3D numerical modeling of geothermal tunnels and receivable energy

Abstract

Nowadays, with development of subway tunnels, heating and cooling stations are playing an important role. Shallow geothermal energy as a renewable time-independent energy source helped significantly to reduce energy consumption for supplying ventilation, heating, and cooling of subway stations. The amount of accessible energy from such modern systems is of great importance in their economic assessment. Hence, this research aims to perform and present finite element modeling to evaluate the amount of exploited energy through ground heat exchangers (GHE) in tunnels. After validation of implemented modeling, the effect of various involved parameters in modeling was studied for a short-term operation of heat exchange. Additionally, the effect of lining isolation with respect to the inner environment of the tunnel was investigated. The results show that by increasing water velocity in the pipe, the total extracted power increased in both isolated and non-isolated conditions. The results can be classified into three main categories. 1. By increasing (a) the spacing between the pipe and inner environment of the tunnel in isolated case, (b) the inner temperature of the tunnel in non-isolated case, and (c) thermal conductivity of soil in both isolate and none-isolate cases, the total extracted power could be improved. The pipe diameter has a fascinating effect on the total extracted power. Firstly, it reduces the amount of extracted power. However, in the case of larger diameters, due to the capability of discharging more water, the amount of extracted power is increased. 2. By increasing (a) the spacing between the pipe and inner environment of the tunnel in non-isolated conditions and (b) pipe thickness in both non-isolated and isolated conditions, the total extracted power is reduced. 3. By increasing the specific heat capacity, density, and porosity of the soil, the total extracted power slightly changes. Indeed, these parameters have a negligible effect on extracted power.

Keywords

• subway tunnels
• shallow geothermal energy
• finite element modelling
• ground heat exchangers (ghe)
• parametric study