Thermal Properties of Backfill Material for Underground Heat Exchange Applications

We are investigating the effect of the thermal properties of backfill material on the efficiency of underground thermal energy systems. We have used aluminum to increase the thermal properties of clay-bentonite and to investigate its potential as a backfill material in underground heat exchange applications. The measurements of thermal properties were made at room temperature using the Transient Plane Source (TPS) technique. The measurements were based on an infinite homogeneous medium using different composites of aluminum and clays in amounts of aluminum concentration ranging from 6 to 25 percent of the sample’s weight. The results confirmed an overall relative increase of 170% and 135% in the thermal conductivity and diffusivity, respectively as the Al content increased by 25%. The estimated average of the thermal transmittance (U-value ) of these samples was in the range from 0.40 to 1.1 W/m2K, which was directly proportional to the relative increase in the measured properties. Our findings can be applied to underground thermal energy systems, ground source heat pumps, and solid dielectric underground transmission lines.


Introduction
Backfilling is vital in the building process, following the ground pouring and fitting utility lines.After each layer is added, the backfill material must be placed in the excavation layers and then compacted [1].The use of clay-bentonite as backfill material in building construction has been widely explored because of its outstanding characteristics, including low hydraulic conductivity, high swelling capacity, and high compressibility [2].However, its low thermal conductivity is a major drawback that restricts its use in other applications.In this paper, we have examined the effect of aluminum on enhancing the thermal properties of clay-bentonite and its potential as a backfill material for underground thermal energy systems.We have used the Transient Plane Source (TPS) method [3] for measuring the thermal properties of the clay-bentonite samples with varying aluminum concentrations [4].Our results show a substantial improvement in the heat conductivity and diffusivity values of the samples as the aluminum content increased.These findings suggest that the addition of aluminum can effectively improve the heat conductivity of the samples and make it a viable option for use in various applications including underground energy-storing structures, geothermal pumps, and horizontal ground heat transfer devices [5].The emphasis of this paper is dedicated to the effect of Al on the heat properties (both conductivity and diffusivity) to improve the heat conduction performance in the soils, particularly, of bentonite clay composites that could be used as backfill materials.Mainly, for underground horizontal heat-exchanging applications.Our selection of aluminum is in line with the typical application of modified aluminum clays as a stabilizer in environmental contexts to improve the sedimentation of particles holding phosphorus [6].
It's worth mentioning that it has been reported recently on improving the thermal properties of nanocomposite by a solution intercalation method [7][8] the thermal conductivity of the nanocomposite increased with increasing aluminum content and reached a maximum value of 0.83 W/mK at 20 wt% aluminum loading, which was 3.3 times higher than that of pure clay-bentonite.The thermal stability of the nanocomposite also improved with increasing aluminum content, as indicated by higher decomposition temperature and lower weight loss.However, one of the principal concerns when incorporating nano clays (e.g., bentonite) into a polymer matrix is the difficulty in uniformly dispersing them in the matrix.These studies suggest that adding aluminum nanoparticles to clay-bentonite can effectively improve its thermal properties, as well as its mechanical and flame-retardant properties.However, some challenges remain, such as achieving uniform dispersion of aluminum nanoparticles in the clay matrix, optimizing the processing parameters, and evaluating the environmental impact of the nanocomposites.

Experimental 2.1 TPS technique
The TPS technique utilizes a resistive heating element.It is a device that converts electrical energy into heat through the process of Joule heating.It is a sensor made of a thin sheet of metal covered on both sides with thin layers of insulating material.The experiment is executed by simultaneously recording the voltage/resistance variations over the TPS element, as its temperature is slightly raised by a constant electrical current pulse.The resistance as a function of time over the element is expressed as where Rr (| 4: at ambient temperature) of the element prior to the start of the transient recording, D is the temperature coefficient of resistance (| 4.0x10 -3 K -1 at ambient temperature), and 'T(t) is the temperature rise as a function of time of the element.The method is based on the analysis of the three dimensions heat flux within the sample.The sample can be considered as an infinite homogeneous medium if the transient recording time is stopped before the thermal wave reaches the sample boundaries and causes edge effects.The measurement of temperature YDULDWLRQ ¨7W ZLWKLQ WKH KHDWHU LV LQIOXHQFHG E\ YDULRXV DVSHFWV VXFK DV WKH WRWDO SRZHU RXWSXW LQ WKH 736 element, the design constraints of the sensor, the thermal conductivity properties of the sample, The thermal conductivity, and diffusivity of a disk-VKDSHG VDPSOHV FDQ EH FDOFXODWHG IURP ¨7W ZKLFK LV GHILQHG E\ the following equation.
Po is the output power, N is the thermal conductivity coefficient of the sample, and a is the sensor's radius.

