Thermal energy storage in compacted soils
In this section rst the thermal energy storage in compacted back ll soil is introduced and then a new method to store thermal energy in embankments is also presented to point out the necessary research investigations have been carried out on it to the date.
Thermal energy storage in compacted back ll soil
In recent years, thermal energy storage in compacted soils in shallow depth of ground has been considered as a new and e cient method due to the ease of access, low cost of construction, less environmental impact and thermal energy e ciency (Gan 2013; Jradi et al. 2017). In this method, the thermal energy can be stored in compacted soil using horizontal heat exchanger loops, and then is released to heat the buildings in winter via heat pumps linked to under oor heating (Figure 1.3).
Elminshawy et al. (2017) investigated experimentally the thermal performance of the earth-air pipe heat exchanger that was inserted in a compacted soil of the Madinah region (Saudi Arabia). The experimental setup consisted of inserting an exchanger tube in a cylindrical sample (Figure 1.4). The soil sample was compacted in the mold with target relative densities of 26, 70 and 91% (T1, T2 and T3 respectively) at an initial temperature of 20 to 25 oC. Then, the air was blown in the heat exchanger tube using a temperature-controlled air blow system. The imposed ow rate was in the range of 0.006 to 0.015.
The results compared the di erence between inlet and outlet air temperature ( T ). Figure 1.5 shows the results performed on soils at di erent densities (T1, T2 and T3) and di erent air ow rates but at a constant inlet air temperature of 50 oC. At air ow rate of 0.006 m3:s 1, the di erence between inlet and outlet air temperature is higher than the other air ow rates and this di erence increased by soil compaction level from T1 to T3. These results show that by increasing density the soil particle contact increases and consequently heat transfer by conduction increases which causes the temperature loss between the inlet and outlet air ow. This study shows that the thermal performance of this system highly depends on the soil density and air ow rate. These observations are in agreement with the study of Hurtado et al. (2012) who investigated the capacity of a compacted soil to store thermal energy from the chimney power plant using an analytical model based on a nite volume procedure. They mentioned that the output power energy was increased by 10% when the soil compaction increased from loose to dense level.
E ciency of thermal energy storage in compacted soil
By storing thermal energy in embankments using horizontal heat exchangers, heat may pass through the compacted soil due to the temperature gradient between the loops and the compacted soil. To increase the long-term e ciency of these systems, a good knowledge of coupled heat and mass transfer in soil, and soil thermal properties are required. In the following, these aspects are investigated in detail.
Coupled heat and mass transfer mechanism in soil
The heat transfer through soil provides the mass transfer. Therefore, in the beginning, the heat transfer mechanism and then the mass transfer in the porous media due to the moisture and temperature gradient are discussed in detail.
Heat transfer mechanism in soil
The main mechanisms which contribute to heat transfer within soils are (Figure 1.10): heat transfer by conduction; heat transfer by convection; heat transfer by radiation.
Heat transfer by conduction is a heat ow passed through the solids and liquids or vapor phases by vibration, the collision of molecules, and free electrons due to the temperature gradient between two di erent points. In compacted soils, the heat ow passes from particle to particle through their contacts. The rate of heat ow will be calculated by Fourier’s Law: Q qcond = A.
where qcond is the heat transfer by conduction (W:m the section of the material (m2). (1.2) 2), Q is the heat ow (J:s 1), A is Heat transfer by convection occurs when a gas or liquid moves from one place to another place following a temperature gradient. The heat convection occurs inside the porosity of the soil, as the solid particles are static, the heat convection has to be distinguished from water ow or air ow inside the soil porosity. Heat transfer by water convection can be described by: qw;conv = cw w w(T T 0) (1.3).
E ect of temperature on soil thermal properties
Some researchers observed that temperature variations could change the soil’s thermal properties. This evaluation may a ect the capacity of heat storage in compacted soils.
