
observed any more, but the peak temperature still exists at the
Figure 4. Variations of the soil temperature under different initial moisture
contents.
distance of 10 cm from the pipe wall of the BHE. The lower
limit where the peak moisture content occurs is determined as
2.02 1022 m3/m3 (or 3.9% of saturation).
As shown in Figure 3, as the heat injection temperature
increases, the peak phenomenon is becoming evident. Further,
the heat injection temperature has a greater impact on the
occurrence time of the peak temperature than that of the peak
moisture content. For instance, when the heat injection temperature is 608C, the peak of the soil moisture content and
temperature occurs at 19.2 and 40.7 h, respectively. However,
International Journal of Low-Carbon Technologies 2011, 6, 187– 192 189
H. Wang and C. Qi
Figure 6. Comparison of the approximate steady-state temperature profile
under different initial soil moisture contents.
Figure 5. Variations of the soil temperature and moisture content under
different distances from the heat source. (a) Temperature; (b) moisture
content.
when the heat injection temperature increases to 808C, the
peak of the soil moisture content and temperature occurs at
14.1 and 13.5 h, reduced by 26.6 and 66.8%, respectively. In
addition, different initial conditions of moisture content may
lead to different steady-state moisture content and temperature
profile at steady state. Under special circumstances, the thermal
performance of HTTS systems is likely to become unstable.
As Reuss et al. [2] indicated, a high injection temperature
may induce the dry-out effect of the soil near the BHE, which is
harmful to the thermal storage performance of the BHE. In his
experiments, the dry front in clay moved to 10 cm distance
from the heat source after 25 days of thermal storage, where the
heat source temperature was kept as a low value of 508C. In fact,
this result is mainly caused by a low hydraulic conductivity of
clay, except for a low heat injection temperature. For fine sand
with a high hydraulic conductivity, however, the above dry-out
phenomenon may come up tens of hours earlier. Therefore, fine
soils (e.g. clay and silty clay) may be better suited for heat transfer at the high temperature boundary, although their thermal
conductivities are lower than those of sandy soils. If sandy soils
have to be used, it is recommended that an intermittent thermal
storage mode of BHEs should be considered during the operation of HTTS systems.
Further, Figure 5 shows the variations of the soil temperature and moisture content under different distances from the
190 International Journal of Low-Carbon Technologies 2011, 6, 187– 192
heat source. It can be seen that as the distance increases, the
peak phenomenon of the soil temperature becomes weak
gradually and lagged in time. For instance, the peak soil temperature at the distance of 10 and 35 cm occurs at 11.8 and
29.4 h, respectively. With respect to the peak phenomenon
of the soil moisture content, it does not occur at the distance
of 35 cm during the experiments. This is mainly caused by a
low-temperature gradient at far field. Therefore, it can be
concluded that the region that the peak phenomenon occurs
frequently is in agreement with the range of typical borehole
diameters of vertical BHEs, i.e.10 –40 cm. Thus, the selection
of backfill materials is crucial for improving the thermal
performance of HTTS systems. The present experimental
results can provide some guidelines for a proper choice of
backfill materials. Here, it is recommended that the pre-mixed
grout (e.g. sand-bentonite, sand-cement, etc.) should be considered for a moderate thermal and hydraulic performance in
order to ensure efficient thermal storage.
Figure 6 compares the approximate steady-state temperature
profile under different initial soil moisture contents. It is easily
observed that the soil temperature profile at the residual moisture content of 0.007 m3/m3 is very close to the theoretical
results completely determined by Fourier’s law of heat conduction. As the soil moisture content increases, the temperature
profile near the BHE (e.g. 10 –30 cm) tends to be deviated
from the heat conduction mode to a different extent. This
indicates that HTTS using the BHE is undergoing a non-linear
process in which heat conduction becomes less dominant,
which can further verify the experimental fact that the coupling of heat and moisture transfer in the unsaturated soil tends
to be enhanced under a high-temperature gradient.
3.2 Effects of the initial soil moisture content and
heat source temperature on the thermal
performance of the BHE
Figure 7 shows the variations of the heat transfer rate of the
BHE under different initial moisture contents, where the heat
Experimental study of HTTS in the unsaturated soil
Figure 7. Variations of the heat transfer rate of the BHE under different
initial soil moisture contents.
