A research team led by Professor Shinya Inazumi from the College of Engineering, Shibaura Institute of Technology, Japan, has developed an innovative framework to improve the design and performance of energy piles – concrete foundation systems that also serve as heat exchangers using geothermal energy – especially in soft clay soils.
As urbanisation increases and climate change accelerates, there’s an urgent need for sustainable and space-efficient solutions for heating and cooling in buildings. One promising solution is to use energy piles. However, in high-density cities such as Tokyo, Bangkok and Manila, where buildings are often constructed on soft clay foundations, engineers face unique challenges in designing these energy piles.
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Energy piles are concrete foundation elements with embedded U-shaped pipes that circulate heat-transfer fluids within them. These heat transfer fluids exchange thermal energy with the surrounding ground. When these elements are connected to ground source heat pumps (GSHPs), they can efficiently heat and cool buildings by using the stable underground temperatures. GSHPs are known to maintain high performance even in fluctuating surface temperatures, unlike conventional air-source heat pumps, which are less efficient in extreme weather – making GSHPs an ideal solution for extreme-temperature climates.
While GSHPs increase efficiency, energy pile systems encounter several challenges. In most cities, soft clay soils are used for construction; these soils are characterised by low permeability (resistance to water flow) and low thermal conductivity (difficulty in transferring heat). In such cases, the accumulation of heat over time can lead to a phenomenon, known as thermal interference, that reduces the efficiency of the entire system.
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To counter this, the researchers used a combined computational and experimental approach, and developed a three-dimensional heat-transfer model. Using finite element models (FEM) via physics-based simulation software COMSOL Multiphysics, the researchers modelled heat transfer around energy piles embedded in soft clay. These simulations were then calibrated using real-world data obtained from a test site in Bangkok. The model analysed several pile groupings ranging from one to nine piles, which operated under various daily time cycles (8–24 hours).
‘We developed a simplified prediction model to help engineers improve energy pile design without the need for expensive computational resources or specialised expertise,’ said Professor Inazumi.
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The results revealed several insights into the performances of the energy piles. First, the grouped configurations exhibited measurable thermal interference, with soil temperatures rising from 2.2 per cent to 15.4 per cent around the closely spaced piles. Estimating this interference at the design stage was considered critical as it can diminish the system’s efficiency.
‘To simplify the process, we introduced practical multiplier factors that allow engineers to predict thermal behaviour using single-pile simulations,’ explained Proessor Inazumi.
The multiplier factors range from 1.6 to 2.9 and can be applied to the results obtained from single-pile simulations, allowing engineers to predict the performance of larger pile groups without the need for complex three-dimensional models. This dramatically reduces the need for full-scale FEM runs, offering a quick, accessible method for thermal performance estimation.
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The study also noted that reducing operational hours could delay the temperature saturation (when the soil becomes too warm to absorb more heat) by 103 hours. Reducing the operational hours also decreased the peak soil temperatures by 29 per cent over five years. Another critical finding was that the piles at the centre get hotter in comparison to those at the edge, suggesting the effect of crowding. These insights suggest that the design of energy pile groups can be optimised by using the provided multipliers and temperature maps. This optimisation strategy can help maintain structural integrity and extend the system’s lifespan.
The model has significant potential for real-world applications. It’s particularly relevant for engineers working in rapidly urbanising cities built on soft soils, where traditional heating, ventilation and air conditioning systems are both energy-intensive and climate-vulnerable. By offering easy-to-use simulation shortcuts validated with real-world data, this research lowers the entry barrier for adopting geothermal systems in Southeast Asia and beyond, paving the way for a cleaner, more sustainable future.
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‘By demonstrating the viability and affordability of geothermal energy systems for dense urban environments, our study addresses the challenges in regional development, contributing to the global climate agenda,’ concluded Professor Inazumi.
The research has been published in Case Studies in Thermal Engineering.