Heat Exchanger Anchors for Thermo-active Tunnels

Shallow geothermal power represents an important energy resource for the heating and cooling of the buildings. Due to relatively low temperature levels encountered at shallow depths in the soil, between 10°C and 20°C, heat pumps are required to process the extracted heat, forming the so called ground source heat pump system. Different types of heat exchangers with the ground were developed in order to optimize the heat exchanges, from simple geothermal loops grouted in boreholes reaching depths up to a couple of hundreds of meters to complex energy geostructures. Indeed, embedding geothermal loops within concrete foundation structures increases the heat exchange efficiency as well as it saves the cost of additional drillings. Recent developments suggested that applying the concept of energy geostructures to tunnel structures that are in contact with the ground should also be efficient. The present study investigates the potential of using tunnel anchors and nails as heat exchangers with the surrounding soil. Two main structures of urban tunnels were investigated. A cut and cover tunnel, whose diaphragm walls are maintained with long anchors, was modelled first. Thermal influence of the soil surface and unsaturated conditions were taken into account because of the shallow depth of the tunnel body. Nevertheless, mechanical implications of the heat extraction on the cut and cover tunnel were neglected because of the low mechanical confinement observed on the structure. Then, an urban bored tunnel was investigated. Soil conditions encountered at this depth were assumed always saturated and the thermal influence of the surface was neglected. Mechanical implications of the heat exploitation were assessed because of the high confinement of the bored tunnel body induced by the soil weight. Different types of heat exploitation cycles were tested for the different configurations. The heat extraction is based on the external air temperature in order to meet a simplified building heat demand. Cycles with and without heat injection were also investigated. All the exploitation cycles were optimized in order to reach a temperature threshold in the ground to prevent freezing it. Next, comparisons between extracted and injected heat of the different cycles allow drawing an optimum exploitation method. It is found that injecting heat during the hot period is necessary for the cut and cover tunnel as the natural heat reload isn’t high enough to ensure the sustainability of the heat storage. Conversely, the bored tunnel beneficiates from an increased natural heat reload, turning the heat injection into a more expensive solution. Furthermore, mechanical implications of the heat exploitation on the bored tunnel are found to be more significant when injecting heat. This shows the importance of a thermo-mechanical design of such a system. Finally, considering heat injection or not, it is estimated that heat extraction ranges from 0.6 to 1.2 MWh per year and per meter of cut and cover tunnel, and from 2.8 to 4.0 MWh per year and per meter of bored tunnel.

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