Figure 8. Variations of the steady-state heat transfer rate of the BHE with the
initial soil moisture content.
injection temperature is kept at 608C. It can be seen that the
heat transfer rate of the BHE drops rapidly during the first 30 h,
and then gradually tends to be steady. As the initial soil moisture content increases, heat transfer tends to be enhanced to a
different extent. For instance, the steady heat transfer rate of the
BHE is merely 25.5 W/m for the residual soil moisture content
(or zero saturation), but reached 47.2 and 82.5 W/m when the
initial soil moisture content is 0.046 m3/m3 (or 11.6% of saturation) and 0.062 m3/m3 (or 16.4% of saturation), respectively.
Further, Figure 8 compares the steady-state heat transfer
rate of the BHE under different initial soil moisture contents.
It can be seen that when the initial soil saturation is lower than
7% or higher than 25%, the thermal performance of the BHE
is affected weakly by the initial moisture content. The former
can be explained by the fact that heat conduction is dominant.
For the latter, those individual liquid bridges among the soil
pores tends to be connected together and finally forms a continuous flow at a nearly saturated or completely saturated state,
where heat advection instead of heat conduction will have a
major impact on the thermal performance of the BHE. It is
found that when the initial soil saturation ranges from 7 to
25%, the heat transfer of the BHE depends strongly on the
moisture content, which is related with the enhanced vapor
diffusion process [15,16]. In this mechanism, local
Figure 9. Comparison of the heat transfer rate of the BHE under different
heat source temperatures.
condensation and evaporation occur frequently at isolated
liquid bridges in the unsaturated soil. Thus, these liquid
bridges, instead of forming a barrier to vapor diffusion,
enhance the vapor diffusion rate, by reducing the effective diffusion path length. Therefore, this also indicates that under a
high-temperature gradient, the coupling of heat and moisture
transfer in the unsaturated soil tends to be enhanced. It should
be paid much attention during the design of an HTTS system,
especially in the soil with a low saturation.
The heat injection temperature is another major factor affecting the thermal performance of the BHE. As shown in Figure 9,
the heat transfer rate of the BHE increases greatly as the heat
injection temperature increases. For instance, when the heat
injection temperature increases from 60 to 808C, the steady-state
heat transfer rate of the BHE varies from 82.5 to 181.4 W/m. This
indicates that the soil will be heated up to a high enough temperature in summer. For common GSHP systems, the space
heating of buildings in winter has to be satisfied by means of heat
pump units, due to a relative low soil temperature [17]. For
HTTS systems, by contrast, a higher soil temperature is very
helpful for the heat extraction of the BHE in winter. In this situation, the energy extracted from the soil can be used directly for
radiant floor heating of buildings, thereby reducing the operating
time of heat pump units effectively. Thus, the energy efficiency of
the whole HTTS system can be improved greatly.
Finally, it should be noted that it usually takes a very long
time to reach a completely steady heat transfer state in practical
HTTS systems, which depends greatly on the soil type and the
heat source temperature. From this point of view, the test results
obtained above are valid for a short-term thermal storage, considering the size limitation of the present laboratory experimental setup. Further long-term experiments on a full-scale HTTS
system are still needed based on the present study.
4 CONCLUSIONS
For the design of HTTS systems, it is crucial for a better understanding of the thermal performance of BHEs in the soil. In this
International Journal of Low-Carbon Technologies 2011, 6, 187– 192 191
H. Wang and C. Qi
paper, a small-scale laboratory experimental setup using a vertical BHE is built and dynamic changes of the soil temperature
and moisture content during the thermal storage process are
analyzed. The effects of the heat injection temperature and
initial moisture content on the thermal performance of the
BHE are discussed. From the experimental results mentioned
above, the following preliminary conclusions can be obtained:
(i) At the first thermal storage stage, both the soil temperature and the soil moisture content near the heat source
may appear a temporary peak varied in time and space
field. Its occurrence depends mainly on the initial soil
moisture content, the heat injection temperature and the
distance from the heat source.
