The importance of boundary conditions on the modelling of energy retaining walls

Shallow geothermal technologies have proven to efficiently provide renewable energy for space heating and cooling. Recently, significant attention has been given to utilising sub-surface structures, primarily designed for stability, to also exchange heat with the ground, converting them into energy geo-structures. This research includes investigations into the feasibility of applying this technology to retaining walls, focusing on the usually neglected interaction between the energy retaining wall and the air inside the underground space it contains (e.g., a building basement, a metro station). Even though soldier pile walls are adopted for the study, the results are applicable for any retaining wall type. Two commonly adopted boundary conditions on the surfaces of the underground structure (thermal insulation and a defined temperature) are used as well as the computationally expensive approach of fully modelling the air inside the underground space. The results show that if these boundaries are not carefully considered, a significant amount of heat can flow into/out of the underground space (up to about 75% in this study). Importantly, adopting inappropriate boundary conditions for these surfaces can result in erroneous and misleading results, a potentially under-designed heating, ventilation and air-conditioning (HVAC) system and subsequently thermal discomfort within these spaces.