Characterization and optimum performance of sand-bentonite hybrid soil in ground heat exchanger / Muhammad Aizzuddin Bin Abdullah

Faculty of Mechanical and Automotive Engineering Technology

Thesis (Doctor of Philosophy) -- Universiti Malaysia Pahang – 2025

Includes bibliographical references

Ground Heat Exchanger (GHE) is an essential system in utilizing geothermal energy because of the system is able to provide environmentally friendly and sustainable heating or cooling options. Improving the thermal performance of the ground around the buried pipes is one of the main obstacles to optimizing the efficiency of GHE systems. The efficiency of this system is primarily determined by the thermal characteristics of the soil near the GHE infrastructure. Thus, the main objective of the study is to create and analyse sand-bentonite hybrid soil (SBHS) on the thermal performance of the GHE system with the optimum working conditions. The key to enhance soil thermal properties involves the strategic use of thermally enhanced backfilling materials (BFM) coupled with the introduction of moisture. In this context, the investigation of SBHS emerges as a promising avenue for the GHE’s performance. Bentonite, known for its thermal enhancement properties, is introduced into the soil to improve its thermal properties. Additionally, moisture content is manipulated to further enhance the overall thermal performance of the soil. This study started with an in-depth analysis of the ground thermal properties of SBHS. This initial phase is followed by evaluating the thermal performance of the GHE system employing the SBHS and ends with optimising operating parameters for enhancing efficiency. One Factor at a Time (OFAT) method is used to unravel the complexities of SBHS. This method examines the impact of changing three critical parameters: sand grain size, bentonite percentage, and moisture percentage. The grain sizes that are considered are 0.154-0.355 mm, 0.355-0.6 mm, 0.6-1.0 mm, 1.0-1.6 mm, and 1.6-2.0 mm. The bentonite percentage is investigated from 0 to 100%, while the moisture content is investigated from 0 to 25%. Experimental analysis involves the utilization of a GHE simulator to assess the system's performance under diverse conditions of ground temperature, input temperature, and flow rate. The subsequent optimization of operating parameters employs Response Surface Methodology (RSM), accompanied by the formulation of regression models. The research outcomes shed light on the consistent thermal properties across all grain sizes of sand, with the 1.6-2.0 mm range exhibiting the highest thermal performance when combined with 8% bentonite and 20% moisture. The SBHS configuration consistently outperforms native soil in terms of air temperature variation, heat transfer rate, and effectiveness across different conditions. As ground temperatures decrease from 27°C to 23°C and input temperatures drop from 35°C to 31°C, SBHS consistently achieves lower air temperatures, higher heat transfer rates, and greater effectiveness than native soil along the GHE pipe. Similarly, with flow rates increasing from 0.01 to 0.03 kg/s, SBHS maintains superior performance in all aspects compared to native soil. This indicates that SBHS has superior heat retention or heat dissipation capabilities compared to native soil, allowing it to maintain cooler air temperatures and potentially enhance the performance of ground heat exchange systems. The optimized parameters for SBHS demonstrate a refined balance, yielding an air temperature variation, heat transfer rate, and effectiveness of 26.49 °C, 79.65 W, and 0.58, respectively, with the highest desirability index of 0.531. The results highlight the significance of soil-thermal dynamics in GHE performance and have implications for the sustainable use of geothermal energy in heating and cooling applications.