Eslami (2014) investigated the evolution of thermal properties of a compacted illitic clay with the variation of density, water content and temperature in the range of 1 to 70 C along the compaction curve (Figure 1.14). This author showed that, increasing the temperature of soil samples from 1 to 70 C causes an increase in the thermal conductivity which is more signi cant in the dry side of the compaction curve. On the contrary, the temperature increase has a negligible e ect on thermal conductivity in the wet side of compaction curve (Figure 1.14a). Thermal volumetric capacity increases with increasing the water content and density, but temperature variation has no remarkable e ect on this parameter. Thermal di usivity is under the role of thermal volumetric capacity and conductivity that showed an increasing trend with temperature increase in the dry side of the compaction curve but after the optimum value of water content, it tends to decrease slightly (Figure 1.14b).
1.3 E ect of temperature on the hydro-mechanical behavior of soil
When the serviceability of the embankments as a thermal energy storage medium starts, the compacted soil will be subjected to seasonal temperature variations. These tempera-ture variations modify the thermo-hydro-mechanical behavior of the compacted soil. To prevent damage and ensure the long-term structure stability, the e ect of the temperature variation on the hydro-mechanical behavior of the compacted soil should be investigated. In this section, rst, the e ect of temperature on hydraulic properties is described. Then consolidation behavior, such as soil thermal volumetric response, preconsolidation pres-sure, and the compressibility parameters, is discussed. Finally, the impact of thermal solicitations on the shear parameters such as cohesion and friction angle, is presented.
Effect of temperature on the hydro-mechanical behavior of soil
The evolution of hydraulic properties, such as hydraulic conductivity and water retention curve under non-isothermal conditions is investigated by several researchers (Cho et al. 1999; Bouazza et al. 2008; Ye et al. 2013). These authors said that the hydraulic con-ductivity increases with temperature increasing. Delage et al. (2011) studied the e ect of temperature in the range of 20 to 90 C on hydraulic conductivity of Boom clay. Ac-cording to the results of triaxial tests, by heating, the hydraulic conductivity increased from 2.5 10 12 to 6.2 10 12 m:s 1 whereas the porosity decreased from 39% to 37.2%. By cooling, the hydraulic conductivity decreased whereas porosity remained unchanged (Figure 1.16). They pointed out that these results may be due to the coupled e ect of changes in water properties and porosity due to the temperature variation. However, Morin and Silva (1984) and Abuel-Naga et al. (2005) reported that the impact of the soil volume change due to the temperature variation is too small to be considered as a reason for the change of hydraulic conductivity at elevated temperature (90 C). With temperature increase, the viscosity of water decreases and leads to an increase in the soil hydraulic conductivity (Hillel 1980).
Table of contents :
