In response to rising energy demand and urban heat island effects in Taipei, an energy raft foundation was constructed beneath a 13-story residential building to provide energy-efficient indoor heating and cooling for the residents while acting as a structural support for the superstructure.
The study investigated the thermal response of the energy raft foundation using three-dimensional coupled thermo-hydraulic finite element analysis. The numerical model was validated against field measurements. Parametric studies were conducted to investigate the influences of ground heat exchanger (GHE) spacing and pattern on heat exchange efficiency.
The study found that GHE pipe spacing and length were crucial in maximizing heat exchange efficiency while minimizing the thermal impact on nearby structures. This research enhances the understanding of the thermal responses of energy rafts and supports the development of sustainable building solutions in dense urban environments.
The paper is published in the journal Tunnelling and Underground Space Technology.
Summers in Taiwan result in high electricity demand due to the heavy use of air conditioning for indoor cooling, which puts pressure on the power grid. The waste heat from these systems, especially the air-source heat pump systems, also raises surrounding temperatures, worsening the urban heat island effect, especially in Taipei.
To address these issues, energy foundations have gained attention as a sustainable alternative. Energy foundations not only support buildings, transferring load from the superstructure to the ground, but also serve as a ground heat exchanger (GHE), exchanging heat with the ground.
Among these, energy raft foundations are a relatively new approach, especially in high-density urban environments like Taipei, and few have studied this type of energy foundation.
This research investigated the thermal responses of an energy raft foundation for a 13-story residential building with a 3-level basement during summer in Taipei. The construction of the energy raft foundation was intended to lower the building's reliance on conventional air conditioning systems.
Ground-source heat pumps (GSHPs) were used to exchange heat between the indoor building and the ground. The system consisted of a primary circuit (i.e., GHE) and a secondary circuit, which served as the heat distribution system within the building.
The GHE comprised 40 high-density polyethylene (HDPE) pipe loops connected in series and laid under the foundation with a spacing of approximately 0.1 to 0.2 m.
A three-dimensional coupled thermo-hydraulic finite element was developed to simulate the thermal responses of the energy raft foundation and the surrounding soil. The model was validated against the measurement data collected from the building site.
The research found that the spacing and layout of the GHE significantly influenced the heat exchange efficiency of the GSHP system. Closely spaced pipes led to thermal interference between loops, reducing efficiency, while broader spacing improved the thermal performance of the energy raft foundation.
However, to maintain the same total pipe length within a fixed area, wider spacing would require a more complex layout or additional space, which may not always be feasible.
The research identified a pipe spacing of 0.5 to 1 m as the most efficient balance between heat exchange efficiency and GHE length. Note that the area covered by the GHE was maintained to be the same.
A smaller spacing (e.g., 0.1 m) allowed for a longer total pipe length and greater surface area for heat exchange, but it also caused heat buildup due to interactions between adjacent pipes, lowering the temperature gradient and overall performance.
Conversely, wider spacing reduced thermal interference but shortened the total pipe length available within the foundation area, limiting the amount of heat that could be transferred.
Therefore, optimal spacing ensures efficient heat transfer without excessive material use or performance loss. The simulations also showed that the thermal influence of the pipes was limited to a small area under no groundwater flow conditions, meaning the system would not affect nearby structures.
This research has been published as part of the Special Issue: Emerging Technologies for a Sustainable Underground Space: Accelerating the Energy Transition and Adaptation to Climate Change.
"This work shows how energy raft foundations can be tailored for effective and energy-efficient indoor heating and cooling in dense cities like Taipei," says Prof. Kuo-Hsin Yang.
More information: Ignatius Tommy Pratama et al, Investigation of the thermal response of an energy raft foundation in Taipei, Tunnelling and Underground Space Technology (2025). DOI: 10.1016/j.tust.2025.106538
Provided by National Taiwan University