Energy performance of diaphragm walls used as heat exchangers

The possibility of equipping diaphragm walls as ground heat exchangers to meet the full or partial heating and cooling demands of overlying or adjacent buildings has been explored in recent years. In this paper, the factors affecting the energy performance of diaphragm walls equipped as heat exchangers are investigated through finite element modelling. The numerical approach employed is first validated using available experimental data and then applied to perform parametric analyses. Parameters considered in the analysis include panel width, the ratio between the wall and excavation depths, heat transfer pipe spacing, concrete cover, heat-carrier fluid velocity, concrete thermal properties and the temperature difference between the air within the excavation and the soil behind the wall. The results indicate that increasing the number of pipes by reducing their spacing is the primary route to increasing energy efficiency in the short term. However, the thermal properties of the wall concrete and the temperature excess within the excavation space are also important, with the latter becoming the most significant in the medium to long term. This confirms the benefits of exploiting the retaining walls installed for railway tunnels and metro stations where additional sources of heat are available

The role of ground conditions on the heat exchange potential of energy walls

Geotechnical structures are being increasingly employed, in Europe as all around the world, to exchange heat with the ground and supply thermal energy for heating and cooling of buildings and de-icing of infrastructure. Most current practical applications are related to energy piles, but embedded retaining walls are now also being adopted. However, analysis and design methods for these new dual use foundations and ground heat exchangers are currently lacking, making it hard to provide estimates of energy availability without recourse to full numerical simulation. This paper helps to fill this gap by using coupled thermo-hydro finite element analysis to develop charts of energy capacity that could be applied at the outline design stage for energy walls. In particular, the influence of ground properties (hydraulic and thermal conductivities), and ground conditions, (groundwater temperature and flow velocity) are investigated with the results showing that the hydrogeological conditions and the temperature difference between the ground source and application temperature are especially important in determining the performance of the energy wall