District scale influence of waste heat on the subsurface conditions: a numerical investigation for Chicago

Since the 20th century, the rate of urbanization in the world has been increasing exponentially and will reach 68% in 2050. In parallel with this expansion, the size and activities of cities are also greatly growing. As a result, cities are depleting available space and energy resources at an alarming rate. In order to be able to respond to these demands, the solution of building more underground structures is being put in place and is already being implemented in some megacities. These infrastructures have demonstrated that human activity is the factor that generates the main heat flow in the underground and that it has a significant impact on the temperature of urban aquifers. Through the use of geothermal energy, the increase in temperature of the subsoil can be ecologically and economically beneficial. However, for the evaluation of the geothermal potential, many factors have to be taken into consideration (geology, human activities, urban planning, etc), making its execution complex. As a result, most of the research already undertaken on this subject proves to be sketchy and incomplete. This work thus proposes a complete study of the influence of the different heat sources on the subsoil. For this purpose, a unique FE THM (Thermo-hydro-mechanical) model simulating the heat losses from underground structures into the ground over a 50-year period has been developed for the Loop District in Chicago (USA). It can also be adapted to any other region for which geological and underground structural data are available. The case study is challenging due to its relevance and location. Indeed, the Loop remains the second largest commercial district in the United States after Midtown Manhattan and statistics show that it consumes one third of Chicago’s annual energy. By this means, the importance and necessity of addressing this topical issue is shown. This study analyses the impact of heat sources on the thermal, mechanical and hydraulic properties of the subsurface while demonstrating a latency time observed between thermal and mechanical effects. The hydraulic investigations carried out have demonstrated the constancy as well as the very low impact of the groundwater flow over the time in Chicago. A detailed study on the interaction of the different heat sources showed that the combined effect of the different heat sources present (basement, subway and ambient air) is the sum of the effects of each of them separately. Finally, it demonstrated that the contribution of the heat sources to the thermal and mechanical variations always undergoes the same process over time and the same final distribution: 70% for the basements, 30% for the underground subway lines and >1% for the ambient air.