Structural optimization of underground pipe network for geothermal heat extraction

Abstract

Geothermal resources can be classified into shallow and deep geothermal resources according to the formation temperatures of reservoirs. Deep geothermal resources can be explored for space heating and power generation, which have been attracted more researching attentions than the shallow geothermal due to their higher coefficient of performance and stability. The traditional ways of heat extraction from deep geothermal reservoirs include briefly: drilling production and injection wells in a confined aquifer or an artificial fractured reservoir in hot dry rocks. In recent years, there exists a few researches on heat extraction using a closed pipe loop without pumping geofluids out of surface, such as the U shaped deep borehole heat exchanger. However, that whether the route design of pipe underground is optimal remains to be answered. This paper proposes a underground pipe network optimal method to improve the heat extraction efficiency through increasing the heat transfer between the circulating working fluid in the loop and reservoir. To this end, a structural topology optimization issue associated with the heat transfer in porous media has to be taken into account. A simplified flow and heat transfer model is adopted to transform the three-dimensional wellbore into one-dimensional pipeline, which can reduce the computational cost. Then, the ground structure method is used to optimize the underground pipe network with multi objectives of minimizing the flow resistance and maximizing the heat transfer efficiency under the constraint of total pipe volume. Adjoint method and method of moving asymptotes are used to solve this topology optimization problem. Influences of different parameters, such as total pipe volume, flow velocity and temperature, on the optimal structure are analyzed to provide reference for the design of underground pipe network. The obtained results are helpful in finding a new method for geothermal exploration and improving the heat extraction efficiency.