D(W)
is the mathematical formula that represents the time-varying temperature rise and describes the heat transfer pattern of a disk-shaped sensor, assuming that the disk is composed of multiple concentric ring sources [3].To simplify calculations, the average temperature change of the sensor is expressed using a GLPHQVLRQOHVV YDULDEOH Ĳ ZKHUH W =(Kt/a 2 ) or W = (t /T) 1/2 .Here, t represents the time elapsed since the beginning of transient heating, T = a 2 /K is the characteristic time, and K is the diffusivity of the sample.

2.2
Samples preparation Five clay samples were tested for thermal conductivity with different amounts of aluminum mixing concentrations (0%, 6%, 8%, 13%, and 20%) of the sample weight.The sample powder was prepared by mixing a ratio of 10 g of Aluminum ) with 20 g of clay in 1 L of water then settling for 24 then decanting the free water and oven-drying the rest.The size of each disk-shaped sample (pellets) is 30 mm in diameter and 10 mm in thickness.The detailed mixing process is given elsewhere [6] To verify the chemical makeup and mineral composition of these samples XR analysis was performed using an acquisition time of 150 s, with 1.2 mm, an X-ray tube voltage of 50 kV, and a current of 0.641 mA.Then the mineralogical and chemical compositions of these samples are calculated from the XRF analysis.The results for a typical sample are depicted in Figure 1 as shown below.For thermal properties measurements, each sample consisted of two polished circular plates to assure thermal contact among the samples and the sensor.

Results and Discussion
Table 1 shows the obtained measurements as the Al concentration increased in clay-bentonite samples.The results indicated a significant increase of 170% and 135% in the heat conductivity and heat diffusivity values, respectively as the aluminum content increased by 25% in the sample weight.Thermal conductivity is a measure of how easily heat flows through a material, and it has a significant impact on the thermal response of the backfill material on the efficiency of underground heat exchange applications such as underground heat pumps and underground electrical transmission lines(cables).Higher thermal conductivity in the soil allows for greater heat transfer between the ground and the cables or horizontal pipes, which can result in more efficient operation and lower energy costs.Conversely, lower thermal conductivity in the soil can reduce the efficiency of the system, leading to excessive heating of the cables and to higher energy consumption of the pipes.Our measurements were performed using different amounts of aluminum concentration ranging from 6 to 25 percent of the sample weight on a uniform basis.
The results indicated a significant increase in heat conductivity.Thermal diffusivity is an essential physical parameter that indicates how a medium responds to a thermal disturbance.It measures how fast heat spreads through a material and can also affect the thermal reaction of backfill materials on either the performance of transmission cables or horizontal underground loop heat exchanging pipes.Thus, measurements of this parameter can uncover the dynamics behind different heat transfer mechanisms and the impact of microstructure on the thermal reaction of backfill materials.In other words, soils with higher thermal diffusivity allow heat to spread faster, which can help to maintain a more consistent temperature profile in the ground and improve the overall efficiency of the system.Our measurements of this parameter show an overall increase in thermal diffusion.This is consistent with desired properties that will reduce the rate of excessive heating of the cables and/or maintain a desired stable temperature profile, more details about the U-values for cables are given in Ref. [9].To sum up, the heat properties of the soil, such as its heat conductivity and heat diffusivity, which is an important parameter for thermal dissipation dynamics that has a significant impact on the thermal response of underground heat exchange applications [10 & 11].This will lead to a more consistent temperature profile and better functioning of the geophysical ecosystem.In addition to its use for underground power transmission cables.In the case of nanocomposites [13] the stability and reusability of the composite due to the corrosion and aggregation of aluminum nanoparticles [14 ]

Conclusions
We have used the TPS technique for instantaneous measurements of thermal conductivity, and thermal diffusivity of Al. added clay-bentonite backfill materials.The addition of aluminum to claybentonite backfill materials can significantly increase both thermal conductivity and thermal diffusivity.The study found that adding aluminum to clay-bentonite increased its thermal conductivity by 170% and its thermal diffusivity by 135%.This can lead to more efficient operation and lower energy costs in underground heat exchange applications such as underground heat pumps and underground electrical transmission lines.Besides, it will maintain a more consistent temperature profile on the ground and improve the overall efficiency of the geophysical ecosystem.

Fig. 1 .
Fig. 1.The compositions of mass percent of a typical sample of modified clay.

Table 1 .
The average value of five trials of the heat properties for each clay sample modified with aluminum.