Thermal Conductivity Testing of Energy Piles: Field ... - ASCE Library

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3Via Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, ... calibrated the numerical model with a thermal conductivity field test.
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Thermal Conductivity Testing of Energy Piles : Field Testing and Numerical Modeling Tolga Ozudogru1, Tracy Brettmann2, C. Guney Olgun3, James R. Martin II3 and Aykut Senol1 1

Civil Engineering Department, Istanbul Technical University, Maslak, Istanbul, Turkey email : [email protected], [email protected] 2 Berkel & Company Contractors, 2330 Precinct Line Road, Richmond, TX 77406; email: [email protected] 3 Via Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA, USA email : [email protected], [email protected] ABSTRACT Heat exchange capacity of an Energy Pile is a key parameter in the design of these elements as ground sourced heat exchangers. In most cases, field thermal conductivity tests are necessary to verify design assumptions similar to running a conventional pile load test to prove pile capacity. Current standards for measuring the heat exchange performance of geothermal systems are limited and do not directly apply to Energy Piles. Most importantly, current guidelines limit the maximum diameter of a tested geothermal heat exchange element to 6 inches. We have developed a 3D numerical model to simulate thermal conductivity testing and calibrated the numerical model with a thermal conductivity field test. This model will allow us to perform a series of numerical analyses to test the validity of the assumptions underlying current thermal conductivity testing procedures. INTRODUCTION Ground source heat pump (GSHP) systems utilize the constant temperature and thermal storage capacity of the ground for heating and cooling of buildings. Added benefits of these systems include reduced energy use and thus significantly lowered heating and cooling costs compared to conventional air sourced systems. Furthermore, Energy Piles are finding increased use in the last decade as a hybrid system where they serve as foundation support elements and at the same time can be utilized as heat exchangers for heating and cooling of the structure. In this relatively new technology, GSHP systems are incorporated into the building’s deep foundations as the circulation tubes are placed inside the piles or drilled shafts. As a result, the additional cost of geothermal borehole drilling for loop placement is offset by this combined use and installation costs are significantly reduced.

 

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One of the key parameters in the design and dimensioning of ground sourced heating and cooling systems is the thermal characteristics and heat exchange capacity of the in-situ soil and rock formations. Experimental and analytical procedures for evaluating the thermal conductivity of ground sourced heat exchange systems were developed for geothermal borehole loops. In this traditional approach, a relatively deep small-diameter well is drilled, installed with a geothermal circulation loop system and the borehole is backfilled with a mixture of sand, bentonite and/or cement. Heat exchange capacity of these systems is evaluated with in-situ thermal conductivity testing. This test is performed by water circulation where a constant heat exchange rate is maintained while the temperature changes in the inlet and outlet fluid are monitored. The rate of temperature change is then used to estimate the thermal conductivity of the system using a variety of line source analogs and numerical methods. The line source method assumes an infinitely long linear heat source in modeling heat conduction. This theory starts becoming inapplicable as the diameter of the heat source gets larger as in the case of a typical Energy Pile. Another limitation of the line source method is that it ignores end effects, which may start becoming more pronounced as the heat source becomes relatively short. We performed analytical studies to investigate the applicability of conventional procedures to evaluate the thermal conductivity of Energy Piles. We used a well-documented thermal conductivity test (Brettmann et al., 2010) and developed a numerical model to simulate a thermal conductivity test. In these analyses, thermal conductivity testing was simulated with a 3-Dimensional finite element model that considered the components of the system as well as the heat exchange operations during the test. Results of the numerical model were compared with the performed thermal conductivity test and the model was calibrated. The analyses were able to capture the heat exchange behavior of the Energy Pile during thermal conductivity testing. This model will further enable us to perform parametric analyses to evaluate the limitations of current testing procedures and help us develop refined methods and guidelines. IN SITU THERMAL CONDUCTIVITY TEST Thermal conductivity test for geothermal applications were developed by the American Society of Heating, Refrigeration, and Air Conditioning (ASHRAE) (Kavanaugh et al., 2001). Thermal conductivity test is performed circulating fluid within the geothermal loop while heat is injected into the system at a constant rate. General recommendation is to apply 15W-25W of heating per foot length of the borehole. This is done by arranging a set of heaters for the desired heat injection rate and a flow rate is selected in conjunction with the length of the borehole loop. A picture of the thermal conductivity test equipment used for testing is shown in Figure 1. Ground temperature increases as heating energy is injected into the ground through the geothermal loop. Temperature of the circulating water is monitored over time along with the rate of energy injection. Progression of the inlet and outlet fluid temperatures is then used to evaluate thermal conductivity of the system. There are a variety of methods for the analysis thermal conductivity test data. These methods include the Line Source (LS) Method, Cylindrical Heat Source

