Energy Piles and GSHP Systems Key to Sustainable Smart Cities

By Muhammad OsamaReviewed by Laura ThomsonJun 19 2024

In a recent article published in the journal Smart Cities, researchers explored the integration of energy piles and ground source heat pump (GSHP) systems in the development of smart cities. They emphasized understanding thermal movement within the soil, particularly in the soft marine clays common in Southeast Asia, for the optimal implementation of GSHP systems.

Background

GSHP systems, also known as shallow geothermal systems, utilize the constant geothermal energy below the earth's surface to provide heating and cooling for indoor environments. These systems meet the thermal requirements for heating, ventilation, and air-conditioning (HVAC) systems. They are particularly advantageous in urban settings due to their significant reductions in greenhouse gas emissions and electricity consumption compared to traditional air source heat pump systems.

About the Research

In this paper, the authors explored the integration of energy piles, which serve as structural supports and components of the GSHP system, enhancing overall infrastructure efficiency and sustainability.

The study employed a one-dimensional (1D) finite difference method (FDM) to model the temperature distribution around energy piles and assess the impact of soil thermal conductivity and density variations on system efficiency. This approach enabled a simplified yet effective analysis, facilitating the evaluation of soil properties on GSHP performance.

The researchers focused on two critical parameters: apparent thermal conductivity and soil density. The model assumed that thermal conductivity increases proportionally with the rise in temperature in the heat exchanger connected to the GSHP system. Variations in soil density reflected different geographical locations in Southeast Asia where GSHP and energy pile systems could be deployed.

The energy piles were modeled as cylindrical structures with a diameter of 0.3 m and a length of 12 m, embedded in a soft clay layer 10 to 15 m thick. Each energy pile contained a U-tube heat exchanger with a total length of 20 m, divided into inlet and outlet sections. Depending on the depth, the heat exchanger operated in cooling mode, with a constant temperature boundary ranging from 33 to 37 °C.

Based on previous measurements in Bangkok, the background soil temperature was assumed to be 29 °C. The authors utilized four different thermal conductivity functions derived from previous studies and laboratory tests to represent the temperature-dependent thermal conductivity of clay. Three different soil density values were used to represent density-dependent thermal conductivity based on typical soft marine clays in Southeast Asia.

The methodology involved a comprehensive temporal analysis covering short-term assessments within the first week and long-term perspectives over a year. The temperature dynamics around the soil adjacent to energy piles were meticulously examined. Input parameters, including the physical and thermal characteristics of materials and soil, were collected from an extensive literature review of GSHP systems in Southeast Asia, design manuals, and reputable textbooks. Moreover, the model formulation was further enhanced by an in-depth study conducted at a physical GSHP test site and an energy pile installation at Kasetsart University, Thailand.

Research Findings

The outcomes revealed a cyclical pattern in temperature variations around the energy pile, showing a gradual upward trend that indicated heat accumulation within the soil mass over time. This phenomenon was primarily influenced by heat conduction and convection mechanisms. Specifically, temperature variations decreased progressively at distances of 0.15 m, 0.30 m, 0.45 m, and 0.60 m from the pile center, reflecting a reduction in the thermal gradient due to the energy pile's operating cycles.

The authors observed remarkable stability in the temperature distribution around the energy piles over the studied periods despite variations in soil thermal conductivity. This suggested that the design of these systems could afford a degree of flexibility concerning the thermal properties of the soil. Moreover, the impact of soil density variations on temperature predictions was explored.

The results indicated that, as soil density increased, the effect on soil temperature showed a decreasing trajectory. Specifically, for every 10% increase in soil density, there was a corresponding average temperature decrease of 0.007°C at a 0.3 m radius from the energy pile. This underscored the critical role of soil density in determining the thermal efficiency of subsurface energy systems.

Applications

This study has significant implications for smart city construction. Integrating GSHP systems and energy piles can reduce electricity consumption, mitigate greenhouse gas emissions, and improve the efficiency of HVAC systems, demonstrating a promising path toward sustainable urban living environments.

This research addresses the challenges posed by tropical climates and soft marine clays, providing valuable insights for engineers and urban planners seeking to deploy these advanced systems in similar urban settings.

Conclusion

In summary, the researchers highlighted the importance of integrating GSHP systems into sustainable and resilient urban infrastructure, considering climate variability and environmental change. Future work should optimize GSHP system configurations and operating parameters to maximize geothermal energy use in urban environments. This included exploring advanced modeling methodologies, such as the finite element method (FDM), to improve ground temperature distribution prediction accuracy and system performance.

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