1 Literature review on thermal energy storage in soils
1.1 Dierent methods of thermal energy storage in soils
1.1.1 Aquifer thermal energy storage method
1.1.2 Borehole thermal energy storage method
1.1.3 Thermal energy storage in compacted soils
18.104.22.168 Thermal energy storage in compacted backll soil
22.214.171.124 Thermal energy storage in embankments
1.1.4 Conclusion of thermal energy storage methods in geological mediums
1.2 Eciency of thermal energy storage in compacted soil
1.2.1 Coupled heat and mass transfer mechanism in soil
126.96.36.199 Heat transfer mechanism in soil
188.8.131.52 Mass transfer mechanism in soil
1.2.2 Soil thermal properties
1.2.3 Eect of temperature on soil thermal properties
1.2.4 Summary of expected thermal properties to store thermal energy in com- pacted soil .
1.3 Eect of temperature on the hydro-mechanical behavior of soil
1.3.1 Eect of temperature on hydraulic properties
1.3.2 Eect of temperature on consolidation behavior
184.108.40.206 Thermal volumetric response
220.127.116.11 Eect of temperature on preconsolidation pressure
18.104.22.168 Eect of temperature on the compression and swelling indices .
1.3.3 Eect of temperature on shear parameters
1.3.4 Conclusion of temperature eect on hydro-mechanical soil behavior
1.4 Numerical thermo-hydro-mechanical investigation
1.4.1 Thermo-hydraulic theoretical equations
22.214.171.124 Soil surface energy balance
126.96.36.199 Soil surface water balance
188.8.131.52 Hydrothermal transfer in subsurface soil
1.4.2 Thermo-mechanical constitutive models in saturated state
1.4.3 Thermo-mechanical constitutive models in unsaturated state
1.4.4 Application of numerical models
2 Measurement of the thermal properties of unsaturated compacted soil by the transfer function estimation method
2.2 Materials and methods
2.2.1 Material properties
2.2.2 Transfer function estimation method (TFEM)
2.2.3 Water content and density prole measurements
2.2.4 Other methods for measuring the thermal properties
2.3 Modelling .
2.3.1 The TFEM method
2.3.2 Single-needle probe method
2.3.3 The centred hot plate method
2.4 Results and discussion
2.4.1 Water content and density proles
2.4.2 Sensitivity analysis of the TFEM method
184.108.40.206 Inuence of the initial value of the thermal diusivity
220.127.116.11 Inuence of the uncertainty of the temperature variations
18.104.22.168 Inuence of the distance variations on thermal diusivity estimation .
2.4.3 Thermal properties estimated with the TFEM
2.4.4 Comparison with the other measurement methods
3 Eect of monotonic and cyclic temperature variations on the mechanical be- havior of a compacted soil
3.2 Soil properties, devices, and specimen preparation
3.2.1 Soil properties
3.2.2 Device and specimen preparation: oedometric tests
3.2.3 Device and specimen preparation: direct shear tests
3.3 Experimental programs
3.3.1 Consolidation program
22.214.171.124 Monotonic thermo-mechanical paths
126.96.36.199 Cyclic thermo-mechanical paths
3.3.2 Direct shear program
188.8.131.52 Monotonic thermo-mechanical paths
184.108.40.206 Cyclic thermo-mechanical paths
3.4 Experimental results
3.4.1 Thermo-mechanical results for oedometric tests
220.127.116.11 Monotonic thermo-mechanical oedometric results
18.104.22.168 Thermal cycles eect on the volumetric variation of studied com- pacted soil .
3.4.2 Thermo-mechanical results for direct shear test
22.214.171.124 Monotonic thermo-mechanical direct shear results
126.96.36.199 Cyclic thermo-mechanical direct shear results
3.5 Discussion .
3.5.1 Temperature eect on consolidation parameters
3.5.2 Volumetric response due to the temperature cycles
3.5.3 Temperature cycles eect on consolidation parameters
3.5.4 Heating or cooling and temperature cycles eect on shear characteristics .
3.5.5 Engineering implications of results
4 A numerical study into eects of soil compaction and heat storage on thermal performance of a Horizontal Ground Heat Exchanger
4.2 Hydrothermal behavior of the studied soil
4.3 General conditions of the numerical simulations
4.3.1 Geotechnical conditions
4.3.2 Boundary and meteorological conditions
4.3.3 Initial hydrothermal conditions
4.3.4 Pipe and its carryinguid .
4.4 Comparison of performances of HGHE installed in the local and compacted back- ll soils .
4.5 Heat storage eect on the performance of HGHE installed in the compacted back- ll soil .
4.5.1 Studied scenarios and installation depths
4.5.2 Simulation results
4.6 Comparison of dierent studied scenarios
4.8 Appendix .
General conclusion and perspectives
A Thermal conductivity of nonwoven needle-punched geotextiles: eect of stress and moisture
A.2 Materials and methods
A.2.1 Physical properties of geotextiles
A.2.2 Soil properties
A.2.3 Thermal parameters
A.2.3.1 Hot-plate device (steady-state method)
A.2.3.2 Thermal needle probe (transient method)
A.2.4 Compression test
A.3 Experimental results and discussion
A.3.1 Geotextile thermal conductivity vs. thickness
A.3.2 Thermal conductivity of geotextiles vs. vertical stress
A.3.3 Thermal conductivity of wet soil combined with geotextile
A.3.3.1 Theoretical estimation
A.3.3.2 Experimental measurement
B Resume etendu
B.2 Etat de l’art sur le stockage de l’energie thermique dans les sols
B.3 Mesure des proprietes thermiques d’un sol compacte non sature par la methode d’estimation de la fonction de transfert
B.4 Eet des variations de temperature monotones et cycliques sur le comportement mecanique d’un sol compacte
B.5 Calcul de performances d’un echangeur de chaleur horizontal en sol compacte
B.6 Conclusion .