 

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Method, ORNL/Shonder Method or the OSU Numerical Method (Kavanaugh, 2010). These analytical methods typically result in similar thermal conductivity values for a given set of test results. It must be noted that thermal conductivity test gives an average value for the thermal conductivity of the overall system as the thermal characteristics of the different soil/rock layers in the formation, the borehole backfill and the circulation loop material are collectively reflected in the test results and thus the evaluated thermal conductivity. Thermal conductivity test was originally developed to assess the heat exchange capacity of geothermal boreholes. As Energy Piles are installed more frequently, it is becoming necessary to develop testing procedures and analysis methods to evaluate the thermal conductivity of such systems which are geometrically different than boreholes. Inherent limitations prevent the direct application of existing testing and evaluation methods for Energy Piles. For example, ASHRAE limits the thermal conductivity test to a maximum borehole diameter of 6 inches, which makes it practically inapplicable for Energy Piles. Also it is recommended to disregard the first 5 hours of test data when analyzing the results to allow for temperatures to stabilize within the geothermal borehole. It is also conceivable that more time will be necessary for this initial temperature stabilization when thermal conductivity testing is performed on larger diameter geothermal elements such as Energy Piles.

Figure 1. Thermal Conductivity Test Equipment Thermal Conductivity Test Site The thermal conductivity test site is located at Berkel regional office in Richmond, Texas, within the Beaumont geologic formation along the Gulf Coast of Texas. The soils at the site consist of Pleistocene aged deposits of shallow coastal river channels and flood plains. There is a complex stratification sands, silts and clays at the site and vicinity as a result of the frequent changes in the courses of the river channels during this period. The clays at the site are overconsolidated due to  

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desiccation that resulted when the water table was lowered during the Second Wisconsin Ice Age. These clays are predominantly composed of montmorillonite, kaolinite, illite as well as fine ground quartz. The sands are composed of quartz, feldspar and occasional hornblende. The sands vary in density from medium dense to very dense, and are sometimes lightly cemented (Brettmann et al., 2010). Subsoil investigations performed at the test site reveal a stratified soil profile where a stiff to very stiff clay layer extends from the ground surface to a depth about 9.8 m (32 ft). A layer of very dense sand is located below extending to a depth of 17.4 m (52 ft). Beneath this, there is a 1.5 m (5 ft) thick layer of medium stiff clay which is underlain by a medium dense to very dense sand which extends down the depth of boring completion at 21.3 m (70 ft). The groundwater at the site was at a depth of 3.3 m (11 ft) below ground surface. A sketch of the soil profile at the test site is shown below in Figure 2. 0.0 m

3.3 m

GWT

CLAY

APGE Pile D=300 mm L=18.3 m

9.8 m

SAND

17.4 m 18.9 m

CLAY SAND

21.3 m

Figure 2. Soil profile at the test site Laboratory tests were performed on soil samples the grout used for the test pile. The results of the laboratory tests are summarized in Table 1. Laboratory thermal conductivity testing was performed using the thermal needle test following the guidelines in the ASTM5334 (2008). The tested soil sample in the upper clay layer was retrieved from levels below the groundwater table. Thermal conductivity at the upper levels has not been measured separately.