(ii) As the distance from the heat source increases, the peak
phenomenon of the soil temperature becomes weak gradually and lagged in time. The region that the peak phenomenon occurs frequently matches the range of typical
borehole diameters of vertical BHEs. The selection of backfill materials is crucial for the design of HTTS systems.
(iii) For the soil with a high hydraulic conductivity, the
dry-out effect may come up tens of hours earlier. Fine
soils are better suited for heat transfer at the high temperature boundary. If sandy soils have to be used, an intermittent operation mode should be considered.
(iv) As the initial soil moisture content increases, the temperature profile near the BHE tends to be deviated gradually
from the heat conduction mode and heat transfer tends to
be enhanced to a different extent. Especially, when the
initial soil saturation ranges from 7 to 25%, the heat transfer
rate of the BHE depends strongly on the moisture content.
(v) As the heat injection temperature increases, the heat transfer rate of the BHE increases greatly. This is favorable for
the heat extraction of the BHE in winter, thereby reducing
the operating time of heat pump units and improving the
total energy efficiency of HTTS systems.
ACKNOWLEDGEMENTS
The authors are grateful for the supports provided by the
National Natural Science Foundation of China (No. 50609020)
and the National Natural Science Foundation of Tianjin
192 International Journal of Low-Carbon Technologies 2011, 6, 187– 192
(No. 10JCYBJC08500). Especially, thanks the anonymous
reviewers for their valuable suggestions to improve the quality
of our final paper.
REFERENCES
[1] Sanner B, Karytsas C, Mendrinos D. Current status of ground source heat
pumps and underground thermal energy storage in Europe. Geothermics
2003;32:579– 88.
[2] Reuss M, Beck M, Muller J. Design of seasonal thermal energy storage in
the ground. Sol Energy 1997;59:247– 57.
[3] Oliveti G, Arcuri N. Prototype experimental plan for the inter-seasonal
storage of solar energy for the winter heating of buildings: description of
plant and its function. Sol Energy 1995;54:85 – 97.
[4] Yumrutas R, Unsal M. Analysis of solar aided heat pump systems with
seasonal thermal energy storage in surface tanks. Energy 2000;25:1231 – 43.
[5] Ozgener O, Hepbasli A. Experimental performance analysis of a solar
assisted ground-source heat pump greenhouse heating system. Energ
Buildings 2005;37:101– 10.
[6] Gabrielsson A, Bergdahl U. Thermal energy storage in soils at temperature
reaching 908C. J Sol Energ T ASME 2000;122:3 – 8.
[7] Wang H, Qi C. Performance study of underground thermal storage in a
solar-ground coupled heat pump system for residential buildings. Energ
Buildings 2008;40:1278– 86.
[8] Wang H, Qi C, Wang E, et al. A case study of underground thermal
storage in a solar-ground coupled heat pump system for residential buildings. Renew Energ 2009;34:307– 14.
[9] Zeng H, Diao N, Fang Z. Heat transfer analysis of boreholes in vertical
ground heat exchangers. Int J Heat Mass Transfer 2003;46:4467 – 81.
[10] Piechowski M. Heat and mass transfer model of a ground heat exchanger:
theoretical development. Int J Energy Res 1999;23:571 – 88.
[11] Thomas H, King S. Coupled heat and mass transfer in unsaturated soil-a
potential-based solution. Int J Numer Anal Met 1992;16:757 –73.
[12] Basha H, Selvadurai A. Heat-induced moisture transport in the vicinity of
a spherical heat source. Int J Energy Res 1998;22:969– 81.
[13] Veldkampa E, Obrienb J. Calibration of a frequency domain reflectometry
sensor for humid tropical soils of volcanic origin. Soil Sci Soc Am J
2000;64:1549– 53.
[14] Brooks R, Corey A. Properties of porous media affecting fluid flow.
J Irrigat Drain Div 1966;72:61– 88.
[15] Philip J, De Vries D. Moisture movement in porous materials under temperature gradients. Trans Am Geophys Union 1957;38:222– 32.
[16] Cass A, Campbell G, Jones T. Enhancement of thermal water vapor diffusion in soil. Soil Sci Soc Am J 1984;48:25– 32.
[17] Leong W, Tarnawski V, Aittomaki A. Effect of soil type and moisture
content on ground heat pump performance. Int J Refrig 1998;21:595 –606.