[Article] Hygrothermal Behavior of Energy Diaphragm Walls and Their Interaction with Adjacent Underground Space

Designing underground structures with ground-source heat pumps requires more than thermal modeling; moisture matters too. In our paper published in Underground Space (Elsevier/KeAi), we investigate the coupled heat and moisture transfer in energy diaphragm walls (EDW) and their impact on the hygrothermal conditions of adjacent underground spaces.

📌 Citation: Zhou, X., Cao, X., Leng, Z., Zeng, C., Yuan, Y., & Attia, S. (2025). Hygrothermal behavior of energy diaphragm walls and the induced heat and moisture interaction with adjacent underground space. Underground Space. Volume 23, August 2025, Pages 193–219. https://doi.org/10.1016/j.undsp.2025.01.006

🌍 Why this matters

Energy diaphragm walls are increasingly used in basements, metro stations, underground car parks, and shallow tunnels as part of ground-source heat pump systems. Yet most existing research focuses only on thermal and mechanical performance; largely ignoring the coupled moisture transfer process that is inevitable in high-humidity underground environments.

Ignoring moisture transfer leads to systematic underestimation of heat flux, mischaracterization of hygrothermal loads, and potential risks to occupant comfort and structural performance.

📊 What this study does differently

✔ Develops a 1D coupled heat and moisture transfer numerical model using the Finite Difference Method (FDM) in MATLAB

✔ Uses temperature and relative humidity gradients as driving potentials

✔ Investigates both heat injection (summer) and heat extraction (winter) scenarios

✔ Simulates long-term seasonal heating–cooling cycles over multiple years

✔ Conducts an orthogonal sensitivity analysis across key parameters: EDW operating temperature, indoor air temperature, and indoor relative humidity

✔ Validates the model against laboratory experiments, published literature, and the EN 15026 benchmark standard

🔍 Key findings

• The colder the wall surface, the more humid it is — heat extraction increases relative humidity at the inner surface by up to 21%.

• Neglecting moisture transfer underestimates heat flux by more than 3.43% in heat extraction and more than 3.90% in heat injection seasons.

• Sensible heat flux dominates, reaching 18.7 W/m² in summer and −27.4 W/m² in winter — over ten times greater than latent heat flux.

• Moisture flux peaks in transition seasons, driven by complex interactions between saturated vapor pressure and relative humidity at the wall surface.

• The peak latent heat flux in conventional walls is reduced by 14.7% due to EDW operation.

• Indoor relative humidity is the most influential parameter on water vapor flux across all seasons.

• Moisture flux can be considered fully compensated over seasonal cycles, while heat flux is only partially compensated.

📌 What this means

When applying EDW systems in underground engineering, the hygrothermal load induced by combined heat and moisture flux through the wall surface must not be overlooked. Building energy models, HVAC design strategies, and underground space conditioning standards should incorporate coupled hygrothermal analysis, particularly in high-humidity environments. This work provides a methodological foundation for more accurate energy and comfort assessments of underground spaces equipped with geothermal wall systems.

👏 Author team: Xu Zhou, Xiaoling Cao, Ziyu Leng, Chao Zeng, Yanping Yuan, Shady Attia

🏛 Institutions: Southwest Jiaotong University (Chengdu, China) and the Laboratory of Building Materials, University of Liège (Belgium)

📘 Full article https://doi.org/10.1016/j.undsp.2025.01.006

📚 Learn more about our research https://www.sbd.uliege.be/

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