 

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Table 1. Summary of laboratory test results Dry Unit Moisture Depth Weight Sample Content (m) ρdry (pcf) / w (%) (kg/m3) Clay 6.1 21.1 107.7 / 1730 Sand 13.7 14.0 108.6 / 1740 Clay 18.3 28.0 96.3 / 1540 APGE Pile grout 0.0 – 18.3 7.3 118.9 / 1910

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Thermal Conductivity k (W/m*K) 2.22 4.05 2.09 1.35

Test Pile Setup The test pile was installed using Auger Pressure Grouted (APG) pile installation technique. APG piles are installed by rotating a hollow stem, continuous flight auger into the ground down to the desired tip elevation. When the required depth is reached, a high strength, fluid grout is pumped under pressure through the hollow stem auger. The auger is withdrawn slowly by rotating clockwise as the pumping continues to both maintain the head of grout and avoid any intrusion of water or soil into the grout column. APG piles installed with geothermal pipe loops are called Auger Pressure Grouted Energy (APGE) piles™. A total of three piles are installed within this testing program (Brettmann et al., 2010). The results reported here are for the 30 cm (12 inch) diameter AGPE test pile extending to a depth of 18.3 m (60 ft). This test pile is equipped along the full length with two pairs of 25 mm (1-inch) diameter pipe loops with U-bend couplers at the bottom. The high density polyethylene (HDPE) pipe loops were attached to a 25 mm (1-in) diameter full length center steel bar using a 12.7 cm (5-in) diameter plastic spacer. The centers of the pipes were roughly 7.6 cm (3 in) from the center of the pile, and were separated apart from each other approximately 7.6 cm (3 in). The two pipe loops were installed on diametrically opposite sides of the pile as shown in Figure 3. Three thermistors were placed within the test pile at depths of 6.1 m, 13.7 m and 18.3 m. The pile was installed using a standard grout mix design that would result in a compressive strength of 4,000 psi (27.5 MN/m2) in 28 days. A non-shrink additive was also included in the mix design and no grout shrinkage was observed in the pile. The initial water temperature from the local source was 24°C (76°F). The average initial soil temperature measured from the thermistors was 22°C (71°F). This ground temperature is consistent with reported mean ground temperatures and seasonal climate data from this area (Turner and Doty, 2007). The thermal conductivity tests were run for 3 to 4 days. Temperature of the circulated water and the temperature of the center of the pile were measured at 5 minute intervals during the testing period. The heat injection rate was also continuously recorded during testing. Details of the individual thermal conductivity tests on the three piles are reported in Brettmann et al. (2010). In addition, thermal conductivity tests were performed for the group of three test piles as reported in Brettmann and Amis (2011).

 

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Figure 3. Typical pipe loops with U-bend couplers and location of thermistors NUMERICAL SIMULATION OF THERMAL CONDUCTIVITY TESTING OF AN ENERGY PILE Numerical analyses were performed to investigate the heat transfer mechanisms involved during an in-situ thermal conductivity test. A 3-dimensional model was developed to represent the components of the Energy Pile and the thermal conductivity testing was simulated using COMSOL (2010). In this model, heat transfer mechanisms consist of heat conduction within, Energy Pile (30 cm diameter APGE), the HDPE pipe loops and the soil around the test pile, as well as the convective heat transfer within the circulation fluid in the loops. A detailed description of the general model and the analysis methodology are summarized in Abdelaziz et al. (2010). The model consists of six domains in a three dimensional geometry. The soil layers and the elevation of the ground water table are inferred from the borehole information. The Energy Pile has two pairs of pipe loops with U-bend couplers. The pipes are modeled using thin conductive shell elements ignoring the wall thickness of the pipes due to meshing difficulties. However, the pipe thickness is considered in the numerical model by separately evaluating the thermal resistance of the HDPE pipe. The soil around the pile is modeled using cylinder elements. The diameter and the depth of the soil domains were selected to be 20.0 m and 25.0 m, respectively. These boundaries are adequate as it is known from the measurements that there is no change

 

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in the initial soil temperature 2.6 m apart from the energy pile during testing Brettmann and Amis (2011). Finite element model of the modeled energy pile and the surrounding soils are schematically shown in Figure 4.

Figure 4. 3D finite element model and the mesh used in the analyses Soil and energy pile properties are assigned with the help of laboratory thermal conductivity tests conducted on selected soil samples and the grout. Volumetric heat capacities (ρCp) were evaluated using the equation below (Kavanaugh, 2010). Thermal properties of the soils and the Energy Pile in the numerical model are summarized in Table 2.





where w is the moisture content of soil.

 



(1)

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Table 2. Summary of assigned material properties Thermal Depth Density Heat Capacity Conductivity (m) ρ (kg/m3) Cp (J/kg*K) k (W/m*K) 0.0-3.3 1750 1.70 950 3.3-9.8 2095 2.22 1019 9.8-17.4 1984 4.05 898 17.4-18.9 1971 2.09 1110 18.9-25.0 1984 4.05 898 0.0-18.3 2049 1.35 909

Domain 1-Clay 2-Clay 3-Sand 4-Clay 5-Sand Energy Pile

Definition of the Model: Initial and Boundary Conditions General transient heat transfer is governed by the equation: ∙



(2)

This equation has been solved in each domain with the initial condition: , , ,0

(3)

where Tinitial is the undisturbed soil temperature, which is 22°C in accordance with the measured ground temperature at the test site. The energy equation of the circulation fluid flow is shown as: ∙





(4)

The coupling between the tubes and the circulation fluid was achieved with the boundary condition: ∙

(5)

The interfaces between different segments of the model were assigned continuity boundary condition both for temperature and heat flux. Exterior boundaries of the model were (bottom and sides) thermally insulated. The measured inlet temperature during testing was input into the numerical model during the analysis. The numerical model solves the above equations using the assigned initial and boundary conditions. The calculated outlet temperatures are compared with the measured outlet temperatures are shown in Figure 5 and these are in good agreement. This indicates that the numerical model can capture the heat exchange process with reasonable accuracy. This model will further be used to run simulations for a variety of conditions that reflect typical Energy Pile geometries. Most importantly, we will be able to investigate the applicability of current procedures for larger diameter systems with different loop configurations.

 

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Figure 5. Comparison of 3D finite elements analysis with the test measurements CONCLUSIONS One of the key parameters in the design and dimensioning of ground sourced heating and cooling systems is the thermal characteristics and heat exchange capacity of the in-situ soil and rock formations. Traditionally thermal conductivity testing is used to evaluate the heat exchange capacity of such systems. Current guidelines limit the use of these tests to 6 inch diameter boreholes. For this purpose we developed a numerical modeling approach and calibrated it with the results of a thermal conductivity test. It appears that the developed numerical model is able to capture the heat exchange during testing and it gives good results. Additional analyses are being performed using this numerical model. Several other geometries are being considered and these will help in the evaluation of current thermal conductivity test procedures. Recommendations will be developed to account for these different conditions not originally considered in the conventional approach. ACKNOWLEDEMENTS This material is based upon work supported by the National Science Foundation under grants CMMI-0928807 and CMMI-1100752. This support is greatly appreciated. Any opinions, conclusions or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the National Science Foundation.

 

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REFERENCES Abdelaziz, S.L., Olgun, C.G. and Martin II, J.R. (2011) “Design and Operational Considerations of Geothermal Energy Piles.” Geo-Frontiers, ASCE Conference, March 13-16 2011, Dallas, Texas. ASTM (2008). “D5334 : Standard Test Method for Determination of Thermal Conductivity of Soil and Soft Rock by Thermal Needle Probe Procedure”, American Society for Testing and Materials. Brettmann, T., Amis, T., and Kapps, M., (2010). "Thermal Conductivity Analysis of Geothermal Energy Piles." Proceedings of the Geotechnical Challenges in Urban Regeneration Conference, Deep Foundations Institute, London, UK, 6p. Brettmann, T., Amis, T. (2011). "Thermal Conductivity Analysis of Geothermal Energy Piles." Proceedings of the Geotechnical Challenges in Urban Regeneration Conference, Deep Foundations Institute, London, UK, 6p. COMSOL (2010). “Introduction to COMSOL Multyphysics: Version 4.1” Reference Manual and Tutorial, COMSOL Inc, Burlington MA. Kavanaugh, S. (2010). “Determining Thermal Resistance”, ASHRAE Journal, American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Vol. 52, No. 8. Kavanaugh, S.P., Xie, L. and Martin, C. (2001). “Investigation of Methods for Determining Soil and Rock Formation from Short Term Field Tests”, ASHRAE1118TRP, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., 77 p. Turner, W.C., and Doty, S. (2007). “Energy management handbook.” 6th Edition, CRC Press, 924 p.

 

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