Multi-objective optimization of a pipe energy pile with heat exchanger pipes subjected to chaotic advection

Abstract

Shallow geothermal energy extraction through an energy pile system relies on the constant temperature available below the earth's surface at shallow depths. The energy pile system transfers the heat stored in the earth to a place of utility for space heating and cooling through fluid circulation, thus extracting geothermal energy. In this study, a 3D energy pile model comprising a helical heat exchanger pipe (HHEP) was developed and validated with the existing experimental data using a FEM-based software package. Further, it was hypothesized that replacing HHEP with a chaotic configured helical heat exchanger pipe (CHEP) would enhance thermal performance. The numerical analyses supported the hypothesis, where there was about a 15 % increase in heat release rate for energy piles containing CHEP. The above results form a basis for exploring the simulation results through laboratory scale model tests. Thus, an experimental setup was fabricated to conduct geothermal energy tests on a scaled pipe energy pile (PEP) model. The investigation was carried out by considering three design-input parameters: flow rate, pitch, and inlet temperature at three different levels with water and ethylene glycol as heat exchanger fluid (HEF) in a 4:1 ratio. The heat release rate of the PEP (Qp), heat flux per unit length of HEP (qpl), soil excess temperature (θs), total thermal resistance (TTR), and energy consumed (EC) were analyzed. Furthermore, multi-objective optimization using the MO-Jaya algorithm and multiple attribute decision-making (MADM) R-method were used to predict an optimal set of design-input parameters. Finally, a confirmation test was conducted to test the competence of the predicted parameters. The percentage variation between the predicted design input parameters and the experimental values was